Multiscale interfacial engineering strategies for inorganic all-solid-state lithium batteries

Abstract

All-solid-state lithium batteries (ASSLBs) offer exceptional energy density and safety, yet interfacial instability at both cathode and anode remains a major challenge. This review pioneers a unified, multiscale framework that systematically dissects interfacial failure mechanisms across mainstream inorganic solid-state electrolytes (SSEs)—including oxides, sulfides, and halides—under multiphysics coupling. Unlike previous studies focusing on isolated materials or single strategies, we provide a critical, side-by-side comparison of leading interface engineering routes—electrode engineering, interlayer construction, electrolyte regulation, and integrated dual-interface design—emphasizing their respective merits, limitations, and application boundaries. Furthermore, advanced characterization techniques are summarized to reveal dynamic interfacial evolution and validate the effectiveness of various engineering approaches, thereby bridging fundamental understanding with practical design. Finally, future perspectives highlight multiphysics characterization, machine-learning-assisted interface material screening, and scalable process integration. This review offers a comprehensive, comparative, and practical guide to interface innovation in high-performance ASSLBs.

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Broader context

With the global push toward carbon neutrality and the rapid integration of renewable energy, advanced storage systems are required to deliver higher energy density, enhanced safety, and scalable manufacturing. While commercially mature, conventional lithium-ion batteries are intrinsically limited by the flammability of liquid electrolytes and their poor compatibility with high-capacity electrodes, restricting their suitability for next-generation applications such as electric vehicles and grid storage. ASSLBs enabled by thermally stable SSEs offer improved intrinsic safety, structural robustness, and access to the full capacity of lithium metal. However, the solid–solid nature of the electrode–electrolyte interface gives rise to coupled electrochemical, chemical, and mechanical challenges. Poor physical contact, interfacial reactions, and uncontrolled lithium dendrite growth critically limit the rate capability and long-term cycling stability of ASSLBs. Overcoming these issues requires holistic interface design strategies that integrate structural optimization, interfacial chemistry regulation, and advanced multiscale characterization. This review examines three major classes of SSEs—oxides, sulfides, and halides—systematically analyzing their interfacial contact behavior, electrochemical compatibility, and dendrite-related failure modes. It further summarizes mainstream interface engineering approaches and clarifies their applicability across different SSE systems, providing mechanistic understanding and practical guidance for the development of high-performance ASSLBs.

1. Introduction

Against the backdrop of carbon neutrality and the rapid expansion of renewable energy, energy storage technologies are facing unprecedented demands for both enhanced performance and intrinsic safety.^1–3 Conventional lithium-ion batteries, limited by their energy density and the safety risks associated with liquid electrolytes, can no longer meet the growing requirements for high energy and high reliability in electric transportation, grid regulation, and large-scale energy storage systems.^4,5 In this context, ASSLBs have emerged as a promising candidate for next-generation high-energy-density storage systems, owing to the ultra-high theoretical specific capacity (3860 mAh g⁻¹) and the lowest electrochemical potential (−3.04 V vs. SHE) of lithium metal, as well as the enhanced thermal stability and safety enabled by replacing flammable liquid electrolytes with SSEs.^6–9 Within ASSLBs, SSEs not only serve as ionic conductors but also fulfill critical roles in interfacial contact, mechanical support, and dendrite suppression, with their properties directly governing the overall energy output and cycling stability of the battery.^10,11 SSEs can be broadly classified into organic polymer-based and inorganic crystalline systems, depending on their chemical composition and phase structure.^12–15 Polymer electrolytes based on Poly(ethylene oxide) (PEO) and Poly(methyl methacrylate) (PMMA) offer good flexibility and film-forming ability, facilitating intimate contact with electrodes. However, their inherently limited ionic conductivity and electrochemical stability hinder their compatibility with high-voltage cathodes or lithium metal anodes.^16–19 In contrast, inorganic SSEs exhibit highly ordered crystalline frameworks and diverse topological Li⁺ conduction pathways, providing fast and well-defined ionic transport through interconnected tetrahedral and octahedral sites. Their rigid inorganic lattices and strong ionic bonding confer high thermal and electrochemical stability, enabling operation over wide voltage windows (>4 V) and elevated temperatures without degradation. Meanwhile, the high mechanical modulus of inorganic SSEs ensures structural stability and stable electrode–electrolyte contact, effectively suppressing lithium dendrite penetration.^20–22 Depending on their anionic frameworks and crystal symmetries, inorganic SSEs can be divided into three major families: oxide, sulfide, and halide systems, each possessing unique conduction mechanisms and structural advantages (Table 1). Oxide SSEs (e.g., Li₇La₃Zr₂O₁₂, Li_1.3Al_0.3Ti_1.7(PO₄)₃) feature strong structural rigidity, broad electrochemical stability, and excellent compatibility with high-voltage cathodes; sulfide SSEs (e.g., Li₁₀GeP₂S₁₂, Li₆PS₅Cl) offer superionic conductivity up to 10⁻² S cm⁻¹, high deformability, and facile processability; while halide SSEs (e.g., Li₃InCl₆, Li₃YCl₆) combine high ionic conductivity (∼10⁻³ S cm⁻¹) with exceptional oxidative stability and favorable mechanical compliance.^23–33 These collective merits endow inorganic SSEs with the ability to simultaneously achieve fast Li⁺ transport, wide electrochemical operation windows, and robust mechanical reliability, establishing them as one of the most promising material systems for next-generation solid-state batteries.

Table 1 Summary of inorganic solid-state electrolytes and their key properties

Electrolyte system	Type	Crystal structure	Advantages	Disadvantages	Ref.
Oxide SSEs	Garnet	Cubic/Tetragonal (e.g. Li7La3Zr2O12)	High ionic conductivity (10^−2–10^−3 S cm^−1)	High sintering temperature	23
			Wide electrochemical window	Li2CO3 surface formation
			Stable vs. Li metal	Poor Li wettability
			Good mechanical strength
	Perovskite	Layered ABO3 structure (e.g. Li0.33La0.56TiO3)	High bulk conductivity (∼10^−4 S cm^−1)	Ti^4+ reduction by Li	24
			Dense microstructure	Large grain-boundary resistance
			Stable framework
	LISICON	Orthorhombic/Tetragonal (e.g. Li4SiO4)	Stable	Moderate conductivity (∼10^−6–10^−4 S cm^−1)	25
			Low-cost	Brittle and rigid
			Easy synthesis
	NASICON	Rhombohedral/Orthorhombic (e.g. Li1.3Al0.3Ti1.7(PO4)3)	Good chemical stability	Poor compatibility with Li (Ti^4+/Ge^4+ reduction)	26
			Air-stable	High interfacial impedance
			Strong mechanical strength
Sulfide SSEs	Glasses state	Amorphous Li2S–P2S5 glass network	High formability	Low ionic conductivity (∼10^−4 S cm^−1)	27
			Good interparticle contact	Poor structural stability
			Simple synthesis
	Glasses-ceramics state	Mixed amorphous-crystalline (e.g. Li7P3S11)	High ionic conductivity (∼10^−3 S cm^−1)	Moisture sensitive	28
			Low interfacial resistance	Narrow electrochemical window
			Easy densification
	Thio-LISICON	Monoclinic (e.g. Li10GeP2S12)	Superionic conductivity (∼10^−2 S cm^−1)	Air-sensitive (H2S release)	29
			3D Li^+ pathways	Unstable vs. Li metal
			Soft and ductile
	Li-argyrodite	Cubic (e.g. Li6PS5Cl)	3D Li^+ migration channels	Limited chemical stability	30
			Tunable anion disorder	Interfacial decomposition with electrodes
			High conductivity (∼10^−3 S cm^−1)
Halide SSEs	LISICON-like	Trigonal (e.g. Li3YCl6)	High oxidative stability (>4 V)	Anisotropic Li^+ diffusion (1D channels)	31
			Low migration barrier	Moderate ionic conductivity (∼10^−3 S cm^−1)
			Tunable by cation substitution
			Good deformability
		Orthorhombic (e.g. Li3LuCl6)	Air-tolerant	Low ionic conductivity (∼10^−4 S cm^−1)	32
			Stable framework	Hygroscopic under humid conditions
			Compatible with high-voltage cathodes
		Monoclinic (e.g. Li3InCl6)	High ionic conductivity (2–3 × 10^−3 S cm^−1)	Unstable vs. Li metal	33
			3D Li^+ migration network	Moisture-sensitive
			Facile wet-chemical synthesis	High In cost

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However, the inherent structural rigidity and electronic bandwidth narrowing resulting from the high lattice order and chemical stability of inorganic SSEs also give rise to a series of interfacial challenges. On one hand, these materials often exhibit significant mismatches with electrode components in terms of mechanical modulus, electrochemical potential, and lattice structure. Such disparities lead to stress concentration, abrupt ionic flux transitions, and distorted electric field gradients at solid–solid interfaces, ultimately causing poor interfacial contact, ion transport bottlenecks, and localized electrochemical instability.^34–38 On the other hand, their intrinsically low electronic conductivity, while beneficial for suppressing the propagation of parasitic reactions, also promotes the formation of space charge layers (SCL) and electron–ion decoupling at the interface, further exacerbating interfacial polarization and the heterogeneity of interphase evolution.^39–41 Moreover, under extreme chemical potential conditions such as at the interfaces with lithium metal or high-voltage cathodes, the active metal centers in inorganic SSEs (e.g., Ti⁴⁺, Ge⁴⁺, In³⁺) are often subject to valence state changes, leading to the formation of mixed ionic-electronic conducting (MIEC) phases. These transformations result in increased interfacial impedance and continuous performance degradation.^42–44 Therefore, establishing stable interfaces between inorganic SSEs and electrodes has become a key scientific challenge in advancing ASSLBs from laboratory research toward practical engineering applications.

Under this background, numerous strategies have been proposed in recent years to engineer stable interfaces between inorganic SSEs and electrodes, which can be roughly summarized into the following aspects: (1) introducing interfacial buffer layers to create graded transitions in electrochemical potential and mechanical modulus, using materials such as alloys, oxides and polymers.^45–49 (2) modifying the electrode materials via surface coating, elemental doping, or crystal structure reconstruction to suppress interfacial reactivity and enhance compatibility with ionic flux.^50–54 (3) applying surface in situ chemical pretreatment, elemental tuning, or grain boundary engineering to the SSEs in order to enhance its tolerance to extreme chemical environments.^55–60 (4) engineering composite lithium metal anodes or incorporating three-dimensional host structures to relieve deposition stress, enhance interfacial wettability, and enable uniform lithium plating.^61–65 Despite substantial advances at the material level, most existing approaches remain confined to a single class of SSEs or isolated physicochemical factors. Current reviews typically focus on specific SSE families or individual interfacial engineering strategies, lacking a comprehensive framework for cross-system comparison and critical evaluation. More importantly, the advantages, limitations, and practical boundaries of various interfacial strategies have rarely been systematically summarized, hindering the transfer and generalization of interfacial mechanism understanding. In addition, conventional methods often fall short in addressing increasingly complex application scenarios—such as high current densities, extreme temperatures, or the integration of high-voltage cathodes—thus exposing core challenges in stability, universality, and scalability.

To address these gaps, this review proposes, for the first time, a multiscale, multiphysics-coupled analytical framework that systematically dissects the key interfacial failure mechanisms in inorganic ASSLBs, comprehensively covering the three major classes of SSEs: oxides, sulfides, and halides. By integrating interfacial challenges at both the cathode and anode—such as structural mismatch, interfacial reactions, electronic leakage, and lithium dendrite evolution—with the intrinsic properties of each SSE system and their coupling mechanisms under multiphysical fields, this review offers a holistic and in-depth analysis. Furthermore, this work categorizes and critically compares state-of-the-art interfacial optimization strategies from four key dimensions: electrode engineering, interlayer construction, electrolyte regulation, and integrated dual-interface design (Fig. 1). Of particular note, the review provides a thorough comparative analysis of the strengths, limitations, and applicability of each strategy—not only elucidating their fundamental mechanisms but also clarifying their material compatibility and potential for scalable engineering. These systematic comparisons and critical insights contribute to the establishment of a unified and logically coherent framework for interfacial design, alongside actionable guidance for engineering practice. Together, they lay a solid theoretical foundation for achieving high energy density and long cycle life in ASSLBs, while clearly highlighting the distinctive innovation and integrated perspective of this review in research scope, methodological approach, and strategic synthesis.

Fig. 1 Diverse approaches to engineering stable interfaces in inorganic ASSLBs.

2. Cathode–electrolyte interface

2.1. Challenges

In ASSLBs, the interfaces between the cathode and SSEs represent a critical bottleneck that limits both energy density utilization and long-term cycling stability. Despite the distinct properties and application merits of the three major SSE families—oxides, sulfides, and halides—they face several common interfacial challenges when integrated with high-voltage cathodes, including mechanical mismatch, chemical instability, and ion transport barriers induced by SCL (Fig. 2).

Fig. 2 Schematic illustration of various interface issues in inorganic ASSLBs.

2.1.1. Interfacial physical mismatch. During charge and discharge, cathode active materials undergo lattice expansion and contraction. If the SSE lacks sufficient mechanical compliance, structural degradation such as interfacial cracking, delamination, or pore formation can easily occur. Oxide SSEs, owing to their high mechanical strength and rigidity, struggle to form continuous contact with cathode particles during densification, and are prone to microcrack formation and contact failure during cycling due to localized stress concentrations.^66,67 In contrast, sulfide and halide SSEs generally exhibit lower Young's modulus and better formability, enabling more intimate interfacial contact during initial cold pressing.^68,69 However, their long-term interfacial stability is constrained by their respective intrinsic structures and mechanical response behaviors. For sulfide SSEs, the intrinsically low shear modulus and weak interparticle adhesion render them susceptible to microstress disturbances induced by cathode lattice reconstruction or phase transitions during cycling. These localized stresses often lead to particle debonding and gradual pore formation, thereby disrupting interfacial integrity.^70,71 Halide SSEs, on the other hand, display pronounced brittleness. The presence of low-coordination halide anions (e.g., Cl⁻) weakens the lattice bonding energy and stress absorption capacity, which facilitates grain displacement and structural relaxation during electrode expansion. This results in the evolution of heterogeneous void regions at the interface, undermining the efficiency of electron–ion synergistic transport.^72,73 In composite cathode systems, both classes of SSEs are typically co-processed with high-surface-area carbon conductors and oxide-based active materials. However, pronounced disparities in interfacial wettability and mechanical compatibility across the three-phase boundaries often lead to the formation of isolated non-contact regions and electrochemically inactive “dead zones”. These discontinuities interrupt the electron–ion co-migration channels, significantly increasing interfacial resistance and polarization.^74,75 Consequently, although flexible SSEs exhibit favorable interfacial conformity during the initial densification stage, the progressive accumulation of mechanical stress and mismatch among the multiple constituents during prolonged cycling has emerged as a key bottleneck limiting interfacial stability and energy utilization efficiency.

2.1.2. Interfacial chemical instability. The chemical instability of various types of SSEs at the cathode side arises from differences in their intrinsic electrochemical stability windows, crystal structure characteristics, and interfacial reaction kinetics, reflecting distinct material-dependent features.

Oxide SSEs generally exhibit a wide electrochemical stability window and excellent thermal robustness, demonstrating strong oxidation resistance under high-voltage cathode conditions.^76,77 Nevertheless, achieving sufficient densification and high ionic conductivity often necessitates high-temperature sintering to promote grain growth and eliminate porosity. During this process, interdiffusion and interfacial reactions may occur between the oxide SSEs and cathode active materials (e.g., LiCoO₂), causing the emergence of undesirable secondary phases such as LaCoO₃ and LiTi₂(PO₄)₃—where LaCoO₃ acts as an ionic insulating phase impeding Li⁺ transport, while LiTi₂(PO₄)₃, though a NASICON-type Li⁺ conductor, can undergo reduction of Ti⁴⁺ to Ti³⁺, leading to irreversible lithium consumption and interfacial instability—which disrupt the structural integrity of the interface.^78,79

Sulfide SSEs offer high room-temperature ionic conductivity and good moldability, which facilitate the formation of dense structures and continuous lithium-ion transport pathways, thereby enabling low interfacial resistance. Although their thermodynamic electrochemical stability window is relatively narrow (approximately 1.7–2.2 V vs. Li⁺/Li), sulfide SSEs can still stably operate with high-voltage cathodes because, during the initial charging process, limited interfacial redox reactions occur between the oxidized cathode surface and the sulfide framework. These reactions generate a thin layer of partially oxidized species (e.g., Li₂S, Li₃P, and Li–S–O/P–S–O compounds) that electronically isolate the bulk electrolyte while maintaining Li⁺ conduction.^80,81 Once this layer forms, it kinetically suppresses further electron transfer and electrolyte decomposition, thus acting as a self-passivating, metastable interphase that effectively extends the apparent electrochemical stability window beyond the thermodynamic limit. However, this kinetic stabilization is not permanent. During prolonged cycling, mechanical stress and repeated lithiation/delithiation induce microcracks at the electrode–electrolyte interface, exposing fresh reactive surfaces that locally rupture the passivation layer. Meanwhile, high-voltage operation continuously generates reactive oxygen species, which further attack P–S and Ge–S bonds in the sulfide framework, triggering progressive oxidative degradation and producing electronically insulating by-products such as Li₂SO₄ and Li₃PO₄. These insulating phases accumulate at the interface, thickening the cathode electrolyte interphase (CEI) and increasing interfacial impedance.^82,83 Ultimately, localized electron penetration and spatially inhomogeneous reactions lead to excessive polarization, loss of ionic percolation pathways, and gradual deterioration of interfacial stability.

Halide SSEs have recently attracted significant attention as emerging electrolyte systems, owing to their excellent ionic conductivity and theoretical stability against high-voltage cathodes. However, recent studies have revealed that halide SSEs still face risks of interfacial chemical evolution under practical operating conditions. Under high electrochemical potentials, the release of high-energy lattice oxygen from the cathode generates a strongly oxidative environment that can trigger the oxidation of halide ions (Cl⁻ or Br⁻) within the electrolyte, converting them into neutral Cl⁰ or Br⁰ species. These neutral intermediates are often volatile or chemically unstable, compromising the electronic insulation of the electrolyte, inducing electron leakage, and driving continuous evolution of the interfacial electronic structure.⁸⁴ In parallel, high-valence transition metal ions (e.g., Ni³⁺/Ni⁴⁺) present in high-voltage cathodes possess strong electrostatic modulation capabilities. These species can trigger valence fluctuations and charge redistribution of halide anions at the interface, promoting the gradual formation of non-uniform reaction layers. Such interfacial inhomogeneity distorts the potential landscape and hinders efficient lithium-ion transport.⁸⁵ Therefore, although the interfacial reaction kinetics of halide SSEs are generally slower than those of sulfide counterparts, the progressive accumulation of degradation products during cycling can still result in impedance growth and capacity fading over time.

2.1.3. Ion transport barriers induced by SCL. At the interface between the SSE and cathode, differences in lithium-ion chemical potential drive the redistribution of ions and electrons, leading to the formation of a SCL.⁸⁶ This interfacial region typically exhibits local lithium-ion accumulation or depletion, accompanied by potential distortion and the development of internal electric field, which significantly impedes cross-interface lithium-ion transport. In oxide SSEs, the formation and stabilization of the SCL are governed by their intrinsic physicochemical characteristics. Owing to their wide bandgaps, extremely low electronic conductivity, and dense, rigid oxygen frameworks, electronic charge compensation during interfacial potential equilibration is significantly suppressed. As a result, charge redistribution primarily proceeds through ionic defect species, leading to a lithium-ion-depleted region on the electrolyte side. Moreover, the strong ion-lattice interactions and limited dielectric polarizability of oxide SSEs slow defect relaxation and weaken interfacial potential screening, thereby establishing and maintaining a pronounced electric field gradient across the interface. Although this interfacial region is typically only a few nanometers thick, it markedly reduces the concentration of mobile lithium ions and increases interfacial resistance, making its blocking effect on cross-interface lithium-ion transport particularly pronounced under conditions of incomplete interfacial contact or defect accumulation.⁸⁷ For sulfide SSEs, the relatively narrow electrochemical stability window increases their vulnerability to lithium extraction and parasitic product generation at high-voltage cathode interfaces. These processes give rise to a remarkable lithium-deficient space charge region, while the buildup of electronically and ionically insulating phases further hinders lithium-ion transport and elevates the interfacial potential barrier, representing a major source of stagnant interface dynamics.⁸⁸ In contrast, halide SSEs—owing to their higher electrochemical stability and enhanced dielectric polarizability—tend to form a thinner SCL with limited lithium concentration gradients and weaker interfacial electric field distortion. As a result, their impact on interfacial ion transport is comparatively moderate.⁸⁹ In the overall design of ASSLB interfaces, the SCL has been recognized as a critical factor governing interfacial ion transport and rate performance. Therefore, its suppression requires coordinated strategies such as interfacial potential alignment, defect concentration control, and optimization of dielectric properties.

In summary, the multifaceted interfacial evolution at the cathode side originates from the synergistic effects of mechanical mismatch, electrochemical imbalance, and charge redistribution. Although the specific manifestations vary depending on the types of SSEs, they fundamentally converge on two critical issues: the disruption of structural continuity and the obstruction of lithium-ion transport pathways. A deeper mechanistic insight into interface degradation across different SSE systems, coupled with the deliberate construction of interfacial microenvironments that offer mechanical compliance, chemical robustness, and electrostatic compatibility, will be pivotal for enabling the long-term stability of cathode interfaces in high-performance ASSLBs.

2.2. Optimization strategies

In ASSLBs, the interfacial stability between high-voltage cathodes and SSEs is a critical factor governing overall cell performance. To address common challenges at the cathode–SSE interface—such as mechanical stress mismatch, interfacial chemical reactivity, and impeded ion transport—various interfacial engineering strategies have been proposed. These approaches can be broadly categorized into three main types: cathode engineering, interlayer construction and electrolyte bulk optimization. The following sections provide a detailed discussion of each approach, accompanied by comparative analyses of representative pathways and case studies.

2.2.1. Cathode engineering. The structural integrity and interfacial compatibility of cathode materials play a pivotal role in determining the interfacial stability and electrochemical performance of ASSLBs. Accordingly, structural optimization and interfacial regulation strategies centered on the cathode itself have emerged as key approaches to enhancing battery performance. This section provides a comprehensive overview of cathode engineering strategies, focusing on microstructural design, functional interface construction, and the synergistic configuration of composite electrodes.
Single-crystal design. Given that grain boundaries in polycrystalline materials often act as initiation sites for stress concentration and structural degradation during cycling, the use of single-crystal particles has been proposed as an effective approach to improve the structural robustness of cathode materials. For instance, in contrast to polycrystalline counterparts where anisotropic lattice expansion often induces microcrack formation and interfacial delamination, micron-sized single-crystal LiNi_0.83Co_0.11Mn_0.06O₂ (SC-N83) particles effectively suppress these degradation mechanisms, thereby exhibiting excellent cycling stability even at high voltages (up to 4.4 V) (Fig. 3a).⁹⁰ However, due to the limitations of large single-crystal particles in terms of ion transport kinetics and interfacial contact, the development of smaller single-crystal cathodes has gained increasing attention. Tian et al. reported that ∼1 µm single-crystal LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) particles not only display superior structural integrity compared to conventional large-particle designs, but also offer enhanced interfacial contact and electrochemical stability under high-loading and high-rate conditions, attributed to their larger specific surface area and shorter diffusion pathways.⁹¹

Fig. 3 (a) The structural evolution and cycling performance of the PC-N83 and SC-N83 electrodes. Reproduced with permission.⁹⁰ Copyright 2021, Elsevier. (b) Schematic illustration of the surface reconstruction of LCO. Reproduced with permission.⁹² Copyright 2024, John Wiley and Sons. (c) Schematic diagram of the synthesis process for Li–Ta–O–F (LTOF) oxyhalide coating through an in situ gas–solid reaction. Reproduced with permission.⁹⁷ Copyright 2024, John Wiley and Sons. (d) Schematic of the preparation process and principle of LCO@PPy. Reproduced with permission. Reproduced with permission.¹⁰³ Copyright 2024, Royal Society of Chemistry.

Structural and surface regulation. Although single-crystal engineering helps preserve the internal structural coherence of cathode particles, the surface remains highly susceptible to severe interfacial side reactions under high-voltage conditions. To address this, Zhang et al. proposed a surface lattice doping strategy for LiCoO₂ (LCO) cathode.⁹² By introducing AlPO₄ precursor and inducing Al³⁺ ion doping into the surface lattice of the cathode through high-temperature annealing, cation-disordered layer with a rock-salt-like structure is formed (Fig. 3b). This Al/Co/Li mixed atomic layer exhibits low electrochemical activity and exceptional interface stability, effectively inhibiting the oxidative decomposition of Li₃InCl₆ under high voltage and maintaining the structural integrity and reversible lithium-ion transport capability of the cathode–SSE interface at 4.5 V. However, doping strategies are primarily oriented toward lattice-level modifications and remain insufficient to mitigate direct interfacial degradation between the cathode and SSE. To tackle this issue, the construction of physical and chemical buffering layers on particle surfaces is still imperative.

In this context, inorganic coating layers have emerged as a preferred approach.^93–98 Zhang et al. employed a gas–solid interfacial reaction to construct a CoO/Li₂CO₃ dual-phase layer on the surface of LCO (SR-LCO), which effectively passivates reactive oxygen species and blocks the diffusion of active anions such as S²⁻ and PS₄³⁻ from the sulfide electrolyte.⁹⁶ As a result, the SR-LCO cathode retains 84.63% of its capacity after 400 cycles at 0.2C. Unfortunately, despite the advantages of facile fabrication and in situ reconstruction, this strategy still faces challenges under long-term high-voltage cycling, such as interfacial stress concentration and compositional inhomogeneity arising from the dual-phase crystalline structure. To further improve interfacial structural continuity and regulation precision, Zhang et al. employed volatile TaF₅ to react with surface residues on the cathode, thereby in situ forming an amorphous Li–Ta–O–F coating (Fig. 3c).⁹⁷ This layer is ion-conductive but electronically insulating, enabling the battery to retain 80.4% of its capacity after 200 cycles. It is worth noting that although the amorphous inorganic interfacial layer significantly mitigates chemical instability, it still exhibits limitations in terms of strain accommodation and interfacial compliance. To this end, flexible organic coatings have been proposed as a complementary strategy.^99–103 Zhou et al. constructed an organic interfacial layer using polypyrrole (PPy), where −NH groups form hydrogen bonds with lattice oxygen, suppressing oxygen release and enabling the in situ formation of a Li₂O–LiOH buffer layer to mitigate the oxidative decomposition of the SSE (Fig. 3d).¹⁰³ More importantly, the flexible structure of PPy significantly enhances physical contact between the cathode and the SSE, effectively accommodating volume changes during cycling. As a result, the NCM@PPy cathode achieves a capacity retention of 81.2% after 300 cycles. Since neither inorganic nor organic coatings alone can simultaneously ensure chemical stability and mechanical flexibility, Su et al. employed molecular layer deposition (MLD) to construct an aluminum-glycerol (Al-GL) inorganic–organic hybrid coating on the surface of NCM811 (Fig. 4a).¹⁰⁴ This coating shows an elastic modulus of 0.17 GPa and demonstrates excellent adhesion and strain adaptability, effectively preventing structural cracking and electrolyte delamination. Thus, the battery retains 88% of its capacity after 100 cycles at 0.2C (Fig. 4b).

Fig. 4 (a) Schematic diagram of the functioning mechanism for the Al-GL coating. (b) The long-term cycling performance tests of the bare NCM811, Al-GL-5@NCM811, Al-GL-10@NCM811, and Al-GL-15@NCM811 at 0.2C. Reproduced with permission.¹⁰⁴ Copyright 2023, Elsevier. (c) The scheme of dual-modified layer strategy for LCO thin film. (d) The cycling stability of bare LCO and Ti@LCO@LCPO under the rate of 2C at 10 °C. Reproduced with permission.¹⁰⁶ Copyright 2024, Elsevier. (e) Schematic illustration of LMO₃ (M = Ta and Nb) polarization in ASSBs. (f) Corresponding HAADF-STEM image for modified NCM. Reproduced with permission.¹⁰⁷ Copyright 2024, American Chemical Society.

In fact, isolated coating or doping strategies still face constraints in addressing structural degradation and interfacial instability under high-voltage conditions. As a result, synergistic designs that combine bulk doping with surface modification have gradually emerged as a mainstream approach.^105–107 Qiu et al. stabilized the lattice structure through Ti doping and introduced a LiCoPO₄ coating layer epitaxially matched with the LCO lattice, thereby constructing a Ti@LCO@LiCoPO₄ cathode with dual stabilization effects (Fig. 4c).¹⁰⁶ This design markedly boosts cycling stability at high voltage and achieves a capacity retention of 75% after 500 cycles under 2C at 10 °C (Fig. 4d). In contrast to conventional static regulation strategies that rely on lattice doping and surface coating to achieve interfacial stability, Dai et al. proposed a dynamic “chemical competitive diffusion” mechanism, where heteroatoms with distinct migration kinetics and bonding strengths spontaneously distribute along a bulk-to-surface gradient during annealing.¹⁰⁷ Ta⁵⁺ ions, possessing stronger Ta–O bonding and higher lattice-oxygen affinity, remain anchored within the bulk to suppress oxygen evolution and enhance structural integrity. In contrast, Nb⁵⁺ ions, with higher diffusivity, migrate outward and react with residual Li to form a self-assembled LiNbO₃ piezoelectric layer (Fig. 4e and f). This diffusion-driven reconstruction produces a graded interface endowed with spontaneous polarization that regulates the SCL, aligns interfacial electric fields, and promotes Li⁺ transport across the cathode–electrolyte interface. With this regulation, the NCM cathode paired with Li₁₀SnP₂S₁₂ electrolyte demonstrates significantly enhanced capacity retention and cycling stability at 4.5 V.

Functional network and structural integration. In composite cathode systems, even with effective surface regulation of active particles, the continuity of electronic and ionic pathways, along with the mechanical cohesion of the entire electrode, remains critical to interfacial performance and long-term stability. Although conventional carbon black exhibits high electronic conductivity, its large specific surface area and low crystallinity introduce numerous structural defects and oxygen-containing functional groups, which tend to generate localized electron accumulation regions at the carbon–electrolyte interface, thereby triggering the electrochemical decomposition of sulfide SSEs. To address this issue, Kim et al. systematically investigated the interfacial chemical stability of carbon materials with different degrees of crystallinity.¹⁰⁸ They compared amorphous carbon black (CB), graphitized carbon black (GCB) obtained via high-temperature treatment, and carbon nanofibers (CNF) possessing long-range ordered sp² carbon frameworks in Li₆PS₅Cl (LPSC) systems (Fig. 5a). The results reveal that high-crystallinity carbons (such as GCB and CNF), owing to their more ordered graphitic layers, lower surface defect densities, and reduced chemical reactivity, effectively suppress charge-transfer-induced electrolyte decomposition, thereby reducing interfacial resistance and improving cycling stability. On this basis, to further balance the construction of conductive networks with electrolyte stability, Saqib et al. proposed a hybrid conductive architecture comprising a minimal amount (0.3 wt%) of carbon nanotubes (CNT) and a higher proportion (4.7 wt%) of CNF, establishing a synergistic long-short electron transport network (Fig. 5b).¹⁰⁹ This approach avoids the electrolyte instability typically induced by conventional carbon additives such as Super P, while delivering superior rate capability and capacity retention, thereby validating the importance of optimizing both the geometry and loading ratio of carbon components in composite cathode systems. To further eliminate interfacial instability associated with carbon-based conductors, Fang et al. proposed the use of Ti₂O₃ as a substitute for conventional conductive carbon (Fig. 5c).¹¹⁰ Ti₂O₃ not only exhibits metallic-level electronic conductivity (10–10² S cm⁻¹) but also effectively scavenges lattice oxygen released from high-voltage cathodes during cycling, thereby suppressing interfacial oxidative reactions. A composite cathode constructed with Ti₂O₃, NCM811, and LPSC demonstrates excellent interfacial stability, delivering an initial capacity of 192 mAh g⁻¹ and retaining 86.5% of its capacity after 140 cycles. These results confirm the dual functionality of non-carbon conductors in enhancing both interfacial stability and electronic conductivity in ASSLB systems.

Fig. 5 (a) Schematic showing cell structure containing sulfide electrolyte and different types of conductive materials. Reproduced with permission.¹⁰⁸ Copyright 2025, Elsevier. (b) Schematic illustration highlighting the mechanisms of ionic and electrical conductivities in a cathode composite. Reproduced with permission.¹⁰⁹ Copyright 2024, American Chemical Society. (c) Schematic illustration, showing the interfaces in NCM-00, NCM-C, and NCM-Ti₂O₃ cathodes. Reproduced with permission.¹¹⁰ Copyright 2023, Elsevier. (d) Schematic diagram of fabricating slurry-cast NMC811 cathode. (e) Schematic diagram of 1270PIB improves the integrity of cathode that facilitates the Li transport over cycling. Reproduced with permission.¹¹² Copyright 2024, John Wiley and Sons. (f) Schematic illustration of homogeneous cathode microstructure evolution during charging. Reproduced with permission.¹¹⁴ Copyright 2024, Springer Nature.

Beyond conductive additives, binders also play an essential role in determining the film-forming quality, ionic transport pathways, and interfacial stability of composite cathodes.^111–113 Li et al. found that high-molecular-weight polyisobutylene (PIB) facilitates particle adhesion and strain-buffering capability (Fig. 5d and e), resulting in 44.77% increase in capacity under high mass loading conditions (8.51 mg cm⁻²).¹¹² Unlike other high-molecular-weight binders, PIB possesses a non-polar hydrocarbon backbone that is chemically compatible with sulfide SSEs and the non-polar solvent (toluene) used in slurry processing. This unique combination enables uniform particle wetting, continuous coating formation, and robust chain entanglement, thereby strengthening interparticle adhesion and suppressing interfacial voids. Nevertheless, the absence of ionic conductivity in PIB limits its overall performance enhancement. Toward this end, Charlesworth et al. developed a single-ion-conducting polycarbonate binder—poly(5-methyl-5-allyloxycarbonyl-trimethylene carbonate) (PMAC)—which combines excellent mechanical flexibility with interfacial adaptability, significantly improving lithium-ion transport and cycling stability.¹¹³

Still, the aforementioned strategies still rely on multiphase composite architectures, where the separation of electronic and ionic pathways and the complexity of interfacial configurations hinder structural integration and long-term stability. To address these limitations more fundamentally, intrinsic structural design of cathode materials offers a more holistic solution. Cui et al. proposed a “homogeneous cathode” concept and developed a Li_1.75Ti₂(Ge_0.25P_0.75S_3.8Se_0.2)₃ material featuring zero-strain characteristics and intrinsic mixed conductivity (Fig. 5f).¹¹⁴ Unlike conventional layered oxide cathodes that suffer from anisotropic lattice expansion and interfacial delamination, this sulfide-based Ti–P–S–Se framework originates from LiTi₂(PS₄)₃, a three-dimensional open network composed of edge-sharing TiS₆ octahedra and PS₄ tetrahedra. The partial substitution of P by Ge optimizes Li⁺ diffusion channels (D_Li⁺ = 6.5 × 10⁻⁸ cm² s⁻¹), while S-to-Se anion modulation narrows the bandgap from 1.29 eV to 0.52 eV, achieving high mixed ionic-electronic conductivities (σ_Li⁺ = 0.2 mS cm⁻¹, σ_e = 225 mS cm⁻¹). Owing to its low Young's modulus (17 GPa) and reversible Ti⁴⁺/Ti³⁺/Ti²⁺ and P⁵⁺/P^x+ redox couples, Li_1.75Ti₂(Ge_0.25P_0.75S_3.8Se_0.2)₃ undergoes only 1.2% volume change during cycling, maintaining structural integrity and interfacial conformity. Batteries constructed solely from this material, without any conductive or ionic additives, exhibit over 20 000 stable cycles and an energy density of 390 Wh kg⁻¹, pushing the boundaries of conventional cathode structure and functionality.

2.2.2. Interlayer construction. Building on structural stabilization and surface protection in cathode engineering, the construction of interlayers serves as a critical bridge between the cathode and SSE, further improving interfacial contact and chemical compatibility. To address the requirements of different application scenarios, researchers have developed various interlayer strategies—including polymer and molten salt interlayers—to meet the multifaceted demands of interfacial wettability, chemical passivation and thermal stability.
Polymer interlayers. As one of the earliest approaches explored, polymer interlayers have been widely employed to construct initial interfaces due to their inherent flexibility and film-forming capability. Wang et al. developed a PLC₆₀ interlayer, composed of PEO, lithium salt, and C₆₀, which forms an interfacial film rich in Li_xPO_yF_z, LiP_xF_y, and C₆₀F_n species on the surface of NCM811 cathodes, effectively suppressing cathode structural degradation (Fig. 6a and b).¹¹⁵ However, such interlayers generally suffer from low ionic conductivity, narrow electrochemical windows, and poor tolerance to extreme temperatures, limiting their applicability under harsh conditions. To improve transport properties and adaptability, Wu et al. designed an ether-based gel polymer interlayer featuring a flexible ion-conducting network, which boosts interfacial wettability and ion transport (Fig. 6c).¹¹⁶ The interlayer displays a room-temperature ionic conductivity of 2 mS cm⁻¹, enabling the cell to stably cycle for over 100 cycles at 0.3C (Fig. 6d).

Fig. 6 (a) Structural formulas and calculated HOMO/LUMO energies (eV) of the components of the catholyte buffer layer (PEO, LiTFSI, LiDFP, and C₆₀). (b) Schematic diagram showing the positive effects of C₆₀ additives on the catholyte interlayer and the NCM811 cathode. Reproduced with permission.¹¹⁵ Copyright 2024, American Chemical Society. (c) The diagram of the in situ polymerization process and optical photos. (d) Battery cycling of LFP@GPE/LSPCl/3D LSLL coin cells using different type of GPE interlayer at a rate of 0.3C. Reproduced with permission.¹¹⁶ Copyright 2024, Elsevier. (e) Structure for the solid-state battery with (Li,K,Cs)FSI molten salt. (f) Long cycling performance of the Li//LFP batteries modified by (Li,K,Cs)FSI molten salt at the rate of 1C at 60 °C. Reproduced with permission.¹¹⁷ Copyright 2023, American Chemical Society.

Molten salt interlayers. Although gel polymer interlayers enhance interfacial ion transport and adhesion, their organic matrices still suffer from limited thermal stability and poor oxidative resistance, hindering long-term operation under high-temperature or high-voltage conditions. To address these challenges, Yu et al. constructed a partially molten interlayer based on a low-melting (Li,K,Cs)FSI system, which features a wide electrochemical stability window of up to 4.5 V and can adaptively fill interfacial voids while forming continuous ion-conduction pathways at operating temperatures (Fig. 6e).¹¹⁷ This strategy achieves the cell to retain over 80% of its capacity after 1000 cycles at 60 °C and 1C, demonstrating outstanding thermal stability and electrochemical compatibility (Fig. 6f).

2.2.3. Electrolyte bulk optimization. In addition to cathode engineering and interlayer construction, optimizing the intrinsic properties of the SSE to enhance interfacial compatibility with high-voltage cathodes is also a key strategy for improving the overall performance of ASSLBs. Halide SSEs, in particular, offer unique advantages for bulk structural tuning due to their open crystal structures and compositional tunability. Consequently, a range of interface-friendly electrolyte design strategies—such as aliovalent doping, interface-guided bulk modification and amorphization engineering—have predominantly focused on halide systems. This not only highlights the engineering feasibility driven by their intrinsic material properties but also offers valuable insights for the design of other electrolyte families.
Aliovalent doping. Regulating lithium-vacancy concentration and local electronic environments through aliovalent substitution induces controlled lattice distortion that generates low-barrier Li⁺ migration pathways and optimizes the electronic potential landscape of the anion framework, thereby achieving concurrent enhancement of ionic conductivity and oxidative stability.^118–120 For instance, in the Li_3−xLu_1−xZr_xCl₆ system, substituting Lu³⁺ with Zr⁴⁺ precisely tunes the Li/vacancy ratio, yielding a high ionic conductivity of 1.5 mS cm⁻¹ and a low activation energy of 0.285 eV.¹¹⁹ The balanced distribution of Li⁺ and vacancies within the hexagonal close-packed (hcp) anion framework minimizes structural bottlenecks and ensures continuous Li⁺ transport through face-sharing octahedra, while Zr⁴⁺-induced lattice contraction and electrostatic perturbation strengthen the Cl–M bonds (M = Lu³⁺ or Zr⁴⁺), thereby suppressing anion oxidation and enhancing high-voltage stability. This electrolyte shows nearly decay-free cycling over 1000 cycles with high-voltage cathodes. Furthermore, incorporating multiple metal cations (e.g., Y, Er, Yb, In, and Zr) produces a high-entropy electrolyte (Li_2.75Y_0.16Er_0.16Yb_0.16In_0.25Zr_0.25Cl₆), whose configurational-entropy-driven cation disorder endows the lattice with distortion-stabilized characteristics (Fig. 7a and b).¹²⁰ This entropy-mediated structural distortion confines Cl⁻ vibrations and elongates Li–Cl bonds, promoting Li⁺ activation and inhibiting Cl⁻ oxidation. As a result, this high-entropy halide achieves both high ionic conductivity (1.171 mS cm⁻¹) and an expanded electrochemical stability window (4.6 V), markedly improving interfacial compatibility and structural robustness.

Fig. 7 (a) Neutron diffraction patterns and the corresponding refinement of HE-LIC. (b) Graphical representations of a HE-LIC unit cell from the refinement, showing MCl₆ (M = Y, Er, Yb, In, Zr) octahedral and LiCl₆ octahedral framework. Reproduced with permission.¹²⁰ Copyright 2024, Springer Nature. (c) Schematic illustration of the CEI evolution in Li₂YCl_2.5Br_1.5O_0.5 (2LO-0.5) ASSLBs. Reproduced with permission.¹²³ Copyright 2025, John Wiley and Sons. (d) RMC fit (red line) to the experimental G(r) (gray circle) of Li₃ZrCl₄O_1.5. Several possible basic building blocks in Li₃ZrCl₄O_1.5 are shown on the left side. On the right side, the schemes show the connectivity that leads to the edge-sharing and corner-sharing Zr-centered polyhedra in Li₃ZrCl₄O_1.5, respectively. Reproduced with permission.¹²⁴ Copyright 2024, American Chemical Society. (e) Illustration of a hydrate-assisted synthesis route for aluminum-based oxychloride SSEs. (f) The ionic conductivity of the LiAlOCl-ab1 SSEs at 30 °C. (g) Arrhenius plots of LiAlOCl-981 (a = 9, b = 8) and LiAlOCl-681 (a = 6, b = 8) SSEs. Reproduced with permission.¹²⁵ Copyright 2024, John Wiley and Sons.

Interface-guided bulk modification. Unlike conventional doping strategies that originate from crystal lattice design, interface-induced bulk modification emphasizes the design of spontaneous reaction pathways at the operating interface.^121–123 For example, Gao et al. introduced F⁻ into a Ta-based oxychloride electrolyte, leading to the in situ formation of a LiF-rich passivation layer at the interface, which effectively suppresses the oxidative decomposition of Cl⁻.¹²² Liu et al. incorporated O²⁻ into Li₃YCl₃Br₃ to form a Y–O covalent network, which induces the formation of a Y₂O₃-dominated passivation layer at the interface, successfully preventing detrimental reactions between cathode-derived active oxygen species and Y³⁺ (Fig. 7c).¹²³ As a result, the battery retains 80.6% of its capacity after 1000 cycles. This class of strategies enables the in situ construction of stable interphases through “pre-programmed interfacial reactions”, providing an effective buffer against high-voltage electrochemical environments.
Amorphization engineering. Compared to the localized rigidity and ion channel constraints of crystalline electrolyte structures, amorphous structures exhibit stronger interfacial adaptability and diffusion flexibility due to their short-range order and long-range disorder characteristics. In Li₃ZrCl₄O_1.5, oxygen incorporation increases the amorphous content to 89.5%, producing predominantly corner-sharing Zr–O/Cl polyhedra that form a three-dimensional (3D) interconnected network (Fig. 7d).¹²⁴ This configuration weakens lattice constraints and lowers the Li⁺ migration barrier (E_a = 0.10 eV vs. 0.12 eV in crystalline Li₂ZrCl₆), thereby enhancing ionic conductivity from the 10⁻⁴ S cm⁻¹ level to (1.3 ± 0.1) × 10⁻³ S cm⁻¹ at 25 °C. The flexible 3D network accommodates local stress, ensuring intimate contact and chemical stability with high-voltage cathodes. Building on this concept, Wang et al. synthesized amorphous-rich LiAlOCl via a hydrate-assisted route that introduces [Al_aO_bCl_c]^{(2b+c−3a)−} polyanionic clusters into a disordered LiCl-like matrix (Fig. 7e).¹²⁵ The resulting material contains 59 wt% amorphous component, exhibits Li⁺ conductivity >1 mS cm⁻¹ (E_a = 0.48 eV) (Fig. 7f and g), and maintains a wide electrochemical stability window up to 4.8 V. The combination of reduced structural rigidity and mixed O/Cl coordination diversifies Li⁺ environments, weakens Li⁺–X⁻ interactions, and stabilizes the amorphous network under high-voltage operation. These quantitative comparisons confirm that amorphization—by flattening the energy landscape for Li⁺ diffusion and providing elastic structural buffering—establishes a direct link between structural flexibility and high-voltage interfacial stability.

In summary, optimization strategies for the cathode–SSE interface can be broadly categorized into three pathways: cathode engineering, interlayer construction, and electrolyte bulk optimization (Table 2). In terms of general applicability, single-crystal cathode design, structural and surface regulation, as well as interlayer construction, are the main strategies commonly adopted for all three types of SSEs (oxides, sulfides, and halides). Specifically, mainstream cathode materials such as LCO and NCM typically encounter interfacial challenges including structural mismatch, oxygen release, and related side reactions when paired with inorganic SSEs. Additionally, NCM—particularly with high nickel content and polycrystalline structures—is prone to microcrack formation during cycling, further exacerbating interfacial discontinuity and chemical instability. To address these issues, single-crystal strategies, by reducing grain boundaries and microcracks, significantly enhance the mechanical robustness of cathode structures, with especially pronounced benefits for NCM. However, in the case of rigid oxide SSEs, achieving intimate contact between single-crystal particles and the electrolyte remains difficult, and interfacial resistance continues to be a limiting factor. Moreover, direct contact between single-crystal cathodes and SSEs tends to induce more severe high-voltage interfacial side reactions in sulfide systems; for oxide SSEs, interdiffusion and secondary phase formation during high-temperature processing are more prevalent, compromising interfacial electrochemical stability. On this basis, structural and surface regulation of the cathode—such as doping or introducing physical/chemical buffer layers—not only helps maintain the intrinsic cathode structure but also effectively suppresses interfacial side reactions, thereby improving overall interfacial stability. Nevertheless, the ability of these methods to accommodate strain is limited and may introduce compositional inhomogeneity. Building upon this, interlayer construction employing flexible, dense, and chemically passivating designs stands out for its ability to buffer stress, enhance wettability, and inhibit parasitic reactions. However, its long-term chemical stability and ionic conductivity under extreme conditions such as high temperature or voltage still require systematic optimization.

Table 2 Summary of optimization strategies for cathode–SSE interfaces

Interface optimization strategies	Cathode material	SSE type	Modification approaches	Initial capacity (mAh g^−1)	Cycling performance	Ref.
Single-crystal design	LiNi0.83Co0.11Mn0.06O2	Li9.54Si1.74P1.44S11.7Cl0.3	Grain-boundary-free particles	∼150	85.1% after 500 cycles at 0.5C	90
	LiNi0.8Co0.1Mn0.1O2	Li9.54Si1.74P1.44S11.7Cl0.3	Small particle (∼1 µm)	∼138	100% after 500 cycles at 1C	91
Surface lattice doping	LiCoO2	Li3InCl6	Al^3+ doping	72.4	88.5% after 2000 cycles at 3C	92
Inorganic coating	LiNi0.88Co0.11Al0.01O2	Li6PS5Cl	Li3YCl6 coating	∼185	84.7% after 200 cycles at 0.5C	93
	LiNiO2	Li6PS5Cl	LixAlyZnzOδ coating	∼203.08	83.1% after 200 cycles at 0.2C	94
	LiCoO2	Li10GeP2S12	Li7.5La3Zr1.5Co0.5O12 coating	121	96% after 100 cycles at 0.2C	95
	LiNi0.8Co0.1Mn0.1O2	Li1.3Al0.3Ti1.7(PO4)3	La4NiLiO8 coating	171.49	89.47% after 100 cycles at 0.1C	98
	LiCoO2	Li6PS5Cl	CoO/Li2CO3 coating	149.7	84.63% after 400 cycles at 0.2C	96
	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	Li–Ta–O–F oxyhalide coating	151.5	94% after 500 cycles at 1 mA cm^−2	97
Organic coating	LiCoO2	Li5.5PS4.5Cl1.5	Polyaniline (PANI) coating	∼131	85.5% after 200 cycles at 0.5C	99
	LiNi0.83Co0.11Mn0.06O2	Li6PS5Cl	Poly((4-vinyl benzyl)trimethylammonium bis(trifluoromethanesulfonylimide)) (PVBTA-TFSI) coating	∼185	86% after 100 cycles at 0.1C	100
	LiNi0.83Co0.12Mn0.05O2	Li6PS5Cl	Succinonitrile (SN)-based coating	168	80% after 100 cycles at 0.1C	101
	LiNi0.95Co0.04Al0.01O2	Li6PS5Cl	Poly(propylene carbonate)-based ion-conductive polymer (PPC-ICP) coating	125	84% after 100 cycles at 0.5C	102
	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	Polypyrrole (PPy) coating	133.2	81.2% after 300 cycles at 2C	103
Inorganic–organic hybrid coating	LiNi0.8Mn0.1Co0.1O2	Li9.54Si1.74P1.44S11.7Cl0.3	Al-GL coating	174.3	88% after 100 cycles at 0.2C	104
Lattice stabilization and surface coating	LiNi0.9Co0.05Mn0.05O2	Li6PS5Cl	B doping and coating	∼185	90.7% after 300 cycles at 0.5C	105
	LiCoO2	LiPON	Ti doping and LiCoPO4 coating	∼170	75% after 500 cycles at 2C	106
	LiNi0.8Co0.1Mn0.1O2	Li10SnP2S12	Ta^5+ doping and Nb^5+ migration to form LiNbO3 coating	∼198	100% after 100 cycles at 0.2C	107
Conductive network design	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	Hybrid conductive network with CNTs (0.3 wt%) and CNFs (4.7 wt%)	∼150	90.1% after 50 cycles at 0.5C	109
Non-carbon conductor integration	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	Ti2O3 as a substitute for carbon conductors	192	86.5% after 140 cycles at 0.1C	110
Binder optimization	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	high-molecular-weight PIB	206.25	90.42% after 90 cycles at 0.1C	112
	LiNi0.8Co0.1Mn0.1O2	Li6PS5Cl	Lithium borate polycarbonate binder	203	94% after 300 cycles at 1.75 mA cm^−2	113
Homogeneous structure design	Li1.75Ti2(Ge0.25P0.75S3.8Se0.2)3	Li6PS5Cl	Key elements introduced (Ge, Se)	∼175	70% after 20 000 cycles at 2.5C	114
Polymer interlayer	LiNi0.8Co0.1Mn0.1O2	Li1.4Al0.4Ti1.6(PO4)3	PLC60 interlayer	150.3	85% after 200 cycles at 0.5C	115
	LiFePO4	Li6PS5Cl	Ether-based gel polymer interlayer	159.5	96% after 100 cycles at 0.3C	116
Molten salt interlayer	LiFePO4	Li6.4La3Zr1.4Ta0.6O12	Inorganic (Li,K,Cs)FSI ternary system	∼151	81.4% after 1000 cycles at 1C	117
Aliovalent doping	LiNi0.7Co0.1Mn0.2O2	Li3.25InCl5.75O0.25	O doping to replace partial Cl in Li3InCl6	224.97	82.9% after 100 cycles at 0.2C	118
	LiMn2O4	Li2.5Lu0.5Zr0.5Cl6	Zr substitution in Li3−xLu1−xZrxCl6	∼70	100% after 1000 cycles at 0.3C	119
	LiCoO2	Li2.75Y0.16Er0.16Yb0.16In0.25Zr0.25Cl6	High-entropy doping with multiple metal cations (Y, Er, Yb, In, Zr)	∼131	88.9% after 500 cycles at 0.5C	120
Interface-guided bulk modification	LiNi0.9Co0.05Mn0.05O2	Li2.5ZrCl5F0.5O0.5	F^−/O^2− anion engineering to form F-rich CEI	∼113	70.4% after 2000 cycles at 2C	121
	LiCoO2	nLi2O–TaCl5 (LTOC)-10%F	Introduction of F^− to form a LiF-rich passivation layer	∼115	81% after 300 cycles at 1 mA cm^−2	122
	LiNi0.83Co0.11Mn0.06O2	Li2YCl2.5Br1.5O0.5	Pre-oxidation to form Y2O3-based CEI	208	80.6% after 1000 cycles at 0.5C	123
Amorphization engineering	LiNi0.83Co0.11Mn0.06O2	Li3ZrCl4O1.5	Amorphization of Li2ZrCl6via O incorporation	∼138	90.1% after 300 cycles at 1C	124
	LiNi0.88Co0.09Mn0.03O2	LiAlOCl	Hydrate-assisted synthesis of aluminum-based oxychloride SSEs	∼163	84.86% after 1500 cycles at 0.5C	125

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For sulfide and halide SSEs, their lower modulus, greater interfacial compliance, and wider compositional tunability not only allow the adoption of general interface engineering methods but also enable a range of differentiated optimization strategies. Incorporating composite conductive networks and non-carbon conductors facilitates continuous electronic pathways, suppresses electron leakage and side reactions typically induced by high-surface-area carbon materials, and achieves synergistic transport of electrons and ions—thereby greatly enhancing interfacial stability and rate performance. However, it is essential to ensure chemical compatibility between the components and structure of the conductive network and the SSE, as incompatibility may trigger the formation of new interfacial byproducts. In addition, binder optimization improves particle adhesion and strain buffering, while single-ion-conducting designs enable directional Li⁺ transport, further stabilizing the interface. Nevertheless, such binders still face challenges in terms of ionic conductivity, functional synthesis, and scalable application. Furthermore, homogeneous cathode architectures with zero strain and intrinsic mixed conductivity help eliminate three-phase boundaries and transport separation. They are highly compatible with flexible SSEs, though the related materials and processing technologies still need further advancement. At the same time, electrolyte bulk optimization, by tailoring the composition and crystalline/amorphous structure of the SSE, can significantly enhance its compatibility with high-voltage cathodes. In particular, halide SSEs, with their open frameworks and excellent tunability, exhibit unique engineering advantages in terms of aliovalent or high-entropy doping, interface-induced passivation, and amorphization strategies.

Overall, sulfide and halide SSEs, owing to their lower modulus, higher formability, and excellent compositional tunability, offer broader engineering latitude and innovation potential for cathode interfacial modification. By contrast, oxide SSEs, which are rigid, thermally robust, and possess wide electrochemical windows while typically requiring high-temperature densification, are better served by a combined strategy of single-crystal design, precise structural and surface regulation, and construction of thin, high-voltage-stable interlayers (Fig. 8). Single crystallization helps alleviate interfacial stress concentrations and crack initiation; surface doping, graded architectures, and inorganic-dominant hybrid coatings effectively suppress interdiffusion and parasitic reactions at elevated temperature and high voltage, while band alignment and polarization tuning mitigate SCL effects. Interlayers should be thin, dense, and stable under high voltage, and the fraction of flexible organic constituents should be limited to avoid undermining solid–solid interfacial continuity. Accordingly, strategies must be tailored to the intrinsic attributes of each SSE, with precise matching of materials and processing, to fully unlock the energy density and rate capability of high-voltage cathodes and advance the practical deployment of ASSLBs.

Fig. 8 Suitability comparison of cathode–SSE interface engineering strategies in different SSE systems.

3. Anode–electrolyte interface

3.1. Challenges

Anodes in SSBs often experience substantial volumetric fluctuations and high reactivity during cycling, which readily trigger interfacial structural disruption and performance degradation upon contact with SSEs. The associated failure mechanisms at the interface primarily stem from two key factors: First, due to mechanical mismatch, surface contamination, or accumulation of reaction byproducts, a stable and conformal interface is difficult to achieve, interrupting continuous ionic pathways and increasing interfacial resistance. Second, localized defects can serve as nucleation sites for lithium dendrites, which propagate into the SSE bulk driven by electronic leakage and concentrated mechanical stress, ultimately leading to internal short circuits and structural breakdown (Fig. 2).

3.1.1. Interfacial physical mismatch. In ASSLBs, different types of anodes—including lithium metal and silicon-based materials—can experience pronounced mechanical incompatibility with SSEs during cycling. This incompatibility arises from the large disparities in elastic modulus, volumetric strain, and chemical reactivity between the two components, making it difficult to maintain long-term structural continuity and stable ionic transport across the interface.

Taking lithium metal as a representative example, its intrinsically low modulus leads to severe mechanical mismatch when interfaced with most inorganic SSEs. Due to insufficient compaction, surface roughness, or the accumulation of reaction-induced byproducts, the interface often fails to form a continuous and stable ionic transport pathway. For instance, garnet-type oxides such as Li₇La₃Zr₂O₁₂ (LLZO) possess excellent chemical stability but often exhibit interfacial voids due to their high hardness and poor wettability, which hinders intimate contact with lithium metal. In addition, the interfacial layer at the Li–LLZO interface typically forms through reactions between lithium and surface impurities such as carbonates and hydroxides, generating byproducts like Li₂CO₃, LiOH, and Li₂O. These contamination phases differ markedly in ionic conductivity and structural order, with Li₂CO₃-rich regions in particular exhibiting poor Li-ion conductivity, which can lead to localized current density accumulation, electronic leakage, and anisotropic ion migration pathways—ultimately exacerbating interfacial instability.¹²⁶ By comparison, sulfide SSEs, with their lower Young's modulus and superior compressibility, are more likely to form intimate contact with lithium metal under cold-pressing conditions, typically exhibiting lower initial interfacial resistance. However, their strong reductive reactivity with lithium often leads to the formation of porous and brittle interphases (e.g., Li₂S, Li₃P), whose ionic conductivity and transport behavior are governed by the distribution and crystallinity of these phases—crystalline Li₃P provides preferential Li⁺ conduction pathways due to its ordered lattice, whereas amorphous Li₂S acts as a bottleneck with lower mobility, resulting in a spatially heterogeneous percolation network that promotes nonuniform current distribution and compromises interfacial integrity during cycling.^60,127,128 Similar to sulfide-based systems, halide SSEs demonstrate favorable initial densification. However, their interfacial interactions with lithium metal remain non-negligible. Upon contact, high-valence metal cations (e.g., In³⁺, Y³⁺) are prone to reduction, leading to elemental metal precipitation and microstructural reorganization at the interface, thereby weakening interfacial compactness. Concurrently, reduction byproducts such as LiX (e.g., LiCl, LiBr) persist as discontinuous and ionically resistive interphases. As a result, the interphase evolves into a heterogeneous metal-in-salt composite, where lithium-ion migration is restricted to narrow percolation channels within the LiX matrix. These tortuous paths, shaped by the spatial distribution of LiX and embedded metal particles, limit effective ionic transport and give rise to increased interfacial resistance and localized overpotentials.¹²⁹

It is worth noting that even in chemically more stable systems, such as silicon-based anodes, interfacial degradation remains inevitable, but the dominant mechanism transitions from chemical reactivity to mechanical mismatch. Unlike lithium metal, silicon undergoes enormous volume changes of up to ∼300% during lithiation and delithiation.¹³⁰ Its rigid structure and significant elastic disparity with inorganic SSEs induce severe interfacial stress accumulation and delamination upon cycling. In situ tomography and finite-element simulations have revealed that the Si–SSE interface is prone to microcrack initiation, contact loss, and fragmentation of ionic transport pathways.¹³¹ Although silicon shows excellent chemical compatibility with inorganic SSEs, stress-driven interfacial fracture caused by volume expansion remains the major bottleneck restricting the long-term stability of silicon anodes in solid-state systems.

Overall, whether arising from interfacial reactions that induce chemical phase evolution or from volume-change-driven mechanical delamination, the multiscale chemo-mechanical mismatch between anodes and SSEs constitutes the fundamental challenge governing interfacial stability and cycling durability in ASSLBs.

3.1.2. Dendrite-induced instability. The nucleation and growth of lithium dendrites represent one of the most destructive interfacial phenomena in ASSLBs, as they can penetrate the electrolyte, induce internal short circuits, and pose significant safety risks. In oxide SSEs, while their inherently high mechanical strength is theoretically capable of suppressing dendrite growth, experimental studies have consistently reported dendritic propagation both along the interface and within the bulk electrolyte. This indicates that mechanical constraint alone is insufficient to prevent dendritic instability. A variety of advanced characterization techniques have elucidated the microscopic origin of this process: synchrotron X-ray computed tomography (XCT) and in situ scanning electron microscopy (SEM) observations revealed that lithium filaments typically initiate from pre-existing microcracks, grain boundaries, or SSE–SSE contacts.¹³² Moreover, Liu et al. further confirmed through solid-state nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) that dendrite nucleation preferentially occurs in low-energy-barrier regions such as grain boundaries and pores.¹³³ These structural defects are often accompanied by local electronic leakage and stress concentration, which synergistically induce localized reduction and deposition of metallic lithium. During repeated cycling, these processes progressively evolve into penetrating dendritic channels through the electrolyte bulk, eventually leading to internal short circuits. At the microscopic level, the grain boundaries in garnet-type electrolytes (e.g., LLZO) are widely recognized as critical weak points for dendrite initiation. The ion transport kinetics across grain boundaries exhibit pronounced heterogeneity and may display limited electronic conductivity due to the segregation of reduced species such as Zr³⁺ and La₂O₃, thereby forming locally mixed-conductive pathways that facilitate lithium nucleation and propagation along the boundaries. Meanwhile, the presence of grain-boundary voids or pores further amplifies local electric-field distortion and mechanical stress concentration, promoting lithium accumulation and reduction at these sites and triggering dendrite penetration along grain boundaries. In summary, dendrite growth in oxide SSEs is not governed by a single factor but is instead a multiscale electro-chemo-mechanical instability driven by the coupling of electronic leakage, microstructural defects, and mechanical stress, rooted in the electrochemical heterogeneity of interfacial and grain-boundary regions.

In contrast, sulfide SSEs possess higher interfacial compliance and lower interfacial impedance, yet their intrinsically low fracture toughness makes them prone to stress-induced cracking during lithium deposition. Consequently, they exhibit a coupled instability in which mechanical fracture and dendrite penetration co-evolve. In situ transmission electron microscopy (TEM) and focused ion beam-scanning electron microscopy (FIB-SEM) observations reveal that dendrites preferentially propagate along stress-induced microcracks, which in turn facilitate further dendrite growth—forming positive feedback between interfacial cracking and metallic penetration. Singh et al. systematically investigated the effect of microstructure on dendrite evolution by tuning the grain size of LPSC and found that both interfacial roughness and bulk defects jointly determine the stress distribution and dendrite propagation.¹³⁴ Fine-grained LPSC exhibits higher fracture toughness and more homogeneous stress transmission, effectively enhancing the CCD, whereas coarse-grained samples suffer from stress localization and crack percolation, leading to pronounced instability. Moreover, during lithium deposition, the formation of MIEC phases such as Li₃P and Li₂S provides additional electronic pathways, accelerating dendrite extension and interconnection within the electrolyte bulk.¹³⁵ Hence, dendrite formation in sulfide SSEs mainly originates from a mechanical-chemical synergistic instability arising from low fracture toughness and interfacial reaction by-products, characterized by a “crack-first, dendrite-following” evolution behavior.

Similar to sulfide SSEs, halide SSEs also feature a mechanically soft and chemically reactive nature, yet their dendrite instability stems from distinct interfacial dynamics. Existing evidence suggests that reduced metal particles and electrically insulating byproducts formed at the Li–halide interface are often heterogeneously distributed, promoting non-uniform lithium deposition.¹³⁶ Once a continuous metallic network develops within the interphase, electrons can penetrate deep into the bulk electrolyte, initiating spontaneous nucleation and internal lithium growth. Moreover, the inherently low shear modulus of halide SSEs is insufficient to effectively relieve localized stress induced by lithium deposition, making the interfacial region prone to expansion and pore accumulation, which further facilitates secondary dendrite formation.¹³⁷ Broadly speaking, though the three classes of SSEs exhibit distinct interfacial characteristics, their dendrite growth mechanisms consistently evolve from interface-initiated processes to bulk-phase penetration, revealing a universal feature of multiscale-coupled instabilities underlying lithium dendrite behavior.

In summary, failure phenomena at the anode–SSE interface highlight the need for synergistic regulation among structural integrity, chemical stability, and electronic insulation. Owing to the limited self-adaptability of solid–solid interfaces, it is often challenging for a single-phase material to simultaneously fulfill the mechanical conformity, interfacial stability, and dendrite suppression requirements. Consequently, the core of interfacial design lies in constructing composite architectures capable of accommodating mechanical deformation, mediating interfacial reactions, and enabling selective transport, thereby achieving integrated stability across structural, chemical, and electronic dimensions within the interfacial region.

3.2. Optimization strategies

To address the common challenges at the anode–SSE interface, including poor interfacial contact, parasitic reactions, and dendrite-induced failure, research has focused on three main directions: functional anode design, interlayer construction and electrolyte engineering. The following sections analyze the mechanistic features and application potential of these strategies across different classes of SSEs.

3.2.1. Functional anode design. Functional engineering of anodes focuses on regulating Li deposition and interfacial stability through structural and compositional optimization. Key strategies include interfacial layer construction, composite anode design, and crystal structure modulation. Representative studies are discussed below to highlight their design principles and performance benefits.
Interfacial layer construction. Functional interfacial layers can be constructed on the Li metal surface via physical coating or in situ chemical reactions, featuring high ionic conductivity, electronic insulation, and chemical stability. These layers effectively improve interfacial wettability, suppress parasitic reactions, and regulate Li deposition behavior.^138–140 For example, Liu et al. proposed an in situ conversion strategy using P₂S₅ as a precursor to construct a dense and uniform Li₃PS₄ passivation layer on the Li surface, which significantly reduces interfacial reactivity.¹³⁹ The resulting P₂S₅@Li symmetric cell exhibits stable cycling for over 500 h at 0.1 mA cm⁻², and the full cell retains 75.5% of its capacity after 400 cycles at 0.5C. However, this strategy primarily relies on the intrinsic chemical affinity of a single inorganic component and lacks multifunctional synergistic regulation, especially in controlling Li nucleation and deposition morphology. To this end, Wu et al. introduced a LiF–LiPO₃–Ag composite interfacial layer, which integrates the electronic insulation of LiF, the amorphous compliance of LiPO₃, and the strong Li affinity of Ag to enable synergistic regulation of electron blocking, interfacial adaptability, and nucleation behavior (Fig. 9a).¹⁴⁰ By integrating multiple functionalities, this strategy overcomes the limitations of conventional single-component passivation layers and significantly expands the tunability of interfacial engineering.

Fig. 9 (a) Schematic illustration of the whole fabrication procedure for the L-A@Li interphase via solution coating method. Reproduced with permission.¹⁴⁰ Copyright 2025, Elsevier. (b) Schematic of LiAl-p. (c) CCD testing of the LiAl-p//LiAl-p symmetric cell, conducting with a fixed Li plating/stripping time of 1 h. Reproduced with permission.¹⁴¹ Copyright 2024, John Wiley and Sons. (d) CCD comparison for three types of solid-state electrolytes with an Li(110) anode strategy and other reported modification methods. RT, room temperature. Reproduced with permission.¹⁴² Copyright 2025, Springer Nature.

Composite anode design. Functional interfacial layers primarily act on the surface and are often insufficient to suppress long-term dendrite growth and structural degradation. Therefore, research has focused on composite anode design to enable synergistic regulation of bulk and interfacial stability. In this design, metallic, nonmetallic, or compound components with specific structural or chemical properties are incorporated into the anode to reconstruct Li-ion transport pathways, mitigate mechanical stress, and regulate Li nucleation and deposition behavior within the bulk phase, thereby significantly enhancing interfacial stability and cycling durability.

In practical implementation, composite anodes are generally developed following two main design strategies. The first involves alloy-type composite anodes, which achieve structural reinforcement and interfacial stability by forming alloys or multiphase interpenetrating structures between lithium and other elements.^{141,143–146} For example, Zhu et al. fabricated a porous Li–Al alloy anode (∼19% porosity) that effectively relieves volumetric stress (Fig. 9b).¹⁴¹ Moreover, surface oxides react in situ with sulfide electrolytes to form a robust inorganic solid electrolyte interphase (SEI), greatly boosting interfacial stability. This structure enables ultra-long cycling over 5000 h at 0.5 mA cm⁻² and achieves a critical current density (CCD) of 6.0 mA cm⁻² (Fig. 9c). Building upon this, Li–Sn alloys integrate relatively high lithium storage capacity with favorable interfacial compatibility.¹⁴³ Prelithiated phases such as Li₂₂Sn₅ can effectively buffer volume changes during cycling and establish stable interfaces with sulfide-based SSEs (e.g., LPSC), thereby mitigating interfacial side reactions. This synergistic effect leads to excellent long-term cycling performance, with full cells retaining 91% of their capacity after 650 cycles. Extending this concept, Li–Ag alloys introduce a dual-phase structure (Li₃Ag/Li_0.98Ag_0.02) that enables both efficient Li transport and interfacial stabilization.¹⁴⁴ During cycling, Li₃Ag serves as a lithiophilic channel to guide uniform Li plating, while Li_0.98Ag_0.02 acts as the primary Li deposition host. Meanwhile, Ag at the interface reacts with LPSC to form a stable, electrochemically inert Ag–P–S–Cl interphase, which effectively suppresses dendrite growth and preserves interfacial integrity.

Beyond conventional metal-based systems, silicon-based alloy architectures have attracted increasing attention for achieving higher energy density and improved chemical stability. In Li–Si alloys, the formation of Li-rich phases (e.g., Li₁₅Si₄, Li₂₂Si₅) provides both high theoretical capacity and chemical compatibility with sulfide electrolytes. The partial lithiation of Si produces a ductile Li–Si framework that maintains intimate contact at the interface.¹⁴⁷ However, the enormous volumetric expansion and associated stress accumulation can still induce particle pulverization, interfacial cracking, and contact loss, leading to impedance growth and capacity fading. To address these limitations, Li et al. developed an In–Si composite anode featuring a low-melting-point In network that forms a 3D mixed ionic-electronic conducting framework during operation.¹⁴⁸ The In–Li alloy network not only enhances ionic and electronic conductivity but also redistributes local stress and stabilizes interfacial contact. This design delivers outstanding cycling stability (98.5% capacity retention after 2000 cycles at 3.69 mA cm⁻²) and high-rate capability (80% capacity retention at 6C), demonstrating the effectiveness of ductile metal reinforcement in semiconductor-based anodes.

The second approach employs reactive components such as metal phosphides or nitrides (e.g., GaP, GaN, Mg₃N₂), which react with Li to form ion-conductive phases and nucleation-inducing domains.^149–151 For instance, Li et al. introduced GaP into molten lithium to in situ generate Li₃P and Li₂Ga, which not only lower the surface tension but also build a lithiophilic network and favorable nucleation structure.¹⁴⁹ The resulting composite anode delivers stable cycling for 5700 h at 0.3 mA cm⁻², and the full cell retains 97.32% of its capacity after 490 cycles at 1C, demonstrating the long-term stability and practical potential of this interfacial engineering strategy.

In summary, alloy-type and reactive-component composite anodes share a common design philosophy: both rely on the formation and evolution of alloy phases to achieve synergistic optimization of structural integrity, ionic transport, and interfacial stability. However, alloy-based anodes are not without limitations. The large volumetric changes and stress accumulation during repeated alloying/dealloying can induce interfacial delamination, pore formation, or structural fatigue, particularly under low stack pressures.¹⁵² These effects may lead to impedance growth and gradual capacity decay. Therefore, future design efforts should aim to balance the structural reinforcement benefits of alloy phases with strategies that mitigate their intrinsic mechanical instability—such as elastic buffering frameworks, graded architectures, or low-yield-strength interlayers—to ensure long-term stability and practical operation in all-solid-state configurations.

Crystal structure modulation. Despite the significant progress achieved by the above composite structural strategies in improving interfacial wettability, ion diffusion capability, and local reaction stability, the intrinsic polycrystalline nature of lithium metal still imposes critical limitations. The presence of discontinuous grain boundaries and anisotropic diffusion behavior within the bulk remains difficult to eliminate, posing persistent risks of dendrite formation and interfacial instability. To further improve the uniformity of lithium deposition and the coherence of ion diffusion, Chen et al. synthesized single-crystal lithium foils with exposed Li(110) facets via a thermally induced recrystallization process.¹⁴² The Li(110) surface features an ultralow self-diffusion energy barrier and a relatively low Young's modulus, which significantly enhance in-plane Li-ion diffusion while mitigating interfacial stress concentration during deposition. In solid-state systems, the single-crystal lithium anode markedly increases the CCD compared to polycrystalline lithium, reaching 6.0, 5.0, and 5.0 mA cm⁻² in LLZTO, LPSC, and Li₃InCl₆ systems, respectively—far exceeding the 0.5–1.0 mA cm⁻² typical of polycrystalline lithium (Fig. 9d). Compared to structural composite strategies, single-crystal lithium fundamentally avoids grain boundary defects and electrochemical anisotropy, thereby enhancing interfacial electric field uniformity and promoting cooperative ion transport. This crystal engineering approach offers an ideal solution for achieving high current density and long cycle life in ASSLBs.

3.2.2. Interlayer construction. In contrast to functionalized anodes—which primarily focus on regulating bulk structure and guiding Li deposition—interlayer strategies emphasize the construction of a responsive “interfacial regulation platform” to reconcile the physicochemical mismatch and electrochemical incompatibility between the electrode and SSE. Based on differences in material characteristics and construction mechanisms, current research has largely evolved along three interlayer design pathways: flexible organic layers, inorganic oxides, and multifunctional composites. The following sections will discuss each in detail.
Flexible organic interlayers. Owing to their excellent film-forming ability and segmental mobility, flexible organic interlayers can effectively mitigate physical mismatches during the early stages of interface formation, making them particularly suitable for systems with porous interfaces or uneven Li deposition. For example, Li et al. employed poly(1,3-dioxolane) (PDOL) as an interlayer to prevent solvent-induced degradation of the Li_5.4PS_4.4Cl_1.6, while its flexible polymer chains adaptively filled interfacial voids (Fig. 10a).¹⁵³ During cycling, the interlayer transformed into a stable SEI rich in Li₂S and LiF, permitting dendrite-free operation for over 1200 h (Fig. 10b). Building upon such structural adaptability, Yang et al. introduced poly(lithium 4-styrenesulfonate) (PLSS) between LLZTO and Li to further enhance interfacial compatibility at the atomic level.¹⁵⁴ The sulfonate (–SO₃Li) groups in PLSS strongly coordinate with metal atoms (La, Zr, Ta) on the LLZTO surface, forming an ion-conductive “bridge” that accelerates Li⁺ transport across the ceramic–polymer interface. This coordination not only ensures intimate contact and rapid ion migration but also endows the interlayer with strong electron-blocking capability, effectively preventing electron tunneling and dendrite initiation within LLZTO. As a result, PLSS-based symmetric cells achieve long-term stable cycling exceeding 4700 h at room temperature.

Fig. 10 (a) The polydioxolane interlayer formed by in situ polymerization provides a robust solid electrolyte interphase between the lithium anode and Li_5.4PS_4.4Cl_1.6. (b) Voltage profiles of the Li/PDOL/LPSC/PDOL/Li cells at a current density of 0.5 mA cm⁻² with a cut-off capacity of 0.5 mAh cm⁻². Reproduced with permission.¹⁵³ Copyright 2024, Elsevier. (c) Schematic diagram for the CAD process to form metal oxide nanofilms. Reproduced with permission.¹⁵⁶ Copyright 2022, American Chemical Society. (d) Multifunctional double-layer with interfacial design coupling lithiophilic and electron-insulating. (e) Interlayers formed at different cooling rates, fast cooling on the left and slow cooling on the right. Reproduced with permission.¹⁵⁷ Copyright 2024, John Wiley and Sons. (f) Illustrations of the in situ formation of F@NMC811/Li₆PS₅Cl/LiMgS_x/Li₃Bi/LiMg. Reproduced with permission.¹⁵⁸ Copyright 2023, Springer Nature.

With structural and chemical compatibility effectively optimized, research attention has shifted toward electronic-level modulation to achieve deeper interfacial stabilization. To this end, Nie et al. designed a biphenyl (BP)–Li composite interlayer, where π–π stacking of BP molecules and Li coordination induce a built-in electric field at the interface, effectively homogenizing local charge density and suppressing dendrite tip effects during Li deposition.¹⁵⁵ Upon electrochemical evolution, the interlayer converted into a gradient SEI with a “rigid core-soft shell” architecture, combining electronic passivation with mechanical compliance, thereby extending the symmetric cell lifespan to over 1800 h.

Inorganic oxide interlayers. Similar to the evolution pathway of cathode interfaces, flexible organic interlayers offer favorable interfacial conformity at early stages but are still constrained by limited thermal stability and inadequate electronic insulation. These challenges have progressively redirected research efforts toward oxide-based interlayers with superior interfacial passivation performance. Representative oxides such as Li₂O, Al₂O₃ and ZnO demonstrate outstanding electronic insulating characteristics and chemical inertness, effectively suppressing electron leakage and enhancing interfacial robustness.^156,159,160 For example, Jiang et al. constructed a bilayer structure on the surface of LLZTO, consisting of a spontaneously formed Li₂O buffer layer and an electronically insulating Li₂O layer deposited via atomic layer deposition (ALD).¹⁵⁹ The buffer layer improves wettability and fills interfacial voids, while the ALD-grown layer effectively blocks electron penetration while maintaining ionic conductivity, enabling Li//Li symmetric cells to cycle stably for over 1000 h at 0.5 mA cm⁻². However, the high cost and energy consumption of ALD hinder its feasibility for large-scale applications. To overcome this limitation, Guo et al. developed a solution-based wet-chemical strategy, in which polyacrylic acid (PAA) was used to coordinate Al³⁺ ions, enabling the formation of a uniform and flexible Al₂O₃ nanolayer on the LLZTO surface (Fig. 10c).¹⁵⁶ This interfacial modification significantly reduces the interfacial resistance from 2079.5 Ω cm² to 8.4 Ω cm², and the resulting ASSLB exhibits a long cycling lifespan comparable to that of conventional lithium-ion batteries.

Unlike these conventional oxides, lithium phosphorus oxynitride (LiPON), combines strong electronic insulation with measurable Li⁺ conductivity (∼0.7–1.0 mS cm⁻¹ at 25–30 °C) and an amorphous, grain-boundary-free structure that promotes conformal wetting and mitigates contact loss. This dual function—passivating electrons while conducting Li⁺—makes LiPON particularly suited to stabilize Li–SSE interfaces rather than acting as a purely inert electronic barrier like Li₂O, Al₂O₃, or ZnO. Su et al. deposited an ultrathin (∼30 nm) LiPON interlayer by radio frequency (RF) sputtering on argyrodite LPSC and demonstrated markedly improved interfacial contact and wettability.¹⁶¹ As a result, the interfacial resistance drops to ∼1.3 Ω cm², symmetric cells exhibit stable Li plating/stripping for >1000 h at 0.5 mA cm⁻², and the CCD reaches 4.1 mA cm⁻² at 30 °C.

Functional composite interlayers. While oxide-based interlayers offer chemical stability, their limited functionality makes them insufficient to accommodate interfacial evolution and regulate dendrite growth. This limitation has stimulated the development of functional composite interlayers. By integrating lithium-conductive phases, electronically insulating components, and buffering structures, these strategies realize spatially resolved multifunctionality and have emerged as a key approach for constructing stable lithium-metal interfaces. Depending on differences in structural design and adaptive response mechanisms, such interlayers can be broadly classified into two representative types: pre-designed configurations and dynamically evolving architectures.

In pre-designed configurations, the spatial arrangement and functional zoning are defined during the fabrication stage, offering clear structural delineation and controllable regulation pathways.^{157,162–166} A representative example is the BiLi₃/LiCl₃ bilayer interlayer (Fig. 10d and e), where BiLi₃, positioned adjacent to the lithium metal, displays strong lithiophilicity and facilitates uniform lithium deposition. LiCl₃, in contact with the SSE, provides electronic insulation and interfacial passivation.¹⁵⁷ This functionally stratified design enables electronic-ionic decoupling, significantly enhancing both the CCD and interfacial stability. However, due to its inherently stable configuration during operation, it lacks dynamic adaptability to evolving interface conditions.

Dynamically evolving architectures harness interfacial reactivity, electrochemical driving forces, or fluidic mobility to achieve the in situ generation, adaptive regulation, and self-healing reconstruction of functionally stratified structures, thereby exhibiting superior interfacial responsiveness and structural stability under dynamic operating conditions.^{158,167–169} For example, Wan et al. proposed a Mg₁₆Bi₈₄ alloy interlayer that undergoes an interfacial reaction with the LPSC, forming an electronically insulating LiMgS_x layer in situ, accompanied by the migration of Mg atoms toward the lithium metal. This process induces the construction of a tri-layer composite structure comprising LiMg–Li₃Bi–LiMgS_x (Fig. 10f).¹⁵⁸ In this configuration, the Li₃Bi layer serves as a lithium-conductive framework that guides lithium deposition into the interlayer and accommodates interfacial volume changes. Meanwhile, the LiMgS_x and LiMg layers provide electronic passivation and mechanical support, respectively, working synergistically to form a gradient-functional interface. Driven by a solid-solution mechanism, this structure effectively suppresses interfacial side reactions and dendrite formation, enabling symmetric cells to cycle stably for over 2700 h at room temperature. While such solid-phase reaction-based designs offer promising adaptability, their inherent rigidity still limits real-time stress accommodation and defect healing during prolonged cycling. To address this limitation, recent research has extended dynamic interlayer design toward liquid-metal-driven adaptive architectures, in which fluidic metallic phases not only ensure conformal contact but also participate in in situ alloying and redox conversion to achieve self-regulating interfacial stabilization. A representative example is the Ga–In liquid metal (G–LM) interlayer, which couples metallic fluidity with electrochemical reactivity to build an intelligent, self-healing interface.¹⁶⁹ The G–LM can conformally wet the Li–LLZTO interface at room temperature, filling nanoscale voids and establishing intimate contact without high-temperature processing. During lithiation, it undergoes in situ phase transformation to form a multiphase alloy-oxide hybrid composed of Li₂Ga, Li₁₃In₃, LiGaO₂, and LiInO₂. The alloy phases provide fast Li⁺ transport channels, while the oxide components homogenize the interfacial electric field and block electronic leakage, collectively functioning as an ionic-electronic rectification layer. Moreover, the liquid metal's intrinsic fluidity allows continuous self-reconstruction under cycling, dissipating interfacial stress and maintaining uniform Li plating/stripping. Consequently, G–LM-modified Li-LLZTO interfaces achieve ultralow interfacial resistance (12 Ω cm²) and long-term stability exceeding 1400 h at 1.0 mA cm⁻².

Overall, this liquid-metal-mediated dynamic interfacial strategy extends the concept of functional composite interlayers from solid-state reaction systems to fluidic, self-regulating architectures capable of in situ phase transformation and autonomous defect healing, providing a robust design paradigm for next-generation dendrite-free ASSLBs.

3.2.3. Electrolyte engineering. The electrolyte not only serves as an ionic transport medium, but its structural characteristics and interfacial behavior also play a critical role in determining contact stability with lithium metal and the inhibition of dendrite growth. As a key strategy for interface regulation, electrolyte engineering enhances the overall system stability through bulk structure optimization and interfacial property improvement. This section provides a systematic overview of representative strategies and underlying mechanisms, focusing on four main approaches: microstructural regulation, doping engineering, functional coating design, and integrated composite architectures.
Microstructural regulation. To reduce the risk of dendrite penetration and improve interfacial uniformity between the SSE and lithium metal, research has first focused on densifying the microstructure of the electrolyte. In oxide-based systems, Accardo et al. developed a high-pressure low-temperature (HPLT) sintering strategy for LLZO garnet electrolytes.¹⁷⁰ By applying a quasi-isostatic pressure of 5.5 GPa at 500 °C, followed by a brief post-annealing at 800 °C, they fabricated dense LLZO ceramics with a relative density above 97% and an impressive ionic conductivity of 3.1 × 10⁻⁴ S cm⁻¹ at room temperature. This process effectively suppresses lithium volatilization and abnormal grain growth while maintaining a homogeneous submicron grain structure, which strengthens the mechanical resistance against dendrite penetration and ensures stable Li–LLZO interfacial cycling. Inspired by these advances, sulfide-based electrolytes have also been optimized through microstructural densification strategies. Liu et al. employed a “pressing-then-sintering” approach to fabricate a densified LPSC nanorod-based electrolyte.¹⁷¹ By enhancing particle uniformity and surface flatness, this strategy effectively minimized interparticle voids and improved the degree of cold-press compaction, significantly increasing the CCD to 1.05 mA cm⁻². Nevertheless, the structure still suffers from broad particle size distribution and disordered crystallographic orientation, which limit its ability to further support higher current densities. To address this, Zhong et al. regulated the crystallization kinetics of Li_5.3PS_4.3ClBr_0.7 to construct well-defined cubic crystallites with uniform particle size and oriented crystallographic alignment, forming a highly ordered three-dimensional stacking structure (Fig. 11a).¹⁷² This crystal morphology facilitates the formation of a denser and more isotropic ionic transport network during cold pressing, resulting in a remarkably high CCD of 3.8 mA cm⁻² (Fig. 11b). Such microstructural homogenization strategies fundamentally eliminate dendrite initiation pathways by promoting structural continuity and provide a stable morphological foundation for subsequent chemical-level interface engineering.

Fig. 11 (a) Schematic of the Li–electrolyte interface with SS-LPSCB and BMAN-LPSCB. (b) Galvanostatic cycling of Li–Li symmetric cells with BMAN-LPSCB electrolyte at step-increased current densities with a step size of 0.2 mA cm⁻² at room temperature. Reproduced with permission.¹⁷² Copyright 2024, John Wiley and Sons. (c) Crystal structures of Li₁₀GeP₂S₁₂ and Li_9.98Ge_0.99Sn_0.01P₂S_11.98F_0.02. Reproduced with permission.¹⁷³ Copyright 2024, John Wiley and Sons. (d) Diagram of interface reaction and Li dendrite growth at the Li–Li_6.16P_0.92In_0.08S_4.88O_0.12Cl interface. Reproduced with permission.¹⁷⁴ Copyright 2024, John Wiley and Sons. (e) A Scheme of the trade-off principle between Li vacancy and concentration toward an optimized ionic conduction for Li_xYI_3+x (x = 2, 3, 4, or 9) SEs. Reproduced with permission.¹⁷⁵ Copyright 2024, John Wiley and Sons.

Doping engineering. Although microstructural optimization improves interfacial densification, chemical incompatibility between the SSE and lithium metal remains a primary cause of interfacial failure—particularly in sulfide and halide systems, where electronically conductive side phases are prone to formation. To address this issue, research has shifted toward doping strategies that enhance the intrinsic stability of the electrolyte and improve interfacial tolerance by modulating the crystal structure and local bonding environment.

Early studies primarily focused on point-defect-mediated doping strategies, in which aliovalent substitution introduces lithium vacancies or modifies local lattice disorder to enhance ionic conductivity and interfacial compatibility. For instance, in Li₁₀SnP₂S₁₂, partial substitution of S²⁻ with Cl⁻ increases lithium vacancy concentration and shortens Li⁺ diffusion pathways along the c-axis, resulting in enhanced ionic conductivity and reduced electronic leakage.¹⁷⁶ Similarly, in Li₁₀GeP₂S₁₂, the substitution of Ge⁴⁺ with Sn²⁺ introduces lithium vacancies to enhance ionic transport, while the replacement of S²⁻ with F⁻ effectively suppresses moisture-induced reactions and H₂S release (Fig. 11c).¹⁷³ Such doping modulates the lattice structure and electronic distribution, effectively lowering the reduction tendency of the electrolyte upon contact with lithium metal and thereby mitigating interfacial side reactions. However, the regulatory effects of these strategies are primarily confined to the bulk phase, with limited capacity to directly control the interfacial reaction processes.

To further enhance interfacial regulation, doping strategies have gradually evolved toward structure-interface co-regulated approaches, wherein dopants not only stabilize the bulk framework but also actively participate in the formation of beneficial interfacial phases. A representative example is the In³⁺/O²⁻ co-doping of LPSC, which introduces InS₄⁵⁻ and PS₃O⁴⁻ structural units to reinforce the framework rigidity while simultaneously inducing the formation of a Li–In alloy layer at the interface (Fig. 11d).¹⁷⁴ This dual function improves both the bulk stability of the electrolyte and the interfacial wettability with lithium metal. In another case, Al³⁺/F⁻ co-doping in the halide electrolyte Li₂ZrCl₆ not only broadens the electrochemical window to 4.04 V, but also promotes the in situ formation of a Li–Al alloy and LiF-based electronically insulating SEI layer at the interface, effectively suppressing electron leakage and interfacial side reactions in synergistic fashion.¹⁷⁷

Building upon previous efforts, recent research has begun to explore defect-tolerant, intrinsically stabilized doping strategies aimed at fundamentally resolving interfacial instability at the structural level. A notable case is the superionic conductor Li₄YI₇ proposed by Zhang et al., whose highly symmetric lattice structure confers excellent defect tolerance and enables three-dimensional lithium-ion transport pathways (Fig. 11e).¹⁷⁵ Upon contact with lithium metal, this material avoids the formation of electrically conductive by-products (e.g., Li–Y alloys) and instead promotes the stable formation of inert YI₆ octahedral units composed of Y³⁺ and I⁻, thereby effectively preventing undesirable interfacial reactions. Benefiting from its intrinsic high ionic conductivity (1.04 × 10⁻³ S cm⁻¹) and interfacial chemical stability, Li/Li₄YI₇/Li symmetric cells achieve over 1000 h of stable, dendrite-free cycling at 0.1 mA cm⁻², indicating outstanding interfacial compatibility and long-term operational reliability.

These advances collectively highlight a critical shift in doping strategies from isolated bulk-phase regulation toward integrated structure-interface engineering. A key takeaway is that effective doping no longer solely relies on enhancing bulk ionic conductivity or structural robustness, but increasingly aims to engineer the interfacial chemistry in situ. Co-doping schemes, in particular, demonstrate the ability to induce favorable interphases—such as Li–In or Li–Al alloys and LiF-rich SEI layers—that simultaneously suppress electron leakage and stabilize electrochemical cycling. Moreover, the development of intrinsically stable frameworks like Li₄YI₇ underscores the importance of lattice symmetry and defect tolerance in achieving durable lithium compatibility without relying on additional interfacial constructs. These insights suggest that future doping strategies should emphasize multifunctional dopant roles—those that synergistically tailor the bulk structure, interfacial reactivity, and electrochemical properties—to enable robust and scalable ASSLB systems.

Functional coating design. While doping strategies enhance interfacial stability primarily by modulating the bulk structure of the material, surface coating approaches act directly on the electrolyte surface and grain boundaries. This method is not only applicable to a wide range of material systems but also offers greater flexibility for practical engineering applications.

In halide-based electrolytes, Liu et al. in situ constructed a dense and uniform LiF–ZrF₄ (LFZ) fluorinated coating on the surface of Li₂ZrCl₆ (LZC) (Fig. 12a).¹⁷⁸ This layer exhibits ultralow electronic conductivity (7.1 × 10⁻¹⁰ S cm⁻¹) and excellent thermal stability, effectively suppressing lithium reduction side reactions and enhancing moisture resistance. As a result, Li/LFZ-LZC/Li symmetric cells demonstrate stable cycling for over 800 h at 0.1 mA cm⁻², with the CCD increased to 1.8 mA cm⁻².

Fig. 12 (a) Schematic diagram of the LCO/LZC/LFZ@LZC/Li ASSLMBs. Reproduced with permission.¹⁷⁸ Copyright 2025, Elsevier. (b) Ameliorated Li⁺-ion diffusion kinetics of a Co_xB-coated LLZTO pellet through the grain boundaries. Reproduced with permission.¹⁷⁹ Copyright 2023, Royal Society of Chemistry. (c) Schematic diagram of the interfacial and bulk chemistry of the PPA-LLZTO pellet. (d) Cycling test of Li/PPA-LLZTO/Li under a current density of 0.2 mA cm⁻² at 65 °C. Reproduced with permission.¹⁸⁰ Copyright 2023, American Chemical Society. (e) Schematic of the interfacial modification of LLZTO by ZrO₂ and Li₂CO₃ co-sintering. Reproduced with permission.¹⁸¹ Copyright 2024, John Wiley and Sons.

Compared with halide SSEs, which are prone to interfacial reactions and moisture-induced decomposition, oxide SSEs such as LLZTO offer superior chemical stability. However, high-temperature sintering often leads to microcracks at grain boundaries and surface defects, which can trigger electron leakage and interfacial degradation. As a result, coating strategies for oxide SSEs place greater emphasis on the synergistic regulation of electronic passivation and grain boundary buffering. Chen et al. introduced an amorphous Co_xB nanocoating on the surface of LLZTO (Fig. 12b).¹⁷⁹ Owing to its excellent electronic shielding capability and moderate interfacial flexibility, the coating effectively alleviates electron leakage and prevents microcrack propagation at the interface, thereby markedly improving the cycling stability of lithium symmetric cells. To further strengthen continuous coverage along grain boundaries and establish efficient ion transport pathways, Xiong et al. employed a polyphosphoric acid (PPA) infiltration method.¹⁸⁰ The PPA penetrates the interparticle voids of LLZTO and in situ forms a Li₃PO₄ coating, which creates an interfacial network that is electronically insulating and ionically conductive (Fig. 12c). This design effectively inhibits dendrite growth, reduces interfacial resistance, and enables stable cycling for over 2000 h (Fig. 12d). However, such chemical infiltration treatments are inherently passive in nature, and their effectiveness is limited by the penetration depth and uniformity of the solution reaction, making it difficult to achieve comprehensive regulation of grain boundary structures and energy band alignment. To overcome these limitations, Zhou et al. employed a co-sintering approach using ZrO₂ and Li₂CO₃ to in situ generate a Li₂ZrO₃ coating at the surface and grain boundaries of LLZTO (Fig. 12e).¹⁸¹ This process removes interfacial impurities and reconstructs the grain boundary structure, thereby significantly enhancing interfacial insulation and stability. The treated LLZTO shows over 2000 h of stable, short-circuit-free cycling in symmetric cells and retains 70.5% of its capacity after 5800 cycles in full cells, demonstrating excellent interfacial durability and strong potential for practical applications.

In air-sensitive sulfide systems, Luo et al. introduced a g-C₃N₄ coating with excellent air stability, which in situ transforms into Li₃N during electrochemical cycling (Fig. 13a).¹⁸² This layer simultaneously provides electronic passivation and lithium-ion conductive, enabling lithium symmetric cells to operate stably for over 1000 h (Fig. 13b). Considering the rigidity of dry coatings, Su et al. further applied a flexible encapsulation using melt-processable polycaprolactone (PCL) (Fig. 13c).¹⁸³ During hot pressing, PCL forms a dense anchoring network that fills interparticle voids. Meanwhile, its C=O and C–O–C groups promote Li⁺ transport via dipole-ion interactions. As a result, the symmetric cell achieves stable cycling for over 1300 h at 0.2 mA cm⁻², offering combined benefits of structural buffering and ionic conductivity.

Fig. 13 (a) Schematic diagram of g-C₃N₄-coated Li₆PS₅Cl (651–x%) SSEs preparation. (b) Cycle performance of Li//Li symmetric cell with 651–5% SSE at 0.2 mA cm⁻². Reproduced with permission.¹⁸² Copyright 2024, John Wiley and Sons. (c) Illustration of the fabrication process of LPSCl-PLI electrolyte. Reproduced with permission.¹⁸³ Copyright 2024, John Wiley and Sons. (d) Batteries containing multifunctional composite sulfide electrolyte. Reproduced with permission.¹⁸⁴ Copyright 2025, John Wiley and Sons. (e) The schematic illustrating the dendrite-scavenging effect by LTLC. (f) Galvanostatic cycling performance of lithium symmetric cells at a 0.5 mA cm⁻². Reproduced with permission.¹⁸⁵ Copyright 2024, John Wiley and Sons.

Integrated composite architectures. When conventional structural modulation and localized interfacial engineering fail to simultaneously address the multiple demands of ionic conductivity, interfacial compatibility, and dendrite suppression, the design of integrated composite electrolyte architectures emerges as a promising solution. This strategy leverages spatial coupling and functional synergy between heterogeneous phases to construct unified electrolyte systems with multidimensional performance advantages, enabling comprehensive optimization of both intrinsic properties and interfacial behavior.

The multifunctional composite sulfide electrolyte (M-CSE) developed by Xu et al. exemplifies this integrated strategy.¹⁸⁴ By co-blending highly conductive Li_5.3PS_4.3Cl_1.7 with passivating Li_9.54[Si_0.5Sn_0.5]PSBrO in a controlled ratio followed by cold pressing, a dense dual-scale microstructure is constructed (Fig. 13d). At the Li interface, the passivating component in situ forms a Li–Sn alloy and multicomponent buffer phases, which effectively absorb local electronic and mechanical disturbances to establish a stable dendrite-suppressing interphase. Hence, Li/M-CSE/Li symmetric cells achieve dendrite-free cycling for 650 h at a high current density of 3.76 mA cm⁻², while full cells retain 95.04% of their capacity after 500 cycles at 1C. Another representative strategy involves the introduction of an interfacial reaction-regulating layer via a sandwiched architecture. Xu et al. embedded the halide superionic conductor Li_0.388Ta_0.238La_0.475Cl₃ (LTLC), which features high ionic conductivity and strong reductive stability, between layers of the LPSC to construct an LPSC/LTLC/LPSC trilayer structure (Fig. 13e).¹⁸⁵ Upon contact with lithium dendrites, the LTLC layer in situ generates Ta and La metal nanoparticles along with LiCl, forming a dendrite-trapping interphase that enables self-passivating interfacial reactions. This structure mitigates the tip effect and delays dendrite penetration pathways. As a result, Li/LPSC/LTLC/LPSC/Li symmetric cells achieve a lifespan exceeding 500 h at 0.5 mA cm⁻²—over 70 times longer than that of conventional LPSC (Fig. 13f)—while full cells maintain 87.6% capacity after 200 cycles, featuring excellent interfacial protection and high-current adaptability.

In summary, the core of anode–SSE interface optimization lies in the coordinated achievement of mechanical adaptability, chemical stability, ionic conductivity, and electronic insulation, with representative strategies and electrochemical metrics summarized in Table 3. Among them, functional anode design and interlayer construction are the two most widely applied and system-compatible strategies for interface regulation. The former, through surface passivation, composite structures, and crystallographic orientation optimization, can effectively modulate lithium nucleation and deposition behaviors, fundamentally suppressing dendrite growth and interface failure; however, challenges remain in terms of fabrication complexity and chemical compatibility between the anode and SSE, and inappropriate material selection may trigger adverse interfacial reactions. The latter, benefiting from its tunable mechanical and chemical properties, demonstrates remarkable immediate effects in stress buffering, wettability enhancement, and electron blocking, with strong adaptability across various inorganic SSE systems. Nonetheless, during prolonged operation, interlayers may face issues such as structural evolution and increased resistance, thereby imposing stricter requirements on the process window for material composition and thickness. Achieving an optimal balance among ionic conductivity, mechanical compliance, and interfacial connectivity is therefore essential.

Table 3 Summary of optimization strategies for anode–SSE interfaces

Interface optimization strategies	Representative system	CCD (mA cm^−2)	Symmetric cell lifespan	Full cell lifespan	Ref.
Interfacial layer construction	P2S5@Li/Li6PS5Cl	0.4	>500 h at 0.1 mA cm^−2	75.5% over 400 cycles at 0.5C	139
	LiF/LiPO3–Ag @Li/Li5.5PS4.5Cl1.5	2.4	500 h at 0.1 mA cm^−2	82.6% after 400 cycles at 0.5C	140
Composite anode design	LiAl-p/Li6PS5Cl	6	5000 h at 0.5 mA cm^−2	83% after 1800 cycles at 1C	141
	LiSn/Li6PS5Cl	2	>1100 h at 1 mA cm^−2	91% after 650 cycles at 1C	143
	Li–Ag/Li6PS5Cl	—	>1000 h at 0.1 mA cm^−2	83.86% after 150 cycles at 0.1C	144
	In–Si/Li5.5PS4.5Cl1.5	—	—	98.5% after 2000 cycles at 3.69 mA cm^−2	148
	Li–GaP/Li6.4La3Zr1.4Ta0.6O12	2.5	>5700 h at 0.3 mA cm^−2	97.32% after 490 cycles at 1C	149
	Li–Mg/Li6.4La3Zr1.4Ta0.6O12	1	>400 h at 0.3 mA cm^−2	82% after 300 cycles at 0.5C	151
Crystal structure modulation	Li(110)/Li6.75La3Zr1.75Ta0.25O12, Li6PS5Cl, Li3InCl6	6.0, 5.0, and 5.0	—	350 cycles at 1C	142
Flexible organic interlayers	Li/PDOL/Li5.4PS4.4Cl1.6	2.8	>1200 h at 0.5 mA cm^−2	95.4% after 200 cycles at 0.1C	153
	Li/PLSS/Li6.5La3Zr1.5Ta0.5O12	1.1	4800 h at 0.1 mA cm^−2	400 cycles at 0.5C	154
	Li/BP–Li/Li3GaF5.3Cl0.7	0.6	1800 h at 0.1 mA cm^−2	100 cycles at 0.5C	155
Inorganic oxide interlayers	Li/Li2O/Li6.4La3Zr1.4Ta0.6O12	2.4	>1000 h at 0.5 mA cm^−2	84% after 600 cycles at 1C	159
	Li/Al2O3/Li6.5La3Zr1.5Ta0.5O12	1	>1000 h at 0.3 mA cm^−2	500 cycles at 1C	156
	Li/LiPON/Li6PS5Cl	4.1	>1000 h at 0.5 mA cm^−2	—	161
Functional composite interlayers	Li/BiLi3–LiCl3/Li6.5La3Zr1.5Ta0.5O12	2.9	>10000 h at 0.3 mA cm^−2	88.5% after 400 cycles at 0.5C	157
	Li/LiSn–LiCl/Li6.5La3Zr1.5Ta0.5O12	0.8	>6000 h at 0.2 mA cm^−2	94.7% after100 cycles at 0.2C	162
	Li/SbLi3–LiCl/Li6.25Ga0.25La3Zr2O12	6	>1000 h at 2 mA cm^−2	94.7% at 1C after 600 cycles	163
	Li/LiSn–Li2S/Li6.5La3Zr1.5Ta0.5O12	0.9	>1500 h at 0.2 mA cm^−2	99.8% after 100 cycles at 1C	164
	Li/LiIn–Li3N/Li6.4La3Zr1.4Ta0.6O12	1.8	>2000 h at 0.5 mA cm^−2	92.4% after 110 cycles at 0.2C	165
	Li/LiMg–Li3Bi–LiMgSx/Li6PS5Cl	2.6	>2700 h at 1.2 mA cm^−2	681 cycles at 5C	158
	Li/LiF–C–Li3N–Bi/Li6PS5Cl	>3.5	>500 h at a step increased current density	854 cycles at 150 mA g^−1	167
	Li/G-LM/LLZTO	2.7	>1000 h at 0.5 mA cm^−2	90% after 200 cycles at 2.32 mA cm^−2	169
Microstructural regulation	HPLT Li7La3Zr2O12	—	150 h at 0.1 mA cm^−2	—	170
	Nanorod-based Li6PS5Cl	1.05	3000 h at 0.5 mA cm^−2	80.3% after 100 cycles at 1C	171
	Li5.3PS4.3ClBr0.7	3.8	>150 h at 3 mA cm^−2	96% after 1000 cycles at 1C	172
Doping engineering:	Li/Li9.9SnP2S11.9Cl0.1	0.15	350 h at 0.05 mA cm^−2	60% after 25 cycles at 0.1C	176
	Li/Li9.98Ge0.99Sn0.01P2S11.98F0.02	2.1	900 h at 0.1 mA cm^−2	80.1% after 600 cycles at 1C	173
	Li/Li6.16P0.92In0.08S4.88O0.12Cl	1.4	3000 h at 0.1 mA cm^−2	82.9% after 100 cycles at 0.1C	174
	Li–In/LPSC–LZC-0.1AlF3	3	>2000 h at 0.5 mA cm^−2	87.5% after 50 cycles at 0.5 mA cm^−2	177
Functional coating design	Li/LiF–ZrF4@Li2ZrCl6	1.8	1 mA cm^−2 for over 400 h	81.9% after 200 cycles at 1C	178
	Li/Co2B@Li6.4La3Zr1.4Ta0.6O12	—	1450 h at 0.1 mA cm^−2	98.51% after 300 cycles at 32 mA g^−1	179
	Li/PPA@Li6.4La3Zr1.4Ta0.6O12	1.8	2000 h at 0.2 mA cm^−2	85.9% after 200 cycles at 0.2C	180
	Li/LLZTO-(ZrO2 and Li2CO3)	2.1	2000 h at 0.8 mA cm^−2	70.5% after 5800 cycles at 4C	181
	Li/g-C3N4-coated Li6PS5Cl	1.5	1000 h at 0.2 mA cm^−2	99.1% after 200 cycles at 0.1C	182
	Li/LPSCl-polycaprolactone-based binder	0.5	1300 h at 0.3 mA cm^−2	89.9% after 300 cycles at 0.1C	183
Integrated composite architectures	Li/Li6PS5Cl-M-CSE	3.76	650 h at 1.5 mA cm^−2	95.04% after 500 cycles at 0.5C	184
	Li/LPSC–Li0.388Ta0.238La0.475Cl3	1.52	>500 h at 0.5 mA cm^−2	87.6% after 200 cycles at 0.5C	185

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By comparison, electrolyte engineering strategies exhibit stronger system dependence. For oxide-based SSEs, microstructural regulation remains the most effective approach, as these materials rely heavily on structural densification to achieve intimate interfacial contact and mechanical integrity. The intrinsic rigidity and high sintering temperature of oxide electrolytes make their interfacial properties particularly sensitive to porosity and grain connectivity; thus, optimizing particle packing, reducing voids, and balancing densification with mechanical compliance are crucial to enhance ionic conduction and suppress dendrite penetration. In contrast, functional coatings and integrated composite architectures are more suitable for sulfide and halide systems, which are typically softer, more deformable, and more chemically reactive. Functional coatings can effectively passivate reactive surfaces, improve chemical compatibility with lithium, and enhance long-term interfacial stability, while integrated composite architectures leverage multiphase spatial coupling to achieve simultaneous improvements in ionic conductivity, interfacial uniformity, and mechanical buffering. Furthermore, doping regulation is primarily applied to sulfide and halide SSEs, where aliovalent ion substitution can tune defect structures, regulate lattice polarizability, and significantly enhance intrinsic ionic conductivity as well as interfacial tolerance. These materials possess relatively open and flexible frameworks that can accommodate defect modulation without compromising structural integrity. In contrast, oxide SSEs exhibit densely packed and rigid lattices with limited defect mobility, restricting the extent to which doping can alter transport properties or interfacial chemistry.

Overall, the regulation of the anode–SSE interface is a multidimensional dynamic equilibrium involving material structure, interfacial chemistry, and processing conditions. The intrinsic properties of different inorganic SSEs dictate both the diversity and limitations of available strategies (Fig. 14), making it difficult for any single approach to address all interfacial failure bottlenecks. Looking ahead, advancing the synergy among materials innovation, interface engineering, and multiscale process integration will enable full-process optimization—from material selection and structural design to manufacturing. This holistic approach ensures unified control of mechanical, chemical, and electrical properties, paving the way for highly efficient and long-lasting ASSLBs.

Fig. 14 Suitability comparison of anode–SSE interface engineering strategies in different SSE systems.

4. Integrated dual-interface design

SSEs often face highly heterogeneous interfacial challenges at both the cathode and anode sides: oxidative decomposition driven by high voltage at the cathode interface, and severe reduction reactions and dendrite penetration at the anode interface. Conventional interfacial engineering typically focuses on stabilizing single interfaces, making it difficult to achieve balanced performance across the entire cell. The integrated dual-interface design strategy enables simultaneous regulation of both interfaces through structural configuration optimization or the introduction of functional materials. Without requiring additional fabrication steps, this approach constructs efficient ion transport pathways and a long-term stable chemical potential platform spanning the entire cell, representing a key avenue toward the practical realization of ASSLBs. According to differences in construction logic and functional mechanisms, current integrated dual-interface design strategies can be categorized into three main pathways: structural integration, functional interlayer regulation, and chemical synergistic modification.

Structural integration

Conventional sintering-based densification and rapid bonding techniques, such as thermal-pulse welding (TPW) or ultrafast Joule-heating sintering (UHS), have been widely employed to construct continuous, low-resistance interfaces between the SSE and electrodes.^186,187 These thermally induced integration methods effectively improve interfacial connectivity and ion-transport continuity without compromising the bulk structure, as exemplified by the LATP-based TPW process, which achieves 185.9 mAh g⁻¹ and 90.9% capacity retention after 100 cycles (Fig. 15a).¹⁸⁶ However, such physical densification approaches mainly focus on improving contact and neglect the electro-chemo-mechanical coupling effects that arise from mismatched volumetric strain between the cathode and anode during cycling. Recent studies by Gu et al. revealed that when electrodes exhibit opposite volume-change directions (e.g., sulfur or NCM811 vs. Li metal), strong inter-electrode stress crosstalk develops within the SSE, leading to grain-boundary cracking and dendrite penetration.¹⁸⁸ In contrast, cathodes such as LiCoO₂, which expand in the same direction as Li metal, induce milder mechanical damage and longer cycling life. Furthermore, incorporating active pressure-control systems can dynamically balance stack pressure and effectively suppress stress accumulation, extending the lifespan of sulfide-based ASSLBs. These findings suggest that structural integration should not be limited to densification alone but must also consider stress-adaptive design, ensuring mechanical compatibility across both interfaces for long-term durability of dual-interface solid-state systems. Even so, structural integration primarily enhances physical contact while offering limited capability to regulate interfacial chemistry and long-term reactivity.

Fig. 15 (a) Schematic of UHS and TPW technique for integrated anode/SSE/cathode. Reproduced with permission.¹⁸⁶ Copyright 2025, Elsevier. (b) Solid-state structure of the Li–SN complex in stick and ball style shows 2D Li–SN molecules interlocked together, forming a long-range ordered 3D Li–SN complex. Reproduced with permission.¹⁸⁹ Copyright 2024, American Chemical Society. (c) Schematic illustrating the comparison of the electrode–LLZTO interfaces. (d) The comparison of cycling performance between in situ polymerization and in situ polymerization-fluorination strategies at different cut-off voltages. Reproduced with permission.¹⁹⁰ Copyright 2025, American Chemical Society. (e) Schematic of the electrochemical reduction of the electrophile, where the reductive electrophiles gain electron and Li⁺ from LPSC SSE or cathode powders upon contact with them, thus being electrochemically reduced on the surface of these materials. r.t., room temperature. Reproduced with permission.¹⁹¹ Copyright 2025, Springer Nature.

Functional interlayer regulation

To overcome the intrinsic limitation of structural integration in governing interfacial chemistry, functional interlayer materials can be introduced to simultaneously enable interfacial passivation and ion transport pathway reconstruction at both interfaces. For instance, Luo et al. proposed the use of lithium-rich succinonitrile (Li–SN45) as a dual-interface modification material (Fig. 15b).¹⁸⁹ This material exhibits high ionic conductivity (3.38 mS cm⁻¹) and excellent resistance to lithium corrosion. With the introduction of the Li–SN45 interlayer, the battery achieves a discharge capacity of 170 mAh g⁻¹ at 20 mA g⁻¹, with 97% capacity retention after 100 cycles. Unfortunately, the functionality of Li–SN45 is primarily concentrated at the anode side and fails to effectively address structural instability and gas evolution at the cathode interface under high-voltage conditions.

Chemical synergistic modification

Building on this, researchers further developed chemically synergistic strategies based on anion chemistry regulation and interfacial self-passivation mechanisms. Umeshbabu et al. introduced Cl⁻ to partially substitute F⁻, constructing a dual-halide SSE, Li₂ZrF_6−xCl_x, which enables lattice parameter tuning and electronic structure optimization.¹⁹² The high electronegativity of F⁻ facilitates the in situ formation of a dense and stable LiF interfacial layer at the anode side, effectively suppressing reduction-induced decomposition. Meanwhile, Cl⁻ doping significantly enhances ionic mobility and structural stability of the crystal, thereby maintaining a wide electrochemical window and improving high-voltage compatibility at the cathode side. However, this strategy mainly relies on intrinsic structural regulation of the material, while offering limited control over the interfacial reactivity at the electrode surface. In this regard, Yin et al. proposed a polymerization-fluorination coupled strategy (Poly-FR), in which initiators induce the in situ formation of LiF-rich interfacial layers at both the cathode and anode sides (Fig. 15c).¹⁹⁰ These layers provide multiple functions, including electronic passivation, structural buffering, and impurity removal, enabling the full cell to maintain 83.4% capacity retention over 400 cycles at 4.5 V (Fig. 15d). Yet, the process involves thermal initiation and monomer infiltration, making the fabrication relatively complex. Furthermore, Zhang et al. reported a class of reductive electrophilic reagents (e.g., diphosphoryl fluoride, DPF), which, when interacting with metal-nucleophilic materials (e.g., LPSC), can extract electrons and Li⁺ from the material's surface.¹⁹¹ This process triggers electrochemical reduction reaction, leading to the in situ formation of dense, ultrathin solid reductive-electrophilic interface (SREI) on the material's surface (Fig. 15e). The growth of the SREI is governed by electron migration and exhibits self-limiting characteristics. Once the interface layer reaches certain thickness, it automatically transforms into an amorphous inorganic interface layer rich in LiF–Li_xP_yO_zF. This interface demonstrates excellent lithium-ion repellency and electron-blocking properties, effectively mitigating lithium dendrite formation, preventing the reduction decomposition of the SSEs, and greatly enhancing compatibility with high-voltage cathodes. As a result, DPF-treated LPSC (DPF-LPSC) exhibits high lithium reversibility and a high CCD (>3.4 mA cm⁻²) when paired with the lithium metal anode. Under high-loading conditions, the Li//LiNi_0.8Co_0.15Al_0.05O₂ full cell achieves stable cycling over 600 cycles with a Coulombic efficiency exceeding 99.9%. Importantly, the construction of this SREI does not require the application of external electric field or complex equipment; it relies solely on the contact reactions between materials. This approach offers highly simplified process and broad material applicability, providing new pathway and theoretical foundation for the design of next-generation interface engineering.

In the pursuit of high-performance ASSLBs, integrated dual-interface design strategies are evolving from structural construction toward chemical synergy and self-regulated interfacial design, reflecting a deeper trend of multi-mechanism cooperative integration. In particular, when addressing critical challenges such as interfacial reactivity asymmetry between electrodes, local stress mismatch, and discontinuous ion transport across interfaces, these strategies not only overcome the limitations of conventional single-interface approaches but also offer systematic solutions for achieving stable operation under high voltage, high current, and high loading conditions. Looking ahead, such strategies are expected to integrate closely with multiscale interfacial characterization, in situ evolution monitoring, and artificial intelligence (AI)-assisted materials design, paving the way for a more precise, intelligent, and scalable new phase in application-oriented interfacial engineering.

5. Advanced characterization techniques in SSBs interface

Understanding the intricate physicochemical processes occurring at solid–solid interfaces is pivotal to advancing ASSLBs. However, the buried nature and multiscale complexity of these interfaces pose significant challenges for direct observation and quantitative analysis. To overcome these barriers, a suite of advanced characterization techniques—spanning spectroscopic, microscopic, synchrotron-based, and neutron or ion beam-based methods—has been developed to probe interfacial structures, reactions, and dynamics with unprecedented spatial and temporal resolution. These approaches collectively enable the visualization of ion transport pathways, mapping of interphase formation, and elucidation of electro-chemo-mechanical coupling effects under realistic operating conditions. The following subsections summarize representative methodologies and their key contributions to understanding interfacial evolution and degradation mechanisms in ASSLBs, providing a comprehensive framework for correlating atomic-scale chemistry with macroscopic electrochemical behavior.

5.1. In situ and operando spectroscopic techniques

In situ and operando spectroscopic techniques have become indispensable tools for elucidating the dynamic interfacial processes governing the performance and degradation of ASSLBs. By enabling real-time observation of chemical, structural, and electronic evolutions under electrochemical operation, these methods bridge the gap between static post-mortem analyses and true interfacial dynamics. Among them, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, solid-state nuclear magnetic resonance (NMR), and X-ray absorption spectroscopy (XAS) provide complementary insights spanning from surface chemistry and lattice vibrations to atomic-scale lithium distribution and local electronic structure. Together, these techniques offer a multidimensional understanding of interfacial reactions, transport phenomena, and degradation pathways, thereby guiding the rational design of stable and high-performance solid-state interfaces.

5.1.1. X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) is a powerful surface-sensitive technique that provides quantitative and chemical-state information on elements at solid–solid interfaces, making it indispensable for studying interfacial evolution in SSBs. Traditional ex situ XPS has revealed interfacial decomposition products such as Li₂S, LiCl, and Li₃P at LPSC-based interfaces, but recent advances in operando XPS have enabled real-time monitoring of potential distribution and species evolution during electrochemical cycling. For example, Zhong et al. developed an operando XPS configuration in which the working electrode and detector were co-grounded, allowing shifts in photoelectron binding energy to directly reflect variations in interfacial electrostatic potential (Fig. 16a).¹⁹³ This approach reveals the formation and modulation of the SCL between LCO and LPSC and demonstrates that LiNbO₃ coatings effectively reduce potential drops and suppress sulfur oxidation at the interface. Moreover, operando XPS combined with electron-beam excitation has been utilized to visualize dynamic Li plating and SEI formation at Li–SSE interfaces. Morey et al. performed operando XPS on a Li–LPSC stack and found that the sulfide electrolyte is reduced to Li₂S, LiCl, and Li₃P while Li deposition occurs almost simultaneously.¹⁹⁴ The study shows that operando XPS captures not only the chemical transitions but also the kinetics of SEI evolution, whereas operando Auger spectroscopy provides higher spatial resolution and morphological visualization. Taken together, XPS—particularly in its operando mode—is a crucial diagnostic tool for quantifying interfacial reactions, mapping potential gradients, and elucidating electrochemical degradation mechanisms in SSBs. It bridges the gap between thermodynamic predictions and real-time interfacial dynamics, offering mechanistic insights that guide the rational engineering of stable solid-state interfaces.

Fig. 16 (a) Schematic diagram of the LCO/LPSC/Li–In batteries for the operando XPS measurements and grounded mode. Reproduced with permission.¹⁹³ Copyright 2024, Elsevier. (b) Three-dimensional views of the surface cracks identifying those filled with Li or LiMg. Reproduced with permission.¹⁹⁹ Copyright 2024, Royal Society of Chemistry. (c) Experimental setup for the in situ NDP measurements. (d) A typical NDP spectrum of ASSLMB w/3D Ti electrode. Reproduced with permission.²⁰⁹ Copyright 2019, Elsevier.

5.1.2. Raman spectroscopy. Raman spectroscopy is a versatile and non-destructive vibrational technique that provides molecular-level insights into the bonding configuration and structural evolution of electrodes, SSEs, and their interfaces in ASSLBs. It is particularly sensitive to the vibrations of anionic groups such as PS₄³⁻ in sulfide SSEs, allowing the identification of chemical transformations and mechanical responses during cycling. Recently, operando Raman spectroscopy, which integrates real-time measurements with electrochemical operation, has emerged as an essential method for probing interfacial dynamics. For example, Chen et al. employed operando Raman spectroscopy to monitor stress-induced shifts in the PS₄³⁻ symmetric stretching mode (∼422 cm⁻¹) at the LPSC–NCM interface.¹⁹⁵ The study reveals that the modulation of the SCL strongly governs Li⁺ transport and interfacial stability. The reversible stress evolution observed in LiNbO₃-coated NCM cathodes demonstrates that controlled SCL formation effectively mitigates interfacial mechanical degradation. Therefore, Raman spectroscopy not only captures chemical bonding changes but also correlates them with electro-chemo-mechanical coupling at buried interfaces, providing crucial insights into the dynamic evolution of interfacial stability in ASSLBs.

5.1.3. Solid-state nuclear magnetic resonance (NMR). Solid-state nuclear magnetic resonance (NMR) spectroscopy provides atomic-level insights into lithium-ion distribution, chemical states, and dynamic evolution within SSEs and electrode interfaces. Liang et al. performed operando⁷Li NMR spectroscopy to directly quantify and monitor inactive lithium species in sulfide-based ASSLBs.¹⁹⁶ The authors constructed anode-free cells using LCO cathodes and four sulfide electrolytes (Li₁₀GeP₂S₁₂, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, LPSC, and Li₇P₃S₁₁) to explore failure mechanisms. Through operando NMR monitoring, they distinguished between “dead Li” (electronically or ionically isolated metallic lithium) and “SEI-Li” (Li⁺ immobilized in the interphase). The analysis reveals that Li₁₀GeP₂S₁₂ reacts immediately with deposited Li, converting all active lithium into SEI-Li, while LPSC and Li₇P₃S₁₁ primarily accumulate dead Li as the main cause of capacity loss.

Moreover, the chemical shift evolution of metallic lithium—from ∼230 ppm (flat deposits) to ∼270 ppm (dendritic structures)—indicates that NMR can sensitively track morphological transitions correlated with dendrite growth and interfacial degradation. The study also identifies two distinct dead Li formation modes: electronically disconnected Li trapped inside the SSE bulk and ionically isolated Li residing on the Cu current collector. Importantly, operando NMR further demonstrates that dendritic lithium exhibits a faster corrosion rate than planar lithium during calendar aging, reflecting its higher reactivity and instability. Collectively, these findings establish solid-state NMR as a unique, nondestructive probe for simultaneously quantifying lithium species, mapping transport pathways, and elucidating dynamic interfacial failure mechanisms in ASSLBs.

5.1.4. X-ray absorption spectroscopy (XAS). X-ray absorption spectroscopy (XAS) was employed as an element-specific and nondestructive probe to unravel interfacial chemical states and degradation mechanisms in ASSLBs. Morino et al. investigated sulfide-based LPSC/LiNbO₃-coated NCM523 cathodes using Nb K- and L₃-edge XAS to monitor the buried coating layer evolution under high-voltage conditions (4.25–4.55 V vs. Li/Li⁺).¹⁹⁷ They found that while the Nb valence state remains at +5, the fine-structure variations correspond to Li–O release from the LiNbO₃ coating, implying partial decomposition that drives oxidative degradation of the sulfide electrolyte near the interface. The persistence of Nb at the interface confirmed by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) indicates that the coating physically remains intact, whereas XAS detects subtle electronic and coordination changes invisible to conventional microscopy. Complementarily, Kazemian et al. applied scanning transmission X-ray microscopy (STXM) coupled with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the Li–Li_1.5Al_0.5Ge_1.5(PO₄)₃ interface in microfabricated thin-film SSBs.¹⁹⁸ Their soft X-ray spectromicroscopy reveals localized Ge reduction and the formation of Li₂O and Ge–Li alloy phases at the anode interface after cycling, whereas the cathodic interface remains chemically stable. These results demonstrate that XAS techniques—especially in situ and spatially resolved modes—enable direct mapping of oxidation states, coordination environments, and interfacial redox processes in buried electrode–electrolyte regions. Overall, XAS provides unparalleled insight into electronic-structure evolution and interphase formation mechanisms, thereby linking local chemistry to electrochemical degradation pathways in ASSLBs.

5.2. Advanced electron microscopy

Advanced electron microscopy enables direct, high-resolution observation of buried interfaces and microstructural evolution in ASSLBs. Techniques such as focused ion beam (FIB) cross-sectioning and cryogenic transmission electron microscopy (Cryo-TEM) provide precise visualization of interfacial structures and degradation processes, offering critical insights into the nanoscale mechanisms that govern battery stability and performance.

5.2.1. Focused ion beam (FIB) cross-sectioning. Focused ion beam (FIB) cross-sectioning was employed to directly access buried interfaces and visualize internal microstructures in ASSLBs with nanoscale precision. By using a finely focused ion beam—typically gallium (Ga⁺) or plasma sources—FIB enables site-specific milling, thinning, and imaging of electrode–electrolyte cross-sections without extensive mechanical polishing. This approach allows researchers to obtain high-resolution structural information from deep within dense SSEs or composite electrodes, which is essential for correlating interfacial morphology with electrochemical behavior.¹⁹⁹ In SSB research, FIB cross-sectioning provides critical insights into the interfacial degradation, void formation, and dendrite initiation processes that occur beneath the electrode surface. When combined with scanning electron microscopy (SEM) or TEM, it reveals features such as void coalescence, crack propagation, and buried lithium deposits that are otherwise inaccessible. Furthermore, advanced plasma-FIB or cryogenic-FIB systems have been developed to minimize ion-beam damage, contamination, and local heating—particularly important when working with beam-sensitive materials like Li metal or sulfide electrolytes.²⁰⁰ Beyond morphological visualization, FIB cross-sectioning can be coupled with analytical tools such as EDS, electron backscatter diffraction (EBSD), or three-dimensional tomography to reconstruct spatial distributions of elements and phases across interfaces (Fig. 16b). These capabilities enable comprehensive mapping of mechanical and chemical heterogeneities within interphases, providing direct evidence for mechanisms such as lithium sub-surface plating, stress accumulation, and interfacial delamination. Overall, FIB cross-sectioning serves as a versatile bridge between microstructural characterization and electrochemical performance evaluation, offering a nanoscale window into the complex interfacial dynamics that govern the stability and reliability of SSBs.

5.2.2. Cryogenic transmission electron microscopy (Cryo-TEM). Cryogenic transmission electron microscopy (Cryo-TEM) was employed to directly visualize the nanoscale interfacial structures in ASSLBs while mitigating electron-beam and air-exposure damage. Unlike conventional TEM, Cryo-TEM preserves the native chemical and structural states of reactive materials such as Li metal and sulfide or oxide SSEs by rapidly freezing samples below their decomposition temperature, thus preventing beam-induced reduction or lithium migration.^201,202 Researchers typically combined Cryo-FIB for site-specific lamella preparation, enabling artifact-free cross-sections of buried interfaces for atomic-resolution imaging.²⁰³ This technique provides unique insights into the morphology, crystallinity, and phase distribution of solid–solid interphases that conventional microscopy cannot capture. Cryo-TEM reveals, for instance, that SEI on Li anode often exhibits mosaic or multilayered nanostructures, consisting of crystalline and amorphous regions, which are preserved only under cryogenic conditions. The method also allows for simultaneous chemical mapping using cryogenic scanning transmission electron microscopy-EDS (cryo-STEM-EDS) or electron energy loss spectroscopy (EELS), enabling correlation between atomic structure and local chemistry. Overall, Cryo-TEM serves as a powerful tool for understanding interfacial formation, degradation, and stabilization mechanisms in ASSLBs. By eliminating beam-induced artifacts and preserving metastable interphases, it enables direct observation of intrinsic structural and compositional gradients that govern ion transport and electrochemical stability across buried electrode–electrolyte interfaces.

5.3. Synchrotron and X-ray-based techniques

Advanced synchrotron and X-ray-based techniques have emerged as indispensable tools for probing the buried interfaces and structural dynamics in ASSLBs. Their high spatial resolution, tunable photon energy, and compatibility with in situ and operando measurements enable direct correlation between electrochemical behavior and crystallographic or morphological evolution. The following sections summarize representative synchrotron and X-ray characterization methods that have been applied to elucidate interfacial reactions, phase transformations, and mechanical degradation processes in ASSLBs.

5.3.1. Synchrotron-based X-ray diffraction (XRD). Synchrotron-based X-ray diffraction (XRD) was employed to investigate structural evolution and phase transformations at buried interfaces in ASSLBs with high spatial and temporal resolution. Owing to its superior photon flux and tunable energy, synchrotron XRD enables detection of subtle lattice distortions, intermediate phases, and strain evolution that conventional laboratory XRD cannot resolve. Researchers used both in situ and operando configurations to follow dynamic changes in electrode–electrolyte interfaces during cycling, revealing correlations between electrochemical reactions, lattice expansion/contraction, and interfacial stress development.²⁰⁴ In particular, grazing-incidence geometries (GI-XRD or GIXS) allow the characterization of near-surface and buried interfacial layers by adjusting the incident angle below the critical value, effectively isolating signals from thin interphases. This approach provides critical insight into the crystallization and amorphization processes of SSEs, as well as the evolution of preferred orientations in composite cathodes under operating conditions.²⁰⁵ The combination of wide-angle (WAXS) and small-angle (SAXS) scattering further enables simultaneous monitoring of lattice-scale and mesoscale morphology variations, such as grain reorientation, stress accumulation, and domain coalescence. Overall, synchrotron-based XRD serves not only as a structural probe but also as a mechanistic tool linking crystallographic evolution to electrochemical functionality. Its capacity for operando and three-dimensional mapping supports quantitative analysis of strain, phase distribution, and texture across interfaces—thereby deepening understanding of how mechanical, chemical, and electrochemical interactions collectively govern the performance and reliability of SSBs.

5.3.2. X-ray computed tomography (XCT). X-ray computed tomography (XCT) is a powerful nondestructive imaging technique that reconstructs three-dimensional structures based on the attenuation of X-rays through a material, enabling direct visualization of buried interfaces and internal features that cannot be accessed by conventional microscopy. In the context of ASSLBs, XCT has been widely employed to investigate interfacial degradation, dendrite propagation, and mechanical failure within solid electrolytes during electrochemical cycling. Researchers have used synchrotron-based in situ XCT to monitor the morphological evolution of lithium and cracks inside sulfide or oxide SSEs during repeated plating-stripping processes.²⁰⁶ By achieving sub-micrometer spatial resolution and rapid scan times, this method allows dynamic tracking of lithium penetration pathways, quantification of crack width and volume, and mapping of stress and strain fields through digital volume correlation. Such studies reveal that lithium deposition preferentially initiates along pre-existing microcracks and grain boundaries and that crack propagation often precedes lithium infiltration, explaining why internal short circuits can occur only after several plating cycles. The ability to distinguish voids, lithium, and solid phases in reconstructed tomograms enables detailed analysis of partially filled cracks and provides direct evidence of the interplay between electrochemical and mechanical processes at buried interfaces.

Recent developments have extended XCT to post-mortem and in situ analyses of interlayer-modified interfaces. For instance, laboratory-scale XCT combined with a high-pressure xenon contrast method successfully differentiated lithium metal from voids, overcoming the challenge of lithium's low X-ray absorption.²⁰⁷ This approach has been applied to visualize lithium deposition morphology in ASSLBs with carbon interlayers formed by different compaction processes. The technique demonstrates that simultaneous compression of the SSE and carbon layer produces intimate interfacial contact and uniform lithium deposition, while sequential pressing results in poor adhesion and dendritic intrusion. Overall, XCT serves as a crucial bridge between electrochemical performance and microstructural evolution in ASSLBs. Its ability to provide real-time, three-dimensional, and quantitative insight into interfacial morphology and internal stress evolution significantly advances the mechanistic understanding of lithium penetration, interlayer function, and failure processes, offering indispensable guidance for rational interfacial engineering in next-generation SSBs.

5.4. Neutron and ion beam analysis

Neutron and ion beam techniques offer powerful tools to visualize and quantify lithium distribution and interfacial evolution in ASSLBs. Their high sensitivity to light elements and excellent spatial or depth resolution enable nondestructive analysis of buried interfaces. Among them, neutron depth profiling (NDP), neutron imaging and tomography, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) provide complementary information across multiple scales, revealing lithium transport pathways and interphase formation processes.

5.4.1. Neutron depth profiling (NDP). Neutron depth profiling (NDP) is a nondestructive analytical technique uniquely suited for quantifying lithium distribution in ASSLBs with high depth resolution. It relies on the nuclear reaction between thermal neutrons and ⁶Li isotopes in the sample. When neutrons are absorbed by ⁶Li, the reaction emits an alpha particle and a Triton (³H). The energies of these emitted particles are directly related to the depth at which the reaction occurred, because they lose energy while passing through the surrounding material before reaching the detector. By measuring the energy spectrum of the emitted particles, researchers can reconstruct the concentration of ⁶Li as a function of depth, providing a detailed lithium profile across buried interfaces and bulk regions.²⁰⁸

In earlier studies, Li et al. employed in situ NDP to investigate lithium plating behavior in Li/LLZTO/Ti SSBs with a 3D Ti framework (Fig. 16c, d).²⁰⁹ They found that most lithium was deposited within the void spaces of the 3D electrode rather than directly at the solid electrolyte interface. The results show that such deposition effectively mitigates interfacial degradation and suppresses lithium dendrite growth. The NDP spectra record the evolution of ³H and ⁴He signals at various energies corresponding to lithium accumulation at different depths, confirming that lithium preferentially fills the 3D voids while its penetration into the electrolyte is significantly reduced. More recently, Westover et al. conducted comparative measurements combining NDP and neutron reflectometry (NR) on model Li-LiPON interfaces.²⁰⁸ They observed that NDP resolved lithium concentration profiles through Li and LiPON layers several hundred nanometers thick but could not distinguish ultrathin interphases below approximately 10 nm. The results indicate that NDP possesses excellent resolution within the 50 nm–1 mm range, while NR provides complementary sensitivity to nanometer-scale variations (0.1–200 nm). Together, these two techniques yield a comprehensive picture of the interfacial structure and lithium distribution across SSB stacks. Overall, NDP provides direct visualization of lithium migration, interphase formation, and dendrite evolution beneath buried interfaces. It demonstrates high sensitivity to light elements, excellent depth resolution, and a nondestructive nature, making it a powerful tool for probing ion transport and interfacial dynamics in ASSLBs, particularly when integrated with complementary neutron or X-ray-based techniques.

5.4.2. Neutron imaging and tomography. Neutron imaging and tomography are key operando visualization techniques for investigating lithium-ion migration, reaction distribution, and interfacial evolution in ASSLBs. Neutron imaging obtains two-dimensional projection images by detecting variations in neutron transmission through the sample, where differences in lithium content cause distinct contrasts due to changes in neutron attenuation coefficients. Furthermore, neutron tomography reconstructs three-dimensional structures by collecting two-dimensional images at multiple angles, thereby visualizing the internal lithium distribution, phase transitions, and interfacial evolution within the sample. This approach is nondestructive, quantitative, and offers high spatial resolution (down to the micrometer scale), making it particularly suitable for studying buried interfaces and thick-electrode architectures. For example, Ji et al. employed operando neutron imaging to reveal the delithiation behavior of a high-mass-loading NMC811 cathode.²¹⁰ They observed that lithium extraction initiated from the side adjacent to the solid electrolyte and gradually propagated toward the current collector, indicating that ion transport heterogeneity is a major cause of performance degradation in thick electrodes. Based on this insight, they proposed an electrolyte gradient design to enhance Li⁺ transport uniformity and improve rate capability.

Technically, neutron imaging and tomography enable real-time monitoring of lithium migration, dendrite propagation, and interfacial reaction fronts within SSBs. The use of isotope labeling (⁶Li/⁷Li contrast enhancement) can further improve image contrast and provide insights into lithium diffusion behavior inside SSEs. Compared with NDP, neutron imaging and tomography offer a broader visualization range and 3D reconstruction capability, while NDP is more suitable for quantitative depth analysis. The integration of these techniques enables multiscale characterization of lithium distribution from the nanometer to micrometer level, providing a powerful approach for elucidating ion transport and interfacial dynamics in SSBs.

5.4.3. Time-of-flight secondary ion mass spectrometry (TOF-SIMS). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful surface-sensitive analytical technique that enables 3D chemical mapping of buried interfaces in ASSLBs with nanometer-scale resolution. By sputtering the sample surface with a pulsed primary ion beam and collecting the emitted secondary ions according to their mass-to-charge ratios, TOF-SIMS provides detailed compositional information of both light and heavy elements with detection limits down to the ppm level. Compared with XPS or TEM, TOF-SIMS bridges the gap between chemical sensitivity and spatial resolution, allowing quantitative visualization of interphase thickness, elemental diffusion, and reaction product distribution.

In the study by Otto et al., in situ TOF-SIMS was employed to investigate Li–SSE interfaces that were prepared by lithium vapor deposition or electrochemical plating.²¹¹ The technique reveals a continuous Li₂S-rich interlayer (∼250 nm) at the Li–LPSC interface, which demonstrates the formation of a chemically stable yet ionically conductive SEI. Depth-profile analyses through micrometer-thick Li layers further enable classification of the interfacial stability of different electrolytes—LLZO (thermodynamically stable), LPSC (SEI-forming), and LATP (mixed-conducting interphase). Similarly, Sastre et al. utilized FIB-TOF-SIMS to visualize the element distribution and lithium gradient in doped LLZO thin films, and the analysis shows a homogeneous elemental distribution and a slight lithium gradient, demonstrating the capability of TOF-SIMS for nanoscale depth profiling of oxide SSEs.²¹² Overall, TOF-SIMS provides 3D chemical mapping of interfacial reactions, elemental diffusion, and degradation layers with nanometer precision, playing a crucial role in elucidating ion transport mechanisms and interfacial evolution processes.

Taken together, these advanced characterization techniques establish a comprehensive toolbox for deciphering the intricate interfacial phenomena in ASSLBs. From atomic-scale spectroscopy and electron microscopy to mesoscale X-ray tomography and macroscopic neutron analysis, each method contributes complementary information toward building a holistic understanding of interfacial chemistry, structure, and dynamics. The integration of in situ, operando, and multimodal approaches enables direct correlation between electrochemical behavior and structural evolution, thus revealing the origins of interfacial instability and guiding the rational design of robust interfaces. Moving forward, the convergence of these high-resolution tools with computational modeling and machine learning is expected to accelerate the development of predictive frameworks for interfacial engineering, paving the way for safer, more durable, and high-energy SSBs.

6. Summary and outlook

Inorganic ASSLBs are considered promising direction for next-generation energy storage systems due to their potential for high energy density and excellent safety. As the critical pathway for energy and material transfer within ASSLBs, the stability of the interface directly determines the cycling life and rate performance of the entire battery system. However, the inherent mismatch between the SSEs and the electrodes at the physical, chemical, and electrochemical levels presents significant challenges. Achieving stable control of the electrode–SSE interfaces under complex and dynamic operating conditions remains central issue in this field of research. In this review, we systematically integrate the coupled interfacial failure mechanisms encountered by inorganic SSEs at both electrode interfaces—including stress mismatch, interfacial reactions, electron leakage, and dendrite growth—and, crucially, conduct a side-by-side and critical comparison of leading interface engineering strategies. Four key approaches—electrode engineering, interlayer construction, electrolyte regulation, and integrated dual-interface design—are comparatively assessed, with special emphasis on their respective advantages, limitations, and application boundaries across different material systems and operational scenarios. This establishes a novel interfacial regulation framework that encompasses structural modulation, chemical stabilization and multifunctional synergy. Multiscale evolution of interfacial degradation and the functional complementarity among various strategies are highlighted, offering a forward-looking and integrated solution for realizing ASSLBs with high energy density and long-term stability. However, despite the significant progress made in interface design, several key issues remain that require further in-depth research and optimization (Fig. 17):

Fig. 17 Current challenges and future solutions for interface engineering in ASSLBs.

(1) Current interface control primarily focuses on static structural design and enhancing chemical stability. However, in actual operating conditions, the interface environment is influenced by the synergistic effects of multiple physical fields, including electric fields, stress fields, concentration gradients, and temperature gradients. This is particularly pronounced under high-rate or wide-temperature range operating conditions. Therefore, there is urgent need to establish an interface characterization and simulation platform that couples multiple physical fields. This should incorporate in situ nano-mechanical-electrochemical coupling characterization methods (such as in situ electrochemical stretching and synchronized nano-indentation-impedance spectroscopy measurements), alongside multi-scale models combining finite element analysis, density functional theory, and phase field methods. Such approach would reveal the coupling evolution mechanisms of interfaces under real working conditions, providing essential physical insights for the design of interface materials and structures.

(2) Although many current interlayers and composite anode designs have shown some success in mitigating lithium dendrite growth, the initiation mechanisms, evolution pathways, and failure thresholds of dendrites remain inadequately understood. This is especially true regarding the issue of bulk-phase dendrite formation caused by electronic leakage, for which direct evidence and quantifiable standards are lacking. Therefore, it is recommended that future studies employ in situ cryo-electron microscopy and cryo-electron tomography techniques to visualize lithium deposition pathways and grain boundary penetration behaviors. In parallel, operando X-ray spectroscopy—such as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), complemented by grazing-incidence X-ray diffraction (GI-XRD) and small-angle X-ray scattering (SAXS) when appropriate—should be used to track real-time changes in interfacial valence states, coordination environments, and decomposition products, enabling correlation between the onset of electronic leakage and the emergence of mixed-conducting interphases. Based on this multimodal evidence, adopting an energy-band-engineering strategy offers a promising pathway forward. By leveraging interface dipole induction, block-copolymer interlayers, and electronegativity-gradient materials, it is possible to simultaneously optimize electronic barriers and ion flux regulation. Operando spectral parameters—such as edge shifts and pre-edge features—can then quantitatively verify band alignment and stability under cycling.

(3) Current designs for interface interlayers, electrode coatings, and doping modifications primarily rely on empirical screening and limited first-principles calculations, which are inefficient and lack transferability. As material databases (such as Materials Project, Open Catalyst Project, etc.) continue to expand, it is imperative to establish multi-parameter high-throughput screening system focused on indicators such as interface compatibility, conductivity, interface reaction enthalpy, and mechanical modulus. By leveraging machine learning and graph neural network (GNN) models, intelligent and collaborative screening of interfacial materials—such as those at cathode-electrolyte and interlayer-lithium metal anode interfaces—can be effectively realized. Furthermore, reverse design algorithms can be developed, combining battery performance objectives (such as high rate capability, long cycle life, low impedance, etc.) to deduce the material compositions and structural parameters that meet specific performance windows, thereby accelerating the transition of interface-building materials from experimental validation to engineering application.

(4) The present dual-interface synergistic construction strategies primarily rely on symmetric interlayers or electrolyte structural modifications to simplify the process and maintain stability at both the anode and cathode interfaces. However, such “homogeneous synergy” approaches are insufficient under extreme operating conditions—e.g., high current densities and low temperatures—where electro-chemo-mechanical coupling intensifies and the inherent differences in chemical potential, reactivity, and mechanical behavior between the two electrodes become more pronounced, limiting gains in rate capability and low-temperature robustness. To align with practical application needs, future designs should pursue “asymmetric yet coordinated” interfaces that are explicitly engineered for extremes: (i) low-temperature, fast-ion pathways and reliable wetting to minimize cold-start interfacial impedance; (ii) strong electronic blocking and Li-flux homogenization to suppress electronic-leakage-induced bulk dendrites at high rates; and (iii) modulus/thermal-expansion-graded architectures with stress-relief or self-healing chemistries to prevent contact loss across wide temperature ranges. A promising strategy involves developing adaptive interface precursors that spontaneously convert during processing or cycling into targeted interphases—such as lithiophilic, electron-insulating layers at the anode and ion-conductive, stable layers at the cathode. This self-evolving design streamlines fabrication while ensuring coordinated interfacial evolution and durable performance, even under high-current and low-temperature operation.

(5) At this stage, laboratory techniques such as atomic layer deposition (ALD) and molecular layer deposition (MLD) are commonly used; however, they are expensive and difficult to scale up for industrial production. To promote the commercialization of ASSLBs, it is essential to accelerate the development of process-friendly interface construction technologies. These should include low-energy, high-throughput engineering strategies, such as spray-pyrolysis-based interlayer construction, fusion of interfaces via molten compression, and the introduction of pressure-sensitive-adhesive-based polymer interfaces. Alongside these efforts, pilot-scale fabrication trials (e.g., roll-to-roll lamination, slot-die/spray coating, tape casting, and hot-press/melt-fusion under dry-room conditions) should be conducted to translate laboratory protocols to manufacturing-relevant throughputs, quantify yield and uniformity over tens-to-hundreds of meters, and establish robust process windows (temperature, pressure, line speed) that preserve interfacial integrity. Additionally, it is important to explore the stability and controllability of the interface layers under real processing conditions, such as rolling and laminating, using statistically designed experiments during pilot runs to assess reproducibility, aging, and thermal excursions. Furthermore, building a comprehensive framework that integrates a cross-scale interface-failure database, process maps, and lifetime prediction models will bridge the gap between laboratory research and engineering practice. Continuous data feedback from pilot production lines can then close the design-manufacturing loop, paving the way for the large-scale deployment of ASSLBs.

In summary, the stable construction of electrode-inorganic SSE interfaces still faces challenges regarding the multi-physical field coupling evolution, unclear dendrite formation mechanisms, insufficient intelligent design of interface materials, and difficulty in controlling the differences between dual interfaces. Moving forward, the focus should be on the synergistic advancement of mechanism analysis, material screening, and process integration, aiming to establish high-performance, scalable interface engineering systems.

Conflicts of interest

There are no conflicts to declare.

Data availability

This article is a review and does not report new original data. All data supporting the findings of this study are available within the cited references.

Acknowledgements

Z. B. thanks the financial support of National Natural Science Foundation of China (No. 22272093), Research Fund for International Scientists National Natural Science Foundation of China (52350710795), and Natural Science Foundation of Shandong Province (ZR2021MB127). N. W. acknowledge support from the Australian Research Council (DP240102926 and FT240100596).

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