Constructing stable cathode interfaces with halide–sulfide dual electrolytes for all-solid-state lithium batteries with enhanced electrochemical performance

Abstract

Interfacial instability between Ni-rich layered oxide cathodes and sulfide electrolytes remains a major bottleneck hindering the development of high-performance all-solid-state lithium batteries (ASSLBs). Conventional coating materials often suffer from low ionic conductivity and poor mechanical deformability, necessitating complex processing or additional interlayers. Halide electrolytes offer good stability, ionic conductivity, and softness, but their poor reductive stability with lithium metal limits their use as standalone solid electrolytes in full cells. In this work, we propose a dual-electrolyte composite cathode strategy by introducing a halide electrolyte, Li₃InCl₆ (LIC), as a functional surface coating for LiNi_0.8Co_0.1Mn_0.1O₂ (NCM). The nanosized Li₃InCl₆ particles synthesized by freeze-drying exhibit high ionic conductivity and uniform particle size distribution, making them effective as interfacial buffer layers. The optimized 15% LIC@NCM composite cathode delivers a high initial capacity of 189 mA h g⁻¹ with a coulombic efficiency of 84.4% at 0.1 C, along with remarkable cycling stability, retaining 114 mA h g⁻¹ after 250 cycles at 0.5 C. Comprehensive electrochemical and spectroscopic analyses confirm that the Li₃InCl₆ coating effectively mitigates interfacial degradation, suppresses side reactions, and facilitates ion transport across the composite interface. This study offers a facile and scalable interface engineering strategy using halide electrolytes to simultaneously enhance lithium-ion transport and interfacial stability in sulfide-based ASSLBs.

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1 Introduction

All-solid-state lithium batteries (ASSLBs) employing inorganic solid electrolytes are widely recognized as next-generation energy storage systems owing to their intrinsic non-flammability and the potential to achieve high energy densities.¹ In comparison with conventional lithium-ion batteries utilizing flammable liquid electrolytes, ASSLBs significantly mitigate the risk of thermal runaway and support the design of compact, high-voltage, and densely stacked architectures, which render them highly promising for practical applications.^2,3 Despite the diversity of material systems and design configurations in current ASSLB development, one of the most critical bottlenecks lies in the poor interfacial stability between the solid electrolyte and electrode materials, which primarily stems from mechanical and electrochemical mismatches at the interface.^4–6 The interfacial behavior is highly dependent on the type of solid electrolyte used. Oxide-based solid electrolytes, such as Li₇La₃Zr₂O₁₂ ⁷ and Li_1.3Al_0.3Ti_1.7(PO₄)₃,⁸ exhibit wide electrochemical stability windows and good compatibility with high-voltage cathodes. However, their high Young's modulus and poor deformability at room temperature often necessitate high-temperature sintering (>1000 °C) or the incorporation of liquid electrolyte to ensure sufficient interfacial contact, increasing processing complexity and often leading to interfacial degradation.^9–11

In contrast, sulfide electrolytes such as Li₆PS₅Cl¹² and Li₁₀GeP₂S₁₂ ⁷ exhibit excellent room-temperature ionic conductivity (≥10⁻³ S cm⁻¹) and superior mechanical compliance, making them well-suited for cold pressing and intimate interfacial contact with electrode particles.¹³ Consequently, sulfide electrolytes have become not only the most extensively studied but are also among the most commercially promising candidates in the field of ASSLBs. Nonetheless, sulfide electrolytes suffer from limited electrochemical stability against high-voltage oxide cathodes, especially Ni-rich layered materials like LiNi_0.8Co_0.1Mn_0.1O₂. The high oxidation potential at the cathode side can trigger the decomposition of sulfides, forming resistive interfacial byproducts such as Li₂S, metal sulfides, and phosphates, thereby increasing interfacial impedance and accelerating capacity fade.^4,14,15 To address this issue, introducing interfacial buffer or coating layers at the cathode–electrolyte interface has become a widely adopted strategy.^16–18 Conventional coating materials, including LiNbO₃,¹⁹ Li₂ZrO₃,²⁰ and Li₄Ti₅O₁₂,²¹ can enhance interfacial stability to some extent. However, these oxides typically suffer from low ionic conductivity and high rigidity. They often require high-temperature treatments to form dense and continuous layers. These stringent synthesis conditions increase manufacturing complexity, thereby limiting their practical effectiveness.^22,23

Recently, halide electrolytes have gained increasing attention as a new class of interfacial materials due to their unique combination of high electrochemical stability, moderate ionic conductivity and mechanical softness.^24–26 Representative chloride-based electrolytes such as Li₃InCl₆,²⁷ Li₃YCl₆ ²⁸ and Li₂ZrCl₆ ²⁹ exhibit oxidation stability up to 4.0–4.5 V vs. Li/Li⁺, enabling compatibility with oxide cathodes like LiNi_xCo_yMn_zO₂(x + y + z = 1) and LiCoO₂.³⁰ Some halide electrolytes even achieve room-temperature ionic conductivities exceeding 10⁻³ S cm⁻¹, comparable to that of their sulfide counterparts.³¹ Moreover, their relatively low mechanical softness allows for cold pressing without additional thermal treatment, greatly simplifying interface fabrication.³² However, the use of halides as bulk electrolytes remains challenging. Their limited reductive stability makes them prone to decomposition upon contact with Li metal or alloy anodes, necessitating a bilayer architecture when used in full cells, which increases cell complexity and reduces energy density.³³ Additionally, many high-performance halides rely on rare-earth elements (e.g., Y, Er, and Sc), leading to high synthesis costs and resource limitations for large-scale applications.^24,33

Considering these trade-offs, deploying halide electrolytes as interfacial coatings rather than bulk electrolytes represents a promising approach to simultaneously enhance interfacial stability and maintain practical processability. Their high oxidative stability and mechanical compliance enable effective passivation of high-voltage cathodes, while their chemical compatibility with sulfides supports seamless ionic transport across the active material–electrolyte interface. This strategy helps suppress parasitic reactions, reduce interfacial impedance, and improve both the initial coulombic efficiency and long-term cycling stability of ASSLBs. Despite their potential, current halide solid electrolyte synthesis methods (e.g., high-temperature solid-state reactions or the liquid phase method) often yield large, agglomerated particles with poor dispersibility, making it difficult to achieve uniform coating on cathode surfaces. Moreover, residual solvents or thermal treatments may induce undesired phase transformations or introduce impurities that compromise the intrinsic properties of the halide coating.²⁵

To overcome these challenges, we propose a freeze-drying route to synthesize nanosized Li₃InCl₆ particles for use as a cathode surface coating. The yielded halide electrolytes with finer and more uniform particle size distribution enable intimate contact with the LiNi_0.8Co_0.1Mn_0.1O₂ cathode to form a continuous, ionically conductive and stable interfacial layer. The resulting Li₃InCl₆ exhibits a high room-temperature ionic conductivity of 2 mS cm⁻¹. Meanwhile, sulfide-based Li₆PS₅Cl is used as the primary bulk electrolyte to provide rapid Li⁺ transport. The synergistic combination of halide coating and the sulfide matrix constitutes a dual-electrolyte strategy that enables superior electrochemical performance in full cells. The ASSLB assembled with the 15% LIC@NCM composite cathode delivers a high initial capacity of 189 mA h g⁻¹ with a coulombic efficiency of 84.4% at 0.1 C. Even after 250 cycles at 0.5 C, the discharge capacity remains well preserved at 114 mA h g⁻¹. Furthermore, this work systematically investigates the structural integrity, electrochemical kinetics, and interfacial chemistry of the ASSLB, providing experimentally validated insights for rational interfacial design in high-performance sulfide-based ASSLBs.

2 Experimental section

2.1 Synthesis of Li₃InCl₆ and Li₆PS₅Cl electrolytes and the composite cathode

2.1.1 Synthesis of Li₃InCl₆ electrolyte. Li₃InCl₆ solid electrolytes were synthesized via three different methods—freeze-drying (FD), the liquid phase method (LP), and high-energy ball milling (BM)—to systematically investigate the influence of the synthetic route on their structural and electrochemical properties. The starting materials, lithium chloride (LiCl, 99.9%, Macklin) and indium chloride (InCl₃, 99.99%, Macklin), were mixed in a stoichiometric molar ratio of Li : In = 3 : 1. For the freeze-drying route, 2 g of precursor salts were dissolved in deionized water and rapidly frozen in liquid nitrogen to form a solid solution. The frozen solution was then subjected to sublimation drying in a freeze-dryer to thoroughly remove residual water. The resulting powder was annealed at 200 °C, 260 °C and 300 °C for 4 h under an inert atmosphere to obtain the final FD-LIC samples. For the liquid phase method, the same 2 g of precursors were dissolved in deionized water and subsequently collected via vacuum filtration. The filtered powder was dried in a vacuum oven at 80 °C, followed by heat treatment under identical conditions (200 °C, 260 °C and 300 °C for 4 h) to yield LP-LIC. For the high-energy ball milling method, 2 g of precursors were mixed with zirconia grinding balls (ball-to-powder mass ratio: 40 : 1) and milled using a Fritsch PM400 planetary mill at 500 rpm for 24 h. The as-milled powders were then annealed under inert conditions at the same temperatures as above to obtain BM-LIC.

2.1.2 Synthesis of Li₆PS₅Cl electrolyte. Li₂S (99.9%, Alfa Aesar), P₂S₅ (99%, Aladdin Chemistry) and LiCl (99.9%, Sigma-Aldrich) were used as starting materials and Li₆PS₅Cl precursor powder was prepared through a ball mill; the precursor powder was pressed into pellets and sealed in a quartz crucible, and then annealed at 550 °C for 6 h. After annealing, the Li₆PS₅Cl pellet was obtained, which was subsequently ground to yield the Li₆PS₅Cl solid electrolyte powder.

2.1.3 Composite cathode fabrication. All cathode processing steps were carried out in an Ar-filled glovebox (H₂O and O₂ < 0.1 ppm) to prevent exposure to moisture and oxygen. First, the commercial LiNi_0.8Co_0.1Mn_0.1O₂ cathode material was mixed with the as-prepared Li₃InCl₆ in defined weight ratios. The mixture was then milled with zirconia balls (5 mm diameter, ball-to-powder ratio of 40 : 1) at 200 rpm for 2 h to enable mechanical coating of Li₃InCl₆ onto the LiNi_0.8Co_0.1Mn_0.1O₂ surface. The resulting Li₃InCl₆-coated LiNi_0.8Co_0.1Mn_0.1O₂ was further blended with Li₆PS₅Cl sulfide electrolyte via manual grinding to obtain a dual-electrolyte composite cathode. The total mass ratio of electrolyte (Li₃InCl₆ + Li₆PS₅Cl) to the active material (LiNi_0.8Co_0.1Mn_0.1O₂) was maintained at 3 : 7 (i.e., 30% total electrolyte content). Specifically, the content of Li₃InCl₆ in the composite cathode was varied at 0% (pure Li₆PS₅Cl), 15%, and 30% (pure Li₃InCl₆), denoted as 0% LIC@NCM, 15% LIC@NCM, and 30% LIC@NCM, respectively.

2.2 All-solid-state lithium battery assembly

The all-solid-state cell assembly was performed entirely in an Ar-filled glovebox (H₂O and O₂ < 0.1 ppm) to avoid moisture-induced degradation. Stainless steel dies and PEEK (polyether ether ketone) molds with a 10 mm inner diameter were used for cell pressing. First, 150 mg of Li₆PS₅Cl electrolyte was loaded into the PEEK mold and compacted under 240 MPa to form a dense electrolyte pellet (10 mm diameter). Then, 3 mg of the composite cathode, consisting of the active material and electrolyte in a 7 : 3 weight ratio with an areal mass loading of 2.7 mg cm⁻², was then uniformly spread onto one side of the pellet and compressed at 240 MPa to ensure intimate contact. Finally, a piece of Li–In alloy foil was placed on the opposite side as the anode and pressed under 360 MPa to complete the full-cell assembly.

2.3 Material characterization

The phase structure and purity of the synthesized Li₃InCl₆ samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance) using Cu K_α radiation over a 2θ range of 10–80°. Microstructural features and elemental distributions were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi S4800) coupled with energy-dispersive spectroscopy (EDS). The chemical states and bonding environments of the elements were probed by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) to identify potential interfacial reactions or shifts in binding energy in the composite electrodes.

2.4 Electrochemical measurements

Electrochemical performance was evaluated via electrochemical impedance spectroscopy (EIS), direct current (DC) polarization, and galvanostatic charge–discharge cycling. The electrolyte mass was ∼200 mg and it was cold-pressed under 360 MPa into dense pellets. Measurements were carried out in the frequency range of 10⁶ Hz to 10 Hz with an amplitude of 10 mV. Electronic conductivity (σ_e) was determined by DC polarization using a constant 0.5 V bias for 1 h on a Solartron 1470E electrochemical workstation, and the steady-state current was used for calculation. Galvanostatic cycling tests were conducted on a LAND CT-2001A battery testing system (Wuhan LAND Electronics) within a voltage window of 2.4–3.7 V (vs. Li–In/Li⁺) to assess rate capability and cycling stability. To investigate the Li⁺ diffusion behavior of the electrode material, the galvanostatic intermittent titration technique (GITT) was carried out at 25 °C. A constant current pulse of 16 mA g⁻¹ was applied for 15 min within the voltage range of 2.4–3.7 V (vs. Li–In/Li⁺), followed by an open-circuit relaxation for 2 h, and this pulse-relaxation sequence was repeated until the entire lithiation/delithiation process was covered.

3 Results and discussion

The overall construction process of the dual-electrolyte composite cathode is schematically illustrated in Fig. 1. First, indium chloride (InCl₃) and lithium chloride (LiCl) were dissolved in deionized water at a molar ratio of 1 : 3 to form a clear and transparent precursor solution. Upon complete dissolution, the solution was rapidly quenched in liquid nitrogen, followed by freeze-drying to remove the solvent and obtain a uniformly dispersed inorganic precursor. A subsequent low-temperature thermal treatment was applied to successfully synthesize the target halide electrolyte, Li₃InCl₆. For comparison, Li₃InCl₆ samples were also synthesized via the conventional liquid-phase method and high-energy ball milling in order to conduct a systematic investigation on how synthesis methods influence the morphology and electrochemical performance. Once the Li₃InCl₆ electrolyte was obtained, it was mechanically mixed with the cathode active material LiNi_0.8Co_0.1Mn_0.1O₂ in a predetermined mass ratio to fabricate Li₃InCl₆-coated cathode particles. This coated cathode was then manually blended with the sulfide electrolyte Li₆PS₅Cl to form a dual-electrolyte composite cathode architecture. Finally, all-solid-state lithium batteries were assembled using a cold-pressing process and systematically evaluated for their electrochemical performance. In addition, to optimize interfacial contact and lithium-ion transport pathways, the mass ratio between Li₃InCl₆ and Li₆PS₅Cl was finely tuned, allowing us to explore the impact of halide-to-sulfide composition on the interface structure and battery performance, ultimately identifying the optimal coating configuration and electrochemical output.

Fig. 1 Schematic illustration of the freeze-drying synthesis of Li₃InCl₆ electrolyte, the fabrication process of the dual-electrolyte composite cathode, and the assembly of the corresponding all-solid-state lithium battery.

The morphology and crystal structure of Li₃InCl₆ synthesized via different methods were first characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). As shown in Fig. S1, the XRD patterns of the samples prepared by freeze-drying (FD), liquid-phase (LP), and ball milling (BM) methods exhibit identical diffraction peak positions, which can be indexed to a monoclinic phase with a space group of C2/m (ICSD No. 04-009-9027).³⁴ This indicates that the fundamental crystal structure of Li₃InCl₆ remains unaffected by the different synthesis methods. Despite their structural similarity, the samples exhibit markedly different particle morphologies, as revealed by the SEM images in Fig. 2a–c. The Li₃InCl₆ particles obtained via the LP and BM methods are relatively large, unevenly distributed, and tend to agglomerate. In contrast, the FD-derived Li₃InCl₆ shows significantly smaller particle sizes with a more uniform distribution and good dispersion between particles. To compare particle sizes, the dynamic light scattering (DLS) particle size analysis was conducted on the Li₃InCl₆ obtained by the three methods, and the corresponding particle size distribution curves are shown in Fig. S2. The results demonstrate that the Li₃InCl₆ particles prepared by liquid-phase and ball-milling methods exhibit larger sizes. The liquid-phase method yields particles with sizes distributed in the range of 310.7–1039.0 nm, while the ball-milling method produces particles ranging from 568.3–4702.0 nm. In contrast, the freeze-drying method results in Li₃InCl₆ particles with a smaller size, primarily distributed in the 229.8–660.9 nm range. These results confirm that the freeze-drying method enables the preparation of smaller Li₃InCl₆ particles. This can be attributed to the rapid freezing of the precursor solution at cryogenic temperatures, suppressing ion diffusion and particle agglomeration. Furthermore, the subsequent sublimation-driven drying process proceeds under mild and controlled conditions, preventing undesirable recrystallization or particle aggregation during solvent removal, thereby enabling precise control over the final particle size and distribution.³⁵

Fig. 2 SEM images (a–c) of Li₃InCl₆ synthesized by (a) freeze-drying, (b) liquid-phase, and (c) ball-milling methods. (d) XRD patterns of Li₃InCl₆-coated LiNi_0.8Co_0.1Mn_0.1O₂ with different mass ratios (15% LIC@NCM and 30% LIC@NCM), along with the corresponding coating electrolyte (FD-LIC). (e) SEM images and EDS mapping of 15% LIC@NCM.

The ionic conductivity of Li₃InCl₆ electrolytes synthesized via different methods was evaluated using electrochemical impedance spectroscopy (EIS), as shown in Fig. S3, with the corresponding extracted conductivity values summarized in Fig. S3d. Among all samples, the freeze-drying method consistently yielded high room-temperature ionic conductivity across various annealing temperatures, reaching a peak value of 2.0 mS cm⁻¹ at a relatively low annealing temperature of 200 °C. In comparison, the samples prepared via liquid-phase or ball-milling methods required annealing at 260 °C to attain their maximum conductivity. This advantage is mainly attributed to the finer particle size and larger specific surface area of the FD-derived precursor, which facilitate crystal formation and reduce the temperature required for complete crystallization of Li₃InCl₆. To further assess the ion transport capabilities of different samples, we conducted temperature-dependent EIS measurements and calculated the corresponding activation energies for ionic conduction (Fig. S4). The results reveal that the FD-LIC sample exhibits the lowest activation energy (0.28 eV), indicating more favorable Li⁺ transport kinetics. Additionally, the electronic conductivity of each electrolyte was measured using DC polarization. As shown in Fig. S5, the FD-LIC demonstrated the lowest electronic conductivity of just 5.04 × 10⁻⁹ S cm⁻¹, slightly lower than that of the LP- and BM-derived counterparts. This suggests that the use of pure water as a solvent during freeze-drying effectively prevents the introduction of organic residues or impurities, thereby enhancing the intrinsic electronic insulation of the material. A low electronic conductivity is a crucial requirement for solid electrolytes, as it helps suppress interfacial side reactions, improves coulombic efficiency, and contributes to enhanced cycling stability and safety in solid-state batteries.³⁶

Given the superior structural purity, high ionic conductivity, and low electronic conductivity of Li₃InCl₆ synthesized via the freeze-drying method, this electrolyte was selected for subsequent cathode coating and full-cell fabrication. Fig. 2d and e depict the representative structural and morphology features of Li₃InCl₆-coated LiNi_0.8Co_0.1Mn_0.1O₂ (denoted as LIC@NCM). The XRD pattern (Fig. 2d) confirms the coexistence of the primary diffraction peaks from both LiNi_0.8Co_0.1Mn_0.1O₂ and Li₃InCl₆, indicating that no significant side reactions occurred during the mechanical ball milling process. This observation further verifies the good chemical compatibility between the halide electrolyte and the layered oxide cathode. SEM images and EDS elemental maps (Fig. 2e) clearly show that Li₃InCl₆ forms a uniform and continuous coating layer on the surface of LiNi_0.8Co_0.1Mn_0.1O₂ particles. This effective coating can be attributed not only to the nanoscale particle size of Li₃InCl₆, but also to its favorable mechanical properties. Specifically, compared to the “hard” LiNi_0.8Co_0.1Mn_0.1O₂ cathode material with a high Young's modulus (∼200 GPa), the “softer” Li₃InCl₆ (∼20 GPa) is more prone to plastic deformation during ball milling, thereby facilitating conformal coverage over the cathode surface. Such morphological and mechanical compatibility provides a robust foundation for enhancing interfacial contact and improving the overall electrochemical performance of the solid-state battery.

To further validate the advantages of the dual-electrolyte composite cathode design, we assembled ASSLBs and systematically evaluated the influence of varying halide electrolyte content on their electrochemical performance. Fig. 3a–c presents the charge–discharge profiles of the cells tested at 25 °C with different halide contents. A clear variation in first-cycle capacity and coulombic efficiency is observed with changing Li₃InCl₆ content. In contrast, the 15% LIC@NCM cell exhibited the highest initial discharge capacity of 189 mA h g⁻¹ and a coulombic efficiency of 84.4%, significantly outperforming the other two groups (155.6/81.6% and 166/76.8%, respectively). It is noteworthy that the first-cycle coulombic efficiency of the 0% LIC@NCM cell (81.6%) was slightly higher than that of the 30% LIC@NCM cell (76.8%). This phenomenon can be attributed to the absence of the Li₃InCl₆ coating, which permitted direct contact between LiNi_0.8Co_0.1Mn_0.1O₂ and Li₆PS₅Cl. Interfacial reactions likely occurred even before the charging process, resulting in a lower initial charge capacity (190.7 mA h g⁻¹) than that of the 15% and 30% LIC@NCM cells. The improvement in charge–discharge capacity after Li₃InCl₆ coating highlights the significance of forming a stable interfacial buffer layer, which effectively mitigates direct contact between LiNi_0.8Co_0.1Mn_0.1O₂ and Li₆PS₅Cl, thereby suppressing undesirable side reactions. However, when the Li₃InCl₆ content was further increased to 30% (30% LIC@NCM), the performance declined, with both capacity and coulombic efficiency reduced. This is likely due to the relatively low ionic conductivity of Li₃InCl₆ (2 mS cm⁻¹) compared to Li₆PS₅Cl (3.5 mS cm⁻¹). An excessive amount of Li₃InCl₆ introduces a thicker interfacial barrier within the cathode, impeding lithium-ion and electron transport, increasing polarization, and limiting capacity utilization. Moreover, the 15% LIC@NCM cell also demonstrated the best cycling stability, retaining a discharge capacity of 157.1 mA h g⁻¹ after 50 cycles (Fig. 3d), which can be attributed to its enhanced interfacial stability. Fig. 3e shows the rate performance of the three cells and the corresponding charge and discharge curves are shown in Fig. S6. As the current rate increased from 0.1 C to 2 C, the 15% LIC@NCM cell consistently delivered the highest discharge capacity across all rates, demonstrating excellent rate capability. Notably, it retained a capacity of 126.2 mA h g⁻¹ even at 2 C, and when the current density returned to 0.1 C, the capacity recovered to 167.1 mA h g⁻¹, indicating good reversibility and structural stability. These results suggest that a moderate Li₃InCl₆ coating not only stabilizes the interface but also optimizes polarization behavior without significantly compromising ion transport, particularly under high-rate conditions. Long-term cycling performance is illustrated in Fig. 3f, and the 15% LIC@NCM cell exhibited an initial discharge capacity of 149.4 mA h g⁻¹ and maintained a capacity of 114 mA h g⁻¹ after 250 cycles, reflecting excellent capacity retention and cycling stability. In contrast, the 0%LIC@NCM and 30% LIC@NCM cells showed lower initial capacities and more pronounced degradation. Notably, the capacity of the uncoated cell (0% LIC@NCM) rapidly decayed to 52.8 mA h g⁻¹ within 250 cycles, indicating that interfacial side reactions remained inadequately suppressed. Taken together, the 15% LIC@NCM configuration demonstrates the most balanced performance in terms of initial coulombic efficiency, rate capability, and cycle life. These findings confirm the critical role of constructing an appropriately thick halide interfacial buffer layer in stabilizing the cathode-electrolyte interface and enhancing the overall performance of sulfide-based ASSLBs.

Fig. 3 Electrochemical performance of ASSLBs employing different composite cathodes. (a–c) Charge–discharge curves of 0% LIC@NCM, 15% LIC@NCM, and 30% LIC@NCM composite cathodes. (d) Cycling performance at 0.1 C. (e) Rate capability and (f) long-term cycling performance at 0.5 C.

To further evaluate the effectiveness of the Li₃InCl₆ coating layer in mitigating interfacial side reactions, X-ray photoelectron spectroscopy (XPS) was conducted on the composite cathodes after electrochemical cycling and compared with the pristine Li₃InCl₆ and Li₆PS₅Cl electrolytes (Fig. 4a–c), aiming to systematically assess the chemical state evolution of different components. As shown in the In 3d spectrum (Fig. 4d), after 50 charge–discharge cycles, the 15% LIC@NCM electrode exhibits In 3d_5/2 and In 3d_3/2 binding energies and peak shapes that remain nearly identical to those of the pristine Li₃InCl₆,³⁷ indicating that Li₃InCl₆ undergoes negligible chemical degradation during cycling. This result confirms the excellent chemical stability of the halide coating and the absence of severe interfacial side reactions with the cathode active material, further demonstrating its favorable interfacial compatibility. In contrast, the high-resolution P 2p and S 2p spectra after cycling (Fig. 4e and f) reveal not only the characteristic PS₄³⁻ signals from Li₆PS₅Cl (P 2p ≈ 131.5 eV, 132.4 eV; S 2p ≈ 161.1 eV, 162.5 eV), but also additional peaks corresponding to decomposition products, such as PO₄³⁻ (∼134.2 eV, 135.3 eV), SO₄²⁻ (∼166.7 eV, 168.5 eV), Li₂S (∼160.5 eV, 161.8 eV), and polysulfides (P₂S_x).³⁸ The appearance of these oxidized and reduced species indicates that Li₆PS₅Cl still undergoes partial decomposition under high-voltage conditions, even in the presence of the Li₃InCl₆ coating. This is likely caused by local discontinuities or insufficient thickness in the coating layer, which expose regions of the sulfide electrolyte to reactive species and trigger interfacial side reactions. A comparison with the uncoated control group (0% LIC@NCM) further supports this conclusion. The intensities of P₂S_x and PO₄³⁻ peaks are significantly stronger (Fig. 4g and h), suggesting that, without the protection of the halide coating, Li₆PS₅Cl is more prone to oxidative decomposition. This phenomenon is likely associated with the release of reactive oxygen species or transition-metal ions from the LiNi_0.8Co_0.1Mn_0.1O₂ cathode at high voltage, which react with the exposed sulfide electrolyte to form electrochemically inactive by-products. These by-products disrupt ion-conducting pathways at the interface, thereby exacerbating interfacial impedance growth and performance degradation. The XPS results clearly demonstrate that the introduction of a Li₃InCl₆ coating layer effectively alleviates interfacial side reactions between high-voltage oxide cathodes and sulfide electrolytes, significantly enhancing interfacial chemical stability, as illustrated in Fig. 4i.

Fig. 4 XPS analysis of pristine electrolytes and composite cathodes after electrochemical cycling. (a) In 3d spectrum of pristine Li₃InCl₆; (b) P 2p and (c) S 2p spectra of pristine Li₆PS₅Cl; (d–f) In 3d, P 2p, and S 2p spectra of 15% LIC@NCM after electrochemical cycling; (g and h) P 2p and S 2p spectra of 0% LIC@NCM after electrochemical cycling; (i) schematic illustration of interfacial side reaction suppression by Li₃InCl_6.

To better understand the influence of interfacial construction on reaction kinetics and structural evolution, differential capacity (dQ/dV) analyses were performed (Fig. 5a–c). The dQ/dV curves reflect phase transitions during lithiation/delithiation, with specific redox peaks corresponding to transitions between hexagonal (H1, H2, and H3) and monoclinic (M) phases (Fig. S7).³⁹ For the uncoated 0% LIC@NCM cell, these peaks gradually diminish and even disappear during cycling, suggesting that severe interfacial degradation compromises the structural reversibility of the cathode. In contrast, the Li₃InCl₆-coated cathodes, particularly the 15% LIC@NCM sample, exhibit sharp and consistent redox peaks across multiple cycles, indicating excellent structural stability and electrochemical reversibility. Moreover, the polarization, reflected by the voltage gap between oxidation and reduction peaks of the H2–H3 transition, decreases progressively from 98 mV (0% LIC@NCM) to 93 mV (30% LIC@NCM), and further to 33 mV (15% LIC@NCM). This trend highlights that a moderate Li₃InCl₆ coating not only stabilizes the interface but also alleviates polarization, enhancing the overall kinetic performance of the ASSLB.

Fig. 5 The dQ/dV curves of ASSLBs with (a) 0% LIC@NCM, (b) 15% LIC@NCM, and (c) 30% LIC@NCM, respectively, derived from charge/discharge profiles at 0.5 C. EIS spectra of ASSLBs with 0% LIC@NCM, 15% LIC@NCM, and 30% LIC@NCM before (d) and after (e) 50 electrochemical cycles, and (f) the corresponding fitted resistance values (R₁, R₂, and R₃) after 50 cycles. (g) GITT profiles of ASSLBs with 0% LIC@NCM, 15% LIC@NCM, and 30% LIC@NCM; the corresponding lithium-ion diffusion coefficients (D_Li⁺) during (h) charging and (i) discharging processes.

Electrochemical impedance spectroscopy (EIS) measurements were conducted on the cells before and after cycling (Fig. 5d and e) to elucidate interfacial reactions on cathode reaction kinetics. Based on the fitting of the Nyquist plots across different frequency ranges, the impedance response can be divided into three components. R₁ represents the overall ohmic resistance of the cell, arising primarily from the ionic resistance of the electrolytes; R₂ corresponds to the solid–solid interfacial resistance within the cathode composite, reflecting the ability of Li⁺ transport across the interfacial layer. R₃ is associated with the charge transfer resistance at the active material/electrolyte interface, which reflects the interfacial ion transport kinetics. Fig. 5f presents a quantitative comparison of the fitted resistance values. Notably, the 15% LIC@NCM cell exhibited the lowest total impedance after cycling, with both R₂ and R₃ significantly reduced compared to that of its 0% and 30% LIC@NCM counterparts. This indicates that the composite interface formed with 15% Li₃InCl₆ not only provides favorable Li⁺ transport pathways but also ensures an electrochemically stable interface.

Galvanostatic intermittent titration technique (GITT) measurements were also conducted to probe the effect of the Li₃InCl₆ coating on Li⁺ diffusion behavior (Fig. 5g). Fig. S8 presents a comparison of the polarization voltages derived from the GITT measurements. The 15% LIC@NCM cell consistently exhibited lower and more stable polarization voltages throughout both charge and discharge processes compared to 0% LIC@NCM and 30% LIC@NCM cells, with average polarization voltages of 38.3 mV (charging) and 55.5 mV (discharging), highlighting its superior kinetic characteristics. Furthermore, the Li⁺ diffusion coefficients calculated from the GITT (Fig. 5h and i) reveal that, despite fluctuations during lithiation/delithiation, the 15% LIC@NCM cathode maintains a higher and more stable diffusivity across the entire voltage range.

4 Conclusion

A practical and efficient interfacial engineering strategy for sulfide-based ASSLBs was demonstrated by employing a nanosized Li₃InCl₆ halide electrolyte as a functional coating layer for Ni-rich cathodes. Unlike conventional coatings that suffer from limited ionic transport and mechanical rigidity, Li₃InCl₆ features high ionic conductivity (2 mS cm⁻¹), favorable deformability, and excellent interfacial compatibility with both LiNi_0.8Co_0.1Mn_0.1O₂ and Li₆PS₅Cl. These properties enable a conformal and stable coating through simple mechanical mixing. ASSLBs employing a 15% LIC@NCM composite cathode exhibit a high initial discharge capacity of 189 mA h g⁻¹ with a coulombic efficiency of 84.4%, along with excellent rate performance and long-term cycling stability. EIS, XPS, and the GITT collectively confirm that the Li₃InCl₆ coating effectively suppresses interfacial side reactions, reduces polarization, and promotes lithium-ion diffusion. This work highlights the critical importance of rational coating material selection and demonstrates the potential of halide-based coatings in advancing practical ASSLB technologies.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data needed to evaluate the conclusions in this paper are present in the main manuscript or the supplementary information (SI). Supplementary information: detailed electrolyte and battery characterization, including XRD patterns, particle size distribution, EIS spectra, DC polarization data, galvanostatic charge–discharge profiles, dQ/dV analysis, and GITT-derived polarization voltages. See DOI: https://doi.org/10.1039/d5se01037c.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (Grant nos. 52172253, U21A2075, 22309194, 52250610214, and 52372244), Ningbo S&T Innovation 2025 Major Special Programme (Grant No. 2021Z122 and 2023Z106), Baima Lake Laboratory Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (LBMHD24E020001), Zhejiang Provincial Key R&D Program of China (Grant No. 2022C01072 and 2024C01095), Jiangsu Provincial S&T Innovation Special Programme for carbon peak and carbon neutrality (Grant No. BE2022007), and Youth Innovation Promotion Association CAS (Y2021080).

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