Exploring electrode/polymer electrolyte interface chemistry and a regulating strategy of interfacial stability: a review

Abstract

Polymer electrolytes have garnered considerable interest as a promising substitute owing to their exceptional mechanical flexibility, and appropriate interfacial compatibility with electrodes. However, the realization of economically viable and industrially scalable solid-state batteries with an elevated energy density and reliable cycling life remains a formidable task. The integration of high-voltage cathodes presents additional challenges, such as polymer electrolyte decomposition, consequential gas discharge, and the formation of an unstable solid–electrolyte interphase (SEI) layer on the lithium metal anode. These issues significantly impact the battery's cycling life and safety, necessitating profound attention towards enhancing the electrochemical stability of polymer electrolytes. Within this comprehensive review, we explore the problems arising from the evolution of the electrolyte/cathode and electrolyte/anode interfaces (e.g., electrochemical decomposition of the electrolyte, reverse cation catalysis, degradation products, etc.), and propose corresponding interfacial remediation strategies (e.g., in situ polymerization, inorganic coatings, etc.). Finally, we describe the persistent challenges and future perspectives aimed at providing strategies for the development of innovative polymer electrolytes capable of realizing high-performance lithium-metal batteries.

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1. Introduction

In recent years, lithium metal has gained considerable attention as a promising anode material due to its unique advantages, including ultra-high theoretical specific capacity (3860 mAh g⁻¹) and extremely low potential (−3.040 V vs. the standard hydrogen electrode).^1,2 Theoretically, solid-state electrolyte (SSE) systems are likely to form stable interfaces due to their non-diffusive properties, which make them compatible with lithium or lithium alloy anodes. Therefore, the development of high-performance SSEs and corresponding interface specifications is key to achieving long-life and high-energy density solid-state lithium metal batteries (SSLMBs).^3–7

As a new type of solid-state battery, in order to achieve its practical application and development, the design of solid-state electrolyte materials must satisfy the following conditions: (1) higher lithium ion conductivity: the ion conductivity of solid-state electrolyte materials should be close to that of liquid electrolytes (10⁻²–10⁻³ S cm⁻¹ at room temperature); (2) low electronic conductivity can ensure that there will be no short circuit inside the battery; (3) wide electrochemical window: the solid electrolyte can be charged and discharged in a wide potential range without side reactions; (4) better lithium ion mobility: it slows down the concentration polarization generated during the cycle of the all-solid-state battery; (5) excellent chemical stability: it ensures that the interface between the electrolyte and the positive and negative materials cannot easily produce side decomposition phenomena; (6) excellent mechanical properties: it can not only withstand the volume change of electrode materials during lithium-ion insertion and removal, but also can easily use thin films (15–50 microns) for cell assembly, and can block the growth of lithium dendrites from a physical level.^5,8,9

The solid electrolytes reported so far are mainly classified into inorganic solid polymer electrolytes,¹⁰ polymer electrolytes,¹¹ gel polymer electrolytes, and composite solid electrolytes.^12,13 The ionic conductivity of some inorganic solid electrolytes has already reached 10⁻² S cm⁻¹, which is close to the ionic conductivity of liquid electrolytes at room temperature. SPEs are usually composed entirely of polymers with liquid solvents added as plasticizers and can be easily prepared by solvent casting, thermoforming, or extrusion techniques. Pioneering researchers Wright and colleagues discovered in the 1970s that the (–CH₂–CH₂–O–)_n structure in poly(ethylene oxide) (PEO) could form ion-conductive complexes with alkali metal ions.¹³ Thereafter, Armand et al. proposed PEO-based polymer electrolytes for lithium-ion batteries,¹³ ushering in a new era of battery polymer electrolytes. Other commercial polymer species such as poly(methyl methacrylate) (PMMA),¹⁴ poly(acrylonitrile) (PAN),¹⁵ polyurethane (PU)¹⁵ and poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (PVDF-HFP)¹⁶ have also been investigated as polymer electrolytes. Since the relatively low ionic conductivity of SPEs at room temperature hinders their further application, various strategies to increase their ionic conductivity are proposed, such as modifying polymer matrices by blending, copolymerization and cross-linking, and introducing inorganic fillers into polymer matrices.

Polymer electrolytes have made rapid progress as the demand for high performance batteries grows. It is absolutely essential to follow and discuss their current status and future trends, especially regarding the increasing interest in the electrochemical stability of SPEs, where the electrochemical stability window (ESW) influences the choice of the relevant electrode system, and SPEs with wider ESWs usually have higher electrochemical stability. Although remarkable improvements have been made in achieving high bulk ionic conductivities in SPEs, this high conductivity is often negated by the high impedance at the interface between the SPE and the electrode.¹⁷ Furthermore, the cause of poor power density and rapid performance degradation can be largely related to the electrode materials/SPE interface, which sets a significant barrier for Li⁺ ions to transport across.¹⁸

In this review, we provide a comprehensive discussion of polymer electrolyte and SSB (solid-state battery) interfacial engineering with respect to the electrochemical properties of the SPE–electrode interface, summarize present interesting reactions at the cathode/polymer electrolyte interface and a detailed discussion of the polymer-interfacial modification strategies. Finally, we provide future perspectives on polymer design and interfacial modification strategies, paving the way for the development of high-performance, fully operable solid-state batteries.

2. Insights into the electrochemical stability of an interface between the electrode and polymer electrolyte

2.1 Electrochemical decomposition of polymer electrolytes

The electrochemical stability window determines the electrolyte's resistance toward unexpected electron transport to control short-circuit and self-discharge phenomena.¹⁹ We consider that the electrochemical window of an electrolyte is dominated by the reduction and oxidation potentials, which are determined by the conduction band maximum (CBM, i.e., the lowest unoccupied molecular orbital (LUMO) energy level) and the valence band minimum (VBM, i.e., the highest occupied molecular orbital (HOMO) energy level), respectively. A larger electrochemical window implies that a larger energy gap is required between the LUMO and HOMO compared with the anodic potential (μ_A) and cathodic potential (μ_C) to ensure the stable operation of the cell. In addition, μ_A and μ_C should be within the energy gap of the polymer electrolyte, i.e., LUMO > μ_A, HOMO < μ_C.²⁰

Once the electrolyte contacts the cathode and anode, the energy levels of the HOMO and LUMO change (Fig. 1(a)).²¹ When the Li metal is in contact with the electrolyte, the electrons in the conducting band easily move to the LUMO of the electrolyte. This electron movement reduces the electrolyte and products such as Li₂O, Li₂CO₃, LiF and ionic liquid related compounds are deposited on the Li metal side. During charging, the valence band energy of the anode increases and approaches the energy level of its LUMO. Electrons move easily from the anode to the electrolyte, after which electrolyte decomposition and SEI formation are accelerated. The interfacial phenomena at the cathode are similar to those at the anode. The electrolyte promotes oxidation reactions between the electrolyte and the cathode. In the contact mode, electrons are transferred from the HOMO of the electrolyte to the conduction band of the cathode. The electrolyte decomposes due to oxidation, eventually forming SEI compounds. In charging mode, electrons escape from the cathode through the circuit and the valence band energy decreases. The electrons move easily from the electrolyte to the cathode and hence the decomposition and conversion of the electrolyte to SEI are accelerated.

Fig. 1 (a) A Li rechargeable secondary battery, and the interface behavior for the electrolyte, cathode and anode (Li metal); above is just the contact mode without charging, the bottom picture shows the charging mode. Reproduced with permission from ref. 21. Copyright 2017, Elsevier. (b) Voltage profile and corresponding mass signals. (c) Mechanisms of the formation of HTFSI and H₂ in a PEO-based solid polymer battery when coupled with an inert carbon composite electrode and a LiCoO₂ composite electrode. Reproduced with permission from ref. 22. Copyright 2020, American Chemical Society. (d) Structure characterization of a LiCoO₂ electrode in LiCoO₂|PEO-LiTFSI|Li cells before and after electrochemical cycling. Reproduced with permission from ref. 23. Copyright 2020, John Wiley and Sons. (e) Leakage current of the Li|LCO cell from top to bottom, EIS results of the Li|LCO cell in the range of 1 MHz–0.1 Hz, high-resolution TEM image of the bare LCO cycling electrode, and representative 2D XANES chemistries of Co on the bare LCO particle. Reproduced with permission from ref. 24. Copyright 2020, Elsevier. (f) A schematic diagram of the interfacial reaction between LFP and the PPC-SPE. Reproduced with permission from ref. 25. Copyright 2020, Royal Society of Chemistry.

2.2 Phase evolution at the polymer electrolyte/cathode interface

2.2.1 Surface catalytic effect of LiCoO₂. Most polymer electrolytes can only be matched with cathodes with a low operating voltage, such as LiFePO₄, instead of high-voltage LiCoO₂ due to their narrow electrochemical windows. The large contact area between the PE and cathode renders PE oxidatively unstable, and susceptible to oxidative decomposition catalyzed by transition metal ions or conductive carbon. In the following, we take LiCoO₂ as an example to obtain insights into these interesting phenomena.

Some researchers used in situ DEMS to study the gas-phase behavior of LiCoO₂|PEO-LiTFSI|Li SPBs.²² According to Fig. 1(b), while PEO possesses an intrinsic gas release potential of 4.5 V (vs. Li⁺/Li), the surface catalytic effect of LiCoO₂ can lead to significant degradation, resulting in a lower onset gas release voltage of 4.2 V (vs. Li⁺/Li). The decomposition of PEO begins with the co-removal of one electron from oxygen and hydrogen. As shown in Fig. 1(c), the initial oxidation step triggers the removal of an electron, resulting in oxygen that is partially oxidized. This process effectively weakens the bond between carbon and hydrogen, causing the expulsion of hydrogen. The expulsion of hydrogen then enables free carbon radicals to shed electrons and transform into carbon positive ions. Then the released H⁺ forms the extremely acidic HTFSI, which further attacks the PEO, leading to the breaking of the PEO bond and the formation of volatile molecules. Like other acids, HTFSI also corrodes LiCoO₂ particles, leading to degradation of the cathode structure and formation of the cathode–SPE interface. Even worse, when HTFSI migrates to the anode side, it readily reacts with lithium metal to form H₂. Degrading the fluidity of the electrolyte may lead to short-circuiting of the battery, which is reflected in a sharp voltage drop.

Some studies have shown a drastic decrease in battery capacity when PEO is paired with LiCoO₂ and operated at 2.7–4.3 V. Jiliang Qiu²³ found that LiCoO₂–PEO cells begin to oxidize at voltages higher than 3.9 V vs. when Li⁺/Li begins to oxidize, the electrolyte continues to thin with increasing cycles, and the highly active LiCoO₂ surface accelerates the oxidation of the PEO electrolyte, resulting in C=O containing esters that exhibit good Li-ion conductivity and proper chemical/physical stability. However, the above reaction may also contribute to the release of oxygen from LiCoO₂, which will cause surface structural remodeling, passivation of the cathode surface and hindering of lithium ion diffusion. What is worse, the hybridization of the ligand oxygen with low-valent Co to form a CoO or Co₃O₄ phase concentrated in the interfacial region in direct contact with the reduced electrolyte, resulting in rapid deterioration of the interfacial properties (Fig. 1(d)).

Zeyuan Li²⁴et al. explained why the active surface of LCO leads to the rapid oxidation of PEO. Severe oxidation of the bare LCO–PEO cell was detected using a leakage current density (leak) test (Fig. 1(e)), with a significant increase in impedance after cycling, and a significantly longer diffusion tail at low frequencies, indicating that the ions diffuse only slowly on the order of ∼10 nm, making it difficult for Li⁺ to be inserted into the bare LCO and hindering the reduction of Co⁴⁺. The reduced crystallinity of the LCO surface, which contains a large number of defects, was observed by TEM (Fig. 1(e)), indicating that PEO oxidation causes damage to the LCO particles, reduces their crystallinity, and slows down the diffusion of ions in the interior.

2.2.2 Reverse cation catalytic reaction. Within lithium-metal batteries, the cathode materials are usually lithium-rich and contain transition metal elements, such as LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)²⁶ and LiFePO₄ (LFP).²⁷ During constant-current charging and discharging of the battery, the transition metal cations undergo redox reactions on the surface of the electrolyte film, resulting in a change of valence state. In addition, an interesting phenomenon, reverse cation catalysis, has been found in the above process. The above phenomenon is described below using LiFePO₄ as an example.

Cui²⁵et al. revealed an unusual reverse cation redox reaction at the LiFePO₄/PPC interface during the first charging at low current density, with the kinetics of the chemical reaction (ν_CR of eqn (2)) between LFP and PPC-SPE being faster than that of the electrochemical reaction (ν_ER of eqn (1)). As in Fig. 1(f), the competitive cation redox relationship leads to overcharging. That is, the phase change of LiFePO₄ to FePO₄ during charging corresponds to the change of oxidation state from Fe²⁺ to Fe³⁺. However, at the LiFePO₄/PPC interface, the cationic reaction for the reduction of Fe³⁺ to Fe²⁺ is induced by the oxidation of PPC rather than the transition metal ions as a source of charge compensation, leading to an obvious overcharging problem. The chemical reaction proceeds as follows.

Fe²⁺ − e⁻ → Fe³⁺ (1)

Fe³⁺ + PPC → Fe²⁺ + PPC⁻ (2)

In the parasitic reactions of (1) and (2), it is the PPC rather than the Fe ions that serve as the source of charge compensation. Therefore, Fe³⁺ plays a catalytic role in the decomposition of the PPC electrolyte, which prolongs the Fe²⁺/Fe³⁺ plateau, and greater capacity SSLMB overcharge. The oxidative decomposition of PPC electrolyte on the cathode surface produced CO₂ gas during the transition from Fe²⁺ to Fe³⁺. Since CO₂ is non-flammable, its production prevents fires, which enhances the safety of solid-state electrolyte membranes.

2.3 Phase evolution at the polymer electrolyte/Li interface

2.3.1 Lithium dendrite. Ideally, lithium is uniformly plated on the electrolyte/lithium interface, inducing only lithium anode thickening. However, an important practical obstacle to the use of lithium metal anodes is their tendency to form inhomogeneous and unstable needle-like lithium crystals, so-called lithium dendrites, during charge/discharge cycling. Based on the classical theory of instability of long-range transport dendrite growth, a high curvature protrusion can generate an enhanced electric field and a high overpotential near the tip, attracting more metal-conducting ions, which leads to self-amplification of lithium deposition on the protruding tip, and ultimately evolves into dendrites.²⁸ Secondly, the periodic expansion and contraction of lithium metal during cycling leads to deterioration of the already poor mechanical contact between lithium and SPE. In polymer electrolytes, interfacial inhomogeneity, limited ionic transport, and low mechanical strength are the main drivers of dendrite growth.²⁹

Two theoretical models have been proposed for the uncontrolled growth of dendrites through a large number of studies and analyses by our forefathers. The first model was proposed by Chazalviel³⁰ and his coworkers, who argued that the growth of dendrites occurs mainly at the PE–Li metal interface, and that the anion concentration near the lithium anode decreases to zero at Sand's time at high current densities. However, the different behavior of the anion and cation concentrations leads to an excess of positive charge on the lithium anode, which generates a local space charge in association with a large electric field. This situation leads to the growth of lithium dendrites, resulting in an unstable interface.

The second model was proposed by Newman et al.³⁰ They considered the effect of physical variables including surface tension on the lithium deposition kinetics and electrolyte modulus. Based on this model, it is predicted that a solid electrolyte with a high shear modulus (G > 7 GPa), approximately twice that of Li metal (∼3.4 GPa), is sufficient to stabilize the electrode–electrolyte interface and prevent dendrite proliferation. In this case, Li dendrite growth at the electrode–electrolyte interface depends on the ion-transfer and mechanical properties of the electrolyte. Viswanathan³¹et al. further established a general criterion for stable electroplating using the shear modulus ratio of SSE to lithium anode (GSSE/GLi) and the molar volume ratio of Li⁺ to lithium anode (V_Li⁺/V_Li). They concluded that stable plating requires the use of SSEs with high (low) Li molar volume and high (low) shear modulus. SPEs have a soft texture and low shear modulus, whereas the formation of Li⁺ solvation “cages” leads to high volume expansion and high V_Li⁺, which results in their inability to inhibit dendrites. In addition, lithium metal surface impurities (e.g., Li₂O, Li₃N, or Li₂CO₃) tend to cause local current density inhomogeneities and promote nucleation of lithium protrusions.³²

Utilizing solid-state polymer electrolytes is a promising approach to mechanically inhibit lithium dendrites. Thus, various strategies have been proposed to inhibit the growth of lithium dendrites, including the construction of a stabilized SEI layer to the lithium anode (e.g., modification of solvents, dissolved salts, and electrolyte additives), the design of single-ion conductive polymer electrolytes (according to a first model), and the fabrication of electrolytes with a high shear-mechanical modulus (according to a second model). These also include the three-dimensional polymer network obtained by Kim³³et al. from the in situ polymerization of trimethylolpropane ethoxylate triacrylate (ETPTA) monomers in combination with vinyl-functionalized silica (VSNP) that achieves high t_Li⁺ and low interfacial impedance at room temperature and an ionic conductivity of 5.2 S cm⁻¹. And the incorporation of small amounts of polymers and inorganic nano-fillers into conventional liquid electrolytes (LEs) inhibits the formation of lithium dendrimer crystals by decreasing the lithium-ion charge-concentration gradient on the lithium-metal surface and growth.

2.3.2 Degradation products. In addition to exploring the effect of lithium dendrites on the battery performance, understanding the compound components of the degradation layer at the lithium-metal electrode interface is critical to the successful development and implementation of SPEs. Most polymers, such as PEO, PPC and PAN-based electrolytes, chemically react with lithium metal during cycling. Polypropylene carbonate (PPC) is degraded into micromolecular segments when in contact with a lithium metal anode. Wang³⁴ used an EIS test to observe the impedance between the polymer electrolyte, PPC and lithium metal. The resistance decreased abruptly after high temperature activation, and the contact between PPC-SPE and lithium metal electrode became tight. The same phenomenon was found in poly(2,3-butylene carbonate) (PBC), polyethylene carbonate (PEC) based solid polymer electrolytes. This phenomenon confirms that the reaction between polycarbonate-based electrolytes and lithium metal electrodes can significantly increase ionic conductivity in two ways. First, cross-section degradation of the polymer reduces the crystallinity of the electrolyte, which promotes Li⁺ transfer and reduces the body resistance of the electrolyte. Second, the degradation products penetrate into the interface to improve the contact between the electrolyte and lithium metal and reduce the interfacial resistance. It was determined by further analyzing the interfacial chemistry that the degradation of PPC can be induced by chemical reaction with lithium metal or trace LiOH on its surface, as shown in Fig. 2(a). The fracture of polymeric PPC tends to occur between lithium carbonate (I) and the polymer primary radical or PPC. The presence of LiOH accelerates the generation of intermediate (I), which generates lithium alcoholate (II) by decarboxylation, along with lithium carbon dioxide radicals. Intermediate II tends to split into lithium alcoholate (III) and a small molecule PC. With the evolution from II to III, a significant accumulation of PC is obtained in the interfacial system. Cellulose can limit the reaction at the interface by physically hindering it. The hydroxyl unit can burst the lithium alkoxide intermediate and inhibit further degradation.

Fig. 2 (a) The left figure shows the resistance change of the Li|CPPC-SPE|Li symmetric cell, and the right figure shows a typical SEM image of the contact interface between CPPC-SPE and lithium metal electrode before and after activation. Reproduced with permission from ref. 34. Copyright 2018, Elsevier. (b) Illustration of the polymer electrolyte films after lithium deposition. Polymer is green, degradation region is purple, where a darker color indicates a higher degree of polymer degradation, small yellow/blue circles indicate decomposed salt, and lithium is grey. Reproduced with permission from ref. 35. Copyright 2021, Royal Society of Chemistry.

Edvin K. W. Andersson³⁵ used soft X-ray photoelectron spectroscopy to study polymer and salt degradation compounds at the interfaces between lithium and three different SPEs (PEO:LiTFSI, PCL:LiTFSI, and PTMC:LiTFSI) (Fig. 2(b)). For all systems, the polymers and salts decompose upon contact with lithium, with PCL:LiTFSI favoring polymer degradation and the PEO and PTMC systems favoring degradation of the TFSI anion. However, the decomposition of PTMC:LiTFSI is more pronounced than that of PEO:LiTFSI and PCL:LiTFSI, and other salt decomposition products are also formed. For example, Li–O–R and Li_xS_yO_z were recognized at the Li metal/SPE interface of all SPEs. LiF was detected by PEO:LiTFSI and PTMC:LiTFSI. Li₂O and Li₃N were detected by PCL:LiTFSI and PTMC:LiTFSI, whereas Li₂S was detected by only PTMC:LiTFSI. It is noteworthy that the decomposed structures formed in PTMC:LiTFSI are more stable to lithium or less permeable to the formed Li⁺ compounds compared to the interfaces formed on the other SPE. A possible factor contributing to this difference is that PTMC:LiTFSI is completely amorphous compared to semi-crystalline PEO:LiTFSI and PCL:LiTFSI, and since the degradation of the crystalline structures requires an additional energy input to disrupt these structures, there should be an additional barrier to prevent the degradation of the crystalline domains of these materials. It is well known that the degradation reactions become more severe at higher temperatures, which is unfortunate because some of these SPEs require higher temperatures to operate.

3. Interfacial defects of electrodes and polymer electrolytes and optimization strategies

The polymer electrolyte, cathode and lithium metal anode are the basic components of SSLMBs, where the polymer electrolyte acts as the ion transport path and diaphragm. During battery charging, ions diffusing from the cathode to the anode have to pass through the cathode–polymer and polymer–anode interfaces, while electrons are transferred from the cathode collector to the anode collector via an external circuit (Fig. 3(a)).^36,37 Thus, for the interfacial contact between the polymer electrolyte and the cathode/anode, the following aspects should be considered, for the cathode: the interfacial contact inside the cathode and the interface between the cathode and the polymer electrolyte film; for the anode: the interfacial contact between the polymer electrolyte film and the lithium metal.³⁸ In the above interfacial contact, the issues are as follows: lithium dendrites are prone to be formed at the interface between the lithium metal anode and the electrolyte film. Moreover, the contact between the cathode material and the electrolyte film is ineffective. Hence, we ought to modify the polymer interfaces in SSBs through the use of polymer coatings to mitigate the above issues (Fig. 3(b)).³⁹

Fig. 3 (a) The architecture of polymer electrolyte based SSLMBs with multiple interfaces. Reproduced with permission from ref. 36. Copyright 2021, Elsevier. (b) Schematic of the solid–solid interface towards polymers including anode/polymer electrolyte interface, cathode/polymer electrolyte interface and interfacial modification by polymer coating in SSBs. Reproduced with permission from ref. 39. Copyright 2020, Elsevier.

3.1 Remediation strategies at the cathode/electrolyte interface

The electrochemical stability of the cathode/electrolyte interface is critical for the stable operation of SSLMBs, which involves many aspects, including side reactions between the cathode and electrolyte, oxidative decomposition of the polymer electrolyte, dissolution of transition metal ions, and aging/evolution of the interface during the long cycle of the cell.⁴⁰ Poor oxidation resistance at the cathode/electrolyte interface is a bottleneck for high-voltage and high-capacity SSLMBs, especially at low voltages (0 V vs. Li⁺/Li) and high voltages (>4.0 V vs. Li⁺/Li).^41,42 In addition to electrochemical stability, slow charge transport (e.g., Li⁺ or electrons) through interfaces or composite electrodes hinders the application of SSLMBs in practical devices. The point-to-point contact between solid state components (e.g., electrolyte, cathode, additives) leads to slow charge carrier transport and high interfacial impedance.⁴³

Researchers have used various approaches to address these issues (e.g., poor electrochemical stability and charge transport at the interface).⁴⁴ The first is to bring the SPE in full contact with the electrode active material by means of in situ polymerization in order to provide a better transport path for lithium ions. In addition, multilayer structures have been widely introduced to protect the electrolyte from reduction/oxidation, such as anode-stabilized polyacrylonitrile (PAN) at the cathode and PEO at the anode.^45,46 Another approach to protect the electrolyte is to apply antioxidant coatings on the cathode material, including inorganic coatings (e.g., lithium aluminum titanium phosphate (LATP) or lithium aluminum germanium phosphate (LAGP)).^43,47–49

3.1.1 In situ polymerization. Current solid electrolytes cannot spontaneously wet the electrodes as liquid electrolytes do, leading to discontinuous ion transport at the cathode/solid electrolyte interface and inside the porous cathode, which leads to large interfacial resistances and low active material utilization.^50,51 These factors limit the achievable energy density and cyclability of SSLMBs.^52,53 To enhance the electrode/solid electrolyte interfacial contact in batteries, in situ polymerization is a common strategy.^54,55

When a liquid precursor is injected into the cell and then polymerized in situ to form an SPE, the SPE can wet and remain in complete contact with the electrode.⁵⁶ For example, Lin⁵⁷et al. developed a novel integrated cathode/thin solid electrolyte (Fig. 4(a)) as thin as 17 μm with sufficient mechanical strength. The interfacial adhesion between the cathode/electrolyte can be enhanced and the interfacial resistance can be significantly reduced by in situ preparation. The solidification process fills the pores inside the cathode, providing a continuous ion transport pathway, thus allowing the use of highly loaded cathodes. Not only that, Li⁵⁸et al. prepared a novel electrolyte by in situ polymerization using a 1,3,5-trioxoalkyl precursor, which enables the formation of a bilayer solid electrolyte interphase on Li metal electrodes and stabilizes LiNi_0.8Co_0.1Mn_0.1O₂-based cathode materials, thus improving the interfacial charge transfer resistance at low temperatures and hindering the growth of lithium dendrites on lithium metal electrodes.

Fig. 4 (a) Schematic diagram of an all-solid-state I-FPG cell and SEM images of the surface and cross-sectional morphology of the FPG electrolyte. Reproduced with permission from ref. 57. Copyright 2021, John Wiley and Sons. (b) The upper panel shows in situ optical microscopy images showing the lithium deposition behavior of self-healing polymer electrolytes, and the lower panel shows Monte Carlo simulations of the kinetics of Li deposition in different electrolytes. Reproduced with permission from ref. 59. Copyright 2022, John Wiley and Sons.

In addition, Li and co-workers⁵⁹ reported the development of self-healing polymer electrolytes for lithium metal batteries with high-voltage nickel-rich cathodes via in situ copolymerization of 2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl) ureido) ethyl methacrylate (UPyMA) and ethylene glycol methyl ether acrylate (EGMEA) monomers (Fig. 4(b)). In the event of electrolyte fracture, intramolecular hydrogen bonding between the ester and urea groups in the UPyMA unit of the polymer chain induces self-healing of the polymer electrolyte. This property confers the electrolyte to spontaneously repair the defects and cavities caused by dendrites at the complex lithium/electrolyte interface, thus inhibiting the growth of dendrites and providing a more stable cathode/SPE interface. Furthermore, the SPE obtained from the above in situ polymerization possesses strong antioxidant properties as well as the ability to induce the generation of a stable CEI film at the cathode material interface in reducing the severe side reactions at the cathode-side interface and to achieve a long term stable cycling at charging cut-off voltages as high as 4.7 V.

3.1.2 Double-layer polymer electrolytes. Achieving >400 Wh kg⁻¹ cell-level energy density in SSB requires a high area capacity cathode of at least 4 mAh cm⁻², which corresponds to a mass loading of ∼21–25 mg cm⁻² for Ni-rich LiNi_xMn_yCo_zO₂ or NMC cathode active material (assuming a capacity of 160–190 mAh g⁻¹, depending on the NMC type).⁶⁰ There are still few reports on the fabrication and testing of SSBs (especially polymer electrolytes) with highly loaded cathodes. Increasing the cathode capacity or thickness in SSBs may present some new challenges, such as higher volume changes during cycling, which can generate dynamic stresses at different interfaces. The design of double-layer structure polymerized electrolyte can choose one side of the polymer electrolyte which is more stable in contact with a metal Li negative electrode, and the other side is compatible with a high-voltage positive electrode material, which avoids the disadvantage of the polymer electrolyte that is unstable under high potential and low potential, and really realizes the strengths and avoids the weaknesses.

Yang and co-workers⁶¹ conducted a comprehensive study on the cell design with a double layer electrolyte/spacer with a linear PEO-based electrolyte layer inserted between the cross-linked polyethylene oxide (PEO)-based electrolyte layer and the cathode layer (Fig. 5(a)). The cross-linked PEO provides higher mechanical stability and suppression of dendrites, while the linear PEO may help provide a mechanically more robust interface with the composite cathode layer, for highly loaded cathodes, which may prevent delamination or contact loss problems at the separator/cathode interface during their cycling within the cell stack and associated stresses. In addition, the linear PEO has properties that perfectly match the roughness of the cathode surface, which means that it eliminates any gaps with the cathode surface, making the interface appear seamless. Furthermore, the linear PEO is also capable of self-healing at 70 °C, which means that the linear-PEO separator can irreversibly fuse with the linear-PEO catholyte in the cathode, thereby preventing interfacial delamination under cycling stress and ensuring favorable interfacial stability.

Fig. 5 (a) Schematic representation of the three different cell designs and the associated benefits and issues during cycling and an SEM image of the cross-section of the complete cell stack of the Li/bilayer electrolyte/NMC811 (6 mAh cm⁻²) cell, disassembled after one charge/discharge cycle. Reproduced with permission from ref. 61. Copyright 2022, American Chemical Society. (b) Surface SEM images of the LCO cathode, the LCO/T-PEO cathode, and the LCO/d-PEO cathode are shown in the upper panel, and the F 1s of the cyclic LCO with T-PEO/T-PEO, and the F 1s and B 1s of the cyclic LCO with d-PEO/T-PEO are shown in the lower panel. Reproduced with permission from ref. 62. Copyright 2023, Elsevier.

The cathode electrolyte layer was constructed by simply dropping a PEO/lithium difluoroborate (LiDFOB) solution on the surface of the LCO cathode and the electrolyte layer near the lithium metal side of the PEO/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was prepared using a casting method, which is suitable for all-solid-state lithium batteries coupled with high-voltage cathodes (Fig. 5(b)).⁶² For the PEO/LiDFOB layer, the introduced LiDFOB helps to form a stable CEI containing Li_xB_xO_y and LiF in situ during cycling, which prevents the oxidation of the PEO electrolyte and facilitates the interfacial lithium-ion transport between the electrolyte and the cathode.

3.1.3 Electrolyte additives. Electrolyte additives are one of the most convenient and cost-effective methods to construct a stable passivation layer SEI/cathode electrolyte interface (CEI) to improve the stability and electrochemical performance of lithium metal anodes and high-voltage cathodes. Various electrolyte additives have been developed, such as fluoroethylene carbonate, vinylidene carbonate,⁶³ sodium sulfite,^63,64 borate,⁶⁵ sulfonate,^66,67 phosphate etc.^68–70 Liu⁷¹et al. proposed a new multifunctional additive, N,O-bis(TMS) trifluoroacetamide (BTA), which can form LiF-rich, Si and N-containing SEI and CEI layers on the anode and cathode surfaces at the same time to inhibit the decomposition of the electrolyte efficiently and to mitigate the capacity decay caused by structural degradation of the NMC811 cathode during high cut-off voltage cycling. Since the BTA molecules have both Si–O and Si–N bonds, they can spontaneously bind to the harmful impurities H₂O and HF molecules in the electrolyte (Fig. 6(a)), allowing them to be removed to stabilize the LiPF₆-based electrolyte. The above results reflect that the multifunctional BTA additives can facilitate the formation of a stable interfacial layer to effectively inhibit electrolyte decomposition, protect the NMC811 positive electrode and modify the positive and negative surface layers. It is foreseeable that it will bring great prospects for the commercial application of LMB.

Fig. 6 (a) The upper panel shows the TEM images of the pristine NMC811 cathode, STD electrolyte/NMC811 cathode and BTA electrolyte/NMC811 cathode after 100 cycles in the cathode, and the lower panel shows the F 1s and Li 1s XPS spectra of the NMC811 cathode after 200 cycles in different electrolytes and the N 1s and Si 2p XPS of the NMC811 cathode after 200 cycles in BTA electrolyte spectra. Reproduced with permission from ref. 71. Copyright 2021, American Chemical Society. (b) The upper panel shows TOF-SIMS analysis of an NCM cathode recovered from an NCM/graphite full cell containing a baseline electrolyte and a 0.5 wt% TMS-ON electrolyte after 400 cycles at 45 °C (charge/discharge rate: 0.5C), and the lower panel shows a 3D visualization and depth profile of the SEIs on the NCM cathode. Reproduced with permission from ref. 72. Copyright 2020, John Wiley and Sons.

In addition, Choi⁷²et al. proposed 3-(trimethylsilyl)-2-oxazolidinone (TMS-ON) as an electrolyte additive, and analyzed the effect of ON on the interfacial structure of the NCM cathode using the surface-sensitive technique, TOF-SIMS. The presence of TMS-ON dramatically changed the interfacial structure of the NCM cathode (Fig. 6(b)). The polar group CN-, which favors lithium ion migration, was detected in the outer layer of the on-derived SEI, and the inner layer was a mechanically stabilizing substance ⁷LiF₂⁻ which helps to inhibit the deposition of transition metals on the graphite anode. Furthermore, the additive prevented the continuous decomposition of the electrolyte at the cathode by forming a more stable CEI to reduce the irreversible phase transition and microcracks at the NCM cathode, constructing a controllable and stable electrolyte–electrode interface.

3.1.4 Inorganic material coating. One solution to achieve SPE coupling with high-voltage cathodes is the interface engineering of SPEs and interfaces with coated cathode active materials, as first reported by Cho⁷³et al. with coated metal oxides (e.g., Al₂O₃,^74,75 ZrO,⁷⁶ MgO,⁷⁷ AlPO₄,⁷⁸etc.), which significantly improve the capacity retention and structural stability of the cathode. However, most metal oxides are not Li-ion conductors, which may lead to high interfacial resistance between the cathode and the electrolyte. In this regard, surface modification with lithium-ion conductors has its unique advantages.

It has been reported that coating the cathode with Al₂O₃,⁷⁹ Li₃PO₄,⁸⁰ polycyanoacrylate (PECA),⁴⁷ and Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP)⁸¹ can block the direct contact between the polymer electrolyte and the cathode material and inhibit the oxidative decomposition of the polymer electrolyte. However, these conventional cathodic protection measures only studied the coating effect at the active material/SPE interface, ignoring the ability of conductive carbon to accelerate the decomposition of SPEs. As a successful example, Sun and colleagues⁸² reported a lithium tantalate (Li–Ta–O) coating using atomic layer deposition (ALD) to stabilize LiCoO₂/PEO at high voltage (4.5 V vs. Li⁺/Li) (Fig. 7(a)). The conductive carbon/PEO interface was also explored, where carbon can accelerate the decomposition of the PEO electrolyte at high charging voltages. After double surface coating of the ionic conductor, the cycling stability of SSLMBs was superior to that of the single coating. In addition, the LCO-coated cathodes obtained higher capacity at higher current densities due to the higher lithium-ion diffusion in the LCO-coated cathodes than in the bare and LCO/carbon coated cathodes.

Fig. 7 (a) The upper figure shows a TEM image of ALD LTO (thickness of ∼5 nm) coating on LiCoO₂ particles for 10 cycles and a schematic of the LCO-coated LiCoO₂ electrode with unprotected conducting carbon. The lower figure shows backscattered electron mode SEM images and schematic diagrams of LiCoO₂ electrodes coated with LCO + CB on LCO + CB coated samples and ALD LTO (thickness of ∼10 nm) coated on LiCoO₂ particles after 20 cycles of Focused Ion Beam (FIB) cutting, where both LiCoO₂ and conductive carbon were protected. Reproduced with permission from ref. 82. Copyright 2012, Royal Society of Chemistry. (b) The upper panel shows the SEM image and EDS elemental map of LAGP-LCO particles, and the lower panel shows the TEM image of LAGP-LCO, with labeled area 1 for LATP coating and labeled area 2 for bulk LCO. Reproduced with permission from ref. 24. Copyright 2020, Elsevier. (c) Illustration of the protection afforded by the combination treatment (B doping and B coating) of a Ni-rich layered cathode. Reproduced with permission from ref. 83. Copyright 2023, American Chemical Society.

Li²⁴et al. demonstrated a nanoscale interface engineering strategy to form a layer of LAGP ceramic electrolyte nanoparticles on the LCO surface by gradable ball milling and sintering methods (Fig. 7(b)). Since the ceramic coating still has grain boundaries and pinholes that induce PEO oxidation, by adding lithium bis(oxalate)borate (LiBOB) as a salt, which is expected to decompose on the cathode surface to form a surface coating that specifically passivates pinholes in the LAGP layer, the synergistic effect of the LAGP ceramic electrolyte coating and the salt decomposition passivation layer together makes Li⁺ easily transportable and forms an energy potential barrier to resist PEO electrolyte oxidation. Moreover, Yu⁸³et al. simultaneously doped and coated boron elements on a nickel-rich Li[Ni_0.9Co_0.05Mn_0.05]O₂ cathode, which altered the microstructure of the cathode and the cathode–SPE interface, respectively, and reduced the occurrence of side reactions, as well as suppressed grain isolation and minimized contact loss (Fig. 7(c)).

3.2 Remediation strategy for the anode/electrolyte interface

For SSLMBs, the lithium anode/electrolyte interface problems are focused on instability at the solid electrolyte interface (SEI), parasitic reactions, high interfacial resistance, and uneven nucleation of lithium on the anode surface leading to dendrite formation, resulting in low coulombic efficiency (CE), rapid capacity decay, and even dendrites that can penetrate the membrane and cause short circuits.^84,85 In addition, the volume expansion of the lithium metal in long cycles will lead to repeated rupture and growth of the fragile solid electrolyte interface (SEI), and the tight interfacial contact between the lithium anode and the electrolyte cannot be maintained, which leads to degradation of the electrochemical performance of SSLMBs.^86,87

For the above problem, various strategies have been proposed in recent decades, such as the construction of high modulus artificial solid electrolyte interface (SEI) layers,^88,89 development of solid electrolytes,^90,91 modification of electrolytes and additives^92,93 and design of 3D hosts, in situ polymerization, and other effective and promising strategies.

3.2.1 In situ polymerization. Solid polymer electrolyte (SPE) films are usually prepared by the solution casting method, which is complex, costly and environmentally unfriendly.⁹⁴ The unavoidable residual traces of solvent in the polymer matrix are often thought to reduce anode stability due to reactions with the anode surface.⁹⁵ More importantly, poor contact at the electrolyte/electrode interface using this method is another common problem.^96,97 Ideally, in situ fabrication of SPE membranes can avoid additional solvent introduction, facilitate improved interfacial stability, simplify the production process of solid-state LMBs, and be compatible with commercial lithium-ion battery manufacturing processes.^34,98,99

The polymerization reactions of many initiators have a negative impact on the electrochemical performance of the assembled solid-state SSLMBs,^100,101 for example, initiators such as azobis(isobutyronitrile) and methyl benzoylformate inevitably introduce parasitic reactions (thermal decomposition of salt and interphase side reactions) in the cell system.^101–106 Sun¹⁰⁷ used ε-caprolactone (ε-CL) ring-opening polymerization to prepare ultrathin PCL-based polymer electrolytes for lithium batteries (Fig. 8(a)). The above problems can be effectively circumvented by using Sn(Oct)₂. Sn(Oct)₂ not only can effectively induce ring-opening polymerization of ε-CL in the cell, but can also react with the lithium metal anode to form a LiSn alloy layer, which effectively prevents dendrite growth.

Fig. 8 (a) A schematic diagram of the in situ PCL polymerization process, top-view SEM image and SEM cross-section of ε-CL before. Reproduced with permission from ref. 107. Copyright 2021, Elsevier. (b) Schematic illustration of the quasi-solid-state polymer LMBs prepared by in situ polymerization, which demonstrates the improvement of the interfacial contact by in situ preparation. Reproduced with permission from ref. 108. Copyright 2023, American Chemical Society. (c) Interface stability of Li|PDEGDA/PVDF FMs|Li cells assembled in situ and ex situ with a current density of 0.05 mA cm⁻² at 60 °C. The insets are optical photographs of the surface of Li anodes that were cycled 358 times and over 2000 h, respectively. The surface morphologies of cycled Li anodes collected from Li|ex situ PDEGDA/PVDF FMs|Li and Li|in situ PDEGDA/PVDF FMs|Li cells. Reproduced with permission from ref. 108. Copyright 2023, American Chemical Society.

Lin¹⁰⁸et al. prepared electrolyte membranes by pre-polymerizing diethylene glycol diacrylate (DEGDA) in situ by thermal initiation in polyvinylidene fluoride backbone (PVDF FMs) electrospinning (Fig. 8(b)). PDEGDA with C–O and C=O groups provides multiple coupling/decoupling transport pathways for Li⁺. The multiple transport modes promote Li⁺ migration within PDEGDA/PVDF FMs QSPEs, contributing to 1.41 × 10⁻⁴ S cm⁻¹ ionic conductivity at 25 °C and a Li⁺ transfer number of 0.454. In addition, compared to the rough and loose morphology presented by the non-in situ Li anode surface, the in situ polymerization takes full advantage of the wetting potential of the monomer on the electrode, improving the interfacial contact, enriching the lithiated oligomers and LiF, which are protective for Li anodes, with significant ability to mitigate Li dendrite growth and improve electrochemical stability (Fig. 8(c)).

3.2.2 Janus interface. In order to achieve solid-state batteries with high safety and specific energy density, high chemical and electrochemical stability of polymer electrolytes with cathodes and Li metal anodes is essential, which requires a rational internal cell structure design and optimized electrolyte composition. Nevertheless, it is widely believed that no polymer has a large enough energy gap between the vacant and occupied electronic states to allow it to remain stable at both electrodes of a high-voltage cell during cycling.^109–111 SPEs with stable Janus interfaces for example, are not only expected to achieve high ionic conductivity without sacrificing mechanical strength and thermal stability, but also meet the unique requirements of cathodes and anodes.

UiO-66-SO₃Li is a metal organic framework (MOF) with a lithium-loving SO₃²⁻ group chemically anchored to an open metal site in the UiO-66 channel, which allows for single Li⁺ conduction due to its electrostatic repulsion and narrow pore size. Ruan¹¹²et al. proposed a Janus electrolyte (JCSSE) with mortise and tenon joints for enhanced interfacial compatibility. The PVDF-HFP/LLZTO layer (LCSEP) toward a high-voltage cathode and PDADMATFSI/UiO-66-SO₃ Li layer (UCSSEP) toward a lithium metal anode are shown in Fig. 9(a). The Janus electrolyte facilitates the regulation of the amount of TFSI⁻ in the Li⁺ first solvation shell and protects the construction of continuous Li⁺ conduction channels for structural Li⁺ diffusion. This will prolong the Sand's time. Due to the adoption of UiO-66-SO₃Li with lithophilic ion-channels, PDADMATFSI-based UCSSEP (the layer attached to the Li metal anode) was supposed to exhibit a single-ion conduction feature. It ensures both close contact with the lithium metal anode and uniform deposition of Li⁺ to ease the formation of the space charge layer and lithium dendrites. For the cathode interface, defluorinated P (VDF-HFP) has a lower HOMO energy level (−11.36 eV), which enhances the compatibility between JCSSE and the cathode.

Fig. 9 (a) Schematic describing the Li⁺ solvation and transport mechanism in CSSEs, and TEM images of NCM particles in JCSSE and UCSSE. Reproduced with permission from ref. 112. Copyright 2023, Elsevier. (b) CEI formation mechanism, TEM image of the NCM622 cathode removed from a NCM622|SPE|Li battery after 10 cycles (left), and a TEM image of the NCM622 cathode removed from a NCM622|SL-SPE|Li battery after 10 cycles red (right). Reproduced with permission from ref. 113. Copyright 2023, Elsevier. (c) SEM images of (upper) CF and (lower) CF@Cu₂Mg electrodes with 1, 3, and 5 mAh cm⁻² amounts of Li deposition. Reproduced with permission from ref. 114. Copyright 2022, American Chemical Society. (d) The top shows the schematic and SEM cross-sectional images of WDC-GDAg with a capacity of 12 mAh cm⁻² at 1 mA cm⁻². The inset shows SEM images of the bottom and top of a WDC-GDAg with a lithium deposition capacity of 12 mAh cm⁻². The bottom shows the schematic and SEM cross-sectional images of the WDC with a capacity of 12 mAh cm⁻² at 1 mA cm⁻². The inset shows SEM images of the top and bottom regions of a WDC with a lithium deposition capacity of 12 mAh cm⁻². Reproduced with permission from ref. 115. Copyright 2022, John Wiley and Sons.

Functional groups in the polymer matrix are designed to effectively modulate the frontier orbital energy levels of the polymer electrolyte and the chemically passivated interface formed in the electrode–electrolyte interface (CEI and SEI components). Silin Chen¹¹³ ingeniously constructed asymmetric bilayer solid polymer electrolytes (SL-SPE), and synthesized structurally integrated asymmetric polymer electrolytes consisting of two functionalized layers by two-step polymerization via ultraviolet (UV) irradiation. The integrated reduction layer is not only kinetically but also thermodynamically stabilized towards the Li metal anode. In addition, PEGDA was used as a “bridge” to connect sulfonyl groups (O@S=O) on the reduction-tolerant layer to obtain an ultrathin oxidation-tolerant layer, which can stabilize the cathode|SPE interface through the formation of a good CEI film (Fig. 9(b)). SL-SPE provides a wide electrochemical stability window of up to 5 V due to the TSFI⁻ which can be reduced at the Li anode to form a LiF and Li₃N containing robust SEI layer, which can effectively prevent uncontrolled parasitic reactions between SL and Li anodes while deploying stable Janus interface properties.

3.2.3 3D hosts. Infinite volume changes in Li metal anodes during discharge/charge may also lead to SEI layer cracking and dead Li formation, resulting in a low CE and poor cycling stability. Designing 3D collectors as Li anode bodies can provide enough space to mitigate the volume change during lithium plating/stripping. In addition, the 3D substrate has good electrical conductivity and a large specific surface area, which can effectively reduce the local electric field density and uniformly regulate the electric field distribution, facilitating uniform Li deposition.^116,117 Various types of 3D materials such as graphene oxide,^118,119 carbon nanofibers,^120,121 metal (e.g., Ni and Cu) foams,^122–124 and free-standing carbon fiber films,^125,126 have been used as hosts for Li metal storage via electrodeposition strategies.

Jiang¹¹⁴et al. constructed Cu₂Mg alloy layers on copper foam (CF@Cu₂Mg) to obtain highly lithium-loving and dendrite free lithium metal anodes by using a simple thermal evaporation and in situ alloying process (Fig. 9(c)). Density functional theory (DFT) calculations show that Cu₂Mg has a larger negative adsorption energy (−1.96 eV) than pure Cu (−1.45 eV), which implies a better lithium affinity of Cu₂Mg. In addition, unlike other lithium-loving compounds containing MgO, Cu₂Mg alloys have excellent chemical and electrochemical stability. They do not undergo lithium alloying reaction during the discharge process, avoiding cracking and failure of the lithium-philic coating, and can reduce the preparation cost when introducing highly lithium-philic Mg components. With the synergistic effect of excellent lithophilicity and high electrochemical stability of the host, we obtained lithium coatings without dendritic lithium dendrites and provided a new direction for the construction of highly lithophilic and electrochemically stabilized magnesium-containing host materials without dendritic lithium metal anodes.

Inspired by the transpiration process of substances in trees in nature, Zhu¹¹⁵et al. obtained gradient-distributed lithophilic dots in low-curvature 3D wood-derived carbon (WDC) frameworks as an effective scaffold for lithium deposition/exfoliation by simple capillary-induced gradient deposition (Fig. 9(d)). The finite element simulation reveals that the gradient-distributed silver nanoparticle-modified WDC framework (WDC-GDAg) follows the ideal design of bottom-up lithium deposition behavior to avoid lithium dendrite growth near the separator, related to the concentration of Li⁺ flux and the redistribution of current density induced by the gradient lithiophilic sites. It maintains the advantage of 3D deposition-regulated scaffolds for mitigating volume changes. This kind of spatially distributed lithiophilic site with a gradient strategy presents a significant step toward practical applications of safe and high-rate performance Li anode materials for next-generation metal batteries. The values of ionic conductivity and the electrochemical stability window of polymer electrolytes with different modification strategies are summarized in Table 1.

Table 1 Ionic conductivity and electrochemical stabilization windows of polymer electrolytes in different strategies

	Materials	Ionic conductivity	Electrochemical stability window (V)	Ref.
In situ polymerization	PVDF/PEO/garnet(PG)	8.2 × 10^−5 S cm^−1 (25 °C)	—	57
	UPyMA/EGMEA	2.2 × 10^−5 S cm^−1 (25 °C)	4.7	59
	In situ copolymerization of CL and LA (CL)	8.9 × 10^−5 S cm^−1 (30 °C)	5.0	107
	PDEGDA/PVDF FMs	1.41 × 10^−4 S cm^−1 (25 °C)	—	108
Inorganic material coating	PEO/Li–Ta–O	1.03 × 10^−4 S cm^−1 (30 °C)	4.2	82
	PEO/PEGDME/LAGP	2.3 × 10^−4 S cm^−1 (25 °C)	4.5	24
Janus interface	P(VDF-HFP)/LLZTO-PDADMATFSI/UiO-66-SO3Li	2.1 × 10^−4 S cm^−1 (25 °C)	5.0	112
	PEO/TEGDME/PEGDA	6.84 × 10^−4 S cm^−1 (30 °C)	5.0	113

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4. Conclusion and outlook

As technology advances, the need for safe and energy-intensive batteries for consumer electronic devices such as electric vehicles, portable devices, and smart grid community systems continues to grow. Currently, SSLMBs made from SSEs are gaining attention mainly because of their ability to overcome the safety shortcomings of conventional LBs associated with liquid electrolyte leakage issues and flammability. Hence, the combination of lithium metal anodes and SPEs is a potential strategy to achieve safe, high-performance and low-cost energy storage systems in the future. SPEs offer suitable interfacial contacts, good electrochemical stability, flexibility, excellent processability and economic availability. In recent years, different coping strategies have been widely explored around the disadvantages of polymer electrolytes and the hindrance of interfacial problems between the electrolyte and electrode, such as the use of in situ polymerization, electrolyte additives and new interfacial construction techniques, molecular design of polymer structures, and electrolyte multilayer development. Significant efforts are still needed to overcome the obstacles and improve the performance of polymer electrolytes. These include the following:

(1) A cell design using a high pressure stabilized polymer contact cathode and a low pressure stabilized polymer contact Li metal anode can effectively extend the electrochemical window and optimize interfacial compatibility.

(2) The new in situ polymerization strategy can focus more on finding interface-friendly initiators, avoiding the introduction of parasitic reactions (thermal decomposition of salts and interphase side reactions) in the cell system, as well as avoiding the introduction of additional solvents that can affect the electrochemical performance of solid-state LMBs.

(3) The ideal polymer electrolyte or polymer coating should provide a stable, flexible and suitably rigid interfacial layer to enhance tight solid–solid electrode/electrolyte interfacial contact, regulate electrode volume expansion and inhibit the formation of lithium metal dendrites.

(4) Develop adaptive and self-healing functional polymeric materials that are responsible for stress by automatically changing strain that helps to suppress lateral reactions and Li dendrite growth by flexibly adjusting the interface to accommodate the volume expansion of Li deposition under plating/exfoliation processes.

Considering the rapidly growing academic and industrial interest in the development of polymer electrolytes and solid-state lithium-based batteries, it is reasonable to expect a major breakthrough in the near future.

Author contributions

Shuru Wu: conceptualization, methodology, and software Chenyu Wang: data curation, and writing – original draft preparation. Shuanghui Li: visualization and investigation. Jingzheng Weng: writing – reviewing and editing.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Leading (Key) Projects of Fujian Province, China (no. 2018H0009, 2022Y0006).

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