2.1.1 Physical contact at the SSEs/cathode interface

Physical contact at the SSEs/cathode interface plays a pivotal role in determining Li⁺ transport kinetics, where inadequate interfacial contact significantly hinders Li⁺ transfer, resulting in compromised electrochemical performance and shortened cycle life in SSBs employing untreated cathode materials. To address this challenge, co-sintering of cathode materials with inorganic SSEs has emerged as an effective strategy. As shown in Fig. 2(a), Wang et al. developed an all-ceramic cathode/electrolyte solid-state battery with ultra-low interfacial resistance by co-sintering LiCoO₂ with Li₇La₃Zr₂O₁₂ (LLZO), where the Li_2.3-xC_0.7+xB_0.3-xO₃ solid electrolyte interphase was formed in situ through reaction with Li₂CO₃ present on both LLZO and LiCoO₂ surfaces [34]. Similarly, Zhang et al. developed an innovative two-step sintering process that repurposed residual Li₂CO₃ on LLZO surfaces. During the first sintering stage, LLZO@Li₂CO₃ was transformed into LLZO@LiCoO₂, with Li₂CO₃ serving as the lithium source, while lithium loss concurrently generated a Li-deficient La₂Zr₂O₇ phase within LLZO (Fig. 2(b)) [35]. The subsequent sintering step with additional Li₂O successfully restored the lithium content, converting La₂Zr₂O₇ back to LLZO. This methodology not only established superior interfacial contact between cathode and electrolyte components but also enhanced Li⁺ transport pathways through both bulk and surface regions of the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) @LiCoO₂ composite. Figure 2(c) demonstrates an innovative asymmetric interfacial modification strategy, combining an ultrathin in-situ polymerized cathode interlayer with a lithiophilic liquid metal coating on the Li-metal anode [36]. Owing to enhanced electrode-electrolyte interfacial contact and improved wettability, the interfacial impedance on both sides was significantly reduced, with the cathode and anode interfacial impedances decreasing to 214 Ω∙cm² and 7.45 Ω∙cm², respectively. Guo et al. demonstrated an effective blade-casting approach for fabricating composite cathodes by directly depositing a uniform LiFePO₄/PVDF/LiTFSI/carbon black slurry onto LLZO solid electrolytes [37]. The resulting architecture achieved efficient ionic transport through the ceramic electrolyte while maintaining continuous electron conduction pathways and excellent cathode-electrolyte interfacial contact, enabling 93% capacity retention after 100 cycles at 0.05C (Fig. 2(d)). Achieving ideal solid-solid interfacial contact between battery components ideally involves integrating electrodes with electrolytes to form monolithic cells. While employing molten lithium as the anode is feasible, the cathode side presents greater challenges due to interdiffusion of detrimental elements during high-temperature processing. Herein, Wen et al. developed an simple and cost-effective strategy involving liquid-phase-sintering-assisted in situ interfacial growth [38] . This approach enables the formation of a monolithic structure between a LiCoO₂ cathode and a LLZTO electrolyte, interconnected via a unique decrystallizing interlayer. Notably, this interlayer comprises Co-doped LLZTO nanocrystals dispersed within an amorphous matrix (termed the NAM region), wherein the amorphous matrix effectively prevents interdiffusion of detrimental elements, while the nanocrystals facilitate Li⁺ transport. Benefiting from this distinctive NAM architecture, the fabricated all-in-one all-solid-state battery exhibits remarkable long-term cyclability (exceeding 500 cycles at 0.2C with > 80% capacity retention).

Figure 2 (a) Schematic of all-ceramic cathode/electrolyte SSBs fabrication. Reprinted with permission from Ref. [34], © 2018, Cell. (b) Process for converting LLZO@Li₂CO₃ to LLZO@LiCoO₂. Reprinted with permission from Ref. [35], © 2020, Springer Nature. (c) The asymmetric interface modification strategy for LLZO. Reprinted with permission from Ref. [36], © 2023, Elsevier. (d) SEM image of the interface between a composite cathode and LLZO electrolyte. Reprinted with permission from Ref. [37], 2015, Elsevier. (e) Schematic illustrating voids between LiCoO₂ and SSEs hindering Li⁺ transport paths. Reprinted with permission from Ref. [40], © 2018, Elsevier.

Volume changes during repeated lithiation/delithiation exacerbate interfacial stress between the electrode material and SSEs [39]. The rigid mechanical coupling in SSBs amplifies contact issues caused by this stress. Tallarek et al. revealed that interfacial voids between LiCoO₂ and SSEs significantly increase Li⁺ transport path tortuosity, resulting in high interfacial impedance (Fig. 2(e)) [40]. Wagemaker et al. employed 2D Li⁺ exchange Nuclear Magnetic Resonance (NMR) spectroscopy to monitor interfacial impedance evolution, where chemical shift variations indicated severe contact degradation at Li₂S/Li₆PS₅Br interfaces caused by cathode volume changes and interfacial side reactions [41] . These findings highlight the necessity for interface optimization considering volume change effects in inorganic SSEs systems. In contrast, organic or composite SSEs maintain better contact with cathodes due to their inherent flexibility [42, 43].

2.1.2 Space charge layer at the SSEs/cathode interface

The "space charge layer" (SCL) theory, originally developed by Wagner et al. in 1972 [44], gained recognition for explaining cathode/SSEs interfacial issues only in 2006 [45]. At mixed ionic/electronic conductor (MIEC) cathode/SSEs interfaces, electrochemical potential differences induce charge redistribution, forming an SCL where carrier concentrations vary significantly. The LiCoO₂/Li₁₀GeP₂S₁₂ (LGPS) system exemplifies this phenomenon: Differences in chemical potential between the oxide cathode and sulfide electrolyte, coupled with Coulombic forces from O^2-, drive Li⁺ migration from the sulfide electrolyte across the interface to the LiCoO₂ surface until electrochemical equilibrium is reached. This creates a Li⁺-depleted SCL within LGPS, exhibiting low ionic conductivity due to compositional imbalance. Simultaneously, as LiCoO₂ is an MIEC, its electrons migrate to the surface to balance the excess Li⁺, initiating a vicious cycle: more Li⁺ migrates from LGPS to the interface, thickening the SCL and increasing interfacial resistance. Electron probe microanalysis provides direct evidence of this mechanism, revealing substantial sulfur enrichment within LiCoO₂ fracture surfaces (Fig. 3(a)) [46]. These findings demonstrate how SCL formation not only increases interfacial resistance but also promotes mechanical degradation in cathode materials.

Figure 3 (a) Electron probe microanalysis of LiCoO₂ particles at the LGPS interface. Reprinted with permission from Ref. [46], © 2021, Royal Society of Chemistry. (b) LiCoO₂/sulfide interface with and without Li₄Ti₅O₁₂ oxide. Reprinted with permission from Ref. [45], © 2006, Wiley. (c) Schematic of lattice mismatch and chemical potential difference distribution at the LiCoO₂/β-Li₃PS₄ interface. Reprinted with permission from Ref. [51], © 2014, American Chemical Society. (d) Difference in Li⁺ exchange activation energy with/without SCL and NMR ion exchange profiles for Li_xV₂O₅/LAGP interface. Reprinted with permission from Ref. [52], © 2020, Cell.

Inserting an ion-conductive but electronically insulating oxide interlayer between the cathode and the SSE is an effective strategy to mitigate the SCL effect, transforming the cathode/SSEs interface into a cathode/interlayer/SSEs interface [47]. The interlayer effectively mitigates the chemical potential difference at the cathode interface while its electronic insulating nature suppresses space charge layer formation at the electrolyte interface. The pioneering work by Takada et al. demonstrated that introducing a Li₄Ti₅O₁₂ interlayer could effectively suppress SCL formation in LiCoO₂/sulfide electrolyte systems (Fig. 3(b)) [45]. This seminal study sparked extensive research into alternative interlayer materials, with LiNbO₃ [47], LiTaO₃ [48], Li₃PO₄ [49], LiAlO₂ [50] emerging as promising candidates. Yoshitaka et al. provided fundamental understanding through density functional theory (DFT) calculations, revealing that the LiNbO₃ interlayer not only promotes favorable Li⁺ adsorption on CoO₆ octahedra but also facilitates efficient Li⁺ transport through available sites in the Li layer [51]. More significantly, this buffer layer creates an atomically coherent interface that eliminates spatial heterogeneity in Li⁺ adsorption, thus preventing non-uniform Li⁺ distribution (Fig. 3(c)). During initial charging, SCL formation leads to increased interfacial resistance. Cheng et al. [52] quantified this effect using NMR spectroscopy to probe Li⁺ transport across Li_xV₂O₅/Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) interfaces. Their results (Fig. 3(d)) show that SCL formation elevates the Li⁺ exchange activation energy from 0.315 eV to 0.515 eV, representing a 63% enhancement that substantially reduces exchange current density and elevates interfacial resistance. These findings provide compelling experimental evidence for the adverse effects of SCL on interfacial charge transfer kinetics.

2.1.3 Interfacial side reactions at the SSEs/cathode interface

The mismatched electrochemical stability windows between SSEs and cathode materials can induce interfacial side reactions, occurring either at open circuit or under applied voltage [53]. The electrochemical window, defined as the voltage range between oxidation and reduction potentials, where wider windows indicate better stability, can be determined via linear sweep voltammetry (LSV) or DFT calculations. Goodenough et al. [54] elucidated the essence of the electrochemical window using molecular frontier orbital theory (Fig. 4(a)). In this framework, the electrolyte's lowest unoccupied molecular orbital (LUMO), corresponding to the conduction band (C.B.) in semiconductors, represents the energy level where it can accept electrons. If the anode chemical potential (μ_A) is higher than the LUMO, electrons transfer from the anode to the electrolyte, driving reduction. Conversely, the highest occupied molecular orbital (HOMO), corresponding to the valence band (V.B.), represents the level where it can donate electrons. If the cathode chemical potential (u_C) is lower than the HOMO, electrons migrate from the electrolyte to the cathode, resulting in oxidation. Thus, a stable electrolyte should possess a LUMO lower than the anode lithiation potential and a HOMO higher than the cathode delithiation potential.

Figure 4 (a) Schematic of electrochemical windows for liquid and solid electrolytes. Reprinted with permission from Ref. [54], © 2013, American Chemical Society. (b) Calculated electrochemical windows for common SSEs. Reprinted with permission from Ref. [55], © 2015, American Chemical Society. (c) Schematic illustration of strategies to overcome each factor of capacity fading in ASSBs. Reprinted with permission from Ref. [61], © 2025, Springer Nature. (d) Schematic diagram of the structure of SSBs. Reprinted with permission from Ref. [62], © 2023, Elsevier.

The conventional assumption regarding the high-voltage stability of inorganic SSEs may require reconsideration. As shown in Fig. 4(b), theoretical calculations indicate that many inorganic SSEs (e.g., LLZO, LISICON, LLTO) possess intrinsic oxidation limits below 4 V versus Li⁺/Li [55]. However, experimentally measured oxidation potentials obtained through cyclic voltammetry (CV) often appear higher due to kinetic limitations. The restricted interfacial contact area with blocking electrodes significantly retards decomposition rates, thereby obscuring the materials' true thermodynamic instability under high-voltage conditions. To address this measurement challenge, Wang et al. developed an innovative Li/LLZO/LLZO-C/Pt configuration, replacing the conventional Li/LLZO/Pt structure for stability window determination [56]. By incorporating carbon into LLZO to form a composite electrode, the researchers enhanced electron transport and increased the active area for charge transfer, which accelerated decomposition kinetics. This approach revealed a distinct oxidation current for LLZO at 4 V, substantially lower than the previously reported 6 V. The discrepancy between measured and theoretical values can be attributed to the oxidation overpotential of LLZO. Interface stability remains a critical challenge in SSBs systems. Banerjee et al. [57] systematically investigated the interface between Li₆PS₅Cl and LiNi_xCo_yAl_1-x-yO₂ (NCA) cathodes using advanced characterization techniques combined with first-principles calculations. Their work identified spontaneous chemical reactions generating side products including Ni₃S₄, LiCl, and Li₃PO₄. For polymer-based SSEs such as polyethylene oxide (PEO), the typical electrochemical window ranges from 0–4 V. Notably, transition metal oxides in the cathode can catalyze irreversible oxidative decomposition of PEO, as evidenced by NMR monitoring of fluorine element distribution. Common strategies to mitigate window mismatch include cathode surface coating and incorporation of ionic conductive additives, which have shown promise in improving interfacial stability [58–60]. Park et al. [61] conducted a comprehensive study on capacity fading mechanisms in Ni-rich NCA cathodes within SSBs. Their work identified three primary degradation pathways: (1) surface reactions at the cathode-electrolyte interface, (2) inner-particle isolation caused by microcracks, and (3) detachment of cathode particles from the SSEs (Fig. 4(c)). Through surface modification (boron coating) and morphology engineering (Nb doping), the researchers demonstrated that surface reactions dominate in cathodes with 80% Ni content, while mechanical degradation (isolation and detachment) becomes predominant in Ni-rich cathodes (≥ 85% Ni). The optimized cathode (SM-Ni90) achieved 80.2% capacity retention after 300 cycles in high-loading pouch cells, though further improvements are needed to address particle detachment for long-life SSBs. Interfacial engineering has emerged as a promising approach to address these challenges. As illustrated in Fig. 4(d), Wang et al. developed a nanoscale LiAlO₂ coating (~5 nm thick) for LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes to improve interface stability in SSBs [62]. This coating effectively functioned as a buffer layer, suppressing both SCL formation and electrolyte decomposition at the cathode/sulfide electrolyte (Li_5.5PS_4.5Cl_1.5) interface. Electrochemical characterization demonstrated that the coating enhanced Li⁺ migration kinetics while reducing interfacial resistance and polarization. Consequently, the LiAlO₂@NCM811 cathode delivered an initial discharge capacity of 139.46 mAh∙g⁻¹ at 0.05C with 82.4% capacity retention after 60 cycles, significantly outperforming uncoated NCM811. X-ray photoelectron spectroscopy (XPS) and Transmission Electron Microscope (TEM) analyses confirmed the coating's effectiveness in stabilizing the interface by minimizing oxidative degradation of the solid electrolyte, highlighting the importance of tailored interfacial design for optimizing Ni-rich cathode performance in SSBs.

2.2.1 Lithium dendrites

The commercialization of lithium metal batteries has faced persistent challenges since their earliest implementation in Moli Energy's liquid Li-MoS₂ system (1980) [63]. Safety concerns stemming from lithium dendrite penetration, leading to internal short circuits and thermal runaway, ultimately forced the discontinuation of this battery. Beyond safety risks, dendritic growth triggers multiple failure mechanisms: (1) continuous consumption of lithium and electrolyte through repetitive SEI formation on newly exposed lithium surfaces, (2) progressive deterioration of Coulombic efficiency, and (3) substantial volumetric fluctuations that disrupt electrode-electrolyte interfaces. These fundamental challenges underscore why dendrite mitigation remains a critical requirement for practical lithium metal batteries [64–66].

Investigating dendrite formation in solid-state systems presents unique experimental hurdles. The inherent opacity of most solid electrolytes and the confined nature of buried interfaces complicate direct observation. Consequently, researchers often draw upon mechanistic insights from liquid electrolyte systems, where characterization is more accessible. Table 1 systematically compares established dendrite growth models and their underlying physical principles across different battery configurations.

The space charge model proposed by Chazalviel [67], derived from Sand's time model for Cu²⁺ deposition [71], remains the most widely accepted framework for explaining dendrite initiation. This model attributes dendritic growth to the formation of space charge layers in dilute electrolytes, where dendrites nucleate when the Li⁺ concentration at the electrode surface is depleted under high current density conditions. The initiation time τ, called Sand time, is given by Eq. (1), where D is the cation diffusion coefficient, e is the elementary charge, C₀ is the initial electrolyte concentration, J is the effective current density, μ_a and ${\mu }_{{\text{Li}}^{+}}$ are the anion and Li⁺ mobilities, respectively. Sand time τ depends empirically on the transference numbers of electrons and Li⁺. Increasing τ (delaying dendrites) can be achieved by reducing J, increasing ${\mu }_{{\text{Li}}^{+}}$, or raising C₀. Strategies like high-surface-area anode structures and electrolytes with high Li⁺ transference numbers ($t_{{\text{Li}}^{+}}$) can suppress dendrites.$\tau =\pi D\frac{\text{e}{C}_0{\left({\mu }_{\text{a}}+{\mu }_{{\text{Li}}^{+}}\right)}^2}{2J{\mu }_{\text{a}}{}^2}$(1)

The heterogeneous nucleation model offers fundamental understanding of the initial lithium deposition process that governs the final morphology of Li metal deposits. Ely et al. [68] identified five nucleation states: nucleation suppression regime, long incubation time regime, short incubation time regime, early growth regime, and late growth regime. Thermodynamically unstable nuclei dissolve in the nucleation suppression regime. Long incubation time regime allows stable growth, whereas short incubation time limits nucleus size distribution beyond a critical potential, after which nuclei exceeding a critical size grow with increasing overpotential. Smoothing the Li surface, improving Li wettability, and controlling deposition overpotential can inhibit dendrites growth.

The surface nucleation-diffusion model elucidates dendrite formation through the interplay of surface energetics and ion transport kinetics. Comparative analysis reveals lithium's inherent dendrite propensity stems from its materials-specific characteristics: While exhibiting diffusion barriers comparable to magnesium (Mg), lithium's weaker single-bond strength results in a diminished free energy difference between bulk and low-dimensional phases [69]. This thermodynamic predisposition, coupled with sluggish surface diffusion, promotes dendritic growth. In contrast, Mg²⁺'s enhanced surface mobility facilitates preferential deposition at existing sites, naturally suppressing dendrite formation. Detailed investigations of common SEI components (LiOH, Li₂O, Li₂CO₃, and LiX) by Arias et al. demonstrate how interfacial chemistry governs deposition morphology [72]. Li₂CO₃, characterized by high Li⁺ diffusion barriers and low surface energy, creates unfavorable conditions for uniform deposition. Conversely, lithium halides (LiX) exhibit more favorable properties: their higher surface energies and reduced diffusion barriers promote two-dimensional growth. These fundamental insights have driven the strategic use of halide additives, particularly fluorides, which have been shown to enhance Coulombic efficiency while significantly suppressing dendritic growth [73, 74].

The formation of the SEI is an inherent consequence of lithium metal's spontaneous reaction with organic electrolytes. This interfacial layer plays a pivotal role in determining deposition morphology, as its mechanical integrity and transport properties govern lithium plating behavior. During repeated cycling, substantial volume changes induce mechanical stress within the SEI, leading to fracture and subsequent formation of localized high-current-density regions that serve as preferential sites for dendritic nucleation [70]. Advanced in-situ TEM characterization by Sacci et al. [75] has provided direct visualization of the dynamic interplay between SEI evolution and lithium deposition. These observations reveal a complex, self-adjusting interface where SEI growth and lithium plating occur concomitantly. Complementary studies by Li et al. demonstrate the critical influence of deposition kinetics on morphological development [76]. Under high overpotential conditions, accelerated SEI formation creates a constrictive environment that accumulates mechanical stress, ultimately leading to root-initiated dendritic growth. In this regime, the increased polarization renders lithium ion diffusion through the SEI as the rate-determining step, promoting one-dimensional dendrite formation. Conversely, more moderate deposition conditions facilitate a balanced growth process. The slower SEI formation kinetics under low overpotentials permit enhanced lithium ion mobility, enabling the cooperative deposition of metallic lithium and SEI components. This synergistic growth mechanism results in denser, more homogeneous lithium morphologies with improved electrochemical performance. These findings collectively highlight the importance of optimizing both SEI properties and deposition conditions to achieve stable lithium metal anodes.

The growth mechanisms of lithium dendrites in SSBs exhibit fundamental differences between polymer and inorganic SSEs systems, necessitating distinct analysis approaches. In polymer-based SSEs systems, dendrite propagation manifests through several characteristic pathways (Fig. 5(a)) [77]. (1) Tip growth through soft polymer domains: Surface protrusions on electrodes generate localized electric field enhancement at their tips, which preferentially directs lithium deposition. Theoretical analysis by Monroe and Newman based on linear elasticity principles indicates that dendritic growth can be effectively inhibited when the polymer electrolyte's shear modulus surpasses double that of metallic lithium [78]. (2) Lateral growth along the Li/Polymer interface: Lateral Li⁺ deposition through the SEI induces interfacial delamination [79]. (3) Substrate-induced growth: Dendritic nucleation initiates at substrate structural defects, ultimately propagating through the polymer electrolyte (Fig. 5(b)) [80]. (4) Charge redistribution-induced growth: At the Li/polymer interface, a low Li⁺ transference number ($t_{{\text{Li}}^{+}}$) induces rapid anion depletion, resulting in the formation of an electrical double layer that facilitates dendrite nucleation [81]. Dendrite suppression can be achieved through enhanced mechanical strength or improved ionic transport properties. Notably, Lodge et al. developed polymer electrolytes with exceptional mechanical properties (1 GPa modulus) and high ionic conductivity (1 mS∙cm⁻¹) using polymerization-induced phase separation [82]. Zhang et al. fabricated a PEO/PEG-LGPS composite SSEs exhibiting both high room-temperature conductivity (9.83×10⁻⁴ S∙cm⁻¹) and elevated $t_{{\text{Li}}^{+}}$ (0.68), which substantially improved the cycling stability of Li symmetric cells [83].

Figure 5 (a) Classification of dendrite growth in polymer electrolytes. Reprinted with permission from Ref. [77], © 2020, Cell. (b) Dendrite penetration within polymer electrolyte and X-ray tomography scan. Reprinted with permission from Ref. [80], © 2014, Springer Nature. (c) Wettability angle difference of LLZO with/without surface Li₂CO₃. Reprinted with permission from Ref. [84], © American Chemical Society. (d) LLZO surface morphology and deposited Li morphology. Reprinted with permission from Ref. [85], © 2017, Elsevier. (e) Schematic illustration of a pore connected to the lithium-solid electrolyte interface through a microcrack and being filled by Li deposition. Reprinted with permission from Ref. [86], © 2023, Springer Nature.

In inorganic SSEs systems, the growth of lithium dendrites can be categorized into the following four types. (1) Dendrite growth induced by discontinuous interfacial contact. Insufficient contact between the lithium metal anode and the inorganic SSEs leads to high interfacial resistance, significantly lowering the electrolyte's critical current density (CCD). In addition, the poor wettability between the SSEs and lithium metal anode further deteriorates interfacial contact. As shown in Fig. 5(c), Sakamoto et al. demonstrated through first-principles calculations that Li₂CO₃ on LLZO surface increases the contact angle with lithium, reducing its lithiophilicity [84]. (2) Dendrite penetration along grain boundaries. The presence of grain boundaries in polycrystalline SSEs critically governs lithium deposition dynamics. Through meticulous microstructure engineering and scanning electron microscopy (SEM) characterization, Sakamoto et al. provided direct experimental evidence of lithium's preferential deposition along LLZO grain boundaries (Fig. 5(d)), indicating that grain boundaries play a critical role in dendrite formation during lithium deposition [85]. (3) Dendrite growth induced by the microstructure (pores and defects) within the SSEs. The microstructure of inorganic SSEs critically governs their mechanical integrity and ionic transport properties. Ning et al. [86] employed operando X-ray tomography to elucidate a two-phase dendrite formation process, where initial nucleation occurs through lithium infiltration into microcracks and pore junctions, followed by propagation via lithium's viscoplastic deformation that extends these defects (Fig. 5(e)). These findings underscore the importance of microstructural engineering in developing dendrite-resistant inorganic SSEs. (4) Dendrite growth inside the bulk of the inorganic SSEs. While inorganic SSEs exhibit high mechanical modulus that should theoretically prevent dendrite penetration, their experimentally measured CCD consistently fall below theoretical predictions. This apparent contradiction was resolved by Wang et al. through operando neutron depth profiling studies of LLZO and Li₃PS₄ systems [87]. Their work revealed that residual electronic conductivity (10⁻⁹–10⁻⁷ S∙cm⁻¹) in these materials enables lithium ion reduction within the electrolyte bulk, facilitating intragranular dendrite formation despite the materials' mechanical robustness.

2.2.2 Interfacial reactions at the SSEs/Li anode interface

(1) Inorganic SSEs/Li anode interface

The chemical stability of SSEs at open circuit is determined by the relative positions of the lithium anode's chemical potential (μ_Li) and the electrolyte's LUMO level. When μ_Li exceeds the SSEs's LUMO energy, spontaneous reduction of the electrolyte occurs. Under electrochemical polarization, the anode's electrochemical potential (μ_Li) follows Eq. (2):${\mu }_{\text{Li}}={\mu }_{\text{Li}}^o-\text{e}V$(2)where V represents the applied voltage and e denotes the elementary charge [88]. This electrochemical potential difference determines the reduction stability window of the SSEs. For inorganic SSEs, contact with Li metal anode leads to reduction of surface high-valence cations, forming resistive interphases. As shown in Fig. 6(a) [89], SSEs/Li interfaces fall into three types: (1) Thermodynamically stable (non-reactive): no reaction occurs. (2) Mixed conductor interphase (MCI): the mixed conductor interphase (MCI), which is both electronically and ionically conductive, grows into the SSEs, leading to severe self-discharge. This phenomenon is common in SSEs containing multivalent cations and necessitates the use of protective layers for prevention. (3) Passivating interphase: electronically insulating but ionically conductive; protects the SSEs if sufficiently thin and electronically blocking.

Figure 6 (a) Types of SSEs/Li reaction interfaces: (1) stable, (2) mixed ionic/electronic conductor (MCI), (3) electronically insulating but ionically conductive metastable interface. Reprinted with permission from Ref. [89], © 2015, Elsevier. (b) Accelerating rate calorimetry (ARC) results and thermal runaway mechanism for four inorganic SSEs. Reprinted with permission from Ref. [96], © 2020, Cell. (c) PVDF-based polymer electrolyte/Li anode interface: (1) AES after adding different Li salts, (2) TOF-SIMS and depth profile of PVDF-LiFSI/Li reaction interface. Reprinted with permission from Ref. [99], © 2019, Wiley.

Most inorganic SSEs exhibit thermodynamic instability against Li metal. The resulting decomposition products are summarized in Table 2. SSEs without transition metals (e.g., Li₃PS₄, Li₆PS₅Cl, LiPON) typically form passivating interphases rich in Li₃P, Li₃N, LiCl, or Li₂S, which effectively terminate further reduction [90, 91]. In contrast, SSEs containing reducible cations such as Ge⁴⁺ or Ti⁴⁺ (e.g., Li₁₀GeP₂S₁₂, LATP, LAGP) generate MCI containing metallic Ge or Ti, facilitating continuous electrolyte reduction and battery failure [92–95]. The reduction process frequently involves lithiation reactions. Cheng et al. [94] identified lithiated Li₃Al_0.3Ti_1.7(PO₄)₃ in the MCI of LATP via XRD, suggesting that indirect decomposition contributes to a broader apparent electrochemical stability window. Similarly, Kang et al. found that LAGP reacts with Li, forming Li₂O, Li₂O₂, Li₂CO₃, and reduced Ge³⁺ [95]. The MCI enables Li⁺ reduction at the SSEs interface, triggering local volume expansion, crack formation, and LAGP fragmentation.

The replacement of flammable liquid electrolytes with non-flammable SSEs represents a promising approach for enhancing battery safety. However, Li et al. conducted accelerated rate calorimetry (ARC) studies that revealed significant variations in thermal stability among common oxide SSEs (LATP, LAGP, LLTO, and LLZO) when in contact with Li metal (Fig. 6(b)) [96]. While LLZO maintained stability, LATP, LAGP, and LLTO exhibited thermal runaway upon lithium contact. The thermal runaway mechanism stems from coupled physicochemical processes. Li metal melting enhances interfacial contact area, promoting faster reaction rates. Concurrently, elevated temperatures induce oxygen evolution from oxide SSEs, which reacts exothermically with molten lithium. This oxygen-mediated pathway dominates thermal runaway initiation in oxide electrolytes. Sulfide SSEs, though structurally different, possess similarly labile bonds that could engender comparable safety issues. These results highlight essential research directions requiring parallel investigation of degradation pathways and thermal stabilization methods for SSEs systems.

(2) Organic SSEs/Li anode interface

The spontaneous reaction between highly reactive Li metal anode and polymer electrolytes leads to the formation of an interphase layer whose characteristics are dictated by the polymer's functional groups and molecular structure. Hydroxyl (-OH) and carboxyl (-COOH) functional groups have been shown to adversely affect interphase stability, whereas (R-(CH₂CH₂O)-H) facilitates the formation of a well-ordered continuous layer that effectively suppresses side reactions [97]. In conventional PEO/LiTFSI systems, interfacial reactions involve CF₃ radical-induced hydrogen abstraction from the polymer backbone or Li-mediated C-O bond cleavage, yielding Li-O-R compounds that contribute to passivation resistance [98]. Advanced characterization techniques including Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) have revealed the nanoscale architecture of in-situ formed interphases between PVDF-based solid-state electrolytes and Li metal anodes [99]. Notably, PVDF-LiFSI systems develop a composite interphase comprising LiF-sulfide-LiOH-Li₂CO₃-Li₂O, which demonstrates exceptional stability combined with desirable ionic conductivity, electronic insulating properties, and mechanical robustness (Fig. 6(c)). These findings underscore the critical influence of lithium salt selection on interfacial characteristics. Systematic investigations of various lithium salts in PEO matrices have yielded important insights. While LiTFSI, LiBOB, and LiNO₃ individually induce negligible interfacial chemical modifications during cycling, their combination generates reactive species including SO₂ and CF₃ [100]. Innovative approaches to interface stabilization have been demonstrated, Wan et al. added Mg₃N₂ to PEO, forming a Li₃N/Mg mixed interface that stabilized the interface and promoted uniform Li deposition [101]. Other strategies to suppress interfacial reactions include polymer surface modification with LiPON [102] and photopolymerized interpenetrating network electrolytes [103].

2.2.3 Interfacial compatibility at the SSEs/Li anode interface

In addition to dendrite formation and parasitic side reactions, the development of high-rate SSBs faces critical challenges in interfacial compatibility, including poor electrode-electrolyte contact/wettability and detrimental morphological changes at the anode interface. As previously discussed, the presence of Li₂CO₃ on LLZO surfaces severely degrades Li metal anode wettability. To address this, Han et al. [104] employed atomic layer deposition (ALD) to coat LLZO with an ultrathin Al₂O₃ layer, dramatically enhancing Li wettability (Fig. 7(a)) and slashing interfacial resistance from 1710 to 1 Ω∙cm². Advanced characterization techniques, including electron energy-loss spectroscopy (EELS) and selected-area electron diffraction (SAED), confirmed the in-situ formation of a highly Li⁺-conductive Li-Al-O phase, identified as Li₂Al₄O₇. Alternative strategies have also demonstrated promise. Huang et al. developed a Li-C composite anode by blending graphite into molten lithium and casting it onto LLZO, achieving a low interfacial resistance of 11 Ω∙cm² [105]. Other effective approaches include metallic Al interlayers [106], Li-Mg alloy anodes [107], and polymer-based interfacial modifications [108]. Beyond interfacial wettability, anode morphology evolution during cycling, driven by substantial volume fluctuations, poses another major challenge, often leading to void formation and interfacial delamination. Sakamoto et al. systematically investigated the effects of stack pressure and current density on interfacial stability during galvanostatic cycling of LLZO-based cells (Fig. 7(b)) [109]. Their work revealed that below a critical pressure threshold, the cell voltage exhibits a sharp increase, with this critical pressure scaling with applied current density. This phenomenon arises when the Li stripping rate outpaces the mechanical replenishment of interfacial contact at low pressures, resulting in void-induced impedance growth. These findings highlight the necessity of co-optimizing both interfacial wettability and mechanical contact integrity in SSBs design, particularly to mitigate degradation from void formation and cyclic volume changes.

Figure 7 (a) Schematic of wettability improvement for LLZO/Li interface. Reprinted with permission from Ref. [104], © 2016, Springer Nature. (b) Voltage response under different pressures during galvanostatic cycling of LLZO symmetric cells. Reprinted with permission from Ref. [109], © 2019, Cell. (c) Schematic of NCM523 composite cathode. Reprinted with permission from Ref. [111], © 2019, Elsevier. (d) Schematic illustration for the architecture of the solid-state battery with LIM ionic conductive agent and its working mechanism. Reprinted with permission from Ref. [112], © 2018, Elsevier. (e) Schematic of Li₃PO₄-LLZO layer constructed on NCM811 surface. Reprinted with permission from Ref. [114], © 2021, Elsevier.