Spatially resolved modulation of the electrode-electrolyte interface in high Li-stoichiometric battery anodes

Carbonate-based electrolyte additive

Unsaturated ester-based electrolyte additives were first reported for stabilizing high-capacity anode materials. Owing to their high reduction potential and reactive functional groups, such as fluoroethylene carbonate (FEC), vinylene carbonate (VC), and dimethylvinylene carbonate (DMVC), undergo preferential electrochemical decomposition at the early stage of cycling. These additives help form an inorganic-based or mechanically stable SEI that can effectively impede the disintegration of the electrode structure. Among these, FEC additives are widely employed due to their ability to generate a LiF-rich interface, which possesses effective electron-insulating properties and SEI breakdown prevention capability due to its mechanical rigidity^[90]. At the atomic level, the decomposition of FEC typically proceeds via single-electron reduction to generate fluoride radicals and ethylene-based fragments at around 1.5 V^[123].

The radicals rapidly react with Li⁺ to form LiF and are included in the inner SEI layer in a nanocrystalline shape, which can modulate crystal structure and chemical functions^[124]. Following the initial reduction, the unsaturated intermediates undergo radical polymerization and subsequent de-fluorination even below 1.0 V, producing additional LiF. Due to their wide bandgap and electronic insulation properties, they serve as effective barriers that suppress further electrolyte reduction by electronically passivating the Si surface. At the same time, their mechanical rigidity improves the structural resilience against repetitive stress from large volume fluctuations. Furthermore, although bulk and non-uniform LiF is known to be ionically resistive, nanoscale LiF domains, particularly at SEI grain boundaries or amorphous interphases, may facilitate localized Li⁺ migration through space-charge-assisted transport or interfacial hopping mechanisms^[125]. These combined effects allow FEC-derived SEIs to act as selective and protective ionic conductors.

Recent studies have elucidated that LiF, generated from FEC decomposition, is not merely confined within the SEI but can also chemically interact with the Si surface, leading to a form of interfacial doping Figure 8A^[126]. This interfacial embedding alters the local chemical environment by stabilizing undercoordinated Si atoms and generating a fluorine-rich boundary layer. As a result, LiF doping disrupts long-range ordering in lithiated Si, leading to local lattice disorder at the interface. This structural modulation increases the energy barrier for the nucleation of crystalline Li₁₅Si₄, a phase known to accelerate electrode degradation due to its large volume change and high stiffness. The suppression of this crystalline phase fundamentally alters the lithiation behavior of Si. Without FEC, the system tends to form c-Li₁₅Si₄ after full lithiation, particularly at low rates or high depths of discharge. However, in FEC-containing systems, the lithiation pathway is redirected toward amorphous Li_xSi phases that are mechanically softer and electrochemically more reversible^[126]. Cryo-transmission electron microscope (TEM) and selected area electron diffraction (SAED) studies confirm the absence of diffraction spots associated with c-Li₁₅Si₄ under such conditions. This pathway shift is beneficial for both mechanical stability and chemical reversibility. Amorphous Li_xSi reduces irreversible Li trapping and promotes more homogeneous delithiation^[127]. Furthermore, the presence of LiF at the interface may regulate local Li⁺ diffusivity and prevent over-lithiation at grain boundaries. In summary, FEC enables a coupled mechanism of surface doping and phase control, offering a chemically grounded route to stabilize both interfacial structure and electrochemical performance in Si anodes^[128].

Figure 8. (A) TEM images and schematic images of FEC electrolyte additive forming LiF dopant into Si crystal structure. Reproduced with permission from ref.^[126]. Copyright 2023, American Chemical Society. (B) Relative Gibbs free energies of DMVC additive and VC dimer trimerization route reacting with VC, DMVC-OCF₃, or DMVC-OTMS. Reproduced with permission from ref.^[134]. Copyright 2021 Author(s), licensed under a Creative Commons CC BY license. (C) Schematic diagram of the selective methylation design of cyclic ether molecules and mechanistic comparison of ring-opening polymerization. Reproduced with permission from ref.^[137]. Copyright 2024, Wiley. (D) TOCF-SIMS secondary ion images of the cycled micron-sized Si electrodes showing selectively dissolving SEI component. Reproduced with permission from ref.^[43]. Copyright 2023, Author(s), licensed under a Creative Commons CC BY license. (E) Conceptual illustration of domino reactions involving the electrochemical reduction of elemental sulfur. Reproduced with permission from ref.^[145]. Copyright 2023, American Chemical Society. (F) TEM-element mapping of Si and mixed cation-based SEI components derived from multivalent ion additive. Reproduced with permission from ref.^[152]. Copyright 2024, American Chemical Society.

On the other hand, a VC-based electrolyte additive has been developed for high-capacity anode materials, where its reduction primarily forms a polycarbonate-based SEI layer. While decomposition of the FEC additive poses a risk of generating HF during radical reaction, which significantly affects the abuse influence on active materials, VC additives have the advantage of producing negligible harmful byproducts from their decomposition. In particular, VC additives decompose via a two-electron reduction mechanism, opening the vinylene ring and subsequent polymerization into polycarbonate-type structures^[129]. The initial reduction typically occurs at a relatively high potential (1.2~1.4 V vs. Li/Li⁺), forming linear or crosslinked poly (VC) chains on the anode surface. These polymeric species integrate into the SEI as electronically insulating yet highly ionically permeable layers^[130]. Therefore, VC-derived SEI tends to be thinner and more elastic than FEC-derived SEI, offering improved conformal coverage over expanding Si surfaces. As a result, previous work has shown that only 1% VC addition to the electrolyte can promote the formation of initial VC-derived SEI at higher potentials and inhibit the reduction decomposition of electrolyte solvent^[131]. However, due to its compact structure and lack of intrinsic defects, the VC-derived SEI exhibits relatively low Li⁺ conductivity, which can limit high-rate performance despite its excellent interfacial stability^[103].

Similar additives with VC that contain vinyl groups are reported as vinyl ethylene carbonate (VEC)^[132] and methylene ethylene carbonate (MEC)^[133]. While VC undergoes rapid electrochemical ring-opening polymerization, additional alkyl substituents in MEC and VEC stabilize the vinyl group and suppress uncontrolled radical polymerization. Therefore, cells employing VEC and MEC as electrolyte additives demonstrated significantly lower concentrations of ROCO₂Li in the SEI, as confirmed by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) analysis, compared to those using VC^[133]. The reduced presence of these unstable carbonate species correlates with enhanced SEI stability, leading to improvements in cycle retention and rate capability. Further, to overcome the limitations of VC and FEC, DMVC derivatives were rationally designed by introducing functional side groups such as trimethylsilyloxy (-OTMS) and -OCF₃ to tailor their reduction behavior and interfacial reactivity Figure 8B^[134]. Upon one-electron reduction, DMVC undergoes selective ring-opening at the vinylene site, generating carbon-centered radicals that proceed through either homocoupling (path I) or hetero-radical recombination with co-additives (path II). These radical pathways promote thermodynamically favorable C-C bond formation, leading to the growth of cross-linked polycarbonate networks that serve as mechanically adaptive SEI layers. In particular, OTMS-containing species contribute to PF₅ scavenging and effectively suppress the formation of HF and alkyl carbonate byproducts. The resulting SEI exhibits a spatially graded architecture, composed of a soft, polymer-rich outer layer and a rigid, LiF/SiO_x-enriched inner region. This heterogeneous modulus distribution enables anisotropic stress dissipation and prevents interfacial fracture, thereby enhancing the mechanical and electrochemical stability of high-capacity Si anodes under repeated volume changes.

Non-carbonate organic-based electrolyte engineering

Compared to conventional carbonate-based additives, ether-based and other non-carbonate organic electrolyte components offer key advantages for stabilizing Li-alloying anodes, though their low oxidative stability poses a challenge for compatibility with high-voltage cathodes^[135]. Such stabilization is closely associated with the ability of these components to modulate the local solvation environment and determine the preferential reduction of electrolyte species at the anode surface. Their lower viscosity and weaker Li⁺ coordination promote anion-rich solvation structures, facilitating salt-anion decomposition and the formation of an inorganic-rich SEI^[135]. Beyond solvation control, these additives often undergo selective decomposition or generate reduction products that contribute to the formation of desirable SEI components. For example, 1,3-dioxolane (DOL) derivatives can undergo electron-triggered ring-opening polymerization to form conformal, elastic SEI that effectively buffers volume changes in Si anodes^[136]. Unlike carbonates, which tend to fragment into gaseous or brittle species, non-carbonate molecules are more chemically tunable and frequently contain polymerizable or surface-reactive groups, enabling in situ interfacial network formation. These mechanistic benefits collectively allow for improved interfacial stability, chemical uniformity, and mechanical resilience in high-capacity anode materials. Additionally, selective methylation of DOL at specific ring positions enables precise modulation of its electrochemical decomposition pathways and SEI structure^[137]. In particular, 2-methyl-DOL (2MDOL) and 4-methyl-DOL (4MDOL) undergo electron-induced ring-opening reactions, but with markedly different outcomes [Figure 8C]^[137]. DFT analysis reveals that 4MDOL exhibits greater ring strain and stronger hyperconjugation, which facilitate more efficient anionic polymerization into longer-chain oligomers, resulting in a highly crosslinked and elastic SEI matrix. In contrast, 2MDOL exhibits lower ring strain and weaker hyperconjugation, producing shorter polymer chains and a less cohesive SEI structure^[137]. The SEI derived from 4MDOL exhibits superior nanophase separation, comprising flexible ether-rich domains and discrete LiF clusters, which effectively buffer Si volume changes and enhance interfacial stability.

In addition to constructing inorganic-based SEI by decomposition of electrolyte additives, SEI engineering via direct decomposition of electrolyte solvent is also considered an effective strategy^[138]. Adopting tetrahydrofuran (THF) and fluorinated co-solvents such as 2-methyltetrahydrofuran (2-MeTHF) or Bis(2,2,2-trifluoroethyl) ether (BTFE) enables selective formation of LiF-based SEI due to their reductive instability at low potentials^[139,140]. In particular, fluorinated solvents undergo preferential electron-induced cleavage of C-F or P-F bonds near the anode surface, leading to the localized generation of fluoride species. These species readily react with Li⁺ to precipitate LiF at the interface. The solvent decomposition pathway competes with salt-derived SEI formation and can dominate in low dielectric media or localized high-concentration environments, yielding a uniform and robust interphase. Instead of controlling electrolyte decomposition to induce selective interphase formation, selective SEI dissolution was introduced as a new strategy for designing desirable SEI. By incorporating a high donor-number solvent, γ-butyrolactone (GBL), into the electrolyte, soluble organic SEI species such as lithium ethylene dicarbonate (LEDC), lithium ethyl carbonate (LEC), and lithium fluorophosphates undergo gradual dissolution [Figure 8D]^[43]. This process shifts the dynamic equilibrium at the interface, driving the transformation of the SEI composition toward thermodynamically stable, insoluble inorganic species such as LiF. Additionally, the removal of low-modulus organic phases suppresses interfacial creep and fracture, while exposing reactive sites that promote further inorganic deposition. The dissolution-driven evolution results in a mechanically coherent and ionically conductive SEI tailored by interfacial solubility control rather than direct decomposition pathways.

Across carbonate and non-carbonate electrolyte components, SEI regulation differs in how electrolyte species are delivered to the anode surface, decomposed, and retained within the near-surface interfacial region^[141]. Carbonate-based components generally couple electrolyte reduction with SEI construction near the anode surface, as their decomposition products are retained, polymerized, or incorporated into LiF-rich, polycarbonate-rich, or compositionally graded interphases^[134]. Non-carbonate components can redistribute this sequence between the near-surface electrolyte and the growing SEI by regulating Li⁺ coordination, anion participation, reductive decomposition, or fluorinated solvents, ring-opening polymerization, or selective dissolution^[43,142]. Accordingly, SEI formation is governed by the identity of the initially reduced species, the local availability of reactive electrolyte species, and the retention or removal of decomposition products within the interphase. Such spatially differentiated control is important for Li-alloying anodes, where SEI chemistry must remain coupled with interfacial continuity during repeated volume changes

To mechanically stabilize and achieve rapid Li⁺ transportation, introducing heteroatom-based compounds into the SEI layer is considered an effective strategy. For instance, sulfur-containing additives such as 1,3-propane sultone (PS) decompose to generate Li₂SO₃ and Li₂S upon electrochemical reduction, which contribute to a flexible yet robust SEI architecture^[143]. These sulfur-rich species provide enhanced mechanical adaptability during Si volume fluctuations and facilitate Li⁺ transport through the SEI. In particular, SiO_x-based anodes benefit from early passivation by sulfur-containing additives, which selectively reduce at lower potentials, suppressing further solvent decomposition^[144]. However, conventional approaches for designing sulfur-containing additives often relied on multistep or complicated synthesis steps, which were sophisticated and unsuitable for industrial scaling. To address this, recent work suggested a sulfur atom-mediated domino reaction-derived SEI layer. As illustrated in Figure 8E, the introduction of elemental sulfur (S₈) initiates a sequence of reduction-triggered reactions known as a domino process^[145]. Upon early-stage reduction, S₈ decomposes to generate reactive sulfur-centered intermediates, which undergo subsequent transformation into alkyl sulfate and sulfonate species. These intermediates readily graft onto the Si surface or pre-existing SEI components. In parallel, conventional solvents such as FEC and EC are reduced at lower potentials, contributing LiF and polycarbonate components, respectively. The resulting gradient interphase exhibits a sulfur-rich outer layer comprising Li-alkyl sulfates and elastic polymeric domains that provide mechanical compliance, while an inner region rich in LiF ensures dense ionic blocking. Such sulfur-mediated layering imparts high interfacial elasticity, as confirmed by a uniform increase in Derjaguin-Muller-Toporov (DMT) modulus measured by atomic force microscopy (AFM). Additionally, the dense inner structure and elastic outer layer mitigate the growing excessive SEI layer; the domino-reaction-induced SEI layer exhibits uniform and thin morphology in the TEM image.

Inorganic-based electrolyte additive

As well as organic compounds used for electrolyte additives, inorganic additives (salt, metal-ligand material, siloxane) were also explored to enhance the mechanical robustness and chemical stability of SEI^[146]. In contrast to conventional organic additives, which typically generate polymeric or carbonate-based SEI components through electrochemical decomposition, metal-containing additives offer distinct mechanistic advantages by participating in the formation of inorganic-rich interfacial layers. Specifically, these additives can decompose metal fluorides, oxides, or silicates, which can serve as thermodynamically/mechanically stable and ionically conductive SEI constituents. In certain decomposition routes, metal ions can also be incorporated into the SEI, forming metal-doped or alloy-derived interphases that further suppress interfacial degradation^[147]. These inorganic phases act as effective physical barriers against continuous electrolyte decomposition as well as facilitate more uniform Li⁺ transport across the electrode-electrolyte interface, thereby improving both the electrochemical stability and the mechanical robustness of the SEI under the large volume fluctuations. Thereby, metal-based additives have emerged as promising candidates for stabilizing high-capacity anode materials.

Salt-based additives were employed to enable facile decomposition and direct integration into the SEI structure and to enhance its stability and chemical functionality. Heteroatom boron-nitrogen-containing salts act as effective precursors for artificial SEI formation, improving interfacial crack prevention and ion transport. Among boron-containing salts, lithium bis(oxalato)borate (LiBOB) and lithium difluoro(oxalato)borate (LiDFOB) decompose preferentially at higher potentials than conventional electrolyte solvents, yielding borate, oxalate, and fluoroborate fragments. These decomposition products form a crosslinked, inorganic-rich interphase composed of Li-O-B and B-F networks. Such structures provide high chemical stability, mechanical rigidity, and resistance to further electrolyte reduction^[148]. For instance, lithium metaborate (LiBO₂) and lithium tetraborate (LiB₄O₇) exhibit a Young’s modulus 2 times higher than that of LiF and a lower activation energy for Li⁺ transportation, favorable for high-capacity anodes^[149]. In particular, chemical reduction of LiDFOB promotes the formation of lithium oxalate (Li₂CO₃) and crosslinked oligoborate-based polymers. XPS analysis of SEI formed in LiDFOB-containing electrolytes reveals a higher ratio of B-F to B-O bonding states, consistent with polymeric boron-containing structures and suppressed inorganic LiF content. This composition yields a more elastic and stress-buffering interphase that accommodates large volume changes while limiting electrolyte decomposition^[150]. On the other hand, nitrogen-containing salts such as lithium nitrate (LiNO₃) decompose to generate nitrogen-rich species, including lithium nitride (Li₃N) and NO_x fragments^[151]. Li₃N is an effective electron insulator that suppresses further electrolyte decomposition while enhancing Li⁺ transport across the SEI, thus effectively mitigating continuous electrolyte breakdown. Also, the combined use of B- and N-based salts enables the formation of a conformal and ion-conductive SEI that regulates interfacial reactions and improves long-term stability.

Instead of Li-based salts, multivalent metal (K, Ca, Mg)-based salts have also been reported for incorporating metal components into the SEI^[152]. Multivalent metal salts such as Magnesium(II) Bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) and Calcium(II) Bis(trifluoromethanesulfonyl)imide (Ca(TFSI)₂) contribute to interfacial and bulk stabilization of Si anodes through distinct mechanisms. Ca²⁺ reacts with fluorinated species in the electrolyte to form Calcium fluoride (CaF₂), which precipitates as a compact and chemically stable surface layer. This CaF₂-based SEI acts as a physical barrier that suppresses further electrolyte decomposition and inhibits Li loss during storage. Mg²⁺ shows lower surface reactivity but participates in structural stabilization by partially incorporating into the Si during lithiation. This incorporation promotes the formation of Li-Mg-Si ternary phases, which reduce local lattice distortion and enhance the mechanical stability of lithiated Si. Collectively, these processes result in a thinner and more uniform SEI layer that contains Ca and Mg metal, both enhancing cycle life and reducing undesirable side reactions Figure 8F. Additionally, the combination of surface passivation by Ca-containing SEI and bulk phase stabilization by Mg incorporation leads to improved long-term calendaring aging stability even at the elevated temperatures, particularly under extended voltage hold conditions^[152,153]. Beyond compositional SEI control via salt additives, modifying the surface work function of Si electrodes has been reported as an effective strategy to regulate SEI. The native SiO₂ surface of Si-based anodes exhibits a high work function (~6.24 eV), limiting initial electrolyte reduction and SEI formation. The addition of indium acetate (InAc) additives lowers the local work function through surface adsorption and partial In deposition, thereby promoting faster and more uniform EC reduction. Partial lithiation of In to Li-In phases also serves as an electron sink, enhancing reduction kinetics. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and XPS confirm that InAc enables compact SEI formation, suppressing uncontrolled electrolyte decomposition during cycling and storage.

Since the exhibition of high compatibility with Si due to its repeated Si-O-Si structure, siloxane-based compounds demonstrate particularly notable effects on high-capacity anodes, especially in Si anodes. Fluorinated cyclic siloxanes have been reported as effective interfacial modifiers for Si/C electrodes^[154]. Upon electrochemical reduction, they can undergo ring-opening reactions and subsequently condense with surface hydroxyl groups or native silicon oxide (SiO_x). This process leads to the formation of a siloxane-oxide hybrid network that becomes integrated into the SEI. During this transformation, fluorine atoms are released and can form LiF. Rather than creating a continuous LiF film, these reactions tend to yield LiF domains that are locally distributed. The resulting SEI consists of both organic siloxane-derived components and inorganic species such as Si-O-Si and LiF, leading to a heterogeneous and multiphase interfacial structure. This SEI has been described as exhibiting gradients in composition and mechanical properties, with softer organic regions near the electrolyte and stiffer, inorganic-rich regions closer to the electrode surface^[155]. These gradients may contribute to stress redistribution and suppression of mechanical degradation during volume changes of the Si electrode. Such interfacial engineering strategies demonstrate the tunability of silane-derived SEI architectures in both chemical composition and mechanical performance. Notably, (3-Aminopropyl)triethoxysilane (APTES) forms a surface-bound layer via hydrolysis and condensation, introducing C-N bonding and thermally stable SiO₂ domains, which suppress lithium hexafluorophosphate (LiPF₆) hydrolysis and minimize carbonate solvent decomposition at elevated temperatures^[85]. On the other hand, trialkoxyvinylsilane undergoes reductive decomposition prior to bulk solvent breakdown, forming robust Si-O and Si-F-bonded interphases that suppress solvent consumption and reinforce the SEI against volume fluctuations of Si. TOF-SIMS analysis revealed that silane-derived fragments and localized inorganic species such as LiF and Li_xSiO_y exist within the SEI, suggesting a composite and interpenetrating network. Overall, while these diverse inorganic-based electrolyte additive strategies present challenges regarding non-uniform dispersibility or the potential for excessive decomposition, they offer the advantage of enabling precise control over SEI composition and structure, which can be fundamentally modulated through chemical decomposition reactions. Such mechanistically tailored interphases offer a versatile framework to improve the interfacial structure durability and electrochemical stability of high-capacity anodes under extreme cycling conditions. To achieve ultimate interfacial durability in high-capacity Si anodes, the design of electrolyte additives must evolve toward constructing a spatially graded, organic-inorganic hybrid SEI. This multidimensional approach leverages inorganic species to form a rigid, ion-conductive inner barrier, while utilizing organic/polymeric networks as an elastic outer layer to dynamically dissipate stress during severe volume fluctuations.

Carbon coating strategies

Carbon coating is a simple but effective method for stabilizing the interface of a high-capacity anode by enhancing both mechanical strength and electron-transport properties^[156]. Various carbon materials are employed as coating materials, while the degree of carbonization and thickness of the carbon coating critically affect interfacial properties and electrochemical performance of the anode. For instance, heat treatment temperature directly influences carbon ordering, as low-temperature carbonization results in an amorphous carbon structure, while high-temperature treatment induces the formation of more graphitized, ordered carbon layers. Consequently, as shown in Figure 9A, amorphous carbon significantly reduces the internal resistance of the electrode both in the SEI layer and the charge transfer region during cycling. These results indicate that the elasticity and lower Young’s modulus of amorphous carbon effectively release inner stress and stabilize SEI^[157]. Additionally, while excessive carbon coating impedes electron conductivity due to a disordered isotropic structure, a proper carbon coating layer exhibits increased conductivity and effectively inhibits excessive SEI layer generation and assists in the formation of an inorganic-based SEI layer^[158,159].

Figure 9. (A) DRT function for the Si@C electrode, which was carbonized at different temperatures. Reproduced with permission from ref.^[157]. Copyright 2022, American Chemical Society. (B) Schematic images and TEM images of lithiated Gr-Si NPs for both non-defective and defective graphene encapsulation. Reproduced with permission from ref.^[163]. Copyright 2015, Author(s), licensed under a Creative Commons CC BY license. (C) Schematic cross-sectional view showing hoop stress before cycling and after lithiation by finite element modeling (FEM) calculation during lithiation. Reproduced with permission from ref.^[178]. Copyright 2019, Wiley. (D) Structural change of Li doping into conductive polymer coating layer. Reproduced with permission from ref.^[188]. Copyright 2023 Wiley. (E) Integrating SEI into the conductive coating layer enhanced structural stability. Reproduced with permission from ref.^[187]. Copyright 2022, Wiley.

Unlike conventional carbon coating, graphene coating on the high-Li stoichiometric anode has been researched due to the high electronic conductivity and mechanical properties of graphitic carbon^[160]. Coated graphene supports the intrinsic low conductivity of the Li-alloying anode, which leads to stable SEI generation at the initial cycle, and its high mechanical modulus prevents the SEI layer from cracking^[161]. When graphene nanosheets are applied to tin, graphene additionally buffers the large inner stress and maintains continuous electron pathways, thereby mitigating pulverization and preserving interfacial integrity, while their high surface area and lithiophilic defect sites promote uniform Li nucleation and deposition^[162]. However, due to defect sites in graphene, the layered structure easily collapsed, and fractures of the coating layer occurred due to dramatic volume expansion. To address this limitation, a direct chemical vapor deposition approach was developed to grow defect-free and SiC-free graphene layers on the Si surface Figure 9B. By introducing carbon dioxide (CO₂) as a mild oxidant, interfacial reactions between Si and carbon precursors were suppressed, forming uniform graphene layers parallel to the Si substrate^[163]. These defect-free layers accommodate the ~300% volumetric expansion of Si through a sliding mechanism between adjacent layers that consequently leads to non-defective Gr-Si exhibiting stable layering morphology. Further research was achieved by introducing covalent bonding between graphene and Si^[164]. This approach strengthens interfacial adhesion while maintaining dynamic interlayer mobility. The result is a mechanically robust yet adaptive shell that effectively disperses internal stress and mitigates SEI cracking. Collectively, these strategies demonstrate that controlling the structural integrity of graphene and its interaction with Li-alloying anodes are crucial to stabilizing the electrode-electrolyte interface under repeated cycling conditions.

Additionally, carbon coating effectively suppresses the unstable side reaction of the Si anode during calendar aging^[165]. This protective layer minimizes direct contact between the lithiated Si and the electrolyte, thereby hindering undesired interfacial reactions such as LiPF₆ decomposition and subsequent formation of LiF, Li_xPO_yF_z, or Li₂CO₃^[166]. Moreover, the carbon layer interrupts the internal electron-conduction pathway from Li_xSi to nearby graphite or electrolyte components, blocking local electron transfer that otherwise triggers thermodynamically favorable electrolyte reduction. This suppression delays the onset of side reactions that are activated by increased storage temperature and reduced energy barriers in unprotected Si^[167]. Mechanistically, the carbon coating increases the effective energy barrier (ΔE_a) for electron tunneling and solvent decomposition, thus reducing the likelihood of SEI thickening and continuous electrolyte consumption. By stabilizing interfacial energetics and limiting charge carrier leakage, the carbon layer passivates the surface of Li_xSi and mitigates the spatially inhomogeneous interphase growth that typically accelerates during prolonged rest.

Inorganic coating strategies

Inorganic coatings have emerged as one of the most effective strategies to stabilize Si anodes. These coatings act as mechanical buffers, chemical barriers, and ion-conductive layers, thereby preserving electrode integrity and extending cycle life. The mechanisms of their function depend on the coating material’s chemical composition, structural properties, and interaction with Li and electrolyte components. Metallic coatings such as Ni, Cu, Fe, and Ag form conductive shells around Si particles, promoting electron transport and mechanical confinement^[168]. For instance, Ni coatings undergo interfacial alloying to form NiSi phases via thermal diffusion, which not only serve as conductive anchors but also generate chemically bonded interfaces that resist delamination^[169]. Cu, while electrochemically inert, offers ductility and forms conductive networks that facilitate Li⁺ flux and buffer internal stress^[168]. Ag coatings also significantly improve conductivity and reduce SEI reformation^[170].

In contrast, metal oxides such as Al₂O₃, TiO₂, and SiO₂ form passivation layers with high mechanical strength and chemical stability of the Li-alloying anode interface^[171]. Al₂O₃ and ZnO, with their Lewis acid-base characteristics, can scavenge HF and block electron tunneling, thereby preventing electrolyte decomposition while maintaining Li⁺ permeability through nanochannels^[172]. Moreover, their high Young’s modulus allows them to confine Si expansion, while their porous morphology enables Li diffusion. TiO₂-based coatings demonstrate dual advantages of being highly mechanically robust and chemically stable in Li environments^[173]. In core-shell Si@TiO₂ composites, the hollow void between the Si core and TiO₂ shell accommodates volume change, while the outer TiO₂ shell suppresses electrolyte intrusion and SEI instability^[174]. Moreover, TiO₂ can suppress oxygen evolution from the Si-based anode during overcharging and enhance interfacial stability. These coatings also regulate interfacial Li transport and stress buffering, thereby mitigating fracture-driven delamination and preserving both structural and electrochemical coupling across the interface. Further, composite designs incorporating Si embedded in SiO_x matrices offer synergistic benefits by combining high electrochemical activity with structural integrity by absorbing internal stress and preserving structural cohesion. The oxide matrix serves as a robust scaffold that preserves electrical connectivity and mitigates mechanical stress during volume fluctuations^[175]. A particularly promising direction involves forming artificial SEI layers using SiO_x coating. For instance, engineered Si@SiO_x structures incorporate a sacrificial SiO_x shell that reacts with Li to form a stable interphase composed of Li₂O and lithium silicates (Li₄SiO₄, Li₂SiO₃)^[176]. These compounds form a mixed ionic-electronic conductive network that buffers stress and reduces interfacial impedance. Studies have shown that the optimal thickness of the SiO₂ layer provides a good balance between mechanical strength and Li⁺ permeability^[177], while excessively thick layers hinder Li⁺ diffusion, and too thin layers fail to suppress undesirable SEI formation. Furthermore, the multi-layer lamellar structure of the SiO₂ layer effectively acts as a stress-dissipating layer that relieves the inner stress of the active material Figure 9C^[178]. Another critical inorganic coating is silicon oxycarbide (SiOC), which is typically formed through the pyrolysis of siloxane-based precursors. SiOC consists of an amorphous Si-O-C glassy matrix embedded with free carbon, enabling it to participate in Li storage while simultaneously functioning as a mechanically resilient binder^[179]. By bridging adjacent SiNPs and accommodating their volume fluctuations during cycling, SiOC enhances structural integrity. Furthermore, its dense and chemically inert nature suppresses electrolyte penetration and facilitates the formation of a partially self-healing SEI, contributing to long-term interfacial stability.

Functional organic material coating strategies

In recent work, functional polymer coatings have been widely introduced to modify the interface of high-capacity anodes. Non-conductive polymers such as poly(methyl methacrylate) (PMMA) serve as artificial SEI by physically isolating the highly reactive Si surface from LEs. These coatings suppress continuous electrolyte decomposition and inhibit the formation of thick, resistive interphases. PMMA provides a thin, elastic film capable of coordinating solvated Li⁺ through ester groups, promoting ionic conductivity while accommodating the large volume changes of Si^[180]. In parallel, Li-conducting covalent organic frameworks (COF) have demonstrated promise as pre-defined artificial interphases^[181]. These COF coatings possess highly ordered channels that facilitate directional Li⁺ conduction while simultaneously excluding solvent molecules. By regulating Li⁺ flux and shielding the Si surface from direct exposure to the electrolyte, the COF layer promotes the formation of stable SEI components such as LiF and Li_xSiO_y and suppresses excessive decomposition of carbonate and phosphate solvents. Electrochemical measurements have confirmed the absence of initial cathodic decomposition peaks and improved impedance profiles, indicating effective interfacial passivation and controlled SEI evolution^[182]. These ionically selective and mechanically robust interphases significantly improve the stability of the electrolyte interface and minimize the dynamic instability commonly observed in conventional SEI layers.

Functional polymer coatings offer additional advantages by integrating mechanical compliance with simultaneous ion and electron transport^[183,184]. Such materials utilize conjugated backbones and polar side groups to provide Li coordination, redox activity, and charge delocalization. For instance, in situ-polymerized conducting hydrogels have also demonstrated conformal coating of SiNPs, where the hydrophilic polymer matrix swells in the presence of electrolyte to promote Li⁺ conduction while buffering mechanical stress. The polymeric network functions as a dual ion-conductive and protective barrier that regulates interfacial reactions and delays SEI growth. Additionally, π-conjugated polymers such as poly(thiophene) or poly(3,4-ethylenedioxythiophene) can form polaron or bipolaron states, which can coordinate with Li⁺ during cycling, enhancing Li⁺ transport while maintaining electronic connectivity across the interphase Figure 9D^[185,186]. These features allow the conductive coating to act not only as a protective layer but also as a dynamic ion-conductive matrix. Also, self-layered conductive polymer coating strategies by introducing tungstic acid can integrate SEI components such as LiF and Li₂WO₄. Further, incorporating SEI into the conductive layer leads to homogeneous Li⁺ flux and contributes to the formation of uniform, chemically stable hybrid interphases Figure 9E^[187]. These layers mitigate local overpotentials and block direct electrolyte contact, resulting in well-protected Si surfaces with minimized interfacial degradation. Similarly, sponge-like porous conductive polymer coatings leverage their interconnected pore structure to enhance ion diffusion, while the elastic, conductive layer accommodates Si expansion and maintains electrochemical integrity. These coatings enable spatially uniform SEI formation and reduce impedance buildup during long-term cycling^[188]. Collectively, conductive polymer coatings represent a promising strategy for realizing stable and efficient Si anodes by unifying structural adaptability, ionic selectivity, and electrochemical activity within a single interfacial layer.

Conductive polymer binders such as polyfluorene-based copolymers and their thermally hierarchical ordered structure (HOS) actively regulate the SEI formation process by modulating both interfacial chemistry and Li⁺ transport. The polar side chains and conjugated backbone of conductive polymer binder facilitate strong adhesion to native SiO_x and allow for efficient ionic conduction across the interface^[189]. Upon thermal annealing, the binder undergoes a hierarchical π-π stacking transition into an ordered structure, which reinforces interfacial mechanical stability and promotes selective ion and solvent transport^[190]. This ordered arrangement suppresses uncontrolled decomposition of carbonate solvents and directs SEI evolution toward more stable, inorganic-rich compositions. Specifically, HOS inhibits the excessive generation of Li₂CO₃ and ROCO₂Li while enriching the SEI with LiF and Li_xSiO_y components, as confirmed by XPS. This results in a thinner, denser, and more ionically conductive SEI that effectively suppresses parasitic reactions and enhances long-term cycling. Additionally, modulating the chemical structure of the binder can directly influence SEI chemistry by the spontaneous decomposition of the binder itself^[191]. DFT calculations reveal that the LUMO energy level of the fluorinated binder lies below that of ethylene carbonate (EC), diethyl carbonate (DEC), and even their complexes with LiPF₆, inducing preferential reduction at the binder site during initial lithiation. In particular, sulfonyl fluoride (-SO₂F) groups in the fluorinated binder act as electron-accepting moieties that undergo reductive cleavage at low potentials, releasing F^-, which subsequently reacts with Li⁺ to form LiF. This preferential reduction of the binder occurs before electrolyte solvent decomposition, enabling direct LiF formation through the targeted cleavage of F-containing bonds within the binder structure. Molecular dynamics simulations confirm that the binder-integrated SEI exhibits localized LiF clusters with minimal organic content. Unlike traditional SEI derived from random solvent decomposition, this binder-directed mechanism yields chemically homogeneous and mechanically robust interphases with enhanced ionic conductivity, even under lean-electrolyte additive conditions. In conclusion, the rational design of functional coating materials must converge toward a dual-adaptive interfacial strategy: ensuring structural resilience to withstand severe volume fluctuations, while simultaneously governing SEI evolution through intentional electronic passivation and preferential sacrificial reduction [Table 3].

Interfacial instability of high-capacity anode in the solid-state electrolyte system

Despite the promising gravimetric and volumetric capacities of Li-alloying anodes, their application in ASSBs remains hindered by severe interfacial instability, while SSEs offer several intrinsic advantages. Their solid nature enables physical confinement of active materials, which can assist in suppressing the extreme (~300%) volume expansion during operation and mitigating mechanical pulverization/delamination. In addition, the absence of volatile organic solvents enhances thermal stability and safety. While the ionic conductivity of many SSEs tends to be lower than that of conventional LEs, sulfide-based SSEs exhibit room-temperature Li⁺ conductivities on the order of 10^-3 to 10^-2 S cm^-1, approaching that of LEs^[70]. These attributes make SSEs especially promising for high-capacity Li-alloying anodes in next-generation ASSBs. Nevertheless, despite these advantages, Li-alloying anodes suffer from problems, such as the formation of a space-charge layer due to differences in chemical potential and carrier density between the contacting phases. This electrostatically driven region redistributes mobile ions, creating local deviations in stoichiometry and shifting the onset potentials for interfacial redox reactions. Over repeated cycling, such a non-uniform ion distribution can intensify interfacial impedance growth, promote uneven lithiation, and interact synergistically with the mechanical stress field generated by large volume changes^[40]. Moreover, Si and Sb anodes, for example, the alloying/dealloying process often leaves the particles in an unrecovered morphology after delithiation, forming void-shell structures that elevate the charge transfer resistance^[192]. The repeated contraction not only generates mechanical stress but also accelerates interfacial chemical degradation, as the evolving fracture network exposes fresh reactive surfaces to the SSE, promoting continuous interphase growth and altering its composition over cycling^[193]. As illustrated in Figure 10A, XPS analysis of Si anodes cycled in sulfide-based SSEs reveals dynamic changes in chemical states upon operation^[194]. During the first lithiation, the Si 2p spectra indicate a progressive conversion from native SiO₂, Li₄SiO₄-like silicates, and Li_xSi alloys. Simultaneously, the Li 1s and O 1s spectra reflect the accumulation of Li₂CO₃ and Li-silicate species, suggesting ongoing interfacial reactions between SSEs and the reduced Si surface. Upon delithiation, only partial reversibility is observed, with residual Li-silicate phases persisting at the interface, thus contributing to interfacial irreversibility and impedance growth.

Figure 10. (A) XPS spectra of ASSB Si electrode at the pristine electrode, after first lithiation, and after first delithiation. Reproduced with permission from ref.^[194]. Copyright 2020, American Chemical Society. (B) Pros and Cons of the combination of active material/carbon/electrolyte in ASSB. Reproduced with permission from ref.^[192]. Copyright 2023, Wiley. (C) Side reaction between carbon additive and solid electrolyte. Reproduced with permission from ref.^[40]. Copyright 2024, Author(s), licensed under a Creative Commons CC BY license. (D) Side reaction between Si active material and solid electrolyte. Reproduced with permission from ref.^[4]. Copyright 2021, The American Association for the Advancement of Science.

Figure 10B further delineates how these reactions translate into structural degradation over repeated cycling^[192]. In the pristine Si/SSEs interface, lithiation-induced expansion of Si particles (~300%) exerts mechanical stress on the rigid SSEs matrix, leading to crack formation and loss of interfacial contact. This mechanical detachment impedes both electronic and ionic percolation paths, causing non-uniform lithiation and eventual cell failure. The use of Si-SE-C composites has emerged as a strategy to mitigate this issue by incorporating electronically conductive carbon and elastically compliant binders. These composite frameworks buffer volume change and preserve the electrode-electrolyte interface. During lithiation, such architectures exhibit stable Li⁺ conduction channels and enable more homogeneous Li-Si alloying^[192]. Upon delithiation, the structural integrity of the interface is better maintained, suppressing void formation and delamination. This behavior highlights the chemo-mechanical mismatch between expanding Si and rigid SSEs as a primary cause of capacity fading in ASSBs. The formation of interphases such as Li₄SiO₄ and Li_xSi-S compounds can passivate the interface, but often at the cost of increasing interfacial resistance. Moreover, the low fracture toughness of many sulfide-based SSEs like Li₆PS₅Cl exacerbates mechanical failure, where localized interfacial pressure during cycling exceeds the yield strength of SSEs, leading to irreversible morphological degradation.

Influence of chemical degradation in the solid-state electrolyte system

Chemical degradation at the alloy anode/SSE interface heavily depends on the specific chemistry of the solid electrolyte^[40]. In polymer electrolytes, the highly reducing anodes decompose both the polymer matrix and the lithium salt, generating polyether fragments, RoLi species, and sulfonyl derivatives^[195]. Although the compliant nature of the polymer phase can partially accommodate volume changes to maintain interfacial contact, these labile products form a disordered interphase that continuously increases impedance during cycling. Sulfide electrolytes are particularly susceptible to reduction by Li-rich alloy phases, yielding products such as Li₂S, Li₃P, and Li_xPS_y^[196]. While these species can initially passivate the interface, their progressive accumulation severely impedes Li⁺ transport. In contrast, oxide electrolytes exhibit higher intrinsic electrochemical stability; however, their rigid ceramic nature and thermodynamic mismatch with Li-rich alloy surfaces promote the formation of poorly conformal, insulating products like Li₂O, LiOH, Li₂CO₃, and lithium silicates^[73]. Therefore, polymer, sulfide, and oxide SSEs must be differentiated not only by their bulk ionic conductivity but also by the chemical composition, mechanical traits, and transport resistance of their respective interfacial products.

Among these systems, sulfide electrolyte represents a key case where chemical degradation and mechanical instability are strongly coupled. Notably, recent studies have demonstrated that, contrary to earlier expectations, incorporating SSEs and conductive carbon directly into Si anodes can trigger interfacial instability. During lithiation, the Si undergoes alloying to form Li_xSi, which exhibits strong reducing characteristics. This highly reactive alloy phase initiates the electrochemical decomposition of sulfide-based SSEs, particularly LPSCl^[40]. The reduction is driven by Li atoms migrating from the Li_xSi to the electrolyte interface, where they break the P-S and S-S bonds in the PS₄^3- and S^2- units Figure 10C. As a result, decomposition products such as Li₂S and Li₃P are formed at the interface^[197]. Simultaneously, residual SiO_x on the Si surface reacts with Li to yield Li_xSiO_y and SiO₂. These reactions produce a chemically heterogeneous and electronically insulating interphase that impedes Li⁺ transport. This interfacial layer grows progressively with cycling, following a diffusion-limited process. Electrochemical impedance spectroscopy reveals that the interfacial resistance increases proportionally with the square root of time, consistent with Wagner-type diffusion kinetics. Furthermore, this degradation is more severe under high pressure and prolonged resting at low potentials, where the chemical potential of Li is high. Even though no obvious crystalline products are observed at the Si|LPSCl interface in the initial state, high-resolution TEM and XPS analyses after electrochemical cycling confirm the growth of insulating phases. The sulfur 2p spectrum shows an increasing Li₂S signal with each cycle, and the Si 2p spectrum reveals formation of SiO₂ and Li_xSiO_y through side reactions involving SiO_x. The continuous expansion of this interphase not only blocks ion transport but also induces mechanical delamination by weakening interfacial cohesion, particularly under repeated stress induced by Si volume changes.

In parallel, conductive carbon, which is typically added to enhance electrode electronic conductivity, also contributes to electrolyte decomposition in the ASSB system^[4]. Carbon surfaces with high electronic conductivity and surface area facilitate electron transfer to the sulfide electrolyte. Even at moderate voltages, this promotes the reductive decomposition of LPSCl at the carbon-electrolyte interface Figure 10D. However, unlike active materials, carbon does not undergo alloying or interfacial passivation. Instead, it continuously donates electrons to the adjacent SSEs, maintaining the reduction pathway throughout cycling. This persistent degradation leads to a growing accumulation of resistive species, which infiltrate the percolating network of the composite electrode. The resultant effect is the disruption of continuous Li⁺ conduction pathways and increased ionic and electronic resistance throughout the electrode. Moreover, these side reactions not only degrade the electrochemical performance but also weaken the structural integrity of the composite electrode. Additionally, interfacial degradation originating from both Si and carbon leads to progressive mechanical damage such as particle fracture and delamination within the electrode matrix^[198]. The accumulation of interfacial byproducts and localized stress disrupts the continuous network required for efficient ion and electron transport. To address this issue, applying a conformal sulfide electrolyte coating on the conductive carbon has been shown to effectively suppress direct contact between reactive components. This coating limits electron leakage from carbon and buffers mechanical mismatch at the interface, thereby improving both chemical stability and mechanical cohesion in Si-based composite anodes for sulfide-based ASSBs.

Surface crack propagation

In ASSB systems that employ high Li-stoichiometry anodes, interfacial mechanical instability is a critical degradation pathway due to the combined brittleness of active material and sulfide-based SSEs. Cross-sectional observations reveal distinct crack morphologies associated with different stages of electrochemical cycling Figure 14A and B^[231]. During the initial lithiation stage, volumetric expansion of Si generates compressive stress, leading to the formation of vertical cracks oriented perpendicular to the electrode plane. These cracks are typically confined and do not propagate extensively over subsequent cycles. In contrast, delithiation results in heterogeneous contraction within the lithiated Si matrix along interparticle boundaries, inducing lateral cracks that propagate parallel to the interface^[231]. Upon relithiation, these pre-existing cracks are reactivated and widened due to renewed internal strain, leading to irreversible fracture accumulation. This cyclic process progressively fragments the Si structure, weakens interfacial contact with the electrolyte, and increases cell resistance.

Figure 14. (A) Mechanical fracture mechanisms of the high-capacity anode material electrode in ASSB. (B) Contours of damage evolution in a Li_xSi/LPSC simulation domain with a uniform-thickness electrode and a nonuniform-thickness electrode. (A and B) Reproduced with permission from ref.^[231]. Copyright 2024, Author(s), licensed under CC-BY 4.0 license. (C) Ionic conductivity and electrochemical impedance spectroscopy results of prelithiation, modifying surface properties. Reproduced with permission from ref.^[109]. Copyright 2024, Author(s), licensed under a Creative Commons CC BY license. (D) Binder engineering strategies for stabilizing the Si anode in ASSB and its stress distribution modeling. Reproduced with permission from ref.^[239]. Copyright 2024, Wiley.

The electrochemical degradation of micro-sized Li-alloying anodes in ASSBs arises from a complex interplay among Li⁺ diffusion limitations, internal stress accumulation, and mechanically induced fracture. Tao et al. combine experimental observations with finite element modeling to demonstrate that extended Li⁺ diffusion pathways in Si particles generate pronounced concentration gradients^[234]. These gradients lead to spatially heterogeneous chemical potentials and volumetric expansion, which in turn induce the development of both hydrostatic and Von Mises stress distributions. Hydrostatic stress, which represents the isotropic component associated with volumetric deformation, appears as compressive near the particle surface and becomes tensile toward the interior. On the other hand, Von Mises stress, a scalar measure that reflects the material's tendency to yield under deviatoric stress, is concentrated in regions where the mismatch in lithiation-induced expansion is most significant. When the magnitude of localized Von Mises stress exceeds the mechanical fracture threshold of active material, cracks are energetically driven to form and propagate through the particle thickness and gradually lead to structural disintegration. The experimentally observed spatial distribution of fractures shows a strong correlation with the high-stress regions and confirms a direct correlation between internal stress fields and mechanical failure. As cycling progresses, the crack network expands and disrupts ionic and electronic conduction pathways, which electrically isolate portions of the active material from the electrochemical process. This degradation is further aggravated by the rigid nature of SSEs, which restrict volumetric accommodation and inhibit stress relaxation, thereby accelerating crack formation and growth. Although the application of external pressure is effective at suppressing interfacial delamination, it does not address underlying causes, such as Li diffusion limitations and internal stress development^[39]. Therefore, these results highlight the structural and electrochemical limitations of micro-sized Si in ASSB systems and emphasize the need for nanoscale structural tuning and mechanically adaptive electrode designs to achieve long-term durability.

Interfacial stabilization strategies in the solid-state electrolyte system

To mitigate the severe interfacial issues observed in high-capacity anodes interfaced with SSEs, conformal interfacial engineering has been extensively explored. For instance, the direct contact between highly reactive and volumetrically fluctuating Li-alloying surfaces and sulfide electrolytes often leads to undesirable redox side reactions and interfacial phase reconstruction. One effective interfacial stabilization strategy is designing a thin film-type anode. In particular, atomic layer deposition of tin oxynitride (SnN_y) enables the formation of an amorphous, nanostructured matrix in which Li-Sn alloying occurs within a mechanically compliant Li₃N framework^[235]. This conformal coating uniformly covers complex geometries, eliminating local thickness variations that can serve as crack initiation points. The Li₃N matrix exhibits high Li⁺ diffusivity, which not only maintains homogeneous lithiation but also buffers the volume changes of Sn nanoparticles through its comparatively elastic nature. By preventing particle aggregation and constraining anisotropic expansion, the nitride layer suppresses stress concentration at the SSE interface, thereby mitigating crack propagation and preserving electrode-electrolyte contact over extended cycling. Another strategy involves the deposition of a thin ceramic interlayer between the Si anode and the sulfide electrolyte, such as Li₆PS₅Cl^[236]. This electronically insulating oxide suppresses electronic coupling across the interface, as confirmed by DOS calculations, which show a marked reduction in states near the E_F upon SiO₂ insertion^[194]. The suppression of interfacial electronic conduction reduces the driving force for reductive decomposition of the SSEs, thereby stabilizing the chemical environment at the interface. Additionally, the insulating nature of SiO₂ serves as a physical barrier that prevents interdiffusion while still allowing for sufficient Li⁺ transport. As a result, ceramic-coated Si-based full-cell pairing with an LFP cathode enables stable operation. In addition to interfacial passivation, prelithiation of Si has emerged as a complementary strategy to improve electrochemical compatibility with sulfide SSEs^[109]. Electrochemical impedance spectra indicate that higher degrees of lithiation significantly reduce the interfacial resistance Figure 14C. This reduction is attributed to both enhanced ionic and electronic transport properties of Li-rich silicide phases and their abilities to passivate SEI. Moreover, such prelithiated phases promote more uniform lithiation kinetics and suppress the formation of electrochemically isolated domains, which are prevalent in pristine Si anodes. These effects collectively contribute to improved interfacial charge transfer and mechanical coherence, thereby preventing particle pulverization during cycling in ASSB systems.

Another effective strategy for interfacial stabilization lies in the rational design of functional polymeric binders that accommodate the large volume changes of Si while maintaining chemical compatibility with SSEs^[237]. Similar to LE system binder design strategies, a polymer binder consisting of hard and soft segments can buffer mechanical stress cycling by selectively stretching or contracting its segments^[238]. This elastic response reduces particle detachment and interfacial cracking, thereby preserving structural integrity over long-term cycling. In addition to segmental elasticity-based binders, another class of polymer architecture has been developed to simultaneously address mechanical resilience and interfacial conductivity. For instance, dual-chemically functionalized binders incorporating in situ reduced silver nanoparticles (AgNPs) offer a compelling solution by integrating electronic conduction pathways directly within the polymer matrix. Upon reduction of Ag⁺ precursors dispersed in an ether-rich polymer backbone, a percolated Ag network is formed that bridges adjacent Si particles and conductive additives Figure 14D^[239]. This embedded metallic network significantly enhances the overall electronic conductivity of the electrode, ensuring efficient charge transport throughout the cycling process. More importantly, the AgNP-embedded polymer exhibits superior mechanical flexibility, with fracture strains exceeding 750%, allowing it to conformally adapt to morphological changes in the Si electrode while maintaining intimate interfacial contact. The effectiveness of binders is further validated by stress distribution modeling during progressive charging stages. Si-Ag@poly (acrylic-co-poly (ethylene glycol) methyl ether methacrylate) (PAP) anode maintains a uniform and low-stress environment throughout the formation of various Li_xSi phases, indicating efficient stress dispersion. Thereby, the Si-Ag@PAP anode maintains its capacity over 2,000 cycles at an extremely high C-rate at 5C under low-pressure conditions (5 MPa). In contrast, the Si-PAA anode exhibits severe and localized stress accumulation, with stress levels exceeding 500 MPa as lithiation proceeds toward Li₂₂Si₅^[239]. This high internal stress contributes to mechanical failure, such as crack propagation and particle disconnection. The comparative simulation demonstrates that the functional binder effectively buffers internal stresses and suppresses stress-induced interfacial particle cracks by forming a resilient and conductive polymer matrix.

Interfacial ion transport limitation (contact loss)

Beyond the intrinsic cracking problems of high-Li stoichiometry anode materials, failure to establish and maintain interfacial contact with SSEs during operation is a fundamental limitation in ASSB systems. This contact loss, often initiated by mechanical mismatch during repeated lithiation, results in the formation of electrochemically inactive void regions that disrupt ion and electron transport pathways. As a consequence, lithiation becomes increasingly non-uniform, local overpotentials rise, and Li⁺ tends to accumulate in isolated domains. These effects collectively reduce active material utilization and CE while accelerating structural degradation and capacity fading over cycling. In the case of the Si anode, a relatively dense Li_xSi layer forms near the surface of the Si electrode, preserving close contact with the adjacent SSE during the initial lithiation Figure 15A. However, the first delithiation induces structural rearrangement within the Si, accompanied by inhomogeneous contraction^[40]. As this strain becomes concentrated at specific regions, approximately 2 µm-wide voids begin to emerge where mechanical mismatch disrupts contact. Cross-sectional analyses reveal that these voids arise not from superficial separation but from internal detachment, driven by the accumulation of mechanical stress. With continued cycling, these voids grow larger, exceeding 10 µm after 100 cycles, eventually leading to irreversible disconnection of the electrode-electrolyte interface. This mechanical decoupling leads to electrochemically inactive regions and uneven lithiation patterns, thereby reducing the electrode's active area and degrading coulombic efficiency and capacity retention.

Figure 15. (A) TEM images of contact loss between SSE and electrode during cycling. Reproduced with permission from ref.^[40]. Copyright 2024, Author(s), licensed under a Creative Commons CC BY license. (B) Schematic of plane and side views showing major expansion directions along [110] for <100>, <110>, and <111> wafers. Reproduced with permission from ref.^[240]. Copyright 2023, American Chemical Society. (C) TEM images of mixing prelithiated Si enable suppressing electrode contact loss and active material expansion. Reproduced with permission from ref.^[241] Copyright 2025 Author(s), licensed under a Creative Commons CC BY license. (D) Employing an elastic polymer electrolyte enables maintaining contact without external pressure. Reproduced with permission from ref.^[39]. Copyright 2024, Author(s), licensed under a Creative Commons CC BY license.

The disruption of Li⁺ conduction pathways across the interface stems from both physical detachment and stress-induced damage^[39]. Once interfacial voids form, ion transport becomes spatially heterogeneous, and Li⁺ redistributes unevenly, guided by local gradients in chemical potential and mechanical stress. These gradients promote the formation of microcracks within the Si phase, further expanding the mechanically compromised zone. The damaged interfacial region develops into a diffusion layer that exhibits poor ionic conductivity and diminished electrochemical activity. As a result, interfacial resistance rises, and overall charge-transfer kinetics are hindered. The extent of degradation is also affected by the mechanical characteristics of the SSEs. Thermally densified electrolytes, while initially stable, possess high stiffness and low strain tolerance, making them prone to cracking when Si expands. In contrast, quasi-solid electrolytes can accommodate strain through framework deformation, but suffer from instability in preserving continuous contact under dynamic cycling. These effects underscore the importance of coordinated mechanical design. Maintaining interfacial stability in the Si-based ASSB system requires careful balancing of material rigidity, deformation ability, and pressure-driven stress evolution across the cell.

Strategies for maintaining interfacial contact

Various strategies are applied to the high-capacity anode to maintain interfacial contact in the ASSB system. For instance, engineering the crystallographic orientation of Si wafers has emerged as an effective approach Figure 15B^[240]. Among the available orientations, the <110> direction is particularly advantageous due to its large interstitial spacing, which facilitates rapid Li⁺ diffusion along the wafer thickness. This directional Li⁺ transport minimizes accumulation at the interface, thereby suppressing interfacial stress and mechanical failure. Additionally, surface microstructuring, such as KOH-induced grooving morphology, enhances the physical conformity between the Si electrode and the SSEs. These grooves allow the ductile sulfide-based electrolyte to penetrate and interlock with the electrode surface during cell fabrication, promoting uniform interfacial contact and homogeneous lithiation. As a result, the combination of crystallographic alignment and surface modification plays a critical role in preserving electrode integrity and interfacial stability of ultrahigh areal capacity electrodes (≈ 10 mAh cm^-2) under repeated cycling conditions.

To mitigate interfacial degradation without relying on external pressure, a recent study introduced an engineered interfacial architecture composed of a Li₂₁Si₅-rich layer and a carbon-sulfide composite interlayer Figure 15C^[241]. The Li-rich overlayer, formed through thermal pre-lithiation, alleviates local stress and suppresses the chemical reactivity of pristine Si. Simultaneously, the conductive composite beneath the Si electrode promotes homogeneous current distribution and buffers volumetric strain by accommodating interfacial mismatch. Together, these layers reduce stress concentration and inhibit crack formation, allowing pressure-free cycling while maintaining high capacity retention over 300 cycles. Ultimately, Li₂₁Si₅/Si-Li₂₁Si₅ anode-based full-cell paired with LCO cathode enables stable operation over 1,000 cycles. In parallel, a quasi-solid-state electrolyte based on a dual-network polymer infused with a deep eutectic mixture solvent has been proposed as an internal mechanical stabilizer Figure 15D^[39]. The elastic properties of the polymeric electrolyte provide deformability to accommodate Si expansion and contraction, while preserving continuous contact with the electrode surface. This design obviates the need for stack pressure by enabling reversible structural accommodation during cycling, as evidenced by interfacial stress relaxation in finite-element simulations. The system exhibits stable electrochemical performance over extended cycling under extremely low external pressure conditions (< 1 MPa), highlighting the potential of electrolyte engineering as a complementary strategy to Si-side interfacial control.