Over the past decade, in solid-state battery research, the academic community has developed an almost “metric-driven obsession” with the ionic conductivity of solid electrolytes. From sulfide and oxide electrolytes to emerging halide-based systems, the ionic conductivity of solid electrolytes has increased from approximately 10^-4 S·cm^-1 to 10^-2 S·cm^-1, and in some cases has even surpassed that of conventional liquid electrolytes^[1,2]. However, a puzzling phenomenon remains: even when extremely high bulk ionic conductivity is achieved, all-solid-state batteries still suffer from severe capacity fading, rapid increases in internal resistance, and even short-circuit failure during practical cycling^[3-7].

This indicates that ionic conductivity is no longer the sole, or even the dominant, bottleneck to the practical application of solid-state batteries. A solid-state battery is not simply an assembly of materials, but rather a complex chemo-mechanical coupling system^[8]. From this perspective, future breakthroughs are unlikely to come merely from discovering materials with higher ionic conductivity. Instead, greater emphasis should be placed on re-examining and rationally designing the coupled “mechanical-chemical” mechanisms within the battery.

Unlike liquid-based batteries, in which electrodes can be effectively wetted by fluid electrolytes and volume variation can be buffered by their intrinsic fluidity, such physical “soft buffering” is essentially absent in solid-state systems^[9-12]. The failure of solid-state batteries can therefore be understood from three interrelated dimensions [Figure 1]^[13,14]. As illustrated in Figure 1A, the large volume variation of Si can progressively disrupt the intimate contact with Li₆PS₅Cl and promote heterogeneous interfacial reactions, resulting in impedance growth and capacity degradation. Figure 1B further indicates that lithium penetration through solid electrolytes is not solely a mechanical process but can be accompanied by local electrochemical corrosion and crack evolution. Similar chemo-mechanical effects have also been observed in Ni-rich cathodes with different particle microstructures^[15]. Polycrystalline particles are more susceptible to intergranular cracking during calendaring and cycling, whereas single-crystal particles better preserve their structural integrity, although anisotropic lattice strain and interfacial contact degradation may still occur. Collectively, these examples demonstrate that chemical decomposition, mechanical damage, and nonuniform ion transport are strongly coupled during the failure of solid-state batteries. First, during ion insertion and extraction, active materials, such as high-nickel layered cathodes or silicon anodes, undergo severe lattice strain^[16-19]. The resulting periodic volume expansion and contraction can induce microcrack formation at the solid-solid interface, leading to interfacial contact loss, a sharp increase in impedance, and a nonuniform distribution of local current density. Second, although high-modulus ceramic electrolytes have traditionally been considered effective in physically blocking lithium dendrite propagation, this view involves an inherent mechanical paradox^[20,21]. A high modulus is often accompanied by high brittleness, and minor processing defects or cracks generated under cyclic stress may instead become low-resistance pathways for dendrite penetration. Third, local mechanical stress can alter the distribution of chemical potential at the interface, thereby fundamentally affecting interfacial ion-transfer kinetics. Such strong mechano-electrochemical coupling is difficult to accurately capture using conventional static electrochemical models.

To address the above challenges, the design logic of solid-state batteries should be fundamentally reconsidered from the following three dimensions [Figure 2].

The success of solid-state batteries should not be regarded merely as a triumph of electrochemical theory, but rather as the result of deep integration between mechanics and materials science. Future research should shift its focus from the sole pursuit of “ion migration rate” toward the mechanical durability of the interface. In particular, multiphysics models coupling electrical, chemical, mechanical, and thermal fields should be integrated with advanced in situ and operando characterization. For example, operando X-ray tomography can reveal crack propagation and interfacial contact loss, in situ X-ray diffraction can track lattice strain and phase evolution, and integrated pressure or strain sensors can monitor stress changes during cycling. These experimentally measured parameters can be used to calibrate and validate models linking local stress, ion transport, and interfacial degradation, thereby enabling failure prediction and rational interface design. Such an integrated approach may help bridge the gap between laboratory-scale demonstrations and practical solid-state batteries.