Operando X‐Ray Computed Tomography Reveals the Role of Interfacial Nucleation Nanolayers in Suppressing Mechanical Failure in Zero‐Excess Lithium All‐Solid‐State Batteries

Lithium metal (LM) and zero-excess lithium (ZE) anodes offer pathways to increase the energy density of all-solid-state batteries (ASSBs). We employ operando X-ray computed tomography combined with an image subtraction method to visualize lithium plating/stripping morphology, stack mechanical failure, and quantify the lithium reversibility in asymmetric Li₆PS₅Cl (LPSC)-based ASSBs. Lithium metal counter electrode (CE) and copper (Cu) working electrode (WE) emulate LM and ZE interface configurations, respectively. We compare bare Cu and silver-coated Cu (Ag/Cu) WEs under varying current densities. At 0.25 mA cm^-2 _(WE), bare Cu shows edge-localized and non-uniform lithium deposition, while Ag/Cu facilitates more uniform lithium spreading, but results in higher first-cycle irreversibility and lower Coulombic efficiency. Above 0.5 mA cm^-2 _(WE), failure in Li|LPSC|Cu cells initiate at the LPSC|Cu interface via spallation cracks. In contrast, Ag preserves interface integrity at the WE despite lithium initially plates at discrete nucleation spots. However, failure shifts to the Li|LPSC interface, where non-uniform lithium depletion at the CE exposes the underlying Cu, leading to spallation cracks upon subsequent plating. Mechanical finite element simulations support these observations and underscore the critical role of the nucleation layers in mitigating mechanical failure. This study highlights interface engineering as a key strategy to address electro-chemo-mechanical degradation in LM- and ZE-ASSBs.