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 (XCT) combined with an image subtraction method (ISM) to investigate failure mechanisms in asymmetric Li6PS5Cl (LPSC)-based ASSBs, using lithium counter electrode (CE) and copper (Cu) working electrode (WE) to emulate LM and ZE configurations. ISM enables real-time visualization of lithium plating/stripping morphology, stack structure evolution, and lithium reversibility quantification. We compare bare Cu and silver-coated Cu (Ag/Cu) WEs under varying current densities. At 0.25 mA cm-2(WE), both cells cycle reversibly. However, bare Cu shows edge-localized lithium deposition attributed to uneven interfacial contact, while Ag/Cu facilitates more uniform lithium spreading, but leads to a higher irreversible lithium amount and lower Coulombic efficiency. Above 0.5 mA cm-2(WE), both cells short-circuit. In Li|LPSC|Cu, failure initiates at the LPSC|Cu interface via spallation cracks. In Li|LPSC|Ag/Cu, Ag preserves interface integrity at the WE despite lithium initially plating at discrete nucleation spots. However, failure shifts to the Li|LPSC interface due to the non-uniform Li depletion at the CE exposes the underlying Cu, leading to spallation cracks upon subsequent plating. This finding underscores the critical role of the nanoscale nucleation layers in mitigating mechanical failure in ZE-ASSB and highlight interface engineering as a key strategy to address electro-chemo-mechanical degradation in LM and ZE ASSBs.