Operando neutron radiography reveals calendar aging-induced lithium redistribution and back-diffusion in thick-cathode all-solid-state batteries

Calendar aging remains one of the most critical yet poorly understood failure modes in sulfide-based all-solid-state batteries, limiting their reliability and practical deployment. Capacity loss during storage and sustained high-voltage operation is closely linked to lithium redistribution within composite cathodes, where lithium may become immobilized at reactive interfaces, trapped within crystalline domains of active materials, or transport-limited in over-oxidized catholyte regions. However, direct, spatially resolved visualization of this process has remained largely unavailable. Here, we employ operando neutron radiography at Oak Ridge National Laboratory to track lithium transport in real time during open-circuit relaxation and high-voltage holding in high-loading NMC–Li₆PS₅Cl solid-state cells. Open-circuit relaxation proceeds through an initial kinetically controlled stage followed by diffusion-driven equilibration. In contrast, voltage holding maintains a persistent potential gradient that continuously drives lithium migration. Quantitative analysis of spatiotemporal lithium distributions further reveals a solid-state back-diffusion component during voltage holding, indicating that lithium transport is not strictly unidirectional. Moreover, formation protocols designed to maximize first-cycle efficiency through extended voltage holding accelerate irreversible lithium immobilization and compromise subsequent cycling stability. These findings establish calendar aging as a coupled transport–reaction process and highlight the importance of formation strategies that prioritize long-term durability over short-term efficiency.