Visualizing Diverse Lithium Growth and Stripping Behaviors in Anode-Free Solid-State Batteries with Operando X-ray Tomography

Anode-free solid-state batteries could have high energy densities and simplified manufacturing since excess lithium metal is not needed during cell assembly. However, the factors that control lithium growth/stripping at the anode current collector are not well understood. Here, we use operando X-ray microcomputed tomography to comprehensively image and quantify lithium deposition and stripping under various conditions in three different Li|Li6PS5Cl|current collector cells, revealing diverse behavior. A cell with high impedance exhibits extensive lithium filament growth, with filaments that grow around pre-existing pores in the solid-state electrolyte (SSE) rather than lithium filling these pores. Lithium filament formation is partially reversible, with the cracks shrinking as lithium metal is stripped. Uniform lithium deposition is achievable at low current densities in low-impedance cells, whereas higher current densities cause an increase in interfacial roughness, which is correlated with filamentary growth. These results provide insight into filamentary vs. planar lithium growth and highlight that the evolution of lithium is sensitively dependent on SSE microstructure and electrochemical process.