Rate Limitations in Composite Solid-State Battery Electrodes: Revealing Heterogeneity with Operando Microscopy

Solid-state batteries (SSBs) show promise for improving energy density, cycle life, and safety. However, when active material particles are mixed with a solid electrolyte phase, the rate capability of the resulting composite electrode is often limited. As a consequence, trade-offs between energy and power density arise, especially in thick electrodes. Herein, we fabricate graphite/Li6PS5Cl composite electrodes with varying active material fraction and thickness as model systems to probe the mechanisms that limit rate capability in composite SSB electrodes. Using operando optical microscopy, spatial variations in the local state-of-charge of graphite that arise as a result of current focusing are directly observed. Pairing these results with simulations, we identify the electrode properties that limit rate performance, including the electrostatic potential drop within the tortuous solid electrolyte phase and solid-state diffusion within the graphite domains. The results highlight the critical role of microstructure in designing composite SSB cathodes and anodes.