Role and Evolution of FeS₂ Cathode Microstructure in Argyrodite-Based All-Solid-State Lithium–Sulfur Batteries

All-solid-state lithium–sulfur batteries (ASSLSBs) are emerging as a promising alternative for green energy storage, offering high theoretical capacities and energy densities by using inexpensive materials. To date, ASSLSBs commonly suffer from poor cycle life and sluggish reaction kinetics. A promising active material for ASSLSBs is iron disulfide, FeS2, due to its natural abundance, low cost, and high theoretical capacity (894 mAh·g–1). It undergoes a displacement reaction with significant volume changes whose effects can be locally constrained by using small particles. Here, the influence of the positive electrode microstructure on the electrochemical performance of FeS2-based ASSLSBs with Cl-rich argyrodite, Li5.5PS4.5Cl1.5, a mechanically soft sulfide solid electrolyte with high ionic conductivity, is investigated. Composites with different microstructures were prepared using three different processing methods (i.e., hand grinding, ball mill, and mini mill). Their impact on the electrochemical performance was evaluated, revealing that homogeneously submicro-structured composites achieve higher capacities (up to 4.28 mAh·cm–2) and capacity retention (87.2% at the 50th cycle). Furthermore, finely structured composites enhance the in situ formation of active material from the solid electrolyte and increase its accessible reversible capacity. Ex situ analyses (i.e., SEM-EDS and XPS) at different states of charge show that the morphology of FeS2 evolves forming core–shell like submicro-structures.