Challenges of the infiltration method for halide-based solid‑state battery cathodes

Halide solid electrolytes are promising materials for scalable solid-state batteries, but their compatibility with conventional wet-processing methods is still not well understood. In this work, we examine the challenges that arise when a solvent-based infiltration approach is applied to poly crystalline NCM622 cathodes using Li₃YCl₄Br₂ electrolyte solutions. We find that the electrolyte undergoes dissolution and reprecipitation in ethanol and DI water, which significantly lowers its ionic conductivity and leads to the formation of large agglomerates. These solvent-induced structural and morphological changes limit the ability of the electrolyte to penetrate the porous NCM cathode and result in uneven surface coverage. Furthermore, interactions between the solvent, electrolyte, and NCM622 trigger interfacial side-reaction products originating from the partial dissolution of cathode surface species. These reactions increase resistance, disrupt contact between the active material and electrolyte, and ultimately hinder Li-ion transport, causing severe capacity fading. Thus, this study clarifies the fundamental incompatibilities between halide electrolytes and wet-infiltration processing and provides guidance for improving solvent selection, electrode porosity, and interfacial stability in future solid-state battery designs.