Electrolyte-free cathode design for solid-state batteries demonstrated with bifunctional Li2VCl4

All-solid-state batteries have attracted much attention because of the expected high energy density and inherent safety stemming from their nonflammable property. While improving the energy density of the cathode poses a significant challenge, here we introduce a novel battery design strategy to enhance energy density by employing bifunctional cathode material, allowing the weight ratio of the active material to be increased without using an electrolyte for the cathode. By employing lithium-containing vanadium halide Li₂VCl_4, serving as both active material and electrolyte, the all-solid-state battery cell with no electrolyte for the cathode with a capacity approaching the theoretical limit is demonstrated. In addition, we present a guideline for improving capacity retention from the perspective of interfacial stability. Notably, thermodynamic analysis revealed interfacial instability between Li₂VCl₄ and sulfide material. A double-layer separator, incorporating halide materials for the cathode side, was implemented to enhance the interfacial stability and mitigate the capacity degradation. Furthermore, it was found that the rate capability depends on the lithium content in synthesized Li_2-xVCl₄ and does not change with the state of charge significantly. This study will contribute to designing the bifunctional cathode material for an all-solid-state battery and describe its unique properties.