Unveiling Solid-State Electrochemical Oxidation of LiBH4 and Li2B12H12 for High-Voltage All-Solid-State Batteries

Abstract

Hydridoborates are an emerging class of solid electrolytes that offer high ionic conductivity, low density, solution processability, compatibility with metallic anodes, and high oxidative stability. Notably, certain Li+ or Na+ solid electrolytes, consisting of two different cage-like closo-hydridoborate anion species, are compatible with 4 V-class cathodes by forming a sufficiently ion-conductive, passivating interphase. However, the nature of their electrochemical decomposition products and their dependence on electrochemical potentials remain unclear. In this combined theoretical and experimental study, we demonstrate the solid-state electrochemical oxidation of LiBH4 to Li2B12H12 above 2.0 V vs Li+/Li and provide evidence for the successive oxidation of closo-[B12H12]2- anions to larger H-interconnected closo-clusters. This supports the observed trend that larger clusters formed via oxidation are stabilized at higher electrochemical potentials. Notably, the oxidation process from LiBH4 to Li2B12H12 proceeds through the formation of a highly conductive [BH4]−-[B12H12]2- mixed phase, indicating the potential for in situ formation of mixed-anion hydridoborates directly within all-solid-state cells. These insights into solid-state electrochemical decomposition at the solid-solid interfaces are transferable to other hydridoborate systems, regardless of cation species or anion structures, contributing to developing cathode design strategies for high-voltage all-solid-state batteries.