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

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 LiBH 4 to Li 2 B 12 H 12 above 2.0 V vs Li + /Li and provide evidence for the successive oxidation of closo-[B 12 H 12 ] 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 LiBH 4 to Li 2 B 12 H 12 proceeds through the formation of a highly conductive [BH 4 ] − − [B 12 H 12 ] 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 highvoltage all-solid-state batteries.