Electrochemical reductive stability of lithium hydridoborate solid-state electrolytes

Hydridoborates are promising electrolytes for next-generation all-solid-state batteries due to their high ionic conductivity, low density, and chemical stability. To integrate hydridoborates into high-energy-density solid-state batteries, understanding their stability at low potential, i.e., at the anode-electrolyte interface, is essential. Experimental studies, based on linear sweep voltammetry and symmetric cell cycling, have reported apparent electrochemical stability in contact with lithium metal for both closo- and carba-closo-hydridoborates. Here, we re-evaluate the reductive stability of the mixed-anion carba-closo-hydridoborate solid electrolyte Li3(CB11H12)2(CB9H10) using impedance spectroscopy, coulometric titration time analysis, and voltammetry at low scan rates and at an elevated temperature of 60 °C. Our experimental results reveal that the Li3(CB11H12)2(CB9H10) electrolyte is not thermodynamically stable down to 0 V vs. Li+/Li. We confirm our experimental results using first-principles density functional theory and further predict that the closo-hydridoborates Li2B10H10 and Li2B12H12 are likewise unstable but offer wider stability windows compared to the carba-closo-hydridoborates LiCB9H10 and LiCB11H12. Room-temperature measurements show very slow decomposition kinetics of the Li3(CB11H12)2(CB9H10) electrolyte, explaining the apparent stability observed in earlier studies. In contact with a lithium metal anode, the cyclable areal capacity is limited by inhomogeneous lithium stripping/plating, which can be avoided with silicon electrodes. Our results lay the foundation for the rational integration of lithium hydridoborates into high-energy-density all-solid-state batteries.