Metal Borohydrides as Electrolytes for Solid-State Li, Na, Mg, and Ca Batteries: A First-Principles Study

Metal borohydrides are a family of materials that were recently discovered to have extraordinary ionic conductivities, making them promising candidates as electrolytes for solid-state batteries (SSBs). In fact, various groups have measured the ionic conductivities and assembled batteries using specific borohydrides. However, there are no comprehensive studies assessing the thermodynamic properties or discussing the suitability of metal borohydrides as electrolytes in SSBs, especially for beyond-lithium applications. In this work, we investigate the electrochemical stability, interfacial characteristics, mechanical properties, and ionic conductivities of Li, Na, Ca, and Mg borohydrides using first-principles calculations. Our results suggest that Li and Na borohydrides are unstable at high voltages. However, the corresponding decomposition products, i.e., B12H122--containing phases, have wide electrochemical windows which protect the electrolyte, leading to large electrochemical windows as wide as 5 V. In addition, our simulations indicate that metal borohydrides are ductile, suggesting facile processing. However, their low shear moduli may result in metal dendrite formation. For Ca and Mg borohydrides, while they possess reasonably good electrochemical stability, the low cationic diffusivity may impede their practical use. Finally, the anion rotation barrier was shown to correlate with the superionic phase transition temperature, suggesting that anion mixing may be a potential approach to achieve room-temperature superionic conductivity.