Abstract
The goal of this article is to highlight crucial breakthroughs in solid-state ionic conduction in borohydrides for battery applications. Borohydrides, M(z+)B(x)H(y), form in various molecular structures, for example, nido-M(+)BH(4); closo-M(2+)B(10)H(10); closo-M(2+)B(12)H(12); and planar-M(6+)B(6)H(6) with M = cations such as Li(+), K(+), Na(+), Ca(2+), and Mg(2+), which can participate in ionic conduction. This overview article will fully explore the phase space of boron-hydrogen chemistry in order to discuss parameters that optimize these materials as solid electrolytes for battery applications. Key properties for effective solid-state electrolytes, including ionic conduction, electrochemical window, high energy density, and resistance to dendrite formation, are also discussed. Because of their open structures (for closo-boranes) leading to rapid ionic conduction, and their ability to undergo phase transition between low conductivity and high conductivity phases, borohydrides deserve a focused discussion and further experimental efforts. One challenge that remains is the low electrochemical stability of borohydrides. This overview article highlights current knowledge and additionally recommends a path towards further computational and experimental research efforts.