Effects of LiBF4 Addition on the Lithium-Ion Conductivity of LiBH4

Complex hydrides, such as LiBH4, are a promising class of ion conductors for all-solid-state batteries, but their application is constrained by low ion mobility at room temperature. Mixing with halides or complex hydride anions, i.e., other complex hydrides, is an effective approach to improving the ionic conductivity. In the present study, we report on the reaction of LiBH4 with LiBF4, resulting in the formation of conductive composites consisting of LiBH4, LiF and lithium closo-borates. It is believed that the in-situ formation of closo-borate related species gives rise to highly conductive interfaces in the decomposed LiBH4 matrix. As a result, the ionic conductivity is improved by orders of magnitude with respect to the Li-ion conductivity of the LiBH4, up to 0.9 × 10-5 S cm-1 at 30 °C. The insights gained in this work show that the incorporation of a second compound is a versatile method to improve the ionic conductivity of complex metal hydrides, opening novel synthesis pathways not limited to conventional substituents.