Investigation of the stability of metal borohydrides-based compounds LiM (BH^sub 4^)^sub 3^Cl (M = La, Ce, Gd) as solid electrolytes for Li-S batteries

The LiM(BH4)3Cl (M = La, Ce, Gd) compounds are prepared by ball-milling followed by annealing and their electrochemical stabilities are investigated. To validate the use of these compounds as solid-state electrolytes, their stabilities versus Li and Li-In alloy are tested in symmetrical cells and their electrochemical stability windows are studied by cyclic voltammetry. We show that LiCe(BH4)3Cl is the more stable phase without any formation of a resistive layer upon cycling. All-solid-state Li-S batteries using a carbon-sulfur composite as the positive electrode material are then assembled using LiCe(BH4)3Cl as the electrolyte. Reversible electrochemical reaction between Li and sulfur takes place at 45 °C with an initial discharge capacity of 1186 mAh/g of S under a current density of 13 μA/cm2 (i.e. a rate of charge/discharge of C/100). The capacity retention is significant with still a value of 510 mAh/g after 9 cycles showing for the first time the possible use of LiCe(BH4)3Cl as solid electrolyte of Li-S batteries.