EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Lithium borohydride solid-state electrolytes (SSEs) have recently drawn considerable scientific interests owing to the fast Li-ion conduction in their high-temperature hexagonal phase (h-LiBH4), which can be preserved down to room temperature (RT) by partially substituting for [BH4]- with halides such as I-, Cl- and Br-, resulting in RT conductivities in the order of 10-4 S cm-1. Further addition of sulphides forming a glassy borohydride-halide-sulphide composite can increase the RT conductivity to ~10-3 S cm-1. The light-weight LiBH4 provides a good ductility for cells engineering and forms chemically stable interfaces against lithium metal, hence benefiting the development of solid battery systems with high specific energy.

We have previously investigated the Li(BH4)0.75I0.25-Li2S-P2S5 and LiBH4‧LiCl‧P2S5 composite systems for solid-state Li-ion batteries. With high RT Li-ion conductivity of ~ 10-3 S cm-1 as well as promising capacity retention and Coulombic efficiency in the initial cycles, the total cycle number of cells using Li metal and TiS2 electrodes is limited. The complex interface issues, such as the slow transport kinetics across the solid-solid interface, the formation of Li dendrites along the voids in SSEs, and the contact loss due to lattice change (~ 10%) in the layered LixTiS2, are considered responsible.

In this work, we present our recent results dealing with the LiBH4-based solid batteries, and focusing on the interface diagnoses and optimizations to achieve longer cycle life. Composites with varying LiBH4-halide-sulphides ratios are characterized to select a system with the best conductivity. Interface engineering and alloying are applied to improve the SSEs/Li metal contacts. Disordered rock-salt cathode materials that exhibit small lattice change upon lithiation/delithiation are demonstrated for their suitability and compared with the layered TiS2. In addition, post-cycle SEM, XRD and XAFS will be used to monitor the evolution of materials chemistry and morphology.