In Situ Formed Li-B-H Complex Enables High-Performance Solid Electrolyte for Li Batteries

Abstract

All-solid-state Li batteries are considered to be the next-generation high-density energy storage devices. The key material is the solid electrolyte with high Li-ion conductivity and wide electrochemical stability window. LiBH4 is considered as an alternative solid electrolyte due to its high conductivity (10-3 S cm-1 at 120 °C) and good electrochemical stability to Li anode. However, its main disadvantages are the low room temperature conductivity (< 10-7 S cm-1) due to the phase transformation at 110 °C and the poor electrochemical stability at high potential due to the strong reducibility. Searching for new types of Li borohydride beyond LiBH4 with superior Li-ion conducting properties is believed to be an effective strategy to push borohydride-based solid electrolytes towards practical applications. In this work, the Li-B-H complexes are facilely prepared via controllable dehydrogenation of LiBH4 as solid electrolytes for Li batteries. The Li-B-H complex with 7.5 wt% H2 desorption under 3 bar H2 (HP-LiBH2.4) shows exceptionally high Li-ion conductivity (2.7×10-4 S cm-1 at 35 °C) and low activation energy (0.43 eV). The Li-B-H complex possesses the best overall Li-ion conducting properties (in terms of Li-ion conductivity and electrochemical stability) among all borohydride-based solid electrolytes, and thus enables stable cycling of high capacity all-solid-state Li batteries at room temperature. In-depth characterizations demonstrate that the in situ formed interface layer between [Li2B12H11+1/n]n and LiBH4 is believed to be responsible for the high Li-ion conductivity. Moreover, this Li-B-H complex also exhibits excellent electrochemical stability, which enables the stable cycling of Li-TiS2 all-solid-state batteries at room temperature. Our results provide direct evidence for the exceptional Li-ion conductivity and electrochemical stability of Li-B-H complexes. This excellent performance plus the facile fabrication make this class of materials attractive for practical all-solid-state Li battery applications.