Sintering-free preparation of Li7La3Zr2O12–LiBH4-based solid-state electrolytes and their electrical conductivities

Abstract

Solid-state electrolytes with sintering-free characteristics and compatibility with Li metal anodes are desirable for increasing the energy density of lithium-ion secondary batteries. In this study, we developed a composite-type electrolyte two Li-metal compatible materials: conductive Li7La3Zr2O12 (LLZ) and sintering-free LiBH4. Preparation of LLZ powder by hand milling, rather than ball milling, was beneficial to produce dense LLZ–LiBH4 composite. The LLZ–LiBH4 composited mixed by hand did not exhibit the formation of secondary phases at a magic-angle-spinning nuclear magnetic resonance level. Up to a volume fraction of LLZ to 0.8, LiBH4 filled the space between LLZ particles, and the total ionic conductivity increased with increasing LLZ fraction. The LLZ–LiBH4 composite showed 670 times higher room-temperature ionic conductivity at the optimum composition than that of LiBH4 alone. The Li+ conduction behavior of the LLZ–LiBH4 composite changed from a percolation model to a series resistance model by the phase transition of LiBH4 to a high-temperature conductive phase. Furthermore, the current tolerance of Li|LLZ–LiBH4 composite|Li cell was comparable to that of a cell using a well-sintered LLZ electrolyte, suggesting that this composite can be used with Li metal anode.