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

•Dense Li7La3Zr2O12-based composite electrolytes were fabricated at room temperature without sintering.•LiBH4 served as an excellent filler to form the composite electrolyte.•No secondary and interfacial phases were detected at the MAS-NMR level.•The composite showed 670 times higher room-temperature Li+ conductivity than that of LiBH4.•The current tolerance against lithium filament growth was comparable to a well-sintered Li7La3Zr2O12 bulk.