Excess lithium salt functions more than compensating for lithium loss when synthesizing Li6.5La3Ta0.5Zr1.5O12 in alumina crucible

• LLZTO pellets with different amount of excess Li2CO3 were prepared in Al2O3 crucible.
• Effect of excess Li2CO3 on property of LLZTO pellets was investigated systematically.
• Performance of Li-air batteries using LLZTO pellets as electrolytes was investigated.

Garnet type electrolyte “Li6.5La3Ta0.5Zr1.5O12” (LLZTO) was prepared by conventional solid-state reaction in alumina crucibles and excess lithium salt (from 0% to 50 mol%) was added into the starting materials to investigate the effects of excess lithium salt on the property of LLZTO. SEM, XRD and AC impedance were used to determine the microstructure, phase formation and Li-ion conductivity. Cubic garnet with a minor second phase LiAlO2 in the grain boundary was obtained for the pellets with excess lithium salt. As the amount of excess lithium salt increased, more Al element diffused from alumina crucibles to LLZTO pellets and reacted with excess lithium salt to form liquid Li2O–Al2O3 phase in the grain boundary, which accelerated the pellets' densification and reduced lithium loss at a high temperature. Ionic conductivity of LLZTO pellets increased with the amount of excess lithium salt added and leveled off at ∼4 × 10−4 S cm−1 when lithium salt exceeded 30 mol%. The performance of Li-air batteries with hybrid electrolytes, using homemade LLZTO thin pellets as solid electrolytes, was investigated. The LLZTO thin pellet with more excess lithium salt in starting material had a higher density and resulted in better cell performance.