Simultaneously Improved Cubic Phase Stability and Li-Ion Conductivity in Garnet-Type Solid Electrolytes Enabled by Controlling the Al Occupation Sites

Here, we, for the first time, report on the simultaneous enhancement in cubic phase stability and Li-ion conductivity of garnet-type solid electrolytes (SEs) by adding excess Li/Al. The excess Al/Li creates very large grains of up to 170 mu m via the segregation of Al at the grain boundaries and enables preferential Al occupation at 96h sites over 24d sites, a behavior contrary to previous observations. The resulting SE shows improved Li-ion conductivity due to the large grain size and less blocking Li pathway caused by different preferential Al occupation. Surprisingly, it is observed that the cubic phase of the garnet-type SE is transformed to the tetragonal phase on the surface and in the bulk under the applied voltage, and the preferential Al occupation enables its cubic phase stability. Under battery operating conditions, the LLZO SE with excess Li/Al can maintain high ionic conductivity due to the cubic phase stability and large grain size. We clearly demonstrate that the cubic phase stability and ionic conductivity of LLZO can be simultaneously improved by excess Li/Al without any post-treatments. The findings and understanding will provide new insights into practical use of the garnet-type SEs for advanced all solid-state batteries.