Electrochemical Activation of LiGaO: Implications for Ga-Doped Garnet Solid Electrolytes in Li-Metal Batteries.

Ga-doped LiLaZrO garnet solid electrolytes exhibit the highest Li-ion conductivities among the oxide-type garnet-structured solid electrolytes, but instabilities toward Li metal hamper their practical application. The instabilities have been assigned to direct chemical reactions between LiGaO coexisting phases and Li metal by several groups previously. Yet, the understanding of the role of LiGaO in the electrochemical cell and its electrochemical properties is still lacking. Here, we are investigating the electrochemical properties of LiGaO through electrochemical tests in galvanostatic cells versus Li metal and complementary studies via confocal Raman microscopy, quantitative phase analysis based on powder X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and electron energy loss spectroscopy. The results demonstrate considerable and surprising electrochemical activity, with high reversibility. A three-stage reaction mechanism is derived, including reversible electrochemical reactions that lead to the formation of highly electronically conducting products. The results have considerable implications for the use of Ga-doped LiLaZrO electrolytes in all-solid-state Li-metal battery applications and raise the need for advanced materials engineering to realize Ga-doped LiLaZrOfor practical use.