Quantification of Contaminants and Proton Exchange in Garnet Phase Powders and Their Impact on Li+ Transport in Hybrid Electrolytes

Abstract Garnet-phase Li7La3Zr2O12 (LLZO) is a promising solid-state electrolyte in lithium metal batteries but suffers from contaminant formation upon exposure to ambient conditions. We use a suite of analytical methods to quantify this contamination and the resulting protonation of Ta-doped LLZO’s (LLZTO) crystal structure and their removal via heat treatment under argon. We quantify contaminant removal in 3 unique temperature regimes: one where LiOH is removed at lower temperatures (<350 °C) and two where Li2CO3 is removed at intermediate (350–550 °C) and high (>550 °C) temperatures. Removal of LiOH and Li2CO3 in the intermediate regime proceeds through a mechanism involving beneficial H+/Li+ exchange with the LLZTO lattice, whereas high-temperature Li2CO3 removal proceeds through the thermal decomposition mechanism of Li2CO3 to evolve CO2 and deposit solid Li2O. Through these quantification methods, we show that most, but not all, contaminants are removed upon heating to 800 °C, with 97% of Li2CO3 and 85% of LiOH present in the as-given materials being removed. We show that heat treatment in a stagnant atmosphere is less effective than treatment under active gas flow. Re-lithiation of the LLZTO lattice is also found to improve Li+ transport across the LLZTO particle interface when immersed in a Li+-bearing organic electrolyte, and the protonation of LLZTO contributes more to slowing interfacial Li+ transport than the presence of interfacial contaminants.