Instability of the Lithium Garnet Li7La3Sn2O12: Li+/H+ Exchange and Structural Study

We demonstrate for the first time the instability of the lithium garnet Li7La3Sn2O12 toward moisture. In ambient air a spontaneous Li+/H+ exchange occurs leading to Li7-xHxLa3Sn2O12. The lithium released is combined with the atmospheric CO2 under the Li2CO3 form making Li7La3Sn2O12 a CO2 absorbent. The structural study of the exchanged Li7-xHxLa3Sn2O12 phase has been performed on samples obtained by heating at reflux the mother compound in a solution of benzoic acid/ethanol during one week. After chemical analysis of the lithium content and thermal analysis, the compound has been structurally characterized by the Rietveld method from powder and single crystal XRD experiments. It was found to crystallize in the cubic space group Ia (3) over bard with a Li+ distribution comparable to that observed in other cubic lithium garnets: three lithium sites partially occupied (24d, 48g, and 96h). The total ionic conduction of the exchanged garnet has been studied by impedance spectroscopy and compared to that of the mother form Li7La3Sn2O12. This study revealed that after Li+/H+ exchange the conduction remains unchanged.