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H$_2$O and CO$_2$ Surface Contamination of the Lithium-Stuffed Garnet

Understanding the reactivity of ubiquitous molecules on complex oxides has broad impacts in energy applications and catalysis. The garnet-type Li$_7$La$_3$Zr$_2$O$_{12}$ is a promising solid-state electrolyte for lithium(Li)-ion batteries, and it readily reacts with H$_2$O and CO$_2$ when exposed to ambient air. Such reactions form a contamination layer on Li$_7$La$_3$Zr$_2$O$_{12}$ that is detrimental to the battery operations. The strong interactions of Li$_7$La$_3$Zr$_2$O$_{12}$ with H$_2$O and CO$_2$, however, make Li$_7$La$_3$Zr$_2$O$_{12}$ a promising support to catalyze H$_2$O dissociation and CO$_2$ adsorption. Here, using first-principles calculations, we investigate the adsorption and reactions of H$_2$O and CO$_2$ on a Li$_7$La$_3$Zr$_2$O$_{12}$ surface. We show that H$_2$O reacts through the exchange of proton and Li$^{+}$ and produces metal hydroxide species. At high H$_2$O coverage, half of the H$_2$O molecules dissociate while the other half remain intact. CO$_2$ reacts with the Li$_7$La$_3$Zr$_2$O$_{12}$ surface directly to produce carbonate species. We clarify that the individual reactions of H$_2$O and CO$_2$ with Li$_7$La$_3$Zr$_2$O$_{12}$ are more thermodynamically favorable than the co-adsorption of H$_2$O and CO$_2$. Finally, we demonstrate that low temperature and high partial pressure promote the reactions of H$_2$O and CO$_2$ with Li$_7$La$_3$Zr$_2$O$_{12}$. For energy storage application of Li$_7$La$_3$Zr$_2$O$_{12}$, our study guides processing conditions to minimize surface contamination. From a catalysis point of view, our findings reveal the potential of using complex oxides, such as Li$_7$La$_3$Zr$_2$O$_{12}$ as a support for reactions requiring H$_2$O dissociation and strong CO$_2$ adsorption.