First-Principles Study on the Peculiar Water Environment in a Hydrate-Melt Electrolyte.

Aqueous electrolytes have great potential to improve the safety and production costs of Li-ion batteries. Our recent materials exploration led to the discovery of the Li-salt dihydrate melt Li(TFSI)0.7(BETI)0.3·2H2O, which possesses an extremely wide potential window. To clarify the detailed liquid structure and electronic states of this unique aqueous system, a first-principles molecular dynamics study has been conducted. We found that water molecules in the hydrate melt exist as isolated monomers or clusters consisting of only a few (at most five) H2O molecules. Both the monomers and the clusters have electronic structures largely deviating from that in bulk water, where the lowest unoccupied states are higher in energy than that of the Li-salt anions, which preferentially cause anion reduction leading to formation of an anion-derived stable solid-electrolyte interphase. This clearly shows the role of characteristic electronic structure inherent to the peculiar water environment for the extraordinary electrochemical stability of hydrate melts.