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.