Water in Ionic Liquid for Electrochemical Li Cycling

A solution of water in ionic liquid was tested as a practical electrolyte for Li–air batteries. Li metal deposition and stripping were repeatedly performed with an acceptable polarization voltage (<±250 mV) even under 1 vol % of water in the ionic liquid electrolyte. Such an advantageous performance was explained by XPS surface analysis on a Li metal after cycling and the thermodynamic state of water in ionic liquid as a bulk electrolyte. Remarkably, the chemical species (LiF and LiOH) forming at the Li/electrolyte interface did not evolve through the addition of water, suggesting that water did not exist as free water that intensively reacts with Li metal. Furthermore, free water was not thermally observed in the ionic liquid electrolyte, indicating that water may be bound to the electrolyte. The water in an ionic liquid will become a new avenue to design the water-tolerant electrolyte against Li metal.