Effect of water in a non-aqueous electrolyte on electrochemical Mg2+ insertion into WO3

Magnesium batteries are promising candidates for beyond lithium-ion batteries, but face several challenges including the need for solid state materials capable of reversible Mg2+ insertion. Of fundamental interest is the need to understand and improve the Mg2+ insertion kinetics of oxide-based cathode materials in non-aqueous electrolytes. The addition of water in non-aqueous electrolytes has been shown to improve the kinetics of Mg2+ insertion, but the mechanism and the effect of water concentration are still under debate. We investigate the systematic addition of water into a non-aqueous Mg electrolyte and its effect on Mg2+ insertion into WO3. We find that the addition of water leads to improvement in the Mg2+ insertion kinetics up to 6[H2O] : [Mg](2+). We utilize electrochemistry coupled to ex situ characterization to systematically explore four potential mechanisms for the electrochemical behavior: water co-insertion, proton (co)insertion, beneficial interphase formation, and water-enhanced surface diffusion. Based on these studies, we find that while proton co-insertion likely occurs, the dominant inserting species is Mg2+, and propose that the kinetic improvement upon water addition is due to enhanced surface diffusion of ions.