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.