Effect of Adscititious Water on the Mechanism of Mg2+ Intercalation in Tungsten Oxides from Non-Aqueous Electrolytes

Abstract

There is a high demand for low-cost energy storage devices with high energy density and excellent safety. Rechargeable magnesium batteries (RMBs) have the potential to achieve better performance than Li-ion batteries (LIBs) as they allow for the use of a dendrite-free Mg anode, which has about five times the volumetric capacity of the graphite anode in LIBs. One challenge for Mg-ion batteries is the improvement of intercalation kinetics in oxide-based cathode materials. The slow kinetics have been attributed to the higher cation charge/radius ratio of Mg2+­, which results in a high desolvation barrier at the electrode/electrolyte interface and introduces difficult redistribution of charges in the electrode and significant Coulombic interaction between the intercalating species and the host lattice. The addition of water in non-aqueous electrolytes has been shown to improve the kinetics of Mg2+ intercalation, but the mechanism and the effect of water concentration are still under debate. To shed light on these issues, this research focuses on a systematic addition of water into a 0.1 M Mg(ClO4)2 in acetonitrile electrolyte and its effect on Mg2+ intercalation in WO3 and WO3·H2O. Cyclic voltammetry (CV) was used to analyze the electrochemical performance. Solid-state 1H nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma – optical emission spectrometry (ICP-OES), and X-ray diffraction (XRD) were used to study the intercalation mechanisms in the cathodes. Solution 1H and 25Mg NMR and molecular dynamic (MD) simulation were used to study the electrolytes. An improved rate capability as a function of sweep rate and a decreased peak separation of the redox couples for both materials were found as more water was introduced to the electrolyte until the water concentration reached around 14,000 ppm. Based on these studies, we find that the most likely intercalation mechanism in these electrolytes is the co-intercalation of H+ and Mg2+. The findings elucidate how water improves the kinetic performance of Mg2+ intercalation in WO3 and WO3·H2O in non-aqueous electrolytes and is likely general to other oxides benefitting from the addition of water into non-aqueous multivalent electrolytes.