Magnesium Battery Electrolytes with Improved Oxidative Stability Enabled by Selective Solvation in Fluorinated Solvents

Practical Mg batteries require electrolytes that are stable both toward reduction by Mg metal and oxidation by high voltage cathodes. State-of-the-art Mg electrolytes based on weakly coordinating Mg salts utilize standard ether-type solvents (usually glymes) due to their reductive stability. However, the oxidative stabilities of these solvents are less than ideal, leading to difficulties in realizing the high oxidative stabilities of recently developed salts. On the other hand, alternative solvents with greater oxidative stability are typically unable to support Mg cycling. In this work, we report a selective solvation approach involving the combination of glyme and hydrofluoroether solvents. Selective solvation of Mg2+ by the glyme solvent component increases the oxidative stability of the glyme while maintaining sufficient reductive stability of the non-coordinating hydrofluoroether. We show that this approach enables the design of electrolytes with greater oxidative stability than glyme-only electrolytes while retaining enough reductive stability to cycle Mg metal. We also relate the influence of various coordination interactions among the solvents and anions with Mg2+ to their electrochemical stabilities to better inform the design of future electrolytes.

Automated summary

Practical Mg batteries require electrolytes with stability towards both reduction by Mg metal and oxidation by high voltage cathodes. State-of-the-art Mg electrolytes based on weakly coordinating Mg salts use ether-type solvents for reductive stability, but their oxidative stabilities are not ideal. In this work, a selective solvation approach using a combination of glyme and hydrofluoroether solvents is reported to improve the oxidative stability while maintaining the reductive stability necessary for Mg cycling. The study also relates the coordination interactions among solvents and anions with Mg2+ to inform the design of future electrolytes.