Tailoring Loose Mg²⁺ Solvation Structure by Steric and Competitive Solvent Coordination for Fast-Charging Magnesium Batteries

Magnesium batteries are attracting growing interest as next-generation energy storage technology due to their high safety, cost-effectiveness, and resource abundance. However, their development remains limited by sluggish Mg²⁺ transport kinetics at the electrode/electrolyte interface. Herein, we propose an electrolyte design strategy that modulates the Mg²⁺ solvation structure by introducing tetrahydrofuran (THF) as a co-solvent into a borate-based electrolyte, Mg[B(hfip)₄] (MBF) in dimethoxyethane (DME). THF, selected from a series of linear and cyclic ethers, has a comparable dielectric constant and donor number to DME, but its cyclic structure introduces steric hindrance that induces competitive coordination with Mg²⁺. This competition weakens Mg²⁺ − solvent interactions, yielding a more labile solvation structure and enhanced desolvation kinetics. As a result, Mg‖Mg cells employing the optimized MBF/1D1T electrolyte (DME: THF = 1:1, v:v) exhibit a significantly reduced Mg plating/stripping overpotential of 120 mV at 10 mA cm⁻², compared with 316 mV at 8 mA cm⁻² with MBF/DME, along with exceptional cycling stability exceeding 1200 h. Furthermore, representative sulfide cathodes such as CuS and VS₄ demonstrate faster activation and improved high-rate performance in the presence of MBF/1D1T.