Glycol-glyme co-solvent electrolytes enable high-capacity and ultrastable VO2 cathodes in magnesium ion batteries

Rechargeable magnesium batteries (RMBs) are regarded as cost-effective candidates for post-lithium-ion batteries. However, the development of RMBs is hindered by the lack of high-capacity cathodes due to the sluggish Mg²⁺ desolvation at cathode-electrolyte interface and the TFSI^--induced surface passivation in the regular Mg(TFSI)₂/1,2-dimethoxyethane (DME) electrolyte. Herein, we introduced a hydroxyl-rich ethylene glycol (EG) solvent into the ether-based electrolyte to disrupt the unfavorable [Mg(DME)₃]²⁺ complexes and build hydrogen bond networks to faciliate Mg ion migration and suppress TFSI^- decomposition simutaneously. Consequently, the co-solvent electrolyte demonstrates a high reversible capacity of 258 mAh g⁻¹ for VO₂ cathodes with an extremely low capacity degradation rate of 0.0078 % per cycle over 2000 cycles at 500 mAg⁻¹, which rivals the state-of-the-art cathode performance in Mg ion batteries. Practical application of this new electrolyte is evidenced by the high capacities of above 160 mAh g⁻¹ over 50 cycles for the Mg//VO₂ full cells. This work sets a new frontier for effective electrolytes in RMBs with long life and high energy densities.