Anion‐Incorporated Mg‐Ion Solvation Modulation Enables Fast Magnesium Storage Kinetics of Conversion‐Type Cathode Materials

Abstract Rechargeable magnesium batteries (RMB) have emerged as one of the most promising alternatives to lithium‐ion batteries due to the prominent advantages of magnesium metal anodes. Nevertheless, their application is hindered by sluggish Mg‐ion storage kinetics in cathodes, although various structural modifications of cathode materials have been performed. Herein, an electrolyte design using an anion‐incorporated Mg‐ion solvation structure is developed to promote the Mg‐ion storage reactions of conversion‐type cathode materials. The addition of the trifluoromethanesulfonate anion (OTf ⁻ ) in the ether‐based Mg‐ion electrolyte modulates the solvation structure of Mg ²⁺ from [Mg(DME) ₃ ] ²⁺ to [Mg(DME) _2.5 OTf] ⁺ (DME = dimethoxy ethane), which facilitates Mg‐ion desolvation and thus significantly expedites the charge transfer of the cathode material. As a result, the as‐prepared CuSe cathode material on copper current collector exhibits a considerable increase in magnesium storage capacity from 61% (228 mAh g ^‐1 ) to 95% (357 mAh g ^‐1 ) of the theoretical capacity at 0.1 A g ^‐1 and a more than twofold capacity increase at a high current density of 1.0 A g ^‐1 . This work provides an efficient strategy via electrolyte modulation to achieve high‐rate conversion‐type cathode materials for RMBs.