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

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 Mg2+ 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. The incorporation of the trifluoromethanesulfonate anion in the Mg-ion solvation structure of the borate-based Mg-ion electrolyte enables the fast magnesium storage kinetics of the conversion-type cathode materials. The as-prepared copper selenide cathode achieved a more than twofold capacity increase at a high rate and the highest reversible capacities compared to those of the previously reported metal selenide cathodes.

Automated summary

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 in the Mg-ion electrolyte modulates the solvation structure of Mg2+, facilitating Mg-ion desolvation and significantly expediting the charge transfer of the cathode material. This research provides an efficient strategy for achieving high-rate conversion-type cathode materials for rechargeable magnesium batteries.