Strategy for Ultrafast Cathode Reaction in Magnesium-Ion Batteries Using BF₄ Anion Based Dual-Salt Electrolyte Systems: A Case Study of FePO₄

Mg 2+ secondary batteries are remarkably safe, resourceful, and exhibit high energy density. However, the excessively slow reaction kinetics at Mg 2+ -battery cathode materials results in charge–discharge over 10–60 h at room temperature, hindering the performance evaluation and mechanistic analysis of the electrode materials. In this study, we developed a dual-salt electrolyte comprising a conventional magnesium salt, magnesium bis(trifluoromethanesulfonyl)imide [Mg(TFSA) 2 ], and a quaternary ammonium salt, spiro-(1,1′)-bipyrolidinium tetrafluoroborate (SBPBF 4 ), for achieving high-rate performance in the cathode reaction. In a charge–discharge test conducted using a highly defective FePO 4 cathode, the dual-salt system [0.5 M Mg(TFSA) 2 + 0.5–2.0 M SBPBF 4 ] showed a high capacity of over 150 mAh g –1 at 0.5C-rate, even at room temperature. In situ X-ray absorption fine structure measurements demonstrated the Fe 2+ /Fe 3+ redox reaction of the FePO 4 cathode during the charge–discharge, whereas Raman analysis and molecular dynamics simulation indicated that the multiple-anion-coordinated [Mg 2+ –BF 4 – ] structure was more effective in facilitating Mg 2+ insertion/extraction than the [Mg 2+ –TFSA – ] structure, which has a lower number of coordinated anions. These findings indicate that the Mg 2+ insertion/extraction at the cathode/electrolyte interface is drastically improved by using a combination of typically used electrolytic salts as the electrolyte. This strategy enables rapid evaluation of the electrochemical performance of various Mg 2+ -battery cathodes without high-temperature and prolonged operation.