Copper Tetraselenophosphate Cathode for Rechargeable Magnesium Batteries: A Redox-Active Polyatomic Anion Strategy to Design the Cathode Material

Rechargeable magnesium batteries attract interest as advantageous energy-storage devices, but the application is being hampered by the deficiency of suitable cathodes. The traditional method to weaken the interaction between bivalent Mg2+ cations and the cathode material is to increase the anion radius, but excessive expansion of the anion would lead to a decrease of the theoretical capacity and offset the performance improvement. Herein, a new strategy using a redox-active polyatomic anion is developed in terms of copper tetraselenophosphate (Cu3PSe4) fabricated by the PSe43– anion. The covalent P–Se bond facilitates the negative charge delocalization of the PSe43– anion and weakens the interaction with Mg2+ cations, which result in rapid solid-phase Mg2+ diffusion kinetics. The PSe43– anion also provides extra capacities by reversible valence state change of the P element. Cu3PSe4 delivers a high Mg-storage capacity of 225 mAh g–1 at 50 mA g–1 and a superior rate performance of 62 mAh g–1 at 5000 mA g–1, as well as a stable cyclability of 500 cycles. The redox-active polyatomic anion strategy herein opens a new avenue for the exploration of magnesium battery cathodes with a comprehensive consideration of kinetic performance and theoretical capacity.