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 Mg 2+ 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 (Cu 3 PSe 4 ) fabricated by the PSe 4 3– anion. The covalent P–Se bond facilitates the negative charge delocalization of the PSe 4 3– anion and weakens the interaction with Mg 2+ cations, which result in rapid solid-phase Mg 2+ diffusion kinetics. The PSe 4 3– anion also provides extra capacities by reversible valence state change of the P element. Cu 3 PSe 4 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.