A Pyrite Iron Disulfide Cathode with a Copper Current Collector for High‐Energy Reversible Magnesium‐Ion Storage

Owing to its low cost, high theoretical capacity, and environmentally friendly characteristics, pyrite FeS₂ demonstrates promise as a cathode material for high-energy metal-anode-based rechargeable batteries. When it is used in a rechargeable magnesium battery (RMB), the electrode couple exhibits an extremely low theoretical volume change upon full discharge. However, its electrochemical Mg-ion storage is considerably hindered by slow reaction kinetics. In this study, a high-performance FeS₂ cathode for RMBs using a copper current collector is reported, which is involved in cathode reactions via a reversible redox process between copper and cuprous sulfide. This phase transformation with the formation of copper nanowires during discharge activates the redox reactions of FeS₂ via a two-step and four-electron Mg-ion transfer that dominates the cathode reactions. As a result, the as-prepared FeS₂ nanomaterial cathode delivers a significantly enhanced reversible capacity of 679 mAh g^-1 at 50 mA g^-1 . The corresponding energy density of 714 Wh kg^-1 is superior to those of all previously reported metal chalcogenide cathodes in RMBs or hybrid batteries using a Mg metal anode. Notably, the as-assembled FeS₂ -Mg battery can operate over 1000 cycles with a good capacity retention at 400 mA g^-1 .