Revealing the Reaction and Fading Mechanism of FeSe Cathodes for Rechargeable Magnesium Batteries.

Rechargeable Mg batteries (RMBs) are advantageous large-scale energy-storage devices because of the high abundance and high safety, but exploring high-performance cathodes remains the largest difficulty for their development. Compared with oxides and sulfides, selenides show better Mg-storage performance because the weaker interaction with the Mg cation favors fast kinetics. Herein, nanorod-like FeSe was synthesized and investigated as a cathode for RMBs. Compared with microspheres and microparticles, nanorods exhibit higher capacity and better rate capability with a smaller particle size. The FeSe nanorods show a high capacity of 191 mAh g at 50 mA g and a good rate performance of 39 mAh g at 1000 mA g . Ex situ characterizations demonstrate the Mg intercalation mechanism for FeSe , and a slight conversion reaction occurs on the surface of the particles. The capacity fading is mainly because of the dissolution of Fe , which is caused by the reaction between Fe and Cl of the electrolyte during the charge process on the surface of the particles. The surface of FeSe is mainly selenium after long cycling, which may also dissolve in the electrolyte during cycling. The present work develops a new type of Mg intercalation cathode for RMBs. More importantly, the fading mechanism revealed herein has considered the specificity of Mg battery electrolyte and would assist a better understanding of selenide cathodes for RMBs.