Revealing the Reaction and Fading Mechanism of FeSe 2 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 2+ cation favors fast kinetics. Herein, nanorod-like FeSe 2 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 2 nanorods show a high capacity of 191 mAh g -1 at 50 mA g -1 and a good rate performance of 39 mAh g -1 at 1000 mA g -1 . Ex situ characterizations demonstrate the Mg 2+ intercalation mechanism for FeSe 2 , and a slight conversion reaction occurs on the surface of the particles. The capacity fading is mainly because of the dissolution of Fe 2+ , which is caused by the reaction between Fe 2+ and Cl - of the electrolyte during the charge process on the surface of the particles. The surface of FeSe 2 is mainly selenium after long cycling, which may also dissolve in the electrolyte during cycling. The present work develops a new type of Mg 2+ 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.