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.
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