Rechargeable magnesium batteries (RMBs) are a promising large-scale energy-storage technology with low cost and high reliability. However, developing high-performance cathode materials remains the most prominent obstacle because of the insufficient magnesium-storage active sites and unfavorable magnesium cation transport paths, as well as the strong interaction between the cathode material and the bivalent magnesium cation. Herein, ammonium tetrathiomolybdate is demonstrated to be a high-performance RMB cathode material. Ammonium tetrathiomolybdate exhibits a high capacity of 333 mAh g −1 at 50 mA g −1 and a good rate performance of 129 mAh g −1 at 5.0 A g −1 (∼15 C). An amorphous structure with plenty of efficient magnesium-storage active sites and open magnesium transport paths is in situ formed during the first cycle via ammonium extraction. The covalent-like bond between the molybdenum and sulfur delocalizes the negative charge, weakening the interaction with the bivalent magnesium cation and accelerating the kinetics. The covalent-like molybdenum−sulfur bond also promotes the simultaneous redox of molybdenum and sulfur, leading to a high specific capacity. The present work introduces a high-capacity and high-power RMB cathode material, elucidates the origin of the high performance, and provides insights for the design and optimization of RMB cathode materials.

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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 43− 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.

Rechargeable Mg batteries (RMBs) are promising devices for large-scale energy-storage applications, but the lack of cathode materials is hindering the development. Conjugated organic polymers provide more selections by breaking the...

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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 Mg2+ cation favors fast kinetics. Herein, nanorod‐like FeSe2 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 FeSe2 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 Mg2+ intercalation mechanism for FeSe2, and a slight conversion reaction occurs on the surface of the particles. The capacity fading is mainly because of the dissolution of Fe2+, which is caused by the reaction between Fe2+ and Cl− of the electrolyte during the charge process on the surface of the particles. The surface of FeSe2 is mainly selenium after long cycling, which may also dissolve in the electrolyte during cycling. The present work develops a new type of Mg2+ 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.

Hybrid Mg2+/A+ (A = Li, Na, and K) batteries are promising energy-storage devices combining the merits of metallic Mg anode and A+ intercalation cathodes. Mg2+/Li+ co-intercalations into the cathodes have...