ABSTRACT
Multivalent cation batteries are attracting increasing attention in energy-storage applications, but reversible storage of highly polarizing multivalent cations is a major difficulty for the electrode materials. In the present study, charge-delocalizing Mo3S13 cluster-based materials (crystalline (NH4)2Mo3S13 and amorphous MoSx) are designed and investigated as cathodes for rechargeable magnesium batteries. Both of the cathodes show high magnesium storage capacities (296 and 302 mAh g-1 at 100 mA g-1) and superior rate performances (76 and 80 mAh g-1 at 15 A g-1). A high area loading of 3.0 mg cm-2 could be achieved. These performances are of the highest level compared with those of reported magnesium storage materials. Further mechanism study and theoretical computation demonstrate the magnesium storage active sites are the bridging disulfur groups of the Mo3S13 cluster. The valence state of bridging disulfur decreases/increases largely during magnesiation/demagnesiation along with breaking/formation of the sulfur-sulfur bond, which makes the Mg-association/dissociation highly reversible. The sulfur-sulfur bond breaking and formation provides high reversible capacities. Prominently, the valence state increase and sulfur-sulfur bond formation of the bridging disulfur during charge weakens the bonding with Mg2+, significantly assisting the magnesium dissociation. The present study not only develops high-performance magnesium storage cathode materials but also demonstrates the importance of constructing favorable magnesium storage active sites in the high-performance cathode materials design. The findings presented herein are of great significance for the development of electrode materials for the storage of multivalent cations.
ABSTRACT
Hybrid Mg2+/A+ (A = Li, K, or Na) batteries are promising energy-storage devices combining the merits of a metallic Mg anode and A+ intercalation cathodes. Mg2+/Li+ co-intercalations into the cathodes have been reported, and it is believed that Li-intercalation would reduce the activation energy of the Mg-intercalation and improve the kinetics. Herein, a new understanding of the Mg2+/Li+ and Mg2+/Na+ co-intercalations is revealed for layered TiS2 from the point of view of the thermodynamics and kinetics. In addition to Li-, Na- and Mg-intercalations, Mg-Li (Mg-Na) mixed and Mg-Li (Mg-Na) interlaced intercalations can also be observed. The Mg-Li (Mg-Na) mixed intercalation is co-intercalation of Mg2+ and Li+ (Na+) into the same TiS2 layer, whereas the Mg-Li (Mg-Na) interlaced intercalation means Mg2+ and Li+ (Na+), respectively, intercalate into the adjacent TiS2 layers. The Li- (Na-) intercalation has faster kinetics, whereas the Mg-Li (Mg-Na) interlaced intercalation structure is more stable thermodynamically. The Mg-Li and Mg-Na interlaced intercalations would gradually become the predominant principles of the TiS2-based hybrid Mg2+/Li+ and Mg2+/Na+ batteries, respectively. With this stabilization, both of the hybrid Mg2+/Li+ and Mg2+/Na+ batteries show high capacities, good rate capabilities and stable cycling.
ABSTRACT
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 (â¼15 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.
ABSTRACT
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 191â mAh g-1 at 50â mA g-1 and a good rate performance of 39â mAh g-1 at 1000â 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.
