Thermodynamics and kinetics of Mg 2+ /Li + and Mg 2+ /Na + co-intercalation into layered titanium disulfide.

Hybrid Mg 2+ /A + (A = Li, K, or Na) batteries are promising energy-storage devices combining the merits of a metallic Mg anode and A + intercalation cathodes. Mg 2+ /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 Mg 2+ /Li + and Mg 2+ /Na + co-intercalations is revealed for layered TiS 2 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 Mg 2+ and Li + (Na + ) into the same TiS 2 layer, whereas the Mg-Li (Mg-Na) interlaced intercalation means Mg 2+ and Li + (Na + ), respectively, intercalate into the adjacent TiS 2 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 TiS 2 -based hybrid Mg 2+ /Li + and Mg 2+ /Na + batteries, respectively. With this stabilization, both of the hybrid Mg 2+ /Li + and Mg 2+ /Na + batteries show high capacities, good rate capabilities and stable cycling.