Thermodynamics and kinetics of Mg2+/Li+ and Mg2+/Na+ co-intercalation into layered titanium disulfide

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.