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

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.