Oxygen vacancies and Li⁺ pre-embedding for magnesium storage in Anatase TiO₂ toward magnesium/lithium hybrid batteries

In the co-intercalation type of magnesium-lithium hybrid batteries (MLHBs) system, the insertion of Li⁺ enhances the electrochemical activity of the Chevrel phase cathode materials for Mg²⁺. However, the mechanism behind the synergistic diffusion mechanism of mixed multivalent ions still needs further research regarding other non Chevrel phase cathode materials. The present study is the first to reveal the diffusion mechanism of Li⁺/Mg²⁺ in oxygen vacancy anatase TiO₂ (TiO_2–x). Pristine TiO₂ exhibits strong electrochemical inertness to Li⁺ and Mg²⁺, oxygen vacancies induce the preferential insertion of Li⁺ to form Li_yTiO_2–x structure. This in turn activates the electrochemical activity of TiO₂ to Mg²⁺. Experimental results illustrate that the oxygen vacancies not only improve the intrinsic electronic conductivity of TiO₂, but also weaken the electrostatic shielding effect between Mg²⁺ and lattice oxygen during the diffusion process. Li_yTiO_2–x compounds formed by Li⁺ preferentially intercalation can further reduce the diffusion energy barrier of Mg²⁺. The other hand, reduced graphene oxide (rGO) is used as the substrate to further improve the structural stability of TiO_2–x. The prepared TiO_2–x/rGO composite (denoted as NTR) as MLHBs cathode delivers outstanding lithium-magnesium co-storage performance (253.77 mAh g⁻¹ at 0.1 A g⁻¹; the capacity is 127.3 mAh g⁻¹ at 0.5 A g⁻¹ after 750 cycles). The present research emphasizes that the optimization of the charge environment during the diffusion of Mg²⁺ is an important factor in achieving the rapid diffusion of Mg²⁺. This modification strategy opens a new way for other transition metal oxides (TMOs) to be used as co-intercalated MLHBs cathode materials.