Higher Capacity and Voltage Enabled by Stable Ions Dispersedly Occupied Co-Intercalation Phase in Rechargeable...

Multivalent-ion batteries hold great promise as next-generation energy storage systems, yet efficient intercalation of multivalent ions into cathode frameworks remains challenging. Typically, the discharge capacity derived from Mg²⁺ intercalation into TiO₂ cathode is inferior due to the sluggish Mg²⁺ migration ability and the instability of the intercalated phase. Herein, the specific stable ion intercalation stoichiometry is determined through convex hull analysis in three different polymorphs of TiO₂ (anatase, brookite, rutile) and finds that anatase and brookite TiO₂ cathodes show higher capacities in the Mg-Li dual-salt system than those in the Mg salt electrolyte. First-principles calculations indicate that there exist the most energetically stable co-intercalation phases Li_0.1875Mg_0.0625TiO₂ in anatase TiO₂, and Li_0.125Mg_0.0625TiO₂ in brookite TiO₂, using the Mg-Li dual-salt system. Moreover, the stable co-intercalation phases are explained in terms of the dispersedly occupied Mg²⁺ and Li⁺ distribution, which effectively mitigates electrostatic repulsion and reduces the system energy, enabling an enhancement of battery voltage. Experimentally, the voltage plateaus of anatase and brookite TiO₂ in Mg-Li dual-salt electrolyte are ≈0.15 and 0.21 V higher than those in Li salt electrolyte, and much higher than those in Mg salt electrolyte, respectively. These findings provide facile guidelines toward the capacity and voltage enhancement for multivalent ion batteries.