High performance of Mg2+/Li+ hybrid ion batteries achieved through TiO2-x@TiOF2 heterostructure cathodes: Experimental and computational insights

Abstract

The TiO_2-x@TiOF₂ heterostructure was fabricated via an HF-assisted hydrothermal method followed by subsequent thermal treatment. In Mg²⁺/Li⁺ hybrid ion batteries, the TiO_2-x@TiOF₂ heterojunction exhibited outstanding electrochemical performance, including a high initial discharge capacity of 275.8 mAh g⁻¹ at 500 mA g⁻¹, excellent rate capability of 128.6 mAh g⁻¹ at 1000 mA g⁻¹, and superior cycling stability with a capacity retention of 62.5 % after 1000 cycles at 1000 mA g⁻¹. First-principles calculations indicate that the migration barriers for Mg²⁺ and Li⁺ in the TiO₂@TiOF₂ heterostructure are lower than those in pure TiO₂ and TiOF₂. The introduction of oxygen vacancies further reduces these migration barriers, confirming that both the heterostructure and the oxygen vacancies play crucial roles in enhancing the electrochemical performance. Furthermore, post-characterization following prolonged cycling revealed that the energy storage mechanism of the TiO₂@TiOF₂ is based on the co-intercalation of Mg and Li ions. The capacity decay under long-term cycling conditions is primarily attributed to the incomplete de-intercalation of the intercalated ions. This work provides a novel approach for designing high-performance cathode materials for Mg²⁺/Li⁺ hybrid ion batteries by in situ constructing heterostructures and introducing oxygen vacancies in transition metal oxides.