Construction of TiO2/TiOF2 heterojunction as a cathode material for high-performance Mg2+/Li+ hybrid-ion batteries

Anatase TiO2 has attracted significant interest as a cathode material for Mg-ion batteries or Mg2+/Li+ hybrid-ion batteries. However, owing to the semiconductor property and slower Mg2+ diffusion kinetics it still suffers from poor electrochemical performance. Herein, a TiO2/TiOF2 heterojunction consisting of in situ formed TiO2 sheets and TiOF2 rods, was prepared by adjusting the amount of HF in the hydrothermal process, and used as cathode of Mg2+/Li+ hybrid-ion battery. The TiO2/TiOF2 heterojunction prepared by adding 2 mL HF (TiO2/TiOF2-2) exhibits high electrochemical performance, with a high initial discharge capacity (378 mAh/g at 50 mA/g), an outstanding rate performance (128.8 mAh/g at 2000 mA/g), and good cycle stability (capacity retention of 54 % after 500 cycles), which is much superior to that of Pure TiO2 and Pure TiOF2. The reactions of Li+ intercalation/detercalation in the TiO2/TiOF2 heterojunction are revealed by investigating the evolution of the hybrids during different electrochemical states. Moreover, theoretical calculations prove that the Li+ formation energy in the TiO2/TiOF2 heterostructure is much lower than that of TiO2 and TiOF2, demonstrating that the heterostructure plays a crucial role in the enhanced electrochemical performance. This work provides a novel method to design cathode materials with high performance by constructing heterostructure.