Two-dimensional composite of D-Ti₃C₂T_x@S@TiO₂ (MXene) as the cathode material for aluminum-ion batteries

MXenes, the two-dimensional layered materials, are widely used in electrochemical storage devices and exhibit excellent electrochemical performances. Aluminum-ion batteries are favored by researchers around the world due to our urgent need for emerging clean energy; the study of cathode materials is most important in aluminum-ion battery research. Herein, we have prepared a two-dimensional layered composite of D-Ti₃C₂T_x@S@TiO₂ by a simple method, which shows excellent electrochemical performance in aluminum-ion batteries. The discharge specific capacity is 151.3 mA h g^-1 after 120 cycles, which is about 80.0 mA h g^-1 higher than that of Ti₃C₂T_x in aluminum-ion batteries, and the capacity retention rate is 72.3%. The Coulomb efficiency is about 85% during steady cycling. The reasons for its excellent electrochemical performances are explored herein, and it is proved by DFT calculations that Ti₃C₂S is more stable than Ti₃C₂OH in aluminum-ion batteries. The elemental sulfur reacted with the exposed Ti-ions to form some Ti-S bonds, which play a supporting role in maintaining the original structure of the material and preventing the superposition of layers between the materials. Some TiO₂ nanoparticles were grown in situ on the surface of the material by further oxidation treatment, which improved the stability of the material. In addition, we studied the charge/discharge mechanism of the aluminum-ion battery using Ti₃C₂T_x as the cathode material based on the changes in the contents of Al and Cl, and the change in the valence of Ti-ions. The results show that [AlCl₄]^- was intercalated/de-intercalated in the layers of the cathode material during the charge/discharge processes.