Bulk Ti3C2Tx anodes for superior sodium storage performance: the unique role of O-termination

Abstract

The regulation of surface O-termination of Ti₃C₂T_x MXenes is a fascinating problem in the development of anodes for sodium ion batteries. However, the preparation of bulk Ti₃C₂T_x anodes with O-rich termination (O-Ti₃C₂T_x) is still a great challenge, due to their poor antioxidant properties in air. Moreover, the electrochemical behaviors of bulk O-Ti₃C₂T_x are less clear. In this study, bulk O-Ti₃C₂T_x anodes were prepared by a simple “hot melting-decomposition” strategy. The melt organic matter during heat treatment isolates bulk Ti₃C₂T_x from air, resulting in bulk Ti₃C₂T_x remaining stable up to 300 °C in air. Furthermore, we revealed the Na⁺ storage mechanism of bulk O-Ti₃C₂T_x electrodes by studying the structural evolution of both O-Ti₃C₂T_x and bulk Ti₃C₂T_x electrodes during the charge and discharge process. Surprisingly, it was found that bulk O-Ti₃C₂T_x easily evolved into active Ti₃C₂T_x nanoparticles, while bulk Ti₃C₂T_x formed an amorphous matrix. The unique role of O-termination in stabilizing nanocrystals and the anti-aggregation effect of the as-formed Na₁₆Ti₁₀O₂₈ nanocrystals produced at the edge of Ti₃C₂T_x nanoparticles resulted in enhanced sodium storage performance in bulk O-Ti₃C₂T_x electrodes. The bulk O-Ti₃C₂T_x electrode can still deliver a reversible capacity of 153 mA h g⁻¹ after 2500 cycles even at a current density as high as 1 A g⁻¹, showing superior sodium storage performance. The proposed method could be a general approach to improve the antioxidant properties of MXenes in air. Moreover, our work lays the foundation for O-termination regulation of MXenes and their electrochemical mechanism.