Influence of surface termination groups on electrochemical charge storage of MXene electrodes

Two-dimensional transition-metal carbides/nitrides (MXenes) have been intensively investigated as electrode materials for electrochemical energy storage devices, such as batteries and supercapacitors, due to their high capacitance, high-rate capability, and good cycle stability. Although MXenes possess various surface termination groups (e.g., -O, -OH, -F, -Cl, and -Br) that directly interact with adsorbed cations to exhibit charge transfer, the influence of each surface termination group on the electrochemical properties of MXene remains elusive because of difficulty in achieving exclusively modified termination. Herein, we report the electrochemical properties of MXenes with surface termination groups controlled by using fluorine-based aqueous solutions and molten salts as etchants. In aqueous electrolytes, Ti₃C₂Cl _x and Ti₃C₂Br _x synthesized using molten salts show no electrochemical activity in contrast to Ti₃C₂T _x (T = O, OH, F, and Cl). Meanwhile, in a nonaqueous electrolyte, Ti₃C₂Cl _x and Ti₃C₂Br _x undergo amorphization upon the initial lithiation. These results suggest that the -O, -OH, and -F terminations play an important role in the electrochemical properties of MXene electrodes.