Exploring the Potentials of Ti₃C_iN_2–iT_x (i = 0, 1, 2)-MXene for Anode Materials of High-Performance Sodium-Ion Batteries

Two-dimensional (2D) MXenes, including carbides, nitrides, and carbonitrides MXene, have been proved to be a possible candidate as anode materials of sodium-ion batteries. This paper focuses on the electronic properties and the electrochemical performance of nitrides MXene. First, density functional theory simulations were utilized to disclose the geometric structure and electronic properties, Na diffusion path, and storage behaviors of titanium carbonitrides Ti3CNTx, nitrides MXene Ti3N2Tx, and carbides MXene Ti3C2Tx with oxygen terminations, predicting the more excellent performance of Ti3N2O2 than Ti3C2O2. Also, then the structure characterization and electrochemical performance experiments of Ti3C2Tx and Ti3CNTx were conducted to verify the theoretical predictions and test the cycling performances. The superior performance of Ti3N2O2 originates from the stronger connection of O–Ti–N than that of O–Ti–C, resulting in the stackings of Ti3N2O2 being tighter and the interlayer spacings being larger than that of Ti3C2O2, which is advantageous to sodiation and desodiation. The capacity of Ti3CNTx increased again to 145 mAh/g after 35 cycles at a current density of 20 mA/g, which demonstrated a better rate performance than Ti3C2Tx corroborated by the diffusion barriers of the theoretical calculation results. Ti3CNTx exhibits a good cycling performance of 110 mAh/g (≈60% of the initial value) after 200 cycles, which is better than that of 87 mAh/g (≈51% of the initial value) of Ti3C2Tx. It is worth noting that all these performances ensure that nitride MXene is more suitable as the anode material of Na-ion batteries than carbide MXene. These findings are conducive to expanding the MXene family and promoting their application in energy storage applications.