Surface-engineered Ti₃C₂T_x MXene enabling rapid sodium/potassium ion storage

MXene with expanding interlayer and tunable terminations emerge as promising candidates for metal ion storage. Herein, we develop a facile urea decomposition strategy to obtain ultrathin nitrogen-modified Ti₃C₂T_x (N-UT-Ti₃C₂T_x ) with optimized terminations as anode for sodium/potassium ion storage. Experimental results have shown that NH₃ molecules produced by urea pyrolysis could introduce two types of nitrogen modifications in Ti₃C₂, function substitution for -OH (FS) and surface absorption on -O (SA). During subsequent hydrothermal and heating processes, the nitrogen atoms in situ substitute the lattice carbon in Ti₃C₂ (LS). Further, the effects of these nitrogen modifications in Ti₃C₂ on diffusion kinetics of Na⁺ and K⁺ are investigated by first-principles calculations. The superior Na⁺ storage performances of the N-UT-Ti₃C₂T_x anode are the main attribute of the nitrogen modification of LS in Ti₃C₂, while the excellent K⁺ storage performances come from the synergistic effects of the nitrogen modifications of FS and LS in Ti₃C₂. This work emphasizes the effectiveness of surface engineering of nitrogen modifications and optimized terminations for improving the electrochemical performances of Ti₃C₂T_x and inspires the design of heteroatom modified MXenes for energy storage.