Nitrogen doping has been proven to be a facile modification strategy to improve the electrochemical performance of 2D MXenes, a group of promising candidates for energy storage applications. However, the underlying mechanisms, especially the positions of nitrogen dopants, and its effect on the electrical properties of MXenes, are still largely unexplored. Herein, a comprehensive study is carried out to disclose the nitrogen doping mechanism in Ti 3 C 2 MXene, by employing theoretical simulation and experimental characterization. Three possible sites are found in Ti 3 C 2 T x (T = F, OH, and O) to accommodate the nitrogen dopants: lattice substitution (for carbon), function substitution (for-OH), and surface absorption (on-O). Moreover, electrochemical test results confirm that all the three kinds of nitrogen dopants are favorable for improving the specific capacitance of the Ti 3 C 2 electrode, and the underlying factors are successfully distinguished. By revealing the nitrogen doping mechanisms in Ti 3 C 2 MXene, this work provides theoretical guidelines for modulating the electrochemical properties of MXene materials for energy storage applications.

for electron and charge transfer. Therefore, the OV-T n QDs@ PCN/S cathode delivers a superb long-term cycling stability (88% capacity retention over 1000 cycles at 2C) under a S-mass loading of 2.2 mg cm −1 and an E/S ratio of 10 µL mg −1 . In addition, the cathode exhibits good Li + storage at high S-mass loading (4.8 mg cm −1 ) and lean electrolyte (E/S ratio: 4.5 µL mg −1 ), demonstrating its great potential for practical implementation. Our strategy may be extended to other MXenes (e.g., Ti 3 CNT x , Nb 2 CT x , and V 2 CT x ) and pave the way to realize the facile synthesis of QDs with rich OVs for advanced Li-S batteries.

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The achievement of chemical diversity and performance regulation of MAX phases primarily relies on solid solution approaches. However, the reported A-site solid solution is undervalued due to their expected chemical disorder and compliance with Vegard's law, as well as discontinuous composition and poor purity. Herein, we synthesized high-purity Ti 2 (Sn x Al 1−x )C (x = 0-1) solid solution by the feasible pressureless sintering, enabling us to investigate their property evolution upon the A-site composition. The formation mechanism of Ti 2 (Sn x Al 1−x )C was revealed by thermal analysis, and crystal parameters were determined by Rietveld refinement of X-ray diffraction (XRD). The lattice constant (a) adheres to Vegard's law, while the lattice constant (c) and internal free parameter (z M ) have noticeable deviations from the law, which is caused by the significant nonlinear distortion of Ti 6 C octahedron as Al atoms are substituted by Sn atoms. Also, the deviation also results in nonlinear changes in their physicochemical properties, which means that the solid solution often exhibits better performance than end members, such as hardness, electrical conductivity, and corrosion resistance. This work offers insights into the deviation from Vegard's law observed in the A-site solid solution and indicates that the solid solution with enhanced performance may be obtained by tuning the A-site composition.