Ultra-Efficient Synthesis of Nb₄ C₃ T_x MXene via H₂ O-Assisted Supercritical Etching for Li-Ion Battery

Nb₄ C₃ T_x MXene has shown extraordinary promise for various applications owing to its unique physicochemical properties. However, it can only be synthesized by the traditional HF-based etching method, which uses large amounts of hazardous HF and requires a long etching time (> 96 h), thus limiting its practical application. Here, an ultra-efficient and environmental-friendly H₂ O-assisted supercritical etching method is proposed for the preparation of Nb₄ C₃ T_x MXene. Benefiting from the synergetic effect between supercritical CO₂ (SPC-CO₂ ) and subcritical H₂ O （SBC-H₂ O）, the etching time for Nb₄ C₃ T_x MXene can be dramatically shortened to 1 h. The as-synthesized Nb₄ C₃ T_x MXene possesses uniform accordion-like morphology and large interlayer spacing. When used as anode for Li-ion battery, the Nb₄ C₃ T_x MXene delivers a high reversible specific capacity of 430 mAh g^-1 at 0.1 A g^-1 , which is among the highest values achieved in pure-MXene-based anodes. The superior lithium storage performance of the Nb₄ C₃ T_x MXene can be ascribed to its high conductivity, fast Li⁺ diffusion kinetics and good structural stability.