Ultra-Efficient Synthesis of Nb 4 C 3 T x MXene via H 2 O-Assisted Supercritical Etching for Li-Ion Battery.

Nb 4 C 3 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 H 2 O-assisted supercritical etching method is proposed for the preparation of Nb 4 C 3 T x MXene. Benefiting from the synergetic effect between supercritical CO 2 (SPC-CO 2 ) and subcritical H 2 O （SBC-H 2 O）, the etching time for Nb 4 C 3 T x MXene can be dramatically shortened to 1 h. The as-synthesized Nb 4 C 3 T x MXene possesses uniform accordion-like morphology and large interlayer spacing. When used as anode for Li-ion battery, the Nb 4 C 3 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 Nb 4 C 3 T x MXene can be ascribed to its high conductivity, fast Li + diffusion kinetics and good structural stability.