Additive-Free Anode with High Stability: Nb₂CT_x MXene Prepared by HCl-LiF Hydrothermal Etching for Lithium-Ion Batteries

MXenes, represented by Ti₃C₂T_x, have been widely studied in the electrochemical energy storage fields, including lithium-ion batteries, for their unique two-dimensional structure, tunable surface chemistry, and excellent electrical conductivity. Recently, Nb₂CT_x, as a new type of MXene, has attracted more and more attention due to its high theoretical specific capacity of 542 mAh g^-1. However, the preparation of few-layer Nb₂CT_x nanosheets with high-quality remains a challenge, which limits their research and application. In this work, high-quality few-layer Nb₂CT_x nanosheets with a large lateral size and a high conductivity of up to 500 S cm^-1 were prepared by a simple HCl-LiF hydrothermal etching method, which is 2 orders of magnitude higher than that of previously reported Nb₂CT_x. Furthermore, from its aqueous ink, the viscosity-tunable organic few-layer Nb₂CT_x ink was prepared by HCl-induced flocculation and N-methyl-2-pyrrolidone treatment. When using the organic few-layer Nb₂CT_x ink as an additive-free anode of lithium-ion batteries, it showed excellent cycling performance with a reversible specific capacity of 524.0 mAh g^-1 after 500 cycles at 0.5 A g^-1 and 444.0 mAh g^-1 after 5000 cycles at 1 A g^-1. For rate performance, a specific capacity of 159.8 mAh g^-1 was obtained at a high current density of 5 A g^-1, and an excellent capacity retention rate of about 95.65% was achieved when the current density returned to 0.5 A g^-1. This work presents a simple and scalable process for the preparation of high-quality Nb₂CT_x and its aqueous/organic ink, which demonstrates important application potential as electrodes for electrochemical energy storage devices.