Sandwich-like N-doped carbon nanotube@Nb₂C MXene composite for high performance alkali ion batteries

Sodium (Na) and potassium (K) ion batteries are promising for the next-generation energy storage equipment, but compared with lithium (Li) ion battery, their requirements for suitable host materials are much tougher, due to their large ions. 2D accordion-like MXene materials have been studied a lot as electrode materials for Na/K ion battery for their vast interlamellar spacing, but they suffer from layers’ restacking. Herein, a sandwich-like N-doped carbon nanotube@Nb₂C (N-CNT@Nb₂C) has been synthesized, where N-CNT fibers are sandwiched between MXene layers to fix the whole structure, enhance electrical conductivity, and increase interlamellar spacing. After nitrogen doping, carbon nanotubes own higher conductivity and sites to penetrate N-CNT walls. Consequently, N-CNT@Nb₂C shows excellent electrochemical performance in Li, Na, K batteries, such as steady cycling performance for more than 500 cycles. We also test its performance with liquid K–Na alloy as anode of K ion battery. Due to the dendrite-free character of liquid anode, it exhibits better electrochemical performance than with solid K anode. The N-CNT@Nb₂C promotes the finding of more suitable electrode materials for alkali ion batteries, deepens understanding of their inner mechanisms, and facilitates their commercialization.