Surface modification and in situ carbon intercalation of two-dimensional niobium carbide as promising electrode materials for potassium-ion batteries

• The surface-modified and in-situ carbon-intercalated Nb 2 CT x is synthesized. • A method is firstly reported to solve the problem of decrease in the interlayer distance of Nb 2 C during annealing. • Nb 2 C/C exhibits superior electrochemical performance as anode material for KIBs. • Unique architecture boosts the ion accessibility and kinetics, and stability. • The possible charge-discharge mechanism of K + in the Nb 2 C/C is revealed. Potassium ion batteries (KIBs) have a great potential in large scale energy storage because of their cost advantages and similar working mechanism to lithium ion batteries (LIBs). However, the larger ionic radius of K + presents great challenges to find suitable electrode materials for K + insertion/extraction. Herein, the surface-modified and in-situ carbon-intercalated Nb 2 CT x MXene is rationally designed and synthesized as electrode for KIBs. The as-prepared Nb 2 C/C has larger effective interlayer distance ( i.e. gallery height), higher specific surface area and electrical conductivity, and lower concentration of surface functional groups. As a result, Nb 2 C/C exhibits a significantly increased coulombic efficiency (67.6% for Nb 2 C/C, 28.5% for Nb 2 CT x ) and its specific capacity is two times higher than that of Nb 2 CT x at 0.02 A·g −1 , three times at 0.1 A·g −1 . The Nb 2 C/C delivers an initial specific capacity of 397.9 mAh·g −1 at 0.02 A·g −1 and 338.1 mAh·g −1 at 0.1 A·g −1 , and maintains 80.0% after 100 cycles at 0.02 A·g −1 and 76.2% after 200 cycles at 0.1 A·g −1 , respectively. Notably, Nb 2 C/C possesses outstanding cyclic stability and rate performance at various current densities, outperforming most of reported MXene-based anodes for KIBs. The excellent potassium storage performance of Nb 2 C/C can be ascribed to the abundant active sites exposed to electrolyte, rapid diffusion kinetics of K + and few side reactions at the electrode/electrolyte interface. This work proposes an effective strategy for the MXene-based materials to maximize their potential applications in various fields such as batteries, supercapacitors and catalysts.