Nb₂C MXenes with surface chalcogenation and halogenation as high capacity electrode materials for lithium-ion batteries

MXenes show extremely competitive potential applications in electrode materials for lithium-ion batteries (LIBs) due to their excellent specific surface area, high electrical conductivity, and compositional tunability. However, limited interlayer spacing and undesired surface functional group on MXene surface impede the Li-ions accessibility and mobility. Herein, the structural, electronic, mechanical and electrochemical properties of the representative Nb2C MXene with surface chalcogenation and halogenation resulting in the formation of Nb2CT2 (T=O, S, Se, Cl and Br) materials as anodes for LIBs were investigated using first-principles calculations based on density functional theory. The results reveal that Nb2CT2 can exhibit metallic conductivity with improved mechanical strength, which renders the enhanced rate performance and endures the repeated lattice expansion and contraction during charge/discharge process, respectively. In particular, Nb2CS2 and Nb2CCl2 render the enhanced Li-ion storage and mobility with a theoretical Li storage capacity of 613.76 mA .h/g and 597.79 mA .h/g and diffusion energy barrier of 0.275 eV and 0.294 eV, respectively. Moreover, chalcogenation and halogenation yield the expanded interlayer spacing, which improve the Li-ions accessibility in Nb2CT2. The results demonstrate that sulfurized and chlorinated Nb2C MXenes are the promising anode materials with high capacity, low diffusion barrier and lower open circuit voltage for next-generation LIBs.