Role of surface structure on li-ion energy storage capacity of two-dimensional transition-metal carbides

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc₂C, Ti₂C, Ti ₃C₂, V₂C, Cr₂C, and Nb₂C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb₂C and f-Ti ₃C₂ at 673 and 773 K in vacuum for 40 h and in situ X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti₃C₂. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti₃C₂ revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr₂C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc₂C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal.