First-principles prediction of a two-dimensional vanadium carbide (MXene) as the anode for lithium ion batteries.

Exploring two-dimensional anode materials that can utilize the storage capacity and diffusion mobility of Li ions is at the heart of lithium ion battery (LIB) research. Herein, we report the results of ab initio electronic structure calculations on the storage capacity and diffusion mobility affinities of Li ions adsorbed onto nondefective and defective MXene V2C monolayers. It is found that Li ions strongly chemisorb on the two sides of the V2C surface with a preferential adsorption site at the hollow center of the honeycomb structure. The binding profile and open-circuit voltage calculations reveal that the Li/V2C structure exhibits a specific capacity as high as 472 mA h g-1 at the Li2V2C stoichiometry, a value relatively high compared with those of the typical anode materials including graphite (372 mA h g-1). Furthermore, the diffusion barrier of a Li ion over the V2C surface is identified to be no more than 0.1 eV, which is a few times smaller than that of graphene and graphitic anodes. In addition, during the lithiation and delithiation processes, the change in the lateral lattice is quite small, only about a 2% increase at the full lithiation of Li2V2C, implying a good cycling performance. Importantly, these intriguing findings are very robust against the intrinsic structural and atomic defects including local point vacancies and biaxial compressive and tensile strains. More specifically, the presence of a monovanadium vacancy enhances the binding energy up to 3.1 eV per Li ion, which is about a 30% enhancement compared with the defect-free Li/V2C structure, and reduces the activation barrier by about 2 meV; meanwhile, these binding and diffusion mobility features can be improved even more when the lattice constant of the V2C monolayer is expanded. These results thus suggest that MXene V2C could be a promising anode material with high capacity and high rate capabilities for next generation high-performance LIBs.