Mxenes As High Capacity Electrodes Materials for Metal (Li, Na, K, Ca)-Ion Batteries

Two-dimensional (2-D) materials are capable of handling high rates of charge in ion batteries since metal ions do not need to diffuse in a 3-D lattice structure. However graphene, which is the most well-studied 2-D material, is known to have no Li capacity. Here, adsorption of Li, as well as Na, K, and Ca on Ti₃C₂, one representative MXene, is predicted by first-principles density functional calculations. In our study, we observed that these alkali atoms exhibit different adsorption energies depending on the coverage. The adsorption energies of Na, K, and Ca decrease as coverage increases, while Li shows little sensitivity to variance in coverage. This observed relationship between adsorption energies and coverage of alkali ions on Ti₃C₂ can be explained by their effective ionic radii. A larger effective ionic radius increases interaction between alkali atoms, thus lower coverage is obtained. Our calculated capacities for Li, Na, K, and Ca on Ti₃C₂ are 447.8, 351.8, 191.9, and 320.0 mAh/g, respectively. Compared to materials currently used in high-rate Li and Na ion battery anodes, MXene shows promise in increasing overall battery performance.