Dopant-enhanced sodium and potassium-ion adsorption and diffusion in two-dimensional titanium disulfide

Two-dimensional (2D) titanium disulfide (TiS$2$⁠) is the lightest transition-metal dichalcogenide (TMD). It exhibits relatively better adsorption and diffusion of sodium (Na) and potassium (K) ions than other TMDs, such as MoS$2$ (molybdenum disulfide) and ReS$2$ (rhenium disulfide), making it a promising anode material for alkali-ion batteries. Previous studies have found that doping significantly enhances the adsorption and diffusion capabilities of 2D TMDs. For the first time, this work reports the adsorption of Na and K ions on doped TiS$2$ monolayers using first-principles calculations, where the Ti atom is substituted by 3d-transition metals, including iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Metal-atom doping induces remarkably stronger binding of alkali ions on the surface of TiS$2$⁠, with adsorption energies ranging from $−$2.07 to $−$2.48 eV for Na and $−$2.59 to $−$3.00 eV for K. The diffusion barrier energies for alkali ions decrease in the proximity of the doping site and increase as the ions travel away from the doping site for Fe-, Co-, and Ni-doped TiS$2$⁠. The average open circuit voltage increases dramatically when Na ions are adsorbed on Fe-doped TiS$2$ (by 62%) and Co-doped TiS$2$ (by 61%), while K ions result in a moderate improvement of 9% and 8%, respectively. These findings suggest that metal-atom doping considerably improves the electrochemical properties of 2D TiS$2$⁠, potentially enabling its use as anode materials in Na- and K-ion batteries.