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

Two-dimensional (2D) titanium disulfide (TiS2) 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 MoS2 (molybdenum disulfide) and ReS2 (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 TiS2 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 TiS2, 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 TiS2. The average open circuit voltage increases dramatically when Na ions are adsorbed on Fe-doped TiS2 (by 62%) and Co-doped TiS2 (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 TiS2, potentially enabling its use as anode materials in Na- and K-ion batteries.