Ti3C2O2 MXene has been proposed as a promising electrode material for alkali-ion batteries owing to its tunable physical and chemical properties without sacrificing the excellent metallic conductivity. However, it still suffers from low specific capacity due to its limited interlayer spacing, especially for a larger ion like sodium (Na). Sulfur doping was suggested as a viable strategy to improve the electrode’s storage performance. Herein, first-principles calculations and kinetic Monte Carlo (kMC) simulations were carried out to study the role of S doping on Li/Na intercalation. Based on experimental findings, two different doping sites, C (SC) and O (SO), with various S concentrations were reported and therefore used as the models in this study. Computations reveal that S doping on both C and O sites improves the electronic conductivity of the MXenes as their densities of states at the Fermi level are increased. In addition, the doped MXenes reveal an expanded lattice parameter in the normal direction, which agrees with experimental observations. However, only the SO-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the SO-doped MXenes improves stabilities and transport kinetics of ion intercalation as indicated by their significantly lower insertion energies and diffusion barriers when compared with those of the pristine system. The kMC simulations were carried out to account for anisotropic diffusion in the SO-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the SO-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li­(Na) are improved by 4(8) orders of magnitude upon SO doping. A fundamental understanding of the role of S doping on the improved capacitive kinetics serves as a good guide for developing MXene-based electrode materials for Li- and Na-ion batteries.

Vanadate-based compounds, in particular LiV3O8, are promising candidates for cathode materials of Li-ion batteries. Thanks to their open-layered structures and the various possible oxidation states of the V metal center, LiV3O8 can effectively accommodate Li ions and store electron potential. To further improve the transport kinetics of the cathode, in this work, we used the first-principles method to explore the effects of oxygen vacancies on Li insertion, Li diffusion, and electronic conduction in the form of polaron hopping. We find that the polaron is mobile in the [010] direction with an effective hopping barrier, E a,eff, of 0.11 eV and is sluggish in other directions with an E a,eff of 0.56 eV. Such anisotropic conduction is also observed in experiments. Interestingly, unlike other transition metal oxides, formation of a polaron negligibly affects Li insertion and diffusion, where the charge transport kinetics is solely limited by the ion movement with an E a,eff of 0.50 eV. The introduction of an oxygen vacancy, V O, creates two polarons at the two nearby V centers where at least one of them is relatively mobile (E a,eff = 0.16 eV) contributing to electronic conductivity of the materials. At low V O concentrations of up to 1%, the most stable V O exists far from the Li diffusion path and does not affect the ion transport kinetics. In contrast, if the V O concentration increases to 2–3%, the second most stable V O starts to form at the diffusion path, which greatly diminishes Li diffusion. Hence, it is suggested that controlling the low concentration of V O within 1% can enhance electronic conductivity by increasing the concentration of charge carriers while maintaining the ion diffusivity of the LiV3O8 cathode.