Promoting Electrochemical Performance of TiCO MXene-Based Electrodes of Alkali-Ion Batteries via S Doping: Theoretical Insight.

TiCO 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 (S) and O (S), 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 S-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the S-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 S-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the S-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li(Na) are improved by 4(8) orders of magnitude upon S 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.