Insight into the Effect of Oxygen Vacancies on Ion Intercalation and Polaron Conduction in LiV3O8 Cathodes of Li-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,E-eff, of 0.11 eV and is sluggish in other directions with an E-a,E-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,E-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,E-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.

Automated summary

In this study, the effects of oxygen vacancies on Li insertion, Li diffusion, and electronic conduction in LiV3O8 cathode materials were explored using the first-principles method. It was found that oxygen vacancies can affect the formation and mobility of polarons, and therefore influence the electronic conductivity and ion transport kinetics of the material. It is suggested that enhancing electronic conductivity and maintaining ion diffusivity can be achieved by controlling the V-O concentration.