First-Principles Analysis of Li Intercalation in VO₂(B)

Bronze-phase vanadium dioxide VO₂(B) is notable as a promising cathode material for high-capacity Li ion batteries. While various forms of VO₂(B) nanostructures have been experimentally investigated, a comprehensive theoretical understanding of the lithium storage mechanism is still lacking. In this research, we examine the redox mechanism, structural evolution, electronic characteristics, and the kinetics of Li ion diffusion in VO₂(B) by first-principles calculations. The impact of the surface environment on the Li ion insertion properties is also investigated. Our study demonstrates that, although VO₂(B) is capable of incorporating one Li per formula unit from a thermodynamic perspective, its actual capacity can be much less due to kinetic factors. Energy barrier calculations reveal a pronounced compositional dependence of Li diffusion in bulk VO₂(B): the b axis tunnels, which account for the excellent Li diffusivity at low Li content, become blocked when Li concentration goes up. The equilibrium surfaces are predicted to be oxygen-rich in an oxidizing atmosphere, which would lead to a steep gradient of Li concentration near the surfaces and further impede Li intercalation into the bulk. Yet, these limitations can be circumvented by facilitating the diffusion of Li in the c direction that is still feasible at high Li content and stabilizing the stoichiometric terminations of (0 0 1) and (1 1 0) facets, which can alleviate the load of excess Li near the surfaces. Our theoretical insights can provide a rationale for the different experimental values of specific capacity in the literature and inspire new strategies to optimize the electrochemical performance of VO₂(B).