Computational Investigation of Li Insertion in Li3VO4

Abstract

Parallel electrochemical reactions in low-voltage anode LiVO represent a fundamental barrier for fully reliable mechanistic studies even using the most advanced electrochemical and structural characterization techniques. Aiming to unravel the lithium insertion mechanism in LiVO anodes, we have investigated by density functional theory a total of 33 LiVO configurations (x = 0, 0.5, 1, 1.5, 2, 2.5, 3). The key aspect of the proposed Li insertion mechanism is the structural rearrangement of the host-LiVO as larger amounts of Li ions are incorporated, due to ion size and electrostatic effects. We found that for 0 < x < 2 the LiVO phases are energetically stabilized by the distortion of the initial hexagonal package of the oxygen array (H1 → H2 transformation). Specifically, a very stable intermediate H2-LiVO phase is formed after a biphasic region at 0.7 V. The close structural relationship between H1-LiVO and H2-LiVO, with a moderate volume expansion of 4%, supports an insertion reaction as the main mechanism for the reversible cycling of LiVO anodes in the range 0 < x < 2. Insertion of a third Li ion in LiVO would produce a reconstructive phase transformation to an antifluorite-type LiVO structure (H2→ C transformation). The low predicted voltage for such a process (0.14 vs Li/Li) and the major structural rearrangements (20% volume variation) make unlikely the reversible insertion of 3 Li ions in LiVO. The calculated compositional-voltage profile and X-ray diffraction patterns are in agreement with experimental observations.