Computational Investigation of Li Insertion in Li3VO4

Parallel electrochemical reactions in low-voltage anode Li3VO4 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 Li3VO4 anodes, we have investigated by density functional theory a total of 33 Li3+xVO4 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-Li3VO4 as larger amounts of Li ions are incorporated, due to ion size and electrostatic effects. We found that for 0 < x < 2 the Li3+xVO4 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-Li5VO4 phase is formed after a biphasic region at 0.7 V. The dose structural relationship between H1-Li3VO4 and H2-Li5VO5, with a moderate volume expansion of 4%, supports an insertion reaction as the main mechanism for the reversible cycling of Li3+xVO4 anodes in the range 0 < x < 2. Insertion of a third Li ion in Li3VO4 would produce a reconstructive phase transformation to an antifluorite-type Li6VO4 structure (H-2 -> 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 Li3VO4. The calculated compositional voltage profile and X-ray diffraction patterns are in agreement with experimental observations.