Redox-Induced Structural Change in Anode Materials Based on Tetrahedral (MPn₄)x⁻ Transition Metal Pnictides

The study presents a combined experimental and computational study of Li+ extraction/insertion mechanisms in cubic-fcc transition metal pnictides LixMPn4 (MPn = TiP, VP, VAs). Exhibiting high specific capacities (Cmax = 830 mA h g-1) at a potential close to 1 V, these materials have been recently proposed as promising negative electrode materials for Li-ion battery. In situ XRD and first-principle electronic structure calculations including full structural relaxations are used to correlate the reversible structural phase transition observed in oxidation/reduction, to a loss/reconstruction of the unit cell fcc symmetry. The mechanism is rationalized in terms of a M−Pn bond contraction/elongation of the tetrahedral (MPn4)x- electronic entities, on which initial structures are built. This unusual redox-induced mechanism occurs without any significant electrode volume change, thus solving one of the major drawbacks of anodes.