Increased Conductivity in the Metastable Intermediate in LixFePO4 Electrode

With increasing concerns about energy and environmental issues, lithium ion batteries are now penetrating into large-scale applications such as electric vehicles. As an electrode reaction process, it is generally believed that two-phase reaction with structural rearrangement and large lattice mismatch impedes high-rate capability. However, LixFePO4, with its two-phase reaction between LiFePO4 and FePO4, exhibits an exceptional high-rate performance. In this article, after confirming the existence of a single-phase reaction even under moderate rates, we demonstrate an approximately 2 orders of magnitude increase of the conductivity for the quenched intermediate Li0.6FePO4. In addition to the widely accepted strain relaxation effect at the two-phase interface, the dramatically increased conductivity due to polaron/lithium carrier density increase in the intermediate phase should be highlighted as an important factor to accelerate the electrode reaction of olivine LixFePO4.