Evolution and Migration of Lithium-Deficient Phases during Electrochemical Delithiation of Large Single Crystals of LiFePO₄

In a lithium ion battery, lithiation and delithiation of active materials occur during charging and discharging, and diffusion of Li (or Li vacancy) and related phase transformation in the active material greatly affect battery performance. Evolution and migration of Li-deficient phases at surfaces of single crystals of LiFePO4 (2 × 3 × 6 mm in size) during electrochemical delithiation were studied by using Raman microscopy and in situ wide-view scanning Raman spectroscopy. Domains of phases with Li vacancies (LV phases) of a few micrometers or less in sizes formed in the LiFePO4 (LFP) phase during delithiation at a low potential of 4 V vs Li/Li+, and LV phases migrated at a rate corresponding to Li diffusivity of 3 × 10–9 cm2/s. In contrast, at a high potential of 4.5 V, an FePO4 (FP) phase grew in addition to the LV phase, and the rates of growth of the FP phase and migration of LV phases were characterized by quite different Li diffusivities of 1.4 × 10–11 cm2/s and close to 10–7 cm2/s, respectively. The very different rates suggest mechanisms for diffusion strongly coupled to phase mixing/separation kinetics depending on the concentration of Li, which provides implications for the development of superior active materials for high-performance batteries. Such phenomena probably extend to other phase-separating battery materials. This work demonstrates a unique method for in situ observation of phases with dilute vacancies and their large-scale migration.