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. T...