Microscopic mechanism of path-dependence on charge–discharge history in lithium iron phosphate cathode analysis using scanning transmission electron microscopy and electron energy-loss spectroscopy spectral imaging

Abstract We revisited the “path-dependence” problem, i.e. , the differing polarization observed in LiFePO 4 cathode charge/discharge curves depending on the electrochemical treatment history of the material. The phase distributions of the active material particles with different charge/discharge histories in the LiFePO 4 cathode were investigated through spectral imaging (SI) using scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). The STEM-EELS-SI experiments revealed that LiFePO 4 (LFP) and FePO 4 (FP) almost always coexist in the individual primary particles of Li 0.5 FePO 4 (50% state of charge (SOC)) electrodes, forming core/shell structures. This is unlike the conventional domino-cascade model, in that LFP-inside/FP-outside is observed in the lithium-extracted Li 0.5 FePO 4 particles, whereas FP-inside/LFP-outside is seen in the lithium-inserted Li 0.5 FePO 4 particles. We examined the particle-size dependence of the core/total volume ratio of each Li 0.5 FePO 4 particle, and the essential features of the path-dependent charge/discharge curves were semi-quantitatively reproduced by a simple static model that considered the ionic conductivity of the shell phase.