A hybrid phase-transition model of olivine LiFePO4 for the charge and discharge processes

Generally, it is well known that the lithium-ion diffusion in olivine LixFePO4 is correlated with the phase transition dominated by a domino-cascade model instead of a core–shell model. Based on the experiment result, we firstly found that the linear Warburg region of electrochemical impedance spectra should tilt with a small angle of 30° for the domino-cascade model, while the core–shell model should correspond to a traditional angle of about 45°. Considering that the measured Warburg impedance varies along the charge and discharge processes, we firstly proposed a hybrid phase-transition model as a combination of the domino-cascade model and the core–shell model to describe the olivine LixFePO4. And this hybrid model was possibly attributed to both the strong anisotropy in the bulk of olivine LixFePO4 and the different properties between the lithiated and delithiated phases. Potentially, this micro-mechanism could be extended to some other two-phase electrode active materials in lithium ion batteries. ► The Warburg impedance is dependent on the phase and the applied potential. ► The domino-cascade model leads to a tilt angle of 30° in the Warburg region. ► A hybrid phase-transition model of domino-cascade model and core–shell model. ► This hybrid model is due to the strong anisotropy and the two-phase difference.