Mechanism Research of Charge–Discharge Process for Graphite/LiFePO4 Battery

After summarizing the basis of previous studies, the structural evolution of positive and negative active materials of activated graphite/LiFePO4 battery during the first charge–discharge process and the first charge–discharge process without activation were studied in detail by XRD method. Qualitative phase analysis shows that there is a LiFePO4 → LiFePO4 + Li1−xFePO4 → Li1−xFePO4 + FePO4 → FePO4 process on the positive side, and the crystal structures of LiFePO4, Li1−xFePO4, and FePO4 are the same, so it is not a structural phase transition, but a kind of pseudo-structural phase transition. The quantitative phase analysis shows that this phase transition is asymmetry or hysteresis effect of discharge. On the negative side, LiC6 is not formed at the beginning, but there is a process of C–Li solid solution → LiC24 → LiC12 → LiC6, and it is found that the crystal structures of LiC24, LiC12, and LiC6 are the same as that of 2H-graphite. On the basis of these experimental results, the de-intercalation theoryCharge and discharge processDe-intercalation theory of charge-discharge process. of graphite/LiFePO4 battery during charge and discharge is proposed, and the de-intercalation behavior of Li atoms in positive and negative electrodes and conductive theory of during charge and discharge are described in detail.