Degradation Mechanisms of the Graphite Electrode in C6/LiFePO4Batteries Unraveled by a Non-Destructive Approach

C6/LiFePO4 (LFP) batteries have been cycled at various temperatures (20 and 60°C) and State-of-Charge (SoC) ranges (0–30, 35–65, 70–100% and 0–100%). Electromotive force (emf) curves have regularly been determined by regression extrapolation of the measured voltage discharge curves. A non-destructive approach to quantitatively determine the graphite electrode decay has been proposed on the basis of dVEMF/dQ curves. It is concluded that the graphite inaccessibility is more pronounced at lower SoC (0–30%) than at higher SoC. The graphite electrode decay at 35–65%, 70–100% and 0–100% is identified to be negligible at moderated temperatures but becomes significant at elevated temperatures. The graphite electrode degradation has been confirmed by XPS and Raman spectroscopy, supporting the conclusions drawn from the dVEMF/dQ results. A model is proposed to explain the various graphite degradation mechanisms.