Degradation Mechanisms of the Graphite Electrode in C-6/LiFePO4 Batteries Unraveled by a Non-Destructive Approach

C-6/LiFePO4 (LFP) batteries have been cycled at various temperatures (20 and 60 degrees 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 dV(EMF)/d(Q) 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 dV(EMF)/d(Q) results. A model is proposed to explain the various graphite degradation mechanisms. (C) 2016 The Electrochemical Society. All rights reserved.