Abstract:

The capacity degradation patterns and mechanisms of a cell comprising graphite negative electrodes and {LiMn2O4 (LMO) + LiNi1/3Mn1/3Co1/3O2 (NMC)} blended positive electrodes were investigated and analyzed. The contributions of positive and negative electrodes to the full cell degradation were distinguished by
measuring the charge/discharge curves, direct current resistance and electrochemical impedance spectroscopy, via embedded lithium metal reference electrode, as a function of cycle number during cycle aging. The results show that 1) In the blended positive electrodes, the lithiation of NMC was hindered by the progressive loss of lithium inventory as well as the growing graphite impedance. Gradually, LMO became the predominant capacity contributor in the blended positive electrodes in the later stage of cycle aging; 2) During cycle aging, the cell capacity faded along with the impedance increase largely at the negative electrode. These results provide evidence for improving the cycling performance of graphite/{LMO + NMC} cells.

Key words: electric vehicles, power batteries, degradation mechanism, blended positive electrodes, reference electrodes, LMO (LiMn2O4), NMC (LiNi1/3Mn1/3Co1/3O2)