||The charge-discharge cycling effect on the battery capacity decay for commercial LMO-NMC lithium-ion battery has been studied with the same charge rate and various discharge rates. Three sets of batteries were all charged at 0.5C but discharged at 0.5C, 1C and 2C, respectively, until decaying to 70% of the original capacity at 25oC. The batteries were set to rest for three hours after each charge and discharge process and the reference performance test (RPT) was done every 100 cycles. Similar experiments were carried out again without rest and RPT.
Experimental results show that with increasing number of cycling, the capacity decreases, the positions of the peaks in incremental capacity (IC) curve shift toward the high voltage side, whereas the heights of the peaks decrease. The shifting of the peak position is due to the loss of lithium ions and formation of SEI, whereas the decrease of the peak height is due to the loss of active material. With RPT after each 100 cycles and 3-hour rest after each charge and discharge, the end of life (EOL), defined as the cycling number of decaying to 70% of original capacity, are 197, 236 and 276, for discharging at 0.5C, 1C and 2C, respectively. This indicates that the EOL increases with increasing discharge rate. One of the possible reasons is that the commercial battery used in this study is designed for large current discharging.
Without RPT and rest, the EOLs increase to 317, 463 and 1000+ (over 1000), for discharging at 0.5C, 1C and 2C, respectively. This indicates that the rest after the discharge results in significant capacity decay.
According to the experimental results, we have developed a mathematical model for the effect of cycling number and rest time on the capacity of battery. The parameters in the model have been obtained by nonlinear data regression. Comparison between model calculations and experimental data show that both are consistent with R2>0.98, indicating the model can be used to estimate the battery capacity decay during charge-discharge cycling.