Cell-to-cell variation of calendar aging and reversible self-discharge in 18650 nickel-rich, silicon–graphite lithium-ion cells

The origin of the voltage imbalance in lithium-ion battery packs is often linked to different self-discharge rates. However, there is no information regarding the variance of self-discharge currents nor possible implications for cells connected in series. In this study, initial cell parameter distributions were analyzed via differential voltage analysis (DVA) and variations of calendar aging and reversible self-discharge rates of 24 commercial 18650 nickel-rich/SiC cells were statistically evaluated. After initial characterization, all cells were stored for 10 month at 70% state of charge (SOC) at 25 °C, whereby last two weeks of storage were used for reversible self-discharge determination via voltage decay. The results obtained in this work reveal that initial capacity and impedance variations do not depend on electrode balancing, nor storage capabilities of NMC, silicon or graphite. Furthermore, the evaluation of calendar aging showed, that all cells exhibited constant but different aging rates. However, whereas the relative variance of the impedance remained almost the same, the relative variance of the capacity almost doubled during the storage. The average self-discharge current amounted to 4.2 µA (3.5 Ah cell) with the relative variance of 10%. Model based assessment of different self-discharge currents revealed almost no influence on the voltage imbalance in battery packs.