The Effect of High Charge Rate Cycling on Coulombic Efficiency Measurements
Smith et al.  showed that as the charge rate of Li-ion cells was increased from ~C/100 to ~C/20, the coulombic efficiency (CE) increased such that (1 – CE) = k*t, where k is a constant that increases with temperature and t is the time of one charge-discharge cycle. Extending this idea to higher cycle rates means that at very high rates the coulombic efficiency approaches 1.00000 since there is very little time per cycle for degradation. However, it is well known that lithium plating can occur at high charge rates with graphite negative electrodes [6, 7] and this would lower the coulombic efficiency. It has been shown that the coulombic efficiency of plating and stripping metallic lithium from graphite electrodes is 0.97-0.98  compared to the coulombic efficiency of intercalating and de-intercalating lithium from a graphite electrode which is > 0.995 . Therefore as the charge rate continues to increase past the onset of lithium plating, the coulombic efficiency should begin to depart further from 1.00000 as a larger fraction of the lithium is involved in plating/stripping instead of intercalation/de-intercalation reactions.
Figure 1 shows CE versus charge rate results for two temperatures that shows the time dependent degradation regime at low rates on the left (low rates) and the regime governed by lithium plating on the right (high rates).
This study will show results of high charge rate cycling on Li[Ni1/3Mn1/3Co1/3]O2 (NMC)/graphite cells at different temperatures to more thoroughly understand the shape of these curves and the impact of lithium plating on precision measurements of coulombic efficiency and endpoint capacity slippage. This behavior needs to be well understood as many applications for Li-ion batteries, including electrified vehicles, desire high charge rates.
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