Investigation of significant capacity recovery effects due to long rest periods during high current cyclic aging tests in automotive lithium ion cells and their influence on lifetime

In this work for automotive lithium ion cells, the influence of prolonged rest periods of up to 5 days on the available capacity and the long term aging trend during cyclic aging tests is presented. Automotive prismatic cells with NMC/graphite chemistry and over 25 Ah capacity are investigated by means of various cycle tests. The test matrix includes a variation of possible influencing factors like current rates during cycling as well as temperature and state of charge during rest periods, rest period length and number of cycles between rest periods. A significant capacity recovery during rest periods is measured after cycling at moderate temperatures which has only been reported for low temperature lithium plating experiments before. Rest periods were conducted at 100% state of charge, thus the anode overhang effect can be excluded as the cause for the observed capacity increase. Voltage relaxation tests indicate that lithium plating occurs due to high current rates. Furthermore, a microscopic model is proposed to explain the long time constant of the limited exponential capacity increase during rest periods. Under these plating conditions a significant increase in cell cycle lifetime is observed if regular rest periods or standard characterizations are applied.