Calendar aging mechanism of NCM811/graphite-SiOx pouch cells at different temperatures

Considering that a personal car spends about 95% of its life in parking mode, calendar aging can have a significant impact on battery life. High temperature storage is a common method for rapid evaluation of battery calendar life. In order to obtain reliable results of high temperature accelerated aging test, it is necessary to study the aging mechanism of battery stored at different temperature conditions. In this paper, the calendar aging experiment of graphite-SiOx/NCM811 pouch cells were carried out within the temperature range of 25-55℃. The differential curve analysis and post-mortem analysis were used to explore the aging mechanism. The results show that the calendar aging of pouch cells is mainly caused by the loss of lithium inventory and the loss of cathode active materials. When cells store at higher temperature, the loss of lithium inventory and the loss of cathode active materials increase, while the loss of anode active materials remains relatively unchanged. Based on various test results, it can be inferred that the parasitic reactions on the surface of electrode lead to the loss of lithium inventory and the increase of SEI. With the increase of storage temperature, this side reaction continues and consumes more lithium inventory. It is worth mentioning that when storage at 55℃, the microcracking develops and even breaks some of the secondary particles of cathode materials. Therefore, the pouch cells suffer severe loss of cathode active materials, which makes it inappropriate to accelerate the aging of batteries at such high temperature.