Cell aging can be divided into cycling aging and calendar aging. Calendar aging refers to all degradation processes that affect a cell regardless of charge-discharge cycling. It plays a crucial role in various lithium-ion battery applications, especially in scenarios where active operation is considerably shorter than idle periods, such as in electric vehicles. As a result, for lithium-ion cells in most devices, calendar aging occupies a considerable amount of time during the lifetime (10.3390/en14175220, 10.1109/EMCT.2017.8090361), and contributes more than cycling aging to the cell’s aging (10.1016/j.ensm.2023.103147) Therefore, a comprehensive understanding and accurate measurement of the calendar aging process is crucial for predicting battery lifetime and assessing the state of health.
A traditional calendar aging test method is the OCV test. Usually, cells are stored in a stable condition and the capacity is checked regularly (10.1016/j.ensm.2023.103147, 10.1149/2.0411609jes). This OCV method is very time-consuming and inefficient because most of the time is used to wait but not measure data (10.1149/1945-7111/ac6f88). Researchers spent several months (10.1149/2.0411609jes) or even years (10.1016/j.est.2018.01.019) in waiting for cells’ aging.
The float, as a new calendar aging test method, was first reported by Lewerenz et al. (10.1016/j.jpowsour.2017.03.136) and is also known as potentiostatic holds. The testing system keeps the voltage of the cells constant. Once the voltage decreases due to aging, the test system would recharge the cell to maintain the voltage. The current, that is charged into the cell, relates to the aging process in the cells (10.1016/j.jpowsour.2017.03.136).
However, the float current does not directly relate to the charge loss on the anode (10.1149/1945-7111/ac6f88). The float current is the result of voltage change. The charge that the system recharges to the cells does not have to equal the charge loss on the anode. Because the cathode potential also increases during float charging. Then the anode potential does not have to return the original value, which means the float current will not completely compensate for the charge loss on the anode.
This work applied float tests on three-electrode cells. During the experiments, we monitored float current, cell voltage, and anode potential, as shown in attachment 1. By combining the open circuit curve of electrodes, the aging speeds on the anode and cathode were successfully calculated. Furthermore, the capacity loss was precisely calculated by the aging speeds on two electrodes and verified by the final check-up tests as shown in attachment 2.