Calendar aging plays a crucial role in various lithium-ion battery applications, especially in scenarios where active operation is mostly at low currents or only a few hours a day. Thus, calendar aging is a significant driver of cell degradation during the lifetime (10.3390/en14175220, 10.1109/EMCT.2017.8090361), and may contribute more than cycling to the cell’s aging (10.1016/j.ensm.2023.103147). 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, it is used to wait but not measure data (10.1016/j.est.2018.01.019，10.1149/1945-7111/ac6f88). The float current analysis was first reported by Lewerenz et al. (10.1016/j.jpowsour.2017.03.136). The test keeps the voltage of the cell constant. The current during the float test relates to the aging process in the cells (10.1016/j.jpowsour.2017.03.136).

This work applied float tests on three-electrode cells. The Galvanostatic Intermittent Titration Technique (GITT) is used to obtain the OCV curves that can relate the electrodes‘ potential to the state of charge (SOC). We monitored float current, cell voltage, and anode potential. The raw potential data is plotted in Figure a. The potentials of electrodes are marked on the OCV curves, and the corresponding charge value (Q) can be found as shown in Figure b. By calculating the Q change over time, the aging effect on electrodes can be quantified. I_SEI and I_CL in Figure d show the calculated aging speed on the anode and cathode. On anode, it is the solid electrolyte interphase formation that is well known. On cathode, it has been recently proven to be cathode lithiation (10.1149/1945-7111/ad4821). Furthermore, the capacity loss was precisely calculated by the aging speeds on two electrodes and verified by the final check-up tests. By matching the cathode potential SOC to the anode potential SOC, the absolute capacity and loss of active lithium can also be precisely predicted.