During the use of lithium batteries, the actual available capacity will continue to decrease compared with the rated capacity when it leaves the factory, that is, capacity fading occurs. Any side reaction that consumes lithium ions can lead to a change in the balance of lithium ions in the battery that is irreversible and can accumulate over multiple cycles, adversely affecting battery performance. A battery charge and discharge once is called a cycle, and the cycle life is an important indicator to measure the battery life performance. The root cause of the factors affecting the cycle life of lithium batteries is that the number of lithium ions involved in energy transfer is constantly decreasing. However, the total amount of lithium elements in the battery has not decreased, but the “activated” lithium ions are less, they are imprisoned in certain places or the transmission channels are blocked, and cannot freely participate in the process of charging and discharging.
Charge and discharge curve The charge and discharge curve refers to the curve of the battery’s voltage, current, capacity, etc. changing with time during the charging and discharging process of the battery. The information contained in the charge-discharge curve is very rich, including capacity, energy, working voltage, and voltage platform, the relationship between electrode potential and state of charge, etc. The main data recorded during the charge and discharge test is the time evolution of current and voltage. Many parameters can be obtained from the basic charge and discharge data of batteries that have undergone different cycles. The subsequent analysis is basically based on extracting data from the charge and discharge curves. The process of analysis. Typical cycle charge and discharge curves will change as the cycle progresses, the battery capacity decays and the charge and discharge curves will change.
In order to see the change of the charge-discharge curve more intuitively, the ordinary charge-discharge curve can also be made into a curve of cumulative capacity. From the first charge, the abscissa is the capacity or specific capacity. The discharge capacity starts from the charge capacity, and the discharge capacity is a Negative value, gradually decreasing. After each step, the capacity of the previous step is used as the starting point, the capacity is positive during charging, and gradually increases; the capacity is negative during discharging, and gradually decreases.
Constant voltage charging current and time lithium batteries are usually discharged at different currents during use, and often cannot experience a complete and stable discharge process. This incomplete discharge process will affect the subsequent charging process. The battery charging process is generally a cross-current-constant voltage mode CC-CV, which consists of two continuous processes: CC charging and CV charging, with constant current until the battery voltage reaches the nominal maximum voltage. Subsequently, the battery enters the constant voltage charging mode, and the charging voltage remains constant until the charging current gradually decreases to the cut-off current, as shown in Figure 7. Whether the battery is fully discharged or not, the dynamic characteristics of the CV phase can well reflect the health information of the battery. In addition, charging data in the CV phase can be comprehensively monitored through the BMS. Therefore, the kinetics of CV charging can be used to study the battery aging law. During the charging process of the battery, as the CC charging progresses, the lithium ions inside the electrode material migrate from the positive electrode to the negative electrode under the action of the load current. The concentration of lithium ions in the anode gradually increases with time and reaches a maximum at the beginning of the CV phase. In the CV stage, as the load current decreases, the lithium ions in the electrolyte gradually intercalate into the negative electrode, and the concentration decreases rapidly. When the CV phase ends, lithium-ion insertion is complete, and the concentration of lithium ions in the anode drops to almost the same level as that in the cathode. In the CV stage, the loss of active lithium is more common and obvious than in the CC step, and the literature reports that the active lithium loss is 5.5% and 94.5% in the CC stage and the CV stage, respectively.