Lithium-ion (Li-Ion) battery technology has been undergoing a period of intense development and improvement, mostly due to the growth of the electric vehicle (EV) market. Since the charging time of EV is still far from the time it takes to refuel gas-powered cars, this is definitely a key parameter that needs to be reduced.
Fast battery charging is a true challenge for researchers because the technology has to be capable of rapidly charging Li-Ion batteries while maintaining good cell cycle life and minimizing cell degradation.
Researchers from Sungkyunkwan University Korea presented a new approach in rapid charging pattern in a paper published in Applied Sciences. The team’s new rapid-optimization pattern improves the cycle life of the cells by 45 percent compared to conventional fast-charging patterns and solves the issue of capacity drop in the second half of cell life during rapid charging.
Conventional typical fast charging patterns use a single or multiple step constant-current constant-voltage (CCCV) charging processes. To speed up the charging process, conventional methods use constant high current in the area in which battery capacity is still low. The charging current is decreased in steps as the state of charge (SoC) increases (as the battery is charging), thus minimizing cell degradation. These methods do not take into consideration environmental variables affecting the battery, nor the impacts of internal degradation, which influence rapid cell degradation.
Other fast-charging methods that have recently been introduced include pulse charging and boost charging, which include intermittent pauses to avoid lithium plating. Plating is caused by high lithium ion concentration around the anode due to high charging current. A drawback of these methods is the use of higher charge currents to compensate for the pause time, resulting in rapid battery capacity losses. Generally, rapid charging design involves the proper balance between charging time and battery cycle life.
Since the applications of Li-Ion batteries, and the environments in which they are used, are becoming increasingly diverse, understanding battery behavior is key to developing proper battery charging methods. EV batteries feature high energy density and performance, and, thus, are more complex and have higher packing density. Predicting battery characteristics is more challenging as the cycle life is increased. Therefore, understanding EV Li-Ion batteries in the context of a charging pattern is difficult to achieve through the usual approaches, such as using simulations to predict battery performance based on lab analysis.
To improve its estimates, the Sungkyunkwan University team used differential voltage analysis (DVA) to determine changes in voltage with varying capacitance (CV method) dV/dQ (dV=the voltage increase, dQ=the capacity increase). The peak values and sudden changes in the dV/dQ graph indicate the active material losses and chemical changes.
Two new charging methods are combined to create an efficient charging pattern, constant voltage step charging pattern and a constant time (CT) pattern. The CV fast-charging pattern allows for self-charge control to reflect cell degradation and reduces cycle life degradation. CT pattern controls the C-rate of the charging step, based on the capacity degradation of the battery cycle, to ultimately adjust the charging time to the initially set rapid-charging time.
“We here report our study on cell resistance characteristics based on DVA, and further propose two adaptive rapid charging patterns that reflect the intrinsic characteristics of cells in a specific SoC, in addition to considering variations in degradation characteristics as the battery cycle life progresses,” the team said.
The experiments performed on the lithium cobalt oxide/graphite 18650 cylindrical 3.3 Ah cell have shown that the number of feasible cycle numbers for the new fast-charging method with a 20percent capacity degradation increased by 61.7percent, when compared to conventional CCCV step charging. The new method has resulted in no capacity drop at the beginning of the cycle. The new rapid charging method is effective in controlling undesirable side reactions between the anode and electrolyte, which prevents capacity loss.
The researchers concluded, “We, therefore, expect that our method will be a good candidate in the development of rapidly charging electric vehicle battery packs while maintaining suitable cell cycle lives”.