With the rapid expansion of the electric vehicle market, problems related to electric vehicles are becoming increasingly prominent, and one of the most serious problems is the long charging time, so the demand for fast charging is becoming increasingly urgent. In addition, to meet the needs of some cold regions, the low-temperature performance of batteries is also of concern. Therefore, the demand for fast charging at low temperatures is constantly increasing. However, whether it is fast charging or working at low temperatures, it will lead to rapid degradation of battery capacity. Therefore, it is urgent to study the impact of low-temperature fast charging on battery degradation to solve this problem.

Test

The performance testing conditions of the battery are: at -25 ° C, charge and discharge at a rate of 2 C according to the constant current constant voltage charging and discharging system (CC-CV), record the actual capacity every 10 cycles, and perform a mixed pulse power characteristic test (HPPC). The purpose of the HPPC test is to determine the internal resistance of the battery. The changes in capacity and internal resistance of the battery during long cycles are shown in Figure 1.

At the beginning of the cycle, the low-temperature capacity of the battery decays rapidly, but in subsequent cycles, the battery decay slows down to a certain extent. In addition, when measuring the standard capacity of the battery at room temperature, it was found that the battery capacity was partially restored, and the recovery rate gradually increased with the increase of the number of cycles. It can reach 80-90% after 200 cycles. The internal resistance of the battery shows the opposite trend. In the early stage of the cycle, the internal resistance increases linearly and rapidly, but slowly after 200 cycles.

Figure 1 Diagram of the relationship between battery capacity and number of cycles.

• C_-25 °C:The actual discharge capacity measured every 10 cycles at  -25 ° C;
• C_23 °C: the standard capacity measured in the third cycle at room temperature at 23 ° C;
• R_d: Pulse discharge impedance;
• R_C: Pulse charging impedance

Figure 2 shows the proportion of constant current discharge capacity to the total discharge capacity at low temperatures (under the low-temperature CC-CV system, the discharge process includes constant current discharge and constant voltage discharge). It can be seen that the constant current discharge capacity gradually decreases and a sharp drop occurs at about 120 cycles, accounting for only 15% of the total discharge capacity.

Figure 2 CC discharge capacity and its ratio to the total discharge capacity of the cycle (C_cc: CC discharge capacity; C_cc ratio: The proportion of CC discharge capacity to total discharge capacity)

As can be seen from the above, 120 cycles is a critical point. Therefore, the battery after 120 cycles and long cycles (250 cycles) was disassembled and analyzed. As shown in Figure 3, the silver white lithium metal particles almost cover the entire negative electrode, and their distribution is uneven, with more at the edges, which is related to the uneven current distribution and heat generation. Figure 4 shows the SEM image of the electrode, which shows that as the number of charge and discharge cycles increases, the number of lithium dendrites also gradually increases, making graphite particles less visible. This indicates that lithium precipitation is the main reason for battery failure under low temperature and high rate conditions. In addition, it also indicates that lithium will preferentially deposit at the edge of the electrode.

Figure 3 The negative electrode after different cycles

Figure 4 SEM images of the negative electrode surface after different cycles (A, a; D, d): 0 turns; (B, b; E, e): 120 turns; (C, c; F, f): 250 turns (A-C): Edge; (D-F): Center (a – f): The corresponding enlarged image of (A – F)

In order to study the evolution process of lithium deposition, the profile of the negative electrode was analyzed, as shown in Figure 5. The deposition of lithium metal is very uneven, and the deposition layer at the edge is significantly thicker. After 120 cycles, the thickness of the edge layer is 15 µ m, while the thickness of the central area is only 7 µ m. And as the number of cycles increases, the deposited lithium metal layer becomes thicker. Subsequently, XPS analysis was conducted on the negative electrode surface.

The presence of carbides could not be detected on the electrode surface, indicating that the graphite surface had been completely covered by other substances. The large amount of lithium compounds detected were SEI components produced by the reaction between lithium metal and electrolyte. After deep etching of the negative electrode, the presence of elemental lithium was discovered, once again proving that lithium metal is one of the reasons for battery failure.

Figure 5 SEM images of negative electrode cross-sections after different cycles (A-C) Edge; (D-F): Center (a – f): The corresponding enlarged image of (A – F)

Conclusion

This work delves into the capacity degradation mechanism of lithium-ion batteries under low temperature and high rate conditions. Under this operating condition, the battery exhibits some special phenomena, such as a sharp drop in constant current discharge capacity during the mid cycle, lower charging capacity than discharge capacity, and battery capacity recovery. The research results reveal that lithium deposition is the main cause of these characteristic behaviors and the problem of battery failure.

The specific manifestation is that lithium deposition increases the internal resistance of the battery, resulting in a decrease in the discharge capacity of the battery. At the same time, the decrease in electrolyte conductivity, electrode reaction, and ion solid-phase diffusion further increase the internal resistance of the battery, resulting in a sharp decrease in battery capacity. However, due to the active nature of the deposited lithium metal, it can still dissolve and return to the positive electrode during discharge, resulting in a discharge capacity higher than the charging capacity. Moreover, when the ambient temperature increases, it can still be embedded into the graphite negative electrode, causing the capacity to recover.