We all know that the capacity of a lithium-ion battery decreases with each charge-discharge cycle, directly resulting in a decline in battery performance. So, what factors affect the performance of lithium-ion batteries?
Many factors influence the capacity of lithium-ion batteries, including operating temperature, charge/discharge current, and charge/discharge cutoff voltage. The mechanisms causing capacity degradation in lithium-ion batteries can be categorized into three types: increased internal resistance and polarization, loss of active materials at the positive and negative electrodes, and loss of lithium (Li).
Different external factors have varying effects on these three aspects. For example, lithium-ion batteries made of LiFepO4 material have excellent cycle performance, but different usage conditions have a significant impact on their cycle life. Tests have shown that 15C pulse discharge and 15C continuous discharge have completely different effects on 26650 lithium-ion batteries. The capacity of a 26650 lithium-ion battery undergoing 15C pulse discharge decreases very rapidly; after 40 charge-discharge cycles, it can no longer discharge at 15C, but it can still discharge at 1C. In contrast, the capacity of a battery undergoing 15C continuous discharge decreases more slowly; after 60 cycles, it can still discharge at 15C, but the capacity decay rate at 1C is faster than that at 15C pulse discharge.
The impact of increasing the rate of lithium-ion batteries on battery performance
Figure 1. The effect of rate increase on battery performance of lithium-ion batteries
Mechanism analysis concludes that the 15C pulse discharge battery produces more LiF in the SEI film of the negative electrode. LiF further hinders lithium ion diffusion, causing a rapid increase in the battery's Li+ diffusion resistance and charge exchange resistance. This results in excessive polarization voltage during charging and discharging, leading to a rapid decrease in the high-current discharge capability of LiFepO4.
Research on the impact of charging strategies on the lifespan degradation of lithium-ion batteries can better guide our lithium-ion battery design. This paper investigates the impact of different charging control strategies on the lifespan degradation of lithium-ion batteries, studies their application mechanisms, and proposes a lifespan degradation model for lithium-ion batteries. Experimental research shows that when the charging current and cutoff voltage exceed certain values, the degradation of lithium-ion batteries will be greatly accelerated. To reduce the degradation rate of lithium-ion batteries, appropriate charging and discharging currents and cutoff voltages must be selected for different systems.
The impact of rate discharge on battery performance
Figure 2. Effect of rate discharge on battery performance
The data shows that as the charging rate increases, the degradation rate of lithium-ion batteries also increases rapidly. The curve slopes reveal three distinct degradation stages: a rapid initial degradation stage (Stage 1), a slower, stable stage (Stage 2), and a later, accelerated degradation stage (Stage 3). Research on the degradation mechanism in these three stages suggests that Stage 1 may be due to the consumption of Li+ ions during SEI film growth, leading to a faster degradation rate. In Stage 2, the SEI film structure stabilizes, resulting in a more stable internal structure and a slower degradation rate. In Stage 3, as the battery ages, active material loss occurs, reducing the active electrode interface and making the battery highly sensitive to current. Figure C shows the experimental results on the effect of different cutoff voltages on the battery degradation rate. The results indicate that increasing the charging cutoff voltage to 4.3V leads to a sharp deterioration in battery cycle performance, while lowering the charging cutoff voltage effectively improves cycle performance.
The effect of rate capability on internal resistance of lithium-ion batteries
Figure 3. Effect of rate of increase on internal resistance of lithium-ion battery
The dynamic internal resistance analysis of the battery is shown in Figure a. From the test results in Figure a, when the charging current is less than 1C, the dynamic internal resistance of the battery changes almost the same trend with battery cycling. However, when the charging current exceeds 1C, the rate of increase in the dynamic internal resistance increases rapidly with the increase in charging rate. From the test results in Figure b, when the charging cutoff voltage is 4.3V, the increase in the battery's dynamic internal resistance is very rapid, indicating that a high cutoff voltage will deteriorate the battery's kinetic conditions. When the cutoff voltage is 4.1V and 4.2V, the increase in the battery's dynamic internal resistance is relatively slow.
From the above analysis, we can observe that both the charging current and the charging cutoff voltage have a certain value. When the charging current or voltage exceeds this value, it leads to accelerated battery degradation. For the battery mentioned above, this value is 1C and 4.2V. When the charging current and cutoff voltage exceed this value, battery degradation is accelerated. When they are below this value, increasing the charging current and cutoff voltage does not significantly increase the rate of battery degradation. Mechanism studies on the influence of charging current and cutoff voltage on battery degradation rate show that when the charging current is below 1C, the main impact is on the loss of positive and negative electrode active materials, while when the cutoff voltage is below 4.2V, the main impact is on lithium loss. When the charging current and cutoff voltage are above these values, the loss of both positive and negative electrode active materials and lithium loss are significantly accelerated.
