3.Experimental results and analysis
3.1 Changes in Electrical Parameters of Low Temperature Aging Batteries
3.1.1 Open Circuit Voltage (OCV) Characteristics
Open circuit voltage (OCV), as a core indicator reflecting the state of charge (SOC) of a battery, is extremely sensitive to temperature changes. Research has found that within the temperature range of 0 ℃ to 45 ℃, the OCV corresponding to 0-100% SOC remains stable at 2.95V-4.17V. But when the temperature drops below 0 ℃, the OCV in the low SOC region (SOC<50%) significantly increases. For example, an aging battery that has undergone 150 cycles at -20 ℃ has an OCV of 3.66V at 0% SOC, which is 0.32V higher than that of a newer battery. At the same time, the higher the degree of battery aging, the flatter the OCV-SOC curve, indicating the need to optimize the OCV model in low-temperature environments to improve SOC estimation accuracy.
3.1.2 Variation law of internal resistance
The internal resistance of a battery is directly related to its heat generation efficiency and charge discharge performance. HPPC test data shows that a decrease in temperature can lead to a sharp increase in internal resistance: at -20 ℃, the new battery’s ohmic resistance (109.0m Ω), polarization resistance (44.2m Ω), and total internal resistance (153.3m Ω) are 5.6 times, 3.3 times, and 4.7 times higher than at 25 ℃, respectively, with ohmic resistance being more sensitive to temperature changes.
The aging process further exacerbates the temperature response of polarization resistance: after 150 cycles, the polarization resistance of the battery reaches 5.7 times that of 25 ℃ at -20 ℃. In addition, the proportion of Ohmic resistance in the total internal resistance shows a trend of first increasing and then decreasing with decreasing temperature. When the temperature drops below the threshold, polarization effect becomes the dominant factor in the increase of internal resistance.
3.2 The impact of low temperature on battery aging characteristics
3.2.1 Capacity attenuation law
Low temperature environment significantly suppresses battery performance by reducing electrolyte conductivity and lithium ion diffusion rate, accelerating capacity loss. 150 cycles of experimental data show a strong negative correlation between temperature and capacity retention rate: the capacity retention rate is 83.3% at 25 ℃, while it is only 74.2% at -20 ℃. At extreme low temperatures, the initial discharge voltage of the battery rapidly decays, dropping from 3.06V to 2.56V after 150 cycles, approaching the cut-off voltage of 2.5V; At the same time, the maximum voltage rebound decreased from 3.59V to 3.26V, highlighting the safety hazards of high rate charging and discharging of aging batteries in low-temperature environments. Suggest comparing the capacity change curves of -20 ℃, -10 ℃, and 25 ℃ within 0-150 cycles to visually present the low-temperature accelerated capacity decay trend.
3.2.2 Differences in Internal Resistance Growth
The cyclic experiment reveals the nonlinear effect of temperature on the increase of battery internal resistance: in the range above 0 ℃, the lower the temperature, the faster the increase in ohmic resistance (15.4% increase at 25 ℃ and 19.7% increase at 0 ℃), which is related to the thickening of SEI film caused by lithium coating; However, at -20 ℃, the increase in ohmic resistance slows down to 12.4%, possibly due to the improvement of local conductivity caused by polarization induced heat generation. In contrast, the polarization resistance continues to increase with decreasing temperature, with an increase of 28.9% at -20 ℃ (only 13.5% at 25 ℃). It is worth noting that the total internal resistance growth reaches its peak at -10 ℃ (20.8%), indicating that this temperature is the key critical point for battery aging prevention and control.
3.3 The Effect of Charge Discharge Rate on Low Temperature Aging
3.3.1 Correlation between Capacity Changes and Ratios
Under low temperature conditions, there is a complex relationship between charge discharge rate and battery capacity stability. High rate charging and discharging can exacerbate electrode polarization, leading to increased capacity fluctuations; The internal resistance heat generated by moderate magnification can actually alleviate the inhibition of electrochemical reactions by low temperatures. Through 25 cycles of experimental data, it can be seen that under -20 ℃ conditions, the capacity retention rates during 3C charging and discharging are 85.7% (charging) and 87.0% (discharging), respectively. When the discharge rate is in the range of 0.33C-0.5C, higher rates increase electrode activity due to heat generation, resulting in a decrease in capacity decay rate; But after exceeding 0.5C, such as 3C high rate charging and discharging, the polarization effect is significantly enhanced, accelerating capacity decay. After 150 cycles, the 3C charging capacity is only 2396.9mAh, confirming that long-term use at high rates will significantly shorten battery life.
3.3.2 Relationship between internal resistance growth and multiplication rate
At -20 ℃, the effect of charging rate on battery internal resistance is much higher than that of discharging process. When charging at 0.25C-0.66C, the polarization resistance increases by 6.3% (from 9.1% to 15.4%), which is about twice the increase in ohmic resistance; Within the range of 0.25C-0.5C, the total internal resistance increased from 6.0% to 9.1%, while the increase tended to flatten out between 1C-1.5C. This indicates that the optimal balance between charging efficiency and internal resistance control can be achieved within the charging range of 1C-1.5C.
3.4 Effects of DOD/DOC on Low Temperature Aging
3.4.1 Correlation between Capacity Attenuation and Voltage Window
At low temperatures (-20 ℃), the charge discharge voltage window (DOD/DOC) is a key factor affecting the battery cycle life. Experimental data shows that the charging cut-off voltage is positively correlated with capacity decay: batteries with 4.2V cut-off charging experience a capacity decay of 17.5% after 150 cycles, while batteries with 3.8V cut-off only experience a decay of 2.1%; The lower the discharge cut-off voltage, the more significant the capacity loss. However, a narrow voltage window (such as 3.4V-4.2V) can exacerbate polarization phenomena and accelerate capacity decay. Therefore, it is recommended to use a charging and discharging voltage window of 3.0V-4.0V under low temperature conditions to reduce the damage of overcharging and discharging to the battery.
3.4.2 Relationship between Internal Resistance Growth and Voltage Window
-At 20 ℃, the depth of discharge (DOD) directly affects the growth rate of battery internal resistance. When the discharge cut-off voltage drops from 3.4V to 2.5V, the increase in ohmic resistance jumps from 3.5% to 10.3%, and the polarization resistance increases from 3.5% to 7.5%; When using a narrow voltage window cycle of 3.4V-4.2V, the polarization resistance increases by 7.7%. This indicates that both too deep discharge depth and too narrow voltage window will accelerate the increase of internal resistance, emphasizing the necessity of balancing wide voltage window and shallow charging and discharging strategies in low-temperature environments.
3.5 Thermal runaway characteristics of low-temperature aging batteries
Low temperature aging batteries that are fully charged (100% SOC) pose a high risk of thermal runaway. In the experiment, the aging battery experienced shell rupture, electrode material spraying, and toxic gas release during thermal runaway, leaving a large amount of charred material in the calorimeter. The key thermal parameters show that after 150 cycles, the highest temperature of thermal runaway of the battery reached 614.2 ℃, with a self heating rate of 349.1 ℃/s, which is almost the same as that of fresh batteries (612.9 ℃), confirming that low-temperature aging did not significantly weaken the thermal runaway energy of the battery. In addition, the severity of thermal runaway is positively correlated with the initial temperature (T2) – the higher the T2 (such as 150 cycles T2=208.6 ℃), the higher the peak temperature and self heating rate of thermal runaway, highlighting the decisive role of self heating accumulation in the T1-T2 stage on the intensity of thermal runaway.
4.Conclusion
This study investigated the aging and safety characteristics of 18650 NCM lithium batteries in a -20 ℃ environment through low-temperature experiments, providing important basis for low-temperature optimization of battery management systems (BMS)
Low temperature accelerated aging, significant changes in electrical parameters: After 150 cycles at -20 ℃, the battery capacity was only retained at 74.2%, polarization resistance surged by 28.9%, and the open circuit voltage state of charge (OCV-SOC) curve tended to flatten, requiring recalibration of the SOC estimation model to ensure accuracy.
The charging and discharging rate has a significant impact: 1-1.5C charging and discharging can utilize internal resistance to generate heat, alleviate polarization, and delay battery aging; However, 3C high rate charging and discharging can cause lithium metal precipitation, accelerating battery performance degradation.
The voltage window needs to be strictly controlled: controlling the charging and discharging voltage within the range of 3.0V-4.0V can achieve the best balance between capacity and lifespan; 4.2V full charge or 2.5V deep discharge will significantly shorten the battery life.
Low temperature safety risks cannot be ignored: even after 150 low-temperature cycles, the temperature of the battery still exceeds 600 ℃ during thermal runaway. It is urgent to strengthen the monitoring and protection functions of BMS for thermal runaway in low-temperature environments.
Subsequent research can use techniques such as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) to analyze the low-temperature aging mechanism at the microscopic level, and construct low-temperature aging models that are suitable for different battery systems, helping to improve the winter operating performance and economy of electric vehicles.