1 Research background
In order to effectively respond to the adverse changes in the environment and climate, reduce greenhouse gas emissions, and achieve the goals of carbon peak and carbon neutrality as soon as possible, the energy structure is rapidly transforming. The electrification of automobiles plays a crucial role in this energy revolution. Power batteries are the main energy storage carriers for electric vehicles, widely used for their high energy and long lifespan advantages. However, in actual vehicle applications, the internal state of the battery changes, leading to a decline in battery performance and a change in its safe working window.
The usage conditions, especially temperature, have a significant impact on the thermal safety of batteries. Currently, the hot summer temperatures pose a serious challenge to the safe use of batteries. Electric vehicles exposed to scorching sun will experience extremely high battery temperatures during charging, which has a serious impact on the safe use of batteries. This work explores the thermal safety evolution of the widely used high nickel ternary lithium-ion batteries under high-temperature usage conditions, revealing the evolution law of thermal safety of lithium-ion batteries.
2 Research contents
A study was conducted on the thermal safety of NMC631 soft pack lithium-ion batteries during high-temperature cycling aging. Adiabatic discharge heat generation test, adiabatic thermal runaway test, and adiabatic overcharging test were conducted when the battery SOH decayed to 100%, 90%, and 80%, respectively. The impact of high-temperature aging on the thermal safety performance of the battery was analyzed from multiple perspectives.
3 Research results
(1) electronic-chemical performance
During the high-temperature aging process, the battery exhibits an approximate linear decay pattern in the early stages of aging. But as the number of cycles increases, the battery SOH accelerates its decay. At the same time, the impedance spectrum of the battery also undergoes significant changes with aging. As the aging degree deepens, the impedance spectrum gradually shifts to the right and the Ohmic impedance gradually increases.
In addition, due to the continuous occurrence of side reactions at the electrode electrolyte interface during the high-temperature aging process, the facial mask impedance and load transfer impedance of the battery interface also increased rapidly, which was shown as a significant increase between the arcs of the middle and high frequency parts. In addition, with the dissolution of transition metals and changes in the cathode structure, the diffusion of lithium ions inside the electrode becomes difficult, and the Weber impedance also significantly increases.
(2) Adiabatic discharge heat generation characteristics
During the high-temperature cycling aging process, internal degradation of the battery continuously occurs, such as the continuous decomposition of the electrolyte and the thickening of the SEI film. These degradation causes the impedance of the battery to continuously increase, further leading to a change in the rate of temperature rise during adiabatic discharge of the battery.
Due to the slight degradation inside the battery during the early stage of high-temperature cycling aging, the temperature rise rate of the battery does not change significantly throughout the entire discharge process. At this point, capacity plays a major role, and capacity decay leads to a decrease in temperature rise throughout the discharge process. After deep aging of the battery, severe degradation occurs inside the battery, resulting in a significant increase in temperature rise rate during adiabatic discharge.
At this point, the rate of temperature rise plays a major role, although the battery capacity decreases, the overall temperature rise significantly increases. Therefore, as the battery capacity decreases, the temperature rise of the battery during adiabatic discharge shows a trend of first decreasing and then increasing.
(3) Adiabatic thermal runaway characteristics
The characteristic temperatures of battery thermal runaway include the starting temperature of self generated heat T1, the triggering temperature of thermal runaway T2, and the highest temperature T3. As the aging degree of the battery deepens, T1 and T2 continuously decrease, indicating that high-temperature aging reduces the thermal stability of the battery. The decrease in T1 is mainly due to the high temperature changing the composition of the SEI membrane, which in turn leads to a decrease in its thermal stability;
And T2 is mainly due to the decrease in thermal stability of the anode and cathode reaction systems. In addition, further activation energy fitting analysis was conducted on the T1-T2 stage, and the fitting results showed that the activation energy of the battery significantly decreased with aging within this temperature range, further indicating that high-temperature aging led to the deterioration of the battery’s thermal stability.
In addition, as the degree of aging deepens, the maximum temperature and maximum temperature rise rate of battery thermal runaway decrease. And there is an exothermic plateau on the temperature temperature rise rate curve. As aging deepens, the length of the exothermic platform gradually shortens. These phenomena indicate that the harm of thermal runaway gradually decreases with battery aging. This is mainly attributed to the continuous consumption of active substances inside the battery during the high-temperature cycling aging process, such as the loss of active lithium and the consumption of electrolyte.
This reduces the amount of participation in violent chemical reactions during the process of severe thermal runaway, resulting in a decrease in the released energy, a decrease in the intensity of the reaction, a decrease in the maximum temperature and maximum rate of temperature rise, and a shortened length of the heat release platform.
(4) Thermal runaway characteristics of adiabatic overcharging
For the process of thermal runaway due to overcharging, the heat generated by aged batteries during this process is lower than that of fresh batteries. For the total heat generated by overcharging and runaway, chemical reaction heat and internal short circuit heat are the main sources of heat. As aging progresses, this portion of heat decreases. Due to the increase in impedance of the battery during high-temperature aging, the time for overcharging to Vip is similar.
Therefore, it can be seen that the Ohmic heat and reversible heat of aged batteries are more than those of fresh batteries. For the thermal runaway triggering of aging batteries, less energy is required, therefore, less side reaction heat is required during this process. Therefore, as the battery ages, the contribution rate of side reaction heat decreases due to the triggering of thermal runaway.