In the current commercial lithium-ion battery products, the charging and discharging process of the battery is often accompanied by the generation of heat. If the battery generates too much heat during the charging and discharging process and cannot dissipate it in a timely manner, significant degradation and degradation of battery performance may occur due to the accumulation of heat during charging and discharging. When the temperature rises to the point where the internal separator of the battery melts, causing a short circuit between the positive and negative electrodes, the battery may pose a risk of explosion and other hazards. Therefore, studying the heat generation law and thermal runaway behavior of batteries during charging and discharging is crucial for examining the safety of batteries.
Usually, during the use of a battery, the temperature of the battery is significantly increased due to heat exchange between air convection, heat conduction, and the surrounding environment. But in order to study the safety performance of batteries, it is necessary to consider the heat generation behavior of batteries in extremely harsh environments – adiabatic environments. In an insulated environment, there is no heat exchange between the battery and the environment, and the heat generated during the battery charging and discharging process is completely limited within the battery system, which is more likely to cause safety hazards to the battery.
The heat generation during the charging and discharging process of lithium-ion batteries can be roughly divided into two parts: reversible heat (Qrev) and irreversible heat (Qirr). By measuring the thermal effect of a battery in an adiabatic state, not only can the heat generation law of the battery during charging and discharging be understood, but the energy balance of the battery during charging and discharging can also be calculated. Researchers used different methods such as calorimetry and electrochemistry to obtain reversible and irreversible heat of batteries, and developed a heat generation model and thermal simulation method for automotive power batteries. Accelerated calorimetry is a method of testing and analyzing the thermal safety of samples under approximately adiabatic conditions.
With the widespread use of lithium-ion batteries, on the one hand, the requirements for battery life and safety are gradually increasing; On the other hand, the cascading utilization of batteries also needs to consider their safety characteristics. Therefore, it is urgent to study the safety characteristics of life cycle batteries, clarify the safety boundary conditions and energy efficiency relationship of batteries throughout their entire life cycle. So far, there have been many studies on the thermal characteristics of new battery cells, but there have been few studies on the impact of aging on battery safety. General aging can be divided into two types: cyclic aging and storage aging.
Researchers used calorimetry to study the aging and thermal runaway characteristics of lithium-ion batteries under different storage processes. They found that key parameters of thermal runaway, such as exothermic starting temperature and thermal runaway starting temperature, increase with battery aging, while the rate of thermal runaway decreases. Researchers have also studied the heating abuse behavior of working and failed lithium-ion batteries under cyclic aging, high-temperature storage, and room temperature storage aging conditions, and detected the gas explosion process and toxic gases emitted after abuse.
This article takes lithium cobalt oxide batteries as the research object, uses an accelerating rate calorimeter (ARC) to provide an adiabatic environment, tests the specific heat capacity, heat generation, and thermal runaway of the batteries, and studies the thermal characteristics. Study the charging and discharging processes and thermal runaway processes of batteries in adiabatic environments under different cycling aging cycles, and investigate the influence of cycling aging on the thermal characteristics of batteries.
1 Test
The battery adopts a soft pack battery with lithium cobalt oxide (LCO) as the positive electrode and mesophase carbon microspheres (CMS) as the negative electrode, with a capacity of 6.1 Ah and a working voltage of 3.6 V
1.1 Electrochemical performance testing
Capacity testing
The test temperature is (25 ± 5) ℃, and a charging and discharging test system is used for testing. The battery is charged to 4.2 V at a current of 0.1 C, then switched to constant voltage charging with a full current of less than or equal to 0.01 C, and left to stand for 10 minutes; Discharge the battery at a constant current of 0.1 C to 2.75 V, and repeat the charging and discharging cycles three times to obtain a discharge capacity of 6.1 Ah.
Loop testing
At an ambient temperature of (25 ± 5) ℃, charge the battery at a current of 0.5 C to 4.1 V, then switch to a constant voltage until the current is less than or equal to 0.01 C, let it stand for 10 minutes, and then discharge it at a constant current of 1 C to 2.75 V. Cycle it for 500, 1000, and 1500 cycles according to this system.
DC internal resistance test
At an ambient temperature of (25 ± 5) ℃, discharge the battery at a constant current of 0.5 C, and conduct a continuous DC internal resistance test every 12 minutes of discharge (10% DOD). Using a current of 1.5 C for a 10 second pulse discharge, the voltage values before and at the 5th second of the pulse discharge are taken to calculate the DC internal resistance R of the battery under different states of charge.
1.2 Battery thermal characteristic testing
Specific heat capacity test
Perform a battery specific heat capacity test in an accelerated calorimeter. The battery is always in an adiabatic environment, heated by a constant power heating element, and the temperature curve of the battery over time is recorded. The temperature rise rate of the battery in the adiabatic state is obtained by linear fitting the curve. To ensure the accuracy of the measurement, take two battery pack samples and take the average of the two tests.
Heat generation test
Place fresh or cycled batteries in an accelerated calorimeter, balance the temperature between the battery and the adiabatic chamber, and start the charging and discharging system. Charge and discharge are carried out in an adiabatic environment at different operating currents, and the surface temperature of the battery and the curve of the battery voltage over time during the charging and discharging process are collected
Conduct a thermal runaway test on the battery at 100% SOC in an accelerated calorimeter, run the H-W-S mode in an adiabatic state to heat the battery, and simultaneously detect the battery’s self heating rate. When the self heating rate of the battery is ≥ 0.02 ℃/min, it is considered that a self heating reaction has occurred inside the battery. The instrument stops actively heating and enters adiabatic mode, following the temperature rise of the battery until thermal runaway occurs. Simultaneously collect the surface temperature of the battery and the variation curve of the battery voltage over time during the thermal runaway process.
2 Conclusion
The heat generation and thermal runaway behavior during the charging and discharging process of LCO / CMS batteries were studied using an adiabatic accelerated calorimeter. We studied the heat generation behavior of batteries at different charging and discharging rates, and analyzed the effects of working current and cyclic aging on the heat generation characteristics of batteries. With the cyclic aging of the battery, the internal resistance and capacity loss of the battery increase, and the average charging and discharging heat generation rate and total heat generation of the battery both increase.
Comparing the thermal runaway behavior of batteries before and after cycling, it was found that the starting temperature of self heating slightly increased after cycling aging, while the starting temperature of thermal runaway remained basically unchanged. However, the time from self heating to thermal runaway of batteries was shortened. For the thermal runaway behavior of batteries, it is not only necessary to pay attention to key temperature points such as the starting temperature of self heating and the temperature of thermal runaway, but also to accurately measure the heat generation rate and time of the thermal runaway process, in order to evaluate the thermal runaway behavior of batteries throughout their entire life cycle.
By analyzing the changes in heat generation and heat generation rate during the charging and discharging process of batteries under different cycle aging cycles, the effects of entropy and enthalpy changes on batteries are analyzed. Thermodynamic parameter changes are used as a non-destructive testing method to reflect the degree of battery degradation and evaluate the health status of the battery.