At the end of January, I participated in the THT Accelerating Rate Calorimeter (ARC) technology and application training in Britain in 2018. There are people at the training site, CATL, AVIC Lithium Battery, Guoxuan, BYD and other domestic mainstream lithium-ion battery manufacturers Participate in the exchange and training, all manufacturers are very concerned about the battery safety, and has extensive use of ARC technology to test the thermal characteristics of power battery. Now on this learning summary, and to share with you, the text of the error, please criticism.
ARC is a thermal analysis instrument based on the principle of adiabatic design, developed by Dow Chemical in the 1970s and commercialized by Columbia Scientific in the 1980s. ARC provides a controlled and precise adiabatic environment In the experiment, the calorimeter maintains the temperature synchronously with the sample temperature, and can simulate the internal heat of the battery too late to dissipate the thermal characteristics of the exothermic reaction process, to understand the real work of the battery. ARC has the following characteristics:
① high sensitivity, better than DSC 1 ~ 2 orders of magnitude;
② measurement flexibility, ARC can study the battery components, to test different sizes and models of batteries;
③ different resolution of the reaction is strong, can simulate the thermal runaway situation, given accurate thermal data;
④ can simultaneously get the temperature and pressure curves over time;
⑤ can get a number of thermal parameters, such as the initial decomposition temperature, heat rate, heat of reaction, activation energy.
Figure 1 UK THT's ARC equipment diagram
Figure 1 is a physical photograph of an ARC unit consisting of a furnace body with a heater and a temperature sensor and a control system for adiabatic operation.The furnace consists of a top, a perimeter and a bottom 3 parts each containing 2 One heater and one thermocouple with four heaters and one thermocouple in the perimeter and a thermocouple to control the temperature in the respective area The ARC's adiabatic environment is achieved by keeping the sample chamber in line with the temperature of the adiabatic furnace In order to study the self-heating of the sample in an adiabatic environment, the internal structure shown in Figure 2.
Figure 2 ARC's internal structure diagram
The basic principle of ARC operation is the mode of heating-waiting-search (HWS) (Figure 3). The user should set the start and stop temperature, temperature gradient value and sensitivity value to the system. Chamber, the system first heats the sample to the starting temperature and then into the waiting state in order to bring the sample and calorimeter to the same temperature and reach a thermal equilibrium. After the waiting period, the search mode is entered where the heater is not heating , The system compares the heating rate and the preset sensitivity (usually 0.02 ° C / min) to find out whether there is heat release. If the heating rate is higher than the preset value, the instrument will automatically enter the 'exothermic' state, and the system will put the temperature, The heating rate and pressure data are recorded and the entire process sample is adiabatic and the HWS operating mode logic is shown in Figure 4. If the instrument does not detect an exothermic reaction, the system will automatically switch to heating mode, automatically raising a temperature based on the temperature gradient Gradient, start another round of 'heating - wait - search' until the set final temperature is reached or an exotherm is detected.
ARC can be positive and negative materials, electrolyte thermal reaction temperature, heat release test, you can lithium-ion battery positive and negative materials, electrolyte safety research, analysis led to thermal lithium-ion battery causes. The ARC test of the carbon negative electrode in electrolyte can study the thermal decomposition of the SEI film. In addition to analyzing the thermal characteristics of the battery material, the ARC can also analyze the thermal characteristics of lithium ion batteries of different types, sizes, and uses. State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Feng Xun Ning and others published the first international test using ARC to test the thermal runaway of large-capacity power battery and thermal results.They used ARC technology to test the 25Ah NMC / Gr battery, model Is a 26mm thick VDA specification with two cells paralleled and two thermocouples embedded in the cell: one in the middle of the two coils, one at the relief valve and two more on the battery case A thermocouple, as shown in Figure 5.
Figure 5 test battery thermocouple installation location diagram
The battery into the sample chamber for ARC testing, testing the temperature of the four thermocouples and battery voltage during the test temperature and voltage evolution curve over time shown in Figure 6. The maximum temperature appears in the middle of the two core (Figure 5 In position 1), is 853 ° C. According to Figure 6, there are several common characteristics of the thermal runaway of the battery Temperature:
T1: Self-generated heat starting temperature (generally greater than 100C °)
T2: The rate of temperature rise begins to drop
T3: Thermal runaway trigger temperature (generally greater than 200C °)
T4: the highest temperature of thermal runaway (500C ° -1000C °)
Figure 6 battery thermal runaway process temperature, voltage evolution curve
Figure 7 temperature points corresponding to the rate of temperature rise
According to the three characteristic temperatures shown in Fig. 6 and the temperature rise rates corresponding to the temperature points shown in Fig. 7, the thermal runaway of the NMC battery ARC test process can be divided into six stages:
Stage 1: As the test heating temperature continues to rise, the battery capacity decay, lithium ions from the negative pole.
Stage 2: T1 reaches the temperature, the battery began to spontaneous fever, this stage, due to the continued decline in high-temperature battery capacity, while the negative SEI membrane decomposition, the electrolyte continued to react with the negative surface heat.
Stage 3: The T2 temperature is reached, as shown in Figure 7, and the rate of temperature rise declines due to the heat absorbed and dissolved by the membrane.
Stage 4: due to membrane dissolution, micro-short circuit inside the battery, while negative decomposition consumes electroactive substances, rising rate of temperature rise.
Stage 5: Temperature T3 is reached and thermal runaway is triggered. Due to the melting of the large area of the membrane, a large area of severe short circuit occurs. At the same time, violent reactions such as decomposition of the positive electrode material, decomposition of the electrolyte and decomposition of the binder generate a large amount of heat and soon reach the maximum temperature T4. Generally, the temperature point corresponding to the rate of temperature rise above 1 ° C / s is defined as T3.
Stage 6: Residual reactions take place causing a small temperature rise and the equipment begins to cool the test process.
Figure 8 battery thermal runaway temperature range diagram
The temperature range of each stage and the corresponding internal battery reaction process are shown in Figure 8, which is based on the basic common characteristics of the battery ARC test, ARC technology is widely used to study the battery thermal safety features. In addition, the ARC test can also expand the application, such as Increase thermal imaging cameras, online reaction pressure, gas online testing.
