4.1 Simulation Calculation Model for Battery Heat Generation
The actual heat production of the battery is complicated, the simulation calculation process of battery Thermal runaway is similar to the calculation model of battery heat generation, so some assumptions should be made about the physical properties of the battery itself in the simulation calculation:
(1) The specific heat capacity and thermal conductivity of various materials inside the battery are not affected by the change of ambient temperature and state of charge;
(2) The medium of various materials of the battery is evenly distributed, and the thermal physical parameters remain unchanged. For example, the thermal conductivity coefficient of the same material is equal in the same direction.
(3) During charging and discharging of lithium-ion batteries, the current density is evenly distributed and the heat production rate is consistent at different temperatures.
Through the above assumptions, the energy conservation equation of unsteady heat transfer is obtained.
$$\rho {c}_{p}\frac{\partial T}{\partial t}={\lambda }_{x}\frac{{\partial }^{2}T}{{\partial x}^{2}}+{\lambda }_{xy}\frac{{\partial }^{2}T}{{\partial y}^{2}}+{\lambda }_{zx}\frac{{\partial }^{2}T}{{\partial z}^{2}}+q$$
1
\({\rho }_{k}{c}_{p,k}\frac{\partial T}{\partial t}\) refers to the increase of the thermal mechanical energy of the battery unit within a unit time,\(\nabla \bullet \left({\lambda }_{k}\nabla T\right)\) refers to the heat added to the cells inside the battery due to convective heat transfer by the fluid around the battery, q is the rate of heat production per unit volume of a lithium-ion battery, ρk is refers to the average density of the cell, cp,kR is refers to the average specific heat capacity of lithium-ion battery cells, λk is refers to the thermal conductivity of the lithium-ion battery unit, T is thermal, t is time, \(\rho\) is the average density of the material inside a lithium-ion battery, \(q\) is heat production rate per unit volume of a lithium-ion battery, \({\lambda }_{x}\),\({\lambda }_{y}\)and \({\lambda }_{zx}\)are thermal conductivity of lithium ion battery in three - dimensional orthogonal direction.
Many parameters in the chemical reaction kinetics formula that need to be calibrated need to be measured by multiple groups of experimental equipment. The common test method is constant temperature scanning Calorimetry, and the common experimental equipment is differential scanning calorimetry (DSC). DSC can scan the sample at a constant temperature rise rate, and by comparing the difference between the heating amount of the sample and the reference heating amount, the heat release/absorption of the sample at this constant temperature rise rate can be obtained.
If any battery inside the battery is used as a single battery, the formula for the temperature T of the battery as a function of time t is as follows:
$$\text{T}\left(t\right)=T\left(0\right)+\underset{0}{\overset{t}{\int }}\frac{dT\left(\tau \right)}{d\tau }d\tau$$
2
The temperature rise rate\(\frac{dT\left(t\right)}{dt}\)is determined by the net heat generating power\(\text{Q}\left(t\right) is\)inside the battery, where M is the battery mass and the Specific heat capacity of the battery is \({C}_{p}=1100J.{kg}^{-1}{K}^{-1}\).
$$Q\left(t\right)={Q}_{chem}\left(t\right)+{Q}_{e}\left(t\right)-{Q}_{h}\left(t\right)$$
3
$${Q}_{chem}\left(t\right)={Q}_{SEI}\left(t\right)+{Q}_{anode}\left(t\right)+{Q}_{sep}\left(t\right)+{Q}_{e}\left(t\right)+{Q}_{cath}\left(t\right)$$
4
$${Q}_{cath}\left(t\right)={Q}_{caht1}\left(t\right)+{Q}_{caht2}\left(t\right)$$
5
\({Q}_{chem}\left(t\right)\) is chemical reaction heat generation power, \({Q}_{SEI}\left(t\right)\) is heat generating power of SEI membrane Chemical decomposition, \({Q}_{anode}\left(t\right)\) is the heat generation power of lithium metal embedded inside the negative electrode when it reacts with the electrolyte without the protection of the SEI film, \({Q}_{sep}\left(t\right)\) is the heat absorption power of the diaphragm during melting, \({Q}_{e}\left(t\right)\)is the exothermic power of the overall Chemical decomposition of electrolyte solution, \({Q}_{cath}\left(t\right)\) is thermal power generation during decomposition of ternary cathode materials. Since the reaction of ternary positive electrode has two exothermic peaks\({Q}_{caht1}\left(t\right)\) and \({Q}_{caht2}\left(t\right)\), the two Exothermic reaction are only equal to \({Q}_{cath}\left(t\right)\).
$${Q}_{e}\left(t\right)=\frac{1}{\varDelta t}\left(\varDelta {H}_{e}-\underset{0}{\overset{t}{\int }}{Q}_{e}\left(\tau \right)d\tau \right)$$
6
The calculation of Qe can be obtained from the following equation, \(\varDelta {H}_{e}\) represents the total electrical energy possessed by the battery when an internal short circuit occurs; ∆t represents the average time of electrical energy release. The adiabatic Thermal runaway model can simulate the dynamic characteristics of each chemical reaction, and it is calculated in the model。The maximum temperature of Thermal runaway should be consistent with the experimental results. Its main principle is the Conservation of energy, \(\varDelta \text{t}\) represents the total heat energy released in the process of Thermal runaway; M represents the quality of the battery; \({C}_{p}\) represents the Specific heat capacity of the battery; \(\varDelta T\) represents the maximum temperature rise of battery Thermal runaway, according to formula (2–6), ∆ T = T3-T1; \({\varDelta \text{H}}_{chem}\) represents the total amount of Chemical energy converted into heat energy in the process of Thermal runaway; \({\varDelta \text{H}}_{e}\) represents the total amount of electric energy converted into heat energy in the process of Thermal runaway.
$$\varDelta \text{H}=\text{M}\bullet {C}_{p}\bullet \varDelta T={\varDelta \text{H}}_{chem}+{\varDelta \text{H}}_{e}$$
7
$${\varDelta \text{H}}_{chem}=\sum _{x}\left({C}_{x,0}\bullet \varDelta {H}_{x}\bullet {m}_{x}\right)$$
8
\({\varDelta \text{H}}_{chem}\) is determined by the properties of the material itself and can be calculated from the data in the formula, where \({\varDelta \text{H}}_{chem}\)is approximately 2.87 × 105J. According to the energy conservation equation and the experimentally measured ∆ T, set \({\varDelta \text{H}}_{e}\) approximately 3.17 × 105J.
4.2 Simulation calculation model of battery Thermal runaway
Based on the above analysis results, in order to ensure an adiabatic testing environment and accurately test and obtain battery samples。The temperature rise rate dTS/dt of the product under adiabatic conditions should focus on eliminating the influence of sensor measurement errors, that is, the calorimeter EV-ARC should be calibrated before the experiment. During the calibration process, the heat dissipation environment of the calorimeter shall be as close as possible to the environment when the Thermal runaway test is actually conducted. The heat dissipation of the calorimeter chamber not only needs to consider the heat dissipation from its outer wall towards the environment of the experimental site, but also the impact caused by the heat absorption of the battery sample.
The key points of using EV-ARC to conduct thermal insulation Thermal runaway test of large capacity power battery are:
1) thermal insulation environment maintenance;
2) Accurate detection of heat release rate dT/dt.
In order to meet the above two points, the following work should be carried out during the experiment:
1) Before the formal experiment of thermal insulation Thermal runaway, reduce the impact of sensor measurement error through the calibration scheme of equal heat capacity replacement and sensor mechanical clamping;
2) Try to simulate the internal environment of the calorimetry chamber, including large capacity battery samples, during the calibration process to obtain better heat dissipation compensation calibration results;
3) During formal testing, pay attention to using the sensor mechanical clamping scheme to ensure that the sensor is tightly attached to the surface of the battery.
It should also be noted that the temperature distribution inside the large capacity power battery is uneven, and when Thermal runaway occurs, the temperature, the unevenness of distribution is the greatest. In order to accurately evaluate the total energy released by the large capacity power battery in the process of Thermal runaway, the internal temperature of the large capacity power battery should also be measured, and attention should be paid to analyzing the change of the internal temperature difference during the experiment.
Figure 3 shows the ARC experiment results obtained. From the results in Fig. 3, it can be seen that T1 is the starting temperature of battery self-heating, T2 is the trigger temperature of Thermal runaway, about 200 ℃. At this point, the dT/dt value changes sharply, and T3 is the highest temperature when the battery Thermal runaway occurs. This temperature value is very high, which can be used as the reference temperature set by the battery system protection device. The results of this test are consistent with the simulation results of Thermal runaway of the battery before.
It can be seen from the figure that when the temperature is about 60 ℃, the solid dielectric facial mask (SEI film) on the negative electrode surface begins to decompose. At this point, the negative electrode of the battery loses SEI film protection, and the lithium embedded inside the negative electrode comes into contact with the electrolyte, causing a reaction to release heat and generate a new SEI film. The loss of lithium in the negative electrode of the battery leads to an increase in the negative voltage. In addition, under high temperature conditions, metal ions inside the positive electrode of the battery dissolve in the electrolyte, causing the loss of active substances in the positive electrode and reducing the voltage of the positive electrode. Due to the fact that the voltage of lithium-ion power batteries is equal to the difference between the positive and negative voltage, the voltage of the battery also decreases.
Figure 4 shows the comparison between the simulation calculation results and the experimental results of multiple battery center temperature points during the battery Thermal runaway test. These temperature sensors are embedded inside the battery to be measured. The internal center temperature of the battery is measured. See Table 1 for the simulation calculation values and measurement results inside the battery. It can be seen from the data in Fig. 4 and Table 1 that the simulation calculation results are very close to the measured results of the battery, with an accuracy of more than 90%, Therefore, the simulation calculation model of battery heat generation should be used to analyze the process of Thermal runaway of batteries.
Table 1
Simulation and test results of battery inner temperature
| Cell1 | Cell2 | Cell3 | Cell4 |
Simulation(℃) | 1005.2 | 1005.1 | 1004.8 | 995.7 |
test(℃) | 1055.0 | 998.8 | 957.6 | 940.3 |
Accuracy(%) | 95.3 | 99.4 | 95.3 | 94.4 |
5. System design for preventing battery Thermal runaway
There are many ways to improve the prevention of thermal diffusion in power batteries, and targeted design can be carried out from three aspects: battery cells, battery modules, and battery systems. From the cell level, on the premise of not affecting the basic performance of the battery, adding flame retardants in the battery electrolyte and selecting SEI films that are more resistant to high temperature are all effective measures to reduce the damage value of Thermal runaway. Due to limitations in the length of this article, measures to prevent thermal diffusion at the cell level will not be discussed in detail. The next research focus of this paper is the Thermal runaway prevention measures at the battery module and pack level.
5.1 Battery module thermal runaway prevention design
On the target module for the Thermal runaway test, first, according to the design structure and size specifications of the actual module inside the battery pack, design a heating device with the same appearance size as the module for the battery Thermal runaway test, replace a module in the middle of the module, and embed temperature sensors outside and inside the heating device and the electric core inside the module, The control power supply of the heating plate is connected to the trigger power supply outside the battery pack through a wire, and the heating plate is installed together with the thermal conductive mica sheet. The design of the heating plate in the middle of the module is shown in Fig. 5.
Since the design size of the heating plate in the module is basically the same as the size of the battery cell in the original module, the introduction of the heating plate will not have too much impact on the structure of the battery module under test, thus affecting the deviation of the Thermal runaway experiment from the Thermal runaway effect of the actual battery module structure. Add a 1.0 mm thick Aerogel between the end plate and the PC plate, and the thermal conductivity of Aerogel is extremely low, about 0.025 W/(m · K), which is 12.5% of the PC plate. Added thermal resistance between the battery cells at both ends of the module and the module end plate, effectively blocking heat conduction on the end plate side.
In order to better observe the heating target module of the heating plate, do not accidentally trigger the Thermal runaway of the surrounding modules. Therefore, on the other side of the heating plate, there is also a thermal insulation mica sheet. In this way, during the test, the heating plate only heats the target battery in the module, and does not heat the surrounding battery, so as not to affect the experimental effect.
In the tested module, when the tested power battery leaves the factory, a temperature sensor will be added inside the battery and fluorescent substances will be added to the electrolyte, so that not only the temperature changes of the heating plate and battery shell can be observed during the experiment, but also the temperature changes inside the battery can be measured, providing favorable data support for the design of the scheme to restrain the Thermal runaway of the battery. At the same time, due to the addition of fluorescent substances in the electrolyte, the electrolyte eruption in the case of Thermal runaway of the power battery can be observed and measured after the experiment, which provides an excellent reference for improving the Thermal runaway suppression scheme of the power battery pack.
5.2 Battery system thermal runaway prevention design
In the tested module, when the tested power battery leaves the factory, a temperature sensor will be added inside the battery and fluorescent substances will be added to the electrolyte, so that not only the temperature changes of the heating plate and battery shell can be observed during the experiment, but also the temperature changes inside the battery can be measured, providing favorable data support for the design of the scheme to restrain the Thermal runaway of the battery. At the same time, due to the addition of fluorescent substances in the electrolyte, the electrolyte eruption in the case of Thermal runaway of the power battery can be observed and measured after the experiment, which provides an excellent reference for improving the Thermal runaway suppression scheme of the power battery pack. As shown in Fig. 6.
5.3 Battery system thermal runaway prevention software design
According to the changes of battery temperature, internal resistance, voltage and insulation resistance values obtained during the experiment, the judgment conditions of battery Thermal runaway in the Battery management system are modified. For example, BMS judges the rate of cell voltage reduction, the temperature of the battery shell and the temperature changes on the mica chip for 10 consecutive times, which can find Thermal runaway faults earlier and report them truthfully, reducing the probability of false alarm and failure to report Thermal runaway faults.
As shown in Fig. 7, BMS needs to repeatedly confirm the Thermal runaway signal, compare the measured internal and external temperature of the battery, coolant temperature and flow information, battery insulation, explosion-proof valve pressure and other information, and make a final judgment by repeated comparison to avoid misjudgment of Thermal runaway.