A lithium ion battery protection design method based on thermal runaway propagation simulation

Abstract

The application provides a lithium ion battery protection design method based on thermal runaway propagation simulation. Through the detection, modeling and simulation process, the thermal runaway protection design is improved, which can effectively prevent the spread of battery thermal runaway and protect the safety of the battery system, vehicle and even the human body. In the method, the simulation results are used as the reference basis for the thermal runaway protection design. Compared with the existing technology which relies on experiments, the consumption of manpower and material resources is greatly reduced, the efficiency is significantly improved, and the danger in the experimental process is avoided to the greatest extent. The method is expanded from single thermal runaway modeling to battery protection design, and the method is widely applicable to different types of single batteries and group forms.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery thermal protection design technology, specifically relating to a lithium-ion battery protection design method based on thermal runaway propagation simulation. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles due to their high energy density, high operating voltage, and long cycle life. However, frequent battery system thermal runaway incidents pose serious safety hazards to the use of new energy vehicles. Because of the high energy density requirements of lithium-ion battery systems, once thermal runaway occurs, a large amount of chemical energy contained in the battery is converted into heat energy in a short period of time, causing a rapid rise in the temperature of the entire system. In electric vehicle battery systems, batteries are generally used in groups, so the thermal runaway of one battery can easily affect other nearby batteries, ultimately causing the collapse of the entire system and even leading to serious safety problems. Existing protection designs for battery thermal runaway are mainly based on extensive experiments, which are extremely costly in terms of manpower and resources. Considering the different battery system arrangements, continuous supplementation of experimental data is also necessary, and the inherent dangers of the experiments cannot be avoided. Therefore, it is necessary to provide a lithium-ion battery protection design method that is relatively simplified, widely applicable, and highly safe. Summary of the Invention

[0003] In view of the above-mentioned technical problems in this field, the present invention provides a lithium-ion battery protection design method based on thermal runaway propagation simulation, specifically including the following steps:

[0004] S1. Conduct adiabatic thermal runaway tests on individual battery cells to obtain the temperature rise curves and critical thermal runaway temperature data of individual battery cells under thermal runaway conditions.

[0005] S2. Based on the temperature rise curve obtained in step S1, establish a battery cell thermal runaway model that includes both battery heat transfer and thermal runaway heat generation. Then, based on the geometric structure of the battery pack composed of battery cells, establish a battery pack thermal propagation model.

[0006] S3. Trigger at least one battery cell to experience internal short circuit and thermal runaway in the simulation environment. Simulate the heat generation in the battery pack based on the battery pack thermal propagation model established in step S2, and determine whether thermal runaway is propagating in the battery pack based on the thermal runaway critical temperature data collected in step S1. If the determination is yes, proceed to step S4; otherwise, the process ends.

[0007] S4. Improve the thermal protection structure of the battery pack and repeat the simulation in S3 until the thermal runaway of at least one battery cell no longer propagates in the battery pack. The process ends then.

[0008] Furthermore, in step S1, the adiabatic thermal runaway test specifically utilizes an adiabatic accelerated calorimeter to precisely analyze the heat generation and heat generation rate during the entire adiabatic runaway process of a single battery cell, obtaining the temperature rise-time curve of the battery cell during this process. The thermal runaway critical temperature is specifically the lower of the battery cell's thermal runaway trigger temperature and the battery pressure relief valve opening temperature. The adiabatic thermal runaway test is repeated for the same battery cell at different SOCs to obtain multiple sets of the temperature rise-time curves.

[0009] Furthermore, step S2 establishes a single-cell thermal runaway model that incorporates both battery heat transfer and thermal runaway heat generation factors, specifically including:

[0010] 1) Three-dimensional heat conduction equation:

[0011]

[0012] In the formula, ρ is the battery density, and C p Q represents the constant-voltage heat capacity of the battery, k is the thermal conductivity, and Q is the constant-voltage heat capacity. t For additional calorie input;

[0013] 2) The convection equations between the battery and the external environment obey Newton's law of cooling:

[0014] Q = Ah(T) s -T ∞ )

[0015] In the formula, A(m) 2 ) represents the convective heat transfer area, h(W / (m²) 2 ·K) is the convective heat transfer coefficient, T s T is the temperature of the solid surface. ∞ The temperature of the fluid being furthest from the solid surface;

[0016] 3) Equations for the convective heat transfer coefficient between the fluid and the surface of a fixed boundary, and the surrounding temperature:

[0017] When t > 0,

[0018] In the formula, t is time, λ is thermal conductivity, and n represents direction;

[0019] 4) The total heat generation rate introduced by the four side reactions: SEI film decomposition reaction, negative electrode reaction with solvent, positive electrode reaction with solvent, and electrolyte decomposition reaction:

[0020] Q chem =Q sei +Q ne +Q pe +Q e

[0021] In the formula, Q sei Q ne Q pe and Q e The heat generation rate per unit volume for the four side reactions mentioned above, in W / m³. -3 ;

[0022] The exothermic rates of each side reaction are specifically described using the Arrhenius equation:

[0023]

[0024] In the formula, T is the battery temperature in K; c is a dimensionless quantity representing the content of relevant materials in different components; A, E a The superscript m represents the frequency factor, activation energy, and reaction order c of the corresponding reaction, respectively; H indicates that the reaction is exothermic per unit weight, in J / kg; W is the carbon content per unit volume, in kg / m³. 3 R is the ideal gas constant, R = 8.314 J·mol⁻¹ -1 ·K -1 Different correction terms are set for the formulas corresponding to different materials.

[0025] Furthermore, in step S3, the internal short circuit and thermal runaway of the battery cell can be triggered by first establishing an equivalent circuit model of the battery cell and then setting an equivalent short-circuit resistance bypass in the simulation environment; or by selecting different triggering conditions for the battery cell in the simulation environment, including compression, overcharging, etc.

[0026] Furthermore, the improvement of the battery pack's thermal protection structure in step S4 specifically includes adding flame-retardant materials or phase change materials at corresponding locations in the battery pack based on the simulation results of the battery pack's thermal spread model, as well as adjusting the thickness, area, and location of existing flame-retardant materials or phase change materials.

[0027] The lithium-ion battery protection design method based on thermal runaway propagation simulation provided by this invention improves thermal runaway protection design through the proposed detection, modeling, and simulation process. It effectively prevents the propagation of battery thermal runaway, protecting the battery system, vehicle, and even personal safety. The method uses simulation results as a reference for thermal runaway protection design, significantly reducing manpower and material resources compared to existing experimental technologies, thus greatly improving efficiency and minimizing the risks during experiments. Starting with single-cell thermal runaway modeling, the method extends to the overall battery protection design, demonstrating broad applicability and suitability for different types of single-cell and battery pack configurations. Attached Figure Description

[0028] Figure 1This is the overall step flow of the method provided by the present invention;

[0029] Figure 2 This is a temperature rise diagram of the adiabatic thermal runaway of a single battery cell in an embodiment of the present invention;

[0030] Figure 3 This is a comparison chart of the simulated and experimental temperatures of single-cell thermal runaway in an embodiment of the present invention;

[0031] Figure 4 This is a geometric schematic diagram of the battery module model in an embodiment of the present invention;

[0032] Figure 5 This is a schematic diagram of an internally short-circuited battery model in an embodiment of the present invention;

[0033] Figure 6 This is a simulation result diagram of the thermal spread of the battery module in an embodiment of the present invention;

[0034] Figure 7 This is a simulation diagram of the thermal spread of the battery module after the improvement of the flame-retardant plate and phase change material based on the present invention. Detailed Implementation

[0035] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] This invention provides a lithium-ion battery protection design method based on thermal runaway propagation simulation, such as... Figure 1 As shown, the specific steps include:

[0037] S1. Conduct adiabatic thermal runaway tests on individual battery cells to obtain the temperature rise curves and critical thermal runaway temperature data of individual battery cells under thermal runaway conditions.

[0038] S2. Based on the temperature rise curve obtained in step S1, establish a battery cell thermal runaway model that includes both battery heat transfer and thermal runaway heat generation. Then, based on the geometric structure of the battery pack composed of battery cells, establish a battery pack thermal propagation model.

[0039] S3. Trigger at least one battery cell to experience internal short circuit and thermal runaway in the simulation environment. Simulate the heat generation in the battery pack based on the battery pack thermal propagation model established in step S2, and determine whether thermal runaway is propagating in the battery pack based on the thermal runaway critical temperature data collected in step S1. If the determination is yes, proceed to step S4; otherwise, the process ends.

[0040] S4. Improve the thermal protection structure of the battery pack and repeat the simulation in S3 until the thermal runaway of at least one battery cell no longer propagates in the battery pack. The process ends then.

[0041] In a preferred embodiment of the present invention, the adiabatic thermal runaway test in step S1 specifically utilizes an adiabatic accelerated calorimeter (ARC) to precisely analyze the heat generation and heat generation rate during the entire adiabatic runaway process of a single battery cell, thereby obtaining the temperature rise-time curve of the battery cell during this process. The thermal runaway critical temperature is specifically the lower of the battery cell's thermal runaway trigger temperature and the battery pressure relief valve opening temperature. The adiabatic thermal runaway test is repeated for the same battery cell at different SOCs to obtain multiple sets of the temperature rise-time curves. Figure 2 The diagram illustrates the temperature rise curve of a battery during adiabatic thermal runaway, measured in one embodiment. At temperature T1, the battery exhibits the temperature rise set by the ARC instrument, and the device stops heating, entering an adiabatic state. Subsequently, the battery pressure relief valve opens around temperature T2, triggering runaway. The battery then continues to heat up to the highest measurement point T3. It should be noted that since the battery pressure relief valve releases gas and electrolyte, which can damage the battery system, temperature T2 is chosen as the critical temperature. A comparison between the actual battery temperature and the simulated temperature is provided. Figure 3 As shown.

[0042] In a preferred embodiment of the present invention, step S2 establishes a single-cell thermal runaway model that includes both battery heat transfer and thermal runaway heat generation factors, specifically including:

[0043] 1) Three-dimensional heat conduction equation:

[0044]

[0045] In the formula, ρ is the battery density, and C p Q represents the constant-voltage heat capacity of the battery, k is the thermal conductivity, and Q is the constant-voltage heat capacity. t For additional calorie input;

[0046] 2) The convection equations between the battery and the external environment obey Newton's law of cooling:

[0047] Q = Ah(T) s -T ∞ )

[0048] In the formula, A(m) 2 ) represents the convective heat transfer area, h(W / (m²) 2 ·K) is the convective heat transfer coefficient, T s T is the temperature of the solid surface. ∞ The temperature of the fluid being furthest from the solid surface;

[0049] 3) Equations for the convective heat transfer coefficient between the fluid and the surface of a fixed boundary, and the surrounding temperature:

[0050] When t > 0,

[0051] In the formula, t is time, λ is thermal conductivity, and n represents direction;

[0052] In this embodiment, the corresponding thermophysical parameters of the battery are shown in Table 1:

[0053] Table 1

[0054]

[0055] 4) The total heat generation rate introduced by the four side reactions: SEI film decomposition reaction, negative electrode reaction with solvent, positive electrode reaction with solvent, and electrolyte decomposition reaction:

[0056] Q chem =Q sei +Q ne +Q pe +Q e

[0057] In the formula, Q sei Q ne Q pe and Q e The heat generation rate per unit volume for the four side reactions mentioned above, in W / m³. -3 ;

[0058] The exothermic rates of each side reaction are specifically described using the Arrhenius equation:

[0059]

[0060] In the formula, T is the battery temperature in K; c is a dimensionless quantity representing the content of relevant materials in different components; A, E a The superscript m represents the frequency factor, activation energy, and reaction order c of the corresponding reaction, respectively; H indicates that the reaction is exothermic per unit weight, in J / kg; W is the carbon content per unit volume, in kg / m³. 3 R is the ideal gas constant, R = 8.314 J·mol⁻¹ -1 ·K -1 ;

[0061] Four side reactions occur sequentially with increasing temperature. The SEI film forms during the first battery cycle to prevent the negative electrode from reacting directly with the electrolyte, but it is prone to decomposition and exothermic reactions at 80–120°C. Once the SEI film decomposes, the negative electrode comes into contact with the electrolyte, and the lithium embedded in the negative electrode will undergo an exothermic side reaction with the solvent. As the battery temperature continues to rise, the positive electrode active material will also undergo side reactions with the electrolyte, releasing a large amount of heat. Ultimately, at higher temperatures, the electrolyte will decompose and undergo an exothermic reaction.

[0062] Considering the slight differences in the characteristics and content of components among batteries of different capacities or types, an adjustment coefficient w is introduced in the calculation of the heat generation rate of each side reaction. x Based on the heat generation rate, the following targeted adjustments are made:

[0063]

[0064] In the embodiments of the present invention, the parameters of the thermal runaway side reactions in each model are shown in Table 2:

[0065] Table 2

[0066]

[0067]

[0068] In this example, the three-dimensional geometry of the battery box is shown below. Figure 4 The diagram shows the finite element mesh and exploded geometry of the battery model in this example, including the cover, battery, side plates, bottom plate, steel strip, nickel strip, and top cover. The battery module in this example consists of five individual battery cells.

[0069] In a preferred embodiment of the present invention, regarding the internal short circuit and thermal runaway of the battery cell in step S3, the following first-order RC equivalent circuit model is first established for the battery cell:

[0070]

[0071] By connecting an equivalent short-circuit resistor R in parallel with the original circuit of the normal battery model SC It can effectively describe the electrical characteristics of a battery after an internal short circuit occurs, such as Figure 5 As shown. According to Kirchhoff's laws, the short-circuit current satisfies the following relationship:

[0072]

[0073] In the formula, I is the actual current passing through the battery. load I is the operating current (i.e., the measuring current). SC To pass through the equivalent short-circuit resistance R SC The short-circuit current. R SCBased on experimental estimation, 0.04Ω is selected in this example.

[0074] In this embodiment, a single cell at the edge of the battery pack is configured to trigger an internal short circuit and thermal runaway. Figure 6 This figure illustrates the evolution of the average temperature of all battery cells in a battery pack at 25°C when one cell triggers thermal runaway, as simulated by a thermal propagation model. The figure shows that after a single battery cell triggers internal short-circuit thermal runaway, adjacent cells subsequently trigger thermal runaway. Therefore, current battery protection designs are not effective in preventing the propagation of thermal runaway.

[0075] The improvement of the battery pack's thermal protection structure in step S4 specifically includes adding flame-retardant or phase change materials at corresponding locations in the battery pack based on the simulation results of the battery pack's thermal spread model, as well as adjusting the thickness, area, and location of existing flame-retardant or phase change materials.

[0076] Specifically, this example employs a method of combining aerogel flame-retardant materials and paraffin phase change materials between battery cells, such as... Figure 7 As shown, a 2mm aerogel flame-retardant plate was added between the batteries, and the phase change material was 2mm thick. Internal short circuit was used to trigger battery thermal runaway, and the highest temperature of the adjacent batteries was lower than the thermal runaway trigger temperature T2. Therefore, the phase change material can effectively prevent the spread of battery thermal runaway.

[0077] In particular, it is also possible to consider only adding flame-retardant materials or only adding phase change materials, and to consider the impact of increasing the thickness of the materials on the protection of the battery box.

[0078] It should be understood that the sequence number of each step in the embodiments of the present invention does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0079] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A lithium-ion battery protection design method based on thermal runaway propagation simulation, characterized in that: Specifically, the following steps are included: S1. Conduct adiabatic thermal runaway tests on individual battery cells to obtain the temperature rise curves and critical thermal runaway temperature data of individual battery cells under thermal runaway conditions. S2. Based on the temperature rise curve obtained in step S1, establish a single-cell thermal runaway model that includes both battery heat transfer and thermal runaway heat generation factors, specifically including: 1) Three-dimensional heat conduction equation: In the formula, ρ is the battery density, and C p Q represents the constant-voltage heat capacity of the battery, k is the thermal conductivity, and Q is the constant-voltage heat capacity. t For additional calorie input; 2) The convection equations between the battery and the external environment obey Newton's law of cooling: Q=Ah(T s -T ∞ ) In the formula, A is the convective heat transfer area, h is the convective heat transfer coefficient, and T is the heat transfer coefficient. s T is the temperature of the solid surface. ∞ The temperature of the fluid being furthest from the solid surface; 3) Equations for the convective heat transfer coefficient between the fluid and the surface of a fixed boundary, and the surrounding temperature: When t > 0, In the formula, t is the time variable, λ is the thermal conductivity, and n represents the direction; 4) The total heat generation rate introduced by the four side reactions: SEI film decomposition reaction, negative electrode reaction with solvent, positive electrode reaction with solvent, and electrolyte decomposition reaction: Q chem =Q sei +Q ne +Q pe +Q e In the formula, Q sei Q ne Q pe and Q e The heat generation rate per unit volume for the four side reactions mentioned above, in W / m³. -3 ; The exothermic rates of each side reaction are specifically described using the Arrhenius equation: In the formula, T is the battery temperature in K; c is a dimensionless quantity representing the content of relevant materials in different components; A, E a The superscript m represents the frequency factor, activation energy, and reaction order c of the corresponding reaction, respectively; H indicates that the reaction is exothermic per unit weight, in J / kg; W is the carbon content per unit volume, in kg / m³. 3 R is the ideal gas constant, R = 8.314 J·mol⁻¹ -1 ·K -1 Different correction terms are set for the formulas corresponding to different materials; Then, based on the geometric structure of the battery pack composed of individual battery cells, a thermal propagation model of the battery pack is established. S3. Trigger at least one battery cell to experience internal short circuit and thermal runaway in the simulation environment. Simulate the heat generation in the battery pack based on the battery pack thermal propagation model established in step S2, and determine whether thermal runaway is propagating in the battery pack based on the thermal runaway critical temperature data collected in step S1. If the determination is yes, proceed to step S4; otherwise, the process ends. S4. Improve the thermal protection structure of the battery pack and repeat the simulation in S3 until the thermal runaway of at least one battery cell no longer propagates in the battery pack. The process ends then.

2. The method as described in claim 1, characterized in that: In step S1, the adiabatic thermal runaway test specifically utilizes an adiabatic accelerated calorimeter to accurately measure the heat generation and heat generation rate during the entire adiabatic runaway process of a single battery cell, obtaining the temperature rise-time curve of the battery cell during this process. The thermal runaway critical temperature is specifically the lower of the battery cell's thermal runaway trigger temperature and the battery pressure relief valve opening temperature. The adiabatic thermal runaway test is repeated for the same battery cell at different SOCs to obtain multiple sets of the temperature rise-time curves.

3. The method as described in claim 1, characterized in that: In step S3, the internal short circuit and thermal runaway of the battery cell can be triggered by first establishing an equivalent circuit model of the battery cell and then setting an equivalent short-circuit resistance bypass in the simulation environment; or by selecting the battery cell triggering conditions including compression, overcharging and combination in the simulation environment.

4. The method as described in claim 1, characterized in that: The improvement of the battery pack's thermal protection structure in step S4 specifically includes adding flame-retardant or phase change materials at corresponding locations in the battery pack based on the simulation results of the battery pack's thermal spread model, as well as adjusting the thickness, area, and location of existing flame-retardant or phase change materials.

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