Lithium battery thermal runaway analysis method based on thermal runaway characteristic temperature probability

Abstract

The invention provides a lithium battery thermal runaway analysis method based on a thermal runaway characteristic temperature probability. The lithium battery thermal runaway analysis method comprises the following steps: S1, determining a lithium battery characteristic temperature range; s2, constructing a lithium battery thermal runaway heat production model; s3, taking the initial heat production temperature T1 to the thermal runaway temperature T2 as a first temperature stage, taking the thermal runaway temperature T2 to the maximum temperature T3 as a second temperature stage, and determining the heat production amount of the first temperature stage and the heat production amount of the second temperature stage; dividing the first temperature stage, the second temperature stage and the heat production quantity of the first temperature stage and the second temperature stage into n sub-intervals, and determining the triggering probability of each sub-interval; s4, determining thermal runaway characteristic temperature and thermal runaway heat production based on the two temperature stages and the trigger probability of the heat production, converting the thermal runaway characteristic temperature into activation energy, converting the thermal runaway heat production into reaction enthalpy, and substituting the activation energy and the reaction enthalpy into the lithium battery thermal runaway heat production model to determine a heat production curve of the lithium battery.

Description

Technical Field

[0001] The present invention relates to a method for analyzing lithium batteries, and more particularly to a method for analyzing lithium battery thermal runaway based on the probability of thermal runaway characteristic temperatures. Background Art

[0002] Due to their high energy density and wide applications, such as in electric vehicles, energy storage systems, and consumer electronic devices, lithium batteries have become the core of modern energy storage technologies. However, lithium-ion batteries may experience thermal runaway under extreme conditions, that is, the internal temperature of the battery rises sharply, leading to out-of-control chemical reactions and then causing catastrophic consequences such as fires and explosions. Especially in electric vehicles and large battery energy storage systems, the battery pack consists of multiple battery cells. Once a single cell experiences thermal runaway, it may trigger a chain reaction in the surrounding batteries, resulting in the failure of the entire system and posing a serious threat to personal safety and equipment. Therefore, it is necessary to accurately analyze the thermal runaway of lithium batteries.

[0003] In the prior art, the research on lithium battery thermal runaway usually divides the thermal runaway of the battery into three characteristic temperatures, namely: T1 (initial decomposition temperature), which represents the start of irreversible reactions in the battery; T2 (rapid temperature rise temperature), which represents the intensification of the reaction and a rapid increase in temperature; T3 (maximum runaway temperature), which represents the highest temperature reached by the battery. Then, based on the above characteristic temperatures, the thermal runaway process of the lithium battery is analyzed. However, the thermal runaway process of lithium batteries is random. If the thermal runaway analysis is only based on the three characteristic temperatures, the randomness of lithium batteries is ignored, resulting in low accuracy in the analysis of the lithium battery thermal runaway process and being unfavorable for the thermal management of lithium batteries.

[0004] Therefore, in order to solve the above technical problems, it is urgent to propose a new technical means. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for analyzing lithium battery thermal runaway based on the probability of thermal runaway characteristic temperatures, which takes into account the randomness of lithium battery thermal runaway in the simulation analysis of lithium battery thermal runaway, can more accurately reflect the actual process state of lithium battery thermal runaway, can effectively predict the starting heat generation temperature, thermal runaway temperature, and energy release process of different lithium batteries, effectively improve the accuracy of lithium battery thermal runaway analysis, and provide accurate data support for the thermal management of lithium batteries.

[0006] A method for analyzing lithium battery thermal runaway based on the probability of thermal runaway characteristic temperatures provided by the present invention includes the following steps:

[0007] S1. Determine the characteristic temperature range of the lithium battery, where the characteristic temperatures include the starting heat generation temperature T1, the thermal runaway temperature T2, and the maximum temperature T3, and T3 > T2 > T1;

[0008] S2. Construct a heat generation model for lithium battery thermal runaway;

[0009] S3. Take the starting heat generation temperature T1 to the thermal runaway temperature T2 as the first temperature stage, and the thermal runaway temperature T2 to the maximum temperature T3 as the second temperature stage, and determine the heat generation amounts in the first temperature stage and the second temperature stage; Divide the first temperature stage, the second temperature stage, and the heat generation amounts in the first temperature stage and the second temperature stage into n sub - intervals respectively, and determine the triggering probabilities of each sub - interval;

[0010] S4. Determine the thermal runaway characteristic temperature and the thermal runaway heat generation amount based on the triggering probabilities of the two temperature stages and the heat generation amount, convert the thermal runaway characteristic temperature into activation energy, convert the thermal runaway heat generation amount into reaction enthalpy, and substitute the activation energy and the reaction enthalpy into the lithium battery thermal runaway heat generation model to determine the heat generation curve of the lithium battery.

[0011] Further, determining the triggering probabilities of each sub - interval in the two temperature stages and each sub - interval of the heat generation amount specifically includes:

[0012]

[0013] Where: represents the distribution probability of the i - th sub - interval. When u is 1, it represents the temperature distribution probability of the i - th temperature sub - interval. When u is 2, it represents the heat generation distribution probability of the i - th heat generation sub - interval. represents the triggering probability of the i - th sub - interval. When u is 1, it represents the temperature triggering probability of the i - th temperature sub - interval. When u is 2, it represents the heat generation triggering probability of the i - th heat generation sub - interval.

[0014] Further, determining the critical temperature based on the temperature triggering probabilities of each sub - interval in the two temperature stages specifically includes:

[0015] Construct a temperature trigger, and the temperature trigger judges to generate a simulated trigger probability value P in each sub - interval i 1 ;

[0016] Judge Whether it holds. If so, the current sub - interval i does not trigger thermal runaway. If not, the current sub - interval triggers heat generation or thermal runaway;

[0017] Determine the thermal runaway temperature T tr (i):

[0018] T tr (i)=T i - +(T i - -T i + )×Pi 1 ; where, T i + represents the right boundary value of the temperature of the current temperature sub - interval i, and T i - represents the left boundary value of the temperature of the current temperature sub - interval i.

[0019] Furthermore, the heat production quantity Q is determined based on the heat production trigger probabilities of two temperature stages tr (i) Specifically, it includes:

[0020] Construct a heat production trigger, and the heat production trigger determines a simulated trigger probability value P in each heat production sub - interval i 2 ;

[0021] Judge Whether it holds. If so, there is no heat production in the current heat production sub - interval i. If not, heat production is triggered in the current sub - interval;

[0022] Determine the heat production quantity Q tr (i):

[0023] Among them, represents the right boundary value of heat production of the current heat production sub - interval i, represents the left boundary value of heat production of the current heat production sub - interval i.

[0024] Furthermore, converting the thermal runaway temperature to activation energy specifically includes:

[0025] Construct a function of activation energy and thermal runaway temperature:

[0026]

[0027] Among them, ΔT represents the difference in characteristic temperatures; A represents the pre - exponential factor, E represents the activation energy, and R represents the gas constant.

[0028] Furthermore, substituting the activation energy and reaction enthalpy into the heat production model of lithium - battery thermal runaway to determine the heat production curve of the lithium - battery specifically includes:

[0029] The total heat quantity Q of the lithium - battery is:

[0030] Q = Q gen +Q dis ;

[0031] Among them: Q gen represents the heat production quantity of the lithium - battery, and Q dis represents the heat loss of the lithium - battery through convective and radiative heat transfer;

[0032] Q gen = Q1 + Q2;

[0033] Wherein: Q1 represents the heat generated in the first temperature stage; Q2 represents the heat generated in the second temperature stage;

[0034]

[0035] Wherein: V represents the battery volume; A1 and A2 both represent the pre-exponential factor, ΔH2 and ΔH1 both represent the reaction enthalpy, E1 represents the activation energy in the first temperature stage, E2 represents the activation energy in the second temperature stage, c represents the normalized amount of reactants, and α represents the reaction conversion rate of the lithium battery.

[0036] Advantages of the present invention: Through the present invention, the randomness of lithium battery thermal runaway is considered in the simulation analysis of lithium battery thermal runaway, which can more accurately reflect the actual process state of lithium battery thermal runaway, effectively predict the initial heat generation temperature, thermal runaway temperature and energy release process of different lithium batteries, effectively improve the accuracy of lithium battery thermal runaway analysis, and provide accurate data support for the thermal management of lithium batteries. Brief Description of the Drawings

[0037] The present invention will be further described below in conjunction with the drawings and embodiments:

[0038] Figure 1 is the flow chart of the present invention.

[0039] Figure 2 is the characteristic temperature curve diagram of the lithium battery in the present invention.

[0040] Figure 3 is the activation energy corresponding relationship diagram of the lithium battery in the present invention.

[0041] Figure 4 is the sub-interval distribution probability diagram of the present invention.

[0042] Figure 5 is the trigger probability diagram of the present invention.

[0043] Figure 6 is the curve of the lithium battery temperature changing with time in the specific example of the present invention. Detailed Embodiments

[0044] The following further details the present invention:

[0045] A lithium battery thermal runaway analysis method based on the characteristic temperature probability of thermal runaway provided by the present invention includes the following steps:

[0046] S1. Determine the characteristic temperature range of the lithium battery. The characteristic temperatures include the initial heat generation temperature T1, the thermal runaway temperature T2, and the maximum temperature T3, and T3 > T2 > T1. Among them, the initial heat generation temperature T1 is the initial decomposition temperature of the lithium battery, which indicates that irreversible reactions begin to occur in the lithium battery. The maximum temperature T3 represents the maximum runaway temperature, that is, the highest temperature that the lithium battery thermal runaway can reach. The thermal runaway temperature T2 is the rapid temperature rise temperature, indicating that the reaction intensifies and the temperature rises rapidly. The above three temperatures vary for different brands, different models, etc. Moreover, among the above three characteristic temperatures of the battery, T1 and T2 cannot be directly determined initially, but are within a range. For example, the characteristic temperature T1 of a lithium battery may appear between 80°C and 100°C, and T2 may appear between 120°C and 160°C. However, the specific values are unknown and are only roughly determined based on its process, experimental conditions, etc. Therefore, the following steps are required for analysis and determination;

[0047] S2. Construct a heat generation model for lithium battery thermal runaway;

[0048] S3. Take the temperature range from the initial heat generation temperature T1 to the thermal runaway temperature T2 as the first temperature stage, and the temperature range from the thermal runaway temperature T2 to the maximum temperature T3 as the second temperature stage, and determine the heat generation amounts in the first and second temperature stages. Divide the first temperature stage, the second temperature stage, and the heat generation amounts in the first and second temperature stages into n sub-intervals respectively, and determine the triggering probabilities of each sub-interval. As mentioned above, since initially, we do not know the initial heat generation temperature T1 and the thermal runaway temperature T2, however, for different batteries, the ranges of T1 and T2 can be roughly determined. For example, the range of the initial heat generation temperature T1 of a certain brand series of lithium batteries is [80°C, 100°C], and the range of T2 is [140°C, 160°C]. Then, take [T1, T2) as the first temperature stage and [T2, T3] as the second temperature stage. And in these two temperature stages, we can estimate a range of heat generation amounts. For example, the range of heat generation amount in the first temperature stage is [q1, q2), and the heat generation amount in the second temperature stage is [q3, q4]. However, the specific values of the heat generation are unknown to us, so step S4 is required for determination. When dividing the sub-intervals, [140°C, 160°C], [80°C, 100°C), [q3, q4], and [q1, q2) are all divided into n sub-intervals. For temperature, the sub-intervals are called temperature sub-intervals, and for heat generation, they are called heat generation sub-intervals;

[0049] S4. Determine the thermal runaway characteristic temperature and the heat generation amount during thermal runaway based on the triggering probabilities of two temperature stages and the heat generation amount, convert the thermal runaway characteristic temperature into activation energy, convert the heat generation amount during thermal runaway into reaction enthalpy, and substitute the activation energy and the reaction enthalpy into the heat generation model of lithium battery thermal runaway to determine the heat generation curve of the lithium battery. Through the above method, the randomness of lithium battery thermal runaway is considered in the simulation analysis of lithium battery thermal runaway, which can more accurately reflect the actual process state of lithium battery thermal runaway, effectively predict the starting heat generation temperature, thermal runaway temperature and energy release process of different lithium batteries, effectively improve the accuracy of lithium battery thermal runaway analysis, and provide accurate data support for the thermal management of lithium batteries.

[0050] In this embodiment, the specific method for determining the triggering probabilities of each sub-interval of the two temperature stages and each sub-interval of the heat generation amount is as follows:

[0051]

[0052] Wherein: represents the distribution probability of the i-th sub-interval. When u is 1, it represents the temperature distribution probability of the i-th temperature sub-interval. When u is 2, it represents the heat generation distribution probability of the i-th heat generation sub-interval. represents the triggering probability of the i-th sub-interval. When u is 1, it represents the temperature triggering probability of the i-th temperature sub-interval. When u is 2, it represents the heat generation triggering probability of the i-th heat generation sub-interval.

[0053] Wherein: Determining the critical temperature based on the temperature triggering probabilities of each sub-interval of the two temperature stages specifically includes:

[0054] Construct a temperature trigger, and the temperature trigger judges to generate a simulated trigger probability value P in each sub-interval i 1 ;

[0055] Judge Whether it holds. If so, the current sub-interval i does not trigger thermal runaway. If not, the current sub-interval triggers heat generation or thermal runaway;

[0056] Determine the thermal runaway temperature T tr (i):

[0057] T tr (i) = T i - +(T i - -T i + ) × P i 1 ; wherein, T i + represents the right boundary value of the temperature of the current temperature sub-interval i, Ti - Represents the left boundary value of the temperature of the current temperature sub-interval i. When making a judgment, first judge the first sub-interval. If it is judged, mark the first sub-interval as 1, and then wait for the temperature of the battery to rise to the second sub-interval and continue to make a judgment until the nth sub-interval; still like the above example, when it is judged that the sub-interval between 90 °C and 92 °C in the 80 °C - 100 °C interval appears P i -P tr (i) ≥ 0, then determine the heat generation characteristic temperature T1 in this sub-interval. This temperature is also called the critical temperature. After this temperature is determined, the starting heat generation characteristic temperature is no longer determined. Wait for the temperature of the lithium battery to continue to rise until it reaches the interval where the thermal runaway characteristic temperature T2 is located to determine the thermal runaway characteristic temperature. The determination process is the same as that of T1; the thermal runaway characteristic temperature T2 is also called the critical temperature, and the determination formulas of T1 and T2 are the same.

[0058] Determine the heat generation amount Q based on the heat generation trigger probabilities of two temperature stages tr (i) Specifically includes:

[0059] Construct a heat generation trigger, and the heat generation trigger judges to generate a simulated trigger probability value P in each heat generation sub-interval i 2 ;

[0060] Judge Whether it holds. If so, there is no heat generation in the current heat generation sub-interval i. If not, heat generation is triggered in the current sub-interval;

[0061] Determine the heat generation amount Q tr (i):

[0062] Among them, Represents the right boundary value of the heat generation of the current heat generation sub-interval i, Represents the left boundary value of the heat generation of the current heat generation sub-interval i.

[0063] Among them, in two temperature stages, then two Qs will be determined tr (i), the heat generation amount Q tr (i) of the first temperature stage is used to determine the reaction enthalpy ΔH1 of the first temperature stage, and Q tr (i) of the second temperature stage is used to determine the reaction enthalpy ΔH2. The formulas for the reaction enthalpies of the two temperature stages are the same. The reaction enthalpy is determined by dividing the heat generation amount by the mass of the reactants participating in the reaction of the lithium battery to obtain the reaction enthalpy.

[0064] Converting the thermal runaway temperature to activation energy specifically includes:

[0065] Construct a function of activation energy and thermal runaway temperature:

[0066]

[0067] Among them, ΔT represents the difference in characteristic temperature, A represents the pre-exponential factor, E represents the activation energy, and R represents the gas constant. In the first stage, that is, between the initial heat generation characteristic temperature T1 and the thermal runaway characteristic temperature T2, then T tr (i) is T1, ΔT is T2 - T1. In the second stage, that is, between the thermal runaway characteristic temperature T2 and the maximum characteristic temperature T3, ΔT is T3 - T2, then T tr (i) is T2. By solving the function of activation energy and thermal runaway temperature, the activation energy E1 corresponding to T1 and the activation energy E2 corresponding to T2 can be obtained; and both E2 and E1 are solved through the above function of activation energy and thermal runaway temperature;

[0068] Substituting the activation energy and reaction enthalpy into the heat generation model of lithium battery thermal runaway to determine the heat generation curve of the lithium battery specifically includes:

[0069] The total heat Q of the lithium battery is:

[0070] Q = Q gen + Q dis ;

[0071] Among them: Q gen represents the heat generation of the lithium battery, and Q dis represents the heat loss of the lithium battery through convective and radiative heat transfer;

[0072] Among them: Among them, S represents the surface area of the lithium battery, T cell represents the temperature of the lithium battery, T a represents the air temperature, h represents the convective heat transfer coefficient between the battery surface and the air, ε represents the emissivity, and σ represents the Stefan-Boltzmann constant;

[0073] Q gen = Q1 + Q2;

[0074] Among them: Q1 represents the heat generated in the first temperature stage; Q2 represents the heat generated in the second temperature stage;

[0075]

[0076]

[0077] Among them, t represents time.

[0078]

[0079] Where: V represents the battery volume; A1 and A2 both represent the pre-exponential factor, ΔH2 and ΔH1 both represent the reaction enthalpy, E1 represents the activation energy in the first temperature stage, E2 represents the activation energy in the second temperature stage, c represents the normalized amount of reactants, and α represents the reaction conversion rate of the lithium battery. Among the above, determining the heat generation as a total amount through probabilistic triggering cannot reflect the change of heat generation over time. However, after determining Q1 and Q2 through the reaction enthalpy and activation energy, it can reflect the change of heat generation and time, thereby drawing a heat generation curve of heat generation changing with time, which is convenient for subsequent analysis of the lithium battery.

[0080] The following is further illustrated with a specific example:

[0081] A 10×10 lithium-ion battery module of 18650 model was established, which was used to simulate the thermal runaway phenomenon in a large-scale battery system. First, the characteristic temperature, the distribution interval and probability of ΔH2, as well as the energy release interval and probability in two stages were obtained. According to these characteristic temperatures and energy release conditions, COMSOL Multiphysics 6.2 software was used for simulation. The corresponding relationship of its activation energy is as Figure 3 shown, the heat generation curve of the lithium battery is as Figure 2 shown, the heat generation distribution probability and the thermal runaway trigger probability are as Figure 4 and Figure 5 shown, the temperature curve of 100 battery cells changing with time is as Figure 6 ; the corresponding relationship of its activation energy is as Figure 3 shown, the heat generation curve of the lithium battery is as Figure 2 shown. Through the analysis of these simulation curves, it is observed that there are obvious differences in temperature rise and energy release shown by each battery cell during the thermal runaway process. The heat generation and temperature distribution among battery cells show randomness conforming to the probability distribution.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present invention, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A method for analyzing thermal runaway of lithium batteries based on the probability of characteristic temperature of thermal runaway, characterized in that: Including the following steps: S1. Determine the characteristic temperature range of the lithium battery. The characteristic temperatures include the initial heat generation temperature T1, the thermal runaway temperature T2, and the maximum temperature T3, and T3 > T2 > T1; S2. Construct a thermal runaway heat generation model for the lithium battery; S3. Take the temperature range from the initial heat generation temperature T1 to the thermal runaway temperature T2 as the first temperature stage, and the temperature range from the thermal runaway temperature T2 to the maximum temperature T3 as the second temperature stage, and determine the heat generation amounts in the first temperature stage and the second temperature stage; Divide both the first temperature stage and the second temperature stage and the heat generation amounts in the first temperature stage and the second temperature stage into n sub-intervals, and determine the triggering probabilities of each sub-interval; S4. Based on the triggering probabilities of the two temperature stages and the heat generation amount, determine the thermal runaway characteristic temperature and the thermal runaway heat generation amount, convert the thermal runaway characteristic temperature into activation energy, convert the thermal runaway heat generation amount into reaction enthalpy, and substitute the activation energy and the reaction enthalpy into the lithium battery thermal runaway heat generation model to determine the heat generation curve of the lithium battery.

2. The method for analyzing thermal runaway of a lithium battery based on the probability of characteristic temperature of thermal runaway according to claim 1, wherein: Determining the triggering probabilities of each sub-interval of the two temperature stages and each sub-interval of the heat generation amount specifically includes: Wherein: represents the distribution probability of the i-th sub-interval. When u is 1, it represents the temperature distribution probability of the i-th temperature sub-interval. When u is 2, it represents the heat generation distribution probability of the i-th heat generation sub-interval. represents the trigger probability of the i-th sub-interval. When u is 1, it represents the temperature trigger probability of the i-th temperature sub-interval. When u is 2, it represents the heat generation trigger probability of the i-th heat generation sub-interval.

3. The method for analyzing thermal runaway of a lithium battery based on the probability of characteristic temperature of thermal runaway according to claim 2, characterized in that: Determining the critical temperature based on the temperature triggering probabilities of each sub-interval of the two temperature stages specifically includes: Construct a temperature trigger, and use the temperature trigger to determine a simulated trigger probability value P for each sub-interval i 1 ; Judge whether it holds. If so, the current sub-interval i does not trigger thermal runaway. If not, the current sub-interval triggers heat generation or thermal runaway; Determine the thermal runaway temperature T tr (i): T tr (i) = T i - +(T i - -T i + ) × P i 1 ; where, T i + represents the right boundary value of the temperature of the current temperature sub - interval i, and T i - represents the left boundary value of the temperature of the current temperature sub - interval i.

4. The method for analyzing thermal runaway of a lithium battery based on the probability of characteristic temperature of thermal runaway according to claim 3, characterized in that: Determine the heat production Q based on the heat production trigger probability in two temperature stages tr (i) Specifically include: Construct a heat generation trigger, and the heat generation trigger determines a simulated trigger probability value P is generated in each heat generation sub-interval i 2 ; Determine whether it holds. If so, there is no heat generation in the current heat generation sub-interval i. If not, heat generation is triggered in the current sub-interval; Determine the heat production Q tr (i): Among them, represents the right heat generation boundary value of the current heat generation sub-interval i, represents the left heat generation boundary value of the current heat generation sub-interval i.

5. The method for analyzing thermal runaway of a lithium battery based on the probability of characteristic temperature of thermal runaway according to claim 4, characterized in that: Converting the thermal runaway temperature into activation energy specifically includes: Construct a function of activation energy and thermal runaway temperature: Wherein, ΔT represents the difference in characteristic temperature, A represents the pre-exponential factor, E represents the activation energy, and R represents the gas constant.

6. The method for analyzing thermal runaway of a lithium battery based on the probability of thermal runaway characteristic temperature according to claim 5, wherein: Substituting the activation energy and the reaction enthalpy into the lithium battery thermal runaway heat generation model to determine the heat generation curve of the lithium battery specifically includes: The total heat Q of the lithium battery is: Q = Q gen +Q dis ; Where: Q gen represents the heat generation of the lithium battery, and Q dis represents the heat loss of the lithium battery through convective and radiative heat transfer; Q gen = Q1 + Q2; Wherein: Q1 represents the heat generated in the first temperature stage; Q2 represents the heat generated in the second temperature stage; Wherein: V represents the battery volume; A1 and A2 both represent the pre-exponential factor, ΔH2 and ΔH1 both represent the reaction enthalpy, E1 represents the activation energy in the first temperature stage, E2 represents the activation energy in the second temperature stage, c represents the normalized amount of reactants, and α represents the reaction conversion rate of the lithium battery.

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