Lithium ion battery expansion modeling method coupling gas generation and mechanical deformation

Abstract

The invention discloses a lithium ion battery expansion modeling method coupling gas generation and mechanical deformation, and relates to the technical field of lithium ion battery model construction and simulation methods.The method comprises the following specific steps that 1, a lithium ion battery module is selected, and battery electrochemical parameters, material physical property parameters and battery geometric characteristic parameters are obtained; (2) establishing a three-dimensional geometric model and dividing grids according to the initial parameters and the simulated working conditions, and setting boundary conditions; (3) solving an electrochemical reaction model and a pressure change model in the battery, and calculating the internal pressure of the battery; (4) taking the calculated battery temperature and internal pressure parameters as dynamic boundary conditions, and applying the dynamic boundary conditions to the physical boundary of the inner surface of the battery shell; and (5) solving the finite element-based mechanical model to obtain the stress and strain of the battery, and simulating the expansion behavior of the battery in thermal runaway. According to the method, the expansion behavior of the lithium ion battery caused by internal pressure rise in the thermal runaway process can be calculated, meanwhile, the thermal runaway key parameters can be accurately predicted, and a basis can be provided for safety early warning design of a lithium ion battery module and an energy storage system.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery model construction and simulation methods, and in particular relates to a lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation. Background Art

[0002] As the global demand for renewable energy continues to increase, lithium-ion batteries, as an efficient energy storage solution, have become an important component in the fields of electric vehicles, renewable energy storage, and portable electronic devices. Especially in the context of addressing climate change and achieving sustainable development goals, the application of lithium-ion batteries is becoming more and more extensive, and its market prospects are becoming more and more broad. However, the rapid development of lithium-ion batteries is also accompanied by some potential safety hazards, among which the problem of thermal runaway is particularly prominent. Thermal runaway refers to the acceleration of chemical reactions inside the battery during the charging or discharging process of lithium-ion batteries due to factors such as increased internal temperature and uncontrolled reaction, which leads to the rapid release of heat and gas. As the temperature rises further, the battery may have serious consequences such as fire and explosion. The frequent occurrence of thermal runaway accidents of lithium-ion batteries not only threatens the safety of life and property of users, but also poses a challenge to the healthy development of the energy storage industry. In recent years, many accidents caused by thermal runaway of lithium-ion batteries have occurred worldwide, causing huge economic losses and social impacts, and effective solutions are urgently needed to deal with this problem.

[0003] In order to effectively prevent thermal runaway accidents, early warning technology is particularly important. At present, monitoring physical signals such as battery temperature, voltage and gas concentration is the mainstream early warning method, but these methods often have limitations and cannot fully reflect the complex state inside the battery. Therefore, monitoring based on expansion force signals has gradually received attention. This signal can not only respond quickly in the early stages of thermal runaway, but also be quickly transmitted to surrounding batteries and modules through structural materials, providing support for the overall state monitoring of the system. Although expansion force-based monitoring has obvious advantages, existing studies generally fail to fully consider the interaction between gas generation and expansion behavior, resulting in insufficient understanding of the thermal runaway process. Therefore, it is particularly urgent to develop an efficient modeling method to couple the dynamic process of gas generation and mechanical deformation.

[0004] To this end, the present invention proposes a lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation. Relying on numerical simulation technology, this method establishes a coupling model between gas generation and expansion behavior inside the battery. By simulating the gas generation, temperature changes and the expansion behavior caused by the thermal runaway process in detail, the evolution characteristics of the expansion force and its mutual influence with thermal runaway are deeply analyzed. This invention not only provides a theoretical basis for improving the safety of lithium-ion batteries, but also provides new ideas and methods for the design and optimization of future energy storage systems. Summary of the invention

[0005] The purpose of the present invention is to propose a lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation to address the above-mentioned defects in the prior art. The method calculates the internal pressure change of the lithium-ion battery through a lumped parameter electrochemical reaction model and a pressure change model, and solves the battery expansion behavior during thermal runaway through a finite element-based mechanical model.

[0006] The technical solution is: a lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation, including the following steps: S1, select a lithium-ion battery module, obtain the battery electrochemical parameters, material physical parameters and geometric characteristic parameters of the battery; S2, establish a three-dimensional geometric model and divide the grid according to the initial parameters and simulation conditions, and set boundary conditions; S3, solve the electrochemical reaction model and pressure change model inside the battery, and calculate the internal pressure of the battery; S4, use the battery temperature and internal pressure parameters calculated by S3 as dynamic boundary conditions, and apply them to the physical boundary of the inner surface of the battery shell; S5, solve the finite element-based mechanical model, obtain the stress and strain of the battery, and simulate the expansion behavior of the battery in thermal runaway.

[0007] Furthermore, the basic theory and establishment process of the thermal runaway reaction kinetic equation in S3 include:

[0008] During thermal runaway, the heat released by the electrochemical reaction will cause the battery temperature to continue to rise. This process can be described by the lumped parameter transient energy conservation equation based on the Arrhenius formula:

[0009]

[0010] where ρ is the density, C p is the specific heat capacity, T is the temperature, t is the time, ΔH i is the enthalpy of the side reaction, c i is the dimensionless concentration of active material. dc i / dt can be solved by Arrhenius formula, and the relevant control equations are listed in Table 1.

[0011] Table 1 Control equations of thermal runaway heat generation kinetic model

[0012]

[0013] Furthermore, the basic theory and establishment process of the pressure change model in S3 include:

[0014] The gas production process inside lithium-ion batteries includes electrolyte evaporation and side reaction release. The evaporation rate of the electrolyte is determined by the following formula:

[0015]

[0016] where α l is the volume fraction of the electrolyte in the roll core, l1 and l2 are the geometric parameters of the battery, C is the evaporation coefficient; M e is the molar mass of the electrolyte, ρ v is the vapor density inside the battery, Δ vap H is the enthalpy of evaporation, T sat is the saturation temperature of the electrolyte, which can be calculated as:

[0017]

[0018] Where P represents the pressure inside the battery. For the reaction gases, hydrogen, carbon monoxide, carbon dioxide, methane, ethylene and ethane are mainly considered, and their generation rates are considered to be linear functions of the electrochemical reaction rate and are calculated according to the following formula:

[0019]

[0020] where ω i is the gas generation coefficient, obtained by experimentally measuring the total amount of gas generation.

[0021] Furthermore, the internal pressure and jet dynamics model in step 3 are calculated using a lumped parameter model represented by an ordinary differential equation, where the control equation representing the pressure change is expressed as:

[0022]

[0023] Where R is the molar gas constant, V h is the free space volume inside the battery, and n is the molar mass of gas molecules. For a single gas component, its molar change rate depends on the generation rate and the outflow rate, which can be given by the following formula:

[0024]

[0025] in represents the blocking coefficient, C d is the gas emission coefficient, A v is the battery safety valve area, ρ is the gas density, and u is the gas jet velocity. The jet velocity is calculated by the internal pressure of the battery and can be given by the following equation:

[0026]

[0027]

[0028]

[0029] Where γ represents the heat capacity ratio of the exhaust gas mixture, P v is the pressure at the battery safety valve, P a is the ambient pressure; Ma is the Mach number. By iteratively solving the above equation, the change in the internal pressure of the battery during thermal runaway is obtained.

[0030] The calculation in S3 obtains the battery temperature and internal pressure as dynamic boundary conditions, which participate in the calculation of the finite element model in S5.

[0031] Furthermore, the basic theories based on the finite element model in S5 include:

[0032]

[0033] F is the yield stress of the material, B is the strain hardening coefficient. ε is the strain, and i is the strain hardening exponent. C n is the strain rate sensitivity coefficient. ε' is the strain rate. T is the battery temperature, obtained through the lumped parameter model in step 3. T r is the ambient temperature, T m is the melting point temperature of the battery casing, and j is the temperature effect index.

[0034] Furthermore, the boundaries of the battery are divided into the inner surface of the battery and the outer surface of the battery; for the temperature field, the inner surface of the battery is set as the dynamic temperature boundary condition calculated by the electrochemical reaction model, and the outer surface of the battery is set as the convection heat dissipation boundary condition; for the finite element model, the inner surface of the battery is set as the dynamic force boundary condition, the stress value is the internal pressure of the battery calculated by the pressure change model, and the outer surface of the battery is set as the displacement boundary condition.

[0035] Furthermore, the coupling process of this method is as follows: the temperature of the battery will affect the heat and gas production rate of the side reactions inside the battery, and the heat production rate will promote the further increase of the battery temperature. At the same time, the heat and gas production rates will affect the pressure increase inside the battery. The change in the internal pressure of the battery will exert stress on the inner surface of the battery, which will cause the battery to expand and generate expansion force. This process is calculated in the open source computational fluid dynamics software OpenFOAM. The calculation process of this invention is shown in the attached manual. Figure 1 The coupling process and calculation flow of the thermal runaway modeling method in the present invention are shown in the attached Figure 2 The symbols and terms used in this paper are shown in Table 3 Symbols and terms.

[0036] Table 2 Symbols and terms

[0037]

[0038]

[0039] The advantages of the present invention compared with the prior art are: (1) It makes up for the lack of research on the existing thermal runaway model of lithium-ion batteries that fails to consider the battery expansion process; (2) The modeling method can study the temperature and internal pressure changes of lithium-ion batteries under different abuse conditions by changing a series of parameters, and at the same time can obtain the evolution characteristics and laws of the battery expansion force and volume deformation during the thermal runaway process, and can provide data support for the thermal runaway early warning system based on force signals; (3) The modeling method provides a model framework for lithium-ion battery model developers and simulation researchers, which can provide a basis and guidance for subsequent model development. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Attached Figure 1 is an operational flow chart of a lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation;

[0041] Attached Figure 2 It is the coupling process and calculation flow of the battery expansion modeling method in the present invention;

[0042] Attached Figure 3 is a schematic diagram of model geometry and mesh in an embodiment of the present invention;

[0043] Attached Figure 4 is a curve of battery temperature and internal pressure changes during thermal runaway calculated in an embodiment of the present invention;

[0044] Attached Figure 5 It is the battery expansion behavior during thermal runaway predicted in the embodiments of the present invention. DETAILED DESCRIPTION

[0045] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0046] Example

[0047] Taking a square lithium-ion battery with lithium iron phosphate as the positive electrode material and graphite as the negative electrode material as an example, the expansion behavior of the battery during thermal runaway is simulated to describe the present invention in a comprehensive and detailed manner. This method is not limited to the modeling of lithium iron phosphate-graphite square lithium-ion batteries, but is applicable to the prediction of the expansion behavior of all lithium-ion batteries. The method is mainly divided into the following steps: S1, select a lithium-ion battery, obtain the electrochemical parameters, material physical parameters and geometric characteristic parameters of the battery; S2, according to the initial parameters and simulation conditions, establish a three-dimensional geometric model and divide the grid, and set the boundary conditions; S3, solve the electrochemical reaction model and pressure change model inside the battery, and calculate the internal pressure of the battery; S4, use the battery temperature and internal pressure parameters calculated by S3 as dynamic boundary conditions and apply them to the physical boundary of the inner surface of the battery shell; S5, solve the mechanical model based on finite elements, obtain the stress and strain of the battery, and simulate the expansion behavior of the battery in thermal runaway.

[0048] S1, select a lithium-ion battery, obtain the electrochemical parameters, material physical parameters and geometric characteristic parameters of the battery, and the specific steps include:

[0049] Step 1: Obtain the physical properties and reaction kinetic parameters of the battery according to the method of literature research;

[0050] Step 2: Obtain the geometric characteristic parameters of the battery through measurement methods.

[0051] S2, according to the initial parameters and simulation conditions, establish a three-dimensional geometric model and divide the grid, and set the boundary conditions. The specific steps include:

[0052] Step 1: Establish the geometric model. According to the specifications and dimensions of the battery, build the geometric model and divide the mesh. See the model geometry and mesh for details. Figure 3 The size of the entire calculation area is 148mm×134mm×27mm. The boundary of the calculation model includes the inner surface and the outer surface of the battery.

[0053] Step 2: Set the heat transfer conditions of the battery. The dynamic temperature boundary conditions calculated by the electrochemical reaction model are set for the inner surface of the battery, and the convection heat dissipation boundary conditions are set for the outer surface of the battery. Set the force boundary of the battery. The dynamic force boundary conditions are set for the inner surface of the battery. The stress value is the internal pressure of the battery calculated by the pressure change model, and the outer surface of the battery is set to the displacement boundary conditions.

[0054] S3, solves the electrochemical reaction model and pressure change model inside the battery, and calculates the internal pressure of the battery:

[0055] The changes of battery temperature and internal pressure during thermal runaway were calculated by electrochemical reaction model and pressure change model respectively. Figure 4As shown in the figure. During the preheating stage, the battery temperature and internal pressure will continue to rise. As the battery temperature rises, the evaporation of the electrolyte and the generation of reaction gas will cause the internal pressure of the battery to continue to rise. When the battery pressure reaches the critical value of 1.45×10 6 Pa, the safety valve will open and release the reaction gas and electrolyte vapor, and the battery pressure will drop rapidly. As the battery temperature continues to rise, the battery will enter the thermal runaway stage, the battery temperature will reach a peak of 371.2°C, and the internal pressure of the battery will reach a peak of 2.69×10 5 Pa. Figure 4 The comparison between the simulation results and the experimental results is shown, and it can be found that the simulated temperature evolution curve is quite consistent with the experimental results. In particular, the established model captures the peak temperature of the battery very well, with a relative error of less than 1.0%.

[0056] S4, the battery temperature and internal pressure parameters calculated in step 3 are applied as dynamic boundary conditions to the physical boundary of the inner surface of the battery casing. The specific method is: the electrochemical reaction model and the pressure change model calculate the battery temperature and internal pressure respectively, output and record them every 1 second, and store them in the specified text document under the example folder; in the example, the time series data in the document is read as the dynamic parameter to set the boundary condition.

[0057] S5, solve the mechanical model based on finite elements, obtain the stress and strain of the battery, and simulate the expansion behavior of the battery in thermal runaway, such as Figure 5 As shown in the figure, it can be found that as the battery electrochemical reaction occurs and the internal pressure of the battery increases, the battery shell continues to expand, and the simulation results can simulate this process well.

[0058] Based on the above analysis of the calculation results such as the evolution of characteristic parameters during thermal runaway, it can be seen that the lithium-ion battery expansion modeling method that couples gas generation and mechanical deformation described in the present invention can calculate the expansion behavior of the lithium-ion battery caused by the increase in internal pressure during thermal runaway, and can accurately predict the key parameters of thermal runaway, which can provide a basis for the safety warning design of lithium-ion battery modules and energy storage systems.

Claims

1. A method for modeling lithium-ion battery expansion by coupling gas generation and mechanical deformation, characterized in that: The following steps are involved: S1, select a lithium-ion battery module, and obtain the battery electrochemical parameters, material physical property parameters, and geometric characteristic parameters of the battery; S2, based on the initial parameters and simulation conditions, a three-dimensional geometric model is established and meshed, and boundary conditions are set; S3, solving the electrochemical reaction model and pressure change model inside the battery, and calculating the internal pressure of the battery; S4, applying the battery temperature and internal pressure parameters calculated in S3 as dynamic boundary conditions to the physical boundary of the inner surface of the battery housing; S5, solves the finite element-based mechanical model, obtains the stress and strain of the battery, and simulates the expansion behavior of the battery in thermal runaway.

2. A lithium-ion battery expansion modeling method for coupling gas generation and mechanical deformation according to claim 1, characterized in that: For the temperature field, the inner surface of the battery is set as a dynamic temperature boundary condition calculated by the electrochemical reaction model, and the outer surface of the battery is set as a convection heat transfer boundary condition; For the finite element model, the inner surface of the battery is set as a dynamic force boundary condition, the stress value is the internal pressure of the battery calculated by the pressure change model, and the outer surface of the battery is set as a displacement boundary condition.

3. A lithium-ion battery expansion modeling method for coupling gas generation and mechanical deformation according to claim 1, characterized in that: The coupling process is: the temperature of the battery will affect the heat release rate and gas production rate of the side reactions inside the battery, and the heat production rate will promote the further increase of the battery temperature. At the same time, the heat and gas production rates will affect the pressure increase inside the battery; the change in the internal pressure of the battery will exert stress on the inner surface of the battery, causing the battery to expand and generate expansion force.

4. A lithium-ion battery expansion modeling method for coupling gas generation and mechanical deformation according to claim 1, characterized in that: Through model calculation, the temperature, internal pressure and volume expansion changes of the battery during thermal runaway can be obtained simultaneously.

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