Method for constructing lithium battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model

Abstract

The application provides a lithium ion battery electrochemistry-heat-force-short circuit-thermal runaway coupling model construction method, first, electrochemistry test, heat test and mechanical test are carried out on a sample to obtain relevant physicochemical parameters; then, finite element software is used to respectively establish an electrochemistry model, a heat model, a mechanical model, a short circuit model and a thermal runaway side reaction model, an electrochemistry-heat-force-short circuit-thermal runaway coupling model is built, and relevant parameters are given to the established model; then, according to the actual simulation requirements of the lithium ion battery, the boundary conditions and initial conditions of each physical model are set, and the grid is divided; finally, the sample is tested, the coupling model is calibrated and precision verified based on the measured data, and a lithium ion battery electrochemistry-heat-force-short circuit-thermal runaway coupling model is obtained. The model can simulate the dynamic response of the external and internal characteristics of the battery under the extrusion working condition, and significantly improve the prediction ability of the battery state.

Description

[0001] This application is a divisional application of application No. 202210938703.3, entitled "Lithium ion battery electrochemical-thermal-force-short circuit-thermal runaway coupling model". TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium ion batteries, in particular to a lithium ion battery electrochemical-thermal-force-short circuit-thermal runaway coupling model, a construction method thereof and an application thereof. BACKGROUND

[0003] With the rapid development of China's economy, the consumption of fossil energy has led to energy crisis and environmental pollution for the country, and it is urgent to develop and use new renewable energy to alleviate the current situation. Lithium ion batteries are widely used due to their high energy density, high working voltage, low self-discharge rate, no memory effect and other advantages, and have become the preferred and mainstream power source for new energy vehicles. However, the range and safety problems are key factors affecting the promotion of electric vehicles. The development of high-energy-density battery systems not only alleviates the range anxiety, but also makes the battery safety problem increasingly prominent, seriously endangering the safety of people's lives and property.

[0004] Due to the wide application of lithium ion batteries, in some actual use scenarios, they are easily affected by mechanical load impact, resulting in stress and deformation of the battery. In severe cases, it can cause short circuit of the positive / negative electrode inside the battery to generate a large amount of heat, and then trigger the risk of battery combustion and explosion, etc. At present, the safety requirements and test methods for power storage batteries for power vehicles in China (GBT31485-2015) stipulate that single storage batteries should not explode or catch fire in the extrusion test.

[0005] During the battery design and development stage, completely based on experimental methods will bring a large amount of experimental test work, complex operation process and high cost, and cannot help the researchers to deeply understand the internal electrochemical reaction, heat generation and short circuit mechanism of the battery. Therefore, it is necessary to design a coupling model for estimating and predicting the temperature and voltage dynamic response of the battery under extrusion conditions, so as to provide more internal information of the battery.

[0006] There are some simulation models in the prior art that simulate and predict the thermal runaway behavior of the battery, but these simulation models are only suitable for predicting the influence of conventional electrical or thermal triggers. In addition to the above influencing factors, mechanical, short circuit and other extreme accidental factors often cause thermal runaway of the battery, causing deformation and generating a large amount of heat in a short time, which will cause great damage to the battery. The prediction ability of the simulation model for these extreme or accidental factors is particularly important. SUMMARY

[0007] In view of the deficiencies of the prior art, the application provides a lithium ion battery electrochemistry-heat-force-short circuit-thermal runaway coupling model and a construction method and application thereof, which can simulate the dynamic response of the external and internal characteristics of the battery under a crushing working condition, solve the distribution and change of the voltage, temperature, stress and material of the battery during crushing, estimate and predict the occurrence position, size and thermal runaway temperature evolution of the internal short circuit, save experimental cost, guide battery optimization design, improve development efficiency, and significantly improve the prediction capability of the battery state under extreme or sudden conditions such as mechanical and short circuit conditions.

[0008] To achieve the above object, the technical scheme adopted by the application comprises:

[0009] A lithium ion battery electrochemistry-heat-force-short circuit-thermal runaway coupling model, which is coupled by an electrochemistry model, a heat model, a mechanics model, a short circuit model and a thermal runaway side reaction model.

[0010] The coupling correlation mode between the models comprises: the stress / strain parameters of the mechanics model are correlated with the conductivity of the short circuit model, the internal short circuit resistance of the short circuit model is correlated with the boundary condition of the electrochemistry model, the battery voltage of the electrochemistry model is correlated with the boundary condition of the short circuit model, the temperature of the heat model is respectively correlated with the temperature of the electrochemistry model and the thermal runaway side reaction model, and the internal short circuit heat of the short circuit model, the electrochemical polarization heat, reversible entropy heat and ohmic heat of the electrochemistry model, and the side reaction heat of the thermal runaway model are respectively correlated with the corresponding heat generation terms in the heat model. Through the above coupling correlation, the influence of the battery under sudden conditions such as mechanical and short circuit conditions can be successfully transmitted in real time and accurately in the model, and the simulation accuracy, sensitivity and prediction capability under comprehensive conditions of the model are improved through the stress / strain, internal resistance change, temperature change and comprehensive heat generation parameters.

[0011] As an optional scheme of optimization, the coupling correlation mode between the models can be realized by establishing the following specific models.

[0012] The electrochemistry model is composed of an electrochemistry equation group and an electrochemistry heat generation equation group; the electrochemistry equation group comprises one or more of a solid phase mass conservation equation, a liquid phase mass conservation equation, a solid phase charge conservation equation, a liquid phase charge conservation equation and an electrode reaction kinetics equation, which are used to solve the potential, Li+concentration and concentration distribution and change of side reaction materials in the solid / liquid phase in the battery; and the electrochemistry heat generation equation group comprises one or more of an ohmic heat equation, a reversible entropy heat equation and an electrochemical polarization heat equation, which are used to solve the size of each heat generation component of the battery.

[0013] The thermal model is composed of an energy conservation equation for simulating temperature distribution and change in the battery; the energy conservation equation includes a heat transfer term, a heat generation term, and a heat dissipation term; the heat transfer term is used to describe heat transfer phenomena caused by temperature gradient inside the battery; the heat generation term includes heat sources of electrochemical polarization heat, reversible entropy heat, ohmic heat, internal short circuit heat, and thermal runaway side reaction heat; the heat sources of the heat generation term are all transmitted to the thermal model in real time by other physical models through parameter transmission; and the heat dissipation term is composed of convective heat transfer and radiative heat transfer, and is used to describe heat exchange caused by convective and radiative heat transfer between the battery and the environment.

[0014] The mechanical model is composed of a geometry equation, a physical equation, and a balance differential equation, and is used to simulate the distribution and change of stress and strain of the battery under mechanical load caused by the pressure head.

[0015] The short circuit model is constructed based on Ohm's law, and is used to predict the size, location, and heat generation of the internal short circuit; the conductivity of the battery component is defined as a function relationship related to strain / stress, and the size of the conductivity of each battery component changes in real time with the size of the stress / strain in the mechanical model; the strain / stress is transmitted from the mechanical model to the short circuit model in real time in the form of parameter transmission; and the internal short circuit heat generation follows Ohm's law.

[0016] The thermal runaway side reaction model is one or more of a substance conservation equation, an energy conservation equation, and an Arrhenius equation, and is used to solve the concentration and heat generation of each side reaction substance; the side reactions include one or more of SEI decomposition reaction, graphite and electrolyte reaction, graphite and binder reaction, positive and negative electrode reaction, positive and binder reaction, and positive electrode decomposition reaction.

[0017] The coupling mode of real-time parameter transmission is that the above-mentioned physical models are solved simultaneously, each model transmits the parameters obtained by the current solving to the corresponding physical model in real time and carries out the next step of solving calculation, so as to realize the mutual influence between the physical models, that is, coupling. Further, the battery component includes one or more of a negative electrode current collector, a negative electrode coating, a separator, a positive electrode coating, and a positive electrode current collector.

[0018] Further, the lithium ion electrochemical model, the thermal model, the mechanical model, the short circuit model, and the thermal runaway side reaction model can share the same geometry or establish a corresponding geometry separately. As an example, the electrochemical model geometry can use one-dimensional, the geometry of the thermal model, the mechanical model, and the short circuit model can use one or more of one-dimensional, two-dimensional, or three-dimensional, and the geometry of the thermal runaway side reaction model uses lumped (0-dimensional).

[0019] A method for constructing a lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model, comprising the following steps:

[0020] 1. Perform electrochemical test, thermal test and mechanical test on the sample to obtain relevant physicochemical parameters; including geometric parameters, electrochemical related parameters, thermal related parameters and mechanical related parameters;

[0021] 2. Use finite element software to respectively establish electrochemical model, thermal model, mechanical model, short circuit model and thermal runaway side reaction model, build lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model, and assign the relevant parameters obtained in step 1 to the established model;

[0022] 3. According to the actual simulation needs of lithium ion battery, set the boundary conditions and initial conditions of each physical model, and divide the grid;

[0023] 4. Test the sample to obtain battery electrical, thermal and mechanical test data, calibrate and verify the accuracy of the coupling model based on the measured data, and obtain the lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model.

[0024] After completing the above steps to build the model, further perform the following step 5, which can use the model to simulate and / or predict the lithium ion battery, and can make timely adjustments and optimizations to the battery structure design through the simulation and / or prediction results.

[0025] 5. Adjust the battery size parameters and capacity, and carry out simulation and / or prediction under the same process conditions under the battery extrusion working condition. The specific simulation and / or prediction content can include: (1) predicting the performance of lithium ion battery under extrusion working condition, (2) predicting the safe battery structure of lithium ion battery under extrusion working condition, (3) predicting the safe limit capacity of lithium ion battery.

[0026] Further, the sample of step 1 includes battery samples, electrode samples and material samples; the material samples include positive electrode materials, negative electrode materials and separator materials; wherein the positive electrode material is selected from at least one of nickel-cobalt-manganese ternary material, lithium iron phosphate, lithium cobaltate, lithium nickelate, lithium manganate and lithium manganese phosphate; the negative electrode material is selected from at least one of artificial graphite, natural graphite, mesocarbon microbeads, silicon, silicon monoxide and lithium titanate; the electrode sample is a positive / negative electrode battery pole piece made of raw materials including positive / negative electrode materials to prepare positive / negative electrode coating, and together with the positive / negative electrode current collector; the battery sample is a battery made of raw materials including separator materials, together with the electrode sample. The battery sample, the electrode sample and the material sample exist in different forms, and can provide the required physicochemical parameters in different test items.

[0027] Further, the geometry parameters in step 1 include one or more of the length and width of the battery electrode tab, the length and width of the battery current collector, the position parameters of the battery tab, the thickness of the battery electrode tab (the electrode tab includes the positive / negative electrode current collector and the positive / negative electrode coating), the number of battery electrode tabs, and the length, width and height of the battery.

[0028] Further, the electrochemistry-related parameters in step 1 include one or more of the electronic conductivity of the solid phase material of the test sample, the initial concentration of lithium ions, the volume fraction of the solid phase, the volume fraction of the liquid phase, the particle radius of the solid phase material, the effective conductivity of the liquid phase material, the initial lithium concentration of the electrolyte, the electrochemical reaction rate constant, the charge transfer coefficient, the lithium ion diffusion coefficient of the solid phase, and the lithium ion diffusion coefficient of the liquid phase.

[0029] Further, the thermal-related parameters in step 1 include one or more of the mass, density, specific heat capacity, thermal conductivity, and thermal decomposition temperature of the test sample.

[0030] Further, the mechanical-related parameters in step 1 include one or more of the compression modulus and Poisson's ratio of the lithium ion battery internal material.

[0031] Further, the respective establishment of the electrochemical model, the thermal model, the mechanical model, the short circuit model and the thermal runaway side reaction model in step 2 is as follows:

[0032] Further, the relevant parameters obtained in step 1 are assigned to the established models as described in step 2.

[0033] In the present application, the coupled model in step 2, the mechanical model is used to solve the stress / strain changes inside the battery, the short circuit model calculates the size, position and heat production of the short circuit resistance according to the calculation results of the mechanical model, the electrochemical model solves the potential, material and electrochemical heat production distribution under the condition of short circuit inside the battery, the thermal model is used to calculate the temperature distribution of the battery, and the thermal runaway side reaction model is used to solve the concentration and heat production of the thermal runaway side reaction material.

[0034] Further, the coupling mode of the coupled model in step 2 is as follows:

[0035] a) The input of the mechanical model includes the initial conditions and boundary conditions affected by mechanical action, and the output of the mechanical model is the stress / strain changes inside the battery over time and space;

[0036] b) The input of the short circuit model includes the stress / strain changes over time and space output by the mechanical model and the changes in the battery voltage obtained by the electrochemical model, and the output of the short circuit model includes the size of the battery short circuit resistance, the short circuit position and the changes in the internal short circuit heat production;

[0037] c) the input of the electrochemical model includes the battery short-circuit internal resistance size obtained by the short-circuit model and the temperature change obtained by the thermal model, and the output of the electrochemical model includes the change of the battery voltage and the battery electrochemical heat generation, including ohmic heat, polarization heat and reversible entropy heat;

[0038] d) the input of the thermal model includes the electrochemical heat generation obtained by the electrochemical model, the internal short-circuit heat generation obtained by the short-circuit model and the side reaction heat generation obtained by the thermal runaway side reaction model, and the output of the thermal model is the change of the battery temperature with time and space;

[0039] e) the input of the thermal runaway side reaction model is the change of the battery temperature obtained by the thermal model, and the output of the thermal runaway side reaction model is the heat generation of each side reaction.

[0040] Further, the initial conditions and boundary conditions input by the mechanical model include: the pressing speed of the pressure head, the contact interface between the pressure head and the battery is set as a contact boundary condition, and the bottom surface of the battery is set as a fixed displacement boundary condition.

[0041] Further, the change of stress / strain with time and space input by the short-circuit model is realized by linear projection or linear stretching; the change of the battery voltage (expressed as the battery terminal voltage in the example) input by the short-circuit model is applied at the positive tab of the battery model, and the ground boundary condition is used at the negative tab of the battery.

[0042] Further, the short-circuit model under the battery voltage boundary condition can obtain the corresponding short-circuit current of the short-circuit model, and the short-circuit internal resistance size of the short-circuit model is obtained by the ratio of the battery voltage to the short-circuit current.

[0043] Further, the battery short-circuit internal resistance size input by the electrochemical model is applied by using the boundary condition of the external short-circuit resistance, and the change of the temperature input by the electrochemical model is realized by using the setting method of linear projection or linear stretching.

[0044] Further, the heat generation of each part of the electrochemical model, the internal short-circuit heat generation and the side reaction heat generation input by the thermal model are realized by using the setting method of linear projection or linear stretching.

[0045] Further, the change of the temperature input by the thermal runaway side reaction model is realized by using the setting method of linear projection or linear stretching.

[0046] Further, the temperature obtained by the thermal model is transmitted to the electrochemical model in real time to affect the partial electrochemical reaction kinetics parameters in the electrochemical model, such as the electrochemical effective reaction rate constant, the solid-phase lithium ion effective diffusion coefficient, etc., through the Arrhenius relationship.

[0047] Further, the meshing in step 3 uses one or more of a triangular mesh, a quadrilateral mesh, a tetrahedral mesh, a hexahedral mesh, a pyramid mesh, a wedge mesh, and a hybrid mesh composed of the above mesh types.

[0048] Further, the testing of the sample in step 4 is to obtain battery electrical, thermal, and force test data, and the test conditions include:

[0049] 1. Constant current and / or constant current constant voltage charging or discharging at different rates under multiple temperature conditions, wherein the temperature and rate conditions should be selected as much as possible to cover the normal use conditions of the battery;

[0050] 2. During the charging / discharging process, the battery surface and internal temperature are collected by using external or internal thermocouples, optical fiber sensors, and infrared imagers;

[0051] 3. According to the relevant extrusion test conditions, the battery is subjected to extrusion safety testing, and the internal and / or external temperature of the test battery and the voltage change over time are collected during the testing.

[0052] Further, the adjustment of the battery size parameters and the capacity in step 5 includes increasing the size of the battery in the plane direction and increasing the number of laminated sheets of the battery electrode, the increase of the size of the battery in the plane direction includes the increase of the length and width of the battery electrode, and the increase of the number of laminated sheets of the battery electrode is achieved by alternating the stacking of the positive electrode and the negative electrode and separating them with a separator. By increasing the battery capacity under the same process conditions without changing the thickness of the single electrode, the extrusion safety simulation of the battery is carried out.

[0053] In the present application, the application of the above-mentioned lithium ion battery electrochemical-thermal-force-short circuit-thermal runaway coupling model in designing the structure of the lithium ion battery and predicting the performance of the lithium ion battery under the extrusion condition is also provided.

[0054] In the present application, the application of the above-mentioned lithium ion battery electrochemical-thermal-force-short circuit-thermal runaway coupling model in carrying out the simulation and prediction of the performance of the large-capacity battery under the same process conditions is also provided.

[0055] Compared with the prior art, the present application has the following beneficial effects:

[0056] In this invention, the model and method can be used to estimate, predict, and analyze the results of battery safety extrusion tests. It offers high reliability and low cost, simulating the deformation process of a battery under extrusion conditions, solving for the changes in voltage, temperature, and stress over time and space during extrusion, and predicting the magnitude and location of short circuits and thermal runaway temperatures within the battery. Furthermore, the model can also be used to simulate the extrusion safety performance of large-capacity batteries under the same process conditions, predicting their ultimate extrusion safety capacity. This invention overcomes the limitation of general models that are only applicable to simulations under conventional electrical or thermal triggering factors. It can adapt to and predict thermal runaway phenomena caused by extreme or sudden factors such as mechanical stress and short circuits, and thereby provide battery optimization design results and methods, possessing significant application value. Attached Figure Description

[0057] Figure 1 This is a general flowchart of an example of the present invention.

[0058] Figure 2 This is a schematic diagram of the multiphysics coupling principle in an example of the present invention.

[0059] Figure 3 This is a two-dimensional physical model and its mesh division diagram for an example of the present invention.

[0060] Figure 4 This is a comparison chart of the prediction results of the example model of this invention and the experimental results.

[0061] Figure 5 This is a comparison chart of the prediction results of the example model of this invention and the experimental results.

[0062] Figure 6 This is a diagram showing the volumetric strain distribution and short-circuit region of the diaphragm in an example of the present invention.

[0063] Figure 7 The following are some parameter configurations in the electrochemical model, thermal model, mechanical model, and short-circuit model in the embodiments of the present invention.

[0064] Figure 8 This shows some parameters in the thermal runaway side reaction model in this embodiment of the invention. Detailed Implementation

[0065] To facilitate understanding of the present invention, a more comprehensive description of the invention will be provided below, along with preferred embodiments. However, it should be understood that these embodiments are merely for more detailed explanation and should not be construed as limiting the invention in any way, i.e., not limiting the scope of protection of the invention.

[0066] like Figure 1 The diagram shows an example flow of the electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model of lithium-ion batteries, its construction method, and its application, which may include five steps.

[0067] Step 1 obtains the related physicochemical parameters such as geometry, electrochemistry, heat and mechanics of the sample battery by electrochemical test, thermal test and mechanical test, etc.

[0068] The battery sample is a soft package battery with a geometric size of 70 mm in length, 50 mm in width and 8 mm in height, and the battery electrochemical, thermal and mechanical parameters are shown in Table 1. Figure 7 The radius of the indenter is 75 mm, the Young's modulus is 10 GPa and the Poisson's ratio is 0.3.

[0069] The related parameters of step 1 of the present application are the parameters required for model construction. In the present application, the methods of electrochemical test, thermal test and mechanical test are not strictly limited, and the conventional methods in the art can be used; in addition, the above-mentioned electrochemical parameters, thermal parameters and mechanical parameters can also be obtained by existing technologies (such as theoretical data, literature records, etc.).

[0070] Step 2 uses finite element software to respectively establish an electrochemical model, a thermal model, a mechanical model, a short circuit model and a thermal runaway side reaction model, build a lithium ion battery electrochemistry-thermal-mechanical-short circuit-thermal runaway coupling model, and assign the related parameters obtained in step 1 to the established model.

[0071] In the embodiment of the present application, the electrochemical model adopts a one-dimensional model, the thermal model, the mechanical model and the short circuit model use a two-dimensional model, and the thermal runaway side reaction model adopts a lumped model, and COMSOL software is selected for model building.

[0072] In the present application, the electrochemical model is composed of an electrochemical equation group and an electrochemical heat generation equation group.

[0073] The electrochemical equation group includes a solid phase mass conservation equation, a liquid phase mass conservation equation, a solid phase charge conservation equation, a liquid phase charge conservation equation and an electrode reaction kinetics equation, which are used to solve the potential, Li+concentration and concentration distribution and change of side reaction substances in the solid / liquid phase in the battery.

[0074] In the embodiment of the present application, the subscript s of the parameter indicates the solid phase related parameter, and the subscript e indicates the liquid phase related parameter.

[0075] Solid phase charge conservation equation:

[0076]

[0077]

[0078] In the formula, is the effective electronic conductivity of the solid phase material (S-1) wherein σ sε is the effective electronic conductivity of the solid phase material s γ is the solid phase volume fraction, φ is the correction factor s S is the solid phase potential a r is the specific surface area of the solid phase material s ri is the particle radius of the solid phase material loc is the local current density.

[0079] The cell terminal voltage (V) is defined as the difference between the positive and negative solid phase potentials.

[0080] The liquid phase charge conservation equation is:

[0081]

[0082] where, φ is the effective electronic conductivity of the liquid phase material e R is the ideal gas constant, T is the temperature, F is the Faraday constant, f is the activity coefficient, c e ct is the liquid phase lithium ion concentration + St is the liquid phase ion transference number a r is the specific surface area of the solid phase material loc is the local current density.

[0083] The solid phase mass conservation equation is:

[0084]

[0085] where, ct is the solid phase lithium concentration, t is the time, r represents the radius of the spherical coordinate system established with the center of the spherical particle as the origin, D S is the effective diffusion coefficient of the solid phase lithium ion. s eff

[0086] The liquid phase mass conservation equation is:

[0087]

[0088] where, ε is the liquid phase volume fraction, ct is the liquid phase lithium ion concentration, t is the time, e is the effective diffusion coefficient of the liquid phase lithium ion. e a r is the specific surface area of the solid phase material loc is the local current density, F is the Faraday constant, t + St is the liquid phase ion transference number.

[0089] The electrode reaction kinetics equation is:

[0090] ​​​

[0091]

[0092] η=φ s -φ e -E eq

[0093] In the formula, i loc Let i be the local current density, i0 be the exchange current density, α be the charge transfer coefficient, η be the electrode overpotential, F be the Faraday constant, R be the ideal gas constant, T be the temperature, and k be the kJ / k ... eff c is the electrochemical effective reaction rate constant. s，max c is the maximum lithium-ion concentration in the solid phase. s，surf c represents the lithium ion concentration on the particle surface. e c is the concentration of lithium ions in the liquid phase. e，ref =1mol m -3 For the reference concentration of the electrolyte, φ s For solid-state potential, φ e E is the liquid phase potential. eq This is the open-circuit voltage.

[0094] The electrochemical heat generation equation set includes the Ohmic heat equation, the reversible entropy heat equation, and the electrochemical polarization heat equation, which are used to solve for the magnitude of each heat-generating component in the battery.

[0095] Ohm's heat equation:

[0096]

[0097] In the formula, Q ohm For Ohm heat, The effective electronic conductivity of solid-state materials, φ is the effective conductivity of the liquid phase material. s For solid-state potential, φ e For liquid phase potential, c e This represents the concentration of lithium ions in the liquid phase.

[0098] Reversible entropy heat equation:

[0099] Q rea =S a i loc T dU / dT

[0100] In the formula, Q rea As reversible entropy heat, S a i represents the specific surface area of ​​a solid material. loc Let dU / dT be the local current density, T be the temperature, and dU / dT represent the relationship between the electrode equilibrium potential and temperature.

[0101] Electrochemical polarization heat equation:

[0102] Qact = S a i loc (φ s - φ e -E eq )

[0103] where Q act is the electrochemical polarization heat, S a is the specific surface area of solid phase material, i loc is the local current density, φ s is the solid phase potential, φ e is the liquid phase potential, and E eq is the open circuit voltage.

[0104] The initial conditions and boundary conditions of the electrochemical model are as follows: the initial lithium concentration of the electrolyte is 1000 mol / m 3 , the initial lithium ion concentration of the negative electrode is 30272 mol / m 3 , the initial lithium ion concentration of the positive electrode is 14906 mol / m 3 , the negative electrode current collector is grounded, and the positive electrode current collector boundary adopts an external short-circuit resistance R short , which is obtained from the short-circuit model.

[0105] The heat model is composed of an energy conservation equation for simulating the temperature distribution and change in the battery; the energy conservation equation comprises a heat transfer term, a heat generation term, and a heat dissipation term; the heat transfer term is used to describe the heat transfer phenomenon caused by the temperature gradient in the battery; the heat generation term comprises heat sources of electrochemical polarization heat, reversible entropy heat, ohmic heat, internal short-circuit heat, and thermal runaway side reaction heat; each heat source is transmitted to the heat model in real time by the parameter transmission mode from other physical models; and the heat dissipation term is composed of convective heat transfer and radiative heat transfer, and is used to describe the heat exchange caused by the convective and radiative heat transfer between the battery and the environment.

[0106] In particular, the heat model in the application considers the heat caused by short-circuit and the heat caused by thermal runaway side reaction; preferably, the construction parameters of the heat model comprise short-circuit heat generation and thermal runaway side reaction heat generation.

[0107] The energy conservation equation is as follows:

[0108]

[0109] Q dis = h(T amb -T) + εl(T amb 4 -T 4 )

[0110] where ρ is the density, C P is the specific heat capacity, and K Tis the heat conduction, T is the temperature, t is the time, Q act is the electrochemical polarization heat, Q rea is the reversible entropy heat, Q ohm is the ohmic heat, Q short is the internal short-circuit heat, Q j is the heat generated by thermal runaway side reactions, Q dis is the heat dissipation term, h is the convective heat transfer coefficient between the battery and the environment, T amb is the ambient temperature, ε is the surface emissivity, λ is the Stefan-Boltzmann constant.

[0111] The initial conditions and boundary conditions of the thermal model are as follows: the initial temperature of the thermal model is 292.15 K, the heat exchange boundary between the battery and the pressure head and the environment is convective heat exchange and radiative heat exchange, and the convective coefficient is set to 20 W / (m 2 ·K).

[0112] The mechanical model is composed of geometric equations, physical equations, and balance differential equations, and is used to simulate the distribution and change of stress and strain of the battery under the action of mechanical load caused by extrusion of the pressure head.

[0113] Balance differential equation:

[0114] ∑F i = 0

[0115] In the formula, F i is the sum of stress components in each direction (subscript i respectively represents x direction, y direction and z direction in the rectangular coordinate system).

[0116] Geometric equation:

[0117]

[0118] In the formula, x and y respectively represent x direction and y direction in the rectangular coordinate system, u and v respectively represent displacement in x and y directions, ε x , ε y and γ xy respectively represent x direction strain, y direction strain and xy direction shear strain.

[0119] The physical equation is used to describe the constitutive relation between stress and strain of the material, in particular, the material includes two stages in the compression process: the stage of gradually increasing effective modulus and the linear stage after the material is compacted, and the constitutive relation of the model material in the compression process is as follows:

[0120]

[0121] In the formula, σ i and ε irespectively, where subscript i represents the x direction and y direction in the rectangular coordinate system, E is the compression modulus, E max is the maximum compression modulus, and ε P is the strain after the material is fully compacted (its value is approximately equal to the porosity), and β is a mechanical fitting parameter used to represent the increasing gradient of the relative strain in the hardening stage.

[0122] The initial conditions and boundary conditions of the mechanical model are as follows: the pressure head in the mechanical model is extruded in the thickness direction of the battery at a speed of 5 mm / s (the movement is stopped when the displacement amount of the pressure head reaches 30% of the thickness of the battery), and the bottom surface of the battery is set as a fixed displacement boundary condition.

[0123] In the embodiment of the application, the data of the elastic-plastic model used for the positive / negative electrode current collectors come from a literature (Zhang C, Xu J, Cao L, et al. Constitutive behavior and progressive mechanical failure of electrodes in lithium-ion batteries [J]. Journal of Power Sources, 2017, 357 (Jul. 31): 126-137.).

[0124] In the embodiment of the application, the mechanical properties of other components inside the battery are better than those of the separator, and the separator will reach the rupture condition first during the extrusion of the battery, thereby causing the short circuit of the electrode sheet. In particular, in the application, whether the value of the volume strain of the separator is less than -2.11 is determined as the determination condition for the rupture of the separator, and the volume strain is approximately equal to the sum of three principal logarithmic strains (the principal logarithmic strain has a clear conversion relationship with the strain described in the application, so it is not listed again), and the selection of the determination standard and the strain-related conversion are described in the work of Yuan et al. (Yuan C, Wang L, Yin S, et al. Generalized separator failure criteria for internal short circuit of lithium-ion battery [J]. Journal of Power Sources, 2020, 467: 228360.).

[0125] The short circuit model is constructed based on Ohm's law and is used to realize the prediction of the size, position and heat generation of the internal short circuit; the electrical conductivity of the battery components is defined as a function relationship related to the strain / stress, and the size of the electrical conductivity of each battery component changes in real time with the size of the stress / strain in the mechanical model; the strain / stress is a numerical value that is transmitted from the mechanical model to the short circuit model in a parameter transmission manner; and the heat generation of the internal short circuit follows Ohm's law.

[0126] The Ohm's law equation is as follows:

[0127]

[0128] In the formula, I is the current density of the battery assembly, κ is the conductivity of the battery assembly (a function of stress / strain), and Ψ is the electric field intensity in the battery assembly.

[0129] The internal short-circuit heat generation equation is as follows:

[0130]

[0131] In the formula, Q short is the internal short-circuit heat generation.

[0132] The boundary condition of the short-circuit model is that the voltage obtained by the electrochemical model is applied to the positive tab position of the short-circuit model, and the negative tab of the short-circuit model adopts a ground boundary condition.

[0133] The short-circuit current I short can be obtained by integrating the current density flowing through the positive tab position, that is, the short-circuit internal resistance R short is the ratio of the voltage at the positive tab (i.e., the battery terminal voltage V) to the short-circuit current I short .

[0134] In the parameters involved in the above equations, the superscript eff represents "effective" and represents the real-time reaction state parameter, and the corresponding parameter without the superscript is obtained by theory or sample measurement, and the main parameter results are shown in Table 1 Figure 7 .

[0135] The thermal runaway side reaction model includes a mass conservation equation, an energy conservation equation, and an Arrhenius equation, which are used to solve the concentration and heat generation of each side reaction substance; in the embodiment of the present application, the thermal runaway side reactions include SEI decomposition reaction (SEI), lithium intercalation graphite and electrolyte reaction (An-E), lithium intercalation graphite and binder reaction (An-B), positive and negative reactions (Ca-An), positive and binder reactions (Ca-B), and positive decomposition reactions (Ca). In particular, the Arrhenius equation, the mass conservation equation, and the energy conservation equation in the thermal runaway side reaction model in the embodiment of the present application are as follows:

[0136]

[0137] c j = 1-∫κ j dt

[0138] Q j = m j · ΔH j · κj

[0139] In the formula, κ j For the reaction rate, A j E is a pre-exponential factor. a，j Let R be the activation energy, T be the ideal gas constant, t be the temperature, and c be the time. j n represents the normalized concentration of the side reactants. j m is the reaction order. j ΔH represents the mass of the reactants. j To generate heat in the reaction, Q j Let j represent the heat generated by the thermal runaway side reaction, and j be the corresponding thermal runaway side reaction. The parameters of the thermal runaway side reaction are as follows: Figure 8 As shown, this is based on the paper by Ren et al. (Ren D, Xiang L, Feng X, et al. Model-based thermal runaway prediction of lithium-ion batteries from kinetics analysis of cell components[J]. Applied Energy, 2018, 228: 633-644.).

[0140] Based on the above governing equations, an electrochemical model, thermal model, mechanical model, short-circuit model, and thermal runaway side reaction model were constructed and coupled to establish a coupled electrochemical-thermal-mechanical-short-circuit-thermal runaway model for lithium-ion batteries.

[0141] The mechanical model was used to calculate the location-dependent stress (σ) of each component inside the battery. i ) and strain (ε i The changes in stress / strain are transmitted to the short-circuit model in real time; the short-circuit model calculates the short-circuit location and short-circuit resistance (R) within the battery based on the conductivity (κ) of the position-dependent stress / strain changes in the real-time response mechanics model. short ) and internal short-circuit heat generation (Q) short ); short-circuit resistor (R) short The data is transmitted in real time to the electrochemical model to solve for the battery terminal voltage (V) and electrochemical heat generation (Q). ohm Q rea and Q act Changes in battery terminal voltage (V) are transmitted in real time to the short-circuit model; the heat generation (Q) in the electrochemical model... ohm Q rea and Q act ), heat generation in short-circuit model (Q) short ) and heat generated by side reactions (Q) jwhere j is SEI, An-E, An-B, Ca-An, Ca-B and Ca, respectively, is applied on the thermal model as heat source, and the temperature (T) change inside the battery is solved.

[0142] In particular, the temperature (T) solved by the thermal model influences some electrochemical reaction kinetics parameters in the electrochemical model through the Arrhenius relationship, including the solid-state lithium ion effective diffusion coefficient D s eff and the electrochemical effective reaction rate constant k eff , which are specifically formulated as follows:

[0143]

[0144]

[0145] where D s and k represent the solid-state lithium ion effective diffusion coefficient value and the electrochemical effective reaction rate constant value at the reference temperature T ref , respectively, E a，D and E a，k are the activation energies corresponding to the D s and k parameters, respectively, and R is the ideal gas constant.

[0146] In addition, the temperature (T) in the thermal model is also transmitted in real time to the thermal runaway side reaction model, which determines whether the temperature reaches the conditions for each side reaction to occur and solves the thermal runaway side reaction heat generation (Q j ).

[0147] The coupling between the electrochemical model, the thermal model, the mechanical model, the short circuit model and the thermal runaway side reaction model is achieved in the above manner, and the coupling relationship between the physical fields is detailed in Figure 2 .

[0148] Step 3: According to the electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model constructed using COMSOL in Step 2, set the boundary conditions and initial conditions of each physical model according to the actual simulation requirements of the lithium ion battery, divide the grid and perform solving calculation as shown in Figure 3 , which shows the two-dimensional model and its model grid division, the model includes: pressure head, and battery composed of positive electrode current collector, positive electrode coating, separator, negative electrode coating, negative electrode current collector. The pressure head region adopts triangular grid, which contains 284 domain units and 130 boundary units. The quadrilateral grid is used in the battery geometric domain, which contains 3520 domain units and 3724 boundary units, and the overall average unit mass is 1. Due to the dense grid, it is displayed as a black solid in the figure.

[0149] Step 4: The sample is tested to obtain battery electrical, thermal, and force test data, and the coupling model is calibrated and verified based on the measured data to obtain a lithium-ion battery extrusion working condition prediction model.

[0150] Figure 4 The experimental results of the voltage and temperature of the sample battery during the safety extrusion test are compared with the predicted results of the model. From the experimental data of the battery voltage and temperature, it can be seen that there is no short circuit in the battery during the experiment. The predicted results of the voltage and temperature of the model are very close to the experimental results, which fully verifies the accuracy and reliability of the simulation method and prediction model in the present application.

[0151] After increasing the battery extrusion deformation, the simulation results and experimental results of the voltage and temperature of the battery are as shown in Figure 5 The results show that the maximum internal temperature of the battery can reach 976℃, and the maximum surface temperature of the battery is 860.6℃. The temperature sensor is arranged on the surface of the battery in the experiment, and the internal temperature of the battery will be much higher than this value. This result fully embodies that the model and simulation method in the present application can simulate and predict the thermal runaway phenomenon caused by extrusion.

[0152] Step 5: Based on the calibration results in step 4, adjust the battery size parameters and capacity, and simulate the safety performance of large-capacity batteries under the same process conditions during extrusion, and predict the safety limit capacity. This is specifically reflected in increasing the length and width of the battery, and the increase in the thickness direction is realized by increasing the number of positive / negative electrode sheets. After enlarging the length and width of the battery to 1100mm and 250mm respectively, the battery large-capacity model is simulated under the extrusion working condition after the related operations in step 3 are completed.

[0153] As shown in Figure 6 , the volume strain distribution of the separator and the short circuit area (enlarged view) of the large-capacity battery after extrusion are shown. As can be seen from the figure, the volume strain of the separator near the pressure head is large and short circuit occurs first. With the process of the pressure head being pressed down, the number of layers of the separator that short circuits gradually increases, and the short circuit area gradually increases.

[0154] In the exploration of the safety limit capacity, if the separator breaks under the existing number of sheets, the number of positive / negative electrode stacks is correspondingly reduced, and if the separator does not break under the existing number of sheets, the number of positive / negative electrode sheets is correspondingly increased. After multiple optimization solutions, when the number of positive electrode sheets of the large-capacity battery under this size is less than 140, the battery will not short circuit during the safety extrusion test. The corresponding battery safety limit thickness and capacity are 23.66mm and 1437.4Ah, respectively.

[0155] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for constructing a lithium-ion battery electrochemistry-thermal-mechanical-short circuit-thermal runaway coupling model, comprising the following steps: S1. performing electrochemical, thermal and mechanical tests on a sample to obtain relevant physicochemical parameters, including geometric parameters, electrochemical parameters, thermal parameters and mechanical parameters; S2. using finite element software to respectively establish an electrochemical model, a thermal model, a mechanical model, a short circuit model and a thermal runaway side reaction model, to build a lithium-ion battery electrochemistry-thermal-mechanical-short circuit-thermal runaway coupling model, and to assign the relevant parameters obtained in step S1 to the established model; S3. setting boundary conditions and initial conditions of each physical model according to actual simulation requirements of the lithium-ion battery, and dividing the grid; S4. testing the sample to obtain battery electrical, thermal and mechanical test data, calibrating and verifying the accuracy of the coupling model based on the measured data, and obtaining a lithium-ion battery electrochemistry-thermal-mechanical-short circuit-thermal runaway coupling model; wherein the coupling mode of the coupling model in step S2 is as follows: a) the input of the mechanical model is the initial conditions and boundary conditions including the influence of mechanical action, and the output of the mechanical model is the stress / strain change of the battery over time and space; b) the input of the short circuit model includes the stress / strain change over time and space output by the mechanical model and the battery voltage change obtained by the electrochemical model, and the output of the short circuit model includes the size of the battery short circuit resistance, the short circuit position and the change of the internal short circuit heat; c) the input of the electrochemical model includes the battery short circuit resistance size obtained by the short circuit model and the temperature change obtained by the thermal model, and the output of the electrochemical model includes the change of the battery voltage and the electrochemical heat generation of the battery, including ohmic heat, polarization heat and reversible entropy heat; d) the input of the thermal model includes the electrochemical heat generation obtained by the electrochemical model, the internal short circuit heat obtained by the short circuit model and the side reaction heat generation obtained by the thermal runaway side reaction model, and the output of the thermal model is the change of the battery temperature over time and space; e) the input of the thermal runaway side reaction model is the change of the battery temperature obtained by the thermal model, and the output of the thermal runaway side reaction model is the heat generation of each side reaction.

2. The method according to claim 1, wherein, The sample in step S1 includes a battery sample, an electrode sample and a material sample; the material sample includes a positive electrode material, a negative electrode material and a separator material; wherein the positive electrode material is selected from at least one of nickel-cobalt-manganese ternary material, lithium iron phosphate, lithium cobaltate, lithium nickelate, lithium manganate and lithium manganese phosphate; the negative electrode material is selected from at least one of artificial graphite, natural graphite, mesocarbon microbeads, silicon, silicon monoxide and lithium titanate; the electrode sample is a positive / negative electrode battery pole piece prepared by using raw materials including the positive / negative electrode material to prepare a positive / negative electrode coating, and together with the positive / negative electrode current collector; and the battery sample is a separator prepared by using raw materials including the separator material, and a battery prepared together with the electrode sample.

3. The method according to claim 1, wherein, The geometric parameters in step S1 include one or more of the length and width of the battery tab, the length and width of the battery current collector, the position parameters of the battery tab, the thickness of the battery tab, the number of battery tabs, and the length, width and height of the battery; The electrochemical-related parameters in step S1 include one or more of the solid-phase material electronic conductivity, the initial lithium ion concentration, the solid-phase volume fraction, the liquid-phase volume fraction, the solid-phase material particle radius, the liquid-phase material effective conductivity, the initial lithium concentration of the electrolyte, the electrochemical reaction rate constant, the charge transfer coefficient, the solid-phase lithium ion diffusion coefficient and the liquid-phase lithium ion diffusion coefficient of the test sample; The thermal-related parameters in step S1 include one or more of the mass, density, specific heat capacity, thermal conductivity and thermal decomposition temperature of the test sample; The mechanical-related parameters in step S1 include one or more of the compression modulus and Poisson's ratio of the lithium ion battery internal material.

4. The method of claim 1, wherein the method further comprises: The initial conditions and boundary conditions of the mechanical model input include the pressing speed of the pressure head, the contact interface between the pressure head and the battery is set as a contact boundary condition, and the bottom surface of the battery is set as a fixed displacement boundary condition; The stress / strain changes of the short circuit model input over time and space are realized by linear projection or linear stretching; the change of the battery voltage input by the short circuit model is applied at the positive tab of the battery model, and the ground boundary condition is used at the negative tab of the battery; The short circuit current of the corresponding short circuit model can be obtained under the battery voltage boundary condition of the short circuit model, and the short circuit resistance of the short circuit model is obtained by the ratio of the battery voltage to the short circuit current; The battery short circuit resistance size input by the electrochemical model is applied by the boundary condition of the external short circuit resistance, and the temperature change input by the electrochemical model is realized by the setting method of linear projection or linear stretching.

5. The method of claim 1, wherein the method further comprises: The heat generated by each part of the electrochemical model, the internal short circuit heat and the side reaction heat input by the thermal model are realized by the setting method of linear projection or linear stretching.

6. The method of claim 1, wherein the method further comprises: The change of the temperature input by the thermal runaway side reaction model is realized by the setting method of linear projection or linear stretching.

7. The method according to claim 1, wherein, The temperature obtained by the thermal model is transmitted to the electrochemical model in real time to affect the partial electrochemical reaction kinetics parameters in the electrochemical model through the Arrhenius relationship.

8. The method of claim 1, wherein the method further comprises: The grid division in step S3 uses one or more of triangular grid, quadrilateral grid, tetrahedral grid, hexahedral grid, pyramid grid, wedge grid and mixed grid composed of the above grid types.

9. The method of claim 1, wherein the method further comprises: The test working conditions of the sample in step S4 include: (1) Constant current and / or constant current constant voltage charging or discharging under different rates at multiple temperature conditions, wherein the temperature and rate conditions should be selected as much as possible to cover the normal use conditions of the battery; (2) During the charging / discharging process, the battery surface and internal temperature are collected by using external or internal thermocouples, optical fiber sensors and infrared imagers; (3) According to the relevant extrusion test conditions, the battery is subjected to extrusion safety test, and the internal and / or external temperature and voltage changes over time are collected during the test.

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