Application of lithium battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model

Abstract

The application provides a lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model, which is applied to: (1) design of a lithium ion battery structure and prediction of performance of the lithium ion battery in a crushing working condition; and (2) simulation and prediction of performance of a large-capacity battery in the crushing working condition under the same process condition. The model can simulate dynamic responses of external and internal characteristics of the battery in the crushing working condition by adjusting size parameters and capacity of the battery, and significantly improves the prediction ability of the battery state and the development efficiency and safety of the battery.

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 highlights the battery safety problem, which seriously endangers the safety of people's life and property.

[0004] Due to the wide application of lithium ion batteries, in some actual use scenarios, the batteries are easily impacted and influenced by mechanical load, resulting in stress and deformation of the batteries. 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 and catch fire in the extrusion experiment.

[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 dynamic response of temperature and voltage of the battery in the extrusion working condition, 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 trigger factors. In addition to the above-mentioned 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, and these influences 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 battery external and internal characteristics under extrusion conditions, solve the distribution and change of battery voltage, temperature, stress and material during extrusion, estimate and predict the occurrence position, size and thermal runaway temperature evolution of internal short circuit, save experimental cost, guide battery optimization design, improve development efficiency, and significantly improve the prediction ability of 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 the model in real time and accurately, and the simulation accuracy, sensitivity and prediction ability 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 side reaction material concentration distribution and change in the solid / liquid phase in the battery; 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 internal materials of the lithium ion battery.

[0031] Further, the coupling models in step 2 include separately establishing an electrochemical model, a thermal model, a mechanical model, a short circuit model, and a thermal runaway side reaction model.

[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 coupling models in step 2 include a mechanical model for solving the changes in stress / strain inside the battery, a short circuit model for judging the size, position and heat production of the short circuit internal resistance according to the calculation results of the mechanical model, an electrochemical model for solving the potential, material and electrochemical heat production distribution under the short circuit condition in the battery, a thermal model for calculating the temperature distribution of the battery, and a thermal runaway side reaction model for solving the concentration and heat production of the thermal runaway side reaction material.

[0034] Further, the coupling manner of the coupling models 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 changes in stress / strain inside the battery over time and space;

[0036] b) The input of the short circuit model includes the changes in stress / strain over time and space output by the mechanical model and the changes in battery voltage obtained by the electrochemical model, and the output of the short circuit model includes the size of the battery short circuit internal resistance, the short circuit position and the changes in 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 mesh division 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, step 4 tests the sample to obtain battery electrical, thermal, and force test data, and the test conditions include:

[0049] 1. Perform 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. Perform extrusion safety testing on the battery according to relevant extrusion test conditions, and collect the changes of internal and / or external temperature and voltage over time during the test.

[0052] Further, the method of adjusting the battery size parameters and 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, the increase of the size of the battery in the plane direction includes the increase of the length and width of the battery electrode sheet, and the increase of the number of laminated sheets of the battery electrode sheet is achieved by alternating the stacking of positive and negative electrode sheets and separating them with a separator. By increasing the battery capacity under the same process conditions without changing the thickness of the single electrode sheet, the extrusion safety simulation of the battery is further 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 extrusion conditions is also provided. Since the coupling model is successfully established by the above-mentioned steps S1-S4, the coupling model contains capacity-related parameters such as battery size and number of laminated sheets, so the change of battery capacity can be realized by adjusting these parameters, and the change of the electrical performance and safety performance of the lithium ion battery under extrusion conditions can be predicted by using the coupling model. The simulation results of the coupling model can guide the adjustment of the battery size and capacity parameters in the opposite direction, so as to realize the structure design of the lithium ion battery, and therefore the application can be realized by adjusting the battery size and the number of laminated sheets and other capacity-related parameters in step S5.

[0054] In the present application, the application of the above-mentioned lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model in the performance simulation and prediction of large-capacity batteries under extrusion working conditions under the same process conditions is also provided. The coupling model successfully established through the above-mentioned steps S1-S4 of the present application can have relatively accurate simulation and prediction results, so that the same accurate simulation and prediction results can also be obtained for large-capacity batteries under extrusion working conditions under the same process conditions, and the application can be realized through step S5.

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

[0056] In the present application, the model and method can be used for the estimation, prediction and analysis of the results of battery safety extrusion tests, have high reliability and low cost, can simulate the deformation process of the battery under extrusion working conditions, solve the changes of voltage, temperature and stress with time and space during the extrusion process, and predict the size, position and thermal runaway temperature of the internal short circuit of the battery. At the same time, the model can also be used for the simulation of the extrusion safety performance of large-capacity batteries under the same process conditions, and the prediction of the extrusion limit safety capacity thereof. The model and method of the present application break through the limitation of general models which are only applicable to the simulation and prediction under conventional electrical or thermal trigger factors, can adapt to and predict the thermal runaway phenomenon caused by extreme or sudden factors such as mechanical and short circuit, and accordingly provide the battery optimization design results and methods, and have important application value. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 It is a total flowchart of the present application example.

[0058] Figure 2 It is a multi-physical field coupling principle diagram in the present application example.

[0059] Figure 3 It is a two-dimensional physical model and mesh division diagram in the present application example.

[0060] Figure 4 It is a comparison diagram of the model prediction results and experimental results in the present application example.

[0061] Figure 5 It is a comparison diagram of the model prediction results and experimental results in the present application example.

[0062] Figure 6 It is a volume strain distribution and short circuit area diagram of the separator in the present application example.

[0063] Figure 7 It is a part of parameter condition in the electrochemical model, thermal model, mechanical model and short circuit model in the present application example.

[0064] Figure 8 It is a part of parameter condition in the thermal runaway side reaction model in the present application example. Detailed Implementation

[0065] To facilitate understanding of the present invention, a more comprehensive description 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 intended to limit 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 involves conducting electrochemical, thermal, and mechanical tests on the sample battery to obtain its geometric, electrochemical, thermal, and mechanical parameters.

[0068] The battery sample is a pouch cell with dimensions of 70 mm in length, 50 mm in width, and 8 mm in height. The battery's electrochemical, thermal, and mechanical parameters are shown below. Figure 7 As shown. 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 parameters in step 1 of this invention are those required for model construction. In this invention, the methods for electrochemical, thermal, and mechanical testing are not strictly limited; conventional methods in the field can be used. Furthermore, the aforementioned electrochemical, thermal, and mechanical parameters can also be obtained through existing technologies (such as theoretical data, literature records, etc.).

[0070] Step 2: Use finite element software to establish electrochemical, thermal, mechanical, short-circuit, and thermal runaway side reaction models respectively, build a lithium-ion battery electrochemical-thermal-mechanical-short-circuit-thermal runaway coupled model, and assign the relevant parameters obtained in Step 1 to the established model.

[0071] In the embodiments of the present invention, the electrochemical model adopts a one-dimensional model, the thermal model, the mechanical model and the short-circuit model adopt a two-dimensional model, the thermal runaway side reaction model adopts a lumped model, and COMSOL software is used for model building.

[0072] In this invention, the electrochemical model consists of a set of electrochemical equations and a set of electrochemical heat generation equations.

[0073] The electrochemical equation set includes solid-phase mass conservation equations, liquid-phase mass conservation equations, solid-phase charge conservation equations, liquid-phase charge conservation equations, and electrode reaction kinetic equations, used to solve for the potential in the solid / liquid phases within the battery, and the Li... + Concentration and the concentration distribution and changes of by-reactants.

[0074] In the embodiments of the present application, subscript s represents a solid phase related parameter and subscript e represents a liquid phase related parameter, unless otherwise specified.

[0075] Solid phase charge conservation equation:

[0076]

[0077]

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

[0079] The battery terminal voltage (V) is defined as the difference between the solid phase potential of the positive electrode and the solid phase potential of the negative electrode according to the solid phase charge conservation equation.

[0080] Liquid phase charge conservation equation:

[0081]

[0082] wherein, is the effective conductivity of the liquid phase material, φ e is the liquid phase potential, R is the ideal gas constant, T is the temperature, F is the Faraday constant, f is the activity coefficient, c e is the liquid phase lithium ion concentration, t + is the liquid phase ion transfer number, S a is the specific surface area of the solid phase material, i loc is the local current density.

[0083] Solid phase mass conservation equation:

[0084]

[0085] wherein, c s 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 eff is the effective diffusion coefficient of the solid phase lithium ion.

[0086] Liquid phase mass conservation equation:

[0087]

[0088] wherein, εe is the liquid phase volume fraction, c e is the liquid phase lithium ion concentration, t is time, is the liquid phase lithium ion effective diffusion coefficient, S a is the solid phase material specific surface area, i loc is the local current density, F is the Faraday constant, t + is the liquid phase ion transfer number.

[0089] Electrode reaction kinetics equation:

[0090]

[0091]

[0092] η = φ s - φ e - E eq

[0093] where i loc is the local current density, i0 is the exchange current density, a is the charge transfer coefficient, η is the electrode overpotential, F is the Faraday constant, R is the ideal gas constant, T is the temperature, k eff is the electrochemically effective reaction rate constant, c s，max is the solid phase maximum lithium ion concentration, c s，surf is the particle surface lithium ion concentration, c e is the liquid phase lithium ion concentration, c e，ref = 1 mol m -3 is the electrolyte reference concentration, φ s is the solid phase potential, φ e is the liquid phase potential, E eq 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 the size of each heat generation component of the battery.

[0095] Ohmic heat equation:

[0096]

[0097] where Q ohm is the ohmic heat, is the solid phase material effective electronic conductivity, is the liquid phase material effective conductivity, φ s is the solid phase potential, φ e is the liquid phase potential, c e is the liquid phase lithium ion concentration.

[0098] Reversible entropy heat equation:

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

[0100] wherein Q rea is the reversible entropy heat, S a is the specific surface area of solid phase material, i loc is the local current density, T is the temperature, and dU / dT represents the relationship between the equilibrium potential of the electrode and the temperature.

[0101] The electrochemical polarization heat equation is:

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

[0103] wherein 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 inside the battery; the heat generation term comprises the electrochemical polarization heat, the reversible entropy heat, the ohmic heat, the internal short-circuit heat, and the 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 the short-circuit heat generation and the thermal runaway side reaction heat generation.

[0107] The energy conservation equation is as follows:

[0108]

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

[0110] In the formula, ρ is density, C P is specific heat capacity, K T is thermal conductivity, T is temperature, t is time, Q act is electrochemical polarization heat, Q rea is reversible entropy heat, Q ohm is ohmic heat, Q short is internal short-circuit heat, Q j is heat runaway side reaction heat, Q dis is heat dissipation term, H is heat transfer coefficient of the battery and the environment, T amb is environment temperature, ε is surface emissivity, and λ is the Stefan-Boltzmann constant.

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

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

[0113] The equilibrium differential equation is as follows:

[0114] ∑F i = 0

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

[0116] The geometric equation is as follows:

[0117]

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

[0119] The physical equation is used for describing the constitutive relation of material stress and strain, 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 compression process model material is as follows:

[0120]

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

[0122] The initial condition and boundary condition 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 5mm / 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 collector is from the 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 present 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 battery extrusion process, thereby causing the short circuit of the pole piece. In particular, in the present 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 please refer to 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, which is used to realize the prediction of the size, position and heat production of the internal short circuit; the electrical conductivity of the battery component is defined as a function relationship with 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; the internal short circuit heat production 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 component, κ is the electrical conductivity of the battery component (which is a function of stress / strain), and Ψ is the electric field intensity in the battery component.

[0129] The internal short circuit heat production equation is:

[0130]

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

[0132] The boundary condition of the short circuit model is that the voltage obtained from the electrochemical model is applied to the positive tab position of the short circuit model, and the negative tab in the short circuit model adopts the 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 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] Among the parameters involved in the above equations, the superscript eff represents the "effective" representing real-time reaction state parameters, and the corresponding parameters without the superscript are obtained by theory or sample measurement, and the main parameter results are shown in 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 of each side reaction substance and the heat production; in the embodiment of the present application, the thermal runaway side reaction includes SEI decomposition reaction (SEI), lithium-embedded graphite and electrolyte reaction (An-E), lithium-embedded graphite and binder reaction (An-B), positive and negative electrode reaction (Ca-An), positive and binder reaction (Ca-B), and positive electrode decomposition reaction (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 κ j

[0139] In the formula, κ j is the reaction rate, A j is the pre-exponential factor, R a，j is the activation energy, R is the ideal gas constant, T is the temperature, t is the time, c j is the normalized concentration of the side reaction substance, n j is the reaction order, m j is the reactant mass, ΔH j is the reaction heat, Q j is the heat production of the thermal runaway side reaction, and j is the corresponding thermal runaway side reaction. The thermal runaway side reaction parameters are shown in Figure 8 , which are referred to 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 control equations, the electrochemical model, the thermal model, the mechanical model, the short circuit model and the thermal runaway side reaction model are built and coupled to establish the lithium ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model.

[0141] The position-dependent stress (σ i ) and strain (ε i ) changes of each component inside the battery are calculated using the mechanical model, and are transmitted to the short circuit model in real time; the short circuit model calculates the short circuit position, short circuit resistance (R short ) and internal short circuit heat generation (Q short ) in the battery according to the real-time conductivity (κ) of the position-dependent stress / strain changes in the reaction mechanics model; the short circuit resistance (Q short ) is transmitted to the electrochemical model in real time to solve the changes of the battery terminal voltage (V) and the electrochemical heat generation (Q ohm , Q rea and Q act ), and the battery terminal voltage (V) is transmitted to the short circuit model in real time; the heat generation (Q ohm , Q rea and Q act ) of the electrochemical model, the heat generation (Q short ) of the short circuit model and the heat generation (Q j ) of the side reaction, where j is SEI, An-E, An-B, Ca-An, Ca-B and Ca respectively, are applied as heat sources to the thermal model to solve the temperature (T) changes inside the battery.

[0142] In particular, the temperature (T) solved by the thermal model affects part of the electrochemical reaction kinetics parameters in the electrochemical model through the Arrhenius relationship, including the solid-phase lithium ion effective diffusion coefficient D s eff and the electrochemical effective reaction rate constant k eff , and the specific formula is as follows:

[0143]

[0144]

[0145] In the formula, D s and k represent the solid-phase lithium ion effective diffusion coefficient value and the electrochemical effective reaction rate constant value at the reference temperature T ref , E a，D and E a，k are the activation energies of 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 to the thermal runaway side reaction model in real time, and the thermal runaway side reaction model determines whether the temperature reaches the occurrence condition of each side reaction and solves the heat generation (Q j ) of the thermal runaway side reaction.

[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 realized in the above manner, and the coupling relationship between the physical fields is shown in detail in Figure 2 .

[0148] Step 3: According to the electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model constructed using COMSOL in step 2, the boundary conditions and initial conditions of each physical model are set according to the actual simulation requirements of the lithium ion battery, the grid is divided and the calculation is solved as shown in Figure 3 , which shows the two-dimensional model and the model grid division, and the model includes: a pressure head, and a battery composed of a positive electrode current collector, a positive electrode coating, a separator, a negative electrode coating and a negative electrode current collector. The pressure head region adopts a 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 shown as a black solid in the figure.

[0149] Step 4: Test the sample to obtain battery electrical, thermal and force test data, calibrate and verify the accuracy of the coupling model based on the measured data, and obtain a lithium ion battery extrusion condition prediction model.

[0150] Figure 4 The experimental results of the voltage and temperature of the sample battery under the safety extrusion test are compared with the model prediction results. From the experimental data of the battery voltage and temperature in the figure, it can be seen that there is no short circuit phenomenon in the battery during the experiment, and the prediction results of the model are very close to the experimental results. The results fully demonstrate 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 , and the battery occurs thermal runaway during extrusion. As can be seen from the figure, the increase of the extrusion deformation causes the battery to occur internal short circuit, which leads to the rapid decrease of the voltage to about 0V. At the same time, the internal short circuit is accompanied by a large amount of heat generation, which leads to the thermal runaway phenomenon of the battery. The results show that the highest temperature in the battery can reach 976℃, and the highest temperature of the actual measured battery surface is 860.6℃. In the experiment, the temperature sensor is arranged on the surface of the battery, 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 realize the simulation and prediction of the thermal runaway phenomenon caused by extrusion.

[0152] Step 5 Based on the calibration results in step 4, the battery size parameters and the capacity are adjusted, and the safety performance simulation of large-capacity battery under extrusion working condition under the same process condition is carried out, and the safety limit capacity is predicted. It is embodied in increasing the length and width of the battery, and the increase of the thickness direction size is realized by increasing the number of positive / negative electrode sheet laminations. After the length and width of the battery are enlarged to 1100mm and 250mm respectively, the extrusion working condition simulation is carried out on the large-capacity battery model after the related operation in step 3 is completed.

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

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

[0155] The above only describes the preferred embodiments of the present application and is not used 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. An application of a lithium-ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model in one of the following directions: (1) in designing lithium-ion battery structures and predicting the performance of lithium-ion batteries under extrusion conditions; (2) in conducting simulation and prediction of the extrusion performance of large-capacity batteries under the same process conditions. The coupled model is formed by coupling an electrochemical model, a thermal model, a mechanical model, a short-circuit model, and a thermal runaway side reaction model. The coupling and correlation methods between models include: the stress / strain parameters of the mechanical model are correlated with the conductivity of the short-circuit model; the short-circuit internal resistance of the short-circuit model is correlated with the boundary conditions of the electrochemical model; the battery voltage of the electrochemical model is correlated with the boundary conditions of the short-circuit model; the temperature of the thermal model is correlated with the temperature of the electrochemical model and the thermal runaway side reaction model, respectively; and the internal short-circuit heat generation of the short-circuit model, the electrochemical polarization heat, reversible entropy heat and ohmic heat of the electrochemical model, and the side reaction heat generation of the thermal runaway model are respectively correlated with the corresponding heat generation terms in the thermal model. The specific application method includes the following steps: S1. Perform electrochemical, thermal, and mechanical tests on the sample to obtain relevant physicochemical parameters, including geometric parameters, electrochemical parameters, thermal parameters, and mechanical parameters. S2. Use finite element software to establish electrochemical model, thermal model, mechanical model, short circuit model and thermal runaway side reaction model respectively, build a lithium-ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupled model, and assign the relevant parameters obtained in step S1 to the established model; S3. Based on the actual simulation requirements of lithium-ion batteries, set the boundary conditions and initial conditions for each physical model, and divide the mesh; S4. Test the sample to obtain battery electrical, thermal and force test data, and calibrate and verify the accuracy of the coupling model based on the measured data to obtain the lithium-ion battery electrochemical-thermal-mechanical-short circuit-thermal runaway coupling model. S5. Adjust the battery size parameters and capacity, and conduct simulation and / or prediction of the battery extrusion condition under the same process conditions. The specific simulation and / or prediction contents include: (1) predicting the performance of lithium-ion batteries under extrusion condition, (2) predicting the safe battery structure of lithium-ion batteries under extrusion condition, and (3) predicting the safe limit capacity of lithium-ion batteries.

2. The application according to claim 1, characterized in that, The electrochemical model is used to solve for the potential in the solid / liquid phase of the battery and the Li. + The concentration and concentration distribution and changes of by-reactants, as well as the magnitude of each heat-generating component in the battery; the thermal model is used to simulate the temperature distribution and changes within the battery; the mechanical model is used to simulate the distribution and changes of stress and strain in the battery under mechanical loads; the short-circuit model is used to predict the magnitude, location, and heat generation of internal short circuits. The thermal runaway side reaction model is used to solve for the concentration and heat generation of each side reaction substance.

3. The application according to claim 1, characterized in that, The electrochemical model consists of a set of electrochemical equations and a set of electrochemical heat generation equations; the set of electrochemical equations includes one or more of the following: solid phase mass conservation equation, liquid phase mass conservation equation, solid phase charge conservation equation, liquid phase charge conservation equation, and electrode reaction kinetic equation; the set of electrochemical heat generation equations includes one or more of the following: Ohmic heat equation, reversible entropic heat equation, and electrochemical polarization heat equation. The thermal model is composed of energy conservation equations, which include heat transfer, heat generation, and heat dissipation terms. The heat transfer term describes the heat transfer phenomenon caused by the internal temperature gradient of the battery. The heat generation term includes heat sources such as electrochemical polarization heat, reversible entropy heat, ohmic heat, internal short-circuit heat, and heat from thermal runaway side reactions. The heat sources of the heat generation term are all transferred to the thermal model from other physical models in real time through parameter transfer. The heat dissipation term consists of convective heat transfer and radiative heat transfer, and is used to describe the heat exchange caused by convective and radiative heat transfer between the battery and the environment. The mechanical model consists of geometric equations, physical equations, and equilibrium differentials; The short-circuit model is constructed based on Ohm's law; the conductivity of the battery module is defined as a function related to strain / stress; the strain / stress is a value that is transferred from the mechanical model to the short-circuit model in real time according to the parameter transfer method; the short-circuit model follows Ohm's law. The thermal runaway side reaction model includes one or more of the following: the mass conservation equation, the energy conservation equation, and the Arrhenius equation; the side reactions include one or more of the following: SEI decomposition reaction, reaction of lithium-intercalated graphite with electrolyte, reaction of lithium-intercalated graphite with binder, positive electrode and negative electrode reaction, positive electrode and binder reaction, and positive electrode decomposition reaction.

4. The application according to claim 1, characterized in that, The samples mentioned in step S1 include 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 materials, lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, and lithium manganese phosphate; the negative electrode material is selected from at least one of artificial graphite, natural graphite, mesophase carbon microspheres, silicon, silicon suboxide, and lithium titanate; the electrode samples are positive / negative electrode sheets made by preparing positive / negative electrode coatings using raw materials including positive / negative electrode materials and together with positive / negative electrode current collectors; the battery samples are batteries made by preparing separators using raw materials including separator materials and together with the electrode samples.

5. The application according to claim 1, characterized in that, The geometric parameters mentioned in step S1 include the length and width of the battery electrode, the length and width of the battery current collector, the position parameters of the battery tab, the thickness of the battery electrode, the number of battery electrodes, and one or more of the length, width, and height of the battery. The electrochemical parameters mentioned in step S1 include one or more of the following: electronic conductivity of the solid phase material of the test sample, initial lithium-ion concentration, solid volume fraction, liquid volume fraction, particle radius of the solid phase material, effective conductivity of the liquid phase material, initial lithium concentration of the electrolyte, electrochemical reaction rate constant, charge transfer coefficient, solid-phase lithium-ion diffusion coefficient, and liquid-phase lithium-ion diffusion coefficient. The thermally relevant parameters mentioned in step S1 include one or more of the following: mass, density, specific heat capacity, thermal conductivity, and thermal decomposition temperature of the test sample. The mechanical parameters mentioned in step S1 include one or more of the following: the compressive modulus and Poisson's ratio of the internal materials of the lithium-ion battery.

6. The application according to claim 1, characterized in that, The coupling method of the coupling model described in step S2 is as follows: a) The input to the mechanical model is the initial conditions and boundary conditions that include the effects of mechanical action, and the output of the mechanical model is the change of stress / strain inside the battery with time and space. b) The inputs of the short-circuit model include the stress / strain changes over time and space output by the mechanical model and the changes in battery voltage obtained by the electrochemical model. The outputs of the short-circuit model include the magnitude of the battery short-circuit internal resistance, the short-circuit location, and the changes in internal short-circuit heat generation. c) The inputs of the electrochemical model include the battery short-circuit internal resistance obtained from the short-circuit model and the temperature change obtained from the thermal model. The outputs of the electrochemical model include the battery voltage change and the battery electrochemical heat generation, including ohmic heat, polarization heat and reversible entropy heat. d) The inputs of the thermal model include the electrochemical heat generation obtained from the electrochemical model, the internal short-circuit heat generation obtained from the short-circuit model, and the side reaction heat generation obtained from the thermal runaway side reaction model. The output of the thermal model is the change of battery temperature with time and space. e) The input to the thermal runaway side reaction model is the change in battery temperature obtained from the thermal model, and the output of the thermal runaway side reaction model is the heat generated by each side reaction.

7. The application according to claim 1, characterized in that, The initial and boundary conditions input to the mechanical model include: the pressing speed of the indenter, setting the contact interface between the indenter and the battery as a contact boundary condition, and setting the bottom surface of the battery as a fixed displacement boundary condition. The stress / strain input to the short-circuit model changes with time and space are realized by linear projection or linear stretching; the change in battery voltage input to the short-circuit model is applied to the positive electrode tab of the battery model, and a ground boundary condition is used at the negative electrode tab of the battery. The short-circuit model can be used to determine the short-circuit current under the battery voltage boundary conditions. The short-circuit internal resistance of the short-circuit model can be obtained by the ratio of the battery voltage to the short-circuit current. The battery short-circuit internal resistance input to the electrochemical model is applied using the boundary condition of an external short-circuit resistor, and the temperature change input to the electrochemical model is achieved using a linear projection or linear stretching setting. The heat generation of each part of the electrochemical model input to the thermal model, the heat generation of internal short circuits, and the heat generation of side reactions are all achieved by setting linear projection or linear stretching. The temperature change input to the thermal runaway side reaction model is achieved using a linear projection or linear stretching setting. The temperature obtained from the thermal model is transferred to the electrochemical model in real time, influencing some electrochemical reaction kinetic parameters in the electrochemical model through the Arrhenius relation.

8. The application according to claim 1, characterized in that, The test conditions for testing the sample as described in step S4 include: (1) Conduct constant current and / or constant current and constant voltage charging or discharging at different rates under multiple temperature conditions, wherein the selection of the temperature and rate conditions should cover the normal operating conditions of the battery; (2) During the charging / discharging process, the surface and internal temperature of the battery are collected by using external or internal thermocouples, fiber optic sensors and infrared imagers; (3) Conduct a compression safety test on the battery according to the relevant compression test conditions, and collect data on the changes in internal and / or external temperature and voltage of the test battery over time.

9. The application according to claim 1, characterized in that, Step S5 describes adjusting the battery size parameters and capacity by increasing the battery's planar dimensions and increasing the number of stacked battery electrodes. Increasing the battery's planar dimensions includes increasing the length and width of the battery electrodes. Increasing the number of stacked battery electrodes involves alternating stacking of positive and negative electrodes, separated by a separator. By scaling up the battery capacity under the same process conditions without changing the thickness of individual electrodes, extrusion safety simulation is then conducted.

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