Electrochemical, thermal and capacity attenuation coupling modeling method of lithium iron phosphate battery

Abstract

The invention discloses an electrochemical, thermal and capacity attenuation coupling modeling method for a lithium iron phosphate battery, belongs to the technical field of lithium ion batteries, and solves the problem of how to accurately simulate voltage, temperature and capacity attenuation characteristics of the lithium iron phosphate battery under different working conditions. The method comprises the following steps: establishing a control equation based on a charge conservation equation, a material conservation equation and an electrode kinetic equation, establishing a three-dimensional thermal model based on an electrochemical heat production equation, a solid heat transfer equation and a thermal convection equation of a battery surface and an external environment, and establishing a capacity attenuation calculation equation of a lithium iron phosphate battery model; establishing an electrochemical, heat and capacity attenuation coupling relation equation; according to the invention, simulation of voltage, temperature and capacity attenuation characteristics under different working conditions can be effectively realized, and the working characteristics of the battery can be accurately simulated; meanwhile, the mutual influence and coupling relation among electrochemistry, heat and capacity attenuation in the cyclic charging and discharging process are considered, and the working state of the battery in use can be truly reflected.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries and relates to an electrochemical, thermal and capacity attenuation coupling modeling method for lithium iron phosphate batteries. Background Art

[0002] Lithium iron phosphate batteries have become the main power source for new energy electric vehicles due to their outstanding performance advantages. The capacity decay produced during the cyclic charge and discharge process has an important impact on the performance of the battery. Therefore, it is of great practical significance and practical value to establish a model that can accurately and reliably reflect the working characteristics of lithium iron phosphate batteries, study and analyze the aging mechanism and aging process of lithium iron phosphate batteries, and understand the impact of battery aging on its performance. Summary of the invention

[0003] The technical solution of the present invention is used to solve the problem of how to quickly calculate the voltage, temperature and capacity attenuation characteristics of a lithium iron phosphate battery under different operating conditions.

[0004] The present invention solves the above technical problems through the following technical solutions:

[0005] The present invention provides a method for coupled modeling of electrochemical, thermal and capacity decay of a lithium iron phosphate battery, comprising the following steps:

[0006] Step 1: Establishing control equations based on charge conservation equations, material conservation equations, and electrode kinetic equations in the direction of electrode thickness and active material particles;

[0007] Step 2: Establish a three-dimensional thermal model based on the electrochemical heat generation equation, the solid heat transfer equation, and the thermal convection equation between the battery surface and the external environment;

[0008] Step 3, based on the intercalation reaction of active particles of the lithium iron phosphate battery during the charge and discharge cycle and the reduction reaction of parasitic lithium / solvent, a capacity decay calculation equation of the lithium iron phosphate battery model is established;

[0009] Step 4: Based on the control equation, three-dimensional thermal model, and capacity decay calculation equation of the lithium iron phosphate battery model, establish the electrochemical, thermal and capacity decay coupling relationship equation.

[0010] Furthermore, the charge conservation equation described in step 1 includes a solid phase charge conservation equation and a liquid phase charge conservation equation, wherein the solid phase charge conservation equation follows Ohm's law and is expressed as:

[0011]

[0012] Among them, σ eff is the solid phase effective conductivity, φ s is the solid phase potential, i sis the solid phase current density, x is the electrode thickness direction;

[0013] The boundary conditions of the equation are:

[0014]

[0015] Where i is the battery charge and discharge current density, x=0 is the junction between the negative electrode current collector and the leftmost end of the negative electrode, x=pos is the junction between the positive electrode current collector and the rightmost end of the positive electrode, x=neg is the junction between the rightmost end of the negative electrode and the diaphragm, and x=sep is the junction between the leftmost end of the positive electrode and the diaphragm;

[0016] The charge conservation equation in the liquid phase is:

[0017]

[0018] Among them, κ eff is the effective conductivity of the liquid phase, φ l is the liquid phase potential, R is the molar gas constant, T b is the corrected battery temperature, F is the Faraday constant, Lithium ion liquid phase transfer coefficient, c l is the liquid phase lithium ion concentration, i l is the liquid phase current density;

[0019] The boundary equation is:

[0020]

[0021] Among them, the superscripts “-” and “+” indicate the left and right limits of each interface.

[0022] Furthermore, the material conservation equation described in step 1 includes a solid-phase material conservation equation and a liquid-phase material conservation equation, wherein the solid-phase material conservation equation adopts Fick's second law and is expressed as:

[0023]

[0024] Among them, c s is the solid phase lithium ion concentration, t is the time, r is the radius direction of the solid phase spherical particles, D s is the solid phase diffusion coefficient of lithium ions;

[0025] The boundary conditions are:

[0026]

[0027] Among them, r = 0 represents the center of the active particle, R s is the radius of the solid particle, j r is the local current density at the solid-liquid interface;

[0028] The conservation equation of liquid matter is:

[0029]

[0030] Among them, ε l is the liquid volume fraction, is the effective diffusion coefficient of the liquid phase, a is the surface area per unit volume of the electrode particles;

[0031] The boundary conditions are:

[0032]

[0033] Among them, neg represents the negative electrode region, sep represents the diaphragm region, and pos represents the positive electrode region.

[0034] Further, the electrode kinetic equation described in step 1 is expressed as:

[0035]

[0036] Where i0 is the exchange current density, α a and α c They are used to represent the transfer coefficients of the anode and cathode respectively, η is the surface overpotential of the spherical particles, and i0 can be obtained by the following formula:

[0037]

[0038] Where k0 is the electrochemical reaction rate constant, c s,max is the maximum lithium ion concentration in the active material, c surf is the lithium ion concentration at the solid-liquid interface;

[0039] The surface overpotential, solid phase potential and liquid phase potential are expressed by the following equations:

[0040]

[0041] Among them, U eq is the equilibrium potential of active materials in lithium-ion batteries, R film It is the membrane resistance of SEI film generated by side reactions in lithium-ion batteries.

[0042] Furthermore, the electrochemical heat generation equation described in step 2 is:

[0043]

[0044] Among them, q is the total heat generation rate of the battery, qr is the reversible heat generation rate, qp is the polarization heat generation rate, qj is the ohmic heat generation rate, and f ± is the electrolyte activity coefficient.

[0045] Furthermore, the solid heat transfer equation described in step 2 is:

[0046]

[0047] Among them, ρ is the battery cell density, Cp is the constant pressure specific heat capacity of the battery, and λ′ is the corrected thermal conductivity.

[0048] Furthermore, the heat convection equation between the battery surface and the external environment described in step 2 is:

[0049]

[0050] Where h is the natural convection heat transfer coefficient, T amb is the ambient temperature.

[0051] Furthermore, the capacity decay calculation equation described in step 3 is:

[0052]

[0053] Among them, j SEI is the parasitic reaction current density, H′ is the dimensionless graphite expansion coefficient after correction, K is the expansion coefficient of the volume expansion state of the reactive graphite particles, J is the dimensionless exchange current density of the parasitic reaction, i loc,1c,ref is the local current density corresponding to a 1C discharge rate, α is the electrochemical reduction reaction transfer coefficient, η SEI is the parasitic reaction overpotential, Q SEI is the local accumulated charge caused by the formation of SEI film, and f is a lumped dimensionless parameter based on the properties of SEI film;

[0054] The concentration of SEI film formed in the porous electrode c SEI The calculation equation is:

[0055]

[0056] Among them, v SEI is the stoichiometric number of SEI species in the reaction;

[0057] The local accumulated charge Q caused by the formation of SEI film SEI The calculation equation is:

[0058]

[0059] Among them, A v is the electrode surface area;

[0060] According to the SEI film concentration, the SEI film thickness calculation equation is:

[0061]

[0062] Among them, M p is the molar mass of the SEI film, which is 0.16 kg / mol; p is the product density formed by the parasitic reaction, and its value is 1600kg / m 3 ; δ film,0 is the initial film thickness, which is 1 nm at t=0.

[0063] Furthermore, the coupling equation of the electrochemical model and the thermal model described in step 4 is:

[0064]

[0065] Where D′ is the corrected solid phase diffusion coefficient, T ref is the reference temperature, k′ is the corrected reaction rate constant,

[0066] s E act,D and E act,k0 is the activation energy required for the reaction;

[0067] The coupling equation of the electrochemical model and capacity decay is:

[0068]

[0069] where j sum is the local current density of the battery, which is composed of the current density of the deintercalation reaction and the current density of the SEI film parasitic reaction;

[0070] The coupling equation of the capacity decay and thermal model is:

[0071]

[0072] Among them, C ref is the available capacity of lithium iron phosphate battery at standard temperature, C com is the current battery capacity, θ is the exponential pressure factor, γ is the exponential coefficient of the pressure factor, H′ is the corrected dimensionless graphite expansion coefficient, H is the dimensionless graphite expansion coefficient at the reference temperature, E act,H is the activation energy required for the reaction.

[0073] The present invention also provides a storage medium having a computer program stored thereon, and when the computer program is executed by a processor, the steps of the above-mentioned electrochemical, thermal and capacity attenuation coupled modeling method of the lithium iron phosphate battery are executed.

[0074] The advantages of the present invention are:

[0075] The present invention establishes a control equation based on the charge conservation equation, the material conservation equation and the electrode kinetics equation, establishes a three-dimensional thermal model based on the electrochemical heat generation equation, the solid heat transfer equation and the thermal convection equation between the battery surface and the external environment, establishes a capacity decay calculation equation for the lithium iron phosphate battery model, and establishes an electrochemical, thermal and capacity decay coupling relationship equation; the present invention comprehensively considers the performance changes of the battery during the charge and discharge cycle, the result is accurate, and the reliability is high, which is convenient for researchers to fully understand the charge and discharge cycle performance of the lithium iron phosphate battery; the present invention can effectively realize the simulation of voltage, temperature and capacity decay characteristics under different working conditions, and accurately simulate the working characteristics of the battery; at the same time, the mutual influence and coupling relationship between electrochemistry, heat and capacity decay during the cyclic charge and discharge process are considered, and the working state of the battery in use can be truly reflected. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 This is a flow chart of the electrochemical, thermal and capacity decay coupled modeling method for a lithium iron phosphate battery according to the first embodiment of the present invention;

[0077] Figure 2 This is a diagram showing the electrochemical, thermal and capacity attenuation coupling relationship of the lithium iron phosphate battery of Example 1 of the present invention;

[0078] Figure 3 This is a curve diagram showing the effect of rate on capacity attenuation during the charge and discharge cycle of Example 1 of the present invention;

[0079] Figure 4 This is a curve diagram showing the effect of rate on voltage during the charge and discharge cycle of Example 1 of the present invention;

[0080] Figure 5 This is a curve diagram showing the effect of rate on temperature during the charge and discharge cycle of Example 1 of the present invention;

[0081] Figure 6 This is a relationship diagram of the effect of temperature on capacity attenuation according to the first embodiment of the present invention;

[0082] Figure 7 This is a graph showing the influence of capacity attenuation on voltage according to the first embodiment of the present invention;

[0083] Figure 8 FIG. 1 is a graph showing the influence of capacity attenuation on temperature according to the first embodiment of the present invention. DETAILED DESCRIPTION

[0084] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0085] The technical solution of the present invention is further described below in conjunction with the accompanying drawings and specific embodiments of the specification:

[0086] Embodiment 1

[0087] like Figure 1 As shown, the electrochemical, thermal and capacity decay coupling modeling method of the lithium iron phosphate battery of the embodiment of the present invention includes the following steps:

[0088] Step 1: Establish control equations based on charge conservation equation, material conservation equation and electrode kinetic equation in the direction of electrode thickness and active material particles, so as to describe charge conservation, mass conservation and interfacial electrochemical reactions of lithium ions and electrons in solid and liquid phases.

[0089] The electrode active materials of the one-dimensional lithium iron phosphate battery model are spherical particles with the same radius. The electrochemical model of the one-dimensional lithium iron phosphate battery model is based on the concentrated solution theory and the porous electrode theory. The control equations based on the charge conservation equation, the material conservation equation and the electrode kinetic equation are established from the electrode thickness direction and the active material particle direction, so as to describe the charge conservation, mass conservation and interfacial electrochemical reactions of lithium ions and electrons in the solid and liquid phases. The charge conservation equation involves the conservation of charges carried by electrons and lithium ions in the solid and liquid phases and the conservation of exchange charges; the material conservation equation is used to describe the conservation of the total amount of lithium ions in the process of lithium ions diffusing and migrating in the solid and liquid phases respectively; the electrode kinetic equation is used to describe the process of lithium ion insertion / deinsertion and electron gain and loss at the solid-liquid interface of the positive and negative electrodes.

[0090] The charge conservation equation includes the solid phase charge conservation equation and the liquid phase charge conservation equation, where the solid phase charge conservation equation follows Ohm's law and is expressed as:

[0091]

[0092] Among them, σ eff is the solid phase effective conductivity, φ s is the solid phase potential, i s is the solid phase current density, x is the electrode thickness direction;

[0093] The boundary conditions of the equation are:

[0094]

[0095] Where i is the battery charge and discharge current density, x=0 is the junction between the negative electrode current collector and the leftmost end of the negative electrode, x=pos is the junction between the positive electrode current collector and the rightmost end of the positive electrode, x=neg is the junction between the rightmost end of the negative electrode and the diaphragm, and x=sep is the junction between the leftmost end of the positive electrode and the diaphragm;

[0096] The charge conservation equation in the liquid phase is:

[0097]

[0098] Among them, κ eff is the effective conductivity of the liquid phase, φ l is the liquid phase potential, R is the molar gas constant, T b is the corrected battery temperature, F is the Faraday constant, Lithium ion liquid phase transfer coefficient, c l is the liquid phase lithium ion concentration, i l is the liquid phase current density;

[0099] The boundary equation is:

[0100]

[0101] Among them, the superscripts “-” and “+” indicate the left and right limits of each interface;

[0102] The material conservation equation includes the solid phase material conservation equation and the liquid phase material conservation equation. The solid phase material conservation equation adopts Fick's second law and is expressed as:

[0103]

[0104] Among them, c s is the solid phase lithium ion concentration, t is the time, r is the radius direction of the solid phase spherical particles, D s is the solid phase diffusion coefficient of lithium ions;

[0105] The boundary conditions are:

[0106]

[0107] Among them, r = 0 represents the center of the active particle, R s is the radius of the solid particle, j r is the local current density at the solid-liquid interface;

[0108] The conservation equation of liquid matter is:

[0109]

[0110] Among them, ε lis the liquid volume fraction, is the effective diffusion coefficient of the liquid phase, a is the surface area per unit volume of the electrode particles;

[0111] The boundary conditions are:

[0112]

[0113] Among them, neg represents the negative electrode area, sep represents the diaphragm area, and pos represents the positive electrode area;

[0114] The electrode kinetic equation is expressed as:

[0115]

[0116] Where i0 is the exchange current density, α a and α c They are used to represent the transfer coefficients of the anode and cathode respectively, η is the surface overpotential of the spherical particles, and i0 can be obtained by the following formula:

[0117]

[0118] Where k0 is the electrochemical reaction rate constant, c s,max is the maximum lithium ion concentration in the active material, c surf is the lithium ion concentration at the solid-liquid interface;

[0119] The surface overpotential, solid phase potential and liquid phase potential can be expressed by the following equations:

[0120]

[0121] Among them, U eq is the equilibrium potential of active materials in lithium-ion batteries, R film It is the membrane resistance of SEI film generated by side reactions in lithium-ion batteries.

[0122] Step 2: Establish a three-dimensional thermal model based on the electrochemical heat generation equation, solid heat transfer equation, and thermal convection equation between the battery surface and the external environment

[0123] The three-dimensional thermal model mainly considers the heat generation and heat dissipation of the battery, following the law of conservation of energy. Heat generation includes reversible heat, polarization heat and reaction heat, and heat dissipation includes heat conduction inside the battery and heat convection between the battery surface and the external environment.

[0124] The electrochemical heat generation equation is:

[0125]

[0126] Among them, q is the total heat generation rate of the battery, qr is the reversible heat generation rate, qp is the polarization heat generation rate, qj is the ohmic heat generation rate, and f± is the electrolyte activity coefficient;

[0127] The solid heat transfer equation is:

[0128]

[0129] Among them, ρ is the battery cell density, Cp is the constant pressure specific heat capacity of the battery, and λ′ is the corrected thermal conductivity;

[0130] The heat convection equation between the battery surface and the external environment is:

[0131]

[0132] Where h is the natural convection heat transfer coefficient, T amb is the ambient temperature.

[0133] Step 3: Based on the intercalation reaction of active particles in the lithium iron phosphate battery during the charge and discharge cycle and the reduction reaction of parasitic lithium / solvent, the capacity decay calculation equation of the lithium iron phosphate battery model is established.

[0134] In addition to the intercalation reaction of active particles, the lithium iron phosphate battery is also accompanied by a parasitic lithium / solvent reduction reaction during the charge and discharge cycle: S+Li + +e - →P SEI , where S is the solvent, Li + For lithium ion, e - For electronics, P SEI is the product formed in the reaction;

[0135] Therefore, the capacity decay calculation equation is:

[0136]

[0137] Among them, j SEI is the parasitic reaction current density, H′ is the dimensionless graphite expansion coefficient after correction, K is the expansion coefficient of the volume expansion state of the reactive graphite particles, J is the dimensionless exchange current density of the parasitic reaction, i loc,1c,ref is the local current density corresponding to a 1C discharge rate, α is the electrochemical reduction reaction transfer coefficient, η SEI is the parasitic reaction overpotential, Q SEI is the local accumulated charge caused by the formation of SEI film, and f is a lumped dimensionless parameter based on the properties of SEI film;

[0138] The concentration of SEI film formed in the porous electrode c SEI The calculation equation is:

[0139]

[0140] Among them, v SEI is the stoichiometric number of SEI species in the reaction;

[0141] The local accumulated charge Q caused by the formation of SEI film SEI The calculation equation is:

[0142]

[0143] Among them, A v is the electrode surface area;

[0144] According to the SEI film concentration, the SEI film thickness calculation equation is:

[0145]

[0146] Among them, M p is the molar mass of the SEI film, which is 0.16 kg / mol; p is the product density formed by the parasitic reaction, and its value is 1600kg / m 3 ; δ film,0 is the initial film thickness, which is 1 nm at t=0.

[0147] Step 4: Based on the control equation, three-dimensional thermal model, and capacity decay calculation equation of the lithium iron phosphate battery model, establish the electrochemical, thermal and capacity decay coupling relationship equation

[0148] The electrochemical, thermal and capacity decay coupling relationship equations include: coupling equations between the electrochemical model and the thermal model, coupling equations between the electrochemical model and the capacity decay, and coupling equations between the capacity decay and the thermal model.

[0149] The coupling equation of the electrochemical model and the thermal model is:

[0150]

[0151] Where D′ is the corrected solid phase diffusion coefficient, T ref is the reference temperature, k′ is the corrected reaction rate constant,

[0152] s E act,D and E act,k0 is the activation energy required for the reaction;

[0153] The coupling equation between the electrochemical model and capacity decay is:

[0154]

[0155] where j sum is the local current density of the battery, which is composed of the current density of the deintercalation reaction and the current density of the SEI film parasitic reaction;

[0156] The coupling equation of capacity fading and thermal model is:

[0157]

[0158] Among them, C ref is the available capacity of lithium iron phosphate battery at standard temperature, C com is the current battery capacity, θ is the exponential pressure factor, γ is the exponential coefficient of the pressure factor, H′ is the corrected dimensionless graphite expansion coefficient, H is the dimensionless graphite expansion coefficient at the reference temperature, E act,H is the activation energy required for the reaction.

[0159] like Figure 2 As shown, the electrochemical, thermal and capacity decay coupling diagram of lithium iron phosphate battery, in which the capacity decay is measured by Q SEI The thermal conductivity λ affects the thermal model, which is affected by the temperature T b Correct the dimensionless graphite expansion coefficient H in capacity decay to achieve the coupling between thermal model and capacity decay; the temperature T of the thermal model b Correction of D in electrochemical model s,p , D s,n and k0, while j in the electrochemical model r ,η,φ s ,φ l Factors such as , etc. affect heat generation, thus achieving the coupling of electrochemical and thermal models; R film will affect the η of the electrochemical model, and the φ of the electrochemical model s ,φ l and j r Will change η SEI , which in turn affects Q SEI , realizing the coupling of electrochemistry and capacity decay.

[0160] Model simulation and analysis

[0161] Combining the above electrochemical model, thermal model and capacity decay model, adding the coupling relationship between the three, and building the model in COMSOL Multiphysics finite element simulation software, the simulation results are as follows:

[0162] like Figures 3 to 5 As shown, the working characteristic relationship curves of battery capacity, voltage and temperature are shown when the ambient temperature is 25°C, the charging rate is 0.5C, and the discharging rates are 1C, 2C and 3C respectively; the model constructed by the method of the present invention can accurately reflect the various performances of the battery in terms of capacity, voltage and temperature.

[0163] like Figure 6As shown in the figure, the relationship curve between the battery capacity and the number of cycles is shown when the discharge rate is 1C and the ambient temperature is 15℃, 25℃ and 35℃ respectively. As the temperature increases, the chemical reaction rate inside the battery intensifies and the capacity decay rate accelerates.

[0164] like Figure 7 As shown in the figure, when the ambient temperature is 25℃ and the discharge rate is 1C, the relationship curve between voltage and time at different cycle numbers shows that the capacity of the battery decays as the cycle number increases. Due to the consumption of cyclable lithium to form SEI film, the internal resistance of the battery continues to increase, the discharge voltage curve moves downward as a whole, and the discharge time continues to shorten.

[0165] like Figure 8 As shown in the figure, when the discharge rate is 1C and the ambient temperature is 25°C, the relationship curve between temperature and time at different cycle numbers is as follows. With the increase of cycle number and the aggravation of capacity attenuation, the electrolyte is gradually consumed during the aging process and side reaction gases with low thermal conductivity are produced. The formed gas layer affects the thermal conductivity and causes the battery temperature to rise.

[0166] Embodiment 2

[0167] An electronic device includes a memory and a processor, wherein the memory is used to store a program that supports the processor to execute the electrochemical, thermal and capacity decay coupled modeling method of the lithium iron phosphate battery in Example 1, and the processor is configured to execute the program stored in the memory.

[0168] Embodiment 3

[0169] A storage medium stores a computer program, which, when executed by a processor, executes the steps of the electrochemical, thermal and capacity decay coupled modeling method for a lithium iron phosphate battery in embodiment 1.

[0170] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A coupled modeling method for electrochemical, thermal and capacity decay of lithium iron phosphate batteries, characterized in that: The following steps are involved: Step 1: Establishing control equations based on charge conservation equations, material conservation equations, and electrode kinetic equations in the direction of electrode thickness and active material particles; Step 2: Establish a three-dimensional thermal model based on the electrochemical heat generation equation, the solid heat transfer equation, and the thermal convection equation between the battery surface and the external environment; Step 3, based on the intercalation reaction of active particles of the lithium iron phosphate battery during the charge and discharge cycle and the reduction reaction of parasitic lithium / solvent, a capacity decay calculation equation of the lithium iron phosphate battery model is established; Step 4: Based on the control equation, three-dimensional thermal model, and capacity decay calculation equation of the lithium iron phosphate battery model, establish the electrochemical, thermal, and capacity decay coupling relationship equation.

2. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 1, characterized in that: The charge conservation equation described in step 1 includes a solid phase charge conservation equation and a liquid phase charge conservation equation, wherein the solid phase charge conservation equation follows Ohm's law and is expressed as: Among them, σ eff is the solid phase effective conductivity, φ s is the solid phase potential, i s is the solid phase current density, x is the electrode thickness direction; The boundary conditions of the equation are: Where i is the battery charge and discharge current density, x=0 is the junction between the negative electrode current collector and the leftmost end of the negative electrode, x=pos is the junction between the positive electrode current collector and the rightmost end of the positive electrode, x=neg is the junction between the rightmost end of the negative electrode and the diaphragm, and x=sep is the junction between the leftmost end of the positive electrode and the diaphragm; The charge conservation equation in the liquid phase is: Among them, κ eff is the effective conductivity of the liquid phase, φ l is the liquid phase potential, R is the molar gas constant, T b is the corrected battery temperature, F is the Faraday constant, Lithium ion liquid phase transfer coefficient, c l is the liquid phase lithium ion concentration, i l is the liquid phase current density; The boundary equation is: Among them, the superscripts "-" and "+" represent the left and right limits of each interface.

3. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 2, characterized in that: The material conservation equation described in step 1 includes the solid phase material conservation equation and the liquid phase material conservation equation, wherein the solid phase material conservation equation adopts Fick's second law and is expressed as: Among them, c s is the solid phase lithium ion concentration, t is the time, r is the radius direction of the solid phase spherical particles, D s is the solid phase diffusion coefficient of lithium ions; The boundary conditions are: Among them, r = 0 represents the center of the active particle, R s is the radius of the solid particle, j r is the local current density at the solid-liquid interface; The conservation equation of liquid matter is: Among them, ε l is the liquid volume fraction, is the effective diffusion coefficient of the liquid phase, a is the surface area per unit volume of the electrode particles; The boundary conditions are: Among them, neg represents the negative electrode region, sep represents the diaphragm region, and pos represents the positive electrode region.

4. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 3, characterized in that: The electrode kinetic equation described in step 1 is expressed as: Where i0 is the exchange current density, α a and α c They are used to represent the transfer coefficients of the anode and cathode respectively, η is the surface overpotential of the spherical particles, and i0 can be obtained by the following formula: Where k0 is the electrochemical reaction rate constant, c s,max is the maximum lithium ion concentration in the active material, c surf is the lithium ion concentration at the solid-liquid interface; The surface overpotential, solid phase potential and liquid phase potential are expressed by the following equations: Among them, U eq is the equilibrium potential of active materials in lithium-ion batteries, R film It is the membrane resistance of SEI film generated by side reactions in lithium-ion batteries.

5. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 4, characterized in that: The electrochemical heat generation equation described in step 2 is: Among them, q is the total heat generation rate of the battery, qr is the reversible heat generation rate, qp is the polarization heat generation rate, qj is the ohmic heat generation rate, and f ± is the electrolyte activity coefficient.

6. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 5, characterized in that: The solid heat transfer equation described in step 2 is: Among them, ρ is the battery cell density, Cp is the constant pressure specific heat capacity of the battery, and λ′ is the corrected thermal conductivity.

7. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 6, characterized in that: The heat convection equation between the battery surface and the external environment described in step 2 is: Where h is the natural convection heat transfer coefficient, T amb is the ambient temperature.

8. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 7, characterized in that: The capacity fade calculation equation described in step 3 is: Among them, j SEI is the parasitic reaction current density, H′ is the dimensionless graphite expansion coefficient after correction, K is the expansion coefficient of the volume expansion state of the reactive graphite particles, J is the dimensionless exchange current density of the parasitic reaction, i loc,1c,ref is the local current density corresponding to a 1C discharge rate, α is the electrochemical reduction reaction transfer coefficient, η SEI is the parasitic reaction overpotential, Q SEI is the local accumulated charge caused by the formation of SEI film, and f is a lumped dimensionless parameter based on the properties of SEI film; The concentration of SEI film formed in the porous electrode c SEI The calculation equation is: Among them, v SEI is the stoichiometric number of SEI species in the reaction; The local accumulated charge Q caused by the formation of SEI film SEI The calculation equation is: Among them, A v is the electrode surface area; According to the SEI film concentration, the SEI film thickness calculation equation is: Among them, M p is the molar mass of the SEI film, which is 0.16 kg / mol; p is the product density formed by the parasitic reaction, and its value is 1600kg / m 3 ; δ film,0 is the initial film thickness, which is 1 nm at t=0.

9. The electrochemical, thermal and capacity decay coupled modeling method of a lithium iron phosphate battery according to claim 8, characterized in that: The coupling equations of the electrochemical model and the thermal model described in step 4 are: Where D′ is the corrected solid phase diffusion coefficient, T ref is the reference temperature, k′ is the corrected reaction rate constant, s E act,D and E act,k0 is the activation energy required for the reaction; The coupling equation of the electrochemical model and capacity decay is: where j sum is the local current density of the battery, which is composed of the current density of the deintercalation reaction and the current density of the SEI film parasitic reaction; The coupling equation of the capacity decay and thermal model is: Among them, C ref is the available capacity of lithium iron phosphate battery at standard temperature, C com is the current battery capacity, θ is the exponential pressure factor, γ is the exponential coefficient of the pressure factor, H′ is the corrected dimensionless graphite expansion coefficient, H is the dimensionless graphite expansion coefficient at the reference temperature, E act,H is the activation energy required for the reaction.

10. A storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the electrochemical, thermal and capacity decay coupled modeling method for a lithium iron phosphate battery according to any one of claims 1 to 9 are executed.

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