A simulation analysis method for lithium battery electrodes considering local deactivation of electrode materials

Abstract

The present invention discloses a lithium battery electrode simulation analysis method that considers the local deactivation of electrode materials, which belongs to the technical field of secondary batteries. The method includes the following steps: S1, determining the initial structure of the electrode material; S2, establishing an electrochemical numerical model of the electrode charging or discharging process; S3, establishing a local deactivation judgment function of the electrode material; S4, constructing a finite element analysis model of the electrode material and calculating and solving the electrode charging or discharging process; S5, used to obtain the distribution of electrode active substances and the remaining effective cycle capacity when considering the local deactivation of the electrode material, so as to predict the working state of the lithium battery electrode; the method realizes the quantitative and accurate exploration and analysis of the causes of the local deactivation of the electrode material, the mechanism of its influence on the electrochemical performance and the control method. The present invention predicts the working state of the electrode efficiently and at low cost and guides the design and optimization of the lithium battery electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a lithium battery electrode simulation analysis method taking into account local deactivation of electrode materials. Background Art

[0002] With the rapid development of clean energy, smart grid, electric vehicles, drones and other fields, secondary batteries represented by lithium-ion batteries have become one of the most important energy storage systems. However, the growing demand for energy storage has also put higher requirements on the energy density, cycle life, rate performance, safety, etc. of secondary batteries. The new generation of lithium-ion batteries and lithium metal batteries (lithium-sulfur batteries, etc.) with lithium metal as the negative electrode material have become the research focus in the field of secondary batteries.

[0003] However, the new generation of lithium-ion batteries and lithium metal batteries are facing the problem of irreversible capacity loss and low cycle life, especially under more stringent charge and discharge rate conditions. The problem of capacity diving is particularly prominent. The local deactivation of electrode materials in the battery is one of the main reasons for this irreversible capacity loss. For example, the structural dissociation and expansion peeling of negative electrode materials such as graphite and silicon carbon; the formation of "dead lithium" and lithium powderization in the delithiation process of lithium metal negative electrodes; the overcharging of oxide positive electrodes causes the collapse of the positive electrode structure and gas production leading to pore blockage; the "dead sulfur" problem of sulfur positive electrodes, etc., all cause the local deactivation of electrode materials, which in turn leads to irreversible attenuation of battery cycle capacity.

[0004] In order to achieve the scientific and rational design and optimization of lithium-ion batteries and lithium metal batteries with high cycle life, it is necessary to have more accurate and efficient research and analysis methods for the causes, influencing mechanisms and regulation methods of local deactivation of electrode materials. However, due to the complexity of the microstructure and electrochemical reaction process of electrode materials, as well as the limitations of in-situ characterization techniques for morphological evolution, it is difficult to directly analyze the influence mechanism of local deactivation of electrode materials on battery charge and discharge performance through experiments, and it is difficult to propose effective lithium battery electrode design optimization solutions. Summary of the invention

[0005] The object of the present invention is to provide a lithium battery electrode simulation analysis method considering local deactivation of electrode materials, characterized in that the lithium battery electrode simulation analysis comprises the following steps:

[0006] S1: Determine the initial structure of the electrode material;

[0007] S2: Establish an electrochemical numerical model of the electrode charging or discharging process;

[0008] S3: Establishing the judgment function of local inactivation of electrode materials;

[0009] S4: Construct a finite element analysis model of the electrode material and calculate and solve the electrode charging or discharging process;

[0010] S5: Obtain the distribution of electrode active substances and the remaining effective cycle capacity, predict the working state of the electrode, and guide the design and optimization of the electrode.

[0011] The electrode material in step S1 includes at least one of lithium metal, lithium indium alloy, lithium tin alloy, lithium copper alloy, lithium magnesium alloy, lithium aluminum alloy, lithium sodium alloy, lithium potassium alloy, lithium carbon alloy, lithium silicon alloy, silicon carbon alloy, amorphous carbon, graphite, soft carbon, hard carbon, carbon fiber, lithium titanate, lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, lithium oxide, lithium peroxide, sulfur, and lithium sulfide;

[0012] The initial structure of the electrode material includes at least one of thickness, surface roughness, particle morphology, average particle size, particle size distribution, average pore size, pore size distribution, average surface curvature, surface curvature distribution, porosity, and specific surface area.

[0013] The electrochemical numerical model in step S2 includes at least one of the following: an electrochemical reaction equation at the interface between the electrode material and the electrolyte, a mass transfer equation for lithium ions in the electrode material and the electrolyte, a migration vector equation for lithium ions in the electrode material and the electrolyte, a solid phase potential distribution equation in the electrode material and the current collector, a liquid phase potential distribution equation in the electrode material and the electrolyte, a heat transfer equation for heat in the electrode material and the electrolyte, an interface migration vector equation at the interface between the electrode material and the electrolyte, a level set equation at the interface between the electrode material and the electrolyte, a phase field equation between the two phases of the electrode material and the electrolyte, and a fluid mass transfer equation for the electrolyte.

[0014] The local deactivation determination function f in step S3 is composed of the harmonic function f H , mapping function f based on physical field variables P One or more of them are obtained through basic algebraic operations; the deactivation judgment function is used to determine whether the electrode material at a specified spatial position is in a fully active state, a completely inactivated state or a transitionally active state at a specified time; the deactivation judgment function will be directly inserted into the various equations in the electrochemical numerical model that need to consider the local deactivation of the electrode material through basic algebraic operations as a correction factor.

[0015] The value range of the deactivation judgment function f is a closed interval in the real number domain, with [f 1 , f 2 ] indicates that the appropriate deactivation judgment tolerance δ is set 1 ,δ 2 , the value is a non-negative real number (positive real number), or set an appropriate deactivation judgment critical ratio λ 1 , 2After that, the value is a positive real number, then the activity state of the electrode material at the specified spatial position can be determined by the local inactivation function f;

[0016] The activity state of the electrode material is determined by the local inactivation determination function f:

[0017] When f ≥ f 2 -δ 2 , or f ≥ λ 2 f 2 When , the electrode material at this location is in a fully active state;

[0018] When f≤f 1 +δ 1 , or f≤λ 1 f 1 When , the electrode material at this location is completely inactivated;

[0019] When f 1 +δ 1 <f<f 2 -δ 2 , or λ 1 f 1 <f<λ 2 f 2 When , the electrode material is in a transitional active state.

[0020] The harmonic function f of one of the local deactivation determination functions in step 3 H , can be determined by solving the following Poisson equation:

[0021]

[0022] In the above formula To solve the vector differential operator for scalar gradient and vector divergence, the variable σ is the state coefficient used to distinguish electrode materials with different active states. The state coefficient σ of the region where the fully active electrode material exists can be set to σ 2 , there is no region state coefficient σ of fully active and transitionally active electrode materials 1 , harmonic local deactivation decision function f H The solution process requires the following boundary conditions to be set at the boundary where fully active electrode material exists:

[0023] f H =f H2 (2)

[0024] At the boundary where fully active and transitionally active electrode materials do not exist:

[0025] f H =f H1 (3)

[0026] Among them, as a harmonic function f H f at the upper and lower bounds of the range H2 、f H1 Can be set to any two real numbers.

[0027] The mapping function f of one of the local deactivation judgment functions in step 3 based on the physical field variables P , wherein the physical field variables include at least one variable selected from the group consisting of solid phase potential, liquid phase potential, lithium ion concentration, lithium ion chemical potential, lithium ion electrochemical potential, and electrolyte concentration; the mapping function f based on the physical field variables P Determined by the following method:

[0028] Determine the value range of the physical field variable [P 1 , P 2 ], as a mapping function based on physical field variables f p The domain of f P The upper and lower bounds of the range f P2 、f P1 Through one or more basic algebraic operations of step function, step function with smooth polynomial, inverse tangent function, hyperbolic tangent function, logistic function, sigmoid function, the value range of the physical field variable [P 1 , P 2 ] to f P The upper and lower bounds of the range [f P1 , f P2 The mapping function f P .

[0029] The electrode active material distribution c in step S5 a Determined by the following formula:

[0030] c a =fc (4)

[0031] In the above formula, f is the local deactivation judgment function with a range of [0, 1], and the fully active electrode material corresponds to f = 1, c is the concentration distribution of the electrode material, and the obtained electrode active material distribution c a It is a function of spatial position and time.

[0032] The remaining effective cycle capacity Q in step S5 eff Determined by the following formula:

[0033] Q eff =∫ V qc a dV (5)

[0034] In the above formula, c ais the distribution of electrode active materials, q is the molar capacity of the electrode material, V is the total volume of the electrode material, and the remaining effective cycle capacity Q is obtained. eff is a function of time.

[0035] The electrochemical numerical model in step S2 is specifically to establish an electrochemical numerical model of the discharge process of the metal lithium negative electrode; the discharge process of the metal lithium negative electrode, that is, the dissolution process of the metal lithium deposit, is accompanied by significant surface morphology evolution, so the electrochemical numerical model here will be established based on the phase field theory; the equations involved in the electrochemical numerical model are as follows:

[0036] Phase field equation of metallic lithium and electrolyte:

[0037]

[0038] In the above formula (6), ξ is the phase field order parameter, and the values ​​of ξ are 1 and 0, respectively, indicating the metallic lithium phase and the electrolyte phase. d is the local inactivation judgment function, f d The value of 1 or 0 refers to fully active metallic lithium and completely deactivated metallic lithium, respectively; L σ is the phase field interface mobility; L η is the reaction rate prefactor; h(ξ)=ξ 3 (6ξ 2 -15ξ+10) is an interpolation function. S is the phase field surface energy term, F R F is the phase-field electrochemical reaction kinetic term coupled to the Butler-Volmer electrochemical kinetic equation. S and F R Given by the following formula (7) and formula (8),

[0039]

[0040]

[0041] Among them, f 0 (ξ)=16Wξ 2 (1-ξ) 2 is the double potential well function describing the phase interface, W is the energy barrier of the phase change interface; κ is the gradient energy coefficient controlled by the surface energy anisotropy; α is the positive electrode charge transfer coefficient, is the activation overpotential, E eq are the solid phase potential at the interface, the liquid phase potential at the interface, and the thermodynamic equilibrium potential, respectively. is the activity of lithium ions; F, R, and T are Faraday constant, gas state constant, and system temperature, respectively;

[0042] The mass transfer equation of lithium ions in the electrolyte is:

[0043]

[0044] In the above formula, To solve the vector differential operator for scalar gradient and vector divergence, is the concentration of lithium ions, is the diffusion coefficient of lithium ions in the electrolyte, c s is the lithium atom concentration in the solid phase of metallic lithium; the liquid phase potential distribution equation in the electrolyte:

[0045]

[0046] The liquid phase potential distribution is obtained by solving the above Poisson equation (10).

[0047] The beneficial effect of the present invention is to provide a lithium battery electrode simulation analysis method that takes into account the local deactivation of electrode materials with high calculation efficiency, high calculation accuracy and wide applicability, which is used to obtain the distribution of electrode active substances and the remaining effective cycle capacity when the electrode material is locally deactivated, so as to predict the working state of the lithium battery electrode and guide the design and optimization of the lithium battery electrode, specifically including:

[0048] (1) To address the problem of local deactivation of electrode materials, a local deactivation judgment function was developed as a correction factor to achieve efficient and accurate dynamic simulation of the charge and discharge process of the electrode when the electrode material in the lithium battery is locally deactivated. Through simulation calculations, the distribution of electrode active substances and the remaining effective cycle capacity when the electrode material is locally deactivated can be obtained, and various key data of the battery charge and discharge process can be quantitatively obtained to predict the working state of the lithium battery electrode.

[0049] (2) It provides a highly accurate, easy-to-operate, and highly universal simulation analysis method, which solves the problem that it is difficult to directly analyze the mechanism of the local deactivation process of electrode materials through experiments. Through the simulation analysis method, the structure, composition, charging and discharging conditions of electrode materials can be conveniently screened and optimized in batches, and the causes of local deactivation of electrode materials, the mechanism of influence on electrochemical performance, and the regulation method can be quantitatively and accurately explored and analyzed, and ultimately the efficient design and optimization of lithium battery electrodes can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a flowchart of the simulation analysis method;

[0051] Figure 2 A schematic diagram of the process flow of the metal lithium negative electrode simulation analysis method provided in Example 1;

[0052] Figure 3A schematic flow chart of the lithium cobalt oxide positive electrode simulation analysis method provided in Example 2. DETAILED DESCRIPTION

[0053] The present invention provides a lithium battery electrode simulation analysis method taking into account local deactivation of electrode materials. The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0054] Figure 1 The figure shows a flow chart of a battery electrode simulation analysis method, where the electrode simulation analysis includes the following steps:

[0055] S1: Determine the initial structure of the electrode material;

[0056] S2: Establish an electrochemical numerical model of the electrode charging or discharging process;

[0057] S3: Establishing the judgment function of local inactivation of electrode materials;

[0058] S4: Construct a finite element analysis model of the electrode material and calculate and solve the electrode charging or discharging process;

[0059] S5: Obtain the distribution of electrode active substances and the remaining effective cycle capacity, predict the working state of the electrode, and guide the design and optimization of the electrode.

[0060] Embodiment 1

[0061] Figure 2 A schematic diagram of the process flow of the metal lithium negative electrode simulation analysis method provided in Example 1;

[0062] The figure shows the simulation analysis of the delithiation process of the metal lithium negative electrode in the lithium metal battery and the formation process of "dead lithium". The distribution of active metal lithium and inactive "dead lithium" and the remaining effective cycle capacity will be obtained to predict the working state of the metal lithium negative electrode. The specific simulation analysis process includes the following steps:

[0063] S21: Determine the initial structure of the metallic lithium anode;

[0064] Referring to the morphology of the metallic lithium deposits photographed under a scanning electron microscope, the initial structure of the metallic lithium negative electrode here is rod-shaped metallic lithium deposition. The height of the rod-shaped metallic lithium is 30 microns and the diameter is 5 microns. The rod-shaped metallic lithium is evenly distributed on the current collector at a distribution density of 1 rod per 10 microns.

[0065] S22: Establish an electrochemical numerical model of the discharge process of metallic lithium anode;

[0066] The discharge process of the metallic lithium negative electrode, that is, the dissolution process of the metallic lithium deposit, is accompanied by significant surface morphology evolution, so the electrochemical numerical model here will be established based on the phase field theory. The equations involved in this electrochemical numerical model are as follows:

[0067] Phase field equation of metallic lithium and electrolyte:

[0068]

[0069] In the above formula (6), ξ is the phase field order parameter, and the values ​​of ξ are 1 and 0, respectively, indicating the metallic lithium phase and the electrolyte phase. d is the local inactivation judgment function, f d The value of 1 or 0 refers to fully active metallic lithium and completely deactivated metallic lithium, respectively; L σ is the phase field interface mobility; L η is the reaction rate prefactor; h(ξ)=ξ 3 (6ξ 2 -15ξ+10) is an interpolation function. S is the phase field surface energy term, F R F is the phase-field electrochemical reaction kinetic term coupled to the Butler-Volmer electrochemical kinetic equation. S and F R Given by the following formula (7) and formula (8),

[0070]

[0071]

[0072] Among them, f 0 (ξ)=16Wξ 2 (1-ξ) 2 is the double potential well function describing the phase interface, W is the energy barrier of the phase change interface; κ is the gradient energy coefficient controlled by the surface energy anisotropy; α is the positive electrode charge transfer coefficient, is the activation overpotential, E eq are the solid phase potential at the interface, the liquid phase potential at the interface, and the thermodynamic equilibrium potential, respectively. is the activity of lithium ions; F, R, and T are Faraday constant, gas state constant, and system temperature, respectively;

[0073] The mass transfer equation of lithium ions in the electrolyte is:

[0074]

[0075] In the above formula, To solve the vector differential operator for scalar gradient and vector divergence, is the concentration of lithium ions, is the diffusion coefficient of lithium ions in the electrolyte, C s is the concentration of lithium atoms in the metallic lithium solid phase;

[0076] Liquid phase potential distribution equation in electrolyte:

[0077]

[0078] The liquid phase potential distribution is obtained by solving the above Poisson equation (10).

[0079] S23: Establishing a judgment function for the local deactivation of metallic lithium;

[0080] Here, a harmonic function is used to construct the local deactivation judgment function f d , f d The value of 1 or 0 refers to fully active metallic lithium and fully deactivated metallic lithium, respectively, which is determined by solving the following Poisson equation:

[0081]

[0082] The state coefficient σ of the region where fully active metallic lithium exists is set to 100, and the state coefficient σ of the region where fully active and transitionally active metallic lithium does not exist is set to 1. d The boundary condition on the lithium metal side is 1, and the boundary condition on the electrolyte side is 0; by solving the Poisson equation, the local deactivation judgment function f can be determined d ,

[0083] S24: Construct a finite element analysis model of the metal lithium negative electrode and calculate and solve the electrode discharge process;

[0084] A two-dimensional finite element analysis model was constructed in the finite element calculation software. Taking COMSOL Multiphysics software as an example, a 60 micron × 60 micron calculation domain was created, and the initial structure of the rod-shaped metal lithium deposit was imported into the calculation domain. Then, the corresponding multi-physics field was created, and the phase field equations of the metal lithium and electrolyte, the mass transfer equations of lithium ions in the electrolyte, and the liquid phase potential distribution equation in the electrolyte were set respectively, and the judgment function of the local deactivation of metal lithium was created. After that, the calculation domain was meshed, and the transient results of the model could be solved, and the discharge time was calculated to be 600 seconds.

[0085] S25: Obtain the distribution of active metallic lithium and inactive “dead lithium”, as well as the remaining effective cycle capacity, to predict the working state of the metallic lithium negative electrode.

[0086] The distribution of active metallic lithium is determined by the following formula:

[0087] c a =f d ξ (12)

[0088] The distribution of inactive "dead lithium" is determined by the following formula:

[0089] c d=(1-f d )ξ (13)

[0090] In the above formula, c a 、c d The distribution diagram corresponds to the distribution of active metallic lithium and inactive "dead lithium". From the calculation results, it can be clearly seen that in the later stage of the extraction of metallic lithium, the columnar metallic lithium has a problem of uneven dissolution rate during the process of dissolving and thinning, and causes the columnar metallic lithium to dissolve at the waist, causing the top metallic lithium to detach from the bottom current collector and unable to exchange electrons with the outside world to contribute capacity. This part of the inactivated electrode material causes the capacity loss of the metallic lithium negative electrode.

[0091] In addition, the remaining effective cycle capacity Q eff Determined by the following formula:

[0092] Q eff =∫ V qf d ξc s dV (14)

[0093] Capacity lost by inactive “dead lithium” Q loss Determined by the following formula:

[0094] Q loss =∫ V q(1-f d )ξc s dV (15)

[0095] Combined with the initial capacity corresponding to the initial structure, it can be calculated that the loss capacity accounts for 6% of the initial capacity, that is, the theoretical coulombic efficiency of the metal lithium negative electrode after considering the formation process of inactive "dead lithium" is 94%.

[0096] Embodiment 2

[0097] This embodiment simulates and analyzes the positive electrode structure collapse failure process of the lithium cobalt oxide positive electrode material of a lithium-ion battery during overcharging, and will obtain the distribution of active lithium cobalt oxide and inactive lithium cobalt oxide, as well as the remaining effective cycle capacity, to predict the working state of the lithium cobalt oxide positive electrode. Figure 3 The flowchart of this embodiment includes the following steps:

[0098] S31: Determine the initial structure of the lithium cobalt oxide cathode;

[0099] Looking at the particle size distribution of the lithium cobalt oxide positive electrode material used in the experiment, the average particle size is 10 microns. Referring to the morphology of the surface of the coated lithium cobalt oxide positive electrode plate photographed by a scanning electron microscope, the initial structure of the lithium cobalt oxide positive electrode here is a close-packed array of 10-micron spheres.

[0100] S32: Establish an electrochemical numerical model of the charging process of lithium cobalt oxide cathode;

[0101] The charging process of lithium cobalt oxide positive electrode, namely LiCoO 2 Lithium removal to generate Li 0.5 CoO 2 During the process, when the lithium cobalt oxide positive electrode is overcharged, excess lithium will be released from the lithium cobalt oxide. x CoO 2 When x is less than 0.4, Li x CoO 2 It will decompose into cobalt oxide and be accompanied by the precipitation of oxygen. This example explores the overcharge process of the lithium cobalt oxide positive electrode process. The evolution of the solid-liquid interface here can be ignored. Therefore, the electrochemical numerical model here will be established based on the finite element battery simulation method based on the porous electrode theory. The equations involved in the electrochemical numerical model are as follows:

[0102] The mass transfer equation of lithium ions in electrode materials and electrolytes:

[0103]

[0104] In the above formula, is the concentration of lithium ions, R l is the electrochemical reaction rate of lithium ions, is the mass transfer flux vector of lithium ions, as shown in the following equation.

[0105]

[0106] D l is the diffusion coefficient of lithium ions in the electrolyte, t + is the migration number of lithium ions, F is the Faraday constant, is the liquid phase potential, σ l is the conductivity of the electrolyte.

[0107] Liquid phase potential distribution equation in electrolyte:

[0108]

[0109] In the above formula, Q l is the charge source corresponding to the electrochemical reaction in the electrolyte. The liquid phase potential distribution is obtained by solving the above Poisson equation.

[0110] Solid phase potential distribution equation in electrode material:

[0111]

[0112] In the above formula, Q sis the charge source corresponding to the electrochemical reaction and boundary current density in the electrode material. The solid phase potential distribution is obtained by solving the above Poisson equation.

[0113] The electrochemical reaction equation of lithium cobalt oxide positive electrode is:

[0114]

[0115] In the above formula, f d is the local inactivation judgment function, f d The value of 1 or 0 refers to fully active lithium cobalt oxide and completely inactivated lithium cobalt oxide, respectively. O is the exchange current density of the electrochemical reaction of the lithium cobalt oxide positive electrode, α a , α c are the charge transfer coefficients of the negative and positive electrodes, respectively. is the activation overpotential, R and T are the gas state constant and system temperature respectively. The relationship between exchange current density and concentration is:

[0116]

[0117] Among them, i 0，ref is the exchange current density at the base concentration, c, c s，ref 、c s，max are the concentration, baseline concentration, and maximum concentration of lithium cobalt oxide solid phase, respectively. s，ref =0.5 cs，max , c 0 is the reference concentration of lithium ions in liquid phase.

[0118] S33: establishing a local deactivation judgment function for lithium cobalt oxide;

[0119] Note that in this model, the deactivation of the lithium cobalt oxide cathode material can be expressed as Li x CoO 2 The x in is used as the judgment condition, that is, x can be used as the function for local deactivation judgment after basic algebraic operations. Therefore, the mapping function f based on physical field variables is used here. d , the physical field is the distribution field of lithium-cobalt atomic ratio x, f d The value of 1 or 0 refers to fully active lithium cobalt oxide and completely inactivated lithium cobalt oxide, respectively. d The value of between 0 and 1 refers to transition-active lithium cobalt oxide. Here, the local deactivation judgment function is defined as follows:

[0120]

[0121] S34: Construct a finite element analysis model of the lithium cobalt oxide positive electrode and calculate and solve the electrode charging process;

[0122] Construct a three-dimensional finite element analysis model in the finite element calculation software. Taking COMSOL Multiphysics software as an example, create a calculation domain of 80 microns × 80 microns × 80 microns, and import the initial structure of neatly and tightly stacked lithium cobalt oxide spheres into the calculation domain. Then create the corresponding multi-physics field, set the mass transfer equation of lithium ions in the electrode material and the electrolyte, the liquid phase potential distribution equation in the electrolyte, the solid phase potential distribution equation in the electrode material, the electrochemical reaction equation of the lithium cobalt oxide positive electrode, and create a judgment function for the local deactivation of lithium cobalt oxide. After that, mesh the calculation domain, and solve the transient results of the model, and calculate the discharge time to be 1200 seconds.

[0123] S35: Obtain the distribution of active lithium cobalt oxide and inactive lithium cobalt oxide, as well as the remaining effective cycle capacity, to predict the working state of the lithium cobalt oxide positive electrode.

[0124] The distribution of active lithium cobalt oxide is determined by the following formula:

[0125] c a =fd c (twenty three)

[0126] The distribution of inactive lithium cobalt oxide is determined by the following formula:

[0127] c d =(1-f d )c (24)

[0128] In the above formula, c a 、c d The distribution diagram corresponds to the distribution of active lithium cobalt oxide and inactive lithium cobalt oxide. From the calculation results, it can be clearly seen that in the later stage of the lithium cobalt oxide charging process, due to the limited diffusion rate of lithium ions inside the lithium cobalt oxide particles, the charging process inside the lithium cobalt oxide particles, that is, the delithiation process, is not consistent. At the edge of some particles, the crystal lithium-cobalt atomic ratio x is less than 0.4. This part of the electrode material is in a transitional active state, and the positive electrode structure has collapsed, resulting in a certain loss of lithium cobalt oxide positive electrode capacity.

[0129] In addition, the remaining effective cycle capacity Q eff Determined by the following formula:

[0130] Q eff =∫ V qf d cdV (25)

[0131] Capacity loss of inactive lithium cobalt oxide Q loss Determined by the following formula:

[0132] Q loss =∫ V q(1-fd )cdV (26)

[0133] Combined with the initial capacity corresponding to the initial structure, it can be calculated that the loss capacity accounts for 2% of the initial capacity, that is, the theoretical coulombic efficiency of the lithium cobalt oxide positive electrode is 98% after considering the collapse of the positive electrode structure due to overcharging of part of the lithium cobalt oxide.

[0134] The present invention aims at the problem of local deactivation of electrode materials, develops a local deactivation judgment function as a correction factor, and realizes efficient and accurate dynamic simulation of the charge and discharge process of the electrode when the electrode material in the lithium battery is locally deactivated. The present invention can efficiently simulate and analyze the local deactivation of electrode materials, including the structural dissociation and expansion peeling problems of negative electrode materials such as graphite and silicon carbon; the formation of "dead lithium" and lithium powdering process in the lithium metal negative electrode; the collapse of the positive electrode structure and the pore blockage caused by gas production in the oxide positive electrode; and the "dead sulfur" problem in the sulfur positive electrode. Through simulation calculation, the distribution of electrode active substances and the remaining effective cycle capacity when the electrode material is locally deactivated can be obtained, and various key data of the battery charge and discharge process can be quantitatively obtained to predict the working state of the lithium battery electrode. Through the simulation analysis method of the present invention, the structure, composition, charge and discharge conditions of the electrode materials can be conveniently screened and optimized in batches, the causes of the local deactivation of the electrode materials, the mechanism of influence on the electrochemical performance and the regulation method can be quantitatively and accurately explored and analyzed, and finally the efficient design and optimization of the lithium battery electrode can be realized.

Claims

1. A simulation analysis method for lithium battery electrodes considering local deactivation of electrode materials. It is characterized in that The lithium battery electrode simulation analysis comprises the following steps: S1: Determine the initial structure of the electrode material; S2: Establish an electrochemical numerical model of the electrode charging or discharging process; The electrochemical numerical model in step S2 is specifically to establish an electrochemical numerical model of the discharge process of the metal lithium negative electrode; the discharge process of the metal lithium negative electrode, that is, the dissolution process of the metal lithium deposit, is accompanied by significant surface morphology evolution, so the electrochemical numerical model here will be established based on the phase field theory; the equations involved in the electrochemical numerical model are as follows: Phase field equation of metallic lithium and electrolyte: In the above formula (6), ξ is the phase field order parameter, and the values ​​of ξ are 1 and 0, respectively, indicating the metallic lithium phase and the electrolyte phase; f d is the local inactivation judgment function, f d The value of 1 or 0 refers to fully active metallic lithium and completely deactivated metallic lithium, respectively; L σ is the phase field interface mobility; L η is the reaction rate prefactor; h(ξ)=ξ 3 (6ξ 2 -15ξ+10) is an interpolation function; F S is the phase field surface energy term, F R F is the phase-field electrochemical reaction kinetic term coupled to the Butler-Volmer electrochemical kinetic equation. S and F R Given by the following formula (7) and formula (8), Among them, f 0 (ξ)=16Wξ 2 (1-ξ) 2 is the double potential well function describing the phase interface, W is the energy barrier of the phase change interface; κ is the gradient energy coefficient controlled by the surface energy anisotropy; α is the positive electrode charge transfer coefficient, is the activation overpotential, are the solid phase potential at the interface, the liquid phase potential at the interface, and the thermodynamic equilibrium potential, respectively. is the activity of lithium ions; F, R, and T are Faraday constant, gas state constant, and system temperature, respectively; The mass transfer equation of lithium ions in the electrolyte is: In the above formula, To solve the vector differential operator for scalar gradient and vector divergence, c L i+ is the concentration of lithium ions, is the diffusion coefficient of lithium ions in the electrolyte, c s is the lithium atom concentration in the solid phase of metallic lithium; the liquid phase potential distribution equation in the electrolyte: The liquid phase potential distribution is obtained by solving the above Poisson equation (10); The electrochemical numerical model in step S2 is specifically to establish an electrochemical numerical model of the lithium cobalt oxide positive electrode charging process; The charging process of lithium cobalt oxide positive electrode, namely LiCoO 2 Lithium removal to generate Li 0.5 CoO 2 During the process, when the lithium cobalt oxide positive electrode is overcharged, excess lithium will be released from the lithium cobalt oxide. x CoO 2 When x is less than 0.4, Li x CoO 2 It will decompose into cobalt oxide, accompanied by the precipitation of oxygen; the overcharge process of the lithium cobalt oxide positive electrode process is explored, and the evolution of the solid-liquid interface here can be ignored, so the electrochemical numerical model here will be established based on the finite element battery simulation method based on the porous electrode theory; the equations involved in the electrochemical numerical model are as follows: The mass transfer equation of lithium ions in electrode materials and electrolytes: In the above formula, is the concentration of lithium ions, R i is the electrochemical reaction rate of lithium ions, is the mass transfer flux vector of lithium ions, as shown in the following equation; D l is the diffusion coefficient of lithium ions in the electrolyte, t + is the migration number of lithium ions, F is the Faraday constant, is the liquid phase potential, σ is the electrolyte conductivity; Liquid phase potential distribution equation in electrolyte: In the above formula, Q l is the charge source corresponding to the electrochemical reaction in the electrolyte; the liquid phase potential distribution is obtained by solving the above Poisson equation (18); Solid phase potential distribution equation in electrode material: In the above formula, Q s is the charge source corresponding to the electrochemical reaction and boundary current density in the electrode material; the solid phase potential distribution is obtained by solving the above Poisson equation; The electrochemical reaction equation of lithium cobalt oxide positive electrode is: In the above formula, f d is the local inactivation judgment function, f d The value of 1 or 0 refers to fully active lithium cobalt oxide and completely inactivated lithium cobalt oxide respectively; i 0 is the exchange current density of the electrochemical reaction of the lithium cobalt oxide positive electrode, α a , α c are the charge transfer coefficients of the negative and positive electrodes, respectively. is the activation overpotential, R and T are the gas state constant and system temperature respectively; the relationship between exchange current density and concentration is: Among them, i 0，ref is the exchange current density at the base concentration, c, c s，ref 、c s，max are the concentration, baseline concentration, and maximum concentration of lithium cobalt oxide solid phase, respectively. s，ref =0.5c s，max , c 0 is the reference concentration of lithium ions in liquid phase; S3: Establishing the judgment function of local inactivation of electrode materials; S23: Establishing a judgment function for local deactivation of metallic lithium; Here, a harmonic function is used to construct the local deactivation judgment function f d , f d The value of 1 or 0 refers to fully active metallic lithium and fully deactivated metallic lithium, respectively, which is determined by solving the following Poisson equation: The state coefficient σ of the region where fully active metallic lithium exists is set to 100, and the state coefficient σ of the region where fully active and transitionally active metallic lithium does not exist is set to 1; f d The boundary condition on the lithium metal side is 1, and the boundary condition on the electrolyte side is 0; by solving the Poisson equation, the local deactivation judgment function f can be determined d ; S33: establishing a local deactivation judgment function for lithium cobalt oxide; Note that in this model, the deactivation of the lithium cobalt oxide cathode material is based on Li x CoO 2 The x in the equation is used as the judgment condition, that is, x is used as the function for local deactivation judgment after basic algebraic operations; therefore, the mapping function f based on the physical field variables is used here. d , the physical field is the distribution field of lithium-cobalt atomic ratio x, f d The value of 1 or 0 refers to fully active lithium cobalt oxide and completely inactivated lithium cobalt oxide, respectively. d When the value of is between 0 and 1, it refers to transition-active lithium cobalt oxide; here, the local deactivation judgment function is defined as follows: S4: Construct a finite element analysis model of the electrode material and calculate and solve the electrode charging or discharging process; S5: Obtain the distribution of electrode active substances and the remaining effective cycle capacity, predict the working state of the electrode, and guide the design and optimization of the electrode.

2. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The electrode material in step S1 includes at least one of lithium metal, lithium indium alloy, lithium tin alloy, lithium copper alloy, lithium magnesium alloy, lithium aluminum alloy, lithium sodium alloy, lithium potassium alloy, lithium carbon alloy, lithium silicon alloy, silicon carbon alloy, amorphous carbon, graphite, soft carbon, hard carbon, carbon fiber, lithium titanate, lithium iron phosphate, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, lithium oxide, lithium peroxide, sulfur, and lithium sulfide.

3. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 2, It is characterized in that The initial structure of the electrode material includes at least one of thickness, surface roughness, particle morphology, average particle size, particle size distribution, average pore size, pore size distribution, average surface curvature, surface curvature distribution, porosity, and specific surface area.

4. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The electrochemical numerical model in step S2 includes at least one of the following: an electrochemical reaction equation at the interface between the electrode material and the electrolyte, a mass transfer equation for lithium ions in the electrode material and the electrolyte, a migration vector equation for lithium ions in the electrode material and the electrolyte, a solid phase potential distribution equation in the electrode material and the current collector, a liquid phase potential distribution equation in the electrode material and the electrolyte, a heat transfer equation for heat in the electrode material and the electrolyte, an interface migration vector equation at the interface between the electrode material and the electrolyte, a level set equation at the interface between the electrode material and the electrolyte, a phase field equation between the two phases of the electrode material and the electrolyte, and a fluid mass transfer equation for the electrolyte.

5. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The local deactivation determination function f in step S3 is composed of the harmonic function f H , mapping function f based on physical field variables P One or more of them are obtained through basic algebraic operations; the deactivation judgment function is used to determine whether the electrode material at a specified spatial position is in a fully active state, a completely inactivated state or a transitionally active state at a specified time; the deactivation judgment function will be directly inserted into the various equations in the electrochemical numerical model that need to consider the local deactivation of the electrode material through basic algebraic operations as a correction factor.

6. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 5, It is characterized in that The value range of the deactivation judgment function f is a closed interval in the real number domain, with [f 1 , f 2 ] indicates that the appropriate deactivation judgment tolerance δ is set 1 ,δ 2 , the value is a non-negative real number, or set an appropriate deactivation judgment critical ratio λ 1 , 2 After that, the value is a positive real number, then the local inactivation judgment function f can be used to determine the activity state of the electrode material at the specified spatial position.

7. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 6, It is characterized in that The activity state of the electrode material is determined by the local inactivation determination function f: When f ≥ f 2 -δ 2 , or f ≥ λ 2 f 2 When , the electrode material at this location is in a fully active state; When f≤f 1 +δ 1 , or f≤λ 1 f 1 When , the electrode material at this location is completely inactivated; When f 1 +δ 1 <f<f 2 -δ 2 , or λ 1 f 1 <f<λ 2 f 2 When , the electrode material is in a transitional active state.

8. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The harmonic function f of one of the local deactivation determination functions in step 3 H , can be determined by solving the following Poisson equation: In the above formula To solve the vector differential operator for scalar gradient and vector divergence, the variable σ is the state coefficient used to distinguish electrode materials with different active states. The state coefficient σ of the region where the fully active electrode material exists can be set to σ 2 , there is no region state coefficient σ of fully active and transitionally active electrode materials 1 , harmonic local deactivation decision function f H The solution process requires the following boundary conditions to be set at the boundary where fully active electrode material exists: f H =f H2 (2) At the boundary where fully active and transitionally active electrode materials do not exist: f H =f H1 (3) Among them, as a harmonic function f H f at the upper and lower bounds of the range H2 、f H1 Can be set to any two real numbers.

9. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The mapping function f of one of the local deactivation judgment functions in step 3 based on the physical field variables P , wherein the physical field variables include at least one variable selected from the group consisting of solid phase potential, liquid phase potential, lithium ion concentration, lithium ion chemical potential, lithium ion electrochemical potential, and electrolyte concentration; the mapping function f based on the physical field variables P Determined by the following method: Determine the value range of the physical field variable [P 1 , P 2 ], as a mapping function based on physical field variables f P The domain of f P The upper and lower bounds of the range f P2 、f P1 ; Through one or more basic algebraic operations of step function, step function with smooth polynomial, inverse tangent function, hyperbolic tangent function, logistic function, sigmoid function, the value range of the physical field variable [P 1 , P 2 ] to f P The upper and lower bounds of the range [f P1 , f P2 The mapping function f P ; The electrode active material distribution c in step S5 a Determined by the following formula: c a =fc(4) In the above formula, f is the local deactivation judgment function with a range of [0, 1], and the fully active electrode material corresponds to f = 1, c is the concentration distribution of the electrode material, and the obtained electrode active material distribution c a It is a function of spatial position and time.

10. The lithium battery electrode simulation analysis method considering local deactivation of electrode materials according to claim 1, It is characterized in that The remaining effective cycle capacity Q in step S5 eff Determined by the following formula: Q eff =∫ v qc a dV (5) In the above formula, c a is the distribution of electrode active materials, q is the molar capacity of the electrode material, V is the total volume of the electrode material, and the remaining effective cycle capacity Q is obtained. eff is a function of time.

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