Method for discriminating thermal runaway process of lithium ion battery

Abstract

The invention discloses a lithium ion battery thermal runaway process discrimination method, and belongs to the technical field of lithium ion battery thermal safety, and the method comprises the following steps: S1, building an experiment scene and a model; s2, defining a material of the battery model; s3, establishing a heat transfer physical field of the battery according to the experimental scene and the battery model; s4, establishing an electrochemical-thermal coupling model according to the surface temperature of the lithium ion battery in the thermal runaway process and the heat production power in the thermal runaway process; and S5, observing the temperature change of the lithium ion battery in the thermal runaway process according to a simulation result. According to the method for judging the thermal runaway process of the lithium ion battery, expensive experimental equipment and raw materials do not need to be input, experiments with risks in reality can be safely and accurately verified, and repeatability and reliability of experimental results are ensured while the experimental period is shortened.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal safety of lithium-ion batteries, and in particular relates to a method for distinguishing a thermal runaway process of a lithium-ion battery. Background Art

[0002] Environmental and energy issues are becoming increasingly prominent worldwide. China has taken many measures to save energy and reduce emissions, including policies to guide the development of new energy vehicles. New energy vehicles are in a period of rapid development in my country and even in the world. Lithium-ion batteries have become the most competitive power source for new energy vehicles due to their high energy density, long life, relatively mature technology and many other advantages. Users' concerns about the range of new energy vehicles have prompted battery technology workers to work towards high energy density and large capacity power batteries. However, the improvement of energy density comes at the expense of safety. The safety of lithium-ion battery systems is one of the important issues currently restricting the popularization of new energy vehicles. For the currently widely used lithium-ion power batteries, thermal runaway is the core scientific issue of their safety issues.

[0003] Thermal runaway of lithium-ion batteries refers to an overheating phenomenon in which a chain reaction of exothermic reactions occurs inside the battery, causing a sharp change in the rate of temperature rise of the battery. It is a serious failure of lithium-ion batteries under extreme conditions and may cause fire or explosion. The existing technology for identifying thermal runaway of lithium-ion batteries usually involves monitoring parameters such as battery temperature, voltage, current and internal pressure.

[0004] However, in actual situations, the thermal runaway process of lithium-ion batteries reacts violently, generates high heat power, and easily produces large internal and external temperature gradients. It is difficult to obtain the core temperature of the lithium-ion battery during the thermal runaway process by measurement. In addition, thermal runaway experiments not only have certain safety risks, but also have a long experimental cycle, and the reliability of the experimental results is difficult to verify repeatedly. Therefore, it is urgent to propose a method to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for distinguishing the thermal runaway process of a lithium-ion battery, which does not require the investment of expensive experimental equipment and raw materials, can safely and accurately verify experiments that are risky in reality, shorten the experimental cycle, and ensure the repeatability and reliability of the experimental results.

[0006] To achieve the above object, the present invention provides a method for determining a thermal runaway process of a lithium-ion battery, comprising the following steps:

[0007] S1. Establish experimental scenarios and models;

[0008] Determine the experimental scenario according to the purpose and scenario of the experiment, select the geometric dimensions of the battery to establish a battery model, and set a temperature point probe in the middle of one side of the battery model surface;

[0009] S2. Define the materials of the battery model;

[0010] Open the simulation software and import the positive electrode, negative electrode and electrolyte from the material library;

[0011] S3. Establish the heat transfer physical field of the battery;

[0012] In the simulation software, one side of the battery cell is selected as the heat source surface. The heat power of the heat source surface is W. An external heat source is used for heat transfer on the heat source surface. For the other surfaces, natural convection heat transfer is performed with the ambient temperature. The initial external heat source temperature is the same as the ambient temperature, which is set as T0, and the control equation for battery heat transfer is established;

[0013] S4. Establish an electrochemical-thermal coupling model;

[0014] First, an electrochemical model is established to describe the charge transfer, ohmic loss, ion concentration transfer and side reactions inside the battery. Second, a thermal model is established to describe the temperature distribution and heat transfer of the battery. Third, the electrochemical model and the thermal model are coupled through temperature.

[0015] S5. Observe the temperature changes of lithium-ion batteries during thermal runaway through simulation results.

[0016] Preferably, the positive electrode material in step S2 includes the following properties: electrical conductivity, diffusion coefficient, thermal conductivity, constant pressure heat capacity, density, equilibrium potential, temperature derivative of equilibrium potential, reference concentration, maximum state of charge of electrode, minimum state of charge of electrode, volume change, state of charge, ambient temperature and heat source temperature.

[0017] Preferably, at the beginning of step S3, the temperature of the entire battery is uniform and equal to the ambient temperature. Assume that the heat Q introduced from the outside and the heat Q of the chemical reaction inside the battery are h One part is used for battery internal energy consumption, and the other part is used for surface heat dissipation. Establish the governing equations for battery heat transfer:

[0018]

[0019] Where ρ represents density in kg / m 3 , c p represents the specific heat at constant pressure, with the unit of J / (Kg·K); T represents the absolute temperature, with the unit of K; u represents the velocity vector, with the unit of m / s; q represents the heat flow vector, with the unit of W / m.

[0020] Preferably, step S4 comprises the following steps:

[0021] S401. Calculate the heat Q generated by charge transfer JH, satisfying the following energy equation:

[0022]

[0023] Among them, i I represents electrolyte, i S Represents the electrode current density vector, in A / m 2 ; φ I They represent the electrolyte potential, φ S Represents the electrode potential, in V;

[0024] S402. Calculate the heat Q generated by the electric energy released during the internal short circuit process i , satisfying the following energy equation:

[0025]

[0026] Among them, W i Indicates the active material content; H i represents the reaction enthalpy; c i represents the dimensionless concentration of active material;

[0027] S403. Calculate the heat Q generated by ohmic loss m , satisfying the following energy equation:

[0028]

[0029] S404. Calculate the heat Q generated by ion concentration transfer mix , satisfying the following energy equation:

[0030]

[0031] Where F is the Faraday constant, in m 2 / s; E eq Indicates the equilibrium voltage, in V; D S Represents the electrode ion diffusion coefficient, in m 2 / s;c S Indicates the electrolyte salt concentration in mol / m 3 ; ε S Indicates the electrode porosity, in %;

[0032] S405. Calculate the heat Q released by the side reaction rec , satisfying the following energy equation:

[0033] Q rec =ΣQ i =Q SEI +Q a +Q c +Q ec +Qe +Q PVDF ;

[0034] Among them, the heat generated by the decomposition of the SEI film is Q SEI , the heat generated by the reaction between the negative electrode and the electrolyte is Q a , the heat generated by the reaction between the positive electrode and the electrolyte is Q c , the heat generated by the internal short circuit is Q ec , the heat generated by the decomposition of the electrolyte is Q e The heat generated by the reaction with the binder is Q PVDF The above heat units are W / m 3 ;

[0035] S406. Calculate the internal chemical reaction heat Q h , satisfying the following energy equation:

[0036] Q h =Q JH +∑ m a V,m Q m +Q mix +Q rec ;

[0037] Among them, a V,m Indicates the reaction activity of the reactant, dimensionless unit; Q JH represents the heat generated by charge transfer, Q m represents the heat generated by ohmic loss, Q mix represents the heat generated by the transfer of ion concentration, Q rec Represents the heat released by side reactions.

[0038] Therefore, the present invention adopts the above-mentioned method for distinguishing the thermal runaway process of a lithium-ion battery. Compared with the prior art, the present invention has the following significant beneficial effects:

[0039] (1) The present invention completes the experiment through simulation, which does not require expensive experimental equipment and a large amount of raw materials, and can significantly reduce research costs;

[0040] (2) The simulation of the present invention can be carried out quickly without waiting for the experimental equipment to be prepared and the experimental process to be completed, thus reducing time consumption;

[0041] (3) The simulation of the present invention can be repeated, which helps to verify the stability and reliability of the results;

[0042] (4) The present invention does not generate waste and pollutants during the simulation process, and has little impact on the environment;

[0043] (5) The simulation process of the present invention can accurately control various parameters and explore experimental conditions that are difficult to achieve or dangerous in reality.

[0044] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a model diagram of the A side of the battery cell surface of a method for distinguishing a thermal runaway process of a lithium-ion battery according to the present invention;

[0046] Figure 2 It is a model diagram of the B side of the battery cell surface of a method for distinguishing a thermal runaway process of a lithium-ion battery according to the present invention;

[0047] Figure 3 It is a comparison diagram of simulation data and experimental data of a method for distinguishing a thermal runaway process of a lithium-ion battery according to the present invention; Figure 3 (a) shows the trend of battery temperature changing with time in the first experiment; Figure 3 (b) shows the trend of battery temperature changing with time in the second experiment Figure 3 (c) in the figure shows the trend of battery temperature changing with time in the third experiment.

[0048] Reference numerals

[0049] 1. Heat source surface; 2. Temperature probe. DETAILED DESCRIPTION

[0050] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0051] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0052] Embodiment 1

[0053] like Figure 1-2 As shown, a method for determining a thermal runaway process of a lithium-ion battery of the present invention comprises the following steps:

[0054] S1. Establish experimental scenarios and models;

[0055] The experimental scenario is determined according to the purpose and scenario of the experiment, the geometric dimensions of the battery are selected to establish a battery model, and a temperature point probe 2 is provided in the middle of the B side of the battery model surface;

[0056] S2. Define the materials of the battery model;

[0057] Open the simulation software and import materials such as positive electrode, negative electrode and electrolyte from the material library. The positive electrode material includes the following properties: conductivity, diffusion coefficient, thermal conductivity, constant pressure heat capacity, density, equilibrium potential, equilibrium potential temperature derivative, reference concentration, electrode maximum state of charge, electrode minimum state of charge, volume change, state of charge, ambient temperature and heat source temperature, as shown in the following table:

[0058] Table 1 Properties of positive electrode materials for lithium-ion batteries

[0059]

[0060] S3. Establish the heat transfer physical field of the battery;

[0061] In the simulation software, select the A surface of the battery cell as the heat source surface 1. The heat power of the heat source surface 1 is W. For the heat source surface 1, an external heat source is used for heat transfer. For the other surfaces, natural convection heat exchange is performed with the ambient temperature. The initial external heat source temperature is the same as the ambient temperature and is set to T0. Initially, the temperature of the entire battery is uniform and equal to the ambient temperature. Let the external heat Q and the internal chemical reaction heat Q h One part is used for battery internal energy consumption, and the other part is used for surface heat dissipation. Establish the governing equations for battery heat transfer:

[0062]

[0063] Where ρ represents density in kg / m 3 , c p represents the specific heat capacity at constant pressure, in J / (Kg·K); T represents the absolute temperature, in K; u represents the velocity vector, in m / s; q represents the heat flow vector, in W / m;

[0064] S4. Establish an electrochemical-thermal coupling model;

[0065] During the operation of the battery, the electrochemical reaction will generate heat, which will affect the temperature distribution of the battery, and the temperature change will in turn affect the rate and efficiency of the electrochemical reaction. This interaction needs to be described by an electrochemical-thermal coupling model. To establish an electrochemical-thermal coupling model, first, an electrochemical model is established to describe the charge transfer, ohmic loss, ion concentration transfer and side reactions inside the battery. Second, a thermal model is established to describe the temperature distribution and heat transfer of the battery. Third, the electrochemical model and the thermal model are coupled through temperature. The specific content includes the following steps:

[0066] S401. Calculate the heat Q generated by charge transfer JH , satisfying the following energy equation:

[0067]

[0068] Among them, i I represents electrolyte, i S Represents the electrode current density vector, in A / m 2 ; φ I They represent the electrolyte potential, φ S Represents the electrode potential, in V;

[0069] S402. Calculate the heat Q generated by the electric energy released during the internal short circuit process i , satisfying the following energy equation:

[0070]

[0071] Among them, W i Indicates the active material content; H i represents the reaction enthalpy; c i represents the dimensionless concentration of active material;

[0072] S403. Calculate the heat Q generated by ohmic loss m , satisfying the following energy equation:

[0073]

[0074] S404. Calculate the heat Q generated by ion concentration transfer mix , satisfying the following energy equation:

[0075]

[0076] Where F is the Faraday constant, in m 2 / s; E eq Indicates the equilibrium voltage, in V; D S Represents the electrode ion diffusion coefficient, in m 2 / s;c S Indicates the electrolyte salt concentration in mol / m 3 ; ε S Indicates the electrode porosity, in %;

[0077] S405. Calculate the heat Q released by the side reaction rec , satisfying the following energy equation:

[0078] Q rec =ΣQ i =Q SEI +Q a +Q c +Q ec +Q e +Q PVDF ;

[0079] Among them, the heat generated by the decomposition of the SEI film is Q SEI , the heat generated by the reaction between the negative electrode and the electrolyte is Q a , the heat generated by the reaction between the positive electrode and the electrolyte is Q c , the heat generated by the internal short circuit is Q ec , the heat generated by the decomposition of the electrolyte is Q e The heat generated by the reaction with the binder is Q PVDF The above heat units are W / m 3 ;

[0080] S405. Calculate the internal chemical reaction heat Q h , satisfying the following energy equation:

[0081] Q h =Q JH +∑ m a V,m Q m +Q mix +Q rec ;

[0082] Among them, a V,m Indicates the reaction activity of the reactant, dimensionless unit; Q JH represents the heat generated by charge transfer, Q m represents the heat generated by ohmic loss, Q mix represents the heat generated by the transfer of ion concentration, Q rec Represents the heat released by side reactions.

[0083] S5. Observe the temperature change of lithium-ion batteries during thermal runaway through simulation results;

[0084] like Figure 3 As shown in Figure 1, the trend of battery temperature changing with time in three different experiments or simulations is shown. Each figure consists of two curves: one represents the actual experimental data (solid line) and the other represents the simulation data (dashed line). The experimental data and the simulation data show a high consistency: Figure 3 As shown in (a), in the first experiment, the temperature starts from 0℃, gradually increases to about 120℃, and then gradually cools back to room temperature. The temperature reaches a peak of about 140℃ around 2000 seconds and then gradually decreases; Figure 3 In the second experiment shown in (b), the temperature starts from 0°C, rises rapidly to over 500°C at around 1200 seconds, then drops rapidly and stabilizes at about 50°C; Figure 3In the third experiment shown in (c), the temperature starts from 0°C, then rises rapidly to about 600°C in about 500 seconds, then drops rapidly, and finally remains at a low level, slowly decreasing to about 30°C. These results show that the method for distinguishing the thermal runaway process of a lithium-ion battery of the present invention can simulate the temperature change of the thermal runaway of a lithium-ion battery under different conditions, and shows a high consistency with the experimental data.

[0085] It is worth noting that the contents not elaborated in detail in the present invention are all prior art and are well known to those skilled in the art.

[0086] Therefore, the present invention adopts the above-mentioned method for distinguishing the thermal runaway process of a lithium-ion battery, which does not require the investment of expensive experimental equipment and raw materials, and can safely and accurately verify experiments that are risky in reality, thereby shortening the experimental cycle and ensuring the repeatability and reliability of the experimental results.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for determining thermal runaway process of a lithium-ion battery, characterized in that: The following steps are involved: S1. Establish experimental scenarios and models; Determine the experimental scenario according to the purpose and scenario of the experiment, select the geometric dimensions of the battery to establish a battery model, and set a temperature point probe in the middle of one side of the battery model surface; S2. Define the materials of the battery model; Open the simulation software and import the positive electrode, negative electrode and electrolyte from the material library; S3. Establish the heat transfer physical field of the battery; In the simulation software, one side of the battery cell is selected as the heat source surface. The heat power of the heat source surface is W. An external heat source is used for heat transfer on the heat source surface. For the other surfaces, natural convection heat transfer is performed with the ambient temperature. The initial external heat source temperature is the same as the ambient temperature, which is set as T0, and the control equation for battery heat transfer is established; S4. Establish an electrochemical-thermal coupling model; First, an electrochemical model is established to describe the charge transfer, ohmic loss, ion concentration transfer and side reactions inside the battery. Second, a thermal model is established to describe the temperature distribution and heat transfer of the battery. Third, the electrochemical model and the thermal model are coupled through temperature. S5. Observe the temperature changes of lithium-ion batteries during thermal runaway through simulation results.

2. A method for determining thermal runaway process of a lithium-ion battery according to claim 1, characterized in that: The positive electrode material in step S2 includes the following properties: electrical conductivity, diffusion coefficient, thermal conductivity, constant pressure heat capacity, density, equilibrium potential, temperature derivative of equilibrium potential, reference concentration, maximum state of charge of electrode, minimum state of charge of electrode, volume change, state of charge, ambient temperature and heat source temperature.

3. A method for determining thermal runaway process of a lithium-ion battery according to claim 1, characterized in that: Initially in step S3, the temperature of the entire battery is uniform and equal to the ambient temperature. Suppose the external heat Q and the internal chemical reaction heat Q h One part is used for battery internal energy consumption, and the other part is used for surface heat dissipation. Establish the governing equations for battery heat transfer: Where ρ represents density in kg / m 3 , c p represents the specific heat at constant pressure, with the unit of J / (Kg·K); T represents the absolute temperature, with the unit of K; u represents the velocity vector, with the unit of m / s; q represents the heat flow vector, with the unit of W / m.

4. A method for determining thermal runaway process of a lithium-ion battery according to claim 1, characterized in that: Step S4 includes the following steps: S401. Calculate the heat Q generated by charge transfer JH , satisfying the following energy equation: Among them, i I represents electrolyte, i S Represents the electrode current density vector, in A / m 2 ; φ I They represent the electrolyte potential, φ S Represents the electrode potential, in V; S402. Calculate the heat Q generated by the electric energy released during the internal short circuit process i , satisfying the following energy equation: Among them, W i Indicates the active material content; H i represents the reaction enthalpy; c i represents the dimensionless concentration of active material; S403. Calculate the heat Q generated by ohmic loss m , satisfying the following energy equation: S404. Calculate the heat Q generated by ion concentration transfer mix , satisfying the following energy equation: Where F is the Faraday constant, in m 2 / s; E eq Indicates the equilibrium voltage, in V; D S Represents the electrode ion diffusion coefficient, in m 2 / s;c S Indicates the electrolyte salt concentration in mol / m 3 ; ε S Indicates the electrode porosity, in %; S405. Calculate the heat Q released by the side reaction rec , satisfying the following energy equation: Q rec =ΣQ i =Q SEI +Q a +Q c +Q ec +Q e +Q PVDF ; Among them, the heat generated by the decomposition of the SEI film is Q SEI , the heat generated by the reaction between the negative electrode and the electrolyte is Q a , the heat generated by the reaction between the positive electrode and the electrolyte is Q c , the heat generated by the internal short circuit is Q ec , the heat generated by the decomposition of the electrolyte is Q e The heat generated by the reaction with the binder is Q PVDF The above heat units are W / m 3 ; S406. Calculate the internal chemical reaction heat Q h , satisfying the following energy equation: Q h =Q JH +∑ m a V,m Q m +Q mix +Q rec ; Among them, a V,m Indicates the reaction activity of the reactant, dimensionless unit; Q JH represents the heat generated by charge transfer, Q m represents the heat generated by ohmic loss, Q mix represents the heat generated by the transfer of ion concentration, Q rec Represents the heat released by side reactions.

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