CN110633496B - A Method for Determining Thermal Stress and Temperature During Discharge of Li-ion Batteries Based on Thermal-Mechanical Coupled Model - Google Patents

A Method for Determining Thermal Stress and Temperature During Discharge of Li-ion Batteries Based on Thermal-Mechanical Coupled Model Download PDF

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CN110633496B
CN110633496B CN201910743313.9A CN201910743313A CN110633496B CN 110633496 B CN110633496 B CN 110633496B CN 201910743313 A CN201910743313 A CN 201910743313A CN 110633496 B CN110633496 B CN 110633496B
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王青松
梅文昕
段强领
孙金华
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Abstract

The invention discloses a method for determining thermal stress and temperature in a lithium ion battery discharging process based on a thermal-force coupling model, which relates to the field of lithium ion battery thermal expansion and stress calculation, and the method establishes a thermal expansion model by using a lithium ion battery three-dimensional geometric scale, and comprises the following specific steps: (1) Selecting a single battery cell, and acquiring three-dimensional geometric parameters, mechanical and thermodynamic initial parameters of the single battery cell; (2) Establishing a thermal-force coupling model of a three-dimensional electrical core scale according to a thermal expansion coefficient, a temperature difference and a coupling mechanism of a stress-strain relation; (3) Testing the battery temperature and the lug temperature, and verifying the effectiveness of the model; (4) The temperature distribution and the expansion displacement and the stress along the x, y and z directions of the battery are obtained. The invention provides a certain guidance basis for the expansion model of the battery cell on the macroscopic scale and the expansion behavior and rupture prediction of the battery cell in the charging and discharging process.

Description

一种基于热-力耦合模型的锂离子电池放电过程中热应力和 温度的确定方法A thermal-mechanical coupling model based on thermal stress and stress during discharge of lithium-ion batteries How to determine the temperature

技术领域technical field

本发明属于锂离子电池热膨胀和应力计算领域,具体涉及一种基于热-力耦合模型的锂离子电池放电过程中热应力和温度的确定方法。The invention belongs to the field of thermal expansion and stress calculation of lithium-ion batteries, and in particular relates to a method for determining thermal stress and temperature during the discharge process of lithium-ion batteries based on a thermal-mechanical coupling model.

背景技术Background technique

锂离子电池因其优异的性能广泛应用于电子设备、电动汽车和储能电站的同时,也出现了较多的安全事故。锂离子电池在充放电过程中,尤其在高倍率的充放电过程中,会出现局部高温的现象,而高温就会引起热膨胀,进而导致热应力的产生,当产生的应力较大时,就会伴随电极材料的破裂、脱落,以及由于电芯膨胀导致的电芯各部分接触不良,最终导致内阻增大、容量衰减、电池失效,因此对电池的应力进行预测可以有效地防止电池失效,对锂离子电池的安全性提供了保障。Lithium-ion batteries are widely used in electronic equipment, electric vehicles and energy storage power stations due to their excellent performance, but there have also been many safety accidents. During the charging and discharging process of lithium-ion batteries, especially in the high-rate charging and discharging process, there will be local high temperature phenomenon, and high temperature will cause thermal expansion, which will lead to thermal stress. When the generated stress is large, it will Accompanied by the cracking and falling off of the electrode material, and the poor contact of each part of the battery cell due to the expansion of the battery cell, it will eventually lead to an increase in internal resistance, capacity decay, and battery failure. Therefore, predicting the stress of the battery can effectively prevent battery failure. The safety of lithium-ion batteries is guaranteed.

目前传统的实验方法较难观测到电芯在充放电过程中的膨胀行为以及进行应力的预测,而在电芯宏观尺度上的热膨胀模型由于模型有效性难以检验和模型建立较困难等因素也研究较少。因此,本发明打破这二者的局限性,提出了一种基于热-力耦合模型的锂离子电池放电过程中热应力和温度的确定方法,首先获取电芯几何参数,力学和热力学相关参数,根据热膨胀系数、温差及应力-应变关系的耦合机制建立热-力耦合模型;之后通过实验测定的温度和极耳温度进行模型有效性的验证,确保了模型的精确性;随后得到了电芯沿着x,y,z 三个方向的膨胀位移及应力分布。本发明能够计算锂离子电池在放电过程中电芯所受的应力和温度,并可观测到电芯膨胀行为,可为电芯的膨胀和破裂行为预测提供一定的指导依据。At present, the traditional experimental method is difficult to observe the expansion behavior of the battery cell during the charging and discharging process and predict the stress, and the thermal expansion model on the macro scale of the battery cell is difficult to verify the validity of the model and difficult to establish the model. less. Therefore, the present invention breaks through the limitations of the two, and proposes a method for determining thermal stress and temperature in the lithium-ion battery discharge process based on a thermal-mechanical coupling model. According to the coupling mechanism of thermal expansion coefficient, temperature difference and stress-strain relationship, a thermal-mechanical coupling model was established; then the validity of the model was verified by the experimentally measured temperature and tab temperature to ensure the accuracy of the model; Look at the expansion displacement and stress distribution in the three directions of x, y, and z. The invention can calculate the stress and temperature suffered by the battery core during the discharge process of the lithium ion battery, and can observe the expansion behavior of the battery core, which can provide a certain guiding basis for predicting the expansion and rupture behavior of the battery core.

发明内容SUMMARY OF THE INVENTION

本发明提供了一种基于热-力耦合模型的锂离子电池放电过程中热应力和温度的确定方法,通过建立电芯三维宏观尺度的热-力耦合模型,通过有效地实验验证,最终得到电芯在放电过程中的温度分布,热应力分布以及沿着x,y,z三个方向的膨胀位移分布,揭示了电芯膨胀现象与温度之间的关系。The invention provides a method for determining thermal stress and temperature during the discharge process of lithium-ion batteries based on a thermal-mechanical coupling model. By establishing a three-dimensional macroscopic thermal-mechanical coupling model of the battery cell and through effective experimental verification, the battery is finally obtained. The temperature distribution of the core during the discharge process, the thermal stress distribution and the expansion displacement distribution along the three directions of x, y, and z reveal the relationship between the expansion phenomenon of the core and the temperature.

本发明采用的技术方案为:一种基于热-力耦合模型的锂离子电池放电过程中热应力和温度的确定方法,包含以下步骤:步骤一,选取一种单体电芯,获取其三维几何参数,力学和热力学初始参数;步骤二,根据热膨胀系数、温差及和应力-应变关系的耦合机制,建立三维电芯尺度的热-力耦合模型;步骤三,实验测定电池温度和极耳温度,进行模型的有效性验证;步骤四,得到电池的温度分布和沿着x,y,z三个方向的膨胀位移及应力。The technical scheme adopted in the present invention is: a method for determining thermal stress and temperature in the lithium-ion battery discharge process based on a thermal-mechanical coupling model, including the following steps: Step 1, select a single battery cell, and obtain its three-dimensional geometry Parameters, initial parameters of mechanics and thermodynamics; step 2, according to the coupling mechanism of thermal expansion coefficient, temperature difference and stress-strain relationship, establish a thermal-mechanical coupling model of three-dimensional cell scale; step 3, test the battery temperature and tab temperature, Verify the validity of the model; step 4, obtain the temperature distribution of the battery and the expansion displacement and stress along the three directions of x, y, and z.

步骤二中模型为三维电芯尺度的热-力耦合模型,模型的基本理论包含两个方面,(1) 应力-应变关系(2)能量守恒方程。下面叙述模型的建立过程:The model in step 2 is a thermal-mechanical coupling model of the three-dimensional cell scale. The basic theory of the model includes two aspects, (1) stress-strain relationship (2) energy conservation equation. The process of building the model is described below:

(1)应力-应变关系(1) Stress-strain relationship

将电芯看成一个各向异性的导热体和各向同性的线弹性体。其中应力和温度的耦合通过热膨胀来实现,热应力引起的应变与温度的关系见式(1),其中εij为应变分量,α为热膨胀系数,δij为Dirac delta函数,当i=j时其值为1,否则其值为0,ΔT为温差,由公式可知,当温差越大时,应变越大,因此热应力也越大。Think of the battery as an anisotropic heat conductor and an isotropic linear elastic body. The coupling of stress and temperature is achieved through thermal expansion. The relationship between strain and temperature caused by thermal stress is shown in formula (1), where ε ij is the strain component, α is the thermal expansion coefficient, and δ ij is the Dirac delta function. When i=j Its value is 1, otherwise its value is 0, and ΔT is the temperature difference. According to the formula, when the temperature difference is greater, the strain is greater, so the thermal stress is also greater.

Figure RE-GDA0002214025690000021
Figure RE-GDA0002214025690000021

有热应力存在时应力-应变关系根据式(2)表示,In the presence of thermal stress, the stress-strain relationship is expressed according to formula (2),

Figure RE-GDA0002214025690000022
Figure RE-GDA0002214025690000022

其中σij为应力分量,E为杨氏模量,ν为泊松比。where σ ij is the stress component, E is Young's modulus, and ν is Poisson's ratio.

静水应力(σh)和von Mises应力(σv)由式(3)和式(4)给出:The hydrostatic stress (σ h ) and von Mises stress (σ v ) are given by equations (3) and (4):

Figure RE-GDA0002214025690000023
Figure RE-GDA0002214025690000023

σν=|σrθ| (4)σ ν = |σ rθ | (4)

其中σr为径向应力,σθ为切向应力。where σ r is the radial stress, and σ θ is the tangential stress.

边界条件如下所述:电芯表面四周四个表面设置约束,仅使电池沿厚度方向膨胀,即:The boundary conditions are as follows: constraints are set on the four surfaces around the surface of the battery cell, so that the battery only expands along the thickness direction, namely:

uy=0,uz=0 (5)u y =0, u z =0 (5)

(2)能量守恒方程(2) Energy Conservation Equation

将电芯看成一个各向异性的导热体,热模型控制方程及边界条件列于表1中。其产热遵循Bernardi生热速率方程,见式(7),前者为可逆热,由电极材料熵变产生,取决于熵系数 (dU0/dT)的大小;后者为不可逆热,由电池本身内阻导致。热损失考虑热对流及热辐射。The cell is regarded as an anisotropic heat conductor, and the governing equations and boundary conditions of the thermal model are listed in Table 1. Its heat generation follows the Bernardi heat generation rate equation, see formula (7). The former is reversible heat, which is generated by the entropy change of the electrode material and depends on the entropy coefficient (dU 0 /dT); the latter is irreversible heat, which is generated by the battery itself caused by internal resistance. Heat loss considers heat convection and heat radiation.

表1热模型控制方程及边界条件Table 1 Thermal model governing equations and boundary conditions

Figure RE-GDA0002214025690000031
Figure RE-GDA0002214025690000031

在热模型中,电池等效为一个各向异性的导热体,集总的比热容、密度以及导热系数分别见公式(11)-(14),其中导热系数分为平行于极片(x和z方向)和垂直于极片(y方向:厚度方向)两种。In the thermal model, the battery is equivalent to an anisotropic heat conductor, and the aggregate specific heat capacity, density, and thermal conductivity are shown in formulas (11)-(14) respectively, where the thermal conductivity is divided into direction) and perpendicular to the pole piece (y direction: thickness direction).

Figure RE-GDA0002214025690000032
Figure RE-GDA0002214025690000032

Figure RE-GDA0002214025690000033
Figure RE-GDA0002214025690000033

Figure RE-GDA0002214025690000034
Figure RE-GDA0002214025690000034

Figure RE-GDA0002214025690000035
Figure RE-GDA0002214025690000035

(3)耦合过程(3) Coupling process

应力和热通过电池的温度来进行耦合,当温度变化的时候,电池的温差ΔT也会变化,就会导致公式(1)中热应变的变化,热应变的变化进而导致公式(2)中总应变和总应力的变化,而应力的变化又会反过来导致电池温度的变化,以此来实现热-力模型的耦合。该过程复现到多物理场耦合软件COMSOLMultiphysics中,该耦合过程以及计算过程见图1。本发明中出现的符号及术语见表2。Stress and heat are coupled through the temperature of the battery. When the temperature changes, the temperature difference ΔT of the battery will also change, which will lead to the change of thermal strain in formula (1), and the change of thermal strain will lead to the total The change of strain and total stress, and the change of stress will in turn lead to the change of battery temperature, so as to realize the coupling of thermal-mechanical model. This process is reproduced in the multiphysics field coupling software COMSOLMultiphysics. The coupling process and calculation process are shown in Figure 1. The symbols and terms appearing in the present invention are shown in Table 2.

表2本发明中出现的符号以及术语Symbols and terms that appear in the present invention in table 2

Figure RE-GDA0002214025690000041
Figure RE-GDA0002214025690000041

Figure RE-GDA0002214025690000051
Figure RE-GDA0002214025690000051

步骤三中模型的有效性验证采用如下步骤:The validity verification of the model in step 3 adopts the following steps:

(1)首先将电池表面布置若干热电偶,以测量电池在放电过程中的温度,电池与测力装置连接;(1) First arrange several thermocouples on the surface of the battery to measure the temperature of the battery during the discharge process, and connect the battery to the force measuring device;

(2)通过先恒流后恒压的充电方法将电池充满电,并根据电池材料设置充电截止电压;(2) Fully charge the battery through the charging method of constant current and then constant voltage, and set the charging cut-off voltage according to the battery material;

(3)对电池进行恒流放电,并根据电池材料设置放电截止电压;(3) Perform constant current discharge on the battery, and set the discharge cut-off voltage according to the battery material;

(4)将实验得到的平均温度曲线与模拟值进行比较;(4) compare the average temperature curve obtained by the experiment with the simulated value;

(5)根据上述步骤(4)的结果进行参数校正,得到校正后的热-力耦合模型。(5) Carry out parameter correction according to the result of the above step (4), and obtain a corrected thermal-mechanical coupling model.

本发明与现有技术相比的优点为:1.弥补了传统的实验方法难以预测电池在放电过程中所受热应力的不足,也完善了热-力耦合模型体系;2.建立了三维电芯尺度的热-力耦合模型,既可复现出电池的三维几何结构,又可得到电芯在放电过程中的温度分布和热应力分布;3. 可以动态观测到锂离子电池在放电过程中的膨胀现象和位移变化情况,便于分析在整个放电过程中电池的膨胀行为和应力变化;4.数值模拟方法和模型的建立节约资源和人力,并对锂离子电池的热安全和机械安全具有指导意义;5.该发明方法建立的三维宏观尺度的热-力耦合模型,可通过实验验证模型的有效性,保证了模型的精确度,也为后续就此三维模型进行锂离子电池充放电循环过程的应力和膨胀研究以及多因素研究做了基奠;6.为锂离子电池应力数值模拟研究人员及开发人员提供了研究基础和研究依据,为锂离子电池的热安全和机械安全提供技术支撑。Compared with the prior art, the present invention has the following advantages: 1. It makes up for the deficiency that the traditional experimental method is difficult to predict the thermal stress of the battery during the discharge process, and also improves the thermal-mechanical coupling model system; 2. Establishes a three-dimensional battery cell The scale thermal-mechanical coupling model can not only reproduce the three-dimensional geometric structure of the battery, but also obtain the temperature distribution and thermal stress distribution of the battery cell during the discharge process; 3. It can dynamically observe the lithium-ion battery during the discharge process. The expansion phenomenon and displacement changes are convenient for analyzing the expansion behavior and stress changes of the battery during the entire discharge process; 4. The establishment of numerical simulation methods and models saves resources and manpower, and has guiding significance for the thermal and mechanical safety of lithium-ion batteries 5. The thermal-mechanical coupling model of the three-dimensional macro scale established by the inventive method can verify the validity of the model through experiments, which ensures the accuracy of the model, and also provides a basis for the subsequent stress of the lithium-ion battery charge and discharge cycle process on this three-dimensional model. 6. It provides a research foundation and research basis for researchers and developers of lithium-ion battery stress numerical simulation researchers and developers, and provides technical support for the thermal safety and mechanical safety of lithium-ion batteries.

附图说明Description of drawings

图1为本发明中热-力耦合模型的耦合机制与原理。Fig. 1 is the coupling mechanism and principle of the thermal-mechanical coupling model in the present invention.

图2为本发明的实施例中电池熵系数和内阻随放电深度的变化曲线图。Fig. 2 is a graph showing the variation of the battery entropy coefficient and internal resistance with the depth of discharge in the embodiment of the present invention.

图3为本发明的实施例中模型几何和网格示意图,其中,图3(a)为本发明的实施例中模型几何示意图,图3(b)为本发明的实施例中模型网格示意图。Fig. 3 is the schematic diagram of model geometry and grid in the embodiment of the present invention, wherein, Fig. 3 (a) is the schematic diagram of model geometry in the embodiment of the present invention, Fig. 3 (b) is the schematic diagram of model grid in the embodiment of the present invention .

图4为本发明的实施例中电芯平均温度、正负极极耳温度的实验和模拟对比曲线图。Fig. 4 is an experimental and simulated comparison graph of the average temperature of the battery core and the temperature of the positive and negative tabs in the embodiment of the present invention.

图5为本发明的实施例中电池在不同放电时间(5s,900s,2700s,3600s)下的温度分布图。Fig. 5 is a temperature distribution diagram of the battery under different discharge times (5s, 900s, 2700s, 3600s) in the embodiment of the present invention.

图6为本发明的实施例中在不同放电时间下(0s,900s,1800s,2700s,3600s)电池沿着 x,y,z三个方向所受的von Mises应力曲线图,其中(0,0)点为电芯几何中心。Fig. 6 is the von Mises stress curve that the battery is subjected to along x, y, z three directions under different discharge times (0s, 900s, 1800s, 2700s, 3600s) in the embodiment of the present invention, wherein (0,0 ) point is the geometric center of the cell.

图7为本发明的实施例中电池在不同放电时间(5s,900s,2700s,3600s)下的位移分布图,变形为放大5000倍所示。Fig. 7 is a diagram showing the displacement distribution of the battery under different discharge times (5s, 900s, 2700s, 3600s) in the embodiment of the present invention, which is deformed and enlarged by 5000 times.

图8为本发明的实施例中在不同放电时间下(0s,900s,1800s,2700s,3600s)电池沿着 x,y,z三个方向的膨胀位移变化曲线图,其中(0,0)点为电芯几何中心。Fig. 8 is a curve diagram of the expansion displacement of the battery along the x, y, and z directions under different discharge times (0s, 900s, 1800s, 2700s, 3600s) in the embodiment of the present invention, where (0,0) point is the geometric center of the cell.

具体实施方式Detailed ways

为了便于理解本发明,下文将结合实施例对本发明作更全面、细致的描述,但本发明的保护范围并不限于以下具体的实施例。In order to facilitate the understanding of the present invention, the following will describe the present invention more comprehensively and in detail in conjunction with examples, but the protection scope of the present invention is not limited to the following specific examples.

实施例Example

以105Ah磷酸铁锂/石墨电池为例,对该锂离子电池放电过程中的温度和应力分布进行计算,全面、详细地对本发明作出描述,该方法不仅局限于该类型锂离子电池热-力耦合模型的构建及温度和应力的计算,对所有锂离子电池均适用。该方法主要分为以下四部分:(1) 热-力耦合模型的建立;(2)模型的有效性验证;(3)放电过程中电池的温度分布;(4)放电过程中电池的应力分布及膨胀行为。Taking a 105Ah lithium iron phosphate/graphite battery as an example, calculate the temperature and stress distribution during the discharge process of the lithium-ion battery, and describe the present invention in a comprehensive and detailed manner. The method is not limited to the thermal-mechanical coupling of this type of lithium-ion battery The construction of the model and the calculation of temperature and stress are applicable to all lithium-ion batteries. The method is mainly divided into the following four parts: (1) Establishment of thermal-mechanical coupling model; (2) Validation of the model; (3) Temperature distribution of the battery during discharge; (4) Stress distribution of the battery during discharge and expansion behavior.

1.首先对模型建立部分进行描述,共分为2个步骤,如下所述:1. First, describe the model building part, which is divided into 2 steps, as follows:

步骤一,模型参数获取。根据文献调研的方法获取电芯的力学参数以及材料的热力学参数,该部分参数均列于表3和图2中。Step 1, model parameter acquisition. The mechanical parameters of the cell and the thermodynamic parameters of the material are obtained according to the method of literature research, and these parameters are listed in Table 3 and Figure 2.

步骤二,电芯尺度三维热-力耦合模型的建立。模型几何和网格见图3,模型包含三个计算域:负极极耳(材料为铜),正极极耳(材料为铝),电芯主体(各项同性的弹性体和各向异性的导热体),不考虑电芯内部的分层结构,以简化模型。网格构建采用自由剖分四面体网格的方法,共包含9777个四面体单元,1926三角形单元,206个边单元和24个顶点单元,并且通过了网格独立性检验。Step two, the establishment of a three-dimensional thermal-mechanical coupling model at the cell scale. The model geometry and grid are shown in Figure 3. The model contains three computational domains: the negative tab (the material is copper), the positive tab (the material is aluminum), and the main body of the battery (isotropic elastomer and anisotropic thermal conductivity Body), the layered structure inside the cell is not considered to simplify the model. The mesh construction adopts the method of freely subdividing the tetrahedral mesh, which contains 9777 tetrahedral elements, 1926 triangular elements, 206 edge elements and 24 vertex elements, and has passed the mesh independence test.

表3热-力耦合模型参数Table 3 Thermal-mechanical coupling model parameters

Figure RE-GDA0002214025690000071
Figure RE-GDA0002214025690000071

注:“-”表示该项不存在或不考虑。Note: "-" indicates that the item does not exist or is not considered.

2.模型的有效性验证2. Validation of the model

模型的有效性验证步骤如下:The validation steps of the model are as follows:

(1)首先将电池表面布置若干热电偶,以测量电池在放电过程中的温度,电池与测力装置连接;(1) First arrange several thermocouples on the surface of the battery to measure the temperature of the battery during the discharge process, and connect the battery to the force measuring device;

(2)以1C倍率对电池进行先恒流后恒压的充电过程,充电截止电压为3.65V,将电池充满电;(2) Charge the battery with constant current and then constant voltage at a rate of 1C. The charging cut-off voltage is 3.65V, and the battery is fully charged;

(3)以1C倍率对电池进行恒流放电,截止电压设置为2.65V;(3) Discharge the battery with a constant current at a rate of 1C, and set the cut-off voltage to 2.65V;

(4)将实验得到的正极极耳温度、负极极耳温度和电芯平均温度曲线与模拟值进行比较;(4) Compare the positive pole lug temperature, negative pole lug temperature, and battery average temperature curve obtained in the experiment with the simulated value;

(5)根据上述步骤(4)的结果进行参数校正,得到校正后的热-力耦合模型。(5) Carry out parameter correction according to the result of the above step (4), and obtain a corrected thermal-mechanical coupling model.

从图4中可以看出,电池初始温度和放电结束时的温度,以及整体温度曲线走势与实验值基本一致,放电结束时的温度均在49℃左右,保证了模型的精确度,与实验有所不同的是,实验由于环境条件的影响,温差较大,而模拟由于相对理想的环境,温差较小,但这是在误差允许的范围内,可大致确保模型的准确性。It can be seen from Figure 4 that the initial temperature of the battery, the temperature at the end of discharge, and the trend of the overall temperature curve are basically consistent with the experimental values. The difference is that the temperature difference in the experiment is large due to the influence of environmental conditions, while the temperature difference in the simulation is relatively small due to the relatively ideal environment, but this is within the allowable range of error, which can roughly ensure the accuracy of the model.

3.放电过程中电池的温度分布3. The temperature distribution of the battery during the discharge process

图5为电芯放电5s,900s,2700s和3600s时温度及其等值面分布,在放电过程中电芯内部中心温度最高,表面温度较低,通过等温线分布可以看出温度沿着电芯中心向四周减小,但总体温差较小;且正极极耳的温度高于负极极耳的温度,这是由于正极极耳为铝,负极极耳为铝和铜的复合,正极极耳的导热系数低于负极极耳,导致正极极耳产热较多,因此温度高于负极极耳。这也与下面对应力的分析相对应,温度越高则热膨胀力越大,也即应力越大。Figure 5 shows the distribution of temperature and its isovalue surface when the cell is discharged for 5s, 900s, 2700s and 3600s. During the discharge process, the internal center temperature of the cell is the highest and the surface temperature is relatively low. It can be seen from the isotherm distribution that the temperature is along the cell The center decreases to the surroundings, but the overall temperature difference is small; and the temperature of the positive tab is higher than that of the negative tab. This is because the positive tab is aluminum, and the negative tab is a composite of aluminum and copper. The heat conduction of the positive tab is The coefficient is lower than the negative tab, resulting in more heat generation in the positive tab, so the temperature is higher than that of the negative tab. This also corresponds to the analysis of stress below, the higher the temperature, the greater the thermal expansion force, that is, the greater the stress.

4.放电过程中电池的应力分布及膨胀行为4. Stress distribution and expansion behavior of the battery during discharge

图6电芯在不同放电时间下沿着三个方向的von Mises应力变化曲线,从图中可以看出,放电开始时电芯表面的应力很小,可忽略不计,随着放电的进行,电芯表面的应力增大,变形也增大,直到3600s放电结束时电芯表面的应力最大,最大应力在10000N/m2左右。沿着 x和z方向的应力变化曲线趋势基本一致,电芯中心部位应力较大,中心部位沿着向外出现一段“平台期”,然后在边缘处出现“先下降后回升”的趋势,这是由于边界设置约束,导致应力波动;而沿着y方向的变化曲线较平滑,电芯中部应力最高,向两边逐渐减小,后面的位移变化曲线将对该现象做出进一步的说明。Figure 6. The von Mises stress variation curves of the battery cell along three directions at different discharge times. It can be seen from the figure that the stress on the surface of the battery cell is very small at the beginning of the discharge and can be ignored. The stress on the surface of the core increases, and the deformation also increases, until the end of 3600s discharge, the stress on the surface of the cell is the largest, and the maximum stress is about 10000N/m 2 . The trend of the stress change curve along the x and z directions is basically the same. The stress in the center of the cell is relatively large, and there is a "plateau period" in the center along the outward direction, and then there is a trend of "falling first and then rising" at the edge. It is due to the boundary setting constraints that lead to stress fluctuations; while the change curve along the y direction is relatively smooth, the stress in the middle of the cell is the highest, and gradually decreases to both sides. The following displacement change curve will further explain this phenomenon.

电芯厚度的变化用电芯在放电过程中的位移量来表征,由于仅仅模拟了一次放电过程,所以厚度变化相对较小。图7为电芯在放电5s,900s,2700s,和3600s时位移的变化情况,每个子图包含zy平面和三维立体的位移变化图。从图中可以看出,随着放电时间的增大,电芯整体位移增大,并且在电芯中心部位的位移最大,极耳处的位移也较大,这也与电芯中间部位和极耳处温度较高、所受应力和膨胀力较大相对应,在放电结束时最大位移为3.04μm;从zy平面的位移变化图可以看出,随着放电时间的延长,电芯沿着厚度方向膨胀较明显,且极耳有了明显的膨胀,由该趋势可知,在电芯进行多次循环之后,厚度会有更加明显的变化。图8为不同放电时间电芯沿着三个不同方向(x,y,z方向)的位移变化曲线,图中虚线为每个平面的中心部位。从三个图中可以看出:随着放电时间的增大,沿着三个方向的位移均增大;从沿着x方向的位移变化图可以看出,从电芯中部向外,位移呈现先增大后减小的趋势,峰值点对应位置即极耳的位置,越过极耳后位移减小,这是极耳处的应力集中导致;沿着y方向的位移最大,并且从电芯中间向两边线性增大,直到电芯最外侧位移最大;从沿着z方向的位移变化图可以看出,与沿着x方向的位移变化较相似,右边的峰值点依然是由于极耳的应力集中,而左边的峰值点是由于电池底部约束的设置;沿着x和y两个方向的位移变化呈对称分布,而沿着z方向的位移变化并未对称分布,这是由于电池顶部极耳的存在,极耳部位的应力集中导致沿着z方向的膨胀向极耳区域靠近。The change of the cell thickness is characterized by the displacement of the cell during the discharge process. Since only one discharge process is simulated, the thickness change is relatively small. Figure 7 shows the change of the displacement of the cell during discharge of 5s, 900s, 2700s, and 3600s, and each sub-graph contains the displacement change diagram of the zy plane and the three-dimensional stereo. It can be seen from the figure that as the discharge time increases, the overall displacement of the battery cell increases, and the displacement at the center of the battery core is the largest, and the displacement at the tab is also larger, which is also consistent with the middle part of the battery core and the pole. The temperature at the ear is relatively high, the stress and expansion force are relatively large, and the maximum displacement at the end of the discharge is 3.04 μm; from the displacement change diagram of the zy plane, it can be seen that as the discharge time prolongs, the cell along the thickness The directional expansion is more obvious, and the tabs have obvious expansion. From this trend, it can be seen that the thickness of the cell will change more obviously after multiple cycles. Figure 8 is the displacement change curve of the battery cell along three different directions (x, y, z directions) at different discharge times, and the dotted line in the figure is the center of each plane. It can be seen from the three figures: as the discharge time increases, the displacement along the three directions increases; from the displacement change diagram along the x direction, it can be seen that from the middle of the cell to the outside, the displacement presents The trend of first increasing and then decreasing, the position corresponding to the peak point is the position of the tab, and the displacement decreases after crossing the tab, which is caused by the stress concentration at the tab; the displacement along the y direction is the largest, and from the middle of the cell Increase linearly to both sides until the outermost displacement of the cell is the largest; from the displacement change diagram along the z direction, it can be seen that it is similar to the displacement change along the x direction, and the peak point on the right is still due to the stress concentration of the tab , and the peak point on the left is due to the setting of the battery bottom constraint; the displacement changes along the x and y directions are symmetrically distributed, but the displacement changes along the z direction are not symmetrically distributed, which is due to the top tab of the battery Existence, the stress concentration at the lug part causes the expansion along the z direction to approach the tab area.

Claims (6)

1. A method for determining thermal stress and temperature in a lithium ion battery discharge process based on a thermal-force coupling model is characterized by comprising the following steps:
selecting a single battery cell, and acquiring three-dimensional geometric parameters, mechanical and thermodynamic initial parameters of the single battery cell;
step two, establishing a thermal-force coupling model of a three-dimensional electrical core scale according to a thermal expansion coefficient, a temperature difference and a coupling mechanism of a stress-strain relation; stress and heat are coupled through the temperature of the battery, when the temperature changes, the temperature difference delta T of the battery also changes, so that the change of thermal strain is caused, the change of thermal strain further causes the change of total strain and total stress, and the change of stress in turn causes the change of the temperature of the battery, so that the coupling of a thermal-force model is realized;
step three, measuring the temperature of the battery and the temperature of the lug through experiments, and verifying the effectiveness of the model;
and step four, obtaining the temperature distribution of the battery and the expansion displacement and stress along the x, y and z directions.
2. The method of claim 1, wherein the thermal-mechanical coupling model is a three-dimensional cell macro-scale thermal expansion model, and the model geometry of the thermal-mechanical coupling model comprises a cell body, and positive and negative electrode tabs, and the stress, expansion behavior, and temperature distribution of the cell body and the positive and negative electrode tabs can be obtained.
3. The method for determining the thermal stress and the temperature in the discharging process of the lithium ion battery based on the thermal-mechanical coupling model according to claim 1, wherein the thermal strain is a quantity related to the thermal expansion coefficient and the cell temperature difference according to a coupling mechanism of the thermal expansion coefficient, the temperature difference and a stress-strain relation, so that the coupling of the stress and the heat transfer physical field is realized.
4. The method for determining the thermal stress and the temperature in the discharging process of the lithium ion battery based on the thermal-mechanical coupling model is characterized in that in the third step of the method, the temperature of the battery and the temperature of the tab are determined through experiments, and then compared with simulation data, so that the effectiveness of the model is fully verified.
5. The method for determining the thermal stress and the temperature in the discharging process of the lithium ion battery based on the thermal-mechanical coupling model according to claim 1, wherein a three-dimensional cell-scale macroscopic thermal-mechanical coupling model can observe the temperature distribution of the battery during the discharging process of the cell.
6. The method for determining the thermal stress and the temperature in the discharging process of the lithium ion battery based on the thermal-force coupling model according to claim 1, wherein a three-dimensional cell-scale macroscopic thermal-force coupling model can observe the expansion displacement and the stress of the cell in the x, y and z directions in the discharging process.
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