基于伪二维电化学-热耦合模型的锂电池热特性研究

锂电池放电过程中的产热受电池内部电化学反应和欧姆效应影响,电池产热由电池化学与动力学决定,而电池动力学依赖于电池运行条件和设计参数。锂电池的六个温度依赖性参数对锂电池的放电过程中的产热速率具有影响,包括固相活性颗粒和电解液中的锂离子扩散系数、反应速率常数、电极开路电压、电解液离子电导率、热力学因子和阳离子迁移数。基于LiFePO_4圆柱形电池建立了伪二维电化学-热耦合模型,研究电池在恒流放电过程中的产热速率,以及正极、隔膜和负极各部分的产热速率和所占比例。结果表明,总产热功率随反应热的波动而变化,其中正极电极层中反应热占比最大,负极电极层中极化产热所占比例高于正极,而隔膜中的产热主要来源自欧姆热。不同对流传热系数条件下,电池的表面温度和内部温度差都不同,因此要合理的采取电池热管理措施。     

A novel electro-thermal coupled model of lithium-ion pouch battery covering heat generation distribution and tab thermal behaviours

This paper develops an electro-thermal coupled model for a pouch battery which considers the influence of the electric potential distribution and thermal behaviours of tabs on the distributions of heat generation and temperature. This model reflects the connection between electric potential distribution and the heat generation distribution. Then, the proposed electro-thermal model is used to predict the thermal behaviour of a lithium-ion pouch battery under the conditions of various discharge currents and environmental temperatures. Tests are implemented to verify the prediction precision of the proposed thermal model. The results show that the electro-thermal coupled model can accurately predict both the temperature distribution and its rise. At 1 C discharge rate, the average mean static absolute errors (MSAEs) in the positive and negative tabs for all the ambient temperatures are 1.083 K and 0.377 K, respectively, and the average MSAE in the battery body is only 0.627 K; at the discharge rate of 3.5 C, the respective average MSAEs are 1.643 K for the positive tab, 0.581 K for the negative tab, and 0.889 K for the battery body. Last, the influence of the thermal contact resistance on the prediction of the tab temperature is studied. If it is considered in the thermal model of tabs, the predicted tab temperature is closer to the tested value and the MSAE can be limited within 2.65 K for the positive tab and 0.83 K for the negative tab in the cases researched.     

Simulation and verification of lithium-ion battery temperature changing process

针对软包锂离子电池放电过程中温度变化过程进行研究,依据电池产热基本理论,通过内阻实验及0.5 C放电倍率下的温升实验计算出瞬态生热率曲线,得出电池熵热系数,建立生热速率随放电深度不断变化的瞬态生热模型,基于该模型进行不同放电倍率的温度仿真模拟,并与实验进行对比。结果表明,温度变化模拟结果与实验相吻合,生热率变化模拟结果与实验计算值相符合,模型可以很好地模拟电池在不同放电倍率下的温度变化,对电池温升过程分析及电池热管理过程控制具有指导意义。     

Numerical study on thermal behavior and a liquid cooling strategy for lithium-ion battery

Investigation on the thermal behavior of the lithium-ion battery which includes the temperature response, heat contribution and generation, is of vital importance for their performance and safety. In this study, an electrochemical-thermal cycling model is presented for a 4 Ah 21700 type cylindrical single cell and 3× 3 battery pack and the model is validated by experiment on a single cell. Thermal behavior on a single cell is first analyzed, the results show that the heat generated in the charge is smaller than the discharge, and the polarization heat contributes the most to total heat, especially under higher rate. It can also be concluded from the battery pack that the temperature of the cell inside the battery pack is significantly greater than the external battery, while the temperature difference exists the opposite regular due to the worst heat dissipation of the central cell. Finally, after taking the enhanced liquid cooling strategy, the maximum temperature is 320.6 K that is reduced by 9.38%, and the maximum temperature difference is 4.9 K which is reduced by 69.6% at 2C, meeting the requirements of battery thermal management system.     

Measurements and application of thermal characteristics of soft-packed NCM lithium-ion power battery

锂离子电池的极化内阻是不可逆热测试的关键参数。为了更准确地计算极化内阻,针对三元软包锂离子动力电池,进行了HPPC测试、熵热系数测试、充放电温升测试,采用两种方法对极化内阻进行了计算,一种是通过电压变化量除以电流得到,另一种是通过建立二阶RC模型,结合HPPC测试工况辨识得到。根据两种方法得到的极化内阻,结合Bernardi生热速率模型公式对电池进行了1C充电和0.5C、1C、2C放电下的温度场仿真,并与红外热成像仪记录到的温度分布进行了对比。结果表明：根据二阶RC模型得到的极化内阻进行的仿真与实验数据吻合较好,说明利用二阶RC模型得到的极化内阻更加适用于电池持续充放电过程中的热分析。模型很好地模拟了电池不同充放电倍率下的温度场信息,对电池热分析及热管理可起到指导作用。     

Modeling and validation of temperature changes in a pouch lithium-ion battery at various discharge rates

This paper deals with the thermal modeling of temperature rise in a pouch lithium-ion battery with LiFePO₄ (also known as LFP) cathode material. The developed model represents the main thermal phenomena in the cell in terms of temperature change. The proposed model is validated with the collected experimental data from a module composed of 11 cells. In the conducted experiments, the different charge and discharge rates of 1/2C, 1C, 2C and 2.5C are applied. It is seen that, the increased discharge rates result in increased temperature on the surface of the battery. When the discharge rate is doubled, from 1C to 2C, cell temperatures have risen by 3.5 times. A simplified model for determining the heat generation is developed and validated with the test results.     

Review on heat generation theory model of lithium-ion battery

本文根据近年来锂离子电池产热特性方面的研究,详细阐述了锂离子电池产热的基本原理,并总结了国内外锂离子电池产热模型的研究现状。重点针对电化学-热耦合模型、电-热耦合模型以及热滥用模型进行了详细综述,并在此基础上对锂离子电池热效应的研究和产热模型的建立进行了展望。     

Thermal studies of a lithium-ion battery

In order to characterize the heat generation behavior of a lithium-ion secondary battery during discharge, calorimetry of the battery was carried out. The temperature dependency of the heat generation was studied between 283 and 333 K. An exothermic peak and an endothermic peak were observed around 4 V, except at 333 K. These peaks are caused by a phase transition of the positive electrode material, Li_xCoO₂, between the hexagonal and monoclinic structures, which is observed at around x = 0.5. The heat of discharge consists of mainly two factors, the battery reaction and the electrochemical polarization. In the temperature range between 283 to 333 K, if the battery is discharged at 50 mA, the least heat generation is observed at about 300 K owing to the temperature dependence of both factors.     

Study on electrochemical and thermal characteristics of lithium‐ion battery using the electrochemical‐thermal coupled model

The higher specific energy leads to more heat generation of a battery, which affects the performance and cycle life of a battery and even results in some security problems. In this paper, the capacity calibration, Hybrid Pulse Power Characteristic (HPPC), constant current (dis)charging, and entropy heat coefficient tests of chosen 11‐Ah lithium‐ion batteries are carried out. The entropy heat coefficient increases firstly and then decreases with the increase of the depth of discharge (DOD) and reaches the maximum value near 50% DOD. An electrochemical‐thermal coupled model of the chosen battery is established and then verified by the tests. The simulation voltage and temperature trends are in agreement with the test results. The maximum voltage and temperature error is within 2.06% and 0.4°C, respectively. Based on the established model, the effects of adjustable parameters on electrochemical characteristic are systematically studied. Results show that the average current density, the thickness of the positive electrode, the initial and maximum lithium concentration of the positive electrode, and the radius of the positive electrode particle have great influence on battery capacity and voltage. In addition, the influence degree of the internal resistance of the solid electrolyte interface (SEI) layer, the thickness of negative electrode, and the initial and maximum lithium concentration of the negative electrode on the capacity and voltage is associated with certain constraints. Meanwhile, the influences of adjustable parameters related to thermal characteristic are also systematically analyzed. Results show that the average current density, the convective heat transfer coefficient, the thickness, and the maximum lithium concentration of the positive electrode have great influence on the temperature rise. Besides, the uniformity of the temperature distribution deteriorates with the increase of the convective heat transfer coefficient.     

The review of thermal management technology for large-scale lithium-ion battery energy storage system

大容量锂离子电池储能系统对完善传统电网和高效利用新能源都具有非常重要的作用。为了实现大容量锂离子电池储能系统的高倍率化、长寿命化以及高安全性,高性能电池热管理系统的研发刻不容缓。本文总结了温度对锂离子电池性能的影响规律,综述了空冷、液冷、热管冷却、相变冷却这4种典型热管理技术的研究概况,分析了热管理技术在锂离子电池储能系统中的应用与研究状况。随着锂离子电池储能系统工作倍率的提高,产热量随之增大,对热管理系统的要求也越来越高。下一步的研究工作应围绕空冷系统优化、基于新型冷却介质的液冷系统、经济型热管及多目标优化设计这4方面展开。     

High safety electrolyte for lithium-ion battery

锂离子电池安全性问题的本质是电池内部发生了热失控,热量不断的累积,造成电池内部温度持续上升,其外在的表现是燃烧、爆炸等。因此,锂离子电池的安全性与比能量、使用温度和倍率性能等存在一定的矛盾。电池能量密度越高、倍率性能越快和使用环境越恶劣,其能量剧烈释放时对电池体系的影响就越大,安全问题也越突出。当前锂离子电池电解液一般由低闪点的碳酸酯、对痕量水和温度敏感的LiPF₆和其它添加剂组成,本身具有高度可燃性。同时,电解液与正负极材料之间形成界面膜被认为是电池热失控的起点。因此,电解液改性是提升电池安全性的重要措施。本文分析了离子液体和氟代溶剂等溶剂对电解液安全性的提升效果,对比了多种锂盐对电解液安全性的影响,介绍了阻燃剂、过充保护剂、锂枝晶抑制剂和成膜稳定剂等电解液添加剂对锂电池安全性的改善。最后,从电池整体应用性能的角度出发,讨论了今后高安全性锂离子电池电解液的研发方向。     

锂金属参比电极稳定性研究

锂离子电池商业化应用已有30多年,但目前的电池性能仍不能满足社会发展的需求。为此,须不断开发高性能的电池材料。电化学测量是电池材料开发不可或缺的关键技术。锂金属电极是锂离子电池电化学测量中最常用的参比电极,其电位稳定性将影响电化学测量结果的准确性。报道一种能提高锂金属参比电极电位稳定性的锂金属表面处理方法。将有机锌盐和氟代碳酸酯的混合溶液滴加在锂金属表面,通过锂金属与溶液组分的反应,在锂金属表面形成一层含锌锂合金和氟化锂的混合界面膜,可降低锂溶解/沉积过电位。处理得到的锂金属电极在Li//Li对称电池中用1 mA/cm²的电流及1 mA∙h/cm²的容量恒电流连续充放电,该对称电池的电压稳定时间是未处理电池的2倍以上。这种锂金属电极表面处理方法可提高电极材料电化学性能测量的稳定性,有利于锂离子电池新材料的开发。     

Influence of cycling on the heat-release and thermal runaway of the lithium ion battery under adiabatic condition

随着锂离子电池能量、寿命的提升,对安全性需求也越来越高,温度对电池的寿命和安全有着重要影响。以钴酸锂/中间相碳微球体系电池为研究对象,采用加速量热仪研究了不同工作电流、不同循环老化周期电池的产热特性和热失控行为,电池的发热量随着充放电倍率的增加而增大。通过比较不同循环老化周期电池的产热速率,发现容量衰减速度与直流内阻、产热量之间存在很强的关联性。从热失控行为研究发现,自放热起始温度为105.4℃,随后发生连续自放热,直到温度达到149.7℃热失控起始温度,发生内短路,最终导致电池热失控。循环后电池的热失控过程中自放热和热失控起始温度稍有变化,热失控时间大大缩短。     

In situ observation of lithium dendrite on the electrode in a lithium-ion battery

锂离子电池尽管已成为便携式电子设备的主流电源,也是电动汽车、混合动力汽车等电源的主要选择之一,但依然存在使用过程中因形成锂枝晶而发生内短路的安全隐患。本文设计了一个宏微观实验研究商业用锂离子电池电极材料的充放电循环性能。在常温小电流充放电条件下,实时原位地观测锂枝晶的产生、生长、消融以及死锂残留等过程。实验结果揭示了锂枝晶不仅仅只是大电流过充或低温充电状态下的产物,常温常态小电流充电条件下依然能够生成锂枝晶。实验发现：锂枝晶出现在充电后期,随后直线伸长,尖端区域形貌保持不变;放电时,锂枝晶逐渐消融,尖端区域形貌依然不变,放电结束后电极上有死锂残留。     

Numerical investigation on integrated thermal management for lithiumion battery pack with phase change material and liquid cooling

为满足3 C放电倍率下电池组散热要求,提出了PCM\液冷复合式散热方案,利用有限元分析了液体流速、流道排列方式、铝制框架鳍宽和环境温度对电池组温度的影响。结果表明,增加流速可优化电池组散热性能,但当流速大于0.08 m/s时,流速的增加对散热系统无明显优化;各流速下Type I散热方式效果均为最优且电池组满足散热要求;鳍宽为2 mm时可将电池组最高温度进一步降低1.6℃;当环境温度从38℃增至42℃时,复合式散热系统体现了良好的热稳定性能。     

循环充放电条件下锂离子电池的温度模拟

锂离子电池在充放电过程中产生的热量主要为两部分,即因电化学反应而产生的可逆热和由极化产生的不可逆热。若电池内部温度达到82℃以上时,钴酸锂电池材料将发生热分解,引发一系列不可控化学反应,释放出大量的反应热。本论文在可逆热和不可逆热的基础上,耦合电池材料分解热,采用有限元技术,模拟锂离子电池在充放电过程中不同对流条件以及不同外界温度下电池内部温度的变化,为揭示锂离子电池热失控机制提供理论依据。     

Thermal safety studies of high energy density lithium-ion batteries under different states of charge

The popularity of lithium-ion batteries in electric vehicles has promoted the increase of its energy density, and battery cathode and anode materials have developed rapidly in recent years. As the next generation of material systems, high-nickel-content Li-Ni-Co-Mn oxide cathode and high-silicon-content Si-C anode material systems have a high potential for further application. However, safety is a key indicator for their use in traction batteries. We thus conducted a thermal safety analysis of the pouch cells of such a system for different states of charge and revealed the key factors for the thermal safety evolution of batteries by analyzing the morphology and thermal stability of cathodes and anodes.     

Heterogeneous aging of large-scale flexible lithium-ion batteries based on micro-heterostructures and simulation

To meet application demands of electric vehicles, cathode materials of batteries have to overcome the life limitation and performance attenuation caused by crack propagation on the surface of electrode particles. With the increase of size and power of batteries, the voltage gradient generated by metal foil current collectors with high conductivity cannot be neglected. This study reconstructed a porous microstructure based on images of surface morphologies of lithium manganese oxide particles collected by a field emission-scanning electron microscope. Based on this, a multi-scale and multi-physics simulation model coupling electro-chemo-thermo-mechanical behaviours was developed to predict heterogeneous mechanical stress and capacity loss of a large-scale flexible lithium-ion battery. The results arising from use of the model show that: (1) Lithium in electrode particles cannot be diffused in time under a high-charge and discharge rate, and the capacity loss of the battery is directly proportional to the stress generated on the electrode particles. Capacity loss at discharge rate of 10C is 46% higher than that at the rate of 1C and corresponding stress in the microstructure increases by 16%. (2) In the design of the battery layout adopted in this study, utilization rates of electrodes and temperature fields are highly heterogeneous at the higher rate. Mechanical stress near the tab is 8% higher than that at the bottom edge, and it is speculated that the rate of aging of the tab is 35% higher. (3) Mechanical stress during lithium extraction in the cathode during charge is less than half of that during discharge. Attributed to small influences of material activity and excellent performance of lithium titanate oxide in the anode, capacity loss during charge is only 2%. (4) During discharge, stress in the contact region of between particles is the largest, resulting in the decrease of the activity and the low lithium-ion concentration. This leads to cracks during cyclic charging and discharging, which further decreases the activity of the materials. (5) Heterogeneity in the distribution of lithium-ion concentration with different sizes of particles significantly rises with the rate. The model built in this research couples the analysis of temperature field of a battery cell and stress field of the microstructure, which is conducive to understanding mechanisms underlying performance attenuation of the large-scale flexible lithium-ion battery under high-rate use.     

Thermal behavior of small lithium-ion battery during rapid charge and discharge cycles

The secondary batteries for electric vehicles (EV) generate much heat during rapid charge and discharge cycles at current levels exceeding the batteries’ rating, such as when the EV quickly starts consuming battery power or when recovering inertia energy during sudden stops. During these rapid charge and discharge cycles, the cell temperature may increase above allowable limits. We calculated the temperature rise of a small lithium-ion secondary battery during rapid charge and discharge cycles. The heat-source factors were measured again by the methods described in our previous study, because the performance of the battery reported here has been improved, showing lower overpotential resistance. Battery heat capacity was measured by a twin-type heat conduction calorimeter, and determined to be a linear function of temperature. Further, the heat transfer coefficient, measured again precisely by the method described in our previous study, was arranged as a function of cell and ambient temperatures. The temperature calculated by our battery thermal behavior model using these measured data agrees well with the cell temperature measured by thermocouple during rapid charge and discharge cycles. Also, battery radial temperature distributions were calculated to be small, and confirmed experimentally.     

Modeling stresses in the separator of a pouch lithium-ion cell

The stress in a separator is mainly caused by the lithium diffusion induced deformation in the electrodes and thermal expansion differential between the battery components. To compute the lithium concentration distribution and temperature change during battery operation, multi-physics models have been developed previously. In this work, a macro-scale model for a pouch cell was developed and coupled with the multi-physics models. In this model, the porous battery components were treated as homogenized media and represented with the effective properties estimated using the rule of mixtures. The stress analysis showed that the maximum stress in the separator always emerged at the area around the inner corner of the separator where it wrapped around the edge of an anode and when the lithium-ion battery was fully charged. Numerical simulations were also conducted to investigate the influences of some design adjustable parameters, including the effective friction, electrode particle radii and thickness of the separator, on the stresses in the separator. The results provided the reference conditions for the improvement of separator materials and the design of lithium-ion batteries.