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。     

圆柱形锂离子电池热管理实验研究

保持合适的运行温度是锂离子电池高效、安全、长寿命的保证,因而对其进行有效的热管理是非常有必要的。本文针对圆柱形锂离子电池,设计了嵌套电池表面的方形金属外壳,以强化电池散热和单体电池间传热。对比自然对流条件下电池单体加壳和无壳时不同放电倍率的温升情况、多个电池并联的温升情况,以及不同通风功率下多个电池并联时嵌套不同外壳的温升情况,发现加壳可以有效促进电池（组）散热。另外,设计了电池组内不同单体电池出现放电不均衡情况,以检验嵌套外壳对减小电池组内单体电池间温差的效果,结果表明,自然对流条件下,加壳后单体电池间最大温差可以降低10℃以上。     

The importance of heat evolution during the overcharge process and the protection mechanism of electrolyte additives for prismatic lithium ion batteries

In this work, the rate of heat generation in the overcharge period for 103450 prismatic lithium ion batteries (LIBs) of the LiCoO₂–graphite jellyroll type with a basic electrolyte consisting of 1 M LiPF₆–PC/EC/EMC (1/3/5 in weight ratio) has been found to be more important than the gas evolution which was traditionally considered as the main reason in the overcharge protection mechanism. The cell voltage, charge current, and skin temperature were monitored during the charge process. For a single battery or batteries in parallel, LIBs without any additives is an acceptable design if the cell voltage is not charged above 4.55 V under the common charge program. The rate of heat generation from the polymerization of 3 wt% cyclohexyl benzene (CHB) is high enough to cause the explosion or thermal runaway of a battery, which is not found for an LIB containing 2 wt% CHB + 1 wt% tert-amyl benzene (TAB). In the 12 V overcharge test at 1C, the thermal fuse was broken by the high skin temperature (ca. 80 °C) due to the polymerization of 3 wt% CHB, which was also the case for LIBs containing 2 wt% CHB + 1 wt% TAB. The disconnection of the thermal fuse, however, did not interrupt the thermal runaway of LIBs without any additives because the battery voltage was too high (ca. 4.9 V). The influence of specific surface area of active materials in the anode on the polymerization kinetics of additives has to be carefully considered in order to add correct amount of overcharge protection agents.     

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。     

Thermal safety study of Li-ion batteries under limited overcharge abuse based on coupled electrochemical-thermal model

Many fire accidents of electric vehicles were reported that happened during the charging process. In order to investigate the reasons that lead to this problem, this paper studies the thermal safety of Li-ion batteries under limited overcharge abuse. A 3D electrochemical-thermal coupled model is developed for modeling thermal and electrochemical characteristics from normal charge to early overcharge state. This model is validated by experiment at charge rates of 0.5C, 1C, and 2C. The simulation results indicate that irreversible heat contributes most to temperature rise during the normal charge process, but the heat induced by Mn dissolution and Li deposition gradually dominates heat generation in the early overcharge period. Based on this, a threshold selection method for multistage warning of batteries overcharge is proposed. Among them, level 1 should be considered as a critical stage during the early overcharge process due to the deposited lithium starts to react with electrolyte at the end of level 1, where temperature rate increases to 0.5°C min⁻¹ for 1C charge. While the thresholds of levels depend on charge rate and composition of battery. Furthermore, several critical parameters are analyzed to figure out their effects on thermal safety. It is found that the temperature at the end of overcharge is significantly influenced by the change of positive electrode thickness and solid electrolyte interface (SEI) film resistance. The final temperature increases by 17.5°C and 7.9°C, respectively, with positive electrode thickness ranging from 50 to 80 μm and SEI film resistance increasing from 0.002 to 0.03 Ω.     

Simulation of passive thermal management system for lithium-ion battery packs

A passive thermal management system that uses a phase change material (PCM) is designed and simulated for a lithium-ion (Li-ion) laptop battery pack. The problem of low thermal conductivity of the PCM was significantly improved by impregnating an expanded graphite (EG) matrix with the PCM. The heat generation rate for a commercial 186502.2 Ah Li-ion battery was experimentally measured for various constant power discharges. Simulation of the battery pack, composed of six Li-ion batteries, shows that safe operation of the battery pack during the most extreme case requires the volume of the battery pack be almost doubled to fit sufficient PCM in the pack. Improving the properties of the PCM composite have the potential to significantly reduce the volume increase in comparison to the original battery pack volume.     

3D electro‐thermal modelling and experimental validation of lithium polymer‐based batteries for automotive applications

This article presents an electro‐thermal model of a stack of three lithium ion batteries for automotive applications. This tool can help to predict thermal behaviour of battery cells inside a stack. The open source software OpenFOAM provides the possibility to add heat generation because of Joule losses in a CFD model. Heat sources are introduced at the connectors and are calculated as a function of battery discharge current and internal resistance. The internal resistance is described in function of temperature. Simulation results are validated against experimental results with regard to cooling air flow field characteristic and thermal behaviour of the cell surface. The validation shows that the simulation is capable to anticipate air flow field characteristics inside the battery box. It also predicts correctly the thermal behaviour of the battery cells for various discharge rates and different cooling system conditions. The simulation supports the observation that batteries have a higher temperature close to the connectors and that the temperature increase depends highly on discharge rate and cooling system conditions. Copyright © 2016 John Wiley & Sons, Ltd.     

An alternating current heating method for lithium‐ion batteries from subzero temperatures

An alternating current (AC) heating method for lithium‐ion batteries is proposed in the paper. Effects of current frequency, amplitudes and waveforms on the temperature evolution and battery performance degradation are respectively investigated. First, a thermal model is established to depict the heat generation rate and temperature status, whose parameters are calibrated from the AC impedance measurements under different current amplitudes and considering battery safe operating voltage limits. Further experiments with different current amplitudes, frequencies and waveforms on the 18650 batteries are conducted to validate the effectiveness of the AC heating. The experimental data recorded by appropriate measurement instrument are of great consistence with simulation results from the thermal model. At high frequency, the temperature rises prominently as the current increases, and high frequency serves as a good innovation to reduce the battery degradation. However, efficient temperature rise can be obtained from high impedance at low frequencies. Typically, 600 s is needed to heat up the battery from ?24 °C to 7.79 °C with sinusoidal waveform and approximately from ?24 °C to 25.6 °C with rectangular pulse waveform at 10A and 30 Hz. The model and experiments presented have shown potential value in battery thermal management studies for electric vehicle (EV)/hybrid electric vehicle (HEV) applications at subzero temperatures. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal Management in Hybrid Power Systems Using Cylindrical and Prismatic Battery Cells

Lithium ion (Li-ion) batteries are promising as both alternative and auxiliary power sources in hybrid and electric vehicles. However, the reliable and efficient operation of the Li-ion batteries depends critically on effective thermal management, due to both the high operational thermal loads and the possible range operational conditions. This work therefore studied the issue of thermal management of battery systems under extreme hot conditions. This study combined both experimental testing and high-fidelity computer fluid dynamics (CFD). Due to the difficulty of conducting experiments under extreme conditions, controlled experiments were first conducted so that CFD models could be validated. The validated CFD models were then applied to study various aspects of thermal management issues in battery systems, with an emphasis on comparing the operation of prismatic and cylindrical batteries under extremely hot environments. The results presented include temperature distribution among cells, pump power required, and different geometrical layouts of the cells. These results are expected to provide insights into the design and optimization of battery systems.     

Investigating electrical contact resistance losses in lithium-ion battery assemblies for hybrid and electric vehicles

Lithium-ion (Li-ion) batteries are favored in hybrid-electric vehicles and electric vehicles for their outstanding power characteristics. In this paper the energy loss due to electrical contact resistance (ECR) at the interface of electrodes and current-collector bars in Li-ion battery assemblies is investigated for the first time. ECR is a direct result of contact surface imperfections, i.e., roughness and out-of-flatness, and acts as an ohmic resistance at the electrode-collector joints. A custom-designed testbed is developed to conduct a systematic experimental study. ECR is measured at separable bolted electrode connections of a sample Li-ion battery, and a straightforward analysis to evaluate the relevant energy loss is presented. Through the experiments, it is observed that ECR is an important issue in energy management of Li-ion batteries. Effects of surface imperfection, contact pressure, joint type, collector bar material, and interfacial materials on ECR are highlighted. The obtained data show that in the considered Li-ion battery, the energy loss due to ECR can be as high as 20% of the total energy flow in and out of the battery under normal operating conditions. However, ECR loss can be reduced to 6% when proper joint pressure and/or surface treatment are used. A poor connection at the electrode-collector interface can lead to a significant battery energy loss as heat generated at the interface. Consequently, a heat flow can be initiated from the electrodes towards the internal battery structure, which results in a considerable temperature increase and onset of thermal runaway. At sever conditions, heat generation due to ECR might cause serious safety issues, sparks, and even melting of the electrodes.     

Electrical‐thermal behaviors of a cylindrical graphite‐NCA Li‐ion battery responding to external short circuit operation

Large amount of heat generated during an external short circuit (ESC) process may cause battery safety events. An experimental platform is established to explore the battery electrothermal characteristics during ESC faults. For 18650‐type nickel cobalt aluminum (NCA) batteries, ESC fault tests of different initial state of charge (SOC) values, different external resistances, or different ambient temperatures are carried out. The test case of a smaller external resistance is characterized by a shorter ESC duration with a faster cell temperature rise, whereas the case of a larger external resistance will last for a longer duration, discharge more electricity, and terminate in a slightly higher temperature. The tested batteries of high initial SOCs generally have higher temperature rise rates, smoother changes at the output current/voltage curves, but a smaller discharged capacity. The batteries of low initial SOCs can be overdischarged by the ESC operations. At low temperatures, say 0°C, the ESC process outputs much less electricity than the process at high temperatures, eg, 30°C. The initial low temperature has little effect on reducing the battery overheat due to ESC operations. The battery thermal behavior is of hysteresis property; analysis of heat generations reveals the subsequent increase of battery surface temperature after the completion of ESC discharge is due to the battery material abusive reaction heats. It is found from analytical and numerical analyses that there can have approximately 30°C temperature difference between the battery core and its surface during ESC operations. The interruption of ESC operation is very probably caused by the high battery core temperature, which leads to the destruction of solid‐electrolyte interface (SEI) film.     

Effect of high Ni on battery thermal safety

Frequent accidents involving Li-ion batteries have prompted higher safety requirements for these batteries. In this study, the high-temperature, thermal runaway (TR) characteristic parameters at 100% state of charge (SOC) for cylindrical NCM811 batteries with a high-energy density were compared to the widely commercialized NCM523 batteries. The average TR trigger temperature of NCM811 battery was 157.54°C, which was 20.62°C lower than that of NCM523. Moreover, the average TR maximum temperature of NCM811 battery is 858.22°C, which was 212.81°C higher than that of NCM523. The maximum TR temperature of the NCM811 battery was 1289.53°C. The high Ni batteries exhibited poor thermal stability and severe TR. An increase in the Ni content resulted in increased fluctuations in the battery's internal TR reaction because high Ni batteries have a poor TR consistency and are difficult to accurately control. The TR combustion explosion of the fully charged NCM811 battery lasted for approximately 1.36 seconds. The combustion explosion severely damaged the positive electrode, and there was a collapse of the negative layered structure. The Cu current collector surface melted locally owing to the high temperature. Moreover, Ni, Co, and Mn particles appeared in the Cu current collectors and graphite.     

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.     

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.     

ARC experimental and data analysis for safety evaluation of Li-ion batteries

锂离子电池安全性能可以通过电池热失控过程的量热分析来进行定性和定量评估。电池在不同温度下的放热速率及累计放热量是衡量电池热稳定性的参数。动力电池的量热分析通过绝热加速量热仪进行。本文主要介绍加速量热仪的测试原理和方法、数据分析方法,并对电池安全程度的评估方法提出了建议。     

Practical performances of Li-ion polymer batteries with LiNi0.8Co0.2O2, MCMB,and PAN-based gel electrolyte

The practical performances and thermal stability of Li-ion polymer batteries with LiNi_0.8Co_0.2O₂, mesocarbon microbead-based graphite, and poly(acrylonitrile) (PAN)-based gel electrolytes are reported. The gel electrolyte, which shows a fire-retardance by itself as well as good chemical stability effectively improved thermal stability of the Li-ion polymer battery up to 170 °C. We also found that the mesocarbon microbead-based graphite showed better coulombic efficiency even though the gel electrolyte contained PC and GBL. An evaluation of cell performances showed that the electrodes and the gel electrolyte were promising material for a next-generation Li-ion polymer battery.     

Heat generation in high power prismatic Li‐ion battery cell with LiMnNiCoO2 cathode material

While in use, battery modules and battery packs generate large amounts of heat, which needs to be accounted for. The main challenge in battery thermal management is the correct estimation of heat generation in the battery cell during charging/discharging. In this paper, a method to calculate accurate heat generation in one individual cell is provided. The heat generation is calculated by measuring the overpotential resistances with four different methods and entropic heat generation in the cell. The effect and contribution of entropic heat generation towards the total heat generation in the cell are also calculated and measured. Finally, calorimeter tests are carried out to compare the calculated and measured heat generation. The results indicate that except for direct current resistance measured by impedance spectroscopy, all the overpotential resistances are very close to each other. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal behavior of overcharged nickel/metal hydride batteries

This work provides a two-dimensional thermal model for cylinder Ni/MH battery. Thermal model is developed to analyze the thermal behavior of the battery when charged and overcharged. Quantity of heat and heat generation rate of the battery during charge and overcharge period are studied by quartz frequency microcalorimeter. Heat generation curve is fitted into a function, and heat transport equation is solved. Analysis with the model and experiment show that temperature rise is about 3 °C and difference between the model and the experiment is no more than 0.1 °C.     

A comparative study on the degradation behaviors of overcharged lithium-ion batteries under different ambient temperatures

In the current work, a series of experiments are carried out to investigate the degradation behavior of lithium-ion batteries during overcharge cycling, as well as the influence of ambient temperature on the degradation. In which, different charge cut-off voltages (4.5, 4.8, and 5.0 V) and ambient temperatures (0°C, 20°C, 50°C, and 70 °C) are included. During the overcharge process, the batteries demonstrate severe temperature rises, and several key electrochemical parameters such as the charge capacity, energy density, median voltage, and resistances all increase, revealing the deterioration of heat generation and electrode kinetics. Besides that, batteries exhibit serious degradation behavior during the overcharge cycling, which is presented through the evolution of battery temperature curves, charge voltage curves, and internal resistance curves. Moreover, the severity of degradation exacerbates with the increasing overcharge degree. Finally, it is found that deep-overcharged batteries may be more sensitive to the ambient temperature than slight-overcharged ones, where an abusive temperature can significantly aggravate the corresponding degradation.     

Development of the inorganic composite phase change materials for passive thermal management of Li-ion batteries: material characterization

The battery thermal management system based on phase change materials (PCMs) has proven to be a safe, inexpensive, and high-performance technology, which is currently gaining popularity over the other battery thermal management systems. PCMs are able to absorb heat generated by batteries, prolong battery life, and improve their performance. Organic compounds, such as paraffin, are widely used as phase change materials for the battery thermal management systems; however, there are no published data on the application of inorganic PCMs. Therefore, the main objective of this work is developing of two composite inorganic PCMs based on magnesium chloride hexahydrate and characterizing their properties for passive thermal control of lithium-ion battery packs. Moreover, to have a valid baseline and compare the behavior of two inorganic PCMs with currently commercialized and well investigated organic PCM, paraffin wax was used as reference material for both mixtures. All three PCMs were impregnated into the expanded graphite matrix to enhance their thermal conductivity. The material characterization studies, including thermal properties investigation, density and viscosity measurements, soaking and compression testing, evaluation of thermal expansion, thermal conductivity, and micro X-ray fluorescence analysis, were conducted for all PCMs. The results indicate that both inorganic mixtures are appropriate for thermal management of Li-ion battery packs. Future work with the developed and characterized composite inorganic PCMs will include electrical cycling studies and nail penetration tests to reveal their effectiveness for passive thermal management of Li-ion battery packs.