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.     

Experimental investigation on the essential cause of the degrading performances for an overcharging ternary battery

Ternary power batteries, as the mainstream power sources of electric vehicles, are liable to inducing thermal runaway (TR) with respect to their sensitivity to abusive conditions. Among various abuse conditions, the overcharge of a battery has been considered as the most common and severe case giving rise to thermal safety accidents. In this study, an overcharged battery and a normal battery, both using ternary/graphite electrodes, were investigated and analyzed synergistically through thermal behaviors and electrochemical characteristics. Initially, a series of electrochemical parameters including charge and discharge voltage plateaus, discharged capacity and time at different discharge rates, and internal resistances were carried out. Then, the heat generation behaviors between normal and overcharged batteries were evaluated. Furtherly, the interconnectedness with the electrochemical capacity degradation and heat generation aggravation of the ternary battery after overcharge was analyzed. Besides, the essential causes of the deterioration of electrochemical properties and severe heat behaviors resulting from overcharge were intensively analyzed via microscopic perspectives. In addition, the electrochemical characteristics fading of abused ternary battery triggered by overcharge were investigated, especially under higher temperature (55°C) and ultralow temperature (−20°C) conditions. Therefore, for an overcharged battery, this research not only elaborates the essential causa of the degraded electrochemical and anabatic thermal performance from a materials and thermal science perspective but also provides a foundation for further promoting the safety properties of commercialized power batteries with ternary chemical systems.     

Influence of coupling of overcharge state and short-term cycle on the mechanical integrity behavior of 18650 Li-ion batteries subject to lateral compression

The study on the mechanism of failure and thermal runaway of lithium-ion battery (LIB) induced by mechanical deformation has received considerable attention. LIBs connected in series are easily overcharged in practical applications. However, the influence of overcharging on the mechanical response of LIBs remains unclear. Thus, we investigated the lateral compression performance of cylindrical batteries before and after short-term cycles at various overcharge states. The onset of short circuits in compression tests for all the batteries before and after cycling at 4.2 and 4.3 V occurred at their modulus peaks, while that of the batteries after cycling at 4.4 and 4.5 V occurred at either the modulus fluctuation points or the first major modulus peaks. Thermal runaway accidents occurred on the batteries at all overcharge states after the short circuits were triggered. Moreover, thermal runaway would occur on the batteries charged at 4.2–4.4 V, when their anode tabs are located in the compression area. The thermal runaway risks of the test batteries would reach 100% when the voltages of these batteries exceeded 4.4 V. Results obtained by using a thermal camera revealed that the highest surface temperatures of all the batteries without thermal runaway were lower than 85 °C during the compression processes, whereas those of the batteries with thermal runaway were between 200 °C and 600 °C. Further analysis of the data indicated that the batteries before and after cycling at high overcharge voltages failed at minimal moduli and stresses, and this trend became obvious with the cycling of batteries.     

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.     

Experimental analysis on the degradation behavior of overdischarged lithium-ion battery combined with the effect of high-temperature environment

A set of experiments are performed in the present work to investigate the degradation behavior of lithium-ion battery during overdischarge cycling, as well as the influence of a high-temperature environment on the degradation. Among, different discharge cut-off voltages (1.0, 0.5, and 0.2 V) are included. During the overdischarge process, batteries experience a stage where a violent electro-thermal behavior is exhibited, involving sharp decreases in the voltage and current, and a fierce increase in the surface temperature; moreover, several parameters such as the discharge capacity, energy density, and internal resistances are all increased after overdischarge. Besides, a poor rate capacity and serious capacity degradation can also be seen during the overdischarge cycling, which is further reflected by the evolution of battery surface temperature, charge/discharge voltage, and internal resistances. What is more, it is found that battery electro-thermal parameters, eg, temperature rise, degradation rate, and internal resistances, increase exponentially as overdischarge deepens. Finally, a high-temperature environment is verified to deteriorate the degradation of overdischarged battery.     

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.     

Thermal behaviors of Ni-MH batteries using a novel impedance spectroscopy

In this paper, a novel impedance spectroscopy was used to describe the thermal behaviors of Ni-MH batteries. The impedance functions were derived similarly to electric impedance functions. The square of current was treated as a current equivalent and heat-flow as a voltage equivalent. The impedance spectra of batteries during charge showed that the combination of hydrogen and oxygen increased rapidly when charge rate was higher than 0.5 C. Thermal runaway might happen when battery was charged at temperature above 348 K even at a low charge rate. The cycling test showed that the charge efficiency of battery was the highest after cycling at high-rate for 10–100 cycles and decreased after more cycles. Different batteries showed different thermal behaviors which may be caused by the different structures of batteries.     

Modeling the capacity degradation of LiFePO4/graphite batteries based on stress coupling analysis

A coupling-analysis-based model to predict the capacity degradation of LiFePO₄ batteries under multi-stress accelerated conditions has been developed. In this model, the joint effect on the battery capacity degradation of any 2 out of 5 stress factors, which include ambient temperature, end of discharge and charge voltage (EODV and EOCV), and discharge and charge rate, is studied through coupling validation tests. Coupling generally exists among these 5 stress factors, and the coupling intensity has a certain relationship with the stress levels. There is a critical stress level at which the coupling can be considered negligible, and when the stress level goes higher, coupling aggravates battery degradation exponentially. Additionally, the study also indicates that battery life shows stronger sensitivity to discharge rate and EOCV than to charge rate and EODV. The developed capacity degradation model based on the input of real operating conditions and coupling intensity calibration achieves error less than 15% when the cycling goes into the stable decay period, and the error converges gradually as the cycling continues.     

Influence of float and charge voltage adjustment on the service life of AGM VRLA batteries depending on the conditions of use

Valve-regulated lead–acid (VRLA) batteries with absorptive glass mat (AGM) separators have been in use for over 20 years in different standby applications. These applications are increasingly varied, especially regarding environmental conditions. Standby batteries are not only for use in applications where conditions are strictly defined and controlled (air conditioning) and it is therefore necessary to review and clarify the key parameters for the use of VRLA batteries with respect to the optimum conditions. Several series of chemical and electrochemical reactions occur in VRLA batteries particularly when in a charge or float charge condition. These reactions give specific properties such as minimal water loss (low maintenance) but also create specific precautions for use.VRLA battery functioning is limited by four main phenomena that are positive grid corrosion, irreversible active mass sulfation, active mass degradation by cycling and dry-out by loss of water. Positive grid corrosion is the usual failure mode in float operation or due to persistent overcharge. Irreversible active mass sulfation occurs due to lack of charge. In cycling, dependent upon the frequency and depth of discharge, the active mass undergoes numerous structural changes that cause degradation.These four limiting phenomena define a framework inside which several parameters determine the service life of VRLA batteries. These parameters are commissioning, temperature, and frequency and depth of discharge. Commissioning is necessary to equalise and fully charge the cells before use. Temperature, and temperature dispersion, is the main factor determining the rate of corrosion. The frequency and depth of discharge determine how the active mass is utilised. This paper, by considering these parameters both qualitatively and quantitatively, attempts to indicate how and why to adjust the charge and float voltages to optimise the use of AGM VRLA batteries according to the environmental conditions.     

Investigation on the root cause of the decreased performances in the overcharged lithium iron phosphate battery

The overcharge of the lithium iron phosphate (LiFePO₄) batteries usually leads to the sharp capacity fading and safety issues, especially under low temperature environment. Thus, investigating their root cause originated from the electrode materials is critical for the safety performance optimization and market promotion of the LiFePO₄ batteries. In this work, the electrochemical/thermal behaviors of 18650 LiFePO₄ batteries are investigated after overcharge under room and low temperature of 25°C and ?20°C, respectively. The results demonstrate a decreased electrochemical performance and faster heating rate of the overcharged battery, particularly under harsh working environments such as high discharge rate and low temperature. Coupling with the analyses of the internal resistance, the crystal structure, and microstructure of the electrodes, the root cause is attributed to the damage of the crystal structure and microstructure, which reduce the electron/Li⁺ migrating capability and electrolyte diffusion/transfer efficiency.     

Performance of 26650 Li-ion cells at elevated temperature under simulated PHEV drive cycles

Cylindrical (type: 26650) Li-ion cells (LiFePO₄ cathodes) currently used in the electric vehicles (EVs), plug-in hybrid electric vehicles etc. were subjected to simulated federal urban driving schedule at 25 and 50 °C for performance evaluation. Drive profiles (current versus time) for charge sustaining and charge depleting modes were derived from the federal urban driving schedule velocity profiles considering acceleration, regenerative braking, rolling resistance, drag force etc. for typical plug-in hybrid electric vehicles. In particular, the batteries were cycled extensively at 50 °C under charge sustaining as well as charge depleting modes to monitor capacity values, followed by analyzing the LiFePO₄ cathode material by X-ray diffraction analysis. The capacity degradation was found to be very significant in both the modes with 13 and 19% under charge sustaining and charge depleting modes after 337 and 1007 cycles, respectively at elevated temperature. High frequency resistance values measured by electrochemical impedance spectroscopy were found to increase significantly under high temperature cycling, leading to power fading. As evident from Rietveld analysis, phase change in LiFePO₄ is observed beyond 1000 cycles at elevated temperature under charge depleting mode, with the observation of FePO₄ from the powder diffraction data of the cathodes from the cycled cells. In addition, there was also significant change in crystallite size of the cathode active materials after charge/discharge cycling under charge depleting mode.     

An improved electrothermal-coupled model for the temperature estimation of an air-cooled battery pack

This work establishes an improved electrothermal-coupled model for the estimation of the temperature evolution in an air-cooled pack with three parallel branches and four serial cells in each branch. This model includes the influences of the cells' state of charge (SOC) and temperature on the ohmic and polarization resistances and polarization capacitance. The current distribution in the pack is considered in the model and applied to predicting the inconsistent effect of cell temperature. Moreover, the pipe network theory is used to model the airflow in the pack and the heat convection between the air and the batteries. An experiment is implemented to verify prediction precision in the electrical and thermal parameters of the pack. The results show that the electrothermal model accurately estimates the electrical and thermal performance of the air-cooled pack. The relative error of the pack terminal voltage between the prediction and the experiment is 3.22% under the conditions of a discharging rate is 1.5 C (C denotes the ratio of charging/discharging current to battery capacity), environment temperature of 37°C, and air inlet velocity of 6 m/s. Regarding the prediction error in the temperature, the root mean square errors of most batteries are no more than 0.6°C under the conditions of discharge rates of 1 C and 1.5 C and ambient temperatures of 17°C, 27°C, and 37°C.     

Research on parameter identification and state of charge estimation of improved equivalent circuit model of Li-ion battery based on temperature effects for battery thermal management

The performance and parameters of Li-ion battery are greatly affected by temperature. As a significant battery parameter, state of charge (SOC) is affected by temperature during the estimation process. In this paper, an improved equivalent circuit model (IECM) considering the influence of ambient temperatures and battery surface temperature (BST) on battery parameters based on second-order RC model have been proposed. The exponential function fitting (EFF) method was used to identify battery model parameters at 5 ambient temperatures including −10°C, 0°C, 10°C, 25°C and 40°C, fitting the relationship between internal resistance and BST. Then, the SOC of the IECM was estimated based on the extended Kalman filter (EKF) algorithm. Using the result calculated by the Ampere-hour integration method as the standard, the data of battery under open circuit voltage (OCV) test profile and dynamic stress test (DST) profile at different ambient temperatures has been compared with the ordinary second-order RC model, and the advantages of the SOC estimation accuracy with IECM was verified. The numerical results showed that the IECM can improve the estimation accuracy of battery SOC under different operating conditions.     

Overcharge studies of carbon fiber composite-based lithium-ion cells

Prototype lithium-ion pouch cells of 5.5 Ah have been fabricated with carbon fiber composite anodes, LiCoO₂ cathodes, and LiPF₆ electrolyte to investigate the overcharge characteristics of these cells at the 1C rate. The cells were made with anode to cathode capacity (A/C) ratios of 1.0 and 1.1. The cells were first examined for charge–discharge characteristics at different rates in order to determine the delivered capacity, specific energy and energy density and rate capability, and to ensure that the cells are suitable for overcharge studies. The current, voltage, and temperature responses during overcharge to 12 V were recorded. Maximum temperatures of 65 and 85 °C were observed with the cells with A/C equal to 1.1 and 1.0, respectively. The overcharged cells were dissected in an inert atmosphere and their components were analyzed using scanning electron microscopy and x-ray fluorescence spectroscopy. It is believed that a relatively low amount of heat is generated with carbon fiber composite-based lithium-ion cells and a separator shutdown mechanism is operative in the cell system which prevents fire or explosion during overcharge.     

Small-capacity valve-regulated lead/acid battery with long life at high ambient temperature

Valve-regulated lead/acid (VRLA) batteries are widely used as back-up power sources for telecommunications and UPS. These applications require high-reliability under severe environmental conditions. To meet this demand, the authors' company have developed small capacity (12 V, 15–65 A h at C₂₀/20 rate), long-life VRLA batteries which can endure high ambient temperature. These batteries make use of a new alloy and grid design which has improved resistance to corrosion at the positive plate, while at the same time reduce float current at high temperature. As a result, these batteries have a life expectancy of 13 years at 25°C, and inhibited thermal runaway even under ambient temperatures up to 75°C. The batteries can be installed in outdoor and underground environments.     

Diphenylamine: A safety electrolyte additive for reversible overcharge protection of 3.6 V-class lithium ion batteries

A polymerizable monomer, diphenylamine (DPAn), is reported to act as a safety electrolyte additive for overcharge protection of 3.6 V-class lithium ion batteries. The experimental results demonstrated that the DPAn monomer could be electro-polymerized to form a conductive polymer bridging between the cathode and anode of the battery, and to produce an internal current bypass to prevent the batteries from voltage runaway during overcharge. The charge–discharge tests of practical LiFePO₄/C batteries indicated that the DPAn additive could clamp the cell's voltage at the safe value less than 3.7 V even at the high rate overcharge of 3 C current, meanwhile, this monomer molecule has no significant impact on the charge–discharge performance of the batteries at normal charge–discharge condition.     

Simulation of temperature-compensated voltage limit curves for aerospace NiCd batteries using a first principles' model

Temperature-compensated voltage limits (V/T limits) are routinely used in Low Earth Orbit (LEO) satellites to permit fast charging of the NiCd battery with minimum overcharge and without the problems of thermal runaway during overcharge. The voltage limits are experimentally determined from extensive testing of cells for a proper design of the charge control system to achieve the desired charge/discharge ratio in orbit. Here, we demonstrate the use of first-principles', mathematical models to construct the V/T curves theoretically. The predicted charge/discharge ratios under various orbit conditions such as different states of charge, in-rush currents and temperatures are compared with the experimental data.     

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 runaway behavior and features of LiFePO4/graphite aged batteries under overcharge

In this paper, the overcharge tests of 25 Ah LiFePO₄/graphite batteries are conducted in an open environment and the overcharge-to-thermal-runaway characteristics are studied. The effects of current rates (C-rates: 2C, 1C, 0.5C, and 0.3C) and states of health (SOHs: 100%, 80%, 70%, and 60%) on thermal runaway features are discussed in detail. The overcharge process can be summarized into five stages based on the experimental phenomena (C-rate ≥ 1 and SOH ≥ 80%): expansion, fast venting after safety valve rupture, slow venting, intense jet smoke, and explosion, while the battery cannot explode at lower C-rates and SOHs. The maximum pressure increases with the increase in C-rate or SOH. There are five obvious inflection points in the voltage curve during overcharge process. The V₁ (point B) of aged battery, corresponding to lithium plating on the anode, changes little with C-rates. It is slightly lower than that of the new battery. A sharp drop in voltage (point E) is probably due to the internal short circuit (ISC), caused by the local melting and rupture of the separator. It takes more than 2 minutes from the moment of ISC to thermal runaway regardless of the SOH, indicating that there are a few minutes to take safety measures if the voltage is an indication parameter. The onset temperature of thermal runaway decreases first and then increases as the SOH decreases from 100% to 60% during 1C constant overcharge tests. These results can provide guidance for the thermal management of the whole battery life cycle and the reuse of retired batteries.     

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.