Safety research of 27A battery in the whole life cycle

Lithium-ion batteries have the characteristics of high specific energy and long life, and have gradually become the mainstream of electric vehicle power batteries. With the promotion and application of electric vehicles, safety accidents characterized by thermal runaway occur from time to time, causing property losses to consumers and undermining consumers' confidence in electric vehicles. In order to prevent the occurrence of thermal runaway, the automotive power battery system needs to be designed with corresponding safety, such as a series of thermal runaway alarms and fire extinguishing designs of Tesla cars. In addition, the battery management system also needs to manage the battery safely (voltage management, thermal management), etc., to ensure that the battery operates within the safety window (voltage, temperature, etc.).

Before new lithium-ion batteries are put on the market, they must undergo a series of safety tests, including hot box, overcharge, puncture and extrusion tests, and their safety performance can be guaranteed to a certain extent. However, the design life of electric vehicles is more than 5 years. The "Beijing Demonstration Application of New Energy Passenger Car Production Enterprises and Product Registration Management Rules (Revised in 2015)" issued by Beijing in 2015 requires that the battery warranty period shall not be less than 8 years or 120,000 kilometers. When the battery is aging during use, as its capacity decays and internal resistance increases, the safety performance of the battery (heat resistance, overcharge resistance, etc.) may also change, which in turn causes changes in the battery's safe working window. To ensure the safety of aging batteries, the safety management and monitoring strategies of the battery management system should also be adjusted accordingly, otherwise, after the safety performance of the aging battery deteriorates, safety hazards may occur within the original working window. In addition, with the large-scale promotion and application of electric vehicles, the problem of handling retired power batteries has also emerged. One of the current treatment methods is to use the retired batteries from the car in a cascade manner and continue to use them in energy storage and other fields to fully utilize the residual value of the battery. The safety performance evaluation and safety management of retired batteries are issues that should be focused on during the cascade utilization process.

In view of this, this paper conducts a review and analysis of the evolution of lithium-ion battery safety performance throughout the life cycle. On the basis of reviewing the changes in battery safety performance under conditions such as cycle aging and storage aging in domestic and foreign literature, the relationship between battery aging attenuation mechanism and safety performance changes is summarized, and future research in this field is prospected. It is hoped that it will have certain guiding significance for the thermal runaway prevention design and safety management of battery systems throughout their life cycle, as well as the safety assessment of battery cascade utilization.

Key content introduction

1 Lithium-ion battery safety issues

There are many triggering causes for lithium-ion battery thermal runaway accidents. According to the characteristics of the trigger, they can be divided into three ways: mechanical abuse triggering, electrical abuse triggering, and thermal abuse triggering. Mechanical abuse refers to needle puncture, extrusion, and heavy object impact caused by car collisions, etc. Electrical abuse is generally caused by improper voltage management or electrical component failure, including short circuit, overcharge, and overdischarge, while thermal abuse is caused by overheating caused by improper temperature management. The three triggering modes are not completely independent. Mechanical abuse generally causes deformation or rupture of the battery separator, resulting in direct contact and short circuit between the positive and negative electrodes inside the battery, and electrical abuse. Under electrical abuse, Joule heat and other heat generation increase, causing the battery temperature to rise, developing into thermal abuse, further triggering chain heat generation side reactions inside the battery, and ultimately causing thermal runaway of the battery.

Thermal runaway mechanism of a commercial lithium-ion battery. During thermal runaway, the side reactions of the battery negative electrode begin first, including the SEI (solide electrolyte interphase) membrane decomposition reaction (70-130°C) and the reaction of the lithium-embedded graphite negative electrode with the solvent (120-200°C). The solute LiPF6 in the electrolyte will also decompose at high temperatures to produce PF5, etc. When the temperature rises to around 200°C, the positive electrode material begins to decompose and release oxygen. The decomposition temperature of the positive electrode material depends on the composition and lithium insertion state of the positive electrode. For the commonly used nickel-cobalt-manganese ternary positive electrode [Li(NixMnyCo1-x-y)O2], the higher the nickel content and the lower the lithium content, the lower the decomposition temperature of the positive electrode material. At high temperatures, the positive electrode material and the oxygen it produces are both strong oxides, which will undergo strong redox reactions with the electrolyte and negative electrode materials as strong reducing substances, releasing a large amount of heat, causing a sharp temperature rise in the battery, and further causing reactions such as binder reaction and electrolyte combustion, leading to thermal runaway of the battery.

2 Lithium-ion battery aging and attenuation mechanism

The aging and attenuation of lithium-ion batteries is manifested externally as capacity attenuation and increased internal resistance. Its internal aging and attenuation mechanism includes the loss of active materials in the positive and negative electrodes and the loss of available lithium ions [29–33]. At present, researchers have conducted extensive research on the aging and attenuation mechanism of lithium-ion batteries and have gained a relatively clear understanding.

3 Safety evolution of lithium-ion batteries throughout their life cycle

Under different aging pathways, the aging and attenuation mechanism and external characteristics of the battery are not the same, and the changes in safety performance are also different. Literature [52-53] points out that the evolution of lithium-ion battery safety over the entire life cycle is closely related to the aging attenuation pathway. Based on a large number of surveys of existing research literature, this paper divides the aging attenuation pathway into two types: cycle aging and storage aging. The changes in battery safety performance with aging attenuation under the two working conditions are summarized respectively, and the relationship between the battery aging attenuation mechanism and the evolution of safety performance is further summarized.

3.1 Effect of cycle aging on battery safety performance

3.2 Effect of storage aging on battery safety performance

3.3 Relationship between battery aging attenuation mechanism and safety performance evolution

Conclusion

Safety issues characterized by thermal runaway are an important factor restricting the large-scale application of lithium-ion batteries. Thermal runaway of lithium-ion batteries is usually caused by mechanical abuse, electrical abuse or thermal abuse, and a series of irreversible heat-generating reactions will occur successively inside, including SEI film decomposition reaction, negative electrode and electrolyte reaction, positive and negative electrode redox reaction, etc. When the battery is continuously aged during use, the side reactions inside the battery (SEI film thickening, negative electrode lithium deposition, electrolyte oxidation, etc.) will not only cause the battery capacity to decay and the internal resistance to increase, but also cause the battery's safety performance (heat resistance, overcharge resistance, etc.) to change.

Existing studies have shown that the evolution of lithium-ion battery safety over the entire life cycle is closely related to the aging attenuation pathway. Under normal temperature/high temperature cycle aging, due to the increase in internal resistance, the battery's Joule heat increases during charging and discharging, the electrical abuse resistance decreases, and the battery's thermal stability will also change to a certain extent. The change law is related to the battery's material system and process level; under normal temperature/high temperature storage aging, the battery's electrical abuse resistance will also decrease, but due to the improved stability of the negative electrode's SEI film during storage, the battery's thermal stability will be improved; under low temperature cycle aging, the battery's thermal stability will drop sharply, mainly due to the lithium deposition of the negative electrode. The deposited lithium metal is very active and can react with the electrolyte at a lower temperature, causing the battery's self-heating temperature Tonset to decrease and the self-heating rate to increase sharply, seriously endangering the safety of the battery.

Based on existing relevant research, we can summarize the relationship between the battery aging attenuation mechanism and the change in safety performance. In general, the overcharge resistance of aged batteries will decrease to a certain extent, mainly due to the increase in internal resistance and the reduction of positive and negative active materials. In terms of thermal stability, lithium plating at the negative electrode will cause a sharp decline in the thermal stability of the battery. It is necessary to focus on research and develop charging management methods to prevent lithium plating and real-time detection methods for lithium plating, etc., to fully ensure the safety of the battery throughout its life cycle.

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