The research on lithium-ion batteries began with the rocking-chair battery concept proposed by Armand et al. in 1972. The commercialization began with the lithium cobalt oxide battery launched by SONY in 1991. After more than 30 years of iterative upgrades, it has been maturely applied to consumer electronic products, Power tools and other small-capacity battery markets have shown great application value in electric vehicles, energy storage, communications, national defense, aerospace and other fields that require large-capacity energy storage equipment.

However, since the birth of lithium-ion batteries, safety has always been an important issue limiting their usage scenarios. As early as 1987, the Canadian company Moli Energy launched the first commercial lithium metal battery based on the metal lithium negative electrode and the MoS2 positive electrode. The battery suffered a number of explosions in the late spring of 1989, which directly led to the bankruptcy of the company and also prompted The industry turned to the development of lithium-ion batteries that use intercalation compounds as anodes more stably. After lithium-ion batteries entered the field of consumer electronics, there have been many large-scale recall plans due to battery fire hazards. In 2016, Samsung's Note7 mobile phone in South Korea suffered many fires and explosions around the world, in addition to causing a global recall plan. In addition, "Lithium battery safety" has once again become a social topic of widespread concern. In the field of electric transportation, the safety accidents of power batteries have gradually increased along with the increase in sales of new energy vehicles. According to statistics, there will be more than 200 electric vehicle fires and combustion accidents reported in China in 2021, and the safety of electric vehicles has become a consumer concern. And one of the most concerned issues for electric vehicle companies. In the field of energy storage, more than 30 energy storage power station accidents occurred in South Korea from 2017 to 2021. The explosion accident of Beijing Dahongmen energy storage power station on April 16, 2021 caused the entire power station to burn down and also caused the sacrifice of 2 firefighters, 1 employee is missing. With the increasing application scenarios of lithium-ion batteries, their safety has sparked extensive discussions and research in both industry and academia.

In the early stage of lithium battery development, the industry and academia paid more attention to the essential causes of safety accidents in lithium batteries. Based on long-term accumulation of knowledge, the nature of safety accidents in lithium batteries can be summarized as follows: the battery is overcharged, overheated, impacted The temperature rises abnormally under abnormal use conditions such as short circuit, etc., which triggers a series of internal chemical reactions, causing the battery to gas, smoke, and the safety valve to open. When it occurs, the temperature of the battery rises rapidly and uncontrollably, causing combustion or explosion, resulting in a serious safety accident. This process is also known as the "thermal runaway" of the battery. There are several important chemical reactions in the process of battery from abnormal temperature rise to thermal runaway.

With the wide application of lithium-ion batteries, the research on the safety of lithium-ion batteries has gradually deepened. From the simple description of phenomena and qualitative predictions in the early days, it has developed into the study of safety mechanisms at multiple scales and methods, based on accurate measurement and The numerical model accurately predicts battery safety performance, and finally proposes a comprehensive research strategy for applied solutions. As shown in Figure 3, the current research on battery safety generally starts from understanding the thermal behavior of lithium-ion battery cells, including using various abuse conditions to determine the safe use limit and failure performance of the battery, using adiabatic heat and other means to specifically Analyze the thermal runaway behavior and characteristic temperature of the battery, and use the thermal runaway numerical simulation method to simulate the thermal runaway performance of the battery;

1 Research on thermal stability of materials
The root cause of thermal runaway in lithium-ion batteries is that the materials in the battery are unstable under certain conditions, resulting in an uncontrollable exothermic reaction. Among the currently commercialized battery materials, the ones most closely related to safety are the charged (delithiated) transition metal oxide positive electrode, the charged (lithium intercalated) graphite negative electrode, carbonate electrolytes and separators. The first three are unstable at high temperature and interact with each other, releasing a large amount of heat in a short period of time, while the currently commonly used polymer separators will melt and shrink at 140-150 °C, resulting in the positive and negative electrodes in the battery. Contact, rapid heat dissipation in the form of an internal short circuit. Since the end of the 20th century, researchers have carried out a lot of research on the thermal stability of materials, and developed a research method that uses thermal analysis to understand the thermal behavior of materials, and combines the characterization of morphology, structure, element composition and valence to comprehensively study the internal mechanism. The recent development of computational materials science has also provided new methods and means for predicting the stability of materials from atomic-scale simulations.

1.1 Thermal analysis method
Thermal analysis is the most direct and intuitive method to understand the thermal behavior of materials. It refers to a type of technology that measures the relationship between a certain physical property of a material and temperature or time under a certain program-controlled temperature (and a certain atmosphere). For battery materials, the relationship between mass, composition, and endothermic and exothermic behaviors with temperature is generally concerned. The relationship between mass and temperature can be obtained by thermogravimetric analysis (TGA or TG), and the relationship between endothermic heat and temperature can be obtained by differential scanning calorimetry (differential scanning calorimetry, DSC). TG and DSC can be designed in Simultaneous testing in the same instrument, this method is also known as simultaneous thermal analysis (simultaneous thermal analysis, STA). Instruments such as TG, DSC, and STA usually use a linear heating program, and record the mass, endothermic and exothermic changes of the sample through thermal balances, heat flow sensors, etc. Due to the early development time, the testing technology and equipment engineering level are relatively mature, and it has become a material for understanding. One of the most important tests for stability.

Based on the results of thermal analysis, the initial temperature, amount of reaction and heat release of the phase transition, decomposition or chemical reaction of the material can be determined, but in lithium-ion batteries, the stability and reaction heat of the charged material in the electrolyte environment are often more concerned. . Good thermal stability is a necessary condition for battery materials to enter applications, while heat generation and heat generation rate affect the severity of battery thermal runaway. The crucibles used for conventional thermal analysis samples are generally made of open alumina or open-pored aluminum metal. In order to study the thermal performance of materials in volatile electrolytes, it is necessary to use self-made or specially provided sealed containers by equipment manufacturers.

In addition to DSC and TG, there is also a special thermal analysis method that uses acceleration calorimeter (accelerating rate colorimeter, ARC) to study the onset temperature of the reaction.

1.2 Phase Analysis Technology
During the heating process of battery materials, phase transitions and chemical reactions occur, and their morphology, structure, composition and element valence state may change. These changes need to be characterized and analyzed based on corresponding methods, such as scanning electron microscope (scanning electron microscope) , SEM) to observe the morphological changes of the materials before and after thermal decomposition, and use X-ray diffraction and spectroscopy to study the material structure and element valence evolution. Due to the significant kinetic effects of material thermal decomposition and thermal reactions, in situ testing during heating can minimize the real process of phase change. At present, there are two main types of mature in-situ characterization techniques: one is mass spectrometry (MS) and infrared spectroscopy (IR), which are used in series with thermal analysis instruments, which can monitor the types of gases generated by the decomposition of substances in real time. , to judge the change of chemical composition during the heating process of the material; the other type is in situ X-ray diffraction (XRD), through a special sample stage, the structural change of the material can be measured in real time and in situ during the heating process At present, most of the world's synchrotron radiation light sources and some laboratory-level X-ray diffractometers can realize in-situ variable temperature XRD tests.

1.3 Computational Materials Science
Computational prediction of all properties of materials based on their atomic structures is the ultimate pursuit of computational materials scientists. The thermodynamic stability of a material can be calculated based on density functional theory (DFT). The basis for judging the stability of materials in DFT is whether the energy difference ΔE before and after the reaction is less than 0. If ΔE is less than 0, the reaction can occur, and the reactants are unstable, and vice versa. In general, the gap between the theoretical simulation technology and the experimental technology at the material level is still far from the current stage, which requires the continuous efforts of researchers.

2 Research on thermal safety of cell
A battery cell refers to a battery cell, which is a basic unit device that converts chemical energy and electrical energy into each other, usually including electrodes, separators, electrolytes, casings and terminals. The thermal safety characteristics of cells are one of the most concerned contents in the battery industry. It is the concentrated expression of the thermal stability of battery materials and the basis for developing large-scale battery system safety early warning and protection strategies. Due to the internal structure of the cell, its safety will show some characteristics that are not discussed in pure material research, which makes the safety of the cell have a wider extension and understanding.