As lithium-ion batteries are widely used in aviation, aerospace, new energy vehicles and other fields, their safety risks and problems in the process of use are increasingly prominent. Especially under abuse conditions (such as high temperature, short circuit, overcharge and discharge, vibration, extrusion and impact, etc.), they are prone to smoke, fire and even explosion. Therefore, their safety indicators are also highly valued internationally. At present, a number of international organizations and countries in the world have introduced corresponding lithium battery standards and inspection requirements, and have investigated the safety performance of lithium ion batteries from different perspectives. This paper discusses the safety standards of lithium-ion batteries, analyzes the test purposes and methods of different safety standards, and focuses on the analysis of the test methods and influencing factors of internal short circuit which has a great impact on the safety of power lithium-ion batteries.
1.Safety evaluation system of lithium ion battery
1.1 System classification
At present, foreign standard systems are divided into three categories according to different levels:
(2) National or regional standards, such as the national standards formulated by the American Standards Institute (ANSI), the British Standards Institute (BSI), the German Standards Institute (DIN), the Japanese Industrial Standards Committee (JISC), and the European regional standards (EN) formulated by the European Organization for Standardization (CEN);
(3) Industry standards, such as those formulated by industry organizations such as the American Institute of Electrical and Electronic Engineers (IEEE), the Japan Battery Industry Association (BAJ), and the Underwriters’ Laboratories (UL).
1.2 Safety evaluation and analysis
In recent years, the detection methods for lithium batteries have become more and more perfect, and many safety standards have been introduced at home and abroad, such as GB/T 18287-2000 IEC62133, UL 1642 IEEE1625, etc. In addition, safety standards for lithium battery transportation and use in equipment have also been introduced, such as UN38.3, GB 4943, etc. It can be said that the safety standard of lithium-ion battery has become a crucial and indispensable part of the lithium-ion battery standard. The test items of security standards commonly used today can be divided into four categories.
There are many differences in the content of lithium-ion battery standards due to the different technical basis and purpose of different formulation institutions and makers. Among the foreign lithium-ion battery standards, IEC, UL, IEEE and JIS standards are influential. Relevant standards of IEC are specified in accordance with performance requirements, safety requirements, mechanical tests, transportation safety, etc; UL standards mainly focus on safety tests, many of which, such as combustion tests, are not available in other standards; The IEEE standard focuses on the whole battery system.
There are detailed requirements for the battery cell, package and charger. As the birthplace of lithium batteries, Japan has major lithium ion battery enterprises including Panasonic, Sony, Sanyo, NEC, Hitachi, etc. These enterprises pay special attention to the development of lithium ion battery technology and related standardization work. The BAJ organization where these enterprises are located formulated the Japanese Yujia Standard JS C8714 – in 2007. This standard is a safety standard developed by Japan specifically for lithium ion batteries, and some of its testing content is pioneering, such as mandatory internal short circuit testing.
1.3 Testing organization
At present, the most important security testing institutions in the world are Underwriters Lab – laboratories (UL-1642), United Nations (UN) for Transportation and International Electrotechnical Commission (IEC-Project).
1.4 Analysis of evaluation items
All security tests are designed to simulate a possible abuse situation. For example, acupuncture simulates internal short circuit; The overcharge test simulates the failure of the protection circuit board; The low pressure test in the environmental adaptability test corresponds to the situation that may occur during air transportation.
The difficulty of safety test items for different types of lithium-ion batteries is different. Generally speaking, for sealed lithium-ion batteries, the environmental adaptability test is generally considered to be easy to pass; The external short circuit of small size and small capacity battery is also relatively easy to pass. For those test items that are difficult to pass, the temperature of the battery will rise during the test. Higher temperature will cause or accelerate the chemical reaction in the battery, and eventually lead to the thermal runaway of the battery, explosion, combustion and other conditions. Commercialized lithium ion batteries are small and medium sized batteries with low capacity, which are relatively easy to pass safety tests, even if security issues arise in the hands of end users. The direct harm caused is also small; For the batteries with higher capacity such as EV and HEV being developed, it is far more difficult to pass the safety test than the commercialized batteries.
In addition, once the safety problem occurs, the consequences will be very terrible. Moreover, compared with other electrochemical systems, lithium-ion batteries have some differences, that is, there is no internal protection mechanism for overcharging. Moreover, the battery system is sensitive to water, so it must be completely sealed. Therefore, when the battery is overcharged, a series of irreversible reactions will occur, including excessive lithium stripping from the positive electrode; Lithium metal precipitates from the negative electrode, resulting in lithium dendrites and electrolyte solvent decomposition reaction. While these reactions occur, a large amount of heat is generated and accumulated, which accelerates the thermal decomposition of the positive electrode and the like. At the same time, the precipitation of oxygen and other gases increases the internal pressure. Eventually, it may cause the battery to exhaust (the pressure rise causes the action of the breaking mechanism), and in serious cases, it may cause violent gas leakage, rupture and fire and other safety accidents. For lithium-ion batteries, due to the use of organic solvents, when accidents occur, combustion often occurs.
Lithium-ion batteries have a certain capacity to withstand over-discharge. This is because the side reaction in the battery is simpler than that in the lithium primary battery. At this time, the positive electrode will have the reaction of lithium deposition, while the negative electrode may have the decomposition reaction of electrolyte. Generally speaking, when small high-capacity batteries are used alone, internal short circuit is most likely to occur. The reason is that on the one hand, when designing this kind of battery, it generally pursues high capacity, so it uses thinner diaphragm and high-density electrode. This increases the probability of causing internal short circuit. At the same time, if there are metal particle impurities in the manufacturing process, it may deposit on the surface of the diaphragm or electrode during the circulation process, resulting in direct overlap of positive and negative electrodes; However, large batteries are generally used in combination (for example, the power battery of electric vehicles is more than 200V), so the accident caused by serious overcharging is most suitable.
2.Internal short circuit evaluation method
When the battery has an internal short circuit, a very high current passes through the short circuit position and generates a large amount of heat, which may cause thermal runaway phenomenon, even fire or explosion in the battery. Simulated internal short circuit test is an important aspect of lithium ion battery detection. The safety performance of lithium-ion batteries can be effectively evaluated by evaluating the internal short-circuit performance of lithium-ion batteries through predictive tests and detection methods. Among various standards of lithium ion standards, the detection methods for simulating internal short circuit mainly include rod extrusion test, extrusion test, needle puncture test, heavy object impact test, forced internal short circuit test, NASA method, blunt needle test, etc.
The safety of power battery is the main limiting factor for the development of new energy power battery at present, and the internal short circuit is the most fatal failure in battery safety. Through the study of the evaluation system and the internal short-circuit evaluation, we can fully understand the different test methods and environments. The evaluation results of power battery safety characteristics are also different.
Various countries and regions are constantly improving relevant laws, regulations, standards and testing. To cope with increasingly demanding application environments. At present, China’s power battery, including the laws and regulations related to new energy vehicles, started relatively late, mainly referring to the systems of Europe and Japan. Only with stricter testing standards, more comprehensive testing scope and more complete testing links can the results be more authentic and fair. Only in this way can we really promote the design of power batteries: the attention to safety issues in the production process, promote enterprises to pay attention to the weak links of power batteries with a higher standard, and cover the whole life cycle of batteries with the awareness of battery safety protection, thus effectively reducing the injury and loss caused by battery safety.