Exploration on the Safety and Reliability Assessment of Lithium-ion Battery

On July 20, 2018, the second China New Energy Vehicle Test and Evaluation Technology Development Summit Forum was opened in Pingshan, Shenzhen. The forum was directed by the Shenzhen Pingshan District Science and Technology Innovation Service Agency and was jointly organized by Chongqing Vehicle Inspection Research Institute Co., Ltd. and Electric Vehicle Resources Network. For electric vehicles, the main problems at present include high battery cost, mileage anxiety, and reliability issues. At this forum, Hexiangming, director of the New Energy and Materials Chemistry Research Office of the Institute of Nuclear Energy and New Energy Technology of Tsinghua University, mainly analyzed the safety and reliability of power cells.

At present, due to the large number of safety accidents caused by power cells, it is worth noting that lithium ion batteries that have occurred in safety accidents have previously passed safety certification. The reason for the occurrence is basically caused by unpredictable internal short circuits, which seem to be unable to completely eliminate; Batteries that cause safety accidents are all qualified at the time of manufacture. So, what's the problem?

The concepts of safety and reliability

The safety of power cell is mainly divided into two aspects: abuse heat out of control and self-induced heat out of control. The abuse of thermal runaway is predictable, can be evaluated by testing, and can be improved by protective measures; Self-induced thermal runaway is unpredictable and can not be evaluated through testing, nor can it be completely eliminated through quality management. At present, all safety measures can not completely eliminate the safety hazards of lithium ion batteries.

Mr Hexiangming said that accidents caused by internal short circuits, that is, self-induced thermal runaway, were reliability problems, not product quality problems. Product quality is a set of inherent characteristics of the product, describing the ability of the product to meet the requirements of customers and other interested parties. The inherent characteristics of the product include performance characteristics-performance indicators, speciality-reliability, maintainability, and support. Product reliability is the ability to maintain product performance over time. In other words, to maintain performance for a long period of time is not to fail. Failure or failure can be quickly repaired and is an important quality feature of the product. To ensure high reliability and good maintenance of products, reliability, maintenance design, testing and management should be carried out in product development.

Definition of safety and reliability of lithium-ion batteries

The safety and reliability of lithium-ion batteries refers to the probability that lithium-ion batteries will not lose heat under normal operating conditions. The safety accident of the battery is an extreme failure event of the battery. Reliability estimation is a bottleneck problem in the design of large lithium ion batteries. Hexiangming believes that this is largely due to the fact that people's understanding of batteries is still very limited and there is not enough experimental and theoretical accumulation in this technical research.. However, in the field of engineering, it is very important and necessary to analyze the potential failure mode of batteries and develop corresponding mitigation measures.

Exploration on the Safety and Reliability Assessment of Lithium-ion Battery

Hexiangming said that to avoid potential thermal runaway, the first thing to prevent is to find out the reasons for the potential safety failure of lithium ion batteries, find out the potential test methods for the safety failure of lithium ion batteries, and find out what can prevent or reduce these potential safety failures. Design measures to avoid or reduce post-safety failure damage.

There are many potential reasons for the safety failure of lithium-ion batteries. Hexiangming said that it mainly includes local overheating, negative electrode analysis of lithium, metal particle foreign bodies, diaphragm defects, design/manufacturing defects, electrode deformation, and microshort circuits. When local overheating exceeds T1, the battery may maintain a spontaneous exothermic "chain" chemical reaction that eventually leads to thermal runaway; Lithium and electrolyte reaction exothermic, resulting in local overheating, lithium deposition of lithium dendritic, short circuit, leading to local overheating; Even tiny metal pollutants that are not visible to the naked eye can cause internal short circuits; In terms of diaphragm, it may be due to Kongdusai, resulting in the outflow of lithium around the plug hole; In the polar film, due to the change of internal stress, Polar deformation, or the expansion and contraction of the polar film during the process of charging and discharging, the internal short circuit is caused.

Potential Test and Evaluation Method for Safety Failure of Lithium-ion Battery

Due to the potential causes of safety failure of lithium-ion batteries, they can be divided into four categories: internal short circuit, thermal stability, lithium precipitation, and abnormal polar position. Therefore, the "safety and reliability" of the current lithium-ion battery can be evaluated by microshort circuit test, thermal stability test, lithium extraction test, and CT test. In this way, the reliability and failure of the battery can be found to a certain extent, and the reliability level of the battery can be further improved. Hexiangming believes that Japanese batteries are currently the best in terms of safety and reliability, and China and South Korea have the same level of batteries.

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