On July 20, the 2018 Second China New Energy Vehicle Testing and Evaluation Technology Development Summit Forum was grandly opened in Pingshan, Shenzhen. This forum is under the guidance of Shenzhen Pingshan District Science and Technology Innovation Service Bureau, and is jointly sponsored by Chongqing Vehicle Inspection and Research Institute Co., Ltd. and Electric Vehicle Resource Network. For electric vehicles, the current main problems include high battery costs, mileage anxiety, and reliability issues. At this forum, He Xiangming, director of the New Energy and Material Chemistry Laboratory of the Institute of Nuclear and New Energy Technology, Tsinghua University, conducted an analysis on the safety and reliability of power batteries.
He Xiangming, Director of the New Energy and Materials Chemistry Laboratory, Institute of Nuclear and New Energy Technology, Tsinghua University
At present, there are many safety accidents caused by power batteries. It is worth noting that lithium-ion batteries that have safety accidents have passed safety certification before. The cause is basically an unpredictable internal short circuit. This internal short circuit seems It cannot be completely eliminated; the batteries that cause safety accidents are all qualified products when they are manufactured. So, where is the problem?
The concept of safety and reliabilityThe safety problem of power battery is mainly divided into two aspects: abuse of thermal runaway and self-induced thermal runaway. Among them, abuse of thermal runaway is predictable and can be evaluated through testing and can be improved through protective measures; self-induced thermal runaway is unpredictable and cannot be evaluated through testing, nor can it be completely eliminated through quality management. None of the safety measures can completely eliminate the safety hazards of lithium-ion batteries.
He Xiangming said that the accident caused by the internal short circuit of the power battery, that is, the self-induced thermal runaway, is a reliability problem, not a product quality problem. Product quality is a set of inherent characteristics of a product, describing the ability of the product to meet the requirements of customers and other related parties. The inherent characteristics of the product include performance characteristics-performance indicators, specialization-reliability, maintainability and supportability. Product reliability is the ability to maintain product performance over time. In other words, to maintain performance for a long time is not to fail. It is a very important quality characteristic of a product that it can be repaired quickly without failure or failure. To make products highly reliable and well maintained, reliability and maintainability design, testing and management must be carried out in product development.
Definition of Li-ion battery safety and reliabilityThe safety and reliability of lithium-ion batteries refers to the probability that a lithium-ion battery will not experience thermal runaway under normal conditions of use. A battery safety accident is an extreme failure event of the battery. Reliability estimation is a bottleneck problem in the design process of large-scale lithium-ion battery packs. He Xiangming believes that this is largely because people's understanding of batteries is still very limited, and there is a lack of sufficient experimental and theoretical accumulation in the technical research. However, in the field of engineering, to analyze the potential failure modes of batteries and develop corresponding mitigation measures, such reliability research, testing, and design are very important and necessary.
Exploration of safety and reliability assessment of lithium-ion batteriesHe Xiangming said that to avoid potential thermal runaway, the first thing to do is to prevent it: discover the causes of potential safety failures of lithium-ion batteries, find out potential test methods for safety failures of lithium-ion batteries, and find out what can avoid or reduce these potential safety failures. Measures, design measures that can avoid or reduce damage after safety failure.
There are many potential reasons for the safety failure of lithium-ion batteries. He Xiangming said that they mainly include local overheating, lithium precipitation in the negative electrode, metal particle foreign bodies, diaphragm defects, design/manufacturing defects, pole piece deformation, micro-short circuits and other reasons. When the local overheating exceeds T1, the battery may maintain a spontaneous exothermic "chain" chemical reaction, which will eventually lead to thermal runaway; the reaction between lithium evolution and the electrolyte generates heat, leading to local overheating, lithium precipitation, lithium dendrites, and short circuits. Causes local overheating; even minute metal contaminants that are not visible to the naked eye can cause internal short circuits; on the diaphragm, the hole may be plugged, causing lithium to precipitate around the blocked hole; on the pole piece, because the pole piece is in the charge and discharge process, The internal stress changes, the pole piece is deformed, or the pole piece expands and contracts during charging and discharging, resulting in an internal short circuit.
Potential test and evaluation methods for safety failure of lithium-ion batteriesSince the potential causes of the safety failure of lithium-ion batteries can be divided into four categories: internal short circuit, thermal stability, lithium precipitation, and abnormal pole position, it can correspondingly pass the micro short circuit test, thermal stability test, lithium precipitation test, and CT test. Evaluate the "safety and reliability" of current lithium-ion batteries. In this way, the reliability and failure reasons of the battery can be found to a certain extent, and the reliability of the battery can be further improved. He Xiangming believes that the safety and reliability of Japanese batteries are currently the best, and that China and South Korea have similar battery levels.
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