With the rapid development of large-scale energy storage, smart grid, clean energy vehicles and consumer electronics, people are demanding higher and higher performance, reliability and safety of electronic energy storage devices. Lithium-ion battery has become one of the most widely used and mature electrochemical energy storage devices due to its high energy density, long cycle life and high rate performance. However, some failures may occur during the production, transportation and use of lithium-ion batteries. Even if a single battery fails, the performance and reliability of the entire battery pack will be affected, and even the battery pack will stop working or other safety problems will be caused. Compared with cylindrical and flexible packaging lithium batteries, square lithium batteries are more and more widely used in the field of electric vehicles because of their high reliability and simple structure. In recent years, there have been a number of fire and explosion accidents related to power batteries at home and abroad, and the safety of power batteries has received great attention. Battery short circuit light causes battery leakage, power battery insulation failure, heavy cause power battery system fire.
In this paper, the reason of low insulation check case of a square lithium battery module is analyzed, and the failure process and mechanism of core shell corrosion are discussed, and the improvement method is proposed.1 Case analysis of Low insulation check for a square lithium battery module
1.1 Troubleshooting Process
A power battery inspection found that the insulation value of one module was much lower than the standard value, only 0.113MΩ. The inspection found that the bottom shell of the No.1 battery of the module had obvious signs of electrolyte corrosion, the Busbar plate was well connected, and the Pin pin of the module was normal and no bending. Check the module process data and find that no exception is found. Test the electrical performance of the faulty module and find that the voltage and internal resistance parameters are normal and the insulation is abnormal. The fault module was disassembled, the blue film of each cell was intact, and the electrolyte leakage at the bottom of No. 1 cell was obvious. After the blue film was removed, obvious corrosion points were found. Check the process data of No. 1 cell and no abnormality is found. The measured negative voltage of the core against the shell is 0.596V (specification is not less than 1.5V). The voltage does not recover after opening the shell. The appearance of the two JR is normal, the bottom is not damaged, and there is no lap top cover at the welding of the pole lug. After JR is removed, the measured resistance value of the negative pole and the top cap of the cell is 0, as shown in Figure 1. CT analysis of the top cover of the battery found that there was a metal foreign body between the plastic and the top sheet in the negative pole, and the negative pole was connected with the top cap.
1.2 Cause Analysis
Check the manufacturing process of the cell top cover. The pole, PPS incoming material and pole injection process are the risk processes. There are two main reasons for the short circuit caused by the metal wire of the core top cover. One is the introduction of injection molding accessories: appearance blanking process, knife edge clearance is too large, negative pillar pier pressure produced copper particles; PPS incoming materials are mixed with stainless steel particles, which are embedded into parts PPS during injection molding. The second is the introduction of equipment: the dryer does not carry out closed feeding management in the injection molding machine, which has the risk of metal particles inhaling PPS raw materials in the workshop.
1.3 Failure process analysis
The core top cover insulation layer is mixed with metal chips, the core top cover electric test using PC500V voltage test 2s, the insulation resistance is required to be no less than 200MΩ standard test pass, 500V voltage can not break down the gap, after the core is welded, the negative aluminum block is heated to melt the surface insulation layer (PP material), the metal chips flow to the negative electrode edge with the melting insulation layer; The negative aluminum block insulation layer melts and then cools, the plastic solidifies and shrinks, so that there is a gap between the pole and the top cover sheet; The metal chips move in the gap because of vibration, which will occasionally make the negative column and the top sheet conductive. In the process of continuous charge and discharge, the core shell slowly corrodes, which leads to the core leakage and low insulation value.
2 Corrosion mechanism
The corrosion of square lithium ion core is actually the process of forming aluminum shell into aluminum lithium alloy, which needs to meet two necessary conditions. One is electronic short circuit, that is, the negative electrode of aluminum shell with low potential. The second is ion short circuit, in which the aluminum shell contacts lithium ions in the electrolyte.
Normally, the negative pole of the core is separated from the shell by an insulating layer, and the shell is at a high potential. When there are metal chips between the negative pole and the top sheet, the negative pole and the aluminum shell will have a short circuit, and the corrosion potential of the aluminum shell will be significantly reduced (the initial lithium embedding potential of the aluminum shell is about 1.4V). At the same time, the aluminum shell contacts with the lithium ions in the electrolyte, and the aluminum shell with low potential will have an electrochemical reaction with the lithium ions, as shown in Figure 3.
Electrons are transferred from the negative electrode to the aluminum shell, and lithium ions are embedded in the aluminum shell to form aluminum lithium alloy, which can be divided into three stages: Stage I reduction of aluminum oxide; The formation of stage Ⅱ; Phase III is in theory formed. As the lithium becomes more embedded, it reacts with oxygen and carbon dioxide in the air to form alkali metal salts, which gradually corrodes the aluminum shell. The core is surrounded by mylar cladding insulation, only at the bottom of the positioning hole and closing gap, which is often seen at the bottom of the core corrosion.
3 Preventive Measures
1) Strengthen process control to reduce the risk of introducing metal foreign bodies. High strength steel is used as the insert of the negative pier blanking die to reduce the gap between the cutting edge and the generation of burrs. Increase the shock time to ensure that burrs are fully polished; PPS raw material particles, adding metal separator for separation and packaging before packaging. The suction tube is bound to the dryer to reduce the risk of particle in the adsorption workshop.
2) Strengthen the insulation detection standard of the bottom line of the cell top cover. In this case, the factory electrical test data traced to the faulty core top cover is qualified. It can be seen that the current test standard 500V voltage cannot break down the gap between metal chips, negative pole and the top cover sheet, and the core top cover of this fault type cannot be detected. Under the premise of keeping DC500Vhipot voltage insulation resistance test at 200MΩ, add one 1500V2s2mA high voltage resistance 100% to the finished product test. Forward verification has been done for the finished product test. The CT results of the samples that failed the 1500V test showed the existence of metal wires.
4 Conclusion
It is very easy for metal foreign matter to cause internal short circuit of lithium-ion battery, and short circuit is the most common cause of thermal runaway. Therefore, it can be seen that the control of metal foreign matter in the production process of lithium-ion battery is important. Battery safety issues need the efforts of the majority of technical personnel to promote, so that the control of foreign matter in battery production process is getting better and better.
