Compared with other cylindrical and square batteries, flexible packaging lithium batteries are becoming more and more popular because of their flexible size design and high energy density. Short circuit test is an effective way to evaluate flexible packaging lithium battery. By analyzing the failure model of battery short circuit test, this paper finds out the main factors that affect short circuit failure. Through the example verification under different conditions, the failure model was analyzed, and the suggestion to improve the safety of flexible packaging lithium battery was given.

The short-circuit failure of flexible packaging lithium batteries usually includes leakage, dry cracking, fire and explosion. Leakage and dry cracking generally occur in the weak area of the pole ear package. After testing, the dry cracking of the aluminum-plastic package can be clearly seen. Fire and explosion are more hazardous production accidents, and the cause is usually aluminum plastic dry crack, electrolyte reaction under certain conditions. Therefore, compared with the short-circuit test of flexible packaging lithium battery, the packaging condition of aluminum plastic material is the key factor leading to failure.

In a short-circuit test, the battery‘s open-circuit voltage drops to zero instantly, while a large current passes through the circuit and Joule heat is generated. The magnitude of joule heat depends on current, resistance and time. Although the short circuit current exists for a short time, it can still generate a large amount of heat due to the large current. This heat is released slowly after a short circuit (usually a few minutes), resulting in an increase in battery temperature. With the extension of time, joule heat is mainly lost to the environment, and the battery temperature also begins to drop. Therefore, it is speculated that the battery short-circuit failure generally occurs in the short circuit moment and a short time after.

Flexible packaging lithium batteries in short circuit testing often produce gas bulge phenomenon, this should be caused by the following reasons. The first is the instability of the electrochemical system, that is, when the high current passes through the interface between the electrode and the electrolyte, it causes the oxidation or reduction decomposition of the electrolyte, and the gas products are filled in the aluminum-plastic package. The gas production swelling caused by this reason is more obvious at high temperature, because the side reaction of electrolyte decomposition is more likely to occur at high temperature. In addition, the electrolyte may partially vaporize under the action of Joule heat even if no side reaction of decomposition occurs, especially for the electrolyte components with low steam pressure. The gas production bulge caused by this reason is relatively sensitive to temperature, that is, the bulge basically disappears when the battery temperature drops to room temperature. However, regardless of the cause of gas production, the increase in air pressure inside the battery during short circuit will aggravate the cracking of the aluminum plastic package and increase the probability of failure.

How to improve the design of soft pack lithium battery short circuit

Based on the analysis of the process and mechanism of short circuit failure, the safety of flexible packaging lithium battery can be improved from the following aspects: optimize the electrochemical system, reduce the positive and negative lug resistance, and improve the strength of aluminum-plastic packaging. The optimization of the electrochemical system can be carried out from positive and negative active materials, electrode ratio and electrolyte, so as to improve the ability of the battery to withstand instantaneous large current and short-term high heat. Reducing the resistance of the pole ear can reduce the joule heat generation and accumulation there, and greatly reduce the heat impact on the weak area of the package. Improving the strength of aluminum-plastic packaging can be achieved by optimizing the parameters in the battery manufacturing process, greatly reducing the occurrence of dry cracking, fire and explosion.