Analysis of short circuit faults caused by soft pack lithium batteries and how to improve the design of soft pack lithium batteries with short circuit-CSIP

Compared to other cylindrical and square batteries, flexible packaging lithium batteries are becoming more and more common due to their flexible size design and high energy density. Short-circuit testing is an effective way to evaluate flexible packaging lithium batteries. This article analyses the failure model of battery short-circuit testing to find out the main factors affecting short-circuit failure; analyses the failure model by carrying out example verification under different conditions and gives proposals to improve the safety of flexible packaging lithium batteries.

The short-circuit failure of flexible lithium batteries usually includes liquid leakage, dry cracking, fire and explosion. Leakage and dry cracking usually occur in the weak area of the lug package, where the aluminium-plastic package can be clearly seen to be dry cracked after the test; fire and explosion are more hazardous safety incidents, and the cause is usually a violent reaction of the electrolyte under certain conditions after the aluminium-plastic is dry cracked. Thus, the condition of the aluminium-plastic package is a key factor in failure compared to the short-circuit test for flexible lithium batteries.

During the short circuit test, the open circuit voltage of the battery instantaneously drops to zero, while a high current is passed through the circuit and Joule heat is generated. The magnitude of Joule heat is determined by three factors: current, resistance and time. Although the short circuit current is present for a short period of time, the high current can still generate a significant amount of heat. This heat is slowly released over a short period of time (usually a few minutes) after the short circuit, resulting in an increase in battery temperature. As time goes on, the Joule heat is mainly dissipated into the environment and the battery temperature begins to drop. It is therefore presumed that short-circuit failure of the battery generally occurs at the moment of short-circuit and for a relatively short period of time thereafter.

Soft pack lithium batteries often develop a gas bulge during short circuit testing, which is supposed to be caused by the following reasons. The first is the instability of the electrochemical system, i.e. the high current passing through the electrode and electrolyte interface causes oxidative or reductive decomposition of the electrolyte and the gaseous product fills the aluminium-plastic package. The gas production bulge due to this cause is more pronounced under high temperature conditions, as electrolyte decomposition side reactions are more likely to occur at high temperatures. In addition, even if the electrolyte does not undergo decomposition side reactions, it may be partially vaporised by Joule heat, especially for electrolyte components with low vapour pressure. The gas production bulge caused by this cause is relatively temperature sensitive, i.e. the bulge largely disappears when the cell temperature drops to room temperature. However, regardless of the cause of gassing, the increased air pressure inside the battery during a short circuit will increase the dry cracking of the aluminium-plastic package and increase the probability of failure.

Based on the analysis of the process and mechanism of short circuit failure, the safety of flexible packaging lithium batteries can be improved from the following aspects: optimising the electrochemical system, reducing the resistance of the positive and negative lugs, and improving the strength of the aluminium-plastic package. Optimisation of the electrochemical system can be carried out from a number of angles, including positive and negative active materials, electrode ratios and electrolyte, in order to improve the battery's ability to withstand instantaneous high currents and short periods of high heat. Reducing the lug resistance reduces the generation and accumulation of Joule heat in this area, significantly reducing the thermal impact on the weak areas of the package. Improving the strength of the aluminium-plastic package can be achieved by optimising the parameters of the cell manufacturing process, significantly reducing the occurrence of dry cracking, fires and explosions.