Liquid leakage analysis of soft-pack lithium-ion battery

Introduction

Efficient and reliable energy storage systems are crucial for our modern society. Lithium-ion batteries (LIBs) with excellent performance are widely used in portable electronics and electric vehicles (EVs), but frequent fires and explosions limit their further and more widespread applications. This review summarizes aspects of LIB safety and discusses the related issues, strategies, and testing standards. Specifically, it begins with a brief introduction to LIB working principles and cell structures, and then provides an overview of the notorious thermal runaway, with an emphasis on the effects of mechanical, electrical, and thermal abuse. The following sections examine strategies for improving cell safety, including approaches through cell chemistry, cooling, and balancing, and afterwards describe current safety standards and corresponding tests. The review concludes with insights into potential future developments and the prospects for safer LIBs.

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The manufacture of lithium-ion batteries is mainly divided into three stages of the process, the first is the production of pole pieces, the second is the packaging and production of cells, and the third is the activation and sorting of batteries. Each stage corresponds to several. Important processes. The first stage accounts for most of the battery production cost, the second stage determines the packaging form of the battery, and the third stage determines the battery grouping method and uses stability.

In this article, we will discuss the problem of corrosion and leakage of soft-pack batteries. For soft-pack batteries, the main process that plays the role of battery sealing is the packaging process. The quality of the packaging directly determines the performance of the battery. Simply speaking, the quality of the package is not very clear, and it needs to be analyzed from the type of leakage. Generally speaking, the causes of corrosion and leakage of the aluminium-plastic film can be divided into three categories:

(1)  Chemical corrosion

For lithium-ion batteries, because moisture will react with the electrolyte to generate HF, hydrofluoric acid is a particularly corrosive acid, which will cause serious damage to the aluminium-plastic film, current collector, and cathode materials. The reaction is as follows:

After the aluminium-plastic film is corroded and damaged, the battery will fail.

2. Electrochemical corrosion

The corrosion and leakage of soft-pack batteries caused by electrochemical reactions are difficult to find. Generally, as the use time becomes longer, the corrosion and leakage of the battery will gradually become serious. Due to the long latency time of this failure mode, once it occurs, it is very likely to cause a crisis of customer confidence. There are generally two conditions for electrochemical corrosion to occur:

1. Ion short circuit. An ion short-circuit channel is formed between the aluminium layer of the aluminium-plastic film and the anode.

2. Electronic short circuit. The aluminium-plastic film aluminium layer and the anode form an electronic short-circuit channel.

In this way, the aluminium layer of the aluminium-plastic film encapsulating the cell forms a short-circuit loop with the anode. The anode is the negative electrode of the cell, which is at a low potential part. Once in contact with aluminium, it will cause an electrochemical reaction through the electrolyte with high conductivity. As a result, the aluminium layer is continuously consumed. The two necessary conditions for the battery to corrode during a short circuit are indispensable.

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The place where the ion channel is easy to form is when the PP layer of the edge sealing part is damaged during heat sealing, or the PP layer is damaged when the edge is folded so that the ion channel is formed between the aluminium layer and the negative electrode. The place where the electronic short-circuit channel is easy to be formed is that the package position of the tab is damaged, and the contact between the negative electrode tab and the aluminium layer forms the electronic channel. If the negative electrode extends too much, it is easy to cause direct contact with the aluminium layer during heat sealing, resulting in an electronic short circuit. In addition to the electronic channel inside the cell, the unintentional contact between the negative electrode tab and the aluminium layer is also one of the reasons for the corrosion and leakage of the soft package, and this happens faster. When both the ion channel and the electronic channel are formed, the aluminium layer of the aluminium-plastic film is gradually embedded by lithium ions to form a lithium-aluminium alloy. After the aluminium layer is corroded, moisture can intrude into the battery unscrupulously, resulting in more intense damage, resulting in soft-pack batteries. The appearance of leakage failure.

Whether the battery is cycled at room temperature, cycled at high temperature, or put on hold at high temperature, it will produce different degrees of bulging and gas production. According to the current research results, the essence of causing the flatulence of the cell is caused by the decomposition of the electrolyte. There are two cases of electrolyte decomposition. One is that the electrolyte has impurities, such as moisture and metal impurities, which cause the electrolyte to decompose and produce gas. The other is that the electrochemical window of the electrolyte is too low, resulting in decomposition during the charging process. After the EC, DEC and other solvents of TiO2 get electrons, they will generate free radicals. The direct consequence of the free radical reaction is the production of low-boiling hydrocarbons, esters, ethers, and CO2. If there is too much moisture inside the battery, it will also cause more gas production, which will cause the battery to bulge and cause the weak part of the package to rupture.

Conclusion

Battery abuse, accidental bumping, and packaging failure caused by the battery itself bulging will also lead to battery leakage. Battery abuse, including battery overcharge, over-discharge, high-temperature use, etc., may cause the battery to generate gas, bulge and increase package failure.

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