Technology || Pouch Cell Lithium Battery Gas Causes And Measures

Pouch cell lithium ion battery slightly inflatable phenomenon will affect the use of electrical appliances, reduce battery performance, serious will burst packaging aluminum foil, resulting in leakage corrosion risk. In this paper, combined with the actual production, analysis of the types of gas and possible causes, and put forward solutions for your reference.

I. Types of gas generation

The gas generation of Pouch cell lithium ion battery can be divided into normal gas production and abnormal gas production. Normal gas production refers to the formation process in the cell production process. It is often called formation gas production, which is accompanied by the formation of SEI film. This kind of gas can be temporarily stored in the air bag and discharged in the subsequent process, without significant impact on the cell.

Abnormal gas production means that when the air bag is removed and packaging is completed, there is too much gas due to the abnormal occurrence in the battery. In this case, the gas can not be discharged, causing the battery cell to bulge, affecting the use of electrical appliances, and causing the deterioration of the battery cell performance. When the internal pressure is too large, it is easy to stretch the packaging aluminum foil, resulting in leakage, corrosion and other serious damage. Therefore, understanding the whole process of cell gas production and preventing abnormal gas production is the key to the production of flexible packaging lithium ion batteries.

1 Formation and gas production

Formation gas production refers to the formation process of the battery cell manufacturing process, that is, the first charging process of the battery, the electrolyte oxidation and reduction reaction on the electrode surface, the formation of solid electrolyte film (SEI film) accompanied by gas production; Chen Yikui et al from the 18th Research Institute of China Electronics Technology Group Corporation studied the comparison of gas production and gas composition analysis between positive and negative terminals, and concluded that gas production in the formation stage of batteries is mainly concentrated in the negative terminals. Huang Li et al. from the Xiamen Baolong Battery Research Institute studied in detail the types and quantities of gases produced at different formation voltages.

The results show that gas production is mainly H2 and CO2 below 2.5V. After 2.5V, a small amount of EC began to decompose, and the products were mainly C2H4. After 3V, DMC and EMC in the electrolyte begin to decompose, and gas production includes CH4 and C2H6 in addition to C2H4. When the voltage exceeds 3.8V, C2H4, the product of EC decomposition, basically disappears. When the voltage ranges from 3.0 V to 3.5V, gas production reaches the maximum, indicating that 3.5V is the main film forming region of SEI film.

SEI membrane ion conduction electron conduction, is composed of two layers in structure, the inner layer is dense and stable inorganic layer, the outer layer is porous and loose organic layer, thickness between 2nm and dozens of nanometers, the outer layer of organic products, has a certain flexibility, can improve the mechanical strength and integrity of the whole film, effective barrier solvent molecules in the electrode surface continuous reduction reaction, Therefore, gas production was basically completed after 3.5V due to the blocking effect of SEI film, and gas production decreased rapidly. The decomposition of EC in SEI membrane formation includes one electron reaction and two electron reaction:

One of the electrons reacts to form alkyl lithium carbonate and produces a large amount of ethylene gas. The two-electron reaction mainly forms lithium carbonate and CO gas:

Good electrolyte and suitable material matching can produce excellent and stable SEI film, which can not only effectively block electrolyte decomposition and improve the initial efficiency, but also reduce the amount of gas generated by the dissolution and regeneration of SEI. Therefore, effective selection of materials and electrolytic liquid system can reduce the gas production and improve the comprehensive performance of the battery.

2 Abnormal gas production

There are many factors that can lead to abnormal gas production in the production process of Pouch cell batteries, which can be divided into several categories: first, the cell itself is unstable in film formation. In the subsequent cycle process, the SEI film on the surface of the negative electrode may fall off or become loose, and the SEI film is reconstructed, accompanied by gas generation; Second, the water content inside the cell exceeds the standard. Third, the short circuit in the battery leads to abnormal gas production; Fourth, high temperature stored process gas production; Five is overcharging and overreleasing gas.

Among them, the density and stability of the film forming is the premise to ensure the performance of the cell, and the excessive water content and short circuit in the battery are often problems in the production process. The following is a brief analysis of these cases.

Ii. Analysis of the causes of abnormal gas production

1 Abnormal flatulence of water content

Due to the sensitivity of the whole battery system to water, although a large number of studies believe that the presence of trace water produced LiF makes the performance of SEI film more stable, but when there is excessive water, not only the consumption of lithium salt increases, reduce the battery performance, but also accompanied by a large number of gas production, causing the battery to flatness, resulting in battery failure. When lithium precipitates out of the negative electrode, it will react violently to water and generate heat, leading to more serious safety problems. Therefore, moisture control is the premise of lithium ion battery production, but also the flexible packaging battery in the production process needs to be strictly controlled process parameters.

There are two points in the performance of the excessive water cell: one is that the hydrogen content in the gas component is significantly increased, and the other is that the capacity of the gas cell is smaller than that of the normal cell. This is because water in the cell will have a series of reactions, resulting in a large number of side reaction gas generation, causing flatulence. In the whole reaction process, the water itself is electrolyzed during charging to produce hydrogen; Second, water reacts with the lithium salts in the electrolyte to produce hydrogen fluoride gas, which also corrodes aluminum foil.

It can be clearly seen that hydrogen content in the flatulence cell caused by abnormal water content increases significantly, and HF generated when placed is prone to corrosion reaction with aluminum foil, so HF is not detected in the gas composition.

The introduction of water content causes side reactions inside the battery, resulting in interface damage. Polarization increases in the formation process, easily reaching the charging potential. As a result, the charging time is generally smaller than that of the normal cell, so the charging capacity is smaller than that of the normal cell.

There are many reasons for the excessive water content in the cell, but it can be roughly divided into two categories: one is due to poor packaging, resulting in the water in the air entering the cell; The other is the poor moisture control in the production process, such as the separator is not dried and the electrolyte injection operation; Excessive dry aqueous humor content; The electrolyte introduces water and so on in the process of use. As shown in Figure 2, the water content in the bare cell increases with the increase of time. The vacuum baked bare cell should be sealed with liquid injection in time to prevent moisture from re-entering the separator with the increase of time, resulting in subsequent flatulent.

Variation curve of water content of bare cell with time after vacuum baking

2 internal short circuit flatulence

In the production process of the battery, when there is an internal short circuit point, the local temperature rises sharply, leading to the decomposition of the electrolyte. The analysis of the gas composition of this kind of flatulence battery shows that the content of CO2 increases greatly, which is because the electrolyte reacts with the decomposition product PF5 of LiPF6 in the presence of high temperature and trace water (20) and (21). The amount of CO2 increases significantly and flatulence occurs.

When the internal short-circuit occurs, the temperature can reach more than 200℃ and the separator is cauterized and carbonized. The cauterized short-circuit point can be generally found when the cell is disassembled, and the decomposition of positive Li0.5CoO2 at high temperature produces oxygen, which accelerates the decomposition of the main solvent EC in the electrolyte, as shown in the reaction below, resulting in serious gas swelling in general.

3. High temperature storage and overcharging and expelling gas

In the process of high-temperature storage and overcharge and overdischarge, LiCoO2 is in a metastable state and extremely unstable, resulting in the following decomposition reaction

In addition, when the stability of SEI film is low, the organic layer in contact with the electrolyte on the outer layer of the film will dissolve with the increase of temperature, such as (CH2OCO2Li)2 as the main component of the alkyl lithium ester layer of SEI film, very unstable, easy to occur such as decomposition reaction, gas generation, cell swelling;

4. Measures to inhibit abnormal gas production

In the normal voltage range, the gas production is less, and most of the hydrocarbons, when abnormal gas production occurs, it will produce a large amount of gas, damage the electrode interface structure, resulting in electrolyte decomposition failure, serious break through the packaging area resulting in leakage, corrosion risk. Material design and manufacturing process should be used to restrain abnormal gas production.

First of all, the material and electrolytic liquid system should be designed and optimized

To ensure the formation of dense and stable SEI film, improve the stability of cathode material, and inhibit the occurrence of abnormal gas production. For the treatment of electrolyte, a small amount of film-forming additives are often added to make the SEI film more uniform and dense, so as to reduce the shedding of the SEI film and the swelling of the battery caused by gas production in the regeneration process.

Relevant studies have been reported and applied in practice. For example, Chengsu of Harbin University of Science and Technology reported that the use of film forming additive VC can reduce the phenomenon of battery flatulence. But most of the studies focused on single component additives, and the effect was limited. Cao Changhe et al., East China University of Science and Technology, used VC and PS as a new electrolyte film forming additive, and achieved good results. The gas content of the battery was significantly reduced during high temperature shelved and circulation process.

The research shows that the SEI membrane formed by EC and VC is divided into linear alkyl lithium carbonate. The alkyl lithium carbonate attached to LiC is unstable at high temperature and decomposes into gas (such as CO2), resulting in battery swelling. The SEI membrane formed by PS is lithium alkyl sulfonate. Although the membrane has defects, it has certain two-dimensional structure and is stable under LiC high temperature. When VC and PS are used together, PS forms a defective two-dimensional structure on the negative surface when the voltage is low, and VC forms linear alkyl lithium carbonate on the negative surface with the increase of voltage. The alkyl lithium carbonate fills in the defects of the two-dimensional structure and forms a stable SEI film attached to the LiC with a network structure. The SEI film with this structure greatly improves its stability and can effectively inhibit gas production caused by membrane decomposition.

In addition, due to the interaction between positive lithium cobalt acid material and electrolyte, its decomposition products will catalyze solvent decomposition in the electrolyte, so surface coating of positive material can not only increase the structural stability of the material, but also reduce the contact between positive electrode and electrolyte, and reduce the gas generated by catalytic decomposition of active positive electrode. Therefore, forming stable and complete coating layer on particle surface of cathode material is also a major development direction at present.

Secondly, the manufacturing process parameters should be strictly controlled

To ensure the reliability of packaging and prevent flatulence caused by excessive moisture inside the battery, the control methods are as follows:

(1) The cell is fully dried after winding to prevent excessive moisture content in the film;

(2) Strictly control the time from the cell to the liquid injection after vacuum baking and the humidity of the drying room;

(3) Ensure the tightness of the liquid injection glove box;

(4) Control the content of water and free acid in the electrolyte;

(5) Standardize the storage environment and sealing conditions of electrolyte to prevent excessive water in the process of use and storage of electrolyte;

(6) Using closed mouth pressure formation or external air bag formation after vacuum sealing exhaust;

(7) Adopt multi-step formation and high temperature shelving process to ensure complete gas production;

(8) Improve packaging reliability.