Research on gas tightness detection technology of battery

Abstract: the importance, leakage principle and leak detection method of lithium ion battery leak detection are introduced. The application of gas tightness detection technology in lithium battery industry is analyzed. At last, the problems to be paid attention to and the focus of attention in ensuring the detection effect in production are pointed out.

Introduction of gas tightness detection methods for lithium ion batteries

The leakage of products in all walks of life is very harmful. In the lithium-ion battery industry, poor sealing of products will lead to serious degradation of battery performance, electrolyte leakage, battery bulging and even explosion, which seriously damage the reputation of products. Nowadays, the safety of products is paid more and more attention in the society. Therefore, lithium ion batteries are being paid more and more attention. The advantages and disadvantages of sealing are very important.

The lithium ion batteries in the market are mainly divided into three kinds of plastic shell lithium ion battery, metal shell lithium ion battery and aluminum plastic film soft packing lithium ion battery. The positive pressure bubble leak detection method is used in the prophase lithium ion battery industry, and the detection method can easily cause the deformation of the battery shell to be difficult to recover. Especially for the metal shell and the aluminum plastic film soft packing lithium ion is not suitable, and the detection efficiency and the accuracy rate is low, so it is imminent to explore a suitable leak detection method as soon as possible.

At present, there are several kinds of leak detection methods used in lithium ion battery industry: water inspection and leak detection, helium mass spectrometry leak detection, direct leak detection, flow leak detection, differential pressure leak detection and so on. Because the force between the gas is mainly repulsive force, the leak detection technology is a completely feasible [1] from the leakage mechanism. The overall leak detection equipment on the market is divided into three types: the positive pressure leak detector, the negative pressure leak detector and the helium mass spectrometer leak detector. The helium mass spectrometry leak detector has the highest precision and is widely used in the tightness of sealing products, but it is also very demanding for the sealing and fixture of the pipeline. It needs frequent maintenance and the high detection cost is prohibitive in many industries. Therefore, it is necessary to find an economical and feasible testing method through repeated practice. And ensure that the test results can fully meet the production needs.

Two, an overview of gas tightness detection technology

To get the most effective leak detection method, we must first understand the cause of the leakage. Leakage is caused by the leakage of the product itself, which is divided into two major types: physical leak and molecular leak. The physical leak refers to the crack or bubble. The leakage of the molecular leak refers to the gap between the molecules. For example, the micro penetration of the helium to the quartz glass can be made into a standard leak for the helium spectrum leak detector. According to the type of leakage, the leakage can be divided into two kinds: one is that the workpiece shell has a penetrating crack or tiny hole, and the fluid leaks outward under the pressure of the container inside and outside the container. This leakage is the main source of the leakage of most products. When the material is stored, the performance of the gas storage is poor, which leads to low vacuum or low pressure when the leak is detected. The internal electric core of lithium ion battery can determine whether the product's sealing performance is up to standard.

Figure 1 Schematic diagram of a vacuum differential pressure leak detector

Among them, 1 is the gas source; 2 is the pressure relief valve; 3 is the test pressure sensor; 4 is the pressure exhaust valve; 5 is the balance valve; 6 is the differential pressure sensor; the 7 is the ball valve; 8 is the reference material; 9 is the object; 10 is the ball valve; 11 is the volume corrector; 12 for the balance valve.

The specific detection process is divided into 4 stages: the pumping stage, the balance stage, the testing stage and the decompression stage, as shown in Figure 2.

Figure 2 test process diagram

1. The stage of gas extraction. Valves 4, 5 and 12 are opened at the same time, the vacuum source is connected, vacuuming the tested substance, reference material and gas path until the set pressure is reached.

2, balance stage. Valve 4 closes, cut off the vacuum source, and valves 5 and 12 remain open. The time setting at this stage is very important to ensure that the pressure of the tested substance and the reference material is equal at the end of the phase.

3. Test stage. Valves 4, 5 and 12 are closed at the same time. After setting time, the computer automatically determines the result based on the pressure difference between the tested substance and the reference material.

4, pressure relief stage. Valves 4, 5 and 12 open at the same time. The tested substance and reference material are communicated with the atmosphere. After setting the time, the whole detection process can be completed and the workpiece can be unloaded.

Three. Application of negative pressure air tight leak detection technology in lithium battery industry

At present, there are many kinds of leak detection methods used in the lithium industry on the market, in which the negative pressure leak detection technology is inspected, the product shell becomes invisible, the sealing performance is low, the maintenance is simple, the technology is made up of up to 100 layers of positive and negative electrode, and the structure is compact, so second kinds of leak detection are made. The leakage generated by the leakage must also be paid attention to. When leakage is detected, whether the sum of the leakage produced by these two kinds of leaks meets the standard is the result of leak detection.

There are many factors that affect leakage, including pressure, temperature, impact vibration, chemical corrosion and dirty plugging. Among them, the detection pressure, leakage rate, content product and beat are called the four elements of leak detection. Detection pressure refers to the differential pressure of the tested object. The relationship between pressure and leakage is expressed by formula (1):

(1) Q indicates leakage rate, unit ml/min; VW represents internal volume, unit ml; Delta P indicates detection pressure, unit Pa; t indicates detection time, unit s.

The influence of temperature on differential pressure is large. It can be expressed by formula (2) [2]:

(2) the delta Pt indicates differential pressure, unit Pa; t indicates room temperature, unit temperature; delta T indicates internal temperature change, unit temperature; P represents detection pressure, unit kPa.

According to experience, usually temperature