What Are The Reasons For Leakage At The Seal Of A Pouch Cell?

Soft-pack batteries, with their advantages of high energy density, lightweight design, and flexible structure, have been widely applied in fields such as consumer electronics, new energy vehicles, and energy storage systems. Compared with cylindrical and square batteries, soft-pack batteries use aluminum-plastic composite films as the outer packaging, eliminating the need for rigid metal shells, resulting in higher space utilization and lighter weight. However, the sealing process has become the most critical and prone-to-problematic in production. If the sealing fails, it will directly lead to leakage of electrolyte, water intrusion, internal gas expansion, and contamination of the electrode sheets, seriously affecting battery performance, safety, and cycle life. Therefore, the quality control of the sealing process for soft-pack batteries is an indispensable core procedure in the cell manufacturing process.

The leakage of the soft-pack battery seal is rarely caused by a single factor. In most cases, it is the result of the combined effects of process parameters, material properties, equipment status, environmental conditions, and human operations. Even if all parameters seem to be within the standard range, the accumulation of small deviations will eventually form micro gaps or leakage channels at the seal interface.

1. Inappropriate process parameters: Temperature, pressure, and time are the core factors for sealing.

Heat sealing is the most common sealing method for soft-pack batteries. Its essence is to cause the inner CPP or PP layer of the aluminum-plastic film to melt, flow, and tightly bond together to form a continuous sealing layer. The three parameters - temperature, pressure, and holding time - directly determine the sealing strength and sealing performance. Any deviation in any of these parameters can lead to leakage.

1.Temperature anomaly

When the temperature is too low, the inner layer of the aluminum-plastic film does not melt fully, and the interface bonding strength is weak. It may appear intact to the naked eye, but in fact, there are a large number of micropores, and later it will form leakage channels; when the temperature is too high, it will cause the high molecular material to undergo thermal aging and decomposition, destroying the bonding strength between each layer of the aluminum-plastic film, resulting in delamination, brittleness, and a significant decrease in mechanical strength. It is very likely to crack under external force.

2. Abnormal Pressure

Insufficient pressure will prevent the molten polymers from fully filling the surface depressions, resulting in voids at the bonding interface; excessive pressure will cause the molten layer to be excessively extruded, leading to a thinner sealing layer, local material shortage, and even damage to the aluminum layer, thereby reducing the barrier property. Uneven pressure distribution is a more common problem. Local pressure being too high or too low at the sealing edge will form weak areas with inconsistent bonding strength, which will fail first under vibration, expansion, and compression.

3. Insufficient holding time

The holding time is too short. The material does not have sufficient time to melt and level out before cooling and solidifying, resulting in insufficient sealing strength. If the holding time is too long, it may cause overheating deformation and material degradation, which also affects the sealing reliability. On high-speed production lines, the compression of the production cycle will lead to a decrease in the consistency of the holding time, and the risk of leakage will significantly increase.

ACEY battery heat sealing machine is mainly used in heating type sealing equipment on the top and side of soft packaging lithium-ion batteries. The device uses resistance heating to seal the head, so that the aluminum-plastic film can be melted under a set pressure.

II. Defects in the material itself: The aluminum-plastic film is the foundation of the sealing quality

The sealing performance of pouch batteries is highly dependent on the quality stability of the aluminum-plastic composite film. Even with perfect manufacturing processes, material flaws cannot result in a reliable seal.

1. Uneven thickness of the aluminum-plastic film and coating deviation

The fluctuation in the thickness of the inner heat-sealing layer can result in some areas of the same sealing surface melting fully while others being inadequately sealed, forming local weak points. 

2. The membrane material may have pinholes, scratches, and microcracks.

These defects can occur during the production, cutting, transportation, and loading processes of the membrane material. They are difficult to detect with the naked eye, but they will directly become leakage channels and cannot be remedied through the process.

3. Compatibility issue between the end shield and the sealing edge

When the end shield passes through the sealing area, it will cause a discontinuity in the interface. If the parameters are not optimized for the end shield area, gaps are likely to occur around the end shield, which is a high-risk leakage location.

III. Leakage-causing contamination in sealing zones: The most common cause of leakage in production

In actual production, contamination in the sealing zones is the primary factor leading to leakage. Even a small amount of contaminants can prevent the fusion of polymer layers, forming permanent microchannels.

1.Electrolyte contamination

During the injection, standing, and transportation processes, the electrolyte is prone to splashing or seeping into the sealing area. Its chemical composition can disrupt the molten bonding of the PP/CPP layer, resulting in a sudden drop in sealing strength. Such leaks are often not obvious in the early stage and gradually fail during aging and cycling processes.

2. Dust, fragment debris of the film, metal particles

Dust from the workshop, film fragments, and protrusions from the electrode leads fall into the sealing area. They will form an isolation layer between the two layers of film, preventing complete fusion and leaving gaps.

3. Moisture and Oil Stains

High environmental humidity, oil stains on gloves, and leakage of lubricating grease from equipment can all contaminate the sealing surface, thereby affecting the bonding strength.

IV. Equipment and Tooling Status: Influencing the Uniformity and Stability of Sealing

The accuracy and maintenance level of the sealing equipment and tools directly affect the sealing consistency of each battery cell. Equipment deterioration can lead to batch defects.

1.The sealing knife is not heated evenly.

If the heating rod ages, the temperature control probe drifts, or the surface of the sealing knife is worn or contaminated, it will cause inconsistent temperature distribution on the sealing surface, resulting in local over-sealing or under-sealing.

2. Inconsistent parallelism of the sealing knives, and misalignment of the fixtures

If the upper and lower sealing knives are not parallel, and there is a deviation in the centering of the fixtures, it will result in uneven pressure distribution, causing the sealing edge to be tight on one side and loose on the other. After long-term use, the sealing knives will deform and wear out, which will further increase the risk of leakage.

3. Inadequate cooling control

If the cooling speed after heat sealing is too fast or too slow, it will affect the crystalline state of the polymer, resulting in low sealing strength and easy cracking.

4. Unstable pneumatic/hydraulic system

Pressure fluctuations and response delays result in unstable holding pressure and a variable sealing effect.

V. Operation and Environmental Factors: Hidden Defects Caused by Human Factors and Environment

1. Transportation and Clamping Damage

During the transfer of the battery cells between processes, they may be compressed, scratched, or have creases, which can cause local damage to the aluminum-plastic film and result in hidden leakage points after sealing.

2. Sealing position alignment deviation

If the cell positioning is inaccurate and the sealing width is insufficient, it will result in an insufficient effective sealing width and insufficient strength.

3. Excessive environmental temperature and humidity

An environment with high temperature and high humidity will accelerate the moisture absorption of the aluminum-plastic film and the evaporation of the electrolyte, causing contamination in the sealing area; while a low temperature will affect the melting characteristics of the materials.

VI. Summary: To prevent leakage at the sealing of pouch batteries, a comprehensive control throughout the entire process is necessary.

The leakage at the sealing of pouch batteries is not an isolated incident; it is the result of the combined effects of five factors: process, materials, pollution, equipment, and environment. To reduce the leakage rate at the source during production, a full-process control system must be established.

For battery equipment suppliers, providing stable and reliable pouch battery sealing machines, heat sealing molds, online inspection equipment, as well as accompanying process debugging and on-site support, is the key to helping customers solve the leakage problem. We focus on the export of lithium battery manufacturing and testing equipment. Based on different cell specifications, material characteristics, and production capacity requirements, we can provide customized sealing solutions and a complete set of technical support to help global customers improve sealing yield and ensure battery safety.