AGM separator-The third pole of the AGM battery

For flooded batteries, the basic role of the micropores separator is to isolate the plates with opposite polarity, avoid electrical contact between them, while ensuring high ion conductivity, allowing the ions to move freely between the plates.

The AGM separator used by VRLA battery has the following other functions:

Absorb the electrolyte (the third active substance of the battery) so that it does not flow. Provides relatively large gas transport holes for oxygen diffusion and thus facilitates COC operation.

Ensure a high ion electrical conductivity. Providing a transmission channel for the ion flow, enabling it to transfer between the two types of plates, allowing the redox reactions to proceed quickly.

Limiting the PAM volume expansion, maintaining the plate group pressure, and minimizing the pulsation effect of the positive plate active substances during the circulation.

Figure 14.5 shows the scanning electron microscopy images (SEM) of the AGM separator sample. As can be seen from the figure, the AGM separator is composed of chemical-grade borosilicate glass fibers of 1 to 2 mm in length and varying in thickness (0.1 to 10μm in diameter). The proportion of different fibers determines the balance between different functions of separator and the price of separator. These fibers are hydrophilic and absorb the electrolyte. The smaller fibers (i.e., those with smaller diameters) in the separator have a larger surface area, and forms a smaller inner diameter of the micropores, but at a higher price. The AGM separator also contains 15~18% of PP, PE and other polymer fibers, which improve the mechanical strength of the separator, and promote the formation of gas channels (because these materials are partially hydrophobic), and also reduce the price of the separator. The production process of AGM separator is similar to that of papermaking, which makes it an anisotropic structure. Its structural characteristics are that the aperture of the X-Y plane of the separator is 2-4 μm, while the size of the micropores perpendicular to the X-Y plane is 10-30 μm. The role of the X-Y plane holes is to distribute the electrolyte in the direction of the thickness of the separator and maintain its core absorption rate when the separator is partially filled with electrolyte. Large pores form open gas channels.

Transmission of the gas through the AGM separator

After the oxygen is precipitated from the positive plate, it is transmitted to the negative plate, and then a reduction reaction occurs in the negative plate. The whole oxygen transmission process goes through the following stages.

First, the oxygen forms tiny bubbles in the PAM micropores filled with the electrolyte. The tiny bubbles then gradually merge into discrete bubbles that gradually replace the electrolyte in the micropores of the plate facing the separator. A small fraction of the oxygen in the bubbles reaching the plate surface is dissolved in the electrolyte, while most of the gaseous oxygen remains at the interface of the plate / separator in the form of the bubbles. The AGM separator is a heterogeneous structure, so that oxygen accumulates in areas where the AGM surface fiber density is low (loose structure) or in some vacancies between the plate and the separator (tube electrode / AGM).

The pressure on the plate group can make the contact between the glass fiber surface and the surface of the plate more closely, and promote the oxygen penetration of the separator. There are two possible reaction mechanisms:

1. When the plate group pressure is low, the volume of gas accumulated at the plate /AGM separator interface increases. Under the force of gravity, the air stream will rise vertically. The electrolyte is twice as dense as the gas, pushing the gas up into the upper space of the plate group. So the oxygen will leave the plate group. The vertical flow rate of gas depends on the current passing through the battery, the temperature of the electrolyte, and the condition of the battery (such as a new battery or a long used battery).

2.When the plate group pressure is high, the separator tightly presses the plate, and the air bubbles enter the separator. The bubble moves horizontally to try to increase the gas passage in the separator. The density of the glass fiber structure is not uniform, and the bubbles enter the parts with low fiber density. The bubbles not only move randomly, but also move parallel to and in a direction perpendicular to the surface of the separator. However, the flow mainly moves through the AGM separator to the negative plate where the gas pressure is minimal, and the pressure gradient pushes the oxygen in this direction. Under pressure, the gas displaces the electrolyte in the micropores of the separator, thus forming a gas channel. When a continuous gas channel is formed, the movement of oxygen between the positive and negative plates is accelerated.

During the production of AGM separator for VRLA battery, the thickness of the separator is measured at a standard pressure of 10kPa. In order to improve the contact between the plate and the separator, the plate group (active body) is compressed to reduce the thickness of the separator by approximately 25%. The plate group of the high stationary battery is secured with a plastic bandage before loading into the battery case to maintain the plate group pressure.

In conclusion, the AGM separator are endowed with more functions. These functions are crucial for AGM batteries, as much as the positive plates and negative plates. Plate group to maintain a certain pressure, in addition to achieve oxygen transmission, more important is to ensure the separator conductivity.