Micropore system of the AGM separator

Keywords: AGM separator; Micropore

The performance characteristics of VRLA battery are largely determined by the capillary characteristics of the AGM separator, which is the ability to keep the micropores in the thickness direction of the separator filled with electrolyte and prevent the electrolyte from drying up and causing delamination. These characteristics are affected by the Micropore structure of AGM, especially the distribution of micropores. The expert studied the structure of AGM separator in depth. He performed core absorption measurements on AGM separator made of fine and thick glass fibers. After the AGM separator is cut into strips, it is erected and placed in H2SO4 solution with a relative density of 1.28. The lower part of the separator is immersed in the solution, and the time for the electrolyte core absorption to reach different heights is measured. Figure 14.6 shows the core absorption uptake rate (height/core absorption uptake time) for AGM separator containing 0%, 10%, 50% or 100% fine fibers.

It can be observed from the figure that there is an obvious linear relationship between them. Using Washburn equation and further theoretical analysis, The expert obtained the equation of Laplace as follows:

Where P is the capillary pressure, r is the aperture, γ is the inner surface tension, and θ is the contact Angle. AGM separator with a certain micropore size structure can be produced by changing the ratio between small and large fibers.

FIG. 14.8A shows the pore size distribution of AGM separator samples represented by radii, which are produced by using A low-pressure extrusion process.

As shown in Figure 14.8A, about 90% of the micropores were 10 to 24μm in diameter. These are mainly Z-plane micropores. About 5% of the macropores were between 30 and 100μm in diameter. In VRLA battery, the AGM separator hole system is in close contact with the two kinds of plate hole system. FIG. 14.8B shows the micropore size distribution of active substances in positive and negative plates [17]. For the new and fully formed plates, 80% of the micropore size of the active material is less than 1μm. This value is much lower than the medium aperture of AGM separator. When the plate group is pressed, the AGM separator is close to the plate, thus ensuring close contact between the two surfaces. After the vacuumizing, the filling electrolyte is first absorbed by the plate micropores, and then by the AGM separator micropores. According to technical requirements, the AGM separator shall ensure that 96% of the micropores are filled with electrolyte.

When the plate starts to gas out, the electrolyte in the micropores of the plate is extruded and quickly adsorbed to the micropores of the separator, so that the separator is completely saturated. When the circuit is disconnected, the gas leaves the plate microhole, and the electrolyte drawn by the separator is sucked back into the plate microhole. Thus, only the large-diameter micropores in the AGM partition remain empty, while the plate micropores are filled with electrolyte. Therefore, the parameter "electrolyte saturation" is mainly used for AGM separators.

FIG. 14.9 shows the pore ratio of a 225g/m2 AGM sample as a function of applied pressure (not exceeding 138kPa). The porosity was expressed as the proportion (percentage) between the volume of micropores and the total volume of AGM separator[18].

Under the above pressure, the pore ratio of AGM changes greatly. The wall thickness of the battery case should be large to resist this high pressure. Therefore, the pressure in the X-Y plane of the separator has only a slight effect on the porosity [18].

Why does pressure have such a small effect on the porosity of the AGM separator? The separator glass fiber structure consists of randomly overlapped fibers, but this is mainly in the X-Y plane of the AGM, and the largest hole is in the Z-axis direction perpendicular to the X-Y plane. The effect of separator compression on the hole size along the Z axis is very weak. In fact the expert has confirmed this [16]. However, this is not the case for hole sizes in the X and Y directions. Under the action of pressure, these holes change significantly. Therefore, 15% compression reduces the diameter of the micropore by 50%[19].

In conclusion, the micropore structure determines the nature of AGM separator. The smaller pore of the plate is easier to absorb electrolyte. In the process of charging and discharging, the electrolyte is dynamically distributed in the whole plate group. The plate group is pressed so that the separator is tightly attached to the plate to ensure ion transport and oxygen transport.