4.0 Separators

A separator is a material that is inserted between the positive and negative plates of the cell to prevent short circuiting. The development of separators was essential to the growth of the lead-acid battery industry. They have gone through many changes during their history. The earliest designs of lead-acid battery employed materials such as porous pot, flannel and felt to separate the plates. In the 1880s, perforated hard rubber, India rubber, sponge and cork were used as separators. The first use of wood separators was in 1892 by E.P. Usher and by G.H. Roe and G. Sutro. Wood was the separator of choice for many years. It began to be phased out in favor of rubber, cellulose and PVC separators as recently as the 1940s and 1950s.

The separators used today in lead-acid batteries reveal a great deal of variation in materials, design and in how they are used in the battery assembly. They can be made from plastic, rubber, glass fiber, cellulose and sulfuric acid gel. Today, most separators for flooded types of automotive and industrial batteries are made from microporous polyethylene, while in valve-regulated batteries, absorptive glass mats or sulfuric acid gels are employed. While separators in flooded types of batteries act as a physical barrier between the plates, the separators in valve-regulated batteries perform an additional function acting as a medium for transport of oxygen from the positive to the negative plate.

Separators for flooded batteries have a standardized design a backweb on which ribs are formed. The backweb prevents the plates from touching, while the ribs form channels that contain the electrolyte and allow escape of gas from between the plates. A wide variety of backweb thicknesses and rib configurations are used by the battery industry, depending on the desired properties of the separator and the battery. A typical profile for an automotive battery separator is shown in Figure 4-1. Industrial battery separators have a considerably thicker backweb for longer life and taller ribs to increase the amount of electrolyte between the plates.

 

Figure 4-1: Typical profile of microporous polyethylene automotive battery separator

 

The most common separators for flooded batteries are produced from polyethylene, polyvinyl chloride, phenolic resins and natural rubber. Polyethylene separators are produced from a mixture of high-molecular-weight polyethylene, silica and oil which is extruded into a sheet at high temperature. Ribs are then added by passing the sheet through calender rollers. Some of the oil is then extracted with organic solvents, leaving about 10%-20% remaining in the separator to improve its flexibility and oxidation resistance. The flexibility of microporous polyethylene separators provides a very desirable mechanical advantage. It allows them to be formed into sleeves and pockets for improved isolation of the plates.

Polyvinyl chloride separators are manufactured from a mixture of powdered PVC, silica, water and a solvent. This is extruded at an elevated temperature and calendered to provide the required number and design of ribs. The solvent is extracted in hot water. After drying, a rigid, porous sheet results.

Rubber separators are produced from a blend of rubber, silica and water. These components are blended in a mixer, extruded into a sheet and calendered to form the ribs. The extruded sheet is then vulcanized to produce a hard, rigid sheet. Curing can also be done by cross-linking with an electron beam, producing a more flexible product.

Phenolic separators are produced by blending silica with phenolic resin and then forming this mixture into a sheet on a polyester scrim. Ribs are then extruded onto the resin sheet in a separate operation.

Although separators perform an essential function in ensuring long life, they reduce the capacity and high-rate performance of batteries. This is because they displace electrolyte and add electrical resistance. To reduce these losses, separator manufacturers are constantly working to increase the porosity and reduce the electrical resistance of their products without sacrificing mechanical strength.

The specific properties common to all separators are:

 

4.1 Life

Low acid solubility and oxidation resistance are important to ensure that the separator is capable of functioning over the life of the battery. Battery separators are subjected to both physical and chemical degradation in service. Significant increases in temperature, electrolyte concentration, overcharge and service conditions can accelerate the degradation to some degree. The failure mode is very dependent on the material used to make the separator and the type of battery service. The rate at which the separator degrades in the battery depends on the thickness of the backweb and, in the case of microporous polyethylene separators, on the amount of residual oil. Increasing the amount of oil in the separator improves its oxidation resistance, but if the oil content is too high, it can leach out of the separator and cause problems, such as blocking of the vent caps. There is no industry standard test for physical and chemical degradation of separators, but the following tests are in use by battery and separator manufacturing companies:

 

4.2 Electrical Resistance

Electrical resistance of the separator is a measure of its ability to transfer ions from one electrode to the other. This has a direct effect on the performance of the battery by increasing the internal resistance of the cells. The major contributor to resistance is the backweb and, to a lesser extent, the resistance of the ribs. The resistance of the separator can be expressed as:

 

      Rsep =Rel (t2/p-1)

Where: Rsep = separator resistance (Ω cm)

            Rel = electrolyte resistance (Ω cm)

            T = tortuosity = l/d

            P = porosity

 

Figure 4-2 shows the relationship between backweb thickness and electrical resistance for a typical automotive battery separator. The graph shows that for every 0.001 inch increase in backweb thickness, the electrical resistance increases by approximately 7%. Clearly, separators for batteries designed to operate at high discharge rates must have a thin backweb. However, for industrial batteries which are discharged at much lower rates, a thicker backweb can be employed.

 

Figure 4-2: Electrical resistance of battery separators

4.3 Acid Displacement

Acid displacement results from separators taking up space that could be utilized for additional electrolyte. The amount of acid displaced is the volume of the solid fraction of the separator and can be calculated from:

 

      D = (Vb + Vr) (1-P)

Where: Vb = volume of backweb

            Vr = volume of ribs

            P = porosity

For an automotive battery separator with a 0.008-inch backweb, the acid displacement will be around 95 ml/m2. If the backweb is reduced to 0.006 inch, the acid displacement will be reduced to 80 ml/m2. This provides additional electrolyte to participate in the cell reaction, which increases the capacity. Alternatively, the plates can be spaced closer together while still retaining the same electrolyte volume. This will improve the high-rate performance of the battery. Ideally, the backweb thickness should be as low as possible. However, consideration has to be given to the strength and durability of the separator. This results in a trade-off between performance, strength and life. As a general rule, separators for automotive batteries will have a backweb in the 0.008-0.010 inch range while a backweb of 0.030-0.040 inch is common in industrial batteries.

4.4 Porosity

Porosity is that fraction of the separator volume that is composed of voids that are capable of holding sulfuric acid. The greater the porosity, the greater is the amount of electrolyte that can be retained in the separator and, for a given thickness, the lower the electrical resistance. Not all the pores in a separator are filled with electrolyte. Some are totally enclosed by the separator material. Some are also dead ended, preventing ions from being transported between the electrodes. The pores can be thought of as storage areas holding the inventory of sulfuric acid that participates in the electrochemical reaction. Ideally, the higher the porosity, the better. However, once again, there is a trade-off between the ideal and serviceability.

4.5 Strength

Strength of separators encompasses a number of factors including stiffness, puncture resistance, rigidity, flexibility, brittleness and tensile strength. These factors become most important during assembly, when damage can lead to short circuits in the battery. The use of expanded metal grids with exposed sharp protrusions requires separators that have high puncture resistance, while flexibility is essential for enveloping or sleeving the plates. High tensile strength and flexibility are required for separators that are to be used with automatic wrapping/stacking machines. At the end of the 20th century, automotive batteries were almost exclusively built with enveloped plates that require high flexibility and tensile strength. Therefore, microporous polyethylene separators are by far the most widely used.

4.6 Flexibility

Flexibility can be increased by reducing backweb thickness. However, this leads to reduced stiffness, which can result in problems during assembly. The loss in stiffness in the direction of the ribs is only minor, but is significantly more serious in the cross-rib direction. A reduction in backweb thickness from 0.008-0.006 inches results in a reduction in stiffness of only 2% in the rib direction, but a reduction of 60% in the cross-rib direction (W. Bohnstedt, Journal of Power Sources, 67 (1997) pp. 299-305). A loss of stiffness results in a loss in accuracy of cutting and folding during production of separator pockets. To reduce this problem, separator profiles have been developed with cross ribs that have a considerably lower profile to avoid hindering the release of gas from between the electrodes. With suitable design, cross ribs can compensate for loss in stiffness without significantly affecting the electrical resistance and acid displacement.

4.7 Puncture Resistance

Puncture resistance also depends on backweb thickness. This relationship is shown in Figure 4-3. The puncture resistance can be increased by increasing the ratio of polymer to filler in the material. This, however, increases the cost, reduces the porosity and increases the electrical resistance. The puncture resistance can also be increased by the use of polymers of higher molecular weight, but breakdown of the polymer chains during processing limits the benefits that can be obtained.

 

Figure 4-3: Puncture resistance of battery separators as a function of backweb thickness

 

Industrial battery separators are considerably thicker than those for automotive batteries and are available in a wider choice of materials. In addition, glass mats are often attached to the ribs to reinforce the active material and to prevent it from expanding into the space between the rib and backweb. On the following page in Table 4-1 is a summary of separator properties, as shown in G.H. Brilmyers Journal of Power Sources, 78 (1999) pp. 68-72.

 

 

Separator Type

 

Polyethylene

 

PVC

Hard Rubber

Flexible Rubber

Phenolic Resin

Strength

Excellent

Good

Good

Fair

Good

Rigidity/flexibility/brittleness

Flexible

Rigid

Rigid

Flexible

Rigid

Mean pore diameter (om)

0.10

0.22

0.20

0.06

0.50

Volumetric porosity (%)

55-65

60-70

50-60

45-55

60-70

Water permeability (cc/psi/cm2/min)

0.045

0.13

0.031

0.020

0.049

Backweb thickness (mm)

0.65

0.60

0.70

0.35

0.60

Maximum thickness (mm)

3.5

4.2

5.8

4.1

3.6

Electrical resistance (Ωcm2)

0.120

0.130

0.280

0.250

0.140

Oxidation resistance

Excellent

Excellent

Very good

Good

Good

Resistance to Sb transfer

No

No

Yes

Yes

No

Thermal resistance

Good

Good

Excellent

Good

Good

Rib styles

V/D/S

V/D

V/S

V/S

V/S

Glass mat

Available

Available

Available

Available

Available

 

Table 4-1: Selected properties of lead-acid battery separators          V = vertical; D = diagonal; S = serpentine

4.8 Glass Mat Separators

Glass mat separators are essentially used for valve-regulated lead-acid batteries. These batteries owe their existence to the discovery that if oxygen formed during charging of the positive electrode can be transported to the negative plate, it will be reduced and consequently depolarize the negative plate, thereby preventing hydrogen evolution. No electrolysis of water takes place and water consumption is eliminated. Although it had been known for many years that oxygen that diffused through the electrolyte in flooded cells was reduced at the negative plate, this took place at a very low rate. Milner (P.C. Milner, The Bell System Technical Journal, 49, 7, pp. 1321-1334 (1970)) calculated that the rate of oxygen reduction in a typical standby power cell was 20-35 A per Ampere-hour of cell capacity. In their groundbreaking patent, McClelland and Devitt showed that oxygen could be transferred from the positive to the negative plate through a glass mat that had been partially saturated with electrolyte so that unfilled channels were available for oxygen transport (D.H. McClelland and J.L. Devitt, U.S. Patent 3,862,861 (1992)).  Today, microfiber glass mats are the most common separators used in valve-regulated lead-acid batteries. Like the separators used in flooded systems, they keep the plates separated, but also have the ability to allow oxygen transport from the positive to the negative plate.

The glass fiber mats used in valve-regulated batteries have a porosity greater than 90% and a surface area exceeding 1 m2/g. The large surface area is achieved by use of glass fibers having a very low diameter, around 1 micron. Glass fiber has a zero contact angle with sulfuric acid; therefore, the separators have high capillary forces which result in good wettability. They are compressible and conformable so that they provide support for the electrodes. They are usually made from blends of glass fibers of different diameter that are processed into mats on paper-making machines. Fibers below 1 om give a large surface area and a well dispersed structure of small channels for acid absorption and oxygen transport. But, due to their shortness, they give little tensile strength to the mat. Fibers having a larger diameter improve strength, but are also more brittle and increase the tendency to break when compressed. A typical composition will have a ratio of 20%-30% microfine fibers. This gives an acceptable balance between electrical properties, strength and cost. A simplified view of the glass-fiber-separator structure is shown in Figure 4-4 Typical properties of glass mat separators for valve-regulated batteries are shown in Table 4-2. (W. Bohnstedt, Journal of Power Sources, 78 (1999) pp. 35-40).

 

Figure 4-4: Simplified view of glass microfiber separator structure

 

Basic weight (g/m2)

200

Porosity (%)

93-95

Mean pore size (om)

5-10

Thickness (acid filled)

at 10 kPa (mm)

1.3

 

at 35 kPa (mm)

1.0

Puncture strength (N)

7.5

Table 4-2: Typical properties of glass fiber separators for valve-regulated lead-acid batteries

 

The strong capillary forces in glass mat separators result in good wicking characteristics. Figure 4-5 shows how the percentage of fibers (<1om) affects the rate of wicking.

 

Figure 4-5: Effect of fiber mix on wicking characteristics of glass mat separators

 

The time for the electrolyte to wick to a given height decreases as the percentage of fine fibers in the mat is increased. All voids do not fill uniformly. However, the smaller voids will fill preferentially and the larger voids more slowly. The result is a larger proportion of unfilled large voids in the upper part of the separator. This is the area where the greatest amount of oxygen transfer takes place.

Acid stratification can take place in valve-regulated batteries with glass mat separators just as it does in flooded cells. And for the same reason sulfuric acid formed during charging will diffuse downward and displace acid of lower concentration to the top of the cell. Stratification is difficult to avoid in glass mat separators since the electrolyte is never stirred by the bubbling action of oxygen and hydrogen when the battery is charged. Figure 4-6 shows how the height of the plates in a valve-regulated battery affects the stratification and the capacity (R.E. Nelson, private communication).

 

Figure 4-6: Stratification effects with glass mat separators

 

A viable strategy for reducing stratification in a valve-regulated battery is to turn the battery on its side, which reduces the wicking height. No leakage takes place because the cells contain no free electrolyte.  Figure 4-7 shows how the orientation of the battery affects the capacity of valve-regulated cells during cycling. The graph shows that better retention of capacity is achieved when the cells are oriented on their side. The best result is obtained when the cells are positioned with the shortest side in the vertical orientation. For this reason, many valve-regulated cell installations position the cells on their sides.

 

Figure 4-7: The effect of orientation on the capacity of valve-regulated cells during cycling

 

Among the most important attributes of glass mat separators are compressibility and conformability. Conformability allows the separator to adapt to imperfections in the battery plate and maintain good plate-to-electrolyte contact. When the battery is discharged and charged, the plates will expand and contract. Conformability allows the separator to accommodate these changes without losing contact with the plate.

Recent studies have shown that high levels of compression can improve the cycle life of valve-regulated batteries. The glass fibers essentially act like springs between the battery plates and support the active materials. The support can be increased by using separators with higher density and by using a greater percentage of fine fibers in the glass blend. A two-gram sample of fine fiber (diameter 0.8 om) has approximately 5.6 billion fibers, while a coarse fiber (diameter 3.5 om) has only 28 million fibers.

The degree to which the separator is compressed is an important factor in the design of the battery. The optimum amount of compression has not been determined. It is affected by the type and thickness of the separator and the plate spacing. It is known, however, that too little compression will result in premature battery failure, while too much will compress the spongy lead in the negative plate. The compression is reduced when the cell is filled with electrolyte. Therefore, the compression remaining after filling is the critical value. For example, Zguris (Journal of Power Sources, 67 (1997) pp. 307-313) has reported that the force required to compress a stack of glass mat separators wetted with sulfuric acid of 1.23 specific gravity to a 46% compression was reduced to 60% of the value required to compress the dry material. The amount of compression will affect:

 

 

The ability of the separator to act as a spring is affected by the amount of compression. There is a permanent loss in thickness when the glass mat is compressed. Figure 4-8 shows the effect of different compressive loads on the thickness of the separator compared to when the load is removed. The data shows that a loss in thickness takes place that is proportional to the amount of compression applied.

     

Figure 4-8: The effect of compressive force on the thickness of glass-mat separator material

 

The issue of compression in glass mat separators still requires further work. Clearly, compression has an important effect on the performance and life of the battery. If the compression is too low, battery life will be compromised. If it is too high, compaction of the negative plate will take place, filling will be difficult and the amount of electrolyte will be reduced. In this case, the capacity of the battery will be reduced. The level of compression should take into account the design of the battery, the characteristics of the separator and the degree of saturation desired.

4.9 Gelled Electrolyte

Gelled electrolyte provides an alternative for the valve-regulated battery. As pointed out previously, there are still some unresolved problems with glass mat separators in valve-regulated batteries. Consequently, many valve-regulated batteries use an electrolyte that is immobilized by gelling. The most common gel is formed by the addition of between 5%-8% of silica to the electrolyte. These gels are thixotropic and can be fluidized by mechanical stirring. They can be added to the cell in liquid form and then allowed to gel after the required amount has been added. Initially, the gel provides an impermeable barrier to oxygen transfer, preventing any gas recombination. Overcharge results in water loss which causes the gel to shrink and develop cracks. This enables oxygen to migrate to the negative electrode and be reduced. Once the oxygen cycle has been established, no further water loss takes place. (O. Jache, German Patent 1,671,693 (1967.)) A conceptual view of the recombination process with gelled electrolyte is shown in Figure 4-9.

 

Figure 4-9: Conceptual view of the recombination process with gelled electrolyte

 

Batteries that use gelled electrolyte also use a conventional separator to prevent short circuits and to control the spacing between the electrodes. It is important that these have minimal acid displacement. The combination of the standard microporous separator and gel results in reduced electrolyte volume. Typical data for microporous separators suitable for use in gelled electrolyte valve-regulated batteries are shown in Table 4-3 (W. Bohnstedt, Journal of Power Sources, 78 (1999) pp. 35-40).

 

Backweb thickness (mm)

0.3

Porosity (%)

70

Mean Pore Size (om)

0.5

Acid displacement (ml/m2)

145

Electrical resistance (mς cm2)

120

 

Table 4-3: Typical properties of microporous separators for valve-regulated batteries with gelled electrolyte