There are four major requirements for a separator in a flooded lead-acid battery:
Prevent shorting between electrodes
Stable in sulfuric acid
Allow ionic conduction without dendrite growth
Provide mechanical spacing between electrodes
Historically, lead acid battery separators have included cellulose, polyvinyl chloride, organic rubber, and polyolefins. Today, most flooded lead acid batteries utilize “polyethylene separators” — a misnomer because these microporous separators require large amounts of precipitated silica to be acid-wettable. Silica is responsible for the separator’s electrical properties; polyethylene is responsible for the separator’s mechanical properties. The porosity range for polyethylene separators is 50-65%.
Precipitated silica is combined with ultrahigh molecular weight polyethylene (UHMWPE), process oil and various minor ingredients (e.g., antioxidant, carbon black) to form a mixture that is extruded at elevated temperature through a die to form an oil-filled sheet. The oil-filled sheet is calendered to a controlled thickness with the desired rib pattern. Next, the majority of the process oil is extracted with an organic solvent. The sheet is then passed through a dryer and hot air oven to remove the solvent and leave behind a porous structure. Finally, the sheet is slit at multiple positions to form rolls of microporous polyethylene separators that have the appropriate profile for customers’ battery designs. The term “profile” refers to the width, backweb thickness, rib height, and shoulder design of the separator.
The primary purposes of the polyolefin contained in the separator are to (1) provide mechanical integrity so that the separator can be enveloped at high speeds and (2) to prevent sharp grid wires or plates from puncturing the separator during battery assembly or operation. The hydrophobic polyolefin preferably has a molecular weight that provides sufficient molecular chain entanglement to form a microporous web with high stiffness and puncture resistance. The primary purpose of the hydrophilic silica is to increase the acid wettability of the separator, thereby lowering its electrical (ionic) resistivity. In the absence of silica, the sulfuric acid would not wet the hydrophobic web and ion transport would not occur, resulting in an inoperable battery. Consequently, the silica component of the separator typically accounts for between about 55% and about 70% by weight of the separator, i.e., the separator has a silica-to-polyethylene mass ratio of between about 2.0:1 and about 3.5:1.
After the calendering process, the sheet is cooled so that the oil phase separates from the polymer to form regions that will eventually become pores after solvent extraction of the oil. There is always a controlled amount of oil left in the finished separator because it has a positive impact upon the oxidation resistance of the separator. The residual oil is believed to reside within the UHMWPE fibrils that are dispersed throughout the separator as shown in Figure 2. In this case, the oil serves as a reactive species for scavenging oxygen and other oxidizing agents that can attack the long polymer chains and cause embrittlement of the separator.
After delivery to a lead-acid battery manufacturer, the separator roll is fed to a machine that forms “envelopes” by cutting the separator material and sealing its edges as shown in Figure 3. Next, either a positive or negative grid that is pasted with electrochemically active material is inserted into the envelope to form an electrode package. The electrode package is then alternated with the other grid type to form a stack in which the separator acts as a physical spacer and an electronic insulator between the grids (i.e., electrodes). After making series and parallel connections between the grids, sulfuric acid is then introduced into the assembled battery to facilitate ionic conduction. The battery then goes through an electrochemical formation step prior to final inspection and shipment.