Battery separators wettability

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

Generally, surfactant-coated polyolefin separators are used in batteries based on aqueous electrolytes. The surfactant makes the separator wettable, fadUtating ion migration through the pores of the separator. Radiation grafting of carboxylic functionality has also been shown to be effective in making polyolefin separators wettable ]45, 46]. 

A great variety of polyolefin separator types are now used in Li ion batteries. They must be stable in the organic electrolytes. Typically they may not be properly wetted by the electrolytes of the optimized composition, e. g., mixtures with PC, PE, and others. Therefore some proprietary treatments are needed to provide hydrophilic behavior. Generally, a micro-porous nonwoven morphology with a large surface gives a good wettability. 

Wettability. The separators should wet out quickly and completely in typical battery electrolytes. 

The aqueous batteries use water based electrolytes (e.g., KOH electrolyte for NiCd and NiMH and H2-SO4 electrolyte for lead acid), which are less resistive then nonaqueous electrolytes. Polyolefin materials are generally suitable for use in the manufacture of separators for these batteries, but they are not inherently wettable by aqueous electrolytes. Such electrolytes are therefore unable to penetrate the pores of a separator formed from such a material, so that ion migration through the pores in solution will not occur without modification. This problem is sometimes overcome by treating the polyolefin material with a surfactant, which allows an aqueous electrolyte to wet the material. However, such surfactant can be removed from the surfaces of the polyolefin material when electrolyte is lost from the device, for example during charging and discharging cycles, and it is not subsequently replaced on the material when the electrolyte is replenished. 

Rubber separators have good voltage characteristics, the ability to retard antimony transfer, properties to retard dendrite growth, and good electrochemical compatibility. Due to the hydrophilic properties of the rubber composition, the separators are highly wettable and renewable for the dry-charging process. Paik et al. showed that AGE-SIL (sulfur cured, hard rubber) separators performed well in industrial stationary or traction batteries. FLEX-SIL (electron-beam-cured. flexible rubber separator) separators are suited for deep-cycling batteries, and MICROPOR-... 

There are two broad classes of separators employed in nickel—zinc batteries a main separator, which exhibits resistance to dendrite penetration, and an interseparator, which principally acts as an electrolyte reservoir and wicking layer. Both main and interseparator should be resistant to chemical attack by the alkaline electrolyte and resistant to oxidative attack by nascent oxygen, permanently wettable by the electrolyte, flexible, heat sealable, tear resistant, and inexpensive. 

Most commonly used separator materials for alkaline Zn/Mn02 batteries are nonwoven polymers, such as cellulose, vinyl polymers, polyolefin, and others. The separator materials must be chemically stable in concentrated KOH solutions and electrochemically stable under both oxidizing and reducing conditions in the cell. In addition to its good electronic insulation, physical strength, and porous structure, good wettability to concentrated KOH solutions is especially crucial to provide a good ionic pathway for the battery operation. 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

Again the separator must be chemically stable to the electrolyte and to the active materials at the temperature of operation. It is also necessary for the membrane to have the correct qualities of wettability, selectivity, resistivity and flexibility for the particular battery system. The cost is very variable in lead/acid batteries the target would be pence per square metre while for some Ni/Cd batteries 1 per square metre may be acceptable. 

Porous PE membranes on which a layer of acrylonitrile was deposited were treated with argon plasma. The modified membranes used as a separator for a lithium-ion battery showed improved wettability, electrolyte retention, and interfacial adhesion between the electrodes and the separator, and hence the performance of the battery was also better (Kim et al. 2009). 

Wettability - The separators should wet out quickly and completely in typical battery electrolytes. The lack of wetting results in localized spots of high resistance. 

Polypropylene (PP) and polyethylene (PE) microporous separators (e.g. with 20 jxm thickness and 50% porosity) are used for electrically separating the positive electrode and negative electrode. SEM of microporous separator is shown in Figure 12.1.4. As organic solvents are wettable to PP and PE, the solvents can penetrate into such micropores. The pore size of the separator is normally less than 0.5 (tm, in order to ensure that fine active ceramic particles of electrodes do not pass through the separator. A PP/PE/PP layered separator is often used for practical Li-ion batteries because of a shut down effect. When battery temperature approaches tbe melting point of PE (130°C), micropores of only PE are suddenly closed, and the battery reaction coming from Li-ion transportation is stopped by tbe separator. 

A polyethylene membrane containing silica filler has been developed as a cell separator for maintenance-free batteries. Characteristics of the polyethylene cell separator are shown in Table 7 and are compared with other materials used for the same purpose. The polyethylene cell separator is thinner than the other separators, so that the electrical resistance of the polyethylene separator is much lower than that of the other materials. In addition, the polyethylene separator with silica is readily wettable without any detergent, presumably because of the polarity of the silica filler. The polyethylene separator is heat sealable. This property greatly facilitates battery assembly. 

DuPont polyimide separators are thin, offer lower ionic resistance when filled with electrolyte, are made with higher-temperature stable materials that offer very low shrinkage at high temperatures, and offer very good wettability with typical organic electrolytes used in batteries and capacitor applications. 

Consequently, there was a need for a more stable NiMH separator material to reduce self discharge while still retaining electrolyte crucial for maintaining cycle life. NiMH batteries now have widespread use of what is termed permanently wettable polypropylene. In fact, the polypropylene is a composite of polypropylene and polyethylene fibers where the base composite is hardly wettable to the KOH electrolyte without surface treatment. 

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