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Polymer Separators

A polymer separator is a permeable membrane placed between the anode and cathode of a battery. The main function of a separator is to keep the positive and negative electrodes, the cathode and anode respectively, apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers which are needed to complete the circuit during the passage of current in an electrochemical cell.

Background

Polymer separators are critical components in liquid electrolyte batteries. The separator is placed between the positive and negative electrode in order to prevent physical contact of the electrodes while enabling ionic transport. They are generally comprised of a microporous layer consisting of a polymeric membrane. The separator must be chemically and electrochemically stable towards the electrolyte and electrode materials, while also being mechanically strong enough to withstand the high tension of battery construction. They are important to batteries because their structure and properties considerably affect the battery performance, including the batteries energy and power densities, cycle life, and safety.

History

Unlike many forms of technology, polymer separators were not developed specifically for batteries. They were instead a result of spin-offs of existing technologies, which is why most polymer separators are not optimized for many of the systems they are used in. Even though this may seem unfavorable, most polymer separators can be mass produced at a comparatively low cost, because they are based on existing forms of technologies.

Synthesis

Polymer separators generally fall in the category of microporous polymer membranes. Microporous polymer membranes are usually fabricated from a variety of inorganic, organic, and naturally occurring materials. The pore size in these types of polymer separators is typically larger than 50-100 Å. Materials such as nonwoven fibers (cotton, nylon, polyesters, glass), polymer films (polyethylene, polypropylene, poly (tetrafluoroethylene), poly (vinyl chloride), and naturally occurring substances (rubber, asbestos, wood). There are also ion exchange membranes which are fabricated from polymeric materials that have pores with diameters of less than 20 Å. These are not typically used in batteries because their pore size is too small. The methods for manufacturing the microporous membranes and ion exchange membranes can be divided into two processes: dry process and wet processes.

Dry Process

The dry process is comprised of three steps: extruding, annealing, and stretching. The extruding step is generally carried out at a temperature higher than the melting point of the polymer resin. This is because the polymer resins are melted in order to shape them into a uniaxially orientated tubular film, called a precursor film. The structure and orientation of the precursor film produced depends on the processing conditions and the characteristics of the polymer resin used. In the next step, the annealing process, the precursor polymer is annealed at a temperature slightly lower than the melting point of the polymer. The purpose of this step is to improve the crystalline structure in order to enable the formation of micropores in the final step, stretching. In the final step, stretching, the annealed film is deformed along the machine direction by a process consisting of a cold stretch, a hot stretch, and a relaxation. The cold stretch is used to create the pore structure by stretching the film at a lower temperature with a faster strain rate, and the hot stretch is to increase the size of the pores by further stretching the film at a higher temperature with a slower strain rate. The purpose of the relaxation step is to reduce internal stress within the film. The porosity of the final film depends on the morphology of the precursor film, annealing conditions, and the stretching ratios and conditions.

Wet Process

Similar to the dry process the wet process consists of three steps: the mixing of the polymer resins, paraffin oil, antioxidant and other additives and then heating to produce a homogenous solution, then forcing the heated solution through a sheet die into a gel-like film, and then finally extracting the paraffin oil and other additives with a volatile solvent to form the microporous structure.

Different types of polymers used in batteries

There are specific types of polymers which are ideal for the different types of synthesis.

Ideal Polymers for Dry Processes

The dry process is only suitable for polymers with high crystallinity. These include but are not limited to: semi-crystalline polyolefins, polyoxymethylene, and isotactic poly (4-methyl-1-pentene). One can also use blends of two immiscible polymers, in which at least one polymer has a crystalline structure, such as polyethylene-polypropylene, polystyrene-polypropylene, and poly (ethylene terephthalate) - polypropylene blends.

Ideal Polymers for Wet Processes

The wet process is suitable for both crystalline and amorphous polymers. The separators synthesized by wet processes often use ultrahigh-molecular-weight polyethylene. The use of these polymers enables the batteries to have favorable mechanical properties while also preventing the battery from functioning when it becomes too hot.