Polymer separators

A separator is a permeable membrane placed between a battery's anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell.

Separators are critical components in liquid electrolyte batteries. A separator generally consists of a polymeric membrane forming a microporous layer. It must be chemically and electrochemically stable with regard to the electrolyte and electrode materials and mechanically strong enough to withstand the high tension during 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.

Unlike many forms of technology, polymer separators were not developed specifically for batteries. They were instead spin-offs of existing technologies, which is why most are not optimized for the systems they are used in. Even though this may seem unfavorable, most polymer separators can be mass-produced at a low cost, because they are based on existing forms of technologies. Yoshino et al. of the Asahi Kasei Corporation first developed them for a prototype of secondary lithium-ion batteries (LIBs) in 1983.

Initially, lithium cobalt oxide was used as the cathode and polyacetylene as the anode. Later in 1985, it was found that using lithium cobalt oxide as the cathode and graphite as the anode produced an excellent secondary battery with enhanced stability, employing the frontier electron theory of Kenichi Fukui This enabled the development of portable devices, such as cell phones and laptops. However, before lithium ion batteries could be mass-produced, safety concerns needed to be addressed such as overheating and over potential. One key to ensuring safety was the separator between the cathode and anode. Yoshino developed a microporous polyethylene membrane separator with a “fuse” function. In the case of abnormal heat generation within the battery cell, the separator provides a shutdown mechanism. The micropores close by melting and the ionic flow terminates. In 2004, a novel electroactive polymer separator with the function of overcharge protection was first proposed by Denton, et al. This kind of separator reversibly switches between insulating and conducting states. Changes in charge potential drive the switch. More recently, separators primarily provide charge transport and electrode separation.

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