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Separator (electricity)
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.〔Flaim, Tony, Yubao Wang, and Ramil Mercado. "High Refractive Index Polymer Coatings." SPIE Proceedings of Optical Systems Design. Web〕
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.〔Arora, Pankaj and Zhang, Zhengming (John). “Battery Separators” Chemical Reviews 2004 104 (10), 4419-4462〕
==History==
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.〔Choi, Sung-Seen, Soo Lee, Young, Whan Joo, Chang Goo Lee, Seung, Kyoo Park, Jong and Han, Kyoo-Seung “Electrospun PVDF nanofiber web as polymer electrolyte or separator” Electrochimica Acta 50 (2004) 339–343〕 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, employ the frontier electron theory of Kenichi Fukui〔(Licari, J. J., and B. L. Weigand. "Solvent-Removable Coatings for Electronic Applications." ACS Symposium Series 123 (1980): 127-37. Print.)〕 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.〔Chung, Y. S., S. H. Yoo, and C. K. Kim. "Enhancement of Meltdown Temperature of the Polyethylene Lithium-Ion Battery." Industrial and Engineering Chemistry Research 48.9 (2009): 4346-351. Print.〕 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.〔Feng, J.K., Ai, X.P, Cao, Y.L. et al “A polytriphenylamine-modified separator with reversible overcharge protection for 3.6 V-class lithium-ion battery” Journal of Power Sources 189 (2009) 771–774〕 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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