The separator has two functions to fulfill in the supercapacitor technology:
- it must provide the electronic insulation between the electrode of opposite polarization,
- it must support the ionic conduction from one electrode to the other.
The separator series resistance is proportional to the separator thickness which is in the range of 10-100 m and inversely proportional to its area. The conductivity through the separator is proportional to its porosity (40 - 70 %) and is given by equation.
Where σo is the electrolyteconductance, ρ is the porosity and α is a power factor [i]. Most of the separators available on the market are cellulose based material or porous polypropylene. They are produced in the form of self-standing films. Some other special materials (ceramic, PTFE, etc.) are investigated to overcome the temperature limitation of the classical ones. The film thickness should be as small as possible. Its minimum size is nevertheless limited by the electrical shortening failure risk due to free carbon particles [ii],[iii] which may create a contact between the two electrodes, and by the reduced mechanical tension which may lead to a tearing of the film during the winding process.
[i] Kötz R, Carlen M. Principles and applications of electrochemical capacitors. Electrochimica Acta 2000;45: 2483-98.
[ii] Richner RP. Entwicklung neuartig gebundener Kohlenstoffmaterialien für elektrische Doppelschichtkondensatorelektroden. PhD submitted to the ETHZ, Zurich; 2001 (http://e-collection.ethbib.ethz.ch/ecol-pool/diss/fulltext/eth14413.pdf).
[iii] Wade TL. High power carbon-based supercapacitors. PhD submitted to the School of Chemistry, University of Melbourne; 2006. (http://eprints.infodiv.unimelb.edu.au/archive/00002521/01/High_Power_Carbon-Based_Supercapacitors.pdf)