ABSTRACT
Fundamentally new technologies, many based on advanced particle structures, are successfully under development to improve the energy density of capacitors used for electric systems requiring high power and/or rapid charging. Prototypes with energy greater than 400 J/cm3 are now reported, a value surpassing optimized classic battery technologies (e.g. lead-acid), but still almost an order of magnitude lower than new lithium ion battery technology. Remarkably, the simple algebraic expression for parallel plate capacitors, specifically capacitance proportional to electrode area and dielectric constant and inversely proportional to thickness, is shown to describe most new development. Supercapacitor research is focused on increasing electrode area, and presently there is great interest in the highest surface area, electrically conductive material: graphene. ‘On-chip’ devices for micromachines, etc. focus on decreasing ‘thickness’, and dielectric layers of the order 1 micron are now commercially available. Several approaches are focused on increasing the dielectric constant including the addition of metal particles to standard dielectric materials, and the recent invention of ‘salt solution’ based superdielectric materials, many of which have demonstrated dielectric constants six orders of magnitude higher than any previously recorded for solids. Fundamental changes in capacitor structures have been accompanied by new techniques for determining capacitance, dielectric constant, energy and power density. A critique is presented that concludes not all these techniques are appropriate for the task. Finally, some issues are reconsidered in light of new capacitor developments including the fundamental mechanism of energy storage in capacitors, the limitations on ultimate energy density due to ‘saturation’ and ‘breakdown’, and the physical basis of capacitive roll-off with frequency.
