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A call to action: Establishing performance metrics for supercapacitors

Ragone plots are useful for comparing electrochemical energy storage devices. The energy and power densities in this plot are weight-based. However, converting to a volume-based plot reveals the difficulties in scaling carbon thin film or low density electrodes for medium or large devices. Credit: Gogotsi; Drexel.

Electrochemical capacitors—also called supercapacitors or ultracapacitors—have received a lot of attention in the past decade or so for energy storage because of their ability to deliver large amounts of energy quickly. Batteries, on the other hand, store large amounts of energy, but delivery it slowly.

In a short paper in Science, researchers Gogotsi and Simon, note that some recent papers report energy densities for ECs that are near or even better than the energy densities of batteries. In their paper, they warn that it is important to be careful about comparing metrics. It matters, they say, because “even when some metrics seem to support these claims, actual device performance may be rather mediocre.”

Energy storage devices can be compared using Ragone plots, which map power density (the rate of charge/discharge) against energy density (the storage capacity). The densities are usually presented as weight-based quantities, which Gogotsi and Simon observe may not be accurate for assembled devices “because the weight of the other device components also needs to be taken into account.” Those other components are the same as the components that comprise a Li-ion battery-current collectors, electrolyte, separator, binder, connectors, packaging and carbon-based electrodes.

The carbon electrodes contribute about 30 percent of the total mass of a commercial EC, which means “the energy density of 20Wh/kg of carbon will translate to about 5 Wh/kg of packaged cell.” However, carbon electrodes that are thinner or lighter will reduce energy density even more. For example, an electrode made of the same carbon material as a commercial electrode, but 10 times thinner or lighter, reduces energy density by about one-third, so the 5Wh/kg is reduced to 1.5 Wh/kg.

Gogotsi and Simon make the case that comparing energy and power densities on a volumetric basis would eliminate uncertainty and confusion about performance metrics for ECs. Nanomaterials have a very low packing density, leaving a lot of empty space which the electrolyte can flood, thus increasing the weight of the device without contributing any capacitance. Also, they note that the smaller the device, the less meaningful weight-based metrics are, simply for lack of mass. For example, a carbon nanotube coating electrode contributes negligible weight to the device.

“These systems may show a very high gravimetric power density and discharge rates, but those characteristics will not scale up linearly with the thickness of the electrode, i.e., the devices cannot be scaled up to power an electric car,” they say in the paper.

The authors also caution against relying too heavily on Ragone plots because they convey no information about other important device performance metrics such as cycle lifetime, energy efficiency, in-service temperature range, cost, etc.

The authors conclude with a call to action to the electrochemical energy storage device research community to present energy and power density data in a consistent manner. And, they recommend setting up national and international testing facilities for benchmarking electrodes and EES devices similar to those that exist for evaluating photovoltaics.

“Clear rules for reporting the performance of new materials for EES devices would help scientists who are not experts in the field, as well as engineers, investors, and the general public, who rely on the data published by the scientists, to assess competing claims,” they conclude in the paper.

The paper is “True Performance Metrics in Electrochemical Energy Storage,” Y. Gogotsi and P. Simon, Science, 18 Nov. 2011 (doi: 10.1126/science.1213003)