True Performance Metrics in Electrochemical Energy Storage
Y. Gogotsi * and P. Simon **
Exceptional performance claims for electrodes used in batteries and electrochemical capacitors often fail to hold up when all device components are included.
* Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, PA 19104, USA.
** Universit? Paul Sabatier– Toulouse III, CIRIMAT UMR-CNRS 5085, 118 Route de Narbonne, 31062 Toulouse, France. E-mail: gogotsi@drexel.
А dramatic expansion of research in the area of electrochemical energy storage (EES) during the past decade has been driven by the demand for EES in handheld electronic devices, transportation, and storage of renewable energy for the power grid . However, the outstanding properties reported for new electrode materials may not necessarily be applicable to performance of electrochemical capacitors (ECs). These devices, also called supercapacitors or ultracapacitors , store charge with ions from solution at charged porous electrodes. Unlike batteries, which store large amounts of energy but deliver it slowly, ECs can deliver energy faster (develop high power), but only for a short time. However, recent work has claimed energy densities for ECs approaching or even exceeding that of batteries. Prof. Y. Gogotsi and Prof. P. Simon in that paper, published in Science Magazine, show that even when some metrics seem to support these claims, actual device performance may be rather mediocre.They focused in that paper on ECs, but these considerations also apply to lithium (Li)—ion batteries.
Increasing the energy density of ECs usually comes at the cost of losses in cyclability or power, which are the most important properties of ECs and without which they become mediocre batteries. A major effort has been directed toward increasing the energy density of ECs by either increasing the capacitance of the material, or the operation voltage window, or both.
Some recent publications on graphene and nanotube-based materials have used Ragone plots to argue that supercapacitors can achieve the energy density of batteries.
Reporting the energy and power densities per weight of active material alone on a Ragone plot may not give a realistic picture of the performance that the assembled device could reach because the weight of the other device components also needs to be taken into account. ECs are similar to Li-ion batteries .
A tale of two plots. One way to compare electrical energy storage devices is to use Ragone plots ( 10), which show both power density (speed of charge and discharge) and energy density (storage capacity). These plots for the same electrochemical capacitors are on a gravimetric (per weight) basis in (A) and on a volumetric basis in (B). The plots show that excellent properties of carbon materials will not translate to medium- and large-scale devices if thin-fi lm and/or low-density electrodes are used ( 10).
By presenting energy and power densities in a consistent manner, researchers can facilitate introduction of new materials and fi nd solutions for EES challenges the world faces. National and international testing facilities should be created for benchmarking electrodes and devices similar to the facilities that exist for benchmarking photovoltaics. Clear rules for reporting the performance of new materials for EES devices would help scientists who are not experts in the fi eld, as well as engineers, investors, and the general public, who rely on the data published by the scientists, to assess competing claims. But numerous scientists who have been publishing ridiculous claims about enormous capacity of novel battery materials and energy density of electrochemical capacitors exceeding that of Li-ion batteries in the past couple of years will not like this discourse.