Yonggang Wang

Electrochemical capacitors (i.e. supercapacitors) include electrochemical double-layer capacitors that depend on the charge storage of ion adsorption and pseudo-capacitors that are based on charge storage involving fast surface redox reactions. The energy storage capacities of supercapacitors are several orders of magnitude higher than those of conventional dielectric capacitors, but are much lower than those of secondary batteries. They typically have high power density, long cyclic stability and high safety, and thus can be considered as an alternative or complement to rechargeable batteries in applications that require high power delivery or fast energy harvesting. This article reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, systems, characterization methods, and applications. In particular, the newly developed charge storage mechanism for intercalative pseudocapacitive behaviour, which bridges the gap between battery behaviour and conventional pseudocapacitive behaviour, is also clarified for comparison. Finally, the prospects and challenges associated with supercapacitors in practical applications are also discussed.

Abstract Rechargeable zinc–manganese dioxide batteries that use mild aqueous electrolytes are attracting extensive attention due to high energy density and environmental friendliness. Unfortunately, manganese dioxide suffers from substantial phase changes (e.g., from initial α-, β-, or γ-phase to a layered structure and subsequent structural collapse) during cycling, leading to very poor stability at high charge/discharge depth. Herein, cyclability is improved by the design of a polyaniline-intercalated layered manganese dioxide, in which the polymer-strengthened layered structure and nanoscale size of manganese dioxide serves to eliminate phase changes and facilitate charge storage. Accordingly, an unprecedented stability of 200 cycles with at a high capacity of 280 mA h g −1 (i.e., 90% utilization of the theoretical capacity of manganese dioxide) is achieved, as well as a long-term stability of 5000 cycles at a utilization of 40%. The encouraging performance sheds light on the design of advanced cathodes for aqueous zinc-ion batteries.

Supercapacitor electrode materials must exhibit high specific capacitance and high-rate charge–discharge ability. The ordered whiskerlike polyaniline (PANI) reported here, which was synthesized in situ on the surface of mesoporous carbon by a novel process, is demonstrated to have these properties thanks to its ordered nanometer-sized “thorns” (see figure) and the V-shaped nanopores between them. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2089/2006/c0445_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Truly long-life: A LiFePO4/carbon composite containing a highly crystalline LiFePO4 core with a size of about 20–40 nm and a semi-graphitic carbon shell with a thickness of about 1–2 nm provides both high power and very long cycling life (see picture). The synthetic method can be extended to the preparation of other materials, such as Li4Ti5O12/carbon and Mn3O4/carbon composites. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z802539_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.