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asymmetric paper - A930 Journal of The Electrochemical...

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An Asymmetric Hybrid Nonaqueous Energy Storage Cell Glenn G. Amatucci, * ,z Fadwa Badway, Aurelien Du Pasquier, * and Tao Zheng * Telcordia Technologies, Red Bank, New Jersey 07701, USA A nonaqueous asymmetric electrochemical cell technology is presented where the positive electrode stores charge through a reversible nonfaradaic or pseudocapacitive reaction of anions on the surface of an activated carbon positive electrode. The negative electrode is a crystalline intercalation compound which supports the fast reversible intercalation of lithium ions. Using a positive electrode material of activated carbon and newly developed negative electrode material of nanostructured Li 4 Ti 5 O 12 we obtain a cell which exhibits a sloping voltage profile from 3 to 1.5 V, 90% capacity utilization at 10C charge/discharge rates, and 10-15% capacity loss after 5000 cycles. Electrolyte oxidation on the activated carbon positive electrode was characterized in a Li metal asymmetric hybrid cell by cyclic voltammetry. Oxidation during the anodic scan was found to decrease significantly after surface passivation at high voltage and elevated temperatures. We also introduce the asymmetric hybrid technology in a bonded flat plate plastic cell configuration where packaged energy densities were calculated to be in excess of 20 Wh/kg. In addition, a practical method for three-electrode analysis of Li cells by use of a Ag quasi-reference electrode wire is discussed. © 2001 Telcordia Technologies. @ DOI: 10.1149/1.1383553 # All rights reserved. Manuscript submitted July 5, 2000; revised manuscript received March 25, 2001. Available electronically July 13, 2001. The number of applications requiring mobile power sources has increased dramatically in the past 10 years. The research and indus- trial community has responded by producing batteries of exceptional energy densities and capacitor technology of exceptional power den- sity. Li-ion batteries 1 and capacitors have addressed the extremes of the energy density and power density, respectively. Electrolytic capacitors store energy through the use of a thin insulating oxide film separating the two metal electrodes from the electrolyte. The electrostatic capacity of the capacitor, C ( m F), is governed by the basic equation C 5 S « d @ 1 # where « is the dielectric constant of the dielectric, S (cm 2 ) is the electrode surface area, and d ~ cm ! is the thickness of the dielectric or distance between electrodes. Electrolytic capacitors allow exceptionally fast charge, discharge ~ ms ! , and robustness. 2-5 Although capable of high voltages in excess of 100 V, these capacitors exhibit poor energy densities that severely limit their use in energy storage applications. Electrochemical double-layer or electric double-layer capacitors ~ EDLCs ! 6 have ca- pacitance exceeding that of electrolytic capacitors by one to two orders of magnitude. These electrochemical capacitors consist of porous electrodes having capacitor plate area ~ S ! orders of magni- tude greater than electrolytic capacitors. Charging of the electrode
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