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Complex capacitance analysis was done on the porous carbon electrode-electrolyte interface, where a minor leakage current is involved in addition to the dominant capacitor charging current. Based on the transmission line model, imaginary capacitance profiles ͑C im vs. log f͒ were theoretically derived for a cylindrical pore and multiple pore systems of nonuniform pore geometry. The parallel RC circuit was assumed for the interfacial impedance, where R is the charge-transfer resistance for leakage current and C the double-layer capacitance. The theoretical derivation illustrated that the resistive tail relevant to the leakage current appears in addition to the capacitive peak, which was in accordance with the experimental data taken on the porous carbon electrode. The electric double-layer capacitor ͑EDLC͒ parameters such as the extent of leakage current, total capacitance, and rate capability were visually estimated from the imaginary capacitance profiles. The more quantitative EDLC parameters were obtained by a nonlinear fitting to the experimental data. Electric double-layer capacitor ͑EDLC͒ utilizes the double layer formed at the electrode-electrolyte interface, where electric charges are stored on the electrode surface and ions of counter charge are arranged in the electrolyte side. The most demanding feature for EDLC electrodes is the ideally polarized behavior over a wide potential range. The practical EDLC electrodes, however, suffer from a self-discharge at the charged state that is caused by leakage currents. [1][2][3][4][5] This nonideally polarized behavior should thus be minimized to improve the charge-discharge efficiency and reliability of commercial cells.The leakage current appearing in EDLC electrodes can be analyzed using ac impedance spectroscopy. When impedance data are analyzed using the conventional Nyquist plot, the vertical line in the low-frequency region that is observed for ideally polarized electrodes becomes inclined with an increase in the leakage current. [6][7][8] The extent of leakage current can thus be estimated from the degree of inclination. The problem here, however, is that the inclination is also observed even in ideally polarized electrodes when they have a nonuniform pore geometry that is common in most practical porous EDLC electrodes. [9][10][11][12] In theory, the differentiation between the leakage current and nonuniform pore geometry for the cause of inclination is possible when the measurement is made at very low frequencies. In the extremely low-frequency region, ideally polarized electrodes with nonuniform pore geometry give a vertical line on the Nyquist plot, whereas the leakage current is visualized as a semicircle whose diameter reflects the charge-transfer resistance for leakage reaction. In practice, however, it is difficult to obtain data at such a low frequency due to an instrumental limitation and extremely long measuring time.In this work, the complex capacitance analysis is done on the leakage current involved in porous carbon electrode. From the imagi...

A comparative study is made on the cycle and rate performance of three Fe 2 O 3 -containing electrodes. The first electrode is made from a commercial nanosized ͑Ͻ100 nm͒ Fe 2 O 3 powder, whereas the second is made from a carbon-supported nanosized Fe 2 O 3 ͑Fe 2 O 3 /carbon composite͒. The third electrode is prepared by modifying the second, incorporating the metallic Fe into the Fe 2 O 3 particles ͑Fe 2 O 3 /Fe/carbon composite͒. Three electrodes show the following performance order in the cycle retention and rate capability: nanosized Fe 2 O 3 Ͻ Fe 2 O 3 /carbon composite Ͻ Fe 2 O 3 /Fe/carbon composite. The reverse order is, however, found on both the electrode volume change and the evolution of internal resistance. Thus, the best performance observed with the Fe 2 O 3 /Fe/carbon composite electrode has been ascribed to the presence of a carbon support and metallic Fe, both of which seem to play two important roles: the buffering action against the massive volume change in Fe 2 O 3 particles and providing an electronically conductive network within the swollen electrode layer. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3298891͔ All rights reserved. One of the recent issues in lithium-ion batteries ͑LIBs͒ concerns a high energy density, particularly for the rapidly increasing market of sophisticated electronic devices. Transition-metal oxides ͑M x O y , where M is Co, Ni, Fe, etc.͒ have emerged as a potential negative electrode material for high energy density LIBs because they deliver a specific capacity 2-3 times larger than that of the currently used graphite ͑372 mAh g −1 ͒. 1-6 Such a high capacity results from a reduction of metal ions to their elemental state according to the general reaction MO + 2Li + + 2e = Li 2 O + M 0 . As a result of the reduction reaction, nanosized metal particles that are dispersed in the Li 2 O matrix are formed ͑this is called conversion reaction͒, but are restored back to the original oxides by delithiation. The number of electrons involved in this charge-discharge process ͑that is, capacity͒ is thus determined by the oxidation state of metallic components; for instance, six electrons for Fe 2 O 3 . Among the transitionmetal oxides, iron oxide ͑Fe 2 O 3

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An expanded graphite ͑e-MCMB, mesocarbon microbeads͒ having a wider interlayer spacing ͑d 002 = 0.404 nm͒ than that of common graphites is prepared by heat-treatment of an oxidized MCMB. When the e-MCMB electrode, which gives a negligible capacitance due to a small surface area, is polarized over a certain onset potential ͓4.6-4.8 V ͑vs Li/Li + ͒ for positive and 1.3-1.0 V for negative direction͔, it is electrochemically activated to be a high-capacitance positive and negative electrode for electrochemical capacitor. The activation process involves an ion intercalation into the interlayer space to generate ion-accessible sites. The intercalation is evidenced by the presence of a voltage plateau in the charge-discharge profiles, and by the widening of the interlayer distance ͑by in situ X-ray diffraction study͒ and concomitant electrode swelling ͑by electrochemical dilatometry͒ that occur at the same potential region. The electrochemically activated e-MCMB particles carry slitlike pores of ca. 0.45 nm in the mean interlayer distance, into which ions very likely enter either bare or with partial solvent shells with a mixed adsorption/ intercalation charge storage behavior. A full cell fabricated with two e-MCMB electrodes delivers a volume specific capacitance of 30-24 F mL Until now, electric double-layer capacitors ͑EDLCs͒, which deliver a higher rate capability and longer cycle life as compared to the modern secondary batteries, have been used as the energy storage device for memory back-up systems. [1][2][3][4][5][6] Recently, the markets for EDLCs have been extended to the higher power and higher energy systems such as hybrid electric vehicles. Energy density of the present EDLC, however, does not meet the market's need. Hence, to exploit such new applications, it is necessary to develop electrode materials having a higher energy density than the conventional ones.The energy density of EDLC is given by E = 1/2 CV 2 , where C stands for the capacitance per volume or weight, and V the working voltage. An enlargement in either C or V can thus be the way to achieve a high energy density in EDLCs. 3,5,7 One way to increase the cell voltage is the use of nonaqueous electrolytes. Normally, the cell voltage of EDLCs employing aqueous electrolytes is below 1.2 V, which can, however, be enlarged up to 3.0 V by using nonaqueous electrolytes. The other approach to increase the energy density is the employment of high-capacitance electrode materials, which are normally high-surface-area conductive materials as the electric double layer is formed at the electrode/electrolyte interface.