Complex Capacitance Analysis of Porous Carbon Electrodes for Electric Double-Layer Capacitors

A new analytical methodology, complex capacitance analysis, is developed for porous carbons that are employed as electric double-layer capacitor ~EDLC! electrodes. Based on the transmission line model, the imaginary capacitance profiles (Cim vs. log f) are theoretically derived for a cylindrical pore and further extended to multiple pore systems. The theoretical derivation illustrates that two important electrochemical parameters in EDLCs can be estimated from the peak-shaped imaginary capacitance plots: total capacitance from the peak area and rate capability from the peak position. The usefulness of this analysis in estimating EDLC parameters is demonstrated by applying to two sets of practical porous carbon electrodes. In addition, the penetrability distribution curves that are derived from the experimental imaginary capacitance data using the log-normal assumption and discrete Fourier transform allow us to estimate the pore structure of carbon electrodes.This work was supported by KOSEF through the Research Center for Energy Conversion and Storage

Electrolyte Effects on Spinel Dissolution and Cathodic Capacity Losses in 4 V U/LiJvIn2O4 Rechargeable Cells

Spinel dissolution and cathodic capacity losses in 4 V Li/LiMn204 secondary cells were examined in various electrolyte solutions comprising different solvents and Li salts. It was found that spinel dissolution is induced by acids that are generated as a result of electrochemical oxidation of solvent molecules on composite cathodes. Among various organic solvents, ethers such as tetrahydrofuran and dimethoxyethane were readily oxidized to produce acids whereas carbonates (ethylene carbonate, propylene carbonate, diethylcarbonate) were relatively inert. Consequently, when a spinelloaded composite cathode was charge/discharge cycled in the potential range of 3.6 to 4.3 V (vs. Li/Li), both the acid concentration and the extent of spinel dissolution was much higher in the ether-containing electrolytes as compared to the carbonates. The results, obtained from the chemical analysis on acid-attacked spinel powders and from the open-circuit potential measurement of composite cathodes, indicated that Li and Mn ion extraction is dominant in the earlier stage of acid attack. As the spinel dissolution further continues, however, oxygen losses from the lattice become more important. The combined feature of solvent oxidation and spinel dissolution was also affected by the nature of lithium salts added. Generally, the solvent-derived acid generation was not significant in those electrolytes containing fluorinated salts (LiPF5, LiBF4, and LiA5F6), yet the spinel dissolution in these electrolytes was still appreciable because acids were generated via another pathway; a reaction between the F-containing anions and impurity water.This work has been supported by the Korean Science and Engineering Foundation through the Research Center for Thin Film Fabrication and Crystal Growing of Advanced Materials in Seoul National Universit

Electrochemical Activation of Expanded Graphite Electrode for Electrochemical Capacitor

An expanded graphite (e-MCMB, mesocarbon microbeads) having a wider interlayer spacing (d002 = 0.404 nm) 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 direction], 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 study) and concomitant electrode swelling (by electrochemical dilatometr) 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−1 within 100 cycles for a dry electrode pair at a working voltage of 3.7 V.This work was supported by KOSEF via the Research Center for Energy Conversion and Storage

Solid-State NMR and Electrochemical Dilatometry Study on Li+ Uptake/Extraction Mechanism in SiO Electrode

This work reports the Li+ uptake/extraction mechanism in silicon monoxide (SiO) as the negative electrode in lithium secondary batteries. A combined study of solid-state 29Si- and 7Li-nuclear magnetic resonance (NMR), electrochemical dilatometry, and charge-discharge cycling consistently demonstrates that the SiO2 domain in SiO irreversibly reacts with Li+ to produce lithium silicates and Li2O in the first discharging period, whereas the elemental Si domain reversibly reacts, delivering the same chargedischarge characteristics to those of conventional amorphous Si electrodes. The volume expansion accompanied by the irreversible reaction is less significant than that caused by the lithiation of Si domain. The postmortem analysis made on cycled electrodes reveals a phase segregation between the lithium silicates/Li2O and lithiated Si phase. It is likely that the lithium silicates/Li2O phase plays a buffering role against the volume change of Si matrix, but the crack formation at the phase boundaries and eventual pulverization are still a problem to be solved.This work was supported by KOSEF via the Research Center for Energy Conversion and Storage. We are grateful to the Daegu Center at the Korea Basic Science Institute for helpful discussions and NMR measurements. We also acknowledge Dr. R. Kötz and Dr. P. Novak (Paul Scherrer Institute, Switzerland) for their assistance in fabricating the electrochemical dilatometer

Electrochemical Dilatometry Study on Si-Embedded Carbon Nanotube Powder Electrodes

Si-embedded carbon nanotube (Si–C/NT) powders were prepared by dispersing carbon nanotubes (CNTs) and Si in the tetrahydrofuran solution containing poly(vinyl chloride) (PVC) as a dispersion agent, and then carbonizing the PVC. A better cycle performance was observed with the Si–C/NT containing larger void volume. The origin of this feature was addressed by an electrochemical dilatometry study, where it was found that the electrode swelling becomes less significant when the Si–C/NT possesses a larger void space. It is believed that the void space plays a buffering role against the volume expansion of Si, alleviating the breakdown of electrode integrity.This work was supported by KOSEF via the Research Center for Energy Conversion and Storage, and by the Division of Advanced Batteries in the NGE Program (project no. 10016439). We are grateful to Dr. M. Hahn (Paul Scherrer Institute, Switzerland) for his assistance in fabricating the electrochemical dilatometer

Clonal and genetic structure of Iris odaesanensis and Iris rossii (Iridaceae): insights of the Baekdudaegan Mountains as a glacial refugium for boreal and temperate plants

The main Korean mountain range that stretches from north to south (the Baekdudaegan) has been suggested to harbor an important glacial refugium for boreal and temperate plant species. Under this scenario, we expect high levels of within-population genetic variation and low or moderate degree of among-population differentiation within these species. To test this hypothesis, we examined clonal diversity and levels of allozyme diversity in the boreal Iris odaesanensis and in its temperate congener Iris rossii. In addition, we compiled data on boreal and temperate species whose distribution in the Korean Peninsula is mostly centered in the Baekdudaegan to determine if there is a common pattern. We found lower clonal diversity in I. odaesanensis compared to I. rossii. Both studied species maintained high levels of genetic variation as well as a moderate genetic differentiation (%P = 52.5 and 47.5, A = 1.70 and 1.58, H e = 0.158 and 0.150, and F ST = 0.196 and 0.189 for I. odaesanensis and I. rossii, respectively), in line with what occurs for the species distributed on the Baekdudaegan (n = 14, %P = 46.7, A = 1.73, H e = 0.161, and F ST = 0.190). This study strongly suggests that the Baekdudaegan may have acted as a refugium for boreal and temperate species, in a similar way to the southern Appalachians in the eastern United States

Absolute Upper Bound on the 1-loop Corrected mass of S1S_1 in the NMSSM

We examine in detail radiative corrections to the lightest scalar Higgs boson mass due to the top quark and scalar quark loops in the next-to-minimal supersymmetric standard model (NMSSM). We take into account the nondegenerate state for the top scalar quark masses. In our analysis, the mass matrix of the top scalar quark contains the gauge terms. Therefore our formula for the scalar Higgs boson mass matrix at the 1-loop level includes the contribution of the gauge sector as well as the effect of the top scalar quark mass splitting. Thus we calculate the upper bound on the lightest scalar Higgs boson mass using our formula. We find that the absolute upper bound on the 1-loop corrected mass of the lightest scalar Higgs boson is about 156 GeV.Comment: 12 pages(Latex), 6 Postscript figures are included Figur