Hosull Lee

Kinetics of lithium intercalation into carbon anodes: in situ impedance investigation of thickness and potential dependence

Abstract Electrochemical impedance spectroscopy (EIS) in three-electrode sandwich configuration was applied to investigate the kinetics of Li intercalation into mesocarbon-microbeads (MCMB) based anodes. A new frequency domain model, considering porous macroscopic structure of the active material and spherical diffusion inside the particles, has been applied to analyze the potential and thickness dependence of impedance spectra. Values of several kinetically relevant parameters like specific conductivity of the layer of intercalation material (2.6×10−4 S cm−1), chemical diffusion coefficient of Li-ions in MCMB (2.2×10−9 cm2 s), charge transfer resistance (184 Ω cm2), are obtained from the analysis. Applicability of the proposed model to prediction of time-domain charge curves has been tested using numerical inversion of Laplace transformation (NILT). The time domain modeling led to the conclusion that phase nucleation at the boundaries between Li-rich and Li-poor phases coexistent during intercalation is the rate-limiting step at the initial stage of the impulse-discharge.

Universal battery parameterization to yield a non-linear equivalent circuit valid for battery simulation at arbitrary load

Abstract A method of numerical representation for electrical storage cells based on measurement of wide frequency range impedance spectra at a number of different states of charge and measurement of the depth-of-discharge dependence of equilibrium voltage are developed. Applicability of this method to batteries with various chemistries and sizes are established by comparing numerical prediction to experiment. The model represents all tested batteries with accuracy (less than 1% in average deviation) in processes ranging from time constants of 1 ms to 10 h and from current densities of C/10 to 3C. The method includes the fitting of impedance spectra to physically relevant static linear transmission-line model and the use of parameters determined at different discharge levels to create a non-linear dynamic model. Formalization of the model as a non-linear equivalent circuit enables its direct application as a part of any electric device in digital circuit simulators like SPICE.

The effect of low-temperature conditions on the electrochemical polymerization of polypyrrole films with high density, high electrical conductivity and high stability

High-quality polypyrrole-hexafluorophosphate (PPy-PF6) films with high density (∼1.4 g/cm3), high conductivity (>300 S/cm for unstretched film) and high electrochemical stability are obtained reproducibly by galvanostatic polymerization at low-temperature conditions. The optimal polymerization current density of Jp=0.02–0.05 mA/cm2 was obtained at the polymerization temperature of Tp=−40°C. The surface morphology of the film sensitively varies depending upon the properties of electrode and its surface conditions. The transport measurements characterize the high-density PPy-PF6 film as a disordered metal close to the boundary of disorder induced metal–insulator (M–I) transition. The X-ray diffraction measurements suggest that partially crystalline structure of PPy-PF6 film is related to the transport properties. The uniaxial stretching induces an increase of the conductivity up to ∼930 S/cm in a direction parallel to stretching as well as the anisotropy of conductivity. The comparative studies of thermogravimetric analysis (TGA), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for PPy-PF6 films prepared at room-temperature and low-temperature conditions show that the latter exhibit better thermal stability as well as electrochemical stability under long oxidative polarization.

Nature of the Metallic State in Conducting Polypyrrole

O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/0604-0456

Thermal impedance spectroscopy for Li-ion batteries using heat-pulse response analysis

17.50+.50/0 Adv. Mater. 1998, 10, No. 6[14] a) M. Binggeli, C. M. Mate, Appl. Phys. Lett. 1994, 65, 415. b) D.A.Grigg, P.E. Russell, J.E. Griffith, J. Vac. Sci. Technol. A 1992, 10, 680.c) A. L. Weisenhorn, P. K. Hansma, T. R. Albrecht, C. F. Quate, Appl.Phys. Lett. 1989, 2651.

A novel impedance spectrometer based on carrier function Laplace-transform of the response to arbitrary excitation

Abstract Novel characterization of the thermal properties of batteries have been introduced by defining their frequency-dependent thermal impedance functions. The thermal impedance function can be approximated as a thermal impedance spectrum by analyzing the experimental temperature transient which is related to the thermal impedance function through Laplace transformation. In order to obtain the temperature transient, a process has been devised to generate an external heat pulse with heating wire and to measure the response of the battery. This process is used to study several commercial Li-ion batteries of cylindrical type. Thermal impedance measurements have been performed using a potentiostat/galvanostat controlled by a digital signal processor, which is more commonly available than a flow-meter usually applied for thermal property measurements. Thermal impedance spectra obtained for batteries produced by different manufacturers are found to differ considerably. Comparison of spectra at different states-of-charge indicates an independence of the thermal impedance on the charge state of the battery. It is shown that the thermal impedance spectrum can be used to obtain simultaneously the thermal capacity and the thermal conductivity of the battery by non-linear complex least-squares fit of the spectrum to the thermal impedance model.

Structural aspect of metal-insulator transition in doped conducting polymers

A novel impedance spectrometer has been developed to obtain good quality spectra for electrochemical cells or devices in the minimum time required by the Nyquist theorem for the lowest frequency in the spectrum. A new method, based on a simple instrumental design resolves the known problems of existing methods such as long measurement time and fairly complicated design of the frequency response analyzer (FRA), as well as poor quality of spectra obtained for pulse-response analysis based systems. The spectrometer applies a simple excitation such as a current pulse, instantly connected resistance, or current interrupt, followed by measurement of the response, such as voltage or current against time. The response is fitted to an analytical function (carrier function), and the values of parameters obtained in the fit are applied to the expression of analytical Laplace transformation of this function. The carrier function is selected so that it approximates well the transient response of most electrochemical systems. An analytical expression of the impedance function is obtained by dividing the Laplace-transform of the carrier function by the Laplace-transform of the excitation signal. The frequency dependent impedance spectrum is obtained by evaluating this analytical expression at the required frequencies.

Electrical conductivity and thermopower of phosphoric acid doped polyaniline

Abstract Partially crystalline structure of doped polypyrrole and polyaniline is investigated by the X-ray diffraction experiment and compared with the results from transport measurements. The increase of crystallinity along the direction of polymer chain in pyrrole-hexafluorophosphate (PPy-PF 6 ) film indicates that intrachain disorder plays a critical role in transport in the metallic regime of the metal-insulator ( M-I ) transition. The X-ray structure of doped polyaniline sample in the insulating regime close to the M-I transition boundary suggests that the temperature dependence of electrical resistivity is related to the geometry of aligned polymer chains in crystalline region. In the far insulating regime, no clear relation to transport data is observed in the microscopic crystalline structure possibly due to a strong disorder from large scale inhomogeneity

Transport studies of emeraldine salts protonated by phosphoric acids

Abstract Electrical transport properties of phosphoric acid doped polyaniline are investigated by the measurements of low temperature conductivity and thermopower. Samples were prepared by chemical polymerization of aniline in aqueous solution of phosphoric acid at various ratios of acid to aniline (Z). The conductivity at room temperature increases from 3 S/cm to 40 S/cm, and the thermopower increases from +0.2 μV/K to +7.6 μV/K, as the ratio Z varies from 1 to 6. The low temperature conductivity follows variable range hopping (VRH) temperature dependence, ln σ α - (T0/T)x, where VRH exponent systematically changes from x = 1/2 to 1/4. The thermopower changes from (U-shape to linear temperature dependence as the ratio Z increases. The systematic variation of transport data was analyzed by considering heterogeneous contributions from metallic transport and VRH process in disordered polymeric system.

Electropolymerization of polypyrrole optimized for the behavior of conductivity at low temperature

Abstract Electrical transport properties of polyaniline prepared by chemical oxidative polymerization in the presence of phosphoric acid were investigated by low temperature conductivity and thermoelectric power measurements. As the ratio of acid to aniline is optimized (Z = 6) at the stage of polymerization, the freshly prepared emeraldine salt form of the polyaniline shows (i) relatively high electrical conductivity, σ ~ 40 S/cm at room temperature, (ii) temperature dependence of the conductivity characteristic of the variable range hopping (VRH), In σ ∝ — (T0/T)x, with exponent x = 1/3, and (iii) linear temperature dependence of the thermoelectric power. As the molar ratio Z decreases from 6 to 1, the VRH exponent x systematically changes from 1/3 to 1/2 and the thermoelectric power decreases from + 7.6 to + 0.2 μV/ K. The systematic variation of the transport parameters obtained from the temperature dependence was attributed to combined contributions from the metallic transport and VRH process in the disordered polymeric system.