Seung-Bok Lee

The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode

Abstract The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF 6 -ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge–discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge–discharge curves at 20xa0°C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40xa0°C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li 2 CO 3 to ROCO 2 Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO 2 Li to Li 2 CO 3 , the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge–discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.

Effect of the compactness of the lithium chloride layer formed on the carbon cathode on the electrochemical reduction of SOCl2 electrolyte in Li-SOCl2 batteries

Abstract Effect of the compactness of the lithium chloride layer formed on the carbon cathode on the electrochemical reduction of SOCl2 electrolyte in Li–SOCl2 primary battery was investigated using ac-impedance spectroscopy and potentiostatic current transient technique. From the facts that the straight lines of the Nyquist plots of the ac-impedance spectra and the peak-like runs of the plot of It1/2 versus logxa0t were observed from the pure carbon cathode, it was suggested that the porous layer of lithium chloride deposited on the pure carbon cathode was relatively compact enough to strongly impede the diffusion of SOCl2 through it, and hence the rate-controlling step for overall SOCl2 reduction is changed from the ‘interfacial reaction between the pure carbon cathode and electrolyte’ to the ‘diffusion of SOCl2 through the porous lithium chloride layer’. On the other hand, any of the straight lines of the Nyquist plots of the ac-impedance spectra and of the peak-like courses of the plot of It1/2 versus logxa0t can not be found in the Co–phthalocyanine (Pc)-incorporated carbon cathode. Thus, it was concluded that the porous layer of lithium chloride formed on the Co–Pc-incorporated carbon cathode was relatively porous enough to considerably facilitate the diffusion of SOCl2 through it, and hence the overall reduction rate of SOCl2 is governed by the ‘interfacial reaction between the Co–Pc-incorporated carbon cathode and electrolyte’ throughout the whole discharge of the Li–SOCl2 batteries.

Mechanism of lithium transport through an MCMB heat-treated at 800–1200 °C

Abstract Mechanism of lithium transport through a mesocarbon-microbeads (MCMB) heat-treated at 800–1200xa0°C was elucidated in 1 M LiPF 6 –ethylene carbonate–diethyl carbonate (50:50 vol.%) solution by the quantitative analysis of potentiostatic current transient considering the difference in the relative amount of lithium deintercalation sites having different activation energies for lithium deintercalation. From the coincidence between the current transients experimentally measured and theoretically calculated based upon the modified McNabb–Foster equation along with ‘cell-impedance-controlled’ constraint as the governing equation with the boundary condition, respectively, it is suggested that lithium transport through the MCMB electrode is limited by the ‘cell-impedance’, and at the same time the difference in the kinetics of lithium transport between through the four different lithium deintercalation sites is due to the difference in activation energy for lithium deintercalation between from the four different lithium deintercalation sites present within the MCMB. Moreover, it is realised that since the degree of microcrystallinity of the MCMB is increased with rising heat-treatment temperature, the relative charge amount of lithium deintercalated from the lattice-site is increased, but that amount from the extra-sites is decreased. Thus, the inflexion point, i.e. ‘quasi-current plateau’ in the current transient is less clearly observed with rising heat-treatment temperature.

Effect of surface groups on the electrocatalytic behaviour of Pt-Fe-Co alloy-dispersed carbon electrodes in the phosphoric acid fuel cell

Abstract Effect of surface group on the electrocatalytic behaviour of 10 wt.% Pt–Fe–Co alloy-dispersed carbon (Pt–Fe–Co/C) electrode has been investigated as functions of applied potential and duration in 85% H3PO4 solution of 145°C, using Fourier transform infrared (FTIR) spectroscopy, combined with ac-impedance spectroscopy, potentiostatic current transient technique, and potentiodynamic polarization experiment. It was shown from FTIR spectra that surface group formed in this work mainly comprises carboxyl group and that the formation potential of carboxyl group lies between 600 and 700 mVRHE. From increase of charge transfer resistance (Rct), and decrease of electrocatalytic activity for oxygen reduction with immersion time, it is suggested that above the formation potential of carboxyl group, further formation of carboxyl group on the carbon support around the catalyst particle reduces active surface area of the catalyst particle with immersion time. On the other hand, below the formation potential, dissolution of carboxyl group previously formed on the carbon support raises active surface area of the catalyst particle. In the present study, relationship between electrocatalytic aspect of the electrode, and the amount of carboxyl group formed on the carbon support around the catalyst particle was well discussed with a schematic illustration. The illustrative representation is underlain by formation on and dissolution from the catalyst particle of carboxyl group which cause the rise and fall in circumferential coverage of carboxyl group, respectively and hence the reduction and elevation in active surface area of the catalyst particle.

Determination of the morphology of surface groups formed and PVDF-binder materials dispersed on graphite composite electrodes in terms of fractal geometry

Abstract Morphological structures of surface groups formed and poly-vinylidene fluoride (PVDF)-binder materials dispersed on the PVDF-bonded graphite composite electrode were investigated in terms of fractal geometry using cyclic voltammetry combined with Kelvin probe force microscopy (KFM). When a fractal surface has single fractal geometry consisting of binder materials only, the overall fractal dimension was determined to be 1.82 from cyclic voltammetry based upon the peak current–scan rate relation, which is just the same in value as the individual fractal dimension of binder materials determined from KFM based upon the perimeter–area relation. By contrast, when a fractal surface has multifractal geometry composed of surface groups and binder materials, the overall fractal dimension was determined to be 1.77 from cyclic voltammetry. But the individual fractal dimensions were determined from KFM to distinguish the fractal dimension (1.70) of surface groups from that fractal dimension (1.82) of binder materials. The overall fractal dimension determined from cyclic voltammetry is just the average of the two individual fractal dimensions determined from KFM.

Effect of particle size on the electrocatalytic activity of platinum dispersions in carbon matrix electrodes for phosphoric acid fuel cells

The loss in electrocatalytic activity of Pt particles in carbon matrix electrodes has been experimentally and theoretically investigated as a function of Pt particle size. The measurement of the cathodic potentiostatic current transient showed that a decrease in oxygen reduction current due to carboxyl group formation, relative to the oxygen reduction current in the absence of carboxyl group, increased with a decreasing Pt particle size. This relative value is a measure of the loss in specific activity. A model describing the electrocatalytic activity loss has been proposed by introducing a new parameter, characterising the effective dead active area produced by the carboxyl group formation, relative to the total active area free of the carboxyl group. The agreement of the experimentally determined relative current decrease with the calculated relative value of the effective dead active area confirms the model.

Carbonaceous Materials as Anode Materials for Lithium Ion Secondary Batteries

The present article is concerned with the overview of carbonaceous materials used as anode materials for lithium ion secondary batteries. This article first classified carbonaceous materials into graphite, soft carbon and hard carbon according to their crystal structures, and then summarised the previous works on the characteristics of lithium intercalation/deintercalation into/from the carbonaceous materials. Finally this article reviewed our recent research works on the mechanism of lithium transport through graphite, soft carbon and hard carbon electrodes from the kinetic view point by the analysis of the theoretical and experimental potentiostatic current transients.