Heon-Cheol Shin

The kinetics of lithium transport through Li1−δCoO2 by theoretical analysis of current transient

The kinetics of alkali atom transport through intercalation compounds was theoretically considered through numerical simulation of the current transient and concentration profile across the electrode with time. For the theoretical calculation of those two different points of view, the following were considered: that alkali ion diffusion is a rate-controlling step of alkali atom transport through the electrode subjected to ‘real potentiostatic’ constraint; and that cell-impedance purely governs alkali atom transport. We presented current transient and change in alkali atom content profile across the electrode with time, numerically simulated based upon the two approaches. As an example lithium transport through a carbon-dispersed Li1−δCoO2 composite electrode was examined from the two points of view. From the comparison of experimentally obtained current transients with those numerically simulated, it is suggested that lithium transport during intercalation into and deintercalation from the Li1−δCoO2 composite electrode in the single α phase region are purely governed by cell-impedance. However, the ‘cell-impedance-controlled’ lithium transport during intercalation into the Li1−δCoO2 electrode in the coexistence of two phases α and β is converted into ‘diffusion-controlled’ lithium transport. This transition in transport mechanism can be accounted for in terms of the input flux at the subsurface toward the electrode between by chemical diffusion and by the quotient of potential drop divided by cell-impedance.

Kinetics of double-layer charging/discharging of activated carbon electrodes: Role of surface acidic functional groups

This work involves the role of surface acidic functional groups (SAFG) in the kinetics of double-layer charging/discharging of activated carbon powder electrode specimens in a 30 wt % H 2 SO 4 solution using nitrogen gas adsorption, the Boehm method, ac impedance spectroscopy, a potentiostatic current transient technique, and cyclic voltammetry, For this study, two kinds of as-activated and as-reactivated carbon powder specimens with nonuniformly distributed pore size were prepared, which were characterized by roughly the same pore size distribution, hut by appreciably different concentrations of the SAFG. The resistive and capacitive elements were roughly estimated using complex nonlinear least-squares fitting of the ac impedance spectra to a six-RC-element ladder network. The cathodic current transients and cyclic voltammograms (CVs) were simulated from the circuit analysis based upon the ladder network at a potential step and a potential scan, respectively, during double-layer charging/ discharging using six sets of calculated time constants. Both the simulated current transients and the CVs agreed well in shape and value with those experimentally measured. From the results, we conclude that the SAFG increase the time constant and decrease the rate capability, thus causing considerable retardation of ion penetration into the pores during double-layer charging/discharging of the carbon electrodes.

Lithium transport through Li1+δ[Ti2-yLiy]O4 (y = 0; 1/3 electrodes by analysing current transients upon large potential steps

Abstract Lithium transport through Li 1+ δ [Ti 2− y Li y ]O 4 ( y =0; 1/3) electrodes in the coexistence of a Li-poor phase α and a Li-rich phase β was investigated during electrochemical lithium intercalation by using the potentiostatic current transient technique under large potential stepping. For this purpose, the galvanostatic charge–discharge curve and the cathodic current transient were obtained as functions of the lithium content (1+ δ ) and the lithium injection potential, respectively. The charge–discharge curve showed a potential plateau due to the coexistence of two phases α and β. The values of the quasi-equilibrium potential and the corresponding stoichiometry of the α- and β-phases were determined from the potential plateau. A three-stage current transient was observed as the applied potential step went below the potential plateau, and the second stage of this current was found to be governed by the diffusion-controlled phase boundary movement between the α- and β-phases.

Mechanisms of lithium transport through transition metal oxides studied by analysis of current transients

Abstract Lithium transport through such transition metal oxides as Li1+δ[Ti5/3Li1/3]O4, Li1−δNiO2 and LiδV2O5 was investigated by analysis of current transients. All the experimental current transients in shape deviated markedly from the Cottrell character during the whole intercalation/deintercalation, and the initial current level varied linearly with the applied potential step according to Ohms law. Moreover, it was observed that the current transient during phase transformation is characterised by a ‘current plateau’. The current transient was simulated as a function of applied potential by numerical analysis assuming ‘cell-impedance-controlled’ lithium transport. The numerically simulated current transient featured quantitative behaviour characteristic of non-Cottrell behaviour and exhibited a ‘current plateau’. The lithium transport mechanism through the oxides is discussed in terms of ‘cell-impedance-controlled’ intercalation/deintercalation.

A study on the simulated diffusion-limited current transient of a self-affine fractal electrode based upon the scaling property

Abstract The power law relation I ( t )∝ t − α , α =( D f −1)/2 between current I and time t has been widely used to analyse current transient (chronoamperometric) behaviour during atomic/ionic diffusion towards the electrode with a fractal dimension D f , irrespective of whether the electrode has a self-similar fractal structure or at best a self-affine fractal structure. We show that the self-affine fractal dimension D f,sa (or Hurst exponent H ) is not always the sufficient condition required for describing the atomic/ionic diffusion behaviour to the self-affine fractal electrode: the current transient exhibits a more negative power dependence of current on time before temporal outer cut-off of fractality with increasing morphological amplitude (roughness factor) of the self-affine fractal electrode, rather than a unique power dependence corresponding to D f,sa . It is particularly noted that the current transients from the electrodes with comparatively large amplitudes are roughly characterised by a two-stage power dependence before temporal outer cut-off of fractality. In the present work, a practical method has been suggested to interpret the anomalous current transient from the self-affine fractal electrodes with various amplitudes. This method includes the determination of the apparent self-similar scaling properties of the self-affine fractal structure by the triangulation method.

The kinetics of lithium transport through LiNiO2 by current transient analysis

Abstract The kinetics of lithium transport through Li 1− δ NiO 2 have been investigated in a 1 M solution of LiClO 4 in propylene carbonate using current transient technique. All the cathodic and anodic current transients experimentally measured hardly follow the Cottrell behaviour. From the linear relationship between initial current level in current transient and applied potential step, ‘cell-impedance’ was determined as a function of the electrode potential. ‘Cell-impedance’ with the electrode potential calculated from the current transient, is similar in value to that of internal cell resistance composed of solution resistance, contact resistance, and absorption resistance obtained from the Nyquist plot. Taking the variation of ‘cell-impedance’ with the electrode potential into account, the lithium transport through the Li 1− δ NiO 2 electrode was theoretically analysed by means of numerical simulation of the current transient under the assumption of the ‘cell-impedance controlled’ lithium transport. The current transients theoretically calculated quantitatively shared well those experimentally measured. Lithium transport through the Li 1− δ NiO 2 electrode being even degraded by jumping the electrode potential to the value higher than 4.20 V Li/Li + , proceeds under the ‘cell-impedance controlled’ constraint.

Application of Electrochemical Quartz Crystal Microbalance Technique to Hydrogen/Lithium Insertion into MnO2/LiCoO2 Electrode in Aqueous/Non-aqueous Solution

The hydrogen insertion/desertion into/from MnO2 electrode in aqueous solution and the lithium intercalation/ deintercalation into/from LiCoO2 electrode in non-aqueous solution have been investigated by usingin-situ electrochemical quartz crystal microbalance (EQCM) technique combined with cyclic voltammetry (CV) and galvanostatic charge-discharge experiment. In the case of the MnO2 electrode, the combined cyclic electrogravimetric and CV results indicated that the redox potentials at the transition in oxidation state of manganese ion measured on the cathodic scan are satisfactorily in accord in value with those thermodynamic e-quilibrium potentials calculated in Pourbaix diagram. The positive/negative slope with a constant value in the plot of mass change rate vs. potential means that the reaction is inclined to proceed in the direction of an oxidation/reduction between two phases. From the electrogravimetric curves obtained simultaneously with galvanostatic discharge curves, the discrepancy between the charge and mass variations was discussed in relation with the hydrogen-induced stress. In the case of the LiCoO2 electrode, the cyclic electrogravimetric data obtained simultaneously with CV indicated that neither solvent nor any of other species but lithium ions are intercalated into and deintercalated from the electrode. From the cyclic electrogravimetric curve obtained simultaneously with galvanostatic charge-discharge curve, the discrepancy between the charge and mass variations was discussed in relation with the change of the molar volume and surface roughness of the electrode during the lithium intercalation and deintercalation.

Mechanisms of Lithium Transport through Transition Metal Oxides and Carbonaceous Materials

The kinetics of lithium transport through transition metal oxides and carbonaceous materials has been studied extensively due to its great importance for high power output rechargeable batteries. In most cases, the kinetic analysis and the determination of the chemical diffusivity of lithium in the electrode has been performed assuming the diffusion control concept. This assumes that the diffusion of lithium in the electrode is very slow, while other reactions, including interfacial charge transfer, are too fast to affect the kinetics of lithium transport; thereby lithium diffusion in the electrode is assumed to be the ratecontrolling process of lithium intercalation. However, various kinds of anomalous behavior of lithium transport have been reported in many current transients (CTs) and voltammetric curves obtained with a number of transition metal oxides and carbonaceous materials. In spite of the fact that many researchers