The present work involves the kinetics of lithium transport through transition metal oxide film and composite electrodes.
Chapter Ⅲ is concerned with lithium transport through $Li_{1-δ}CoO_2$ film and composite electrodes studied by analysis of current transient. The effect of cell-impedance on lithium intercalation/deintercalation has been investigated in Chapter Ⅲ-1. The typical current transient and change in lithium content profile across the electrode with time were presented by using numerical simulation based upon the two approaches: Conventional 'diffusion-controlled' lithium transport through the electrode subjected to 'real potentiostatic' constraint and the 'cell-impedance-controlled' lithium transport across the electrode/electrolyte interface. Then, lithium transport through a carbon-dispersed $Li_{1-δ}CoO_2$ 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 $Li_{1-δ}CoO_2$ composite electrode in the single a phase region are purely governed by cell-impedance. However, the 'cell-impedance-controlled' lithium transport during intercalation into the $Li_{1-δ}CoO_2$ electrode in the coexistence of two phases alpha and beta 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.
In Chapter Ⅲ-2, the contribution of the phase boundary between alpha phase and beta phase to lithium intercalation has been considered by using the numerical approach to the moving phase boundary problem. The derivatives of the second stages of the linear and logarithmic current transients in the coexistence of two phases were observed to be characterised by the upward concave shape at relatively low lithium injection potentials, indicating that lithium intercalation proceeds via phase boundary movement. Transition time, $t_{tr(1)}$ and $t_{tr(2)}$, were determined as that time at the local maxima on the derivatives of the linear and logarithmic transients experimentally determined, respectively. These time values correspond to the time at onset and ending of the PBM. The current transient and its derivative were simulated as a function of equilibrium stoichiometry through the numerical analysis for lithium transport under the condition for the potentiostatic lithium injection into the electrode subjected to the limitation placed by the "pinning" of the phase boundary and the impermeable constraint to lithium. The current transient and the derivative of the second stage of the transient numerically simulated fitted qualitatively to those experimentally determined as a function of applied potential in three-stage character and upward concave shape, respectively.
In Chapter Ⅲ-3, lithium intercalation into and deintercalation from sputter deposited $Li_{1-δ}CoO_2$ thin film electrode have been investigated. The experimental cathodic and anodic current transients from thin film electrode exhibited the same behaviour as those from the composite electrode both in the presence of a single phase and in the coexistence of two phases. The value of 'cell-impedance' calculated from the current transient was almost equal to those obtained from electrochemical impedance spectra and galvanostatic discharge curve. The current transients were modelled under the assumption of the 'cell-impedance controlled' lithium insertion/desertion. The current transients theoretically calculated coincided well with those experimentally measured in value and shape.
In Chapter Ⅳ, lithium transport through such transition metal oxides as $Li_{1+d}[Ti_{5/3}Li_{1/3}]O_4$, $Li_{δ-d}NiO_2$ and $Li_{δ}V_{2}O_5$ has been studied 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 Ohm's 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 across the electrode/electrolyte interface. 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 was discussed in terms of 'cell-impedance controlled' intercalation/deintercalation.
In Chapter Ⅴ, the transport of lithium through sputter-deposited lithium cobalt dioxide thin film electrode has been investigated by analysis of cyclic voltammogram (CV). Anodic and cathodic peaks on CV were highly asymmetric each other in shape, and anodic peak current was larger than cathodic one in value. In addition, the anodic peak current $I_anod$ varied linearly with scan rate v to the power of 0.66 to 0.69 (i.e. $I_anod$ ∝ $v^{0.66}$ to $ν^{0.69}$), over the scan rate range of two orders of magnitude, irrespective of the surface roughness of the oxide film. The CVs were simulated as a function of scan rate at various chemical diffusivities $\tilde{D}_{Li}^+$ of lithium in the oxide by numerical analysis, assuming 'cell-impedance controlled' lithium transport across the electrode/electrolyte interface. Especially the numerically simulated CVs at $\tilde{D}_{Li}^+=10^{-10}㎠s^{-1}$ quantitatively shared those experimentally obtained. This $\tilde{D}_{Li}^+$ value was in good accordance with that value determined from electrochemical impedance spectroscopy. The effect of $\tilde{D}_{Li}^+$ on peak current and peak potential during the 'cell-impedance controlled' lithium insertion/desertion, was also discussed.
