The present work is concerned with the electrochemical lithium intercalation into and deintercalation from the $Li_{1-δ}CoO_2$ film electrode prepared by rf magnetron sputtering.
In Chapter Ⅳ-1, lithium transport through the $Li_{1-δ}CoO_2$ film electrode was investigated in a 1 M solution of $LiClO_4$ in propylene carbonate using galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS), and potentiostatic current transient technique. All the experimental cathodic and anodic current transients in the presence of a single phase $Li_{1-δ}CoO_2$ did not follow Cottrell behaviour, but Ohmic behaviour. This means the relationship between the initial current level and the applied potential step obeys Ohm`s law. In addition, the current transients obtained in the coexistence of two phases α and β were characterised in a flatter shape, as compared to those transients in the presence of the respective α phase and β phase. And the plateau current level was proportional to be the difference in value between the applied potential and the plateau potential. From these results, it was suggested that the flux of the lithium ion at the electrode/electrolyte interface during the lithium transport is purely limited by ‘cell-impedance’, not only in the presence of a single phase, but also in the coexistence of two phases. The value of ‘cell-impedance’ calculated from the current transient was almost equal to be those obtained from the impedance spectra and the 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-2, two important factors affecting the ‘cell-impedance controlled’ lithium insertion/desertion were examined: One is the effect of the surface roughness of the oxide on the lithium transport. The current transients obtained from two types of the oxide films with the different surface roughness, clearly showed that the surface morphology of the oxide influences the value and shape of the ‘cell-impedance controlled’ current transients. The current transients were theoretically obtained, based upon the interface between the electrolyte and the electrodes of planar, cylindrical and spherical symmetry subjected to the ‘cell-impedance controlled’ lithium insertion/desertion, and were compared with one another. In addition, the theoretical method of analysing the diffusion towards real irregular surface was discussed. The other is the effect of intercalation-induced stress on lithium transport. From the galvanostatic charge/discharge experiment combined with the electrochemical quartz crystal microbalance technique, the lateral stress induced during lithium intercalation/deintercalation could be estimated as a function of lithium stoichiometry. Role of the intercalation-induced stress in lithium transport was discussed in terms of the stress field gradient across the oxide.
