The present work is concerned with the electrochemical lithium intercalation reaction into anodic vanadium oxide film, porous $V_2O_5$, $LiNiO_2$ and $LiCoO_2$ electrodes.
In chapter III, the electrochemical lithium intercalation reaction into anodic vanadium oxide films in 1 M $LiClO_4$ propylene carbonate solution has been investigated by using the galvanostatic intermittent titration technique and electrochemical impedance spectroscopy(EIS) as a function of electrode potential. Measured impedance spectra were analysed using the complex non-linear least-squares(CNLS) fitting method. The impedance spectra measured in the potential range of 3.6 to 2.6 $V_{Li/Li+}$ showed that the electrochemical intercalation reaction of lithium ions into the anodic oxide film consists of three consecutive steps of charge transfer at the electrolyte/oxide film interface, lithium ion incorporation into the oxide film and diffusion through the oxide film. The charge transfer reaction at the electrolyte / anodic vanadium oxide film interface is mainly affected by the valence change of vanadium ion of the oxide film. In the electrode potential range from 3.0 to 3.6 $V_{Li/Li+}$, intercalated lithium ions are highly accumulated in the near-surface region of the anodic vanadium oxide film. The component diffusivity of lithium in the oxide film was determined to be $10^{-12}$ to $10^{-14}$ ㎠ s^{-1}$ in the lithium content range investigated, due to the increased interaction between intercalated lithium ions.
Also the lithium transport through anodic vanadium oxide film electrode was investigated by using potentiostatic current transient technique. The chemical diffusivities of lithium ion in the fresh anodic vanadium oxide film electrode specimens of various thicknesses were determined as a function of lithium charging potential. The cathodic current transient curves obtained from the fresh anodic oxide film electrode are divided into two stages. The first stage is due to the lithium ion diffusion through the oxide and the second stage is associated with the accumulation of lithium ion at the oxide film / metal interface. The time to the first to second stage transition decreased with decreasing applied potential, suggesting that the lithium transport proceeds not by the diffusion in a single phase, but by the diffusion - controlled movement of boundary separating two phases. The apparent chemical diffusivity of lithium ion in the fresh anodic oxide film was determined to be $10^{-13}-10^{-12}㎠ s^{-1}$ at room temperature; it decreases with decreasing thickness of the oxide film. This is due to a higher interfacial residual compressive stress gradient across thinner oxide film as compared to thicker one. From the measured time exponent value of -1.0 in the cathodic current transients obtained from annealed anodic oxide film electrode it is suggested that the lithium transport through the annealed oxide film proceeds by the interface - controlled movement of phase boundary.
In chapter Ⅳ, the electrochemical lithium intercalation reaction into porous $V_2O_5$, $LiNiO_2$ and $LiCoO_2$ electrodes in 1M $LiClO_4$ propylene carbonate solution has been investigated as a function of lithium content in the oxide electrodes by using X-ray diffractometry(XRD), electrochemical impedance spectroscopy(EIS) and galvanostatic intermittent titration technique (GITT).
Impedance spectra measured in the lithium content range investigated (δ=0-0.8 in $Li_δV_2O_5$) consist of a high frequency depressed arc and a low frequency straight line. The high frequency arc is attributed to the charge transfer reaction at the vanadium oxide/electrolyte interface and the low frequency line is associated with the diffusion of lithium ion through single vanadium oxide phase or with the movement of the boundary separating two phases. The chemical and component diffusivities of lithium ion in vanadium oxide were determined from the low frequency line combined with coulometric titration curve. The variation of the chemical and component diffusivities with lithium content has been discussed in relation to the coulombic interaction between the intercalated lithium ion and the oxide lattice.
The electrochemical impedance spectra of $Li_{1-δ}NiO_2$ electrode showed that the magnitude of the intermediate frequency arc associated with absorption reaction decreases with increasing lithium content,(1-δ), in the range of 0.5 - 0.7. However, $Li_{1-δ}CoO_2$ showed exactly the reverse behaviour. The component diffusivities of lithium ions have a nearly constant value in the order of $10^{-11} cm^2 s^{-1}$ for the both electrodes at room temperature, irrespective of the value of (1-δ), in the range of 0.5 - 0.7. It is suggested that lithium ion diffusion through the both layered oxides is affected by the number of empty site within lithium ion layer, not by the lattice parameter.
In appendix, the effect of the plastic deformation of the ionic conduction in pure silver iodide and silver iodide-alumina composite solid electrolytes was investigated. The ionic conductivities of pure silver iodide and silver iodide - 10 to 40 mol% alumina composites used as solid electrolyte were determined at room temperature by ac impedance spectroscopy as functions of compression pressure and annealing temperature. The ionic conductivities of both the pure and composite silver iodide specimens clearly rose with increasing compression pressure, indicating that structural defects acting as conduction paths are significantly generated by the plastic deformation. The mechanical strength of the as-deformed pure silver iodide specimen was drastically decreased by the annealing treatment at 413 K, whereas those of the as-deformed composite specimens were nearly unchanged. This result indicates that the deformation - induced defects are identified largely as dislocations. The ionic conductivity of the as-deformed pure silver iodide specimen determined during the annealing at 323 K clearly decreased with annealing time, whereas those of the as-deformed composite specimens remained nearly unchanged. It is suggested that the annihilation of the deformation - induced dislocations during the annealing is impeded by dispersed alumina particles.
