The present work involves the lithium transport in carbonaceous electrode and electrochemical reactions at electrode/electrolyte interfaces with emphasis of fractal approach to the morphology of surface groups.
In chapter III, the effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated using galvanostatic charge-discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red (FTIR) spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge-discharge curves at 20℃ evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature 0˚ to 40℃. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to the structural damages. From both the increase of the relative amount of the $Li_2CO_3$ to $ROCO_2Li$, and the decrease of the 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 the transformation of $ROCO_2Li$ to $Li_2CO_3$, 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.
In chapter IV, this article critically evaluates the characteristics of a new in situ spectroelectrochemical cell with an optimised path of IR beam designed at our laboratory for the study on the SEI layer formed between the porous graphite anode and alkyl carbonate solution for lithium ion batteries. The in situ cell was designed in view of the optical principles underlying the way the in situ cell works, to give the depth of penetration of the evanescent infrared (IR) beam through the attenuated total internal reflectance (ATR) crystal into the electrolyte as such a small value ranging from 0.277 to 2.77㎛, that it was possible to minimise the `masking effect` of the ethylene carbonate/diethyl carbonate (EC/DEC) solvent. Moreover, the `local compositional change` which may arise significantly from the `thin layer electrolyte configuration` cell also could be fairly avoided, since only the electrolyte in the vicinity of the electrode composed of the graphite particles is reduced to form the SEI layer to a thickness at most of 0.1㎛ during the application of potentials. Thus, it was possible to measure the in situ FT-IR spectra in the in situ cell which represent the real chemical composition and structure of the SEI layer. Taking the application of the designed in situ cell as examples, this article reports the effect of salt type and electrolyte temperature on the chemical composition and structure of SEI layer between the graphite particles and alkyl carbonate solution with the help of various in situ FT-IR spectra measured.
In chapter V, effect of the compactness of the lithium chloride layer formed on the carbon cathode on the electrochemical reduction of $SOCl_2$ electrolyte in $Li/SOCl_2$ 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 $it^{1/2}$ vs. log t 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 $SOCl_2$ through it, and hence the rate-controlling step for overall $SOCl_2$ reduction is changed from the `interfacial reaction between the pure carbon cathode and electrolyte` to the `diffusion of $SOCl_2$ 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 $it^{1/2}$ vs. log t 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 $SOCl_2$ through it, and hence the overall reduction rate of $SOCl_2$ is governed by the `interfacial reaction between the Co-Pc-incorporated carbon cathode and electrolyte` throughout the whole discharge of the $Li/SOCl_2$ batteries.
In chapter VI, kinetics of lithium transport through a graphite electrode was investigated using galvanostatic intermittent titration technique, ac-impedance spectroscopy and potentiostatic current transient technique. All the experimental current transients measured on the graphite composite electrode did not follow Cottrell behavior, but Ohmic behavior, which means the relationship between the initial current level and the applied potential step obeys Ohm`s law. These experimental findings can be reasonably simulated under the `cell-impedance-controlled` constraint. Thus, it is strongly asserted that the flux of the lithium ion at the electrode/electrolyte interface during lithium transport through the SFG6 graphite electrode is purely governed by `cell-impedance`.
In chapter VII and VIII, mechanisms of lithium transport through a soft carbon electrode and a hard carbon electrode, respectively, were elucidated 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. The current transients experimentally measured coincided well with those transients numerically simulated based upon the modified McNabb-Foster equation as a governing equation and the `cell-impedance-controlled` constraint as a boundary condition. This strongly indicates that lithium transport is governed by `cell-impedance` and at the same time the difference in activation energies for lithium deintercalation between from the four different lithium deintercalation sites existing within the electrode accounts for the different kinetics of lithium transport between through the four different lithium deintercalation sites. Moreover, in the case of the soft carbon electrode, it is realised that since the degree of microcrystallinity of the soft carbon electrode 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.
In chapter IX, morphological structures of surface groups formed and poly-vinyliedene fluoride (PVDF)-binder materials dispersed on the PVDF-bonded graphite composite electrode were investigated in terms of fractal geometry using the 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.
