Lithium transport through non-fractal and fractal $Li_{1-5}CoO_2$ film electrodes using quasi-steady-state and transient electrochemical methods based upon fractal geometry

Abstract

The present work is concerned with lithium transport through non-fractal and fractal $Li_{1-δ}CoO_2$ film electrodes using quasi-steady-state and transient electrochemical methods based upon fractal geometry. In chapter III, stresses generated during lithium transport through the rf sputter-deposited $Li_{1-δ}CoO_2$ films with different thicknesses were investigated by a double quartz crystal resonator (DQCR) technique. In situ resonant frequency changes of the $Li_{1-δ}CoO_2$ -coated AT- and BT-cut quartz crystals were first recorded along with the galvanostatic charge (lithium deintercalation) and discharge (lithium intercalation) curves obtained in a 1 M $LiClO_4-PC$ solution. From the measured resonant frequency changes, the lateral stresses of the $Li_{1-δ}CoO_2$ films were then estimated as a function of lithium stoichiometry (1-δ). Compressive and tensile stresses were developed in the $Li_{1-δ}CoO_2$ films during the lithium deintercalation and intercalation, respectively. The remarkable variation of compressive and tensile stresses with lithium stoichiometry appeared in a two-phase (a Li-poor alpha-phase and a Li-rich β-phase) region. Compressive and tensile stresses decreased in absolute magnitude with increasing film thickness. The contribution of the electrostrictive stress to the total stress was theoretically calculated to be about $2.2×10^{-3}$ %. From the extremely small contribution of the electrostrictive stress, it is strongly suggested that stresses result mainly from the volume contraction and expansion of the $Li_{1-δ}CoO_2$ films due to the lithium intercalation and deintercalation, respectively. Furthermore, it is experimentally confirmed that the relaxation of compressive stress is developed during the lithium deintercalation in a single-alpha-phase region, causing the cracking of the $Li_{1-δ}CoO_2$ films. In chapter IV, ionic diffusion through electrolyte towards self-affine fractal electrode was experimentally investigated under the ...