Volume diffusion and interface transport in LiCoO2 measured by electrochromic absorption

The time-dependent lithium concentration of sputter-deposited lithium cobalt oxide (LCO) layers during delithiation is investigated. For this purpose, functional LCO thin films of well-defined thickness are prepared by ion-beam sputter deposition and delithiated electrochemically under potentiostatic conditions. The lithium concentration of the films during delithiation is determined optically, i.e. by measuring the relative transmission of the films. This electrochromic absorption technique is non-sensitive to electrochemical side reactions, and gives information about the ionic concentration of the LCO films. Therefore, it is used to study the lithium kinetics in the films as a function of their thickness. To evaluate the measurement data, different physical models are discussed. It is found that the experimental data can be accurately described by a thin-film diffusion model, taking into account a limited interface transport. Fitting this model to the measured concentration characteristics gives a lithium diffusion coefficient of D¯=1.04×10-13cm2s-1 and an interface transport coefficient of κ¯=1.59×10-7cms-1.