Electro and Photo-Induced Diffusion and Migration Processes in Nonstoichiometric Lithium Compounds

Abstract

Studying the transport processes in solid intercalation electrodes represents a pressing problem in the solid-state electrochemistry and is simultaneously a prerequisite for the further improvement of lithium-ion and lithium-polymer batteries. The aim of this work is consisted of studying basic laws for kinetic and diffusion processes that occur during the electrochemical intercalation of lithium into carbon and titanium dioxide by means of the development of model electrode systems and the application of a complex of electrochemical methods. In this work there were used such electrochemical methods as potentiostatic and galvanostatic intermittent titration at small concentration perturbations [1-5], cyclic voltammetry [4, 6, 7], electrode impedance spectroscopy [6,7], and photoelectrochemistry [7,8]. The objects under investigation were thinfilm model electrodes that contained no binding or electroconducting additives.It was established that the experimentally obtained chronoamperograms and chronopotentiograms satisfy equations of theory of diffusion with a surface control [9], thus demonstrating the validity of its application for describing transport processes in intercalated materials. A detailed study of the way the chemical diffusion coefficient for lithium D depends on the electrode potential E was performed. It was established that, for the lithium-carbon intercalation compounds, the concentration D , E dependence exhibits an extremum character with a minimum in the region of transition from intercalation compounds of a first step to the intercalation compounds of a second step, which corresponds to a maximum of the intercalation capacity of the material. The correspondence between the extremes in two curves was explained in terms of the Frumkin isotherm. The lithium concentration in the intercalate was shown to hardly effect its own diffusion coefficient in the region of compositions reach in lithium and to have a considerable effect at in the region of dilute phases.It was established that the appearance of the temperature dependence of the lithium diffusion coefficient in carbon varies with the lithium concentration in the intercalate. However, we stress that the temperature effect on D is virtually nonexistent at compositions that lie in the vicinity of the extremum in a D , E curve. This potential region corresponds to the "plateau" exhibited by a charge-discharge curve where there take place successive transients between intercalation steps.The polarization dependence of a kinetic current through the Li x C 6 electrode, which was measured for the first time ever, is fully determined by ion-conducting properties of a surface solid-electrolyte interface (SEI) and obeys the same typical regularities of an ohmic-injector current, which had been established for a passivated lithium electrode in [10]. The concentration and mobility of mobile charge carriers in SEI were determined and found to be dependent on the lithium concentration in the intercalate.Typical peculiarities of frequency spectra for titani...