An investigation of the electrochemical intercalation of lithium into a Li1-delta CoO2 electrode based upon numerical analysis of potentiostatic current transients

Abstract

Lithium insertion into a porous Li1-deltaCoO2 electrode was investigated by numerical analysis of potentiostatic cathodic current transients. As lithium was intercalated, the current transients at first exhibited two-stage behavior in the presence of a single phase. This was later replaced by a three-stage character when a Li-diluted alpha phase coexisted with a Li-concentrated beta phase. From the comparison of derivatives of the experimental logarithmic current transients with those numerically simulated, it is suggested that the chemical diffusivity of lithium ion predominantly determines the shapes of the first stage of the current transients when the two phases coexist and of the later stage of the current transients when only a single phase exists. The derivatives of the second stages of the linear and logarithmic current transients during the coexistence of two phases were observed to be characterized by an upward concave shape, indicating that lithium insertion proceeds via phase boundary movement (PBM). Transition times t(tr(1)) and t(tr(2)) were determined as the times of the local maxima on the derivatives of the experimental linear and logarithmic transients, respectively. These time values correspond to the onset and end of the PBM. The current transient and its derivative were simulated as functions of equilibrium stoichiometry through the numerical analysis for lithium transport under the condition for potentiostatic lithium injection into the electrode subjected to the limitation placed by the 'pinning' of the phase boundary and the impermeable constraint to lithium. The numerically simulated current transient and the derivative of the second stage of the transient qualitatively matched those experimentally determined as functions of applied potential in their three-stage character and upward concave shape, respectively. (C) 1999 Elsevier Science Ltd. All rights reserved.