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

Lithium insertion into a porous Li1-deltaCoO2 electrode was investigated by numerical analysis of potentiostatic cathodic current transients. As lithi um was intercalated, the current transients at first exhibited two-stage be havior in the presence of a single phase. This was later replaced by a thre e-stage character when a Li-diluted alpha phase coexisted with a Li-concent rated beta phase. From the comparison of derivatives of the experimental lo garithmic current transients with those numerically simulated, it is sugges ted that the chemical diffusivity of lithium ion predominantly determines t he 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 logar ithmic current transients during the coexistence of two phases were observe d to be characterized by an upward concave shape, indicating that lithium i nsertion 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 de rivatives of the experimental linear and logarithmic transients, respective ly. These time values correspond to the onset and end of the PBM. The curre nt transient and its derivative were simulated as functions of equilibrium stoichiometry through the numerical analysis for lithium transport under th e condition for potentiostatic lithium injection into the electrode subject ed to the limitation placed by the 'pinning' of the phase boundary and the impermeable constraint to lithium. The numerically simulated current transi ent and the derivative of the second stage of the transient qualitatively m atched those experimentally determined as functions of applied potential in their three-stage character and upward concave shape, respectively. (C) 19 99 Elsevier Science Ltd. All rights reserved.