Electrochemical lithium intercalation into and de-intercalation from porous LiCoO2 electrode by using potentiostatic current transient technique

The lithium-ion transport through a porous LiCoO2 electrode in 1 M LiClO4/propylene carbonate (PC) solution was investigated by using cyclic voltammetry, galvanostatic intermittent charge/discharge experiments and potentiostatic current transient techniques. The apparent chemical diffusivities of the lithium ion were determined as a function of the lithium charging potential during lithium intercalation and de-intercalation. In the lithium charging potential range, not less than the plateau potential with intensive intercalation/de-intercalation, both the cathodic and the anodic current transient curves obtained from the porous oxide electrode are divided into two stages. The first stage is due to the diffusion of the lithium ion through the oxide electrode and the second stage is associated with the accumulation of the lithium ion at the center of the oxide particle. During lithium intercalation, the time from the first to second stage transition decreased with decreasing lithium charging potential. This suggests that the lithium-ion transport during the intercalation proceeds not by the diffusion in a single phase, but by the diffusion-controlled movement of boundary between a concentrated beta-phase and a dilute alpha-phase. The apparent chemical diffusivity of the lithium ion in the porous oxide electrode was determined to be (10(-9)-10(-8)) cm(2) s(-1) at room temperature. During the lithium deintercalation, the apparent chemical diffusivity decreased with decreasing lithium charging potential. The reduced diffusivity value is attributable to a raised lithium content in the oxide electrode. By contrast, during the lithium intercalation the apparent chemical diffusivity increased with decreasing lithium charging potential. The exact opposite dependencies of the lithium-ion diffusivity on the lithium charging potential during the intercalation and de-intercalation were discussed in terms of the phase-boundary movement which is caused by the intercalation-induced stress gradient developed across the alpha/beta-phase boundary. (C) 1997 Elsevier Science S.A.