Stage Transformation of Lithium‐Graphite Intercalation Compounds Caused by Electrochemical Lithium Intercalation

Abstract

The kinetics of the stage transformation of lithium‐graphite intercalation compounds from dilute stage 1 to stage 4 were studied using potential‐step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host material. The current‐transient curve showed a current hump, suggesting that the stage transformation was initiated by the nucleation and growth of stage 4. The phase‐boundary movement was discussed quantitatively using a simple geometric model. The phase boundary progressed in proportion to time during the initial stage. The rate constant was inversely proportional to the product of the interfacial resistance and the geometric edge‐plane area, indicating that the phase‐boundary movement was determined by the rate of the reaction at the graphite/electrolyte interface. In the following stage, the phase boundary advanced in proportion to the square root of time. The parabolic rate constant obtained experimentally was in satisfactory agreement with that calculated using Wagner's classical model which describes the diffusion within two phases separated by a phase boundary. These results indicated that the phase‐boundary movement was initially determined by the rate of the interfacial electrochemical reaction and was controlled thereafter by a diffusion process. © 1999 The Electrochemical Society. All rights reserved.