Stage transformation of lithium-graphite intercalation compounds caused byelectrochemical lithium intercalation

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 or iented pyrolytic graphite was used as a host material. The current-transien t curve showed a current hump, suggesting that the stage transformation was initiated by the nucleation and growth of stage 4. The phase-boundary move ment was discussed quantitatively using a simple geometric model. The phase boundary progressed in proportion to time during the initial stage. The ra te constant was inversely proportional to the product of the interfacial re sistance and the geometric edge-plane area, indicating that the phase-bound ary movement was determined by the rate of the reaction at the graphite/ele ctrolyte 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 Wa gner's classical model which describes the diffusion within two phases sepa rated by a phase boundary. These results indicated that the phase-boundary movement was initially determined by the rate of the interfacial electroche mical reaction and was controlled thereafter by a diffusion process. (C) 19 99 The Electrochemical Society. S0013-4651(98)10-052-6. All rights reserved .