Kinetics of Nucleation and Growth in Two-Phase Electrochemical Reaction of LixFePO4

The kinetics of a two-phase electrochemical reaction in LixFePO4, was investigated by potential-step chronoamperometry under various experimental conditions: amplitude of potential step, direction of potential step, particle size, and thickness of composite electrodes. Only under a small potential step (10 mV) applied to large LixFePO4 particles (203 nm), the chronoamperogram showed a momentary current increase, followed by gradual decline, indicating that the nucleation and growth governed the electrode kinetics. In that condition, the chronoamperogram was analyzed with the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model, which describes the kinetics of phase transition. The obtained Avrami exponent of ca. 1.1 indicates that the phase transition proceeds with a one-dimensional phase-boundary movement, which is consistent with the previously reported mechanism. From the temperature dependence of the obtained rate constant, the activation energy of the phase-boundary movement in LixFePO4 was estimated to be 42 and 40 kJ mor(-1) in cathodic and anodic reactions, respectively.