Kinetics of Nucleation and Growth in Two-Phase Electrochemical Reaction of Li_xFePO₄

The kinetics of a two-phase electrochemical reaction in Li_xFePO₄ 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 Li_xFePO₄ 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 Li_xFePO₄ was estimated to be 42 and 40 kJ mol^–1 in cathodic and anodic reactions, respectively.