Chronoamperometric measurements and modeling of nucleation and growth, and moving boundary stages during electrochemical lithiation of graphite electrode

Chronoamperometric studies of the stage 2 (LiC₁₂) to stage 1 (LiC₆) transition during electrochemical insertion of Li ions into graphite electrodes showed 5 distinct domains in their current vs. time curves. A linear increase of current at the beginning of domain II and a decrease in current with time within domain IY were identified with two-dimensional instantaneous nucleation of cylindrical grains of stage 1 into stage 2, their growth and further coalescence with the formation of the continuous moving boundary. We could calculate the following characteristic parameters: the nucleation time constant, τ_n = 990 s; the density of nucleation sites N_o = 2.5 × 10⁸ cm^-2, the velocity of the lateral growth of cylindrical nuclei, a = 3.6 × 10^-8 cm s^-1 and the thickness of the nucleation layer, d = 0.36 μm. After nucleation stage, the interphase propagates initially linearly with time and then proportionally to the square root of time (domain IV). At 30 °C the speed of propagation of the moving boundary between stage 1 and stage 2, dξ/dt′, is around 1.6 × 10^-7 cm s^-1. The activation energies of a, dξ/dt′ and D₁ obtained from the related Arrhenius plots were found to be 0.13, 0.19 and 0.23 eV, respectively.