Elastically constrained phase-separation dynamics competing with the charge process in the LiFePO₄/FePO₄ system

By using phase-field computer simulations, we have investigated the effects of the coherent strain due to the phase separation in the olivine-type LiFePO₄. In this system, the coherent elastic-strain energy due to the lattice mismatch between LiFePO₄ and FePO₄ phases accompanied by insertion and extraction of Li ions is considered to play a crucial role in the phase separation kinetics during the charge/discharge process. The present phase-field micromechanics simulations reveal several significant features of the LiFePO₄/FePO₄ system accompanying the coherent strain, such as the retardation of the phase separation, the charge rate dependence, the thermodynamic stability of coherent interfaces between dual phases, etc. Nucleation of the new phase is found to be fundamentally unlikely in terms of the elastic strain energy, except in the vicinity of the surface of the particles, and thus the phase separation would be dominated by the spinodal decomposition process. When the nucleus is present precedently, however, the phase separation can proceed in the mixture mode of the domino cascade and spinodal decomposition processes.