Phase transformation and fracture in single Li_xFePO₄ cathode particles: A phase-field approach to Li-ion intercalation and fracture

Modern Li-ion batteries with LiFePO₄ cathodes have been shown to be low cost, non-toxic, have a high theoretical capacity, and high (dis)charging rates. Although LiFePO₄ has advantageous properties for electrical energy storage, it can lose some of its charging capacity when cycled. Researchers have found cracks that develop in LiFePO₄ cathode particles during cycling, and it has been suggested that this is the main cause of the capacity loss. The work presented here develops a multi-physics computational model to investigate the possible causes of fracture in single LiFePO₄ particles. The model combines the recently developed reaction-limited phase-field model for Li-ion intercalation with the phase-field model for brittle fracture. We use our numerical model to simulate single LiFePO₄ cathode particles during galvanostatic discharging as well as under no charging. It was found that because of the phase transformation and two-phase coexistence of LiFePO₄, cracks were able to grow due to large stresses at coherent phase boundaries. Phase nucleation at particle side facets was also examined and we show that pre-cracks grow that follow the high stresses at the coherent interface during charging.