Generalized moving boundary model for charge-discharge of LiFePO4/C cells

Lithium ion cells with electrode materials that undergo phase transitions, like LiFePO4, have unique charge-discharge characteristics. In this work a generalized framework of moving boundary approach is proposed to model the path dependent charge-discharge response of porous electrodes that exhibit multi-phase coexistence. Using the Landau transformation the governing equation in moving coordinate is transformed to fixed coordinate and a suitable path dependent algorithm is devised and is implemented in a multi-physics environment. Simulation results show that tangential propagation of the phase front, often seen in experiments, can be addressed by this model. Incorporation of multi-phase diffusion predicts the characteristic phase separation in LiFePO4 particles. The proposed model successfully captures the charge-discharge asymmetry of LiFePO4 based cells. Efficacy of the proposed approach to model the path dependence of cells with phase change electrodes is demonstrated by simulating the response of LiFePO4/graphite cell subjected to a charge-discharge pulse. Numerical studies are performed to study the effect of important model parameters to enhance cell design and to bring out unique features in the cell response due to multi-phase coexistence.