Temperature Dependent Phase Separation in LiFePO 4 : A CALPHAD-Coupled Chemo-Electro-Mechanical Phase Field Model

Solid state batteries often operate at elevated temperatures to enhance ionic conductivity, yet the impact of these conditions on phase separating electrode materials remains poorly understood. In this paper, the effect of temperature on the LiFePO 4 electrode material is investigated by coupling a CALPHAD thermodynamic database with a chemo-electro-mechanical phase field model that describes the spatial and temporal evolution of the phase separation process. This coupling is achieved through parameterization of a sublattice function on minimized Li–Fe(II) configurations, enabling direct evaluation of temperature dependent chemical potentials. The model shows that the third metastable phase, Li x FePO 4 , is more accessible at higher temperature and that the microstructure after heating and resting the electrode material strongly depends on the state-of-charge. Additionally, the electrode material exhibits improved rate capability under elevated temperature operation. Together, these findings highlight the importance of temperature dependent thermodynamics in predicting phase evolution in LiFePO 4 and other electrode materials undergoing phase separation.