Understanding the effects of the electrochemical and thermal properties of the separator on the battery temperature

Through a combination of modeling and experiments, a detailed understanding of the role of the thermal properties and the electrochemical characteristics of the separators on the evolution of battery temperature during low, moderate and high discharge rates is developed. A microstructure-based heat-transfer model is formulated to predict the thermal conductivity of the separators under dry, wet and coated conditions. A 3D coupled electrochemical-thermal model of the battery is also developed and validated to elucidate the effects of the intrinsic properties of the separators such as thermal conductivity, and ionic conductivity, and extrinsic properties such as battery discharge rate and cooling efficiency of the environment on the battery temperature. It is found that the Al₂O₃-polyacrylic acid (PAA) coated separator mitigates the temperature increase in a Li-ion battery by 20% compared to the uncoated PE separator during high rates of discharge. The electrochemical properties of the separator such as ionic conductivity and diffusivity are enhanced by the PAA binder in the coating while the thermal conductivity of the separator is increased by the Al₂O₃ component in the coating. The enhancement in the electrochemical properties of the separator which reduces the ohmic losses within the separator-electrolyte domain plays a dominant role in moderating the battery temperature while the thermal conductivity enhancement of the separator due to the Al₂O₃-PAA coating plays a secondary role in determining the battery temperature. Compared to a battery with an uncoated separator, the effect of the coated separator in reducing the temperature increase in the battery is most pronounced when the battery is discharged at the highest rate and the ambient surroundings have the lowest cooling efficiency.