3D strongly anisotropic intrinsic thermal conductivity of polypropylene separator

Abstract

In this work, the thermal conductivity in every direction is reported for the first time for the dry separator component made out of polypropylene (PP) that is used in lithium-ion batteries. Herein, a PP separator film is studied dry and in vacuum, as an example of polyolefins separators. These separators are crucial for performance parameters, including cycle life, thermal management, energy and power density, and safety. Hence, a better understanding of the thermal conductivity k in different in-plane directions (parallel and normal direction to the stretching direction in fabrication) as well as the out-of-plane direction is crucial to the overall performance. k of dry PP separator, measured in vacuum, is found four times bigger in the parallel direction (0.217 W⋅m−1⋅K−1) than that in the normal direction (0.043 W⋅m−1⋅K−1), all for the in-plane direction. This strong anisotropy can be explained by the macroscopic and microscopic anisotropic structure and intrinsic anisotropic thermal conductivity of aligned PP. Moreover, the out-of-plane k of the dry PP separator is measured to be 0.036 and 0.025 W⋅m−1⋅K−1 in the air and vacuum, respectively, suggesting good parallel structure of PP in the thickness direction.