Evaluation and comparison of the physical and electrochemical properties of lithium-ion battery separators

The separator is one of the core inner components of the lithium-ion battery, and its performance directly affects the electrochemical properties, life, and safety performance of the battery. In this research, the physical and electrochemical properties of polyethylene (PE), polypropylene (PP) / PE/PP multilayer, (PP), and aluminum oxide ceramic-coated PE (PE–Al2O3) separators were analyzed and compared in detail. Among the separators, the PE–Al2O3 separator exhibited better performance pertaining to puncture strength, thermal stability, wettability, and ionic conductivity. However, its tensile strength is slightly lower than that of the PP/PE/PP separator. The surface of the PE, PP, and PP/PE/PP separators exhibited a large number of submicron pores. Alumina particles were uniformly distributed on the surface of the PE–Al2O3 separator, and a large number of clear pores were presented between the alumina particles. The PP/PE/PP separator exhibited the best tensile properties among the separators in the mechanical direction; the tensile strength of the PP/PE/PP separator was 247.53 MPa, which is higher than that of the PE–Al2O3 (197.58 MPa), PP (119.06 MPa), and PE (147.69 MPa) separators. Meanwhile, the puncture strength of the PE separator was the lowest among the separators at 144.26 N·mm−1 whereas that of the PE–Al2O3 separator was the highest at 426.91 N·mm−1, which was attributed to the high hardness of alumina particles. The melting temperature of the PE–Al2O3 separator was 140.9 ℃, which is lower than that of the PP and PP/PE/PP separators but higher than that of the PE separator. Further, it exhibited no heat shrinkage with heat treatment at 140 ℃ for 1 h. Accordingly, the PE–Al2O3 separator exhibited the best thermal stability compared to the PP, PP/PE/PP, and PE separators. The unique hydrophilic properties of alumina particles improved the wettability between the separator and electrolyte; the wetting angle between the PE–Al2O3 separator and the electrolyte was 12.3°, which was considerably smaller than that for the PP (35.9°), PE (38.3°), and PP/PE/PP (35.3°) separators. The electrolyte wettability of the PE–Al2O3 separator effectively reduced the transmission resistance of lithium ions between the anode and cathode, enabling the separator to exhibit an excellent ionic conductivity (0.719 mS·cm−1). After 100 cycles at 1 C, the capacity retention rate of the PE Al2O3 separator was 91.19%, which was considerably better than the other three types of separators. To summarize, this study reveals that the PE–Al2O3 separator has the best application prospect in high power, long-term stability, and high-safety lithium-ion batteries.