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.