Separators play an important role in battery electrochemistry and safety. In general, a good separator is required to be both physically and chemically stable, avoiding unwanted reactions with strong oxidizing or reducing electrolytes near the cathode and anode surfaces, while maintaining mechanical strength avoiding failures despite the cyclic mechanical stresses during battery operation.[1] Separator is also needed to be wettable by the electrolyte and sufficiently porous to provide low resistance to ion transport and enable fast charging. Currently, most commercial separators based on polyolefin such as polyethylene (PE) and polypropylene (PP) are widely used in lithium-ion batteries (LIBs). However, polyolefin separators suffer from low heat resistance due to their low melting points (usually <160 ℃), which may cause safety concerns. In addition, they are not wetted well by all electrolytes and are often not designed for ultra-fast charging.

Compared to PP or PE, polyetherimide (PEI) has a higher melting point (> 300 ℃). Porous PEI separator membranes can be fabricated via phase inversion method [2] to provide good ion conductivity and a higher heat resistance. Yet, the weak tensile/puncture strength could make it vulnerable towards mechanical damages during battery cycling. PEI also experiences severe shrinkage at high temperatures (>240 °C). To address these issues, we developed a facile and scalable strategy to incorporate a thin layer of ceramic nanowires on both surfaces of the PEI membrane. Heat resistance tests suggest our separators can survive high temperature up to 400 °C, as the coating minimized the shrinkage of the PEI. The inner polymer layer could melt at elevated temperatures, thus forming a shut-down layer between the ceramic layer to further enhance cell safety. These results indicate our nanowire coated separators can allow batteries to operate safely even at extremely high temperatures.

References:

[1] M. F. Lagadec, R. Zahn, and V. Wood, “Characterization and performance evaluation of lithium-ion battery separators,” Nature Energy, vol. 4, no. 1, pp. 16–25, Dec. 2018, doi: 10.1038/s41560-018-0295-9.

[2] J. Liu et al., “SiO₂ blending polyetherimide separator modified with acetylene black/polyvinylpyrrolidone coating layer to enhance performance for lithium‐sulfur batteries,” International Journal of Energy Research, vol. 45, no. 11, pp. 16551–16564, May 2021, doi: 10.1002/er.6902.

[3] Q. Wang, J. Yang, Z. Wang, L. Shi, Y. Zhao, and S. Yuan, “Dual‐Scale Al ₂ O ₃ Particles Coating for High‐Performance Separator and Lithium Metal Anode,” Energy Technology, vol. 8, no. 5, p. 1901429, Mar. 2020, doi: 10.1002/ente.201901429.

[4] P. S. Kim, A. Le Mong, and D. Kim, “Thermal, mechanical, and electrochemical stability enhancement of Al2O3 coated polypropylene/polyethylene/polypropylene separator via poly(vinylidene fluoride)-poly(ethoxylated pentaerythritol tetraacrylate) semi-interpenetrating network binder,” Journal of Membrane Science, vol. 612, p. 118481, Oct. 2020, doi: 10.1016/j.memsci.2020.118481.