Ceramic Composite Separators Coated with Moisturized ZrO2 Nanoparticles for Improving the Electrochemical Performance and Thermal Stability of Li-Ion Batteris

In recent years, the demand for hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs) has increased due to the desire to reduce increased emission of greenhouse gases and consumption of fossil fuels. Lithium ion batteries (LIBs) are regarded as suitable power sources for EV applications because of their high energy and power. However, safety is still an important and serious concern in the development of LIBs for these applications. To improve the safety of these batteries, many researchers have focused on the improvement of the thermal and mechanical properties of separators. The separator plays a key role in ensuring the safety of LIBs by preventing short circuit between the cathode and anode.This implies that the separator must be a good electrical insulator and must possess good mechanical and dimensional stability as well as electrochemical stability toward electrolyte and electrode materials.

   Polyethylene (PE) separators have been widely used in commercial LIBs and have many advantages such as good electrochemical stability, high mechanical strength, small pore size, and thin frames.Despites these properties, the poor thermal stability of PE separators is a disadvantage that needs to be resolved. When exposed to high-temperature environments, PE separators exhibit extensive thermal shrinkage and significant structural degradation, which may trigger internal short circuits in the LIBs. Consequently, the thermal stability of PE separators needs to be further improved to meet the rigorous safety standards for LIBs for EV applications.

   Since the introduction of separators such as safety-reinforced separators (SRS) by LG Chem., various ceramic composite separators have been proposed for practical use in LIBs. For example, Takemura et al. proposed a ceramic composite separator that was made of Al₂O₃ ceramic powder and a polymer binder on a PE framework and that exhibited good thermal stability. Zhang et al. also reported a CaCO₃-based composite separator with excellent thermal stability, in which CaCO₃ ceramic particles coating the separator acted as a heating-resistant material suppressing the thermal shrinkage of the separator.Previous works have largely focused on the improvement of thermal properties of separators by incorporating various ceramic particles. However, the effect of the ceramic particles on the electrochemical performance of LIBs has not yet been intensively studied.

   We herein propose a ceramic composite separator that is coated by a polymeric binder [poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-12wt%HFP) copolymer] and moisturized ZrO₂ nanoparticles. We demonstrate the structural changes of the polymer coating layer induced by the moisture adsorbed on ZrO₂ nanoparticles, and discuss the correlation between the microstructure of the ZrO₂-composite separator and its electrochemical and thermal properties. Our findings can lead to robust ceramic composite separator designs that will improve thermal stability of LIBs for EV applications.

Fig. 1. FE-SEM images of the composite separators with different coatings at different magnifications. (a) A separator coated with only PVDF-12wt%HFP copolymer. (b) The same separator at 1µm. (c) A composite separator coated in PVDF-12wt%HFP copolymer and dried ZrO₂ nanoparticles (moisture content : 100 ppm). (d) The same separator at 1µm. (e)  A composite separator coated in PVDF-12wt%HFP copolymer and moisturized ZrO₂ nanoparticles (moisture content : 3000 ppm). (f) The same separator at 1µm.