New Flame-Retardant Composite Separators Based on Metal Hydroxides for Lithium-Ion Batteries

In order to enhance the thermal stability of conventional polyolefin separators, we propose a novel composite separator based on metal hydroxides. In particular, metal hydroxides not only function as a thermally stable mechanical matrix like alumina (Al₂O₃) or silica (SiO₂) but also flame retardant due to their ability to release absorbed water molecules under some accidental conditions (e.g. higher than 200 ^oC).

 The flame-retardant composite separators are prepared by coating both sides of a microporous polyethylene (PE) membrane with nano-sized metal hydroxide (Al(OH)₃ and Mg(OH)₂) ceramic powders and poly(vinylidene fluoride-co-hexafluoropropylene) PVdF-HFP binder. Their physical properties are evaluated via scanning electronic microscopy (SEM) for surface morphology, the densometer to measure Gurley number (permeability), electrochemical impedance spectroscopy for ionic conductivity, and uptake amount of liquid electrolyte for wettability. In addition, the flame retardancy of composite separator is investigated by measuring the extinction time. Furthermore, the electrochemical properties of composite separators are evaluated with lithium-ion cells composed of a graphite anode and a LiCoO₂ cathode. Comparing to bare separators, the composite separators show improved flame retardancy and rate performances due to water release property and better electrolyte wettability, respectively.

Acknowledgement

This research was financially supported by the Ministry of Education, Science and Technology (MEST) and National Research foundation (NRF) of Korea through the Human Resource Training Project for Regional Innovation (2014066977) and This research was financially supported by Chungcheong Leading Industry Promotion Project of the Korean Ministry of Trade(R0001485).