Development of Flame Retarding Separator for Lithium Ion Battery By Crosslinkable Binder Coating

Abstract

Conventional polyolefin separators undergo thermal shrinkage under abnormal conditions such as overcharging and overheating, and this may induce catastrophic thermal runaway in LIBs, resulting in gas venting, fire, rupture, or explosion. Ceramic-coated separators (CCSs) have been widely used because inorganic materials with high heat resistance protect the polyolefin surface to prevent thermal shrinkage of the separator. However, without the use of highly adhesive and thermally stable polymeric binders, the ceramic coating layer is incapable of functioning properly to maintain the thermal stability of the CCSs when exposed to heat.As one method for securing the thermal stability of the separator, we propose crosslinking of the binder in the ceramic coating layer. This study presents a thermally stable crosslinkable silsesquioxane-based ceramic-coated separator (x-CCS) with flame-retarding capabilities for the fabrication of safe Li-ion batteries. The binder used for crosslinking is ePOSS (inorganic polyhedral oligomeric silsesquioxane with epoxy functional end groups) with high thermal stability, which can form crosslinks with each other through UV irradiation. The x-CCS was fabricated by applying a 3 μm thick composite layer of Al2O3/ePOSS/PVdF-HFP to both sides of a conventional polyethylene separator. This remarkably improved the thermal stability of the separator at 140°C, with the original form of the separator maintained even after an ignition test. Because ePOSS, which has high thermal stability, is crosslinked like a net in the coating layer and maintains its shape after combustion. Although the permeability of the CCS was slightly decreased by the applied coating, the resultant improved wettability towards liquid electrolyte thereby enhancing the ionic conductivity, cycle performance, and rate capability when used in a Li-ion battery.