Development of Separators for Lithium-Ion Cells with High Temperature Melt Integrity

Lithium-ion batteries have been the preferred power source for many small format applications such as cellular phones and notebook computers. However, its safety problems related to thermal runaways are delaying the introduction of lithium-ion batteries into large format systems such as hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). One of the concerns is the possibility of an internal short-circuit caused by the shrinkage of the separator at high temperatures. This project will develop thermally stable separators ¿ i.e., separators that retain their integrity at temperatures of 200 degrees C or higher ¿ for making safe lithium-ion batteries. Phase I demonstrated the feasibility of developing such separators, which showed no shrinkage after heat treatment at temperatures up to 205oC. Lithium-ion cells were then made with the use of these thermally stable separators and evaluated in terms of electric performance. Phase II will (1) refine the recipes and formulations for making these separators; (2) characterize the separators in terms of thermal stability, durability in electrolytes, mechanical properties, porosity, pore size, wettability; (3) evaluate the electric performance of cells built using the separator; and (4) prepare the separator membranes in quantities sufficient to make 100 vehicle-size cells, using an automated production machine. Commercial Applications and other Benefits as described by the awardee: The thermally stable separators should result in safe lithium-ion batteries, enabling such batteries to be used in large format systems such as HEVs, PHEVs, electric vehicles (EVs), and standby power stations. Because of the improved safety and reliability, the battery also could be used for such applications as mobile power sources for space or space-related devices for NASA, the U.S. military, and other federal government agencies.