THE REALITY of driving to work using electric power may only be a hair away. That is about the thickness of the polymer separators used in lithium-ion batteries. Without them, there would be no such batteries or the portability they lend to everything from iPods to power tools. Chemical companies say these essential battery components are ready for the greater challenge of weaning automobiles off gasoline, and several of them are vying to lead the way.

If lithium-ion batteries were a movie, electrode materials would get top billing. Cathodes are based on lithium-transition-metal oxides, such as lithium cobalt oxide (LiCoO2), while a common anode material is lithium-intercalated carbon (LiC6). Dozens of other electrode chemistries have been developed to improve battery safety and energy density. But the principle behind all lithium batteries is the migration of lithium ions, via an electrolyte such as a lithium salt dissolved in an organic solvent, from cathode to anode on charging and in the other direction when discharging.

Separators are more like film directors in the behind-the-scenes role they play. They are electrically insulating porous polymer membranes usually composed of polyolefins in one or more layers. They prevent electrons from passing directly from anode to cathode, sending them off instead to power electronic devices. At the same time, separators let lithium ions pass between electrodes via their pores.
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