In discussing the Volt, we are often concerned with the working of the lithium-ion battery. Still not-yet-ready for prime-time, these types of batteries a great for cars because they can hold a lot of power and energy in a small size and weight.
The batteries work as electrons (electricity) flow between the anode (usually graphite or carbon) and the cathode. The cathodes vary by manufacturer, but A123 uses iron nanophosphate, and LG Chem a manganese spinel. Current is generated as the lithium ions flow through the solution.
An understated player in this electric orchestra is the separator.
The separator is generally a film-like material, made of electrically insulating polymer polyolefins that prevents electrons from flowing directly from anode to cathode, allowing them instead to flow out to the electric motor (in the case of an EV).
These separators have to be porous as well, to allow lithium ions to pass through. The more porous they are, the more energy can travel. On the other hand, they are also the critical determinant of the batteries safety, ruptured membranes (in the case of laptop-type or lithium cobalt oxide batteries) have been implicated in thermal runaway (explosion) events.
Automotive battery applications require specialized separators to deal with the large-format sizes of the cells as well as the need to allow very high energy and power flow, and have high melt strength.
We have heard ExxonMobil has developed a new improved automotive separator. How it works, per the source article is “ExxonMobil uses a wet process to dissolve polyethylene resin in an organic solvent. Evaporation of the solvent leaves behind a porous film. The company’s breakthrough is to coextrude multiple film layers, each of which imparts different characteristics.”
For safety, certain layers can close the pores if temperatures exceed 140 degrees Celsius, effectively shutting down the current flow.
So clearly the separator may very well be the unsung hero of the Chevy Volt’s battery pack after all.
Source (Chemical and Engineering News)
This entry was posted on Tuesday, February 26th, 2008 at 12:54 am and is filed under Battery, Engineering. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.