Sloping Off - March 2005
The Beauty of Bendable Batteries
from an article in the Guardian, 24th Feb 2005, by Alok Jha
Just over 10 years ago (the article begins), Donald Sadoway went for a ride in an electric car. Since then he has dedicated himself to make electric motoring a reality. Sadoway, a professor of materials science at MIT, has invented the "Slimcell", a sandwich of lithium and a type of perspex, which provides as much power as a conventional LiIon battery...but.
"A traditional lead acid battery has a capacity of 35 watt hours per kilogram (Wh/kg). The NiMh batteries that became available in the early 90s and enabled laptop computing, have about 90 Wh/kg. LiIon batteries have about 125 Wh/kg. At 125 Wh/kg you can drive a car 125 miles on a single charge - that's not good enough. You need to go about 250 miles on a single charge before it's going to have widespread appeal."
Sadoway has batteries in his lab that are 300 Wh/kg and he can see the possibility of breaking 400 Wh/kg. How? The Slimcell gets these big energy densities not by increasing the power from the cell, but by ditching weight. He could take little off the electrodes in a LiIon battery because the metal is so light. Instead he focused on the liquid electrolyte, which does not contribute to storage capacity (but is essential in the battery) and has invented a wafer-thin polymer electrolyte that had the mechanical properties of a solid and the electrical properties of a liquid.
The Slimcell's metal/plastic sandwich make it light and easier to manufacture, but also much safer. The battery can't leak and even it is somehow punctured, can carry on working regardless.
Unfortunately the cell is a about 5 years from production (and may face opposition from NiMh producers as well as oil companies, which have investments in hydrogen powered cars) but the benefits are mouth-watering: it is very flexible, so can be moulded into any shape - a non-loadbearing structural component in a car perhaps, reducing the gross weight considerably, the strap of a wristwatch, the fuselage of a model plane (OK, that was the Editor's fantasy).
My dream of a one oz/sq ft indoor model is just 5 years away then. In the meantime I will struggle on with my "traditional" LiIon cells, aware as ever that the next breakthough is just around the corner.... MR
