The Power and Transportation Future - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 250

LITHIUM ION BATTERIES
In the early 1970s, the first non-rechargeable lithium batteries
became commercially available. Lithium is the lightest of the metals
with the greatest electrochemical potential and the largest energy den-
sity for its weight. Attempts to develop rechargeable lithium batteries
failed due to safety problems from the inherent instability of lithium
metal. This work shifted to a non-metallic lithium battery using lithi-
um ions. Although it is slightly lower in energy density than lithium
metal, lithium-ion is safer, provided certain precautions are met when
charging and discharging.
In 1991, Sony introduced the first lithium-ion battery and other
manufacturers quickly followed. Lithium-ion is a low maintenance
battery, there is no memory and no scheduled cycling is required to
extend the battery's life. A protection circuit is used to prevent metal-
lic lithium plating from an overcharge. Some capacity deterioration oc-
curs after a year and lifetime is usually two or three years, but some
lithium-ion packs are known to have served for five years in some ap-
plications.
Manufacturers are constantly improving lithium-ion units with
new and enhanced chemical combinations introduced almost every six
months. These revised batteries may last longer with higher energy den-
sities.
The lithium polymer battery is different from conventional bat-
tery systems in the type of electrolyte used. The original design from the
1970s used a dry solid polymer electrolyte. This is a plastic-like film that
does not conduct electricity but allows ion exchange. The polymer elec-
trolyte replaces the traditional porous separator, which is soaked with
electrolyte. The dry polymer design offers simplifications in fabrication
with a rugged design and safety. However, the internal resistance is too
high to deliver the current bursts needed for many applications.
A compromise is to add some gelled electrolyte. Commercial cells
use a porous polyethylene or polypropylene separator filled with a poly-
mer and gel filling with a liquid electrolyte. They offer improved safety
with more resistant to overcharge and less chance for electrolyte leak-
age.
Battery system efficiency can also be increased by using flywheels
to equalize power demands on batteries during acceleration and hill
climbing.