Hybrid electric
vehicles combine the efficiency of electric vehicles with the power and
longevity of
gasoline-powered
vehicles because they have both a gasoline-fueled conventional internal combustion
engine and an electric motor powered by batteries. Image: Volvo
(October 30, 2015) Hybrid
electric vehicles, cell phones, digital cameras, and the Mars Curiosity rover
are just a few of the many devices that use rechargeable lithium-ion batteries.
Now a team of Penn State researchers has a simple mathematical formula to
predict what factors most influence lithium-ion battery aging.
Lithium-ion batteries function by moving lithium ions from
the negative electrode to the positive electrode and in the opposite direction
when the battery charges. How often and exactly how that battery is used
determines the length of a battery's life. Complex models that predict battery
aging exist and are used for battery design. However, faster, simpler models
are needed to understand the most important factors that influence aging so
that battery management systems in hybrid electric vehicles, for example, can
better control lithium-ion batteries.
"We started out by making models specifically for
Volvo's batteries that were tuned to their specific chemistry and showed that
the models matched experimentally," said Christopher Rahn, professor of
mechanical engineering, Penn State. "We then focused on simplifying the
aging models. Now, we have the ultimate simplified aging model down to a
formula."
According to Rahn, a battery ages, or degrades, whether it
is sitting on a shelf or used. The main cause of lithium-ion battery aging is
the continuous formation of the solid electrolyte interphase (SEI) layer in the
battery. The SEI layer must form for the battery to work because it controls
the amount of chemical reactions that occur in the battery. As the battery is
continually used, however, small-scale side reactions build up at the SEI
layer, which decreases battery capacity -- how much of a charge the battery can
hold. Models allow researchers to understand how different factors affect this
degradation process so that longer-lasting, more cost-efficient batteries can
be made.