2014 Chevrolet Spark EVEnlarge Photo
Aging is a part of life, even for lithium-ion batteries.
And battery life is an important consideration for electric-car owners, especially in these early days before there's sufficient real-world data on actual life of electric-car battery packs over the 10- or 15-year lifespan of a typical vehicle.
Just like any other car part, batteries degrade over time, and so does their ability to hold a charge.
This is well known, but how it happens is less clear.
Researchers at the Helmholtz Zentrum Berlin (HZB) say they have found one explanation for lithium-ion battery aging (via Green Car Congress and ChargedEVs) while testing a "lithium-rich" cathode material.
HZB scientists claim the material they were testing has twice the capacity of today's standard cathodes, as well as the ability to be charged and discharged rapidly or at higher currents.
2013 Nissan Leaf, Nashville area test drive, April 2013Enlarge Photo
"M" represents a transition material, which could be Manganese, Chromium, or Iron in a production version of the test cathode.
HZB notes that this cathode contains smaller amounts of rare, toxic elements like Nickel and Cobalt, which should make it cheaper and greener.
That all sounds good, but in testing a battery with the lithium-rich cathode material suffered voltage reduction after cycling.
Consequently, the amount of energy the battery could discharge after each cycle decreased.
To find out why, scientists used X-ray absorption spectroscopy (XAS) to examine samples of the material--after one charge and after 33 charges--at an atomic level.
The sensitivity of the scanning--it can track the behavior of specific elements within a compound--allowed the researchers to observe changes in the arrangement of atoms within the cathode material's molecules.
Researchers found that repeated charging caused oxygen molecules to rearrange, which had a destabilizing effect on battery chemistry--leading to the poorer performance that was observed.
HZB hopes these findings--and those from the many other scientists and researchers intent on unlocking the secrets to better batteries--will help clarify the process of battery aging and battery lifespan.
Of course, multiple variables interact in real-world electric car use to affect battery life--not only charging but overall use.
A recent study by the Idaho National Laboratory, for instance, found that mileage degrades battery packs more than repeated quick charging, despite conventional wisdom to the contrary.