Why do batteries lose their ability to hold a charge as they age? New research suggests the slow death of a battery might start at the nanoscale.
Yann Guezennec and Giorgio Rizzoni of Ohio State University (OSU) developed new experimental facilities and procedures to charge and discharge commercially-available lithium-ion batteries thousands of times over many months. The tests were designed to mimic a variety of conditions in which these batteries are actually used by hybrid and all-electric vehicles. Some of the batteries were run in hot temperatures such as those experienced in Arizona, while others were tested in colder conditions similar to those in Alaska.
As part of the tests, researchers Bharat Bhushan, Suresh Babu and Lei Raymond Cao studied the materials inside of the batteries to help determine how ageing manifests itself in the structure of the electrode materials.
After the batteries died, the team “dissected” them and used a technique called infrared thermal imaging to search for problem areas in each electrode, which is a 1.5-metre-long strip of metal tape coated with oxide and rolled up like a jelly roll. They then took a closer look at these problem areas using a variety of techniques with different length scale resolutions (for example, scanning electron microscopy, atomic force microscope, scanning spreading resistance microscopy, Kelvin probe microscopy, transmission electron microscopy) and discovered that the finely-structured nanomaterials on these electrodes that allow the battery to rapidly charge and discharge had coarsened in size.
Additional studies of the aged batteries, using neutron depth profiling, revealed that a fraction of the lithium that is responsible, in ion form, for shuttling electric charge between electrodes during charging and discharging, was no longer available for charge transfer, but was irreversibly lost from the cathode to the anode.
“We can clearly see that an aged sample versus an unaged sample has much lower lithium concentration in the cathode,” said Rizzoni, director of the Centre for Automotive Research at OSU. “It has essentially combined with anode material in an irreversible way.”
The centre’s research is being performed in collaboration with Oak Ridge National Laboratory and the National Institute of Standards Technology.
The researchers suspect, but cannot yet prove, that the coarsening of the cathode may be behind the loss of lithium. If this theory turns out to be correct, it could point battery manufacturers in the right direction for making durable batteries with longer lifetimes. Such battery improvements could help accelerate the rollout of electric cars.