Ohio State University’s Center for Automotive Research is studying why batteries lose their ability to hold a charge as they age. In collaboration with ORNL and NIST, the group is focusing on lithium-ion batteries, which are hot in the electric car industry.
Preliminary results from the ongoing study suggest that external conditions such as temperature variations and charging rates play only a minor role in battery life. Aging, the researchers say, begins on the nanoscale level and once the process starts, it is irreversible.
In the lab, batteries are charged and discharged many times in different conditions to mimic different real-world driving environments. Once the batteries were sufficiently aged and degraded, the researchers opened them up to see what was going on inside at the microscopic level. According to a press release by the American Institute of Physics:
When the batteries died, the scientists dissected them and used a technique called infrared thermal imaging to search for problem areas in each electrode, a 1.5-meter-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 (e.g. 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 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 and unaged sample has much lower lithium concentration in the cathode,” says Giorgio Rizzoni, director of the CAR. “It has essentially combined with anode material in an irreversible way.”
The researchers suspect that the coarsening of the cathode may be behind this 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.
Researchers presented their findings last week at the AVS 57th International Symposium and Exhibition.