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Li deposit in current collector linked to battery ageing

Posted: 20 Dec 2012     Print Version  Bookmark and Share

Keywords:current collector  Li-ion  battery electrode 

Researchers from Ohio State University (OSU) have discovered an unexpected factor that affects the performance of batteries commonly used in hybrid and electric-only cars.

The engineers, who are trying to determine the factors that limit battery life, examined used car batteries and discovered that over time, lithium (Li) accumulates beyond the battery electrodes in the current collector—a sheet of copper (Cu) that facilitates electron transfer between the electrodes and the car's electrical system.

According to them, this knowledge could improve design and performance of batteries, stated Bharat Bhushan, professor of mechanical engineering at OSU. The research is an on-going collaboration between Bhushan and Suresh Babu, professor of materials science and engineering and director of the National Science Foundation Centre for Integrative Materials Joining for Energy Applications, headquartered at OSU.

Lithium-ion batteries are the rechargeable batteries used in most hybrid-electric cars and all-electric cars. Inside, Li ions shuttle back and forth between the anode and cathode of the battery to the anode when the battery is charging, and back to the cathode when the battery is discharging. Previously, the researchers determined that, during ageing of the battery, cyclable Li permanently builds up on the surface of the anode, and the battery loses charge capacity. This latest study revealed that Li migrates through the anode to build up on the Cu current collector as well.

"We didn't set out to find Li in the current collector, so you could say we accidentally discovered it, and how it got there is a bit of a mystery. As far as we know, nobody has ever expected active Li to migrate inside the current collector," Bhushan indicated.

Shrikant Nagpure, now postdoctoral researcher at OSU, carried out this research as a part of his doctoral degree. He examined batteries that were aged in collaboration with the university's Centre for Automotive Research, where colleagues Yann Guezennec and Giorgio Rizzoni have studied battery ageing for several years, in collaboration with the automotive industry.

Key to the discovery of lithium in the current collector was collaboration between the OSU team and Gregory Downing, a research chemist at the National Institute of Standards and Technology and an expert on a technique called neutron depth profiling (NDP), a tool for impurity analysis in materials.

Previously, the researchers used NDP to study the cathodes and anodes of six off-the-shelf Li-ion car batteries—one new battery and five batteries that they aged themselves in the laboratory—and found that Li builds up on the anode surface over time.

In the NDP technique, researchers pass neutrons through a material and capture the charged particles that emerge from the fission reaction between neutrons and Li in the electrodes. Since different chemical elements emit a certain signature set of particles with specific energies, NDP can reveal the presence of impurities in a material.

In this latest study, NDP detected the presence of Li in the Cu current collector from one of the aged batteries. The detection was measured as a ratio of the number of Cu atoms in the collector to the number of Li atoms that had collected there. The test yielded a ratio of up to 0.08 per cent, or about one Li atom per 1250 Cu atoms in the collector.

That's a small number, but high enough that it could conceivably affect the electrical performance of the current collector and, in turn, the performance of a battery, Bhushan said. He hopes that battery makers will further investigate this phenomenon and use the information to design new materials that might prevent Li from escaping the electrode material.

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