Researchers from Washington University in St. Louis have determined how to build a safer battery by studying a lithium-ion battery electrode particle by particle.
The team calculated the ‘true current density’ during movement of charges in lithium-ion battery electrodes in a 365 mAh/gcoin cell at any given spot via image analysis using a bench-top optical microscope.
A spokesman for the university told BEST the ‘true current density’ was around 100 times higher than the average current density obtained using the BET surface area (varies from 4mA/cm2 to 0.2mA/cm2).
The team, which looked at graphite anodes as a model system, observed that lithium ions can diffuse into the vacancies in the graphite crystal structure and induce a solid-state phase transformation during battery recharge.
As a result, the induced new phases of graphite— i.e. lithiated graphite— change colour from dark grey, to blue, to red and eventually to gold, as more lithium ions are accommodated.
After analysing thousands of images of the electrode during recharging, the team was able to determine the true current density (a measure of the accumulated charge per unit time on a certain area) in the areas that had changed colour.
The research was published online in the journal Advanced Energy Materials.
The research was conducted at the laboratory of Peng Bai, assistant professor in the Department of Energy, Environmental & Chemical Engineering at the McKelvey School of Engineering at Washington University in St. Louis.
Bai said: “We need to promote more particles to participate in the reaction throughout the whole electrode. Then we can lower the actual local current density and avoid generating hazardous hot spots
“Similar phenomenon was reported before, but only if the dynamics can be explained by a mathematical model can we claim it as the high-fidelity truth.
“But it becomes obvious now that it is the local current density that really matters. The local current density is at least two orders of magnitude higher than the apparent current density.”
Image: PhD student Shubham Agrawal holding an in-situ coin cell, with a glass window used for optical observation at its center. (Photo: Rajeev Gopal)