Researchers at Northwestern University have been improving lithium ion batteries. So far they have come up with vast improvements on the anode side. Next they plan to tackle the cathode.
Harold Kung, professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science, led the research which is published in Advanced Energy Materials. The paper outlines their two major changes to battery technology.
The first change was to increase the capacity of lithium ion batteries by increasing the charge density. The charge density determines how long the battery maintains its charge. The second change was to the charge rate. The teams changes allow batteries to be charged in only 15 minutes.
“We have found a way to extend a new lithium-ion battery’s charge life by 10 times,” said Harold H. Kung, lead author of the paper. “Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today.”
In order to increase the charge density, Kung’s team had to find a way to pack more lithium ions into the same amount of space. Currently one atom thick graphene sheets are used to store the ions but they are limited to one lithium atom per four carbon atoms. Silicon will allow four lithium atoms to one silicon atom, but silicon is unstable.
To solve the instability problem with the silicon, the researchers sandwiched layers of silicon between layers of graphene. That increased the density of the lithium ions thus increasing the battery capacity.
“Now we almost have the best of both worlds,” Kung said. “We have much higher energy density because of the silicon, and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won’t be lost.”
Improving the charge rate meant decreasing the area that a lithium ion must travel. Normally a lithium ion has to travel the entire length of a graphene sheet before it can slide between the sheets, so to speak, coming to rest. Using a “chemical oxidation process” on the graphene sheets created miniscule holes shortening the area that the ion needed to travel and increasing the number of ions that could slip into the silicon at one time.
Those changes would allow lithium ion batteries to stay charged more than a week in smartphones and gadgets and reduces charge time to a mere 15 minutes as opposed to several hours.
Those changes only involved the anode portion of the battery. Now the scientists will start working on improvements to the cathode in order to further improve battery technology. They also plan to work on a method for making lithium ion batteries safer by using an electrolyte to shut off the battery once it reaches a certain temperature.
Batteries with this technology should be hitting the market within the next three to five years. Of course there are other battery technologies that may be hitting the market just about the same time.
