Better Batteries: Powers of Ten

Dean Sigler Electric Powerplants, Sustainable Aviation Leave a Comment

The CAFE Foundation in its Electric Aircraft Symposia has put forth the idea of the 10X battery for many years.  Dr. Seeley therefore found a great deal of excitement in the following news.

Researchers at Northwestern University in Evanston, Illinois have perhaps achieved part of that dream, with a graphene and silicon anode that yields a 10-times-faster charge and can hold a charge 10 times greater than that of a typical lithium-ion battery.

Claiming their technology will be on the market in three to five years, the researchers have published a paper describing the research in the journal Advanced Energy Materials.

A University press release explains.  “’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.’  (Meaning it loses half of its charge ability after a year.)

“Kung is professor of chemical and biological engineering in the McCormick School of Engineering and Applied Science. He also is a Dorothy Ann and Clarence L. Ver Steeg Distinguished Research Fellow,” the release continues.

“Lithium-ion batteries charge through a chemical reaction in which lithium ions are sent between two ends of the battery, the anode and the cathode. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte, and to the cathode; as the battery is recharged, they travel in the reverse direction.

Current battery technology constrains how long a battery can maintain its charge – limited by how many lithium ions can be packed into the anode or cathode – rated as the battery’s charge density.  Its charge rate, or how quickly a battery can be recharged, is also limited by the speed with which lithium ions can move from the electrolyte to the anode.

Northwestern’s press release expands on this.  “In current rechargeable batteries, the anode — made of layer upon layer of carbon-based graphene sheets — can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium: four lithium atoms for every silicon atom. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.

Northwestern's graphene-silicon anode, showing clustering of lithium between graphene sheets and holes permitting rapid transit of ions

“Currently, the speed of a battery’s charge rate is hindered by the shape of the graphene sheets: they are extremely thin — just one carbon atom thick — but by comparison, very long.  During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. And because it takes so long for lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material.”

First, Kung’s team stabilized the silicon. To maintain maximum charge capacity, they sandwiched clusters of silicon between the graphene sheets, creating room for a greater number of lithium atoms in the electrode while exploiting the flexibility of graphene sheets to make up for the expansion and contraction of the silicon during charging and discharging.

Second, Kung’s team used a chemical oxidation process to create 10 to 20 nanometer  holes in the graphene sheets.  These “in-plane defects” – create a “shortcut”for lithium ions to travel to the anode in a more direct way and be stored there by reaction with silicon. This reduces recharging time to one-tenth of normal.

“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.”

The team will next look at the cathode after focusing their research previously on the anode.

The team’s paper is titled “In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries.”  Other authors of the paper are Xin Zhao, Cary M. Hayner and Mayfair C. Kung, all from Northwestern.

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