Sodium (Na) is an earth-abundant and inexpensive element, and shares many properties with lithium. However, graphite, as the most common anode for commercial Li-ion batteries, has been reported to have a very low capacity when used as a Na-ion battery anode; the electrochemical insertion (intercalation) of Na⁺ into graphite is significantly hindered by insufficient interlayer spacing in the graphite to allow the sodium ions—which are larger than lithium ions—to fit.

To address that problem, the University of Maryland team developed a way to expand the graphite material—i.e., to enlarge the interlayer lattice distance to accomodate the larger sodium ions.

The team starts with graphite oxide, a common industrial material formed by exposing graphite to an aggressively corrosive solution that stuffs oxygen between its layers. The oxygen atoms bond with each carbon layer, pushing and holding them apart. However, the resulting material is inevitably “overstuffed,” leaving no room for sodium ions to get in. To make the material suitable for use in Na-ion batteries, some of the oxygen must be removed.

The solution to this second problem was developed by the paper’s first author, Department of Chemical and Biomolecular Engineering (ChBE) graduate student Yang Wen. Wen heats the expanded, oxidized graphite to high temperatures and floods it with argon gas, causing it to decompose. In this process, oxygen bonded to carbon breaks away in the form of either carbon monoxide (CO) or carbon dioxide (CO₂) gas, which is caught up and removed by the argon gas flow.

Schematic illustration of sodium storage in expanded graphite-based materials. Wen et al. Click to enlarge.

Wen’s key discovery was the precise combination of temperature and duration for the reaction. Her technique ensures that enough oxygen atoms have been removed to let the sodium ions in, but enough are left behind to prevent the expanded graphite from collapsing. The process may be likened to jacking up every floor of a multi-storey building to accommodate taller tenants, and then removing excess scaffolding until only the required support beams remain.

Expanded graphite is already commercially available, but industry uses a different method to make it. If they follow Yang’s procedure, they can use it to make expanded graphite suitable for sodium-ion batteries. They won’t be as powerful as lithium-ion batteries. You’ll need more of them to get the same amount of power, but the cost is so much lower it will make up for it.

—UMD Professor Chunsheng Wang

Sodium-ion batteries are also heavier, so for now they’re not suitable for most vehicles and airplanes. But for something like building or grid-level power storage—where they’re just going to sit there—the fact that you get more kilowatt hours per dollar becomes a strong selling point.

—UMD Professor John Cumings

In addition to Wang, Cumings and Wen, the research team included Kai He (now at Brookhaven National Lab), Yujie Zhu (ChBE), Fudong Han (ChBE), and Yunhua Xu (ChBE). Co-authors Isamu Matsuda and Yoshitaka Ishii (both University of Illinois) verified the team’s reduction process.

This research was supported by the Science of Nanostructures for Electrical Energy Storage, the University of Maryland’s Energy Frontier Research Center funded by the US Department of Energy; the Maryland NanoCenter, and the University of Maryland Energy Research Center.

Resources

• Yang Wen, Kai He, Yujie Zhu, Fudong Han, Yunhua Xu, Isamu Matsuda, Yoshitaka Ishii, John Cumings & Chunsheng Wang (2014) “Expanded graphite as superior anode for sodium-ion batteries,” Nature Communications 5, Article number: 4033 doi: 10.1038/ncomms5033