(Phys.org) —As the demand for rechargeable lithium-ion (Li-ion) batteries has grown, the battery industry has found itself facing a problem of supply-and-demand. Lithium is not an abundant element, and most lithium deposits are found in only a handful of countries. Both problems make its long-term availability and cost uncertain. In a paper published in the June 4 issue of Nature Communications, University of Maryland professors Chunsheng Wang and John Cumings explain how a modified version of a Li-ion battery anode could allow manufacturers to replace the lithium with a more common element.
Sodium (Na), an earth-abundant and inexpensive element, shares many properties with lithium, but so far has not been able to replace it. The best strategies for creating Li-ion batteries often can’t be adapted for use in Na-ion batteries, rendering them a laboratory curiosity and keeping them out of the market.
The main problem is the atom’s size. Sodium ions are larger than lithium ions, which limits the kinds of materials that can be used in a Na-ion battery anode, the component into which the positively charged ions flow. Graphite (a form of pure carbon) is among the most superior options, and is also the most common in Li-ion batteries. When creating graphite anodes, lithium ions are easily electrochemically intercalated (embedded) into its layered structure, but for sodium ions it’s a tight squeeze, and the result is a battery with sluggish performance and low capacity.
The solution, Wang and Cumings have discovered, is to increase the space between the individual layers of carbon that make up the graphite. Their 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.