MIT (Massachusetts Institute of Technology) researchers have found a way to improve the energy density of a type of battery known as lithium-air (or lithium-oxygen) batteries, producing a device that could potentially pack several times more energy per pound than the lithium-ion batteries that now dominate the market for rechargeable devices in everything from cellphones to cars.

The work is a continuation of a project that last year demonstrated improved efficiency in lithium-air batteries through the use of noble-metal-based catalysts.

In principle, lithium-air batteries have the potential to pack even more punch for a given weight than lithium-ion batteries because they replace one of the heavy solid electrodes with a porous carbon electrode that stores energy by capturing oxygen from air flowing through the system, combining it with lithium ions to form lithium oxides.

The new work takes this advantage one step further, creating carbon-fiber-based electrodes that are substantially more porous than other carbon electrodes, and can therefore more efficiently store the solid oxidized lithium that fills the pores as the battery discharges.

"We grow vertically aligned arrays of carbon nanofibers using a chemical vapor deposition process. These carpet-like arrays provide a highly conductive, low-density scaffold for energy storage," explains Robert Mitchell, a graduate student in MIT's Department of Materials Science and Engineering (DMSE) and co-author of a paper describing the new findings in the journal Energy and Environmental Science.

"We were able to create a novel carpet-like material - composed of more than 90 percent void space - that can be filled by the reactive material during battery operation," says Yang Shao-Horn, the Gail E. Kendall Professor of Mechanical Engineering and Materials Science and Engineering and senior author of the paper.

In the paper published last year, the team had estimated the kinds of improvement in gravimetric efficiency that might be achieved with lithium-air batteries; this new work "realizes this gravimetric gain," Shao-Horn says.

"Further work is still needed to translate these basic laboratory advances into a practical commercial product," she cautions.