Depiction of the essential functioning of the lithium-air battery. Ions of Li combine with oxygen from the air to form particles of Li oxides, which attach themselves to carbon fibers on the electrode as the battery is being used. During recharging, the Li oxides separate again into Li and oxygen and the process can begin again. Credit: Mitchell, Gallant and Shao-Horn; MIT.

An elusive piece of the alternative energy puzzle has been storage: How does one save it for when it’s needed or carry it around to use where it’s needed? In both cases, energy density is the key parameter and in the latter case, weight matters, too.

We’ve been following MIT associate professor Yang Shao-Horn and her work on lithium-air batteries in posts about alternative catalyst materials and carbon nanotube electrodes. Her group appears to have taken a leap forward in increasing the energy density using aligned carbon nanofiber electrodes that can store four times as much energy on a weight basis than current-technology Li-ion battery electrodes.

The lightweight advantage of lithium-air batteries (or any metal-air battery) comes from replacing a solid electrode like those in typical Li-ion batteries with a porous carbon electrode. Energy is stored when Li ions react with air flowing through the porosity to form Li oxides. The more porous the carbon, the more efficiently Li oxides are stored.

As the battery is used, particles of lithium peroxide form as small dots on the sides of carbon nanofibers (top), and become larger toroidal shapes as the battery discharges (bottom), as seen in these SEM images. Credit: Mitchell, Gallant, and Shao-Horn; MIT

A press release reports that Shao-Horn’s group used chemical vapor deposition to fabricate an electrode of vertically aligned arrays of carbon nanofibers with 90 percent void space, a big increase over the 70 percent void space the group reported achieving last year.

“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,” Shao-Horn says in the press release. That means, according to Robert Mitchell, a graduate student and paper’s first author, that “the carpet-like arrays provide a highly conductive, low-density scaffold for energy storage.”

The gravimetric energy, which is the amount of power that can be stored for a given weight, for these very-low-density electrodes is one of the highest reported to date and demonstrates that “tuning the carbon structure is a promising route for increasing the energy density of lithium-air batteries,” said another graduate student and coauthor, Betar Gallant.

An unexpected finding is that the orderly “carpet” structure of the fibers makes them relatively easy to observe in a scanning electron microscope, and the performance of the electrodes can be monitored at intermediate states of charge. Being able to directly observe the process may shed some light on other vexing issues, such as the degradation observed after many charge–discharge cycles.

These latest results will be published in the August issue of the journal Energy and Environmental Science (see “All-carbon-nanofiber electrodes for high-energy rechargeable Li-O2 batteries,” doi: 10.1039/C1EE01496J).