Improving batteries’ energy storage

Improving batteries’ energy storage

2:43 PM, 26th July 2011
Improving batteries’ energy storage
Testing of the battery.

CAMBRIDGE, US: MIT 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.

The new work takes this advantage one step further, creating carbon-fibre-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 nanofibre using a chemical vapour deposition process. These carpet-like arrays provide a highly conductive, low-density scaffold for energy storage,” explained 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.

During discharge, lithium-peroxide particles grow on the carbon fibre, added, Co-author Betar Gallant, a Graduate Student in MIT’s Department of Mechanical Engineering. In designing an ideal electrode material, she said, it’s important to “Minimize the amount of carbon, which adds unwanted weight to the battery and maximize the space available for lithium peroxide,” the active compound that forms during the discharging of lithium-air batteries.

“We were able to create a novel carpet-like material - composed of more than 90 per cent void space - that can be filled by the reactive material during battery operation,” said Yang Shao-Horn, the Gail E Kendall Professor of Mechanical Engineering and Materials Science and Engineering and Senior author of the paper. The other senior author of the paper is Carl Thompson, the Stavros Salapatas Professor of Materials Science and Engineering and interim head of DMSE.

“Tuning the carbon structure is a promising route for increasing the energy density of lithium-air batteries. The result is an electrode that can store four times as much energy for its weight as present lithium-ion battery electrodes,” said Gallant.

“Further work is still needed to translate these basic laboratory advances into a practical commercial product,” cautioned Shao-Horn.

Because the electrodes take the form of orderly “carpets” of carbon fibre - it is relatively easy to use a scanning electron microscope to observe the behaviour of the electrodes at intermediate states of charge. The researchers said this ability to observe the process, an advantage that they had not anticipated, is a critical step toward further improving battery performance. For example, it could help explain why existing systems degrade after many charge-discharge cycles.

(C) Massachusetts Institute of Technology News

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