A Better Lithium Battery?
M.I.T. researchers design a more energy-dense lithium-based battery
Researchers at the Massachusetts Institute of Technology have come one step closer to replacing the lithium-ion batteries that power phones, laptops and electric cars with a device that stores far more energy for the same weight.
The device is known as a lithium-air or lithium-oxygen battery. Charged lithium atoms react with the oxygen from air flowing through the apparatus, forming lithium peroxide, and deposit on the structure. The peroxide can then be broken down to release electricity.
In a paper published in the journal Energy & Environmental Science, a team of materials scientists and mechanical engineers refined this design and made it hold almost four times the electricity of a lithium-ion battery by weight.
"The key benefit, I think, is energy density," said Robert Mitchell, a Ph.D. student in materials science and engineering. Mitchell and Betar Gallant, a Ph.D. student in mechanical engineering, were the co-lead authors of the study. "In the last couple of years, there has been a flurry of new interest in these batteries," said Mitchell.
The electrode of the battery is formed using carbon nanofibers: thin, hollow cylinders of pure carbon. The nanofibers are grown and aligned, forming a structure not unlike a carpet or a lawn. "The unique morphology of these carbon electrodes creates a low-density scaffold for lithium peroxide," said Mitchell. The open spaces in the structure provide lots of room for electricity to be stored in the form of lithium compounds.
Searching for a proper catalyst
The battery itself is also much lighter than existing rechargeable cells. "The reason has to do with how the cathode works," said Gallant. "We directly react the lithium with oxygen instead of cobalt," the material most commonly used in current-generation lithium-ion batteries. "It can provide a longer range for an electric vehicle for a given weight," she said.
Nonetheless, the new device has some limitations. "One challenge with lithium-air batteries is the poor round-trip efficiency," said Mitchell. "The voltage required to decompose the lithium peroxide is quite large"; i.e., the voltage coming out of the cell is lower than the voltage required to charge it.
The team is investigating some potential solutions. "There's a lot of interest in new catalysts to increase efficiency," said Mitchell. The catalysts would lower the amount of energy needed to break down the lithium peroxide. Mitchell said that practical applications for lithium-air cells are still years off.
However, it isn't just the potential of this research that intrigued Gallant. "What's really exciting about the structure we developed is that we could see lithium peroxide structures develop, grow and degrade," said Gallant. The researchers are hoping to study how the material deposits on the structure, along with tuning the carbon density, in order to continue improving the device's efficiency.