Researchers at the Oak Ridge National Laboratory (ORNL) have come up with a promising design for a lithium-sulfur rechargeable battery that is considerably cheaper and more energy-dense than standard lithium-ions. Using a solid electrolyte rather than a liquid one, the battery is also testing much safer and more durable than previous designs.
Lithium-sulfur batteries are seen by some as the successors of lithium-ions because they are extremely light (they are often used for solar-powered flight), they can reach an impressive energy density, and they are cheaper to produce.
But the technology isn't quite mature yet, and as it turns out, the two major limitations with Li-S batteries have to do with the electrolyte. An electrolyte is a substance that, when mixed with a solvent, releases ions, making it electrically conductive. In batteries, electrolytes transport charge between the two electrodes, converting chemical energy into electrical energy.
In previous Li-S battery designs, the electrolyte used was liquid in nature. This proved a double-edged sword: the liquid electrolyte is an excellent conductor because of how it dissolves the lithium compounds, but this dissolution also causes the battery to break down prematurely. The liquid electrolyte is also flammable, posing serious safety concerns.
But now, researchers may have found a way around these problems.
"Our technology overcomes the capacity fade and safety issues of Li-S technology," Dr. Chengdu Liang, lead author of a paper on the research, told Gizmag. "The battery still performs well after a few hundred cycles, and the volumetric density could be slightly better than Li-ion batteries."
The researchers overcame these barriers by building a solid electrolyte made of lithium polysulfidophosphates (a new class of sulfur-rich materials with good electrical conductivity) to create an energy-dense, all-solid battery that is showing a lot of promise.
Even after 300 charge-discharge cycles at 60°C (140ºF), the battery retained a capacity of 1200 mAh/g (milliampere-hours per gram), compared to the 140-170 mAh/g of a traditional lithium-ion battery (lithium-sulfur batteries, however, only deliver about half the voltage of lithium-ions, so this 8-fold increase actually translates into a 4-fold increase in energy density).
The battery uses elemental sulfur, which is a byproduct of industrial petroleum processing. In other words, the battery could also provide a way to recycle industrial waste into a useful – perhaps even superior – technology.
"The main limitation is the relatively low ionic conductivity of the solid electrolyte," said Liang. "So the power density is lower than Li-ion batteries, but it can be improved with a better solid electrolyte. Moreover, the ceramic structure is brittle, and much optimization is needed."
The technology is still in the early stages of development, but Liang and colleagues are working on ironing out these issue and have filed a patent application for their battery design.
The paper detailing the study was recently published in the journal Angewandte Chemie.
Update 06.10.13: Gizmag wrote back to Dr. Chengdu Liang for more details of the battery's charging and discharging behavior. Here is his response:
"We did not observe self-discharge. A charged cell was put on shelf for over a week, and it still delivered the same capacity. The essence of our all-solid battery design is to eliminate the self-discharge through the all-solid configuration.
"This battery charges slower than Li-ion battery at the current status for a simple reason; the ionic conductivity of both the solid electrolyte and cathode are not high energy to have high current density. Much better performance at elevated temperatures such as 60 degrees C or higher."
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