Researchers at Hanyang University, and their U.S. and Qatar colleagues combined two complementary carbon materials to boost the lithium ion cathode electrode’s conductivity. The team of researchers has designed a cathode for such batteries that holds up to 10 times as much sulfur as other designs without a drop in performance. A test battery made with the new cathode has eightfold higher energy capacity than a typical cellphone battery.

See the text or use the Link to the Research paer below for more info. Image Credit: ACS. Click image for the largest view.

See the text or use the link to the research paper below for more info. Image Credit: ACS. Click image for the largest view.

Arumugam Manthiram of the University of Texas, Austin explained that putting sulfur in battery cathodes presents many engineering challenges.

The team’s research paper has been published in the American Chemical Society’s Energy Letter.

Lithium-sulfur batteries could theoretically store an order of magnitude more energy by weight than today’s lithium-ion batteries because sulfur can incorporate more lithium ions than the standard electrode materials. This could extend the range of electric vehicles and the battery life of portable electronics without adding weight – and sulfur is abundant and cheap.

Manthiram continued, because sulfur has low electrical conductivity, engineers usually compensate by adding conductive, carbon-based compounds to the cathode. But these materials reduce the amount of sulfur that can fit in the battery, lowering the energy density. “We want to increase the sulfur loading while maintaining good performance so that lithium-sulfur batteries can compete,”said Manthiram.

Sun’s team and their colleagues combined two complementary carbon materials to boost the electrode’s conductivity: acetylene black and multiwalled carbon nanotubes. The careful mix of these materials creates a balance between mechanical strength and high conductivity per volume, allowing more incorporation of elemental sulfur without decreasing the conductivity or total energy storage. Other batteries of this type have relied on sulfur compounds, lowering the total sulfur loading.

In their design, the researchers made sure to address another known issue with lithium-sulfur batteries: Sulfur reacts with lithium ions to form lithium polysulfides that can migrate out of the cathode, removing lithium from operation. Using UV-Vis spectroscopy, the researchers confirmed that when the carbon materials were doped with nitrogen, the materials bound to the polysulfides and kept them from escaping. To further help with this, the researchers used chitosan, a polysaccharide found in crustacean shells, as a binder.

To make the cathode, the researchers spread a slurry of the nitrogen-doped carbon materials, chitosan binder, and elemental sulfur on aluminum foil and topped it with a net made of nanotubes to act as a final barrier to keep polysulfides from escaping. “The production cost of our system should be low compared to others,” Sun says. “Our materials are cheap, and we are using existing manufacturing technology.”

The completed cathode contains 10 mg of sulfur per cm2 – much more than the typical 1 or 2 mg/cm2, Manthiram said. When the researchers paired the cathode with a lithium-metal anode to form a battery, the device had an initial capacity of 1,332 milliamp-hours per gram. A typical cell phone battery has a capacity of about 170 mA-hr/g, Manthiram said. After discharging and charging 50 times, the new design maintains 91% of that storage capacity. After further charge cycles, the anode would start to break down; the limited lifetime of lithium-metal electrodes is another engineering challenge.

The third party commentator in the press release, Yuegang Zhang, a battery chemist at the Chinese Academy of Sciences, said it’s significant that the researchers were able to get more sulfur in the battery. But Zhang says they need to figure out how to boost the sulfur content even further to compete with commercial technology.

However Zhang sees this, or even what he knows that isn’t on the market yet, an eight fold increase by weight is a big improvement. The press release didn’t discuss the battery volume, and that could offer some issues. But in the lab showing an 800% increase with nearly a year’s worth of cycles is great news, indeed.


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