NEWS OF THE WEEK
MATERIALS: With three new designs, the batteries could have long lifetimes
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ATTERIES WITH lithium-sulfur electrodes
theoretically could store four times as much energy as conventional lithium-ion batteries. They promise longer play times for electronics and more miles between charges for electric cars. Now, researchers claim that commercialization of these experimental batteries is within reach. With three new materials designs, researchers show that they can extend the batteries’ lifetimes while maintaining a high energy-storage capacity (Nano Lett. 2013, DOI: 10.1021/nl402793z). The lifetimes of lithium-sulfur batteries are normally cut short by physical and chemical defects created as the batteries charge and discharge. The lithium-sulfur cathodes absorb and release large quantities of lithium ions, which cause the electrodes to expand and contract in volume by about 80%, says Yuegang Zhang, a materials scientist at the Chinese Academy of Sciences. The changes can cause the cathode to break, stopping the current flow. Also when lithium and sulfur react, they form lithium polysulfides that are soluble in conventional electrolytes. As this sulfur dissolves from the cathode, the battery’s energy-storage capacity drops. The polysulfides, which are insulators, then deposit on the anode and decrease current flow through the battery. To address these problems, Zhang and his colleagues first used a composite cathode material made by coating sulfur on graphene oxide. The carbon material,
which they reported previously, is conductive and strong, so it increases the cathode’s durability as it expands and contracts (J. Am. Chem. Soc. 2011, DOI: 10.1021/ja206955k). Graphene oxide also binds well to sulfur, preventing the sulfur from dissolving when it reacts with lithium. Next, the researchers used a rubberlike material for the binder that holds parts of the battery together. The conventional binder, polyvinylidene fluoride, is too brittle to withstand the cathode volume changes. Finally, the team picked a new electrolyte consisting of a conventional organic solvent mixed with an ionic liquid, which prevents sulfur from leaching from the cathode. A test battery made with these materials could be charged and discharged 1,500 times with little degradation in performance. Its initial energy-storage capacity was 500 watt-hours per kilogram of battery material. After 1,000 cycles, the storage capacity dropped to 300 Wh/kg. Conventional lithium-ion batteries store about 200 Wh/kg, and the U.S. Department of Energy’s target for electric vehicle batteries is 400 Wh/kg. On the basis of these results, Zhang believes that this technology is ready for commercialization, and he’s seeking industrial partners to scale up the battery design. The work “is a big step forward,” says Jie Liu, a chemist at Duke University, because it shows that the poor long-term stability of lithium-sulfur batteries can be overcome. The next step, he says, is to find a practical anode material for the batteries.—KATHERINE BOURZAC, Special to C&EN YUEGANG ZHANG
BUILDING LITHIUMSULFUR BATTERIES THAT LAST
A researcher connects a lithiumsulfur battery to an electrical circuit to test its storage capacity and longevity.
FERTILIZERS PotashCorp to lay off more than 1,000 workers amid global price declines Potash Corp. of Saskatchewan, a leading producer of potassium, phosphate, and nitrogen fertilizers, says it will reduce its workforce by 18%, or approximately 1,045 employees. The firm is taking the action because of lower-than-anticipated fertilizer demand from the developing world. The market’s softness is a change from 2012, when high prices for agricultural commodities boosted prices and earnings at fertilizer companies. More than half of PotashCorp’s employee cuts will come from operations in Saskatchewan and New Brunswick, where the company
plans to reduce production of potash, a mined salt that is largely potassium chloride. Other layoffs will affect workers at phosphate and nitrogen fertilizer plants in Florida, North Carolina, and Trinidad. Prices for potash have dropped by almost 23% compared with a year ago, according to a recent report from NPK Fertilizer Advisory Service. Analysts say the breakup in July of Belarus Potash Co., a cartel of producers in Belarus and Russia, was a factor in lower prices of the potassium fertilizer. Prices for phosphate and nitrogen have also declined because
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of slow sales in developing countries. It is unclear when prices will recover, according to Steve Hansen, a stock analyst at investment firm Raymond James. “The ongoing Russian potash dispute has cast a dense layer of cloud and fog over the horizon,” he wrote in a report to investors. Two other major North American fertilizer firms, Mosaic and CF Industries, have also reported lower selling prices but have not announced mass layoffs. In November, Mosaic announced the closure of its potash mine in Hersey, Mich.—MELODY BOMGARDNER
