Science Concentrates ENERGY STORAGE
Polyrotaxanes could boost battery performance Polymers prevent silicon anode disintegration Replacing the anode in commercial lithium-ion batteries with a silicon-based version could improve how much charge a battery can hold. But silicon anodes have been difficult to develop because they massively expand and contract as a battery charges and discharges, causing particles in the anode to break apart and the battery to fail. Now, a team led by Ali Coskun and Jang Wook Choi at Korea Advanced Institute of Science & Technology has developed highly elastic polymers that, when added to a silicon anode, can relieve the stress of charging and discharging while holding the silicon particles together (Science 2017, DOI: 10.1126/science.aal4373). The researchers devised a unique polymeric network made of the conventional linear polymer polyacrylic acid covalently linked to polyrotaxanes containing mechanical bonds. In the polyrotaxanes, an amine functionalized polyethylene glycol chain is threaded through a number of cyclodextrin rings. During battery charging, as the silicon anode expands, the rings
major battery maker to test their polymeric pulley system on real battery products. “Mechanical bonds have come to the rescue for the first time in an energy storage context,” says J. Fraser Stoddart of Northwestern University, who received the 2016
freely slide along the chain to dissipate stress, operating like a pulley system. The researchers showed that the molecular netting demonstrated excellent elasticity, withstanding up to about 400% strain. The two main advantages of using this polymeric network are cost and processability, Coskun says. The team was able to make anodes with silicon microparticles, which are easier and cheaper to make in large quantities Highly elastic polyrotaxane-polyacrylic acid polymers hold than the smaller and together silicon microparticles as they expand and contract. more commonly Nobel Prize for his work on rotaxanes and studied silicon nanoparticles, he says. advised Choi as a grad student and Coskun Also, the new polymer accounts for only as a postdoc. The authors’ “ingenious” use 10% by weight of the anode, compared of polyrotaxanes “marks a breakthrough in with the 20% polymer binder typically the performance of marketable lithium-ion used in silicon anodes. The researchers are collaborating with a batteries.”—TIEN NGUYEN
Mass spec might aid bomb-sniffing dogs A device that detects vapors from explosives could help handlers improve canine training Law enforcement has long relied on canines to sniff out explosives. But partially because of differences in each dog’s training, some canines are better at it than others. Now, researchers have developed a device that could improve dogs’ training by analyzing odors from explosives in real time (Anal. Chem. 2017, DOI: 10.1021/acs. analchem.7b00451). Handlers typically present bomb-sniffing dogs with training materials containing TNT and other explosives to teach them to recognize the vapor plumes the materials emit. The handlers then hide the materials and evaluate the dogs’ ability to find them. The problem is that handlers often use multiple training materials during a session, which can lead to cross-contamination of one material with another. This contamination can go undetected, muddling the
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C&EN | CEN.ACS.ORG | JULY 24, 2017
and incorrectly. Some dogs had alerted their handler to explosives in materials that supposedly didn’t contain explosives. But analysis with the mass spec device results of dogs’ performance evaluations. revealed that six of the 68 “blank” samples To improve the validity of training mawere contaminated with explosives. This terials and handlers’ accuracy in assessing finding shows that apparent canine errors canine performance, researchers led by may have in fact resulted from contaminaTa-Hsuan Ong of MIT Lincoln Laboratory tion or other problems. developed a real-time mass spectrometer David Atkinson, chief scientist for that measures at parts-per-quadrillion levexplosives detection research at Pacific els nine compounds found in explosives, Northwest National Laboratory, notes that comparable to or better than dogs’ detecthe researchers tested tion limits. the device on only a few During testing, four With training, dogs can help canines and on a narrow bomb-sniffing dogs com- identify explosives. range of compounds. pleted a training exercise But “it’s a very good to find various hidden start,” Atkinson says. It explosives. The recould make the training searchers then analyzed of these dogs a lot easitraining samples that the er, he adds.—MELISSA dogs had identified correctly—confirming the PANDIKA, special to presence of explosives— C&EN
CR E D I T: A DA PT ED FRO M JA N G WO O K C H O I A N D CO LLEAGUES ( S CH EMATI C ); A N A L . CH E M . (DO G )
FORENSIC SCIENCE
