NMR Method Reveals Hidden Battery Chemistry

By using a novel spectroscopy method to probe batteries under working conditions, researchers have identified critical aspects of the electrochemical mechanism that dictates the ability of a potentially high-powered type of lithium-ion battery to repeatedly store and release charge. Currently in the research phase, the new battery, which features a silicon negative electrode in place of the carbon electrode found in today's batteries, may be able to keep electronics and other types of devices up and running between charges far longer than current lithium-ion batteries.

Cell phones, laptop computers, and other devices draw the power they need from lithium-ion batteries containing a lithium-cobalt-oxide cathode and a carbon (graphite) anode. Switching to a silicon anode could lead to batteries with roughly 10 times the capacity to store charge compared with carbon-based batteries. Yet that potential has not been realized as a result of poorly understood processes that render the silicon electrode incapable of being lithiated and delithiated repeatedly during battery charging and discharging cycles.

To probe those processes in detail, Clare P. Grey and coworkers at the State University of New York, Stony Brook, devised an "in situ" method for cycling a Li-Si battery while it sits inside an NMR probe as they record NMR spectra. The results, which Grey presented on Monday at the American Chemical Society national meeting in Salt Lake City, identify the nature of the various lithium silicide phases formed during battery operation. The group found that one of those phases reacts with the battery electrolyte in a detrimental process that irreversibly robs the electrodes of charge-carrying lithium ions. The researchers determined that treating the electrodes with carboxymethyl cellulose impedes the unwanted reaction (J. Am. Chem. Soc., DOI: 10.1021/ja8086278).

"This is an elegant study of the structural changes that occur in a very important class of relatively new electrode materials for lithium-ion batteries," Linda F. Nazar of the University of Waterloo says. The study identifies previously unknown steps in forming the silicides, as well as a hitherto unknown mechanism of self-discharge involving a highly reactive Li-Si phase, she adds. "The work highlights the importance and the power of such probes of structure and composition to elucidate the processes that occur in lithium-ion cells," she says.