Researchers used the films to create the negative electrode of a lithium-ion cell and test the charge and discharge characteristics of the battery.\u00a0 Graphene films grown on copper did not cycle lithium ions \u201cand the battery capacity was negligible.\u201d\u00a0 Graphene films grown on nickel, though, was able to store and release lithium ions more efficiently.<\/p>\n Singh noted, “We believe that this behavior occurs because sheets of graphene on nickel are relatively thick near the grain boundaries and stacked in a well-defined manner — called Bernal Stacking — which provides multiple sites for easy uptake and release of lithium ions as the battery is discharged and charged.”<\/p>\n Researchers created 10-layer thick tungsten disulfide nanosheets in a second experiment.\u00a0 The material, used as a dry lubricant in automobiles (and on pinewood derby axles), is a dry powder which the team separated into three-atom-thick sheets in a strong acid solution.<\/p>\n The resulting sheets stored and released lithium ions, but differently from the way the graphene sheets did.\u00a0 A conversion reaction in the tungsten\/lithium ion battery causes the tungsten disulfide to dissociate to form tungsten and lithium sulfide during discharge.\u00a0 This involves the transfer of at least two electrons per tungsten atom \u2013 suggesting more energy transfer than single electron transfers common in lithium cells.<\/p>\n Unfortunately, “We also realize that tungsten disulfide is a heavy compound compared to state-of-the-art graphite used in current lithium-ion batteries,” Singh said. “Therefore tungsten disulfide may not be an ideal electrode material for portable batteries.”<\/p>\n The research appeared in a recent issue of the Journal of Physical Chemistry Letters in an article titled “Synthesis of surface-functionalized WS2 nanosheets and performance as Li-ion battery anodes<\/a>.”<\/p>\n Because both projects create nanomaterials in a cost-effective way, they may inspire other researchers to attempt graphene or other film production without the added expense of low-pressure chemical processes. \u00a0The production of multiple layer films also has an advantage.<\/p>\n “Interestingly, for most applications that involve this kind of battery research and corrosion prevention, films that are a few atoms thick are usually sufficient,” Singh said. “Very high quality large area single-atom-thick films are not a necessity.”<\/p>\n The research teams comprised Romil Bhandavat and Lamuel David, both Kansas State doctoral students in mechanical engineering,\u00a0Saksham Pahwa in India, University of Michigan researchers Zhaohui Zhong, assistant professor of electrical engineering and computer science, and Girish Kulkarni, doctoral candidate in electrical engineering.<\/p>\n Work was supported by the National Institute of Standards and Technology and the Kansas National Science Foundation Experimental Program to Stimulate Competitive research program.<\/p>\n Singh plans future research to study how these layered nanomaterials can create better electrodes in the form of heterostructures, which are essentially three-dimensional stacked structures involving alternating layers of graphene and tungsten or molybdenum disulfide.<\/p>\n The research appears in the January issue of ACS-Applied Materials and Interfaces in an article titled “Synthesis of graphene films by rapid heating and quenching at ambient pressures and their electrochemical characterization<\/a>.”<\/p>\n Stacking graphene sheets to gain different properties is the result of a happily accidental discovery<\/a> at the University of California, Riverside, which found that the electrical properties of graphene can be altered by adopting a technique called Bernal trilayer stacking.<\/p>\n![\"\"](\"http:\/\/cafe.foundation\/blog\/wp-content\/uploads\/2013\/02\/large-area-graphene-films-on-Cu-Ni.gif\" "\\"large")
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