Over the last couple of years, research to improve lithium-ion (Li-ion) batteries have been turning to graphene, particularly after researchers at Northwestern University successfully sandwiched a layer of silicon between graphene sheets in the anodes of Li-ion batteries.
But most of the Li-ion battery work being done with graphene to date has depended on high-vacuum environments to create the layered material. Now Gurpreet Singh, a Kansas State University assistant professor of mechanical and nuclear engineering, is leading a team that's looking at faster and cheaper ways of synthesizing the material.
"We are exploring new methods for quick and cost-effective synthesis of two-dimensional materials for rechargeable battery applications," Singh said in a university press release.
The two-dimensional materials to which Singh refers includes not only graphene but also tungsten disulfide nanosheets. In his work with graphene, which was published in the journal Applied Materials & Interfaces (“Synthesis of Graphene Films by Rapid Heating and Quenching at Ambient Pressures and Their Electrochemical Characterization”),Singh’s team was able to create the graphene outside of a vacuum.
The graphene sheets were grown on copper and nickel foils by placing them in a furnace in which a mixture of argon, hydrogen and methane gases was carefully controlled. The researchers quickly heated and cooled the metal foils, forming the graphene films. The entire process apparently takes less than 30 minutes.
When the researchers then used the graphene films to fashion the negative electrode of a Li-ion battery, they discovered that graphene formed from the copper did not cycle lithium ions and had negligible capacity. However, the graphene electrode created from nickel had far superior performance to the copper version.
"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," Singh said in the release.
The second line of research that Singh and his team undertook with tungsten disulfide nanosheets involves conversion-reaction batteries. Conversion-reaction batteries are so named because the materials used in the batteries undergo a conversion reaction when in contact with lithium. A fair amount of work is ongoing to get a better handle on the nature of these conversion reactions, but in the meantime there’s a lot of excitement about the high-capacity capabilities of such batteries.
In the Kansas State research, which was published in the Journal of Physical Chemistry Letters (“Synthesis of Surface-Functionalized WS2 Nanosheets and Performance as Li-Ion Battery Anodes”), Singh and his team developed a process for separating bulk tungsten disulfide into two-dimensional crystals, only three atoms thick. When the material is applied to Li-ion batteries it stores and releases lithium ions in a completely different way from the graphene. When lithium comes in contact with the tungsten disulfide the materials undergo a conversion reaction leaving tungsten and lithium sulfide.
Despite all the new enthusiasm for conversion-reaction batteries, Singh concedes tungsten disulfide may not work for some potential applications of Li-ion batteries based on conversion reactions.
"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 in the release. "Therefore tungsten disulfide may not be an ideal electrode material for portable batteries."