Interview By Will Soutter
In this Thought Leader interview, Dr. Gurpreet Singh from Kansas State University talks to Will Soutter about his work on using 2D nanomaterials such as graphene and tungsten disulfide for electrodes in rechargeable lithium ion batteries.
WS: Can you give us a brief introduction to your work with lithium-ion batteries?
GS: We started working on lithium-ion batteries about 3 years ago. Our primary focus has been on designing the anode (negative electrode) material that can deliver improved electrochemical capacity with reduce polarization losses and no chemical or mechanical degradation over long periods of time. I have two PhD students (Romil Bhandavat and Lamuel David) working on this project.
WS: Why did you choose to study graphene in battery applications?
GS: Our initial work focused on use of molecular precursor derived glass ceramics (such as silicon oxycarbide) as rechargeable battery anodes. These high temperature glasses are unique because of their nanodomain structure consisting of graphene-like carbon domains.
In order to develop these anodes with tailored properties, we thought of first concentrating on understanding Li-ion storage mechanisms in single and multilayered graphene films. And that’s what got us into graphene film based anodes.
WS: How does the graphene manufacturing process you have developed make graphene more viable as a commercial electrode material?
GS: The process that we have developed allows synthesis of few-layer graphene without the use of high vacuum. The total time required for making these films is approximately 20 to 30 minutes. These graphene films can be used for corrosion prevention (grown on copper) and anodes for microbatteries (grown on nickel foil).
WS: What effects make graphene grown on nickel so much more effective than graphene grown on copper?
GS: We believe that sheets of graphene on nickel are relatively thick near the grain boundaries, and based on electron microscopy and spectroscopy data, these sheets are stacked in a well-defined manner called Bernal Stacking, which provides multiple sites for easy uptake and release of lithium ions as the cell is discharged and charged.
WS: How does the performance of your graphene electrodes compare to that of conventional electrode materials in lithium batteries?
GS: The purpose of this work was to study the underlying principles related to insertion and release of Li within individual sheets of few-layer graphene. A typical Li-ion battery anode is made from of a type of graphite slurry (along with conducting agent and binders) coated on a copper foil (current collector), and so a direct comparison with our graphene-based anode may not really be fair.
One clear advantage with graphene/Ni electrode is that it is less prone to delamination and does not require any binding or conducting agents (thereby saving weight). Flexible electrodes for microscale batteries are also possible with our graphene/Ni electrode.
Figure 1. Schematic representation showing likely paths for Li-ions to intercalate in single (SLG), bi (BLG) and few-layer (FLG) graphene on copper (Cu) and nickel (Ni) anodes. Smaller graphene grain size, Bernel stacking, and presence of edge defects in Ni-G specimen are responsible for better uptake and release of Li-ions as the cell is discharged and charged.
WS: You have also published work on tungsten disulfide, another 2D nanomaterial – how do the properties of this material differ from graphene?
GS: Tungsten disulfide (and also molybdenum disulfide) has a similar layered morphology to graphene, but its physical and chemical properties are quite different.
Firstly, each layer in Tungsten disulfide is 3 atoms thick (1 tungsten sandwiched between 2 sulfur atoms), while graphene is a one atom thick layer of carbon.
Secondly, tungsten disulfide is a semi-conductor whilst graphene is an excellent conductor of electricity. Tungsten disulfide is also much heavier and denser.
Now these materials can be combined together in the form of heterostructures and/ or layered composites to unravel some interesting applications - for example, the vertical field effect transistor demonstrated by Nobel Prize winning team of Novoselov and Geim in a recent article.
WS: How does tungsten disulfide differ from graphene when used as an electrode material?
GS: Although both these materials allow uptake and release of Li when used as anodes (approx. 0 to 3 V versus Li/Li+ range), the chemistry is quite different. Li-atoms in graphene or graphite are believed to be stored by means of intercalation - in other words the atoms are physically inserted in between the sheets (approx. one Li atom per six carbon atoms, in a one electron process).
In the case of tungsten disulfide, the majority of the Li-atoms are believed to be stored by means of a chemical conversion reaction, in which Li reacts with tungsten disulfide sheets, dissociating them into tungsten and lithium sulfide nanodomains as the cell is discharged.
Upon charging, after the Li-ions have travelled back to the cathode, nanodomains of tungsten disulfide (approx. 3 to 5 nanometers) are formed at the anode (2 or more electrons get transferred per W atom).
Due to this intrinsic nanodomain structure that is formed after the first cycle, the kinetics of Li-ion transport in the tungsten disulfide anode become slower. Researchers are still trying to get a complete picture of these processes. Prof. C.M. Julien (Université Pierre et Marie Curie), in particular has made some important contributions in the area.
WS: How commercially viable is the fabrication process for tungsten disulfide sheets?
GS: I think this process should be commercially viable in the next few years. At the moment we are trying to optimize the process to get sheets with controlled shape, size and thickness without causing significant damage from interaction with the strong acid.
WS: Do you think we will see these materials in next-generation lithium-ion batteries? How far away is the technology from large-scale commercial applications?
GS: Absolutely. Although it is more likely that we will see graphene-based composites as battery electrodes, where graphene provides both the electrical conductivity and the mechanical flexibility required to reduce the fatigue associated with repeated insertion and extraction of Li-ions. Electrodes based on conversion chemistry are still in their early stages.
As far as large scale commercial applications are concerned, a lot of studies regarding long-term cycleability and safety of these batteries need to be performed. Insufficient testing or safety measures can lead to disasters similar to the one that lead to grounding of the Boeing 787 Dreamliner.
Professor Jeff Dahn at University of Dalhousie is leading the effort in large scale testing of Li-ion batteries.
WS: What areas will you be working on next? Are there any other promising nanomaterials you are exploring?
GS: We will continue to work on graphene and transition metal dichalcogenide film based anodes. In addition, we are also looking at combining the high thermodynamic stability of molecularly derived ceramics with graphene in order to prepare anodes that can withstand extreme abuse particularly when the cell is charged and discharge at extremely high current densities (or C-rates).
WS: Where can we find more information about your research?
GS: You can visit our research group website or follow us on Twitter for recent news and publications.
About Gurpreet Singh
Dr. Gurpreet Singh is assistant professor in the department of mechanical and nuclear engineering at the Kansas State University. He received his M.S. and PhD degrees from the University of Colorado at Boulder, with Prof. R. Raj and Prof. R. L. Mahajan, respectively. Prior to joining K-State, he worked as a postdoctoral associate and guest researcher at Virginia Tech and National Institute of Standards and Technology, respectively.
Singh’s current research is on exploring large-scale synthesis of 1-D (carbon nanotubes) and 2-D (graphene and transition metal dichalcogenides) nanomaterials. Singh is investigating chemical functionalization of these materials with molecular precursor derived high temperature glass for applications in energy conversion (coatings for high-power laser radiometry) and storage devices (robust rechargeable battery anodes).
He is the winner of Kansas NSF-First Award and Big-XII Universities Faculty Fellowship. Dr. Singh is also active in the ceramics community; he has organized symposiums on topics related to controlled processing of nanomaterials at the Materials Science and Technology conference for the last 3 years.