Sodium Ion Battery Technology Takes A New Leap Forward

Gurpreet Singh, assistant professor of mechanical and nuclear engineering at Kansas State University and his student researchers are the first to demonstrate that a composite ‘paper’ made of interleaved molybdenum disulfide and graphene nanosheets can be both an active material to efficiently store sodium atoms and a flexible current collector.
The idea is a new structure for sodium ion battery technology and represents a discovery of flexible molybdenum disulfide electrodes.  The idea opens a new vista on a very attractive technology.
Kansas State University engineers have made a breakthrough in rechargeable
battery applications. The bottom image shows a self-standing molybdenum
disulfide/graphene composite paper electrode and the top image highlights its
layered structure. Image Credit: Gurpreet Singh.
Sulfur and sodium chemistries are often viewed as the leading battery technology for the 21st century.  Their use suggests cheap and abundant materials, high cell energy, high cell voltage, durable in recharge cycling, basically non-toxic, not very heavy, thus suitable for future solid-electrolyte optimizations.  Sodium is particularly attractive, as it should work at almost all ambient temperatures, in some experiments, the cooler the better.
Singh said, “Most negative electrodes for sodium-ion batteries use materials that undergo an ‘alloying’ reaction with sodium. These materials can swell as much as 400 to 500 percent as the battery is charged and discharged, which may result in mechanical damage and loss of electrical contact with the current collector.”
Explaining he said, “Molybdenum disulfide, the major constituent of the paper electrode, offers a new kind of chemistry with sodium ions, which is a combination of intercalation and a conversion-type reaction. The paper electrode offers stable charge capacity of 230 mAh.g-1, with respect to total electrode weight. Further, the interleaved and porous structure of the paper electrode offers smooth channels for sodium to diffuse in and out as the cell is charged and discharged quickly. This design also eliminates the polymeric binders and copper current collector foil used in a traditional battery electrode.”
The research appears in the latest issue of the journal ACS Nano entitled “MoS2/graphene composite paper for sodium-ion battery electrodes”, by lead author Lamuel David, a doctoral student in mechanical engineering and Romil Bhandavat, a recent doctoral graduate.
For the last two years the research team has been developing new methods for quick and cost-effective synthesis of the atomically thin two-dimensional materials of graphene, molybdenum, and tungsten disulfide, in gram quantities, particularly for rechargeable battery applications.
In the latest research, the engineers created a large-area composite paper that consisted of acid-treated layered molybdenum disulfide and chemically modified graphene in an interleaved structured. The research marks the first time that such a flexible paper electrode was used in a sodium-ion battery as an anode that operates at room temperature. Most commercial sodium-sulfur batteries operate close to 300º C (572º F), Singh pointed out.
Singh shares some important reasons for the research about how the effort can affect commercialization.  First is answering how the synthesis of large quantities of single or few-layer-thick 2-D materials can be crucial to understanding the true commercial potential of materials such as transition metal dichalcogenides, or TMD, and graphene.
The research points out a fundamental understanding is needed of how sodium is stored in a layered material through mechanisms other than the conventional intercalation and alloying reaction. Then comes working out how using graphene as the flexible support and current collector for eliminating the copper foil and making lighter and bendable rechargeable batteries.  Having these matters explored would set the stage for optimization and economic investigation.
Singh said, “From the synthesis point of view, we have shown that certain transition metal dichalcogenides can be exfoliated in strong acids. This method should allow synthesis of gram quantities of few-layer-thick molybdenum disulfide sheets, which is very crucial for applications such as flexible batteries, supercapacitors, and polymer composites. For such applications, TMD flakes that are a few atoms thick are sufficient. Very high-quality single-layer flakes are not a necessity.”
The K State team is working to commercialize the technology, with assistance from the university’s Institute of Commercialization. They also are exploring lithium and sodium storage in other nanomaterials.
For real world use sodium is pretty attractive compared to sulfur as a low temperature technology and could be a major cost breakthrough compared to lithium technologies.  For us consumers it will be how much power at what price, weight and size – not a particular chemistry.  Sodium has just taken a large leap forward.  Cheaper, better stronger, lighter, smaller.  The K State team is on a roll.
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