Schematic representation showing the predicted mechanism for Na insertion and extraction into an idealized MoS2/rGO free-standing composite paper electrode. rGO provides the mechanical/ structural stability and high electrical conductivity network to the TMDC undergoing insertion and conversion reactions with Na-ions. Credit: ACS, David et al. SI. Click to enlarge.

Most negative electrodes for sodium-ion batteries use materials that undergo an alloying reaction with sodium, said Gurpreet Singh, KSU assistant professor of mechanical and nuclear engineering and leader of the project. These materials can swell as much as 400 to 500% under cycling, resulting in mechanical damage and degradation of the cell.

Molybdenum disulfide, the major constituent of the new paper electrode, offers a new combination of intercalation and a conversion-type reaction, Singh noted. 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.

… sodium-ion batteries (SIBs) have drawn increasing attention because, in contrast to lithium, sodium resources are practically inexhaustible and evenly distributed around the world while the ion insertion chemistry is largely identical to that of lithium. Also, from an electrochemical point of view, sodium has a very negative redox potential (
2.71 V vs SHE) and a small electrochemical equivalent (0.86 gA h^-1), which make it the most advantageous element for battery applications after lithium. However, many challenges remain before SIBs can become commercially competitive with LIBs. For instance, Na ions are about 55% larger in radius than Li ions, which makes it difficult to find a suitable host material to allow reversible and rapid ion insertion and extraction.

To this end, researchers have proposed a number of high-capacity sodium host materials (negative electrode) involving either carbon or group IVA and VA elements that form intermetallic compounds with Na. … Overall, new electrode design and concepts based on chemistry other than alloying and ion intercalation must also be explored to realize improved performance in Na-ion batteries under normal operating conditions.

… Although layered graphite has been ruled out for sodium-based systems (as Na ions do not tend to form staged intercalation compounds with graphite), a graphene-based free-standing paper-based electrode can provide a porous and flexible support structure for a TMDC [transition metal dichalcogenides such as MoS₂ and WS₂] to undergo a reversible conversion-type reaction with Na ions. It can also act as an efficient electronic current collector, thereby eliminating the need for a metallic substrate…, electrically conducting additives, and polymeric binders that amount to a total of approximately 10% of the cell weight in traditional negative electrodes.

Herein, we provide the first report of (a) synthesis of composite papers from acid-functionalized MoS₂ and reduced graphene oxide flakes, (b) improved capacity and high efficiency reversible Na storage in the self-standing flexible MoS₂/graphene electrodes at room temperature, and (c) mechanical characterization that highlights the high strain to failure in these composite papers.

—David et al.

The team 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 300 degrees Celsius, Singh said.

The electrochemical performance of the crumpled composite paper (at 4 mg cm^–2) was evaluated as counter electrode against pure Na foil in a half-cell configuration.

Singh said the research is important for two reasons:

1. Synthesis of large quantities of single or few-layer-thick 2-D materials is crucial to understanding the true commercial potential of materials such as transition metal dichalcogenides, or TMD, and graphene.

2. Fundamental understanding of how sodium is stored in a layered material through mechanisms other than the conventional intercalation and alloying reaction. In addition, using graphene as the flexible support and current collector is crucial for eliminating the copper foil and making lighter and bendable rechargeable batteries.

The researchers are 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.

Other Kansas State University researchers involved in the project include Lamuel David, lead author on the paper and a doctoral student in mechanical engineering, India, and Romil Bhandavat, recent doctoral graduate.

Resources

• Lamuel David, Romil Bhandavat, and Gurpreet Singh (2014) “MoS₂/Graphene Composite Paper for Sodium-Ion Battery Electrodes,” ACS Nano doi: 10.1021/nn406156b