While lithium-sulfur batteries use the flow of lithium ions in an electrolyte between an anode and a cathode, the way Li-S batteries store the ions is quite different. Li-ion batteries use a process called intercalation to insert the ions between molecules in the electrode. Lithium-sulfur batteries, rely on a multi-step redox reaction with sulfur that results in a number of stable intermediate sulfide ions. This storage process, in theory, reduces limitations of electrode structure—thus enabling higher capacity in similar volumes.

At the negative electrode, lithium is dissolved into solution on discharge and plated out on charge. The solubility of the intermediate sulfide ions depends on the solvent used in the electrolyte, and the voltage vs. discharge capacity profile of the cell thus depends on the solvents used.

Because sulfur is both electrically and ionically insulating, the sulfur must always be in close contact with a conductor. The intermediate sulfur ions can also leak out of the electrodes and into the electrolyte, thereby reducing the electrodes’ active mass. Sion spent a number of years working to increase the sulfur utilization from about 46% to more than 90%.

The University of Waterloo team, led by Dr. Linda Nazar, sought to address those limitations by creating a highly ordered interwoven composite. A conductive mesoporous carbon framework constrains sulfur nanofiller growth within its channels and also generates the essential electrical contact to the insulating sulfur.

The structure provides access to Li⁺ ingress/egress for reactivity with the sulphur, and we speculate that the kinetic inhibition to diffusion within the framework and the sorption properties of the carbon aid in trapping the polysulphides formed during redox.

Polymer modification of the carbon surface further provides a chemical gradient that retards diffusion of these large anions out of the electrode, thus facilitating more complete reaction. Reversible capacities up to 1,320 mA h g^-1 are attained.

The researchers note that the assembly process is simple and broadly applicable, and conceptually can provide new opportunities for materials scientists for tailored design that can be extended to many different electrode materials.

Resources

• Xiulei Ji, Kyu Tae Lee and Linda F. Nazar (2009) A highly ordered nanostructured carbon–sulphur cathode for lithium–sulphur batteries. Nature Materials doi: 10.1038/nmat2460

• Carbon electrodes help form high capacity lithium-sulfur batteries (Chemistry World)

• Karthikeyan Kumaresan, Yuriy Mikhaylik and Ralph E. White (2008) A Mathematical Model for a Lithium–Sulfur Cell. Journal of The Electrochemical Society, 155 (8) A576-A582 doi: 10.1149/1.2937304 (2008)