Researchers from the Gwangju Institute of Science and Technology (GIST) in S. Korea have devised a dual-layered cathode for Lithium-sulfur batteries (LSBs) comprising a sulfur-nanoparticle–graphene composite as the sulfur active layer and a polysulfide absorption layer to minimize the effects of the performance-decaying polysulfide shuttle. Both layers are based on vitamin-C-treated graphene oxide at various degrees of reduction. In a paper in the journal ChemSusChem they report high energy and power densities of more than 600 mAh g−1 for 100 cycles at 1C (1675 mA gsulfur−1).
In a 2010 paper published in the ACS Journal of Physical Chemistry C, a team from Spain reported that Vitamin C was an ideal substitute for hydrazine in the reduction of graphene oxide suspensions.
The production of colloidal suspensions of graphene from graphite oxide typically involves exfoliation of the latter in a suitable solvent (usually water, but also some polar organic solvents), followed by chemical reduction of the dispersed single-layer sheets (graphene oxide sheets) with hydrazine. When the reduction is carried out under controlled conditions, the resulting deoxygenated graphene oxide sheets form stable suspensions without the need of surfactants or any other stabilizers, which is an added advantage. Unfortunately, hydrazine is a highly toxic and potentially explosive chemical, and therefore its use should be avoided in the large-scale implementation of this approach.
We have carried out an exhaustive comparison of the performance of several different reducing agents toward deoxygenation of graphene oxide suspensions, which has not been previously reported in the literature. Most significantly, we find that the efficiency of hydrazine in the reduction of graphene oxide is only matched by vitamin C (ascorbic acid), a natural antioxidant essential for many metabolic functions in living organisms and widely employed as a food additive. Reduction with vitamin C can be made not only in water, but also in some common organic solvents, such as N,N-dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). These findings, together with the nontoxicity of this natural product, suggest that vitamin C represents an ideal substitute for hydrazine in the large-scale production of solution-processable graphene.—Fernández-Merino et al.
Despite the theoretical promise of LSBs, they suffer from several drawbacks such as poor cycle performance and low charge and discharge rates. Capacity fading during long-term cycling is also a serious concern in lithium–sulfur batteries. As the multiple electrochemical reaction steps occur during charge and discharge, the intermediate lithium polysulfide species dissolve readily into the electrolytes.
This “shuttle mechanism” leads to serious performance decay in the cell because of self-corrosion at the lithium anode surface; loss of active material in the cathode, and electrolyte contamination.
To overcome these problems, various approaches to the rational design of sulfur cathodes have been and are being tried to maximize cycle performance at high current density.
The GIST researchers found that by controlling the degree of reduction of graphene using Vitamin C, the dual-layered cathode can increase sulfur utilization significantly owing to the uniform formation of nanosized sulfur particles; the chemical bonding of dissolved polysulfides on the oxygen-rich sulfur active layer; and the physisorption of free polysulfides on the absorption layer.
An intriguing aspect of our work is the synthesis of a high-performance dual-layered cathode by a green chemistry method, which could be a promising approach to LSBs with high energy and power densities.—Kim et al.
Kim, J. W., Ocon, J. D., Kim, H.-S. and Lee, J. (2015) “Improvement of Energy Capacity with Vitamin C Treated Dual-Layered Graphene–Sulfur Cathodes in Lithium–Sulfur Batteries,” ChemSusChem doi: 10.1002/cssc.201500111
M. J. Fernández-Merino, L. Guardia, J. I. Paredes, S. Villar-Rodil, P. Solís-Fernández, A. Martínez-Alonso, and J. M. D. Tascón (2010) “Vitamin C Is an Ideal Substitute for Hydrazine in the Reduction of Graphene Oxide Suspensions” The Journal of Physical Chemistry C 114 (14), 6426-6432 doi: 10.1021/jp100603h