|Schematic of the synthesis steps for a graphene-sulfur composite material, with a proposed schematic structure of the composite. Credit: ACS, Wang et al. Click to enlarge.|
Researchers at Stanford University led by Drs. Yi Cui and Hongjie Dai report the synthesis of a graphene–wrapped sulfur composite material that shows high and stable specific capacities of up to 600 mAh/g over more than 100 cycles. In a paper published in the ACS journal Nano Letters, they suggest that this material represents a promising cathode material for rechargeable Li-ion batteries with high energy density.
Sulfur is one of the promising cathode materials under wide investigation (earlier post) to develop the higher capacity batteries sought by automakers to enable more cost-effective and longer range electromobility. (Earlier post.) Sulfur has a theoretical specific capacity of 1,672 mAh/g, about 5 times higher than that of traditional cathode materials based on transition metal oxides or phosphates. Sulfur also possesses other advantages such as low cost and environmental benignity. Nevertheless, Wang et al. note in their paper, it has been difficult to develop a practical Li-S battery partly limited by the problems of low electrical conductivity of sulfur, dissolution of polysulfides in electrolyte, and volume expansion of sulfur during discharge. These problems cause poor cycle life, low specific capacity, and low energy efficiency.
Various carbon/sulfur composites utilizing active carbon, carbon nanotubes or mesoporous carbon have been made with specific capacity exceeding 1000 mAh/g achieved. However, it remains challenging to retain high and stable capacity of sulfur cathodes over more than 100 cycles.
...In principle, graphene-sulfur composite could lead to improved sulfur cathode materials for Li-S batteries. However, in addition to interfacing sulfur with graphene sheets, it is important to obtain sulfur particles well coated and confined by graphene sheets and meanwhile integrate polymeric (e.g., poly(ethylene glycol) (PEG)) “cushions” in the hybrid structure. These factors could minimize the dissolution and diffusion of polysulfides and accommodate volume expansion effects during discharge, therefore improving the cycling life of the sulfur cathode.—Wang et al.
To create the material, the team wrapped poly(ethylene glycol) (PEG) coated submicrometer sulfur particles with mildly oxidized graphene oxide sheets decorated by carbon black nanoparticles. The PEG and graphene coating layers are important to accommodating volume expansion of the coated sulfur particles during discharge, trapping soluble polysulfide intermediates, and rendering the sulfur particles electrically conducting.
The high specific capacity and good cycling stability make our graphene-sulfur composite a promising potential material for future lithium ion batteries with high energy density. It is worth noting that the graphene-sulfur composite could be coupled with silicon based anode materials for rechargeable batteries with significantly higher energy density than currently possible.—Wang et al.
Hailiang Wang, Yuan Yang, Yongye Liang, Joshua Tucker Robinson, Yanguang Li, Ariel Jackson, Yi Cui, Hongjie Dai (2011) Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium–Sulfur Battery Cathode Material with High Capacity and Cycling Stability. Nano Letters Article ASAP doi: /10.1021/nl200658