Graphene-sulfur composite as stable high energy capacity cathodes for Li-ion batteries
02 July 2011
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| 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.
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| Electrochemical characterization of graphene-sulfur composites. (a) 10th cycle charge and discharge voltage profiles of the graphene-sulfur composite with PEG coating at various rates. (b) Cycling performance of the same composite as in (a) at rates of ~C/5 and ~C/2. (c) Cycling performance of PEG coated sulfur without graphene coating at the rate of ~C/5. (d) Cycling performance of graphene coated sulfur without any surfactant PEG coating at the rate of ~C/5. Credit: ACS, Wang et al. Click to enlarge. |
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.
Resources
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
Another interesting potential possibility for future improved battery electrodes. It would be valuable to have affordable 600 Wh/Kg batteries by 2020 or so for highway EVs.
Posted by: HarveyD | 02 July 2011 at 08:50 AM
Lithium sulfur looks good, they are working out the right combination to get the recharge cycle number up. Similar activities are taking place with lithium air batteries.
Posted by: SJC | 02 July 2011 at 06:56 PM
I am always surprised when new research is presented that is significantly lower performance than previous.
In 2009 Linda Nazar of Waterloo presented papers with similar materials but different configuration. She also showed PEG and non-PEG graphs only instead of only 300 and 600 mAh/g they were 800 and 1100 mAh/g. However her tests were done at even lower power of C/10. For automotive use a much higher power of C2 or greater is desired.
http://www.science.uwaterloo.ca/~lfnazar/2009/Nat_Mat_LiS.pdf
Posted by: Roy_H | 03 July 2011 at 03:51 AM
Roy_H
Cui is at Stanford. Between Stanford and MIT you probably have the best scientific publicists in the country. Cui has been known in batteries mostly for his work on silicon anode materials. Indeed, reading their press releases one would think no one else is working on silicon anodes, but in reality numerous people are and they are more likely than Cui to have a viable product to market sooner. Likewise with sulfur cathodes, others will likely offer alternatives that may be more economically viable. I am not saying that Cui is doing bad work, he is doing good work, it's just that others are too. The press and venture capitalists should quit being so easily manipulated by appearances. This is probably why we are sinking so fast as a nation, we no longer have a critical press, our business guys spent their college years drinking, and everyone is more impressed with the tall good looking person than the one with the best answer. Superficiality abounds.
Posted by: ChrisJ | 07 July 2011 at 09:57 AM