Graphene oxide-sulfur (GO-S) nanocomposite cathodes for high-capacity, stable cycling lithium sulfur batteries
|Reversible capacity vs. current density (rate capability). Credit: ACS, Ji et al. Click to enlarge.|
A team from Lawrence Berkeley National Laboratory and Tsinghua University (China) have synthesized graphene oxide-sulfur (GO-S) nanocomposite cathodes and applied them in lithium/sulfur cells to show a high reversible capacity of 950-1400 mAh g-1 and stable cycling for more than 50 deep cycles at 0.1C (1C = 1675 mA g-1).
The team used a chemical approach to immobilize sulfur and lithium polysulfides via the reactive functional groups on graphene oxide. This approach enabled them to obtain a uniform and thin (around tens of nanometers) sulfur coating on graphene oxide sheets and address some of the challenges retarding the commercialization of Li/S batteries. A report on their work appers in the Journal of the American Chemical Society.
With a theoretical specific capacity of about 1675 mAh g-1 and a theoretical specific energy of 2600 Wh kg-1, elemental sulfur (S) is very attractive as a cathode material for high-specific-energy rechargeable lithium batteries. In addition, sulfur is also inexpensive, abundant, and nontoxic. However, there are a number of challenges in Li/S batteries, Ji et al. note in their paper:
The high electrical resistivity of elemental sulfur; and
The high solubility (in organic solvent electrolytes) of the polysulfide ions that are formed during the discharge/charge processes. The soluble intermediate Li polysulfides can diffuse through the electrolyte to the Li anode where they are reduced to form solid precipitates. These reduced products can also diffuse back to the cathode during recharging. These issues can lead to low active materials utilization, low coulombic efficiency, and short cycle life of the S electrode. The loss of sulfur cathode material as a result of polysulfide dissolution causes significant capacity fading in rechargeable lithium/sulfur cells. (Earlier post.)
In this work, we used a low-cost and environmentally benign chemical reaction deposition strategy to immobilize S on quasi-two-dimensional graphene oxides (GO) to prepare graphene oxide-sulfur (GO-S) nanocomposite cathodes for Li/S cells in ionic liquid-based electrolytes.
...The unique structure of the GO-S nanocomposite can improve the overall electrochemical performance when it is used as a cathode material for Li/S batteries. First, it can accommodate the significant volume changes of S as it is converted to Li2S on discharge, and back to elemental S on recharge. In addition, the partially reduced GO with its large surface area along with ubiquitous cavities can establish more intimate electronic contact with S and avoid aggregation and loss of electrical contact with the current collector. Second, the low-temperature heat-treated GO still contains various kinds of functional groups. These functional groups can have strong adsorbing ability to anchor S atoms and to effectively prevent the subsequently formed Li polysulfides from dissolving in the electrolyte during cycling.—Ji et al.
The team concludes that the same design strategy could be helpful to explore and develop new porous carbon or conductive polymer-based sulfur nanocomposite cathodes for advanced Li/S cells.
Liwen Ji, Mumin Rao, Haimei Zheng, Liang Zhang, Yuanchang Li, Wenhui Duan, Jinghua Guo, Elton J. Cairns, and Yuegang Zhang (2011) Graphene Oxide as a Sulfur Immobilizer in High Performance Lithium/Sulfur Cells. Journal of the American Chemical Society DOI: 10.1021/ja206955k