Rechargeable lithium-sulfur batteries get a boost from graphene

Physorg.com
July 13, 2011

By wrapping tiny sulfur particles in graphene sheets, researchers from Stanford University have synthesized a promising cathode material for rechargeable lithium-sulfur batteries that could be used for powering electric vehicles on a large scale. When combined with silicon-based anodes, the new graphene-sulfur cathodes could lead to rechargeable batteries with a significantly higher energy density than is currently possible.

The researchers, led by Yi Cui and Hongjie Dai from Stanford University, have published their study in a recent issue of Nano Letters.

As the researchers explain in their study, in order to power electric vehicles that are competitive with gasoline-powered vehicles, one of the biggest challenges is improving the energy and power densities of rechargeable lithium batteries. The batteries’ weak spot is currently the cathode materials, which have specific capacities that are much lower than those of the anode materials. (The specific capacities for cathode materials are about 150 mAh/g for layer oxides and 170 mAh/g for LiFe-PO4, while those for anode materials are 370 mAh/g for graphite and 4200 mAh/g for silicon.)

In order to improve the cathode, the researchers turned to sulfur, which has a theoretical specific capacity of 1672 mAh/g, about five times higher than those of traditional cathode materials. Although sulfur has other advantages, such as low cost and a benign environmental impact, it also has some disadvantages. For instance, sulfur is a poor conductor, it expands during discharge, and the polysulfides dissolve in electrolyte. Together, these problems cause a low cycle life, low specific capacity, and low energy efficiency.
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