Lithium batteries have become ubiquitous in modern society. More than likely, every house has several in the different portable electronic devices it's equipped with.
Although there's been tremendous performance improvements in recent years, lithium batteries are still not fully up to the demanding task of powering the next generation of electric vehicles due to low energy and power densities.
The problem lies with the cathodes in these batteries. The specific capacities of the anode materials in lithium batteries are 370 mAh/g for graphite and 4200 mAh/g for silicon. By contrast, the cathode specific capacities are 170 mAh/g for LiFePO4 and only 150mAh/g for layered oxides.
Hence, it is important to improve the cathode's specific capacity while maintaining all the other characteristics that batteries require, such as decent energy efficiency and good cycle life.
Hailiang Wang and his team at Stanford University say they've achieved a significant step towards this goal using sulphur as the cathode material of choice.
Sulphur has potential as it has a theoretical specific capacity of 1672 mAh/g. It also has a number of disadvantages, not least the fact that sulphur is a poor conductor. On top of this, polysulphides tend to dissolve and wash away in many electrolytes while sulphur tends to swell during the discharge cycle causing it to crumble.
But Wang et al say they've largely overcome these problems by creating submicron sulphur particles and coat them in polyethyleneglycol or PEG. This traps polysulphides and prevents them from washing away.
Next, the team wraps the coated sulphur particles in a "graphene cage", rendering the particles electrically conducting and also supporting the particles as they swell and shrink during each charging cycle.
The result is a cathode that retains a specific capacity of more than 600 mAh/g over 100 charging cycles. Such a cathode would immediately lead to rechargeable lithium batteries with a much higher energy density than is possible today. According to Wang et al the graphene-sulfur composite could be coupled with silicon based anode materials for rechargeable batteries with significantly higher energy density than currently possible.
But there is more work ahead. Even though the material maintains a high specific capacity over 100 cycles, the capacity drops by 15 per cent in the process (as can be seen in the graphs below). So a lot of research work is still ahead, focusing on improving on this as they further optimise the material.
The next step then is to create a working battery out of this stuff. Wang and co say they plan to couple it to a pre-lithiated silicon based anode to achieve this.
If it all works out (and that's a significant 'if'). your next car could be powered by Li-S batteries.
For more attend Electric Vehicles Land Sea Air USA 2012.
Also read Electric Vehicle Traction Batteries 2011-2021 and Advanced Energy Storage Technologies: Patent Trends and Company Positioning .
Reference, image source: www.arxiv.org/abs/1107.0109 : Graphene-Wrapped Sulfur Particles as a Rechargeable Lithium-Sulfur-Battery Cathode Material with High Capacity and Cycling Stability
Lead image source: Marine Mineral~
