Boosting Lithium Batteries with Sulfur

I’ve written here a number of times about new developments in energy storage technology.  Improvements in this area are of great importance, both to facilitate the use of renewable energy sources (because the wind does not always blow, nor does the sun always shine), and to improve the performance and range of electric vehicles.

The “Physics arXiv Blog” at Technology Review has an article reporting on some new research aimed at improving the performance of lithium batteries.  These batteries have become very common in contemporary portable electronic gadgets, such as laptop computers, cellular phones, and music players.  In this kind of application, they perform well, and are a significant improvement on the nickle-cadmium (NiCd) battery technology used earlier.  But they still are marginal as the power source for vehicles: they have trouble delivering enough power quickly enough.

A significant part of  the problem is attributable to just one element of the battery: the cathode.

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 [lithium iron phosphate] and only 150mAh/g for layered oxides.

This limits the amount of current that the battery can deliver.

A team at Stanford University, led by Hailiang Wang, has used some clever nanoengineering to adapt sulfur for use as a cathode material.   Though sulfur has some attractive theoretical properties, there are problems with its physical integrity when used in electrodes; moreover, sulfur does not conduct electricity very well.

The researchers addressed these problems by a two-part process.  First, they create sub-micron (micrometre, or 1.0 × 10-6 meter) sulfur particles coated with polyethylene glycol [PEG], which prevents the material from washing away in the cell electrolyte.    Next, they “wrap” the particles in a cage of graphene (a particular form of carbon, which has also been used to make high-speed transistors and, more recently, integrated circuits).  The combination of carbon and sulfur improves both the electrical conductivity and physical integrity of the electrode material.

Although there is still significant work to be done before the technology can be used to make batteries for real-world applications, the initial results are encouraging.

The result is a cathode that retains a specific capacity of more than 600 mAh/g over 100 charging cycles.

This represents about  a four-fold increase compared to current cathode technology, though longevity of the material is still not as good as one would like.  Still, it is encouraging that we are seeing positive results from research on so many different aspects of battery technology.

The Stanford team’s paper is available at the arXiv.org site [abstract, full  PDF download available].

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