Technique may treble lithium battery performance

Researchers at the University of Waterloo reckon they have laid the groundwork for a lithium battery that can store and deliver more than three times the power of conventional lithium ion batteries.

Ontario, Canada – Researchers at the University of Waterloo reckon they have laid the groundwork for a lithium battery that can store and deliver more than three times the power of conventional lithium ion batteries.

Chemists have had high hopes for lithium-sulphur batteries for twenty years. This is partly because combining lithium and sulphur delivers much higher energy densities – but also because sulphur is cheaper than many other materials currently used in lithium batteries.

“The difficult challenge was always the cathode, the part of the battery that stores and releases electrons in the charge and recharge cycles,” said Dr Linda Nazar, who headed the team. “To enable a reversible electrochemical reaction at high current rates, the electrically-active sulphur needs to remain in the most intimate contact with a conductor, such as carbon.”

The Canadian research team cut to the chase by tackling the contact issue at the nanoscale level. Although they say the same approach could be used with other materials, for their proof-of-concept study they chose a member of a highly structured and porous carbon family called mesoporous carbon. At the nanoscale level, this type of carbon has a very uniform pore diameter and pore volume.

Using a nanocasting method, the team assembled a structure of 6.5 nanometre thick carbon rods separated by empty three to four nanometre wide channels, with carbon fibres holding the voids open. To fill the voids, sulphur was heated and melted. Once in contact with the carbon, it was drawn or imbibed into the channels by capillary forces, where it solidified and shrunk to form sulphur nanofibres.

Scanning electron microscope sections revealed that all the spaces were uniformly filled with sulphur, exposing an enormous surface area of the active element to carbon and driving the exceptional test results of the new battery.

“This composite material can supply up to nearly 80 percent of the theoretical capacity of sulphur, which is three times the energy density of lithium transition metal oxide cathodes, at reasonable rates with good cycling stability,” said Dr Nazar.

Dr Nazar said a patent has been filed, and she is reviewing options for commercialization and practical applications.

The finding is reported today in the on-line issue of Nature Materials.