Patent of the month: Feel the buzz

Materials World magazine
,
1 Sep 2018

This month, Dr Jennifer Unsworth, Patent Attorney at intellectual property law firm Withers and Rogers, looks at a patent that improves cathode technology.

A typical lithium-sulphur cell is composed of a lithium-based anode (negative terminal) and a cathode (positive terminal), formed from sulphur-containing material. When manufacturing standard cathodes, sulphur particles are mixed with electrically conductive carbon black, which is intended to improve their ability to carry the flow of current. However, there is often poor electrical contact between the non-conducting sulphur particles and electro-conductive carbon material and this can cause the sulphur to separate from the cathode as the battery is cycled.

In an attempt to improve electrical contact, various modifications to the lithium-sulphur cell have been suggested. For instance, more electro-conductive carbon material could be added to the cathode. However, this solution reduced specific energy and noticeably increased the weight of the cell. Alternative methods include finely grinding sulphur and the electro-conductive material together. However, excessive grinding has been found to yield electrodes with poor porosity. 

Good contact

Oxis Energy Limited, based in Abingdon, Oxfordshire, recently secured a UK patent, GB2533672, which describes a lithium-sulphur cathode having superior charge capacity and better behaviour under cycling, due to improved electrical contact between sulphur and carbon.

The inventors have developed a cathode for a lithium-sulphur cell made from a mixture of particulates containing a composite of electro active, sulphur-containing material and electro-conductive carbon, filled with conductive carbon filler particles dispersed in the composite structure. The electro-conductive material can be in the form of carbon nanotubes, graphene, or carbon nanofibres.

Sulphur-containing material is first melt bonded to electro-conductive carbon. Melt bonding is a manufacturing process where the electro-conductive carbon is kneaded into a molten pool of electro active sulphur material in a co-kneader or a twin-screw extruder. This type of appliance has not previously been used for this purpose, due to significant difficulties in maintaining rheological properties of the sulphur component above 140°C. 

The inventors have found that melt bonding or melt compounding facilitates better electrical contact between the constituents, which gives sulphur easier access to chemical reactions. Another advantage is that the electro-conductive carbon is well dispersed in the sulphur-based matrix, providing more efficient transfer of electricity from the current collector and an active interface for electrochemical reactions to occur. 

It has been demonstrated that battery cells including cathodes that comprise, by weight, 76% sulphur, 11% carbon fibre nanotubes, 11% carbon black, and 2% gelatine, lose only 20% of the initial capacity after 80 charge-discharge cycles, as shown above. This is a significant improvement over the existing lithium-sulphur cell systems. The patent document also describes the influence of sulphur-based cathode morphology on cell properties, which benefits all cell configurations. 

The UK patent application protects not only the composition of matter, but also the process by which this matter is obtained. By wording its patent application in this way, Oxis Energy Limited has protected both aspects of its invention. This could give the company a stronger hand when securing funding for further R&D activities and enhance its market credibility.

Read the full patent here: bit.ly/2O4A97z