New materials for sustainable, low-cost batteries

　　Scientists from ETH Zurich and Empa, led by Maksym Kovalenko, Professor of Functional Inorganic Materials, have identified two new materials that could bring about key advances in the development of aluminium batteries.

　　Made using cheap and abundant raw materials the first is corrosion-resistant and suitable for the conductive parts of the battery; the second is a novel material for the battery's positive pole that can be adapted to a wide range of technical requirements.

　　As the electrolyte fluid in aluminium batteries is extremely aggressive and corrodes stainless steel, and even gold and platinum, scientists have been looking for corrosion-resistant materials for the conductive parts of these batteries. The work being undertaken by Kovalenko and his colleagues involves titanium nitride, a ceramic material that exhibits sufficiently high conductivity. "This compound is made up of the highly abundant elements titanium and nitrogen, and it's easy to manufacture," explains Kovalenko.

　　The scientists have successfully made aluminium batteries with conductive parts made of titanium nitride in the laboratory – it can be easily produced in the form of thin films and also as a coating over other materials such as polymer foils.

　　According to Kovalenko it should also be possible to manufacture the conductors from a conventional metal and coat them with titanium nitride, or even to print conductive titanium nitride tracks on to plastic.

　　The second new material can be used for the positive electrode (pole) of aluminium batteries. Whereas the negative electrode in these batteries is made of aluminium, the positive electrode is usually made of graphite.

　　Kovalenko and his team have found a new material that rivals graphite in terms of the amount of energy a battery is able to store. The material, polypyrene, is a hydrocarbon with a chain-like (polymeric) molecular structure.

　　"A lot of space remains between the molecular chains. This allows the relatively large ions of the electrolyte fluid to penetrate and charge the electrode material easily," Kovalenko explains.

　　One of the advantages of electrodes containing polypyrene is that scientists are able to influence their properties, such as the porosity. The material can therefore be adapted perfectly to the specific application.

　　As both titanium nitride and polypyrene are flexible materials, the researchers believe they are suitable for use in "pouch cells" (batteries enclosed in a flexible film).

　　An increasing amount of electricity is generated from solar and wind energy. However, as electricity is needed even when the sun is not shining and the wind is not blowing, new technologies will be needed, such as new types of batteries, to store this electricity in a cost-effective manner.

　　Although existing lithium-ion batteries are suitable for electromobility due to their low weight, they are expensive and therefore unsuitable for economical large-scale, stationary power storage.

　　Lithium is also a relatively rare metal and is hard to extract - unlike aluminium, magnesium or sodium. Because of this batteries based on one of these elements are seen as a promising option for stationary power storage in the future.

　　However, such batteries are still at the research stage and have not yet entered industrial use.