Today, the challenge posed by energy storage science lies in the discovery and manufacture of new, safer, more cost-effective materials; the struggle to produce batteries with greater capacities and better features is prompting the need to explore new and more advanced materials.

One of the energy storage technologies, or batteries, predominant on the market are lithium-ion batteries used in electrically-driven cars and electronic devices, such as mobile phones and laptop computers. Lithium-ion batteries can store a huge amount of energy and are easy to produce. Yet lithium reserves are limited and we could see a situation in which lithium is in short supply and its price rises.

In this respect, “this work has focussed on sodium batteries. Sodium is an element that, despite its lower energy density when compared with lithium, can be used to produce more cost-effective batteries, since sodium can be extracted from many sources, such as sea water”, said Asier Fernández de Añastro-Arrieta, researcher in the UPV/EHU’s department of Applied Industrial Chemistry and at POLYMAT.

“The main aim of this thesis is to study new polymer materials for sodium batteries. A battery has three components: a cathode, or positive pole, an anode or negative pole, and a permeable material known as an electrolyte that separates these two components. The electrolyte performs two main functions, one to encourage the spread of ions from the cathode to the anode, which enables us to charge or discharge the battery; the higher and more effective the spread of ions is, the faster and more efficient the charging will be. The other function of the electrolyte in a battery has to do with the safety of the device itself. It is crucial for the electrolyte to physically separate the cathode from the anode and that they remain separate throughout the battery’s useful service life, since any contact between the cathode and the anode (due to a possible breakage in the electrolyte) or a leak in the electrolyte could lead to a fault, overheating and, in extreme cases, the explosion of a car or mobile phone battery, as has been seen on various occasions in the media,” explained the researcher.

So, “in this work we developed polymer-based membranes that act as an electrolyte. But not just any polymer electrolyte, but ionogels. Ionogels are materials that combine the best features of polymers –flexibility, low cost and lightness- with the best features of ionic liquids,” said Fernández de Añastro. “Ionic liquids are also,” he went on, “liquids with a great capacity for ion diffusion as they are virtually fireproof liquids. The sum of the polymers and ionic liquids produces an ionogel, a solid, robust, polymer-based membrane with a huge capacity for spreading ions and is a very safe material because of its negligible flammability”.

“In the course of the research we managed to synthesize various types of ionogels with a high liquid content of between 50% and 90%, using various physico-chemical methods with different properties. We also used these materials in prototypes of actual batteries, such as button batteries, and we demonstrated their capacity and good performance," stressed the UPV/EHU researcher.

The researcher pointed out that “what is currently limiting ionic liquids is their high cost; conventional electrolyte liquids present in all our mobile phones are much more cost-effective”. In any case, “in recent years ionic liquids have been found to display excellent properties for a range of applications in industry. So, however much they cost, if their applications are justified, we could find them on the market in a not-too-distant future", said Asier Fernández de Añastro.

Additional information

This research was conducted within the framework of the PhD thesis by Asier Fernández de Añastro-Arrieta (Vitoria-Gasteiz, 1991) entitled Ionogels for sodium rechargeable batteries. It was supervised by David Mecerreyes, deputy director of POLYMAT. The PhD thesis was mostly written up at POLYMAT with the collaboration of the Deakin University of Australia.