Fully Fluorine-Free Liquid Electrolytes for Na-Ion Applications

Sodium-based batteries are interesting candidates, when replacement of the lithium- and lead-based batteries ins taken into consideration. The advantages over lead-based systems is based mainly on the low weight and low impact on the environment of the sodium in comparison to lead. Therefore, use of the sodium-based batteries is interesting when high power density is needed. The main advantage of sodium in comparison to lithium is based on the fact that sodium is both the most abundant alkali metal on Earth. It is worth to remember that application of the Li-ion technology in stationary batteries applications and in the electric vehicles industry is limited due to insufficient sources of lithium. Such problems in case of sodium-based batteries do not occur. Moreover, sodium salts are much cheaper than respective lithium salts.

Up to now, several sodium-based batteries were constructed. First sodium-sulfur battery, unfortunately, exhibiting low energy density, was built in 1960s. This technology was developed and NASICON electrolyte was introduced to it in 2000s. This battery was of superior energy density. On the other hand, both electrodes need to be liquid in such system. Therefore, the operating temperature of such battery have to be higher than 150°C, and the electrode materials in it need to be melted before it starts. This make impossible use of such batteries in applications in which fast start is desired. The same disadvantage has Na-NiCl₂ battery, which also exhibit low energy density.

It is believed that the problems of the high operating temperatures for sodium-based systems can be solved by designing of sodium-ion power sources, per analogiam to the lithium-ion battery technology. The main problem related to this system is to design and obtain the electrolyte which is stable against electrode materials. Naturally, the problems of the Na-ion are similar to this of lithium-ion, however, several novel issues occur. The affinity of the sodium to the fluoride is higher than to lithium, therefore, the decomposition of the salt is more intensive. There are also differences when the affinity of lithium and sodium to the solvent molecules is discussed. Due to this, the simple substitution of the lithium salts with the sodium ones can give not fully sufficient results. Therefore, new electrolytes, containing novel salts and solvents other than carbonates, need to be tested in such systems.

In the presentation, the ion transport properties of the systems containing novel sodium salts with heteroaromatic anion will be presented. The electrochemical properties of the electrolytes will be confronted with the coordinating properties of both cation and anion in the solution and in the solid-state. Also interactions in the salt solvates will be discussed.