New Salts for Sodium-Ion Batteries

The first successful attempt of a sodium battery was undertaken in 1967 by Ford Motor Company (USA) in the sodium-sulfur battery. This innovation was possible from to the discovery by Ford researchers of the favourable Na ionic conduction properties in s-alumina. The growing application of Li-ion and Li-polymer batteries for hybrid electric cars, electric vehicles, mobile devices etc. requires a large amount of lithium in the form of Li2CO3 every year. High price and relatively small world reserves of 13 million tons, forced to look for new materials possible to use in battery technology. Sodium has molar mass and only slightly lower than lithium red-ox potential (2.73V). Low cost (about 4 times lower than lithium) and enormously easier and almost unlimited world-wide reserves makes him a really promising candidate. Metallic sodium does not form dendrites being a key issue if lithium metallic anodes (with gigantic theoretical capacity of 1190 mAh/g) were tested in present batteries. Several secondary advantages only promotes sodium concept for ex. cheaper than aluminum current collectors might be applied. [] The challenge in development of sodium battery technology is to shift temperature of work to room temperature by carful choosing of the salt, what includes anion designing. The anion modification should consider present of specific elements (carbon, nitrogen, optionally fluorine) related to low toxicity and electrochemical resistance, and moreover uniform charge distribution, which courses good dissociation and in consequence high ionic conductivity. The novel, promising concept of the application of new anions is based on the application of “Huckel anions”. The name came from the transposition of the Huckel rule predicting the stability of the aromatic systems. One of the most common examples of this type of anions is 4,5-dicyano-triazole (DCTA). This particular structure is completely covalently bonded and shows very stable 6π (or 10π electron if CN bonds are involved in calculations) configuration. It can be produced from commercially available precursor and even more importantly does not comprise fluorine atoms. Salts of this type of anion were found to exhibit high (~300C) thermal stability. LiDCTA was successfully tested in PEO matrices systems as a promising, improved electrolyte for rechargeable lithium batteries []. Unfortunately DCTA failed as a component of the EC/DMC (1:1) battery electrolyte. This paper presents the properties of newly designed sodium salts e.g. sodium 4,5-dicyano-2 (trifluoromethyl)imidazolate (NaTDI) and sodium 4,5dicyano-2-(pentafluoroethyl)imidazolate (NaPDI) for application in liquid non-aqueous sodium electrolytes. Fig. 1 Structure of tetraglymeNaTDI and PCNaTDI.