Compatibility Studies with New F-Free Electrolyte for Sodium Ion Batteries

Abstract

Up to now sodium battery technology finds application for high temperature systems such as sodium-sulfur and sodium-metal chloride batteries. The challenge, which we took up was synthesis and investigation of the new F-free sodium salts as a base for electrolytes working at room-temperature batteries. While the best conductivities have been shown by fluorinated salts, there is a paradigm shift in research and development. Since fluorinated salts may negatively affect electrolyte safety, cost and environmental friendliness, some researchers are looking for fluorine-free options [1]. Even partial elimination of the salts containing fluorine (both type YFX n-or fluoro-organic) should lead to limiting production and emissions of toxic compounds to the environment. Use of a salt which doesn’t contain fluorine has also significant advantages such as stability of the electrode-electrolyte phase boundary in batteries or lack of formation of lithium fluoride layer at the solid-electrolyte interphase. Three novel sodium salts: sodium pentacyanopropenide, sodium 2,3,4,5-tetracyanopirolate and sodium 2,4,5-tricyanoimidazolate were dissolved in organic solvents for sodium-ion batteries electrolytes. Salts were selected with respect to their fluorine-free composition and diffused negative charges. Due to the unique, non-covalent structural configurations of anions, improved electrolyte properties were observed [2]. Excellent dissociation of salts in carbonate such as PC or mixed carbonates EC/DMC results in high conductivity at room temperature. All presented salts showed high thermal stability and suitable width of electrochemical stability windows between 3 and 5 V. For the very first time new type of electrolyte based on F-free sodium salts were studied versus electrode materials. Their superior electrochemical properties, compared to the reference systems will be presented. [1] Scheers, J., et al. All fluorine-free lithium battery electrolytes. J. Power Sources 251,451-458 (2014)[2] Plewa-Marczewska, A. et al., New Tailored Sodium Salts for Battery Applications, Chem. Mater., 26 (2014), 4908–4914