Intermediate-Temperature Operation of Sodium Secondary Batteries with High Rate Capability and Cyclability Using Ionic Liquid Electrolyte

Abstract

Ionic liquid electrolytes are widely studied to construct safe electrochemical devices owing to their unique properties such as low volatility, low flammability, and wide liquid-phase temperature range.Ionic liquids based on sulfonylamide anions (bis(fluorosulfonyl)amide anion (FSA-) have low melting point, low viscosity, and high ionic conductivitysome and are attractive as electrolytes in secondary batteries. The present study reports physical and electrochemical properties of FSA-based ionic liquid electrolytes for secondary betteries operating from room temperature to intermediate temperature. Intermediate operation of secondary batteries in is available in some circumstances such as an engine compartment of automobiles and is preferable to enhance both the ion transport and electrode reactions. The Na[FSA]-[C3C1pyrr][FSA] (C3C1pyrr+ = N-methyl-N-propylpyrrolidinium) and Na[FSA]-[C2C1im][FSA] (C2C1im+ = 1-ethyl-3-methylimidazolium) ionic liquid systems have wide liquid phase temperature ranges around room temperature and high ionic conductivities [1,2]. Immersion of Na metal in FSA- and TFSA-based ionic liquids clearly showed that FSA-based ionic liquids form stable film on Na metal to prevent futher reduction. Dendrite formation during Na metal electrodeposition is suppressed by increasing temperature. Excellent rate capability and cycle property were observed for the Na2FeP2O7 positive electrode in FSA-based ionic liquids at 363 K.[3] The origin of this performance will be discussed based on transport properties of Na+ by considering transport number and Na+fraction in these ionic liquids. Acknowledgements: This study was partly supported by Advanced Low Carbon Technology Research and Development Program (ALCA) of Japan Science and Technology Agency (JST) and Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) program “Elements Strategy Initiataive to Form Core Research Center”.