Improvement on Electrochemical Performance and Safety Using Mixtures of Ionic Liquid and Conventional Organic Electrolyte for Sodium-Ion Batteries

Abstract

The success of commercializing sodium-ion batteries is inhibited by the implementation of suitable electrolytes that have economic feasibility and stable electrochemical performance. Organic electrolytes such as Na[ClO4]- or Na[PF6]-including organic solvents, EC, PC, DMC, and DEC, are most widely used to investigate the performance of sodium-ion batteries. These electrolytes are widely accepted because of the analogous studies in LIBs, but their flammability can cause reliability issues. Moreover, the SEI layer composed of sodium cations is known to be inhomogeneous and fragile, giving a higher solubility than that based on lithium cations.(1) In this regard, ionic liquid electrolytes are a drawing huge attention to constructing safe electrochemical devices owing to their unique properties such as low volatility and low flammability. Furthermore, recent studies report ionic liquids show similar electrochemical performance at room temperature to organic electrolytes and better interfacial properties.(2-4) Despite this bright improvement, the cost of ionic liquids drags down the advantage of economical sodium-ion batteries.In this study, ionic liquid and organic liquid electrolytes are mixed to adjust the balance of cost, performance, and safety. The [C3C1pyrr][FSA] (C3C1pyrr+ = N-methyl-N-propylpyrrolidinium) ionic liquid and PC organic solvent are used as mother electrolytes with Na[FSA] (FSA = bis(fluorosulfonyl)amide) and Na[ClO4] salts. The physical and electrochemical properties of the various mixing ratio of those mixture electrolytes are investigated for sodium-ion batteries. These mixture electrolytes improve not only flammability but also show better rate and cycling performance. The detailed electrochemical properties are evaluated by comparing reference electrolytes to clarify the dual salts effect and dual solvents (ionic liquids and organic solvent) effect. This performance will be discussed based on half cell tests using a Na2FeP2O7 positive electrode and full cell tests using a hard carbon negative electrode and a NaCrO2 positive electrode.