Poly(vinyl chloride) Ionic Liquid Polymer Electrolyte Based on Bis(fluorosulfonyl)Amide for Sodium Secondary Batteries

Abstract

Non-flammable electrolytes have been extensively studied to improve the safety of energy storage devices. In this study, a new ionic liquid polymer electrolyte (ILPE) prepared by a cast technique using poly(vinyl chloride) (PVC) as the host polymer was examined for sodium secondary batteries. The ILPE containing 50 wt% of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C 2 C 1 im][FSA] ionic liquid showed an ionic conductivity of 5.6 mS cm −1 at 318 K. It remains stable up to 517 K based on 5 wt% loss. Stable sodium metal electrodeposition/dissolution was observed at the cathodic limit of the electrochemical window for the ILPE containing Na [FSA] Lithium secondary batteries are used in various applications such as portable electronics, consumer electronics, and auto motives. 1,2However, the uneven distribution of lithium resources might limit their feasibility in large scale energy storage systems. Consequently, sodium secondary batteries have become one of the most promising alternatives to lithium secondary batteries.3 The key advantages of using sodium-based energy storage devices are the low cost, and the abundant resources and low redox potential of sodium (E o Na + /Na = −2.71 vs. SHE; which is ∼0.3 V higher than E o Li + /Li ). While organic electrolytes are currently used in researching sodium secondary batteries, 4,5 ionic liquids (ILs) offer great advantages in terms of safety, such as low vapor pressure, low flammability, high thermal stability, high conductivity, and wide electrochemical window. 6,7 The physical and electrochemical properties of ILs in Na secondary batteries have been studied in recent works. [8][9][10][11][12] In real application, polymerization of the IL is one important technique to retain the advantages of ILs and avoid leakage. As a type of gel polymer electrolytes (GPE), the resulting "ionic liquid polymer electrolytes (ILPEs)" consists of ILs, the host polymer, and another component for electrochemical reactions (e.g alkali metal salt). 13,14 In ILPE, it is assumed that the IL phase is trapped within the polymer matrix, forming a self-standing polymer electrolyte with the ions moving in the liquid phase. The advantages of ILPE are high processability, high flexibility, and high dimensional stability that lead to the elimination of the separator. In comparison with the conventional separator, ILPE offers better capacity to trap liquid electrolytes, 15 and acts as the separator and electrolyte at the same time.One of the most challenging issues for ILPEs is improving the compatibility among the IL, the host polymer, and the third component (e.g. alkali metal salt). Most studies of ILPE for lithium secondary batteries are based this ternary system; using poly(ethylene oxide) (PEO) as a host polymer. [16][17][18][19][20][21] For sodium ion conducting ILPE, only a few candidate host polymers have been demonstrated, including poly(vinylidene difluoride) (PVdF) and poly(vinylidene fluoridehexafluoropropylene) (PVdF-HFP). 22,23 Very recent research also focused on PEO-based I...

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