Room Temperature Sodium-Sulfur Batteries with Glyme-Na Salt Solvate Ionic Liquid Electrolytes
Authors: Terada Shoshi , Nozawa Risa , Ikeda Kohei , Mandai Toshihiko , Ueno Kazuhide , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

Room temperature ionic liquids (RTILs) are desirable electrolyte materials for advanced batteries because of their unique properties such as non-volatility, non-flammability and high thermal and electrochemical stability. We have reported that glyme-Li salt complexes, comprising equimolar mixtures of a glyme (CH3O(CH2CH2O)
n
CH3) and a Li salt, have similar properties to RTILs and classified as solvate ionic liquids. Certain glyme-Li salt complexes such as lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) and tetraglyme (G4) or triglyme (G3) are liquid at room temperature and show high thermal stability, high lithium ion transference number, and high lithium ion concentration. We demonstrated reversible charge-discharge of lithium secondary batteries using glyme-Li salt complex as an electrolyte. Recently, the similar results were obtained with equimolar mixtures of pentaglyme (G5) and Na[TFSA] abbreviated as [Na(G5)1][TFSA] was able to use as an electrolyte of sodium secondary battery.
Elemental sulfur has been studied as the post-lithium ion battery cathode material for its high theoretical capacity of 1672 mAh g-1. However, the major drawback of this material is the poor cycle stability because of dissolution of lithium polysulfides into typical organic electrolyte. One facile approach to overcome this problem is trapping the sulfur inside a polymer network such as poly(acrylonitrile) (PAN) to prevent the dissolution. By mixing PAN and excess amount of sulfur and heating at around 300 °C, cyclization and dehydrogenation of PAN is promoted by sulfur and results in sulfur contained p-conjugated polymer (PAN-S). The PAN-S has been studied as cathode active material with high specific capacity, good efficiency and cycling stability.
However, most reports of PAN-S dealt with lithium batteries and not so many reports have been done in PAN-S sodium system. In this study, we tested the charge-discharge property of PAN-S composite cathode with [Na(G5)1][TFSA] electrolyte at 30 °C.
The PAN-S was synthesized by mixing PAN and elemental surfur in 1:4 weight ratio and heated at 350 °C for 6 hours under Ar atmosphere. The composite cathode was fabricated by mixing PAN-S, AB, PVA in the weight ratio of 70:20:10 with NMP used as homogenizer to obtain slurry and pasted onto Al sheet. The electrolyte was prepared by mixing pentaglyme (G5) and sodium bis(trifluoromethylsulfonyl)amide (Na[TFSA]) in 1:1 molar ratio ([Na(G5)1][TFSA]) and mixed with a hydrofluoroether (HFE) in 1:4 molar ratio in order to improve ionic conductivity. Anode material Na15Sn4was prepared by mixing sodium metal and Sn powder in 15:4 molar ratio by ball mill for 5 hours, 510 rpm.
The result of charge discharge test is shown in the figure. The initial discharge capacity was about 500 mA h g(PANS)
-1 and charge-discharge capacity after that was around 400 mA h g(PANS)
-1with excellent coulombic efficiency for 30 cycles. Although there are capacity decrease as the cycle number increase, the cycle stability is much improved compared to that of elemental sulfur composite cathode and sodium metal anode system.
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