Physicochemical and electrochemical properties of glyme-Na salt equimolar complex electrolytes were investigated. The pentaglyme (G5) forms complexes with Na salts in 1:1 molar ratio.^1,2 The structures of complexes of G5-Na[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) and G5-Na[FSA] (FSA: bis(fluorosulfonyl)amide were investigated using single crystal X-ray diffraction and ab initio calculations. In the crystals of G5-Na[TFSA] and G5-Na[FSA], G5 molecules wrap around Na⁺ ion and form [Na(G5)]⁺ complex cations. The melting points of [Na(G5)][TFSA] and [Na(G5)][FSA] are 32 °C and 41 °C, respectively.¹ To apply the G5-Na salts complexes to sodium ion batteries as electrolytes at room temperature, the complexes were mixed with a low polar solvent, HFE (HFE: 1,1,2,2–tetrafluoroethyl 2,2,3,3–tetrafluoropropyl ether (CF₂H−CF₂−O−CH₂−CF₂−CF₂H)). The liquid structures of [Na(G5)][TFSA]/HFE and [Na(G5)][FSA]/HFE were elucidated using Raman spectroscopy. The complex structure of [Na(G5)]⁺ complex cation were kept in the liquids, and the HFE hardly participate in the solvation of Na⁺ in the solutions.³ [Na(G5)][FSA]/HFE showed higher ionic conductivity than [Na(G5)][TFSA]/HFE owing to the higher mobility of FSA than that of TFSA.

The reversibility of the Na metal deposition/dissolution changed greatly depending on the electrolyte composition. The Coulombic efficiency of Na metal deposition-dissolution in [Na(G5)][FSA]/HFE reached to 95 % while the efficiency in [Na(G5)][TFSA]/HFE was much lower. X-ray photoelectron spectroscopy (XPS) revealed that a thin and compact SEI layer was formed on the Na metal in [Na(G5)][FSA]/HFE, and the SEI was effective in suppressing the reductive decomposition of the electrolyte.

The charge and discharge properties of hard carbon (HC) anode and Na_0.44MnO₂ cathode of sodium ion batteries in [Na(G5)][FSA]/HFE electrolyte were characterized. Highly reversible charge-discharge of HC anode was possible in [Na(G5)][FSA]/HFE with a high Coulombic efficiency of 99.9%. In addition, the discharge capacity of HC negligibly decreased during 300 charge-discharge cycles. Na_0.44MnO₂ cathode also showed highly stable charge-discharge behavior in [Na(G5)][FSA]/HFE owing to the good oxidative stability of the electrolyte.

Acknowledgements

This study was supported in part by the MEXT program “Elements Strategy Initiative to Form Core Research Center” of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and JSPS KAKENHI (No. 15H03874) from the Japan Society for the Promotion of Science (JSPS).

References

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