Electrochemical Properties of Glyme-Based Complex Electrolytes for Na and Mg Batteries
Authors: Terada Shoshi , Susa Hiroko , Suzuki Soma , Kamei Yutaro , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

Certain glyme-Li salt complexes behave like room temperature ionic liquids (RTILs). Li+ ion forms complex cation with glyme, Gn (CH3O(CH2CH2O)
n
CH3), in 1:1 ratio when the chain length of glyme n = 3 or 4. In the equimolar complex of a glyme and a Li salt, nearly all glymes coordinate to Li+ cations and therefore the solvate cation [Li(glyme)]+ is the only cation in the system.1) Consequently, the equimolar complex behaves as a RTIL consisting of [Li(glyme)]+ and a counter anion, resulting in the low volatility, low flammability, high thermal stability, and electrochemical stability. The equimolar complexes of glyme (n = 3 or 4) and certain Li salts are classified as solvate ionic liquids (SILs).2) The glyme-Li salt SIL can be used as a thermally stable electrolyte in Li secondary batteries.3) To achieve high battery performance, the stability of solvate cation and the dissociation of complex into solvate cation and anion is essential. SILs are composed of metal cation, anion, and ligand. The interaction between the metal cation and ligand needs to overcome that of cation and anion. Otherwise, ion-pair formation takes place significantly and the uncoordinated ligands are generated in the system, resulting in the thermal and electrochemical instabilities and low ionic conductivity.4) Therefore, the choice of ligand and counter anion is crucial to develop stable electrolytes for high performance batteries.

 In this work, the physicochemical properties of glyme-Na salt were studied. Recently, the Na and Mg secondary batteries are attracting much attention owing to high natural abundance of Na and Mg compared to Li. We investigated the effect of chain length of glyme on the dissociativity and electrochemical properties of glyme-Na salt complexes and also compared with those of glyme-Li salt and glyme-Mg salt complexes.5) This study will surely promote the development of such elementally rich and low cost secondary batteries.

 The equimolar mixtures of Na[TFSA] (sodium bis(trifluoromethanesulfonyl)amide: NaN(SO2CF3)2) and tetraglyme (G4) or pentaglyme (G5) was dissolved in hydrofluoroether (HFE: HCF2−CF2−O−CH2−CF2−CF2H) at around 1 mol dm−
3 concentration, at which highest ionic conductivity was obtained, and used as electrolyte for the battery test at 30 °C. The composite cathode was fabricated by mixing Na0.44MnO2 : acetylene black : PVDF = 80 : 10 : 10 (wt%) and pasted on an Al foil. The sodium metal was used as an anode. The charge current density was fixed at 70 mA cm−
2 (0.1C, 12 mA g−1 based on the mass of Na0.44MnO2) and discharge current density was changed to 70 ~ 1350 mA cm−2 for the rate capability test.

 Figure 1 shows the discharge capacity of Na0.44MnO2 as a function of current density at 30 °C. The discharge capacity of the cell with [Na(G5)][TFSA]/HFE is higher than that of one with [Na(G4)][TFSA]/HFE. The cell with [Na(G5)][TFSA]/HFE keeps 77% of the full capacity at 1 C rate (0.65 mA cm−2), while the capacity of cell with [Na(G4)][TFSA]/HFE decreases rapidly as increasing the current density. The discrepancy between G4 and G5 systems can be explained by the difference in ionic conductivity. The interaction between the Na+ and [TFSA]− is mitigated by the coordination of glyme’s ether oxgen atoms to Na+. The dissociativity of the [Na(G4)]+−[TFSA]− is lower than that of [Na(G5)]+−[TFSA]− because G4 having smaller number of ether oxygen atoms. Therefore the ionic conductivity is lower in G4 system, resulting in poor rate performance of the cell. The further detailed investigation on the interactions between the [M(glyme)]
m

+−[TFSA]− and solvate cation stability of [M(G4)]
m

+ (M=Li, Na or Mg) will be reported in the presentation.

Figure 1