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.