New glyme-cyclic imide lithium salt complexes as thermally stable electrolytes for lithium batteries

摘要

New glyme-Li salt complexes were prepared by mixing equimolar amounts of a novel cyclic imide lithium salt LiN(C_2F_4S_2O_4) (LiCTFSI) and a glyme (triglyme (G3) or tetraglyme (G4)). The glyme-Li salt complexes, [Li(G3)][CTFSI] and [Li(G4)][CTFSI], are solid and liquid, respectively, at room temperature. The thermal stability of [Li(G4)][CTFSI) is much higher than that of pure G4, and the vapor pressure of [Li(G4)][CTFSI] is negligible at temperatures lower than 100 ℃. Although the viscosity of [Li(G4)][CTFSI] is high (132.0 mPa s at 30 ℃), because of its high molar concentration (ca. 3 mol dm~(-3)), its ionic conductivity at 30 ℃ is relatively high, i.e., 0.8mS cm~(-1), which is slightly lower than that of a conventional organic electrolyte solution (1 mol dm~(-3) LiTFSI dissolved in propylene carbonate). The self-diffusion coefficients of a Li~+ cation, a CTFSI~- anion, and a glyme molecule were measured by the pulsed gradient spin-echo NMR method (PGSE-NMR). The ionicity (dissociativity) of [Li(G4)][CTFSI] at 30 ℃ is ca. 0.5, as estimated from the PGSE-NMR diffusivity measurements and the ionic conductivity measurements. Results of linear sweep voltammetry revealed that [Li(G4)][CTFSI] is electrochemically stable in an electrode potential range of 0-4.5 V vs. Li/Li~+. The reversible deposition-stripping behavior of lithium was observed by cyclic voltammetry. The [LiCoO_2][Li(G4)][CTFSI][Li metal] cell showed a stable charge-discharge cycling behavior during 50 cycles, indicating that the [Li(G4))[CTFSI] complex is applicable to a 4 V class lithium secondary battery.