High Ionic Conductivity and Moduli Iongels Made with Tetraglyme/Litfsi /Methyl Cellulose for Lithium Batteries

Abstract

The motivation for the development of solid polymer electrolytes (SPEs) arises from their safety advantages over liquid electrolytes. In particular SPEs are non-flammable and avoid the problem of electrolyte leakage, both of which can result in fires/explosions. However, SPEs have lower conductivities at room temperature (RT) compared to those of liquid electrolytes, so there is a need to develop SPE with improved RT conductivities. Recently, glymes, (CH3−O-(CH2−CH2−O)n−CH3) n=3 or 4, have been studied as solvents for non-aqueous electrolytes because of unique properties such as their solvation power (Lewis basicity), relatively long-lived (stable) solvates, and complex formation in certain molar ratios, [M(glyme)x]+, with alkali metal ions. Li(G3 or G4)][TFSI] is molten at room temperature and can be considered as a representative ionic liquid. It has very low volatility, is nonflammable, and has good oxidative stability. Here we report the preparation, ionic conductivity (s), mechanical and thermal properties of iongels made from Li(G4)TFSI and methyl cellulose (MC), in ratios of 90/10, 80/20 and 70/30 Li(G4)TFSI/MC. The ionic conductivity decreases with increase in MC content over the temperature range measured, 10 oC to 90 oC (Figure 1A), while the storage moduli increase with increase in MC content. For 90/10 Li(G4)TFSI/MC at 30 oC , s = 3.78×10-4 S/cm and the storage modulus = 50 MPa(Figure 1B). Thermogravimetric analysis of G4, Li(G4)TFSI, Li(G4)TFSI/MC and MC indicates that iongels are stable until the vaporization of the G4(Figure 2A). DSC data for G4, G4 with TFSI, and G4 with TFSI/MC (Figure 2B ) show that: (i) complexation of Li+ with G4 results in an increase in Tg and a suppression of the melt temperature Tm, and (ii) MC content increases Tg and decreases conductivity (Figure 1A). Figure 1A. Log (conductivity) versus 1000/T for Li(G4)TFSI and Li(G4)TFSI/MC gels. Figure 1B. Storage modulus for 90/10 Li(G4)TFSI/MC as a function of temperature Figure 2A.TGA of G4,Li(G4)TFSI, Li(G4)TFSI/MC and MC Figure 2B. DSC scans (second heating cycle) of Li(G4)TFSI and Li(G4)TFSI/MC References (1) Zhou, S.; Kim, D. Polymers for Advanced Technologies 2010, 21, 797. (2) Henderson, W. A. The Journal of Physical Chemistry B 2006, 110, 13177. (3) Pappenfus, T. M.; Henderson, W. A.; Owens, B. B.; Mann, K. R.; Smyrl, W. H. Journal of The Electrochemical Society 2004, 151, A209. Figure 1