Glyme-based nonaqueous electrolytes for rechargeable lithium cells

Poly(ethylene glycol)dimethyl ethers [(CH3O(CH2CH2O)nCH3, n = 1, 2, 3, and 4)] are generally known as “glymes”. This study examines the conductivity, lithium ion solvation state and charge–discharge cycling efficiency of lithium metal anodes in glyme-based electrolytes for rechargeable lithium cells. 1 M (M: mol l−1) LiPF6 was used as the solute. The properties of the glymes were investigated by using a ternary mixed solvent consisting of n-glyme, ethylene carbonate (EC) and methylethylcarbonate (MEC). This was because the solubility of LiPF6 is far less than 1 M in an n-glyme single solvent. The glyme solutions exhibited higher conductivity and higher lithium cycling efficiency than EC/MEC. The conductivity tended to increase with decreases in ethylene oxide chain number (n) and solution viscosity. The decrease in the solution viscosity resulted from the change in the lithium ion solvation structure that occurred when a glyme was added to EC/MEC. The selective solvation of the glyme with respect to lithium ions was clearly demonstrated by 13C-NMR measurements. The lithium cycling efficiency value depended on the charge–discharge current (Ips). When n increased there was an increase in lithium cycling efficiency at a low Ips and a decrease in the reduction potential of the glymes. When the conductivities including those at low temperature (below 0 °C), and charge–discharge cycling at a high current are taken into account, di- or tri-glyme is superior to the other glymes tested here.