Glyme-based nonaqueous electrolytes for rechargeable lithium cells

Abstract

Poly(ethylene glycol)dimethyl ethers [(CH 3O(CH 2CH 2O) n CH 3, 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) LiPF 6 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 LiPF 6 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 13 C -NMR measurements. The lithium cycling efficiency value depended on the charge–discharge current ( I ps). When n increased there was an increase in lithium cycling efficiency at a low I ps 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.