Improved Li-battery electrolytes by heterogeneous doping of nonaqueous Li-salt solutions

Abstract

The concept of heterogeneous doping is applied to liquid lithium battery electrolytes. For that purpose, surface-acidic oxides (viz. SiO₂ are dispersed in LiPF₆ containing ethylene carbonate/dimethyl carbonate (w/w 1:1). If the volume fraction (φ) is so high that the particles touch, the overall conductivity is significantly enhanced, in particular if the grain size is small. The results are explained by adsorption of the anion and therefore dissociating Li⁺ out of the ion pairs. This is in line with the zeta potential measurements. At lower φ, the effect is negligible owing to the repulsion of the colloids. At higher φ the conductivity is lowered because of inhomogeneity effects and eventually by the insulating nature of the admixtures. In addition to the enhanced overall conductivity, these composites show the favorable mechanical properties of soft matter (“soggy sand” electrolytes) and exhibit high effective viscosities. A significant advantage is that the SiO₂ content can make the use of polymer separators dispensable. Performance tests in Li/Li_xC₆ cells do not show any perceptible interaction of SiO₂ with high Li activity materials.