Doping crystalline polymer electrolytes with glymes

Abstract

Following our discovery that crystalline polymers can support ionic conductivity, in contrast to earlier views, we have investigated raising their conductivity by replacing PEO molecules in the conducting crystalline complexes PEO₆ : LiXF₆, X = P, As, by the glyme molecules CH₃O(CH₂CH₂O)₃CH₃ (triglyme, G3) and CH₃O(CH₂CH₂O)₄CH₃ (tetraglyme, G4). In all cases increasing the value of x in (PEO_1−xGZ_x)₆ : LiXF₆, where Z = 3 or 4 and X = P, As, leads to an increase in conductivity because of conduction through a liquid phase that must be present when the 6 : 1 composition is retained above x = 0. However, in one case, true doping of a glyme within the 6 : 1 crystal structure was observed, (PEO_0.75G4_0.25)₆ : LiPF₆. The increased number of chain ends arising from replacement of the 1000 Da PEO (22 EO units) by G4 increases the number of defects that are present where chain ends meet along the PEO tunnels, within which the Li⁺ ions migrate, thus raising the conductivity compared with the undoped 6 : 1 complex. This is the first example in which a molecular dopant has been introduced into the 6 : 1 crystalline complex and hence the first example of increasing the conductivity in crystalline polymer conductors by such a mechanism.