A QCM analysis of water absorption in lithium polymer electrolytes

Abstract

A quartz crystal microbalance (QCM) has been used to measure water uptake as a function of humidity in a polymer electrolyte consisting of amorphous poly(ethylene oxide) doped with lithium perchlorate. The theory of the technique is reviewed to assess the effect of non-rigidity of sample films and to provide a method of correcting results for viscosity effects by measuring the admittance spectrum close to the resonant frequency. Results show that the mass of 0.1 µm films can be measured to sufficient accuracy without correction but corrections are useful for thicker films. A comparison of results with others obtained on a conventional microbalance shows that the relative mass increases are consistent between the two techniques and that QCM is much less subject to random errors. The water uptake was found to be mainly associated with the lithium ion rather than the polymer. After the absorption of one water molecule per lithium at very low humidity the water : lithium ratio rose linearly to about 6:1 at 60% humidity.