In situ observation of ion concentration profiles in lithium ion-conducting gels

A gel electrolyte was produced by photo-polymerisation of poly(ethyleneglyc ol diacrylate) in an electrolyte mixture containing LiPF6 in propylene carb onate in a cell containing lithium metal electrodes. The cell was placed in a Schlieren optical set-up and a potential difference was applied across t he electrodes. Using the optical set-up, the change in refractive index acr oss the cell could be measured and then translated into concentration value s. In this way, concentration change across the electrolyte could for the f irst time be measured in situ while monitoring the current passing through the cell during the application of an external potential difference. After the applied potential difference had been removed, the decay of the concent ration gradient across the cell and the open circuit potential were monitor ed. Good agreement between the experimental results and the theory was obse rved. The transport number estimated for lithium using the concentration di fference values was in good agreement with the value estimated on the basis of the diffusion coefficients of the anion and the cation. (C) 2000 Elsevi er Science B.V. All rights reserved.