High-resolution study of conductivity and cell potential versus doping concentration in potassium-doped polyacetylene: Correlation with structural transitions

Abstract

A high-resolution quasiequilibrium in situ study of parallel conductivity ${\mathrm{\sigma }}_{\mathrm{\parallel }}$ and open-circuit voltage ${\mathit{V}}_{\mathrm{OC}}$ during potassium doping and dedoping of an oriented (CH${)}_{\mathit{x}}$ electrode is presented. Features in ${\mathit{dV}}_{\mathrm{OC}}$/dy and d${\mathrm{\sigma }}_{\mathrm{\parallel }}$/dy (y=K mole fraction) found previously at y=0.06, 0.12, and 0.15 are confirmed and correlated with recent x-ray and ESR results and with a model of staging via intercalation channels. We also find new features during dedoping at y=0.03 and 0.08 in d${\mathrm{\sigma }}_{\mathrm{\parallel }}$/dy but not in ${\mathit{dV}}_{\mathrm{OC}}$/dy, the former coinciding with a similar ESR feature. We propose that these are signatures of subtle structural effects that do not involve major changes in lattice constants or unit-cell symmetries. We tentatively assign the y=0.03 feature to a metastable ‘‘dilute stage-2’’ phase, by analogy to graphite intercalates. Similarly, three phases with the same stage-1 channel structure are inferred from the observation of three plateaus in ${\mathit{V}}_{\mathrm{OC}}$(y) in the range 0.10<y<0.17 upon dedoping. A maximum ${\mathrm{\sigma }}_{\mathrm{\parallel }}$=17 500 S/cm is found near y=0.12 during the first doping cycle. With successive cycles ${\mathrm{\sigma }}_{\mathrm{\parallel }}$(max) decreases, the maximum attainable y also decreases, and ${\mathit{V}}_{\mathrm{OC}}$(y) approaches the ideal behavior of an intercalation electrode exhibiting first-order phase transitions.