Microstructure and ionic conductivity of LiSn2P3O12-Teflon composites

The ac electrical response is investigated in ionic conductor-polymer compo sites formed of a Li+ ion conductor (LiSn2P3O12) and an insulating polymer (Teflon) taken in different concentrations (29, 44, 60, and 74% by volume o f LiSn2P3O12). The microstructure of the composites, as analyzed by scannin g electron microscopy (SEM), shows an aggregation of LiSn2P3O12 particles g iving rise to two types of clusters: some extended along the composite and some isolated or are surrounded by Teflon. Based on such a microstructure w e propose an equivalent circuit model that matches the frequency response. The relative proportion of the two types of clusters, determined by SEM, is compared with the value of a weighting parameter (f) introduced in the equ ivalent circuit for fitting the electrical data. The composites were also p repared to estimate the ionic conductivity of LiSn2P3O12 because of the dif ficulty of obtaining either sintered ceramic pellets or single crystals of LiSn2P3O12. The ionic conductivity estimated for LiSn2P3O12 is 2 +/- 1 x 10 (-5) S cm(-1) at 120 degreesC. For the analogous LiGe2P3O12 a comparison be tween the ionic conductivity of the ionic conductor deduced from the compos ite, and the ionic conductivity measured on a well-sintered pellet of the p urr ionic conductor has been made. Moreover, the temperature dependence of the conductivity for the ionic. conductor and the overall conductivity for the composite and the well-sintered pellet have been studied. (C) 2001 The Electrochemical Society.