Effect of the phase transition of LiSn2(PO4)(3) on the Li+ ion conduction in LiSn2(PO4)(3)-Teflon composites

A composite LiSn2(PO4)(3)-25% vol Teflon was prepared to prevent breakages in powder pellets due to the phase transition of LiSn2(PO4)(3) and, thus, to study the effect of the phase transition on Li+ conduction. The composite microstructure, as followed by scanning electron microscopy, shows aggregated LiSn2(PO4)(3) particles surrounded by Teflon regions which operate as a skeleton. The imaginary part of the electric modulus shows two peaks at high and low frequency which are ascribed to grain-interior and grain-boundary response, respectively. In the two phases the ionic conductivity inside the grains is about three orders of magnitude larger than that found for the grain boundary. The phase transition affects the activation energy in different ways. It decreases for the grain-interior response from 0.68 eV for the low-temperature phase to 0.34 eV for the high-temperature phase, while it increases from 0.45 to 0.56 eV for the Li+ movement through the grain boundaries. An anomalous increase in activation energy for the grain-interior response in the high-temperature phase before it is transformed into the low-temperature phase has been found.