Structural and electrical characterisation of the NASICON-type Li2FeZr(PO4)3 and Li2FeTi(PO4)3 compounds

In order to develop new electrolytes for all-solid-state rocking chair lithium batteries, the NASICON-type compounds Li2FeZr(PO 4)3 and Li2FeTi(PO4)3 were investigated by powder X-ray diffraction technique and impedance spectroscopy. Li2FeZr(PO4)3 is orthorhombic Pbna (a=8.706(3), b=8.786(2), c= 12.220(5) Å) and Li 2FeTi(PO4)3) is orthorhombic Pbca (a=8.557(3), b=8.624(3), c=23.919(6) Å). They show no phase transitions from RT to 800°C. In the same temperature range logσT vs. 1/T show no slope variations. The activation energies for the ionic conductivity were 0.62 and 0.64 eV for Li2FeTi(PO4), and Li2FeTi(PO 4)3, respectively. In order to better evaluate the present results they were compared with those of α and ß-LiZr 2(PO4)3 phases, which were also prepared and characterised. A change of activation energy from 0.47 eV to 1.03 eV was observed in the case of ß phase, at about 300 °C; attributed to the ß (orthorhombic) ↔ ß' (monoclinic) phase transition. In the α phase the activation energy 0.47 eV in the temperature range 150-850 °C. The Li2FeZr(PO4)3 and Li 2FeTi(PO4)3 compounds can be interesting for applications as solid electrolytes in high temperature (> 300 °C) lithium batteries.