Ionic conduction and vibrational characteristics of Al3+ modified monoclinic LiZr2(PO4)3

Abstract

Effects of Al3+ substitution for Zr4+ in LiZr2(PO4)3 on its structure and lithium ion conduction are investigated. Li1+xZr2-xAlx(PO4)3 samples prepared via a sol-gel route and calcined at 900 °C crystallize in monoclinic structure with P21/n space group and show a reduction in cell volume with an increase in x. Raman spectra showed an increase in broadening of higher frequency v1 & v3 vibrational modes and a spectral weight transfer between v2 & v4 bending modes of PO4 tetrahedra with the increase in Al3+ doping. Analysis of Raman spectra further suggested that the renormalization of the mode frequencies in doped samples is controlled by Li-ion motion via strongly interacting with internal bending modes of PO4 tetrahedra. A significant improvement in ionic conductivity was observed in Al-doped samples, and the highest conductivity of 1.83 × 10−4 Ω−1m−1 and lithium diffusion coefficient of about 5.7 × 10−19 m2s−1 was observed for Li1.25Zr1.75Al0.25(PO4)3 at room temperature. Li transference number suggested that the conductivity in Li1.25Zr1.75Al0.25(PO4)3 is predominantly ionic. Activation energy was found to decrease from 0.58 eV for LiZr2(PO4)3 to 0.47 eV for Li1.25Zr1.75Al0.25(PO4)3. © 2018 Elsevier Ltd