Lithium ionic conductivity and structural studies of NASICON structured Li_0.4Sr_0.3Zr_0.5Ti_1.5(PO₄)₃

Stable solid electrolytes have been the subject of intense research due to the growing demand for energy storage and portable energy sources in rechargeable Li-ion batteries. Sodium (NA) super (S) ionic (I) conductor (CON) abbreviated as NASICON are suitable for use as solid electrolytes due to their excellent ionic conductivity and chemical stability. We have reported the effect of partial doping of Sr at Li and Zr⁴⁺ at Ti⁴⁺ position on the structure and ionic conductivity of composition Li_0.4Sr_0.3Ti₂(PO₄)₃ (LSTP). The composition with formula Li_0.4Sr_0.3Ti_2−xZr_x(PO₄)₃ (x = 0.0, 0.5, 2.0) were prepared by solid state reaction method. The phase and crystal structure were examined using X-ray diffraction (XRD) patterns. The compositions were in rhombohedral phase with the space group R \(\overline{3}\) $\overline{3}$ c. The doping of Zr⁴⁺ in LSTP causes the peaks in the XRD spectra for doped composition (x = 0.5, 2.0) to move towards lower angle. The displacement of the XRD peaks towards lower angle side show the expansion in the lattice of the Zr doped compositions Li_0.4Sr_0.3Ti_1.5Zr_0.5(PO₄)₃ and Li_0.4Sr_0.3Zr₂(PO₄)₃. The presence of stretching and bending modes of PO₄ groups in the prepared compositions was identified by FTIR spectra. The enhanced bulk conductivity of 4.32 × 10^–5 S/cm for Zr doped composition Li_0.4Sr_0.3Ti_1.5Zr_0.5(PO₄)₃ and 3.20 × 10^–5 S/cm for composition Li_0.4Sr_0.3Zr₂(PO₄)₃ is observed at 25 °C. The modulus peaks in the Zr doped composition Li_0.4Sr_0.3Ti_1.5Zr_0.5(PO₄)₃ is displaced towards higher frequencies at 450 °C, indicating the presence of the thermally produced dielectric relaxation phenomenon and charge carrier hopping. The cole cole plot consisting of one semicircle in case of Li_0.4Sr_0.3Ti_1.5Zr_0.5(PO₄)₃ discusses the contribution of bulk and grain boundary resistances and is fitted by equivalent circuit consisting of CPE and R element joined in parallel.