The effects of lithium doping level on the structural, electrical properties of Li+-doped BPO4 solid electrolyte

A series of lithium ion conducting solid electrolytes LixB1−x/3PO4(x = 0.01, 0.05, 0.09, 0.13, 0.17, 0.20) is synthesized by a soft-chemistry route. FTIR and XRD measurements reveal that the electrolyte is pure phase of tetragonal structure. AC-impedance spectroscopy (AC-IS) at room temperature shows that LixB1−x/3PO4 exhibits higher ionic conductivities in the range 0.05 ≤ x ≤ 0.13, beyond which, the ionic conductivities decrease quickly. Maximum ionic conductivity of the LixB1−x/3PO4 reaches 3.35 × 10−5 S cm−1 at room temperature for x = 0.05. Direct current polarizing (DCP) measurement indicates that the decomposition voltage for the solid electrolyte reaches up to 3.7 V. Micro-structure parameters of synthesized LixB1−x/3PO4 samples are calculated by Rietveld refinement of X-ray diffraction spectra. The unit-cell parameters, lattice strain, crystal grain size and ionic conductivities of the samples are correlated with the lithium ion doping level x.