Room temperature synthesis and phase transformation of lithium phosphate Li3PO4 as solid electrolyte

Abstract

We successfully prepared single-phase sample of the low-temperature β-type form of lithium phosphate Li3PO4 by a stoichiometric mixing of LiOH solution and NH4H2PO4 powder at room temperature without heating. The orthorhombic lattice parameters with the space group of Pmn21 for β-Li3PO4 were refined to be a = 6.1281(6) Å, b = 5.2674(5) Å, c = 4.8923(2) Å, and V = 157.92(2) Å3. We also prepared the single-phase sample of high-temperature γ-Li3PO4 by heating the β-Li3PO4 at 650°C. The orthorhombic lattice parameters with the space group of Pmnb for γ-Li3PO4 were refined to be a = 6.12569(5) Å, b = 10.48730(9) Å, c = 4.92957(4) Å, and V = 316.685(4) Å3. The structure transformation from β- to γ-type phase was examined utilizing the XRD measurements. The XRD data confirmed the continuous phase change in the temperature range between 25°C and 650°C. The lattice contraction for the β-Li3PO4 phase was observed between 150°C and 350°C. The phase transformation from β- to γ-Li3PO4 was observed around 450°C. The spherical primary particle size was ca. 20–30 nm order for both β- and γ-Li3PO4 samples. Accordingly, the nano-sized particle Li3PO4 sample can be easily prepared using the present synthetic technique at room temperature.