Solid electrolyte composite Li4P2O7–Li3PO4 for lithium ion battery

The researches on solid electrolyte have been significantly increasing due to the safety problem in lithium ion battery. The lithium phosphates are chosen due to environmentally friendly. In the present study Li4P2O7 was synthesized by solid state reaction using NH4H2PO4 and Li2CO3 with the ratio 1:2 at various temperatures of 600 °C, 800 °C and 900 °C. The products were characterized by x-ray diffraction, scanning electron microscopy and impedance spectroscopy. The x-ray diffraction showed that all samples consisted of two phases. It was found that the products consisted of 52.44% Li4P2O7 and 47.56% LiPO3; 93.56% Li4P2O7 and 6.44% Li3PO4; and 46.27% Li4P2O7 and 53.67% Li3PO4 under the synthesizing temperature of 600 °C, 800 °C and 900 °C, respectively. The highest ionic conductivity of 3.85 × 10−5 S/m was achieved for composite Li4P2O7–Li3PO4 with the highest content of 93.56% Li4P2O7. This conductivity is higher compared with single phase of LiPO3, Li3PO4 and Li4P2O7. The increase in ionic conductivity may be due to the mixed anion effects related to the phosphate networks, and it also corresponds to the existence of anorthic phase Li4P2O7 with the space group P −1 (2). The crystal lattice analysis showed that the reactant Li4P2O7 consisted of diphosphate groups P2O7. The lithium tetrahedral LiO4 were linked to P2O7 groups formed a continuous framework containing large voids, available for Li+ ion transport, and thus it exhibited high conductivity. A composite Li4P2O7–Li3PO4 is a promising solid electrolyte for solid state battery.