Phase Transition of Li3PO4 Solid Electrolyte for Secondary Batteries in Vacuum: A Real-Time Synchrotron X-ray Scattering Study

The phase transition of Li3PO4 solid electrolyte for secondary batteries in vacuum has been studied by using real-time synchrotron X-ray scattering. According to the vacuum heating results, the crystal β-Li3PO4 phase was stable only from room temperature to 400°C. The crystal γ-Li3PO4 phase appeared at 410°C and the crystal β-Li3PO4 phase completely disappeared at 430°C. The transition from the β-Li3PO4 to the γ-Li3PO4 phases occurred primarily at 410°C, which is 40°C lower than the transition temperature of 450°C in air. The decrease in the phase transition temperature in vacuum, compared to that in air, is attributed to a relatively high concentration of oxygen vacancies. As the isothermal annealing time at 390°C was increased to 8 hours, the β-Li3PO4 phase was completely transitioned into the γ-Li3PO4 phase. Our study revealed the detailed transition behavior from the β-Li3PO4 to the γ-Li3PO4 phases during real-time annealing in vacuum.