Evaluation of α₁-LiVOPO₄, β-LiVOPO₄, and α-LiVOPO₄ synthesized from a same precursor by hydrothermal method

We succeeded in producing three phases of lithium vanadyl phosphate (α₁-LiVOPO₄, β-LiVOPO₄, α-LiVOPO₄) from a same precursor, synthesized by hydrothermal method. The phases were formed simply by controlling sintering temperature. We studied the relationship between their electrochemical performance and particle characterization using powder X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). In-situ heating XRD revealed the relationship between sintering temperature and generated phases of LiVOPO₄; α₁-LiVOPO₄ appeared at low temperature and we were able to extract over 80% of lithium (131 mAh/g) from large particles (about 500 nm). β-LiVOPO₄ appeared between 420 and 690°C, and at 0.1 C showed the highest discharge capacity of the three phases (140 mAh/g). The single phase of α-LiVOPO₄ appeared at over 720°C. Discharge capacity of α-LiVOPO₄ was half of theoretical capacity, but rate performance was the best of the three phases. EELS mapping of Li at full charge state revealed that only below 200 nm could particles of α-LiVOPO₄ contribute to the electrochemical property. This paper reveals that particles of α-LiVOPO₄ below 200 nm should be considered as a prospective candidate for next-generation phosphate cathode materials.