ε- and β-LiVOPO₄: Phase Transformation and Electrochemistry

ε- and β-LiVOPO₄ were synthesized from the same precursor at different temperatures in an air atmosphere. ε-LiVOPO₄ is obtained at 400 and 700 °C. The 700 °C sample has better purity and crystallinity, but the 400 °C sample has a little better electrochemical performance due to its smaller particle size and the conducting carbon residue in the sample. β-LiVOPO₄ is formed between the above two temperatures, which gives slightly lower capacity than that of the ε-LiVOPO₄ sample, indicating higher kinetics of the lithium reaction for the ε phase than those of the β one. The phase transformation from ε to β then back reversibly to ε was also observed by ex situ X-ray diffraction. This thermal study verifies that ε-LiVOPO₄ is the more stable phase for LiVOPO₄; however, reaction kinetics control the phases formed at lower temperatures.