Exploring high performance VOPO4 for lithium batteries: A comparison between β and δ polymorphs

The β and δ polymorphs of orthorhombic VOPO4 are prepared by an oxalic acid assisted sol–gel method and a reflux method, respectively. Apparently, the structural and morphological characteristics have effect on the electrochemical properties. The potential plateaus in the flat charge/discharge profiles of the β and δ polymorphs are observed at 4.05/3.9 V and 4.02/3.85 V, respectively, which is in accordance with the two phase process for lithium de/intercalation. The β polymorph has a compact crystal structure and its chemical bond is stronger than that of the δ polymorph, which makes the capacity of the β polymorph lower than that of the δ polymorph when the current density is lower than 0.5 C. However, because the positive effect of the favorable morphological characteristic of the β polymorph overtakes the negative effect of its compact structure, the capacity of the β polymorph becomes higher than that of the δ polymorph when the current density is higher than 1 C. Besides the lower charge transfer resistance of the β polymorph, its charge transfer resistance decrease faster than that of the δ polymorph upon cycling.