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

The beta and delta 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 beta and delta 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 beta polymorph has a compact crystal structure and its chemical bond is stronger than that of the delta polymorph, which makes the capacity of the beta polymorph lower than that of the delta polymorph when the current density is lower than 0.5 C. However, because the positive effect of the favorable morphological characteristic of the beta polymorph overtakes the negative effect of its compact structure, the capacity of the beta polymorph becomes higher than that of the delta polymorph when the current density is higher than 1 C. Besides the lower charge transfer resistance of the beta polymorph, its charge transfer resistance decrease faster than that of the delta polymorph upon cycling. (C) 2013 Elsevier B.V. All rights reserved.