Structure and electrochemical properties of vanadium doped for lithium iron phosphate cathode

Abstract

LiFePO4 and vanadium doped LiFePO4 were successfully prepared by sol- gel method. The concentrations of vanadium were varied by 0.01, 0.03, 0.05, 0.10, 0.15, and 0.2 wt %. Both doped and pure LiFePO4 were calcined at 400 and 600°C. XRD results showed that by changing the dopant concentration and calcination temperature, the crystallinity changed. As the concentration of dopant increased the size of crystallite size become larger. The peaks pattern shows orthorhombic structure for LiVFePO4 upon incorporation of vanadium, and changed to hematite at low calcination temperature. As the calcination temperature increase the size of particle become larger while smaller particle were obtained at low temperature. FESEM analysis showed that at low temperature, smaller particles tend to agglomerate. Agglomeration of several grains produced larger particles which might be due aggregation of very small crystal grains during calcinations. Upon addition of vanadium to LiFePO4, larger particles size was obtained. Similar ionic radius of vanadium with iron may cause the expansion of crystal lattice, resulting in larger particle size. EDX analysis confirmed the existence of each element in the samples except for Li which cannot be detected as the atomic number is less than five. In charge/discharge electrochemical test, the obtained discharge capacities were far from the theoretical value and not suitable for commercialization of lithium ion batteries. Poor performance of the prepared samples may be due to the formation of solid electrolyte interface layer and cracking causes the loss of lithium and active materials from the sample.