Citric acid-assisted synthesis and characterization of doped LiCoVO 4

Abstract

For the first time, a citric acid complex method has been used to prepare doped LiCoVO 4. The powders synthesized by the citric acid method have a relatively small particle size and particle size distribution compared to powders prepared by conventional solid-state reaction. As dopants, Fe, Cr and Cu were studied. As starting materials, Li 2CO 3, CoCO 3·H 2O and NH 4VO 3 were used. The formed citric acid complex consists of two complexes, i.e., Li 2[Co(C 6H 5O 7) 2] and [(VO) 2C 6H 7O 7] 2−. The TGA and the HT-XRD results showed that the formation of doped LiCoVO 4 occurred between 250 and 300 °C. For the analysis, the powders were synthesized at 500 °C for 2 h in air, resulting in a particle size, which is between 200 and 400 nm. Raman spectroscopy was carried out for the doped oxides and it was found that the substitution of Co for a dopant caused shifts of various vibration bands. Shifts were observed for the Li–O–M band located at 475 cm −1, and for the broad bands located at 786 and 810 cm −1 which are attributed to the stretching vibrations of the VO 4 tetrahedron. X-ray diffraction patterns were recorded and revealed a single-phase material. However, the 8 and 10 mol% Fe-doped LiCoVO 4 revealed a minor impurity of Fe 2O 3. The electrochemical results showed for the powders with a dopant concentration higher than 2 mol% the cyclic voltammograms show a new oxidation peak on the first scan. This new peak was attributed to the oxidation of Co 2+ to Co 3+, which has migrated from the octahedral to the tetrahedral site during Li extraction. The initial discharge capacity of 76 mA h/g obtained with the 6 mol% Fe-doped LiCoVO 4 dropped rapidly with cycling as was observed for the undoped oxide. The submicron-based doped LiCoVO 4 results in an enhanced initial charge capacity; however, the capacity fades quite rapidly due to the structural instability of the material.