Citric acid-assisted synthesis and characterization of doped LiCoVO4

For the first time, a citric acid complex method has been used to prepare doped LiCoVO4. 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, Li2CO3, CoCO3·H2O and NH4VO3 were used. The formed citric acid complex consists of two complexes, i.e., Li2[Co(C6H5O7)2] and [(VO)2C6H7O7]2−. The TGA and the HT-XRD results showed that the formation of doped LiCoVO4 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 VO4 tetrahedron. X-ray diffraction patterns were recorded and revealed a single-phase material. However, the 8 and 10 mol% Fe-doped LiCoVO4 revealed a minor impurity of Fe2O3. 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 Co2+ to Co3+, 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 LiCoVO4 dropped rapidly with cycling as was observed for the undoped oxide. The submicron-based doped LiCoVO4 results in an enhanced initial charge capacity; however, the capacity fades quite rapidly due to the structural instability of the material.