Synthesis And Characterizations Of Fe_xV₂O₅ And Cr_xV₂O₅ Cathode Materials For Lithium Secondary Batteries

Vanadium pentoxide, V₂O₅, has been widely used as the cathode material for commercial lithium batteries, due to its high energy densities, low price and good safety properties. The layered structure of V₂O₅ makes it much favorable forLi+ insertion and extraction. However,it has been stated that pristine V₂O₅ is not an appropriate cathode material for lithiumbatteries because of its poor capacity retention. The capacity fading mechanism of V₂O₅ has been attributed to several reasons such as the low electronic conductivity and poor structural stability of the material. Recently, intensivework has been done to overcome these shortcomings encountered in the applications of V₂O₅. As an improvement, cation doped M_xV₂O₅ (M = Ag,Cu, etc.) showed better cycling performance than the un-doped V₂O₅. In this work, we prepared crystalline Fe and Cr and doped V₂O₅ by a simple sol-gel method with oxalic acid as a chelating agent .Their structure and electrochemical properties were investigated by XRD, SEM, Raman, FTIR and electrochemical charge-discharge cycling.1. For Fe_xV₂O₅ , the orthorhombic Fe_xV₂O₅ was synthesized by a simple sol-gel method with oxalic acid as a chelating agent , using Fe_xV₂O₅ and Fe(NO₃)₃·9H₂O as raw materials . The precuesor of the Fe:V ratios was 0.05:1 was sintered in different temperatures and times .XRD study showed that the crystal structure of Fe_xV₂O₅was almost independent of the sintering time and sintering temperature , so the sample sintered at 400℃for 10h had the appropriate structure. Fe_xV₂O₅ (x=0.05,0.1,0.12,0.15) was then calcined at 400 oC for 10h at the ambient condition , in order to study the effect of the content of Fe doping on V₂O₅ . XRD study showed that Fe_xV₂O₅ (x=0.05,0.1,0.12) has orthorhombic structure with space group Pmmn.Fe_0.05V₂O₅ and Fe_0.12V₂O₅ was selected to study the effect of Fe and the content of Fe doping on the structure and electrochemical properties of V₂O₅ . It is well known that V₂O₅ is not dissolvable in water. However, during the material preparation we observed that V₂O₅ dissolved in oxalic acid solution and the solution color changed to blue. This occurred because oxalic acid was not only a chelating agent but also a reducing agent, which caused the reducing of V5+ to V4+. The V4+ ions have a typical color of blue.The short range local structure of Fe_0.05V₂O₅ and Fe0.12V₂O₅was studied by Raman and FTIR spectra. the Raman spectra showed that a small shoulder band is observed at 956 cm-1 which is close to the V-O1 stretching. Such a shoulder band is not attributed to orthorhombic V₂O₅ but has been observed for oxygen deficient CuV2O6-δand lithiated Li_xV₂O₅ and LixCuV2O6. According to these reports, this shoulder band can be assigned to V4+-O1 stretching. The V4+-O1 stretching mode locates at a lower wavenumber with respect to that of V5+-O1 because of its weaker bond strength. Therefore, the observation of V4+-O1 stretching mode confirms the existence of V4+ ions in the samples. The formation of V4+ cations generated by Cr doping in V₂O₅ will result in higher electronic conductivity for Fe_xV₂O₅ than for V₂O₅. In addition, it is noticed that the relative intensity of V4+-O1stretching with respect to V5+-O1 stretching of Fe0.12V₂O₅ is much larger than that of Fe_0.05V₂O₅. This suggests that the V4+ concentration in Fe0.12V₂O₅is higher than that in Fe_0.05V₂O₅. Both samples exhibit the typical Raman pattern as that of V₂O₅. The IR bands at 1022 and 482 cm?1 are attributed to the V–O1 vibrations, and the bands at 829 and617 cm?1 correspond to the V–O3 and V–O2 vibrations, respectively. Beside these, two weak IR bands observed at 1619 and 3452 cm?1 for the material prepared at 400 ?C are assigned to the residual water in the material.Electrochemical charge-dishcarge cycliability of the compounds were tested in the voltage range of 4.0 and 2.0V at a rate of 0.1 C , 0.5 C and 1C . Therefor the Cr_0.1V₂O₅ material which contained more V4+ ions and Fe3+ ions . Since the material Fe0.12V₂O₅ exhibited much better electrochemical performance than Fe_0.05V₂O₅. The first charge curves were characterized by four plateaus, which delivered an initial discharge capacity of about 343.8mAh/g at a rate of 0.1C. The cycling performance of Fe0.12V₂O₅ exhibited good potentials for battery use at high-rate conditions. At a very high rate of 1C, the material still showed good cycling performance, which maintained a reversible discharge capacity 176.5 mAh g^-1 over20 cycles.2. For Cr_xV₂O₅ , the orthorhombic Cr_xV₂O₅ (x=0.05,0.1,0.15,0.2,0.3) also was synthesized at 400℃for 10h by a simple sol-gel method with oxalic acid as a chelating agent. the TGA plots of the precursor material for Cr_0.1V₂O₅ show that a slight weight increase appears in the temperature region between 400 oC and 600 oC. As mentioned above, the oxidation state of vanadium in the precursor material was V4+. Based on this, we speculate that partially V4+ ions in the precursor might remain in the material after heat treating the precursor at 400 oC. Cr_0.05V₂O₅ and Cr_0.1V₂O₅ was selected to study the effect of Fe and the content of Fe doping on the structure and electrochemical properties of V₂O₅ . In addition, the diffraction peaks of Cr_0.1V₂O₅ are broader than those of Cr_0.05V₂O₅. This reveals that Cr_0.1V₂O₅has lower crystallinity than Cr_0.05V₂O₅..The short range local structure of Cr_0.05V₂O₅ and Cr_0.1V₂O₅ also was studied by Raman and FTIR spectra. the Raman spectra showed that a small shoulder band is also observed at 956 cm-1 which is assigned to V4+-O1 stretching. Therefore, the observation of V4+-O1 stretching mode confirms the existence of V4+ ions in the samples. The formation of V4+ cations generated by Cr doping in V₂O₅ will result in higher electronic conductivity for Cr_xV₂O₅ than for V₂O₅. In addition, it is noticed that the relative intensity of V4+-O1stretching with respect to V5+-O1 stretching of Cr_0.1V₂O₅ is much larger than that of Cr_0.1V₂O₅. This suggests that the V4+ concentration in Cr_0.1V₂O₅ is higher than that in Cr_0.1V₂O₅. Both samples exhibit the typical Raman pattern as that of V₂O₅. Beside those, two weak IR bands observed at 1619 and 3452 cm?1 for the material prepared at 400 ?C are assigned to the residual water in the material.The cyclic voltammograms of the sample showed that the formation of some phases was significantly depressed by Cr doping. Therefore, Cr_xV₂O₅ was expected to showbetter cycling performance than pure V₂O₅ because of the presence of less number of phases. The potential intervals between the cathodic/anodic redox couples are 0.09 V, 0.11 V, 0.39 V for Cr_0.1V₂O₅, and 0.12 V, 0.19 V, 0.44 V for Cr_0.05V₂O₅. The smaller potential intervals of Cr_0.1V₂O₅ indicate good electrochemical reversibility of the material .Electrochemical charge-dishcarge cycliability of the compounds were tested in the voltage range of 4.0 and 2.0V. the material Fe_xV₂O₅ exhibited much better electrochemical performance than V₂O₅. Cr_0.1V₂O₅ exhibited much better electrochemical performance than Cr_0.05V₂O₅, which delivered an initial discharge capacity of about 271.4mAh/g. Cr_0.05V₂O₅ exhibited poor capacity retention. The discharge capacity after fifty cycles is below 100 mAhg-1 at each current density. Especially, there are big fluctuations on the capacity retentions especially at higher current densities. Comparing with previous studies on pristine V₂O₅, it seems that slight Cr doping (Cr_0.05V₂O₅) does not cause significant improvement in the electrochemical performance of V₂O₅. Conversely, the Cr_0.1V₂O₅ sample exhibited much better cycling performance. The Cr_0.1V₂O₅ sample exhibited well-defined S-shaped potential profiles at the 15th cycle. This indicates that Cr doping in V₂O₅ improved the material's structural stability,leading to the better cycling performance of the doped materials during short term cycling. The Cr_0.1V₂O₅ sample had no plateaus at the 50th cycle. This indicates that Cr doping in V₂O₅ can not improved the material's structural stability during long term cycling.