Synthesis And Electrochemical Properties Of Metal Doped V₂O₅ Cathode Materials For Li-ion Batteries

The development of Li-battery technology is critical for advancements in a variety of applications ranging from hybrid electric vehicles to consumer electronics. One of the challenges for impoving the performance of lithium ion batteries to meet the increasing requirements for energy storage is the development of suitable catode materials. V₂O₅ is one of interest in the applications of cathode materials for lithium-ion batteries. It has some advantages such as high specific capacity and energy density, low cost, and easy preparation. However, it has been demonstrated that pure crystalline V₂O₅ is not an appropriate cathode material because of its poor capacity retention and poor high-rate performance. In this work, we prepared cation doped vanadium pentoxides (CrxV₂O₅ and Al_xV₂O₅) using a soft chemical method, and gave a systematical study on their crystal structure and electrochemical properties.Cr³⁺ and Al³⁺ doped V₂O₅ were synthesized by a sol-gel method assisted by oxalic acid. Thermogravimetric analysis was used to study the synthetic route of the material. X-ray diffraction confirmed the best conditions for synthesis of phase pure cation doped V₂O₅. FTIR and Raman spectra showed that cation doping increased the structural stability of V₂O₅, which would improve the electrochemical properties of the materials. X-ray photoelectron spectroscopy showed that a low level of V4+ ions were still contained in the doped materials. In addition, the morphology features of the materials were studied by scanning electron microscopy. It is showed that the particle size increased with heat treatment temperature. The sample sintered at 350oC for 5h contained homogeneous small particles with an average particle size about 60 nm. We studied the effects of Cr³⁺ and Al³⁺ doping on the electrochemical properties of V₂O₅ through galvanostatic charge-discharge and cyclic voltammetry experiments. It reveals that cation doping and nano preparation can prevent the irreversible structure transition of V₂O₅ during Li⁺ ion insertion/extraction. However, it seems that slight cation doping does not cause significant improvement in the cycling life of V₂O₅.The kinetic properties of Li⁺ ion is one of the most important factors for the electrochemical performance of cathode materials. In this study, cyclic voltammetry, electrochemical impedance spectra and potentiostatic intermittent titration technique were employed to evaluate the Li⁺ ion diffusion coefficients and correlative kinetic properties of cation doped V₂O₅. It was found that cation doping did not significantly improve the Li⁺ ion diffusion coefficients of V₂O₅. This suggests us to prepare nano materials to improve the rate capability of V₂O₅ because nano materials provide higher specific surface area and shorter pathways for lithium ions, leading to efficient Li⁺ ion diffusion. Following this suggestion, we prepared Al_0.2V₂O₅ nanoparticles. Electrochemical experiments showed that the material showed significant superior performance with high capacity, good rate capability and excellent cycling performance.On all accounts, this work showed that a combinative technique of cation doping and nano preparation is the most effective way to improve the overall perfroamce of V₂O₅. The new findings presented in this thesis are very useful for further applications of V₂O₅ in rechargeable lithium batteries.