Preparation And Characterizations Of LiNi_1/3Mn_1/3Co_1/3O₂ And LiNiVO₄ Cathode Materials For Lithium Ion Batteries

The main content of this thesis includes: Preparation, physical properties and electrochemical performance studies of layered Ni-Mn based cathode materials LiNi_1/3Mn_1/3Co_1/3O₂ for lithium ion batteries; Studies on the preparation, surface-modification and structural properties of 5V cathode material LiNiVO₄; Studies on the kinetic behavior of electrode processes.LiNi_1/3Mn_1/3Co_1/3O₂ has been introduced by Ohzuku and J. R. Dahn as a candidate for new cathode materials. This material has attracted intense interests because the combination of Ni, Mn and Co provide advantages such as high reversible capacity, good rate capability, mild thermal stability, low cost and low toxicity. In this work, LiNi_1/3Mn_1/3Co_1/3O₂ compound was prepared by a simple wet chemical process. TGA/DTA analysis was employed to determine the chemical reaction process and appropriate calcining temperatures. The results of XRD refined show that a higher annealing temperature caused a smaller Li⁺/Ni²⁺ site exchange in the material lattice, which suggests that this material has better electrochemical performance. Raman scattering and FTIR absorption spectra analysis suggests strong structural disorder within the octahedral [MO6] units due to different masses and bonding of the M-O bonds, as well as the Li⁺/Ni²⁺ site exchange in the crystals. XPS analysis indicates that a lower annealing temperature than 900oC leads to the formation of oxygen deficient LiNi_1/3Mn_1/3Co_1/3O₂-δ. The Li-ion storage mechanism and electrochemical performance of the LiNi_1/3Mn_1/3Co_1/3O₂ were investigated systemically by cyclic voltammograms and charge-discharge experiments, respectively.Lattice structures, morphology, magnetic and electrochemical properties of LiNiVO₄ were systemically studied by XRD, Raman/FTIR spectra, SEM, SQUID and charge-discharge experiments. Extensive investigations have shown that the electrochemical performance of a cathode material is closely dependent on its structural properties. XRD reflects the long-range ordering of crystal lattice, whereas Raman/FTIR spectra are much more sensitive to the short-range environment of coordinative units than XRD is. The structural properties of the as-prepared and electrochemically Li-extracted LiNiVO₄ were studied by XRD and Raman/FTIR spectra. The particle surface of LiNiVO₄ was modified with Al₂O₃ by sol-gel method in order to improve its electrochemical performance.Electrochemical impedance spectroscopy (EIS) was used to study the kinetic behavior of LiNi_1/3Mn_1/3Co_1/3O₂ and LiNiVO₄ electrode. In order to study the Li⁺ ion transmission mechanism in interface region of the electrode, Equivalent circuit of LiNi_1/3Mn_1/3Co_1/3O₂ and LiNiVO₄ electrode during de-intercalation process was built. It was confirmed that kinetic rate-determining step was dominated mainly by the charge transfer process as the LiNi_1/3Mn_1/3Co_1/3O₂ was charged above 4.4V. The faster capacity fading in the potential region of 2.5-4.6 V than that in 2.5-4.4 V has been attributed to the large charge transfer resistance at potentials above 4.4 V.