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Authors: Singh, Gurpreet
Issue Date: 2009
Abstract: Environmental friendly and cheaper cathode material for lithium ion battery has been chosen as a material to work upon. Various experimental and computational investigations have been carried out on pristine and doped LiMn2O4. Chromium (Cr) and Magnesium (Mg) have been chosen as the dopants. Change in morphology from truncated octahedron to perfect octahedron has been observed with the increase in Cr content in LiMn2O4. Decrease in the particle size from — 250 nm to — 80 nm has been observed with increasing dopant concentration. Apart from the decrease in lattice parameter of the cubic spinel structure with the increase in Cr content, no major change has been observed in the crystal structure. No change in the space group symmetry is observed for the entire doping range of Cr, i.e., LiCrxMn2_.04; 0.0-5x<0.5. From the electrochemical performance of the materials it has been observed that Cr suppresses the fall in capacity in the 4 V range, and the voltage step which occur in the 4 V regions also gets disappear with the increase in Cr content. Decrease in capacity has been observed with the increase in Cr content. Impedance spectroscopy results show that the bulk impedance increases and hence electrical conductivity of the material reduces with the increase in dopant concentration. The electrical conductivity of the material has been observed to be of the order of 10-5 S/cm. Coexistence of two phases having space groups of Fd3m and P4332 in the Mg doped LiMn2O4 spinet has been observed. X-ray powder diffraction (XRD) studies show that the crystal structure of LiMgxMn2_,(04 for x < 0.25 is a single phase cubic spinel which has space group of Fd3m. The cubic spinel structures having space group of Fd3m and P4332 are found to coexist in the compound for x = 0.25. The structure becomes single phase cubic spinel with space group P4332 for x > 0.25. Microscopic observations have shown the particle size ranging from 100 to 350 nm. It has been observed that the particle size decreases with the increase in Mg content. Thermal studies show an exponential decay relationship between the Mg doping content and the decomposition temperature to form nonstoichiometry (LiMg„Mn2_x04_8) in the air atmosphere. Fourier transform infrared spectroscopy shows the increase in number of vibrational bands with the increase in Mg content, which indicates ordering of the ions in case of ordered spinel structure and consequent reduction of the space group symmetry from Ohl to O. Effect of citric acid content on the various structural and electrochemical properties of layered LiNi1i3Mn13C01/302 has been studied. Average particle size of — 200 nm has been observed with higher citric acid content. First cycle coulombic efficiency of —93% has been observed for R'=3 (where R'= ratio of citric acid to metal ions) in the voltage range of 4.6 V to 2.5 V. First principles calculations have been performed for the doped lithium manganese oxide spinels. Effect of doping spinet LiMn2O4 with Cr and Mg has been studied using the first-principles spin density functional theory within GGA (generalized gradient approximation) and GGA+U. GGA and GGA+U based calculations predict different ground states for pristine LiMn2O4 and same ground state for doped systems. Body centered tetragonal phase was found to be the ground state structure using GGA and face centered orthorhombic using GGA+U for LiMn2O4. Both GGA and GGA+U based calculations for LiM0.5Mn1 504 (M= Cr or Mg) show base centered monoclinic as ground state and for LiMMnO4 (M= Cr or Mg) body centered orthorhombic as ground state. We find that GGA based calculations show the pristine LiMn2O4 as metallic while GGA+U based calculations predict it to be insulating. For doped spinels, GGA based calculation predicts the ground state to be half metallic while GGA+U predicts it to be insulating or metallic depending on the doping concentration. Calculation based on GGA+U predicts insulator-metal-insulator transition as a function of doping in case of Cr and in case of Mg the ground state is found to go from insulating to a half metallic state as a function of doping. Analysis of the charge density and the density of states suggest a charge transfer from the dopants to the neighboring oxygen atoms and manganese atoms. We have calculated the Jahn-Teller active mode displacement Q3 for doped compounds using GGA and GGA+U. The bond lengths calculated from the calculations using GGA+U are found to be in better agreement with the experimental bond lengths. Based on the bond lengths of metal and oxygen, we have also estimated the average oxidation states of the dopants
Other Identifiers: Ph.D
Appears in Collections:DOCTORAL THESES (MMD)

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