Electronic properties and structural properties of Li (1 - chi) FePO 4 (X = 0 , 0.5 ,1)

Abstract

Lithium iron phosphate (LiFePO4) is currently applied as a cathode material for lithium ion batteries. However, its electronic properties and the mechanism by which lithium ions are extracted (delithiation) or inserted (lithiation) into the lattice are still not completely understood. In this thesis the electronic and structural properties of Li(1-x)FePO4 (LiFePO4 x = 0, FePO4 x = 1) have been investigated using valence and core loss electron energy loss spectroscopy (EELS) and high resolution transmission electron microscopy (HRTEM). In the first part of this thesis we present the study of the electronic structure before and after delithiation, LiFePO4 and FePO4 respectively. This is accomplished using valence EELS (VEELS), core-loss EELS and bandstructure calculations. We show that the changes in the electronic structure between FePO4 and LiFePO4 are quite significant such that FePO4 can be considered to be a charge transfer insulator while the LiFePO4 can be considered to be a Mott-Hubbard insulator. In LiFePO4 the energy states at the top of the valence band and bottom of the conduction band are dominated by Fe 3d states. These states form the lower Hubbard band (LHB) and the upper Hubbard band (UHB) respectively. Delithiation is characterized by shifting of the iron (Fe) 3d bands to lower energies and increased hybridization between the Fe-3d and oxygen (O) 2p states. The second part of this thesis is concerned with structural studies on partially delithiated (Li(0.5)FePO4). Examining the lattice parameter distribution in partially delithiated LiFePO4 grains we find lattice parameters for FePO4 and LiFePO4 phases. This shows that at a partially delithiated state both phase do co-exist in the same grain. These results support the one dimensional model (1D) of delithiation which states that extraction of Li ions proceeds through the ion channels parallel to the [010] direction resulting in the formation of lithiated and delithiated domains in the same grain