Electronic and Structural Properties of Li(1-x)FePO4 (X = 0 , 0.5 ,1)

Day of Ph.D. defence: 31.05.2010

 

Abstract

Lithium ion batteries are widely used for energy storage in many electronic devices. It is well known that the ability of a lithium ion battery to store energy is dependent on the properties of the materials which are used to construct the battery (see M.Winter and R.J. Brodd, Chem. Rev. 104, 4245 (2004)). These materials include those used as the anode, cathode,and electrolyte. 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 EELS (VEELS), core-loss EELS, bandstructure calculations and high resolution transmission electron microscopy (HRTEM).

The VEELS spectra of FePO4 are characterized by interband transitions peaks found between 0-20 eV which is not observed in LiFePO4. The observed VEELS spectra were fully analyzed using band structure calculations and calculated dielectric functions. It was in particular possible to assign the observed interband transitions as originating from bands situated just below the Fermi level which have mainly O-2p character. From coreloss EELS it observed that the O-K edge in FePO4shows a pre-edge peak that is observed above the threshold of the main O-K edge. This pre-edge peak is not observed in the core EELS spectra of LiFePO4. We show that the position of the pre-edge peak is determined by a charge transfer process which shifts the position of the Fe 3d bands with respect to the conduction band. We also show that the intensity of the pre-edge peak is related to the changes in Fe3d-O2p hybridization as a result of lithium ion extraction from the LiFePO4 lattice. 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. As a consequence, in FePO4 the valence states are now dominated by O-2p states and the UHB in the conduction band and the LHB in the valence band shifted to lower energies. These changes in the electronic structure are experimentally observed in the VEELS and core-loss EELS of LiFePO4 and FePO4.

Ph.D. thesis