Comparison of the electronic structures of four crystalline phases ofFePO4

Abstract

LiFePO 4 in the olivine structure is a promising cathode material for Li ion batteries. During normal battery operation, an olivine form of FePO 4 is produced. In addition to the olivine form, FePO4 is known to form in a quartz-like structure, a high pressure CrVO4-like structure, and a monoclinic structure. We report the results of a detailed density functional study of the electronic structures and total energies of these four crystalline structures of FePO 4. Partial density of states analysis of the four materials finds them all to be characterized by strong hybridization between the Fe and O contributions throughout their upper valence bands, consistent with recent X-ray spectroscopy studies of olivine FePO4. Results obtained using the local density approximation for the exchange-correlation functional find the olivine structure to be more stable than the quartz-like structure by 0.1 eV which is in good agreement with recent calorimetry experiments.

Mechanisms ofLi+diffusion in crystallineγ- andβ−Li3Du1, Holzwarth2 2007Phys. Rev. BSelf Cite995510View full textAdd to dashboardCite

“…2 show that the two crystals have very similar density of states profiles, with the ␤ form showing slightly smaller bandwidths than those of the ␥ form. As found in other phosphate materials, 15,16,42 the lowest bands correspond to the O 2p bonds with the P 3s states ͑near −7 eV͒ and O 2p bonds with the P 3p states ͑near −5 eV͒. The upper portion of the valence band has primarily O 2p character relative to the P-O bonds.…”

“…14 The pseudopotentials for Li, P, and O were constructed using the USPP code 14 and tested for agreement with calculations using other methods and codes. 15,16 For most of the calculations, the form of the exchange-correlation functional was chosen to be the local density approximation ͑LDA͒, 17 although some calculations were performed using the generalized gradient approximation ͑GGA͒. 18 The calculations for single unit cells of the perfect crystals were performed using plane wave expansions for pseudo-wave-functions with wave vectors ͉k + G͉ 2 ഛ 64 Ry.…”

“…16 The experimental measurements 4,33-35 vary less than 1% among themselves. The calculated fractional atomic positions are insensitive to the form of the Although we do not know of experimental measurements of these relative energies, the sign ͑indicating that the ␤ structure is the more stable͒ is consistent with differential thermal analysis measurements.…”

Laser‐induced phase changes in olivine FePO4: a warning on characterizing LiFePO4‐based cathodes with Raman spectroscopy

“…[24,25] One possible explanation is that the local structure of disordered FePO 4 is more similar to α-FePO 4 than the olivine phase. That is, most of the iron atoms in the amorphous FePO 4 are tetrahedrally coordinated like the α-FePO 4 phase and not octahedrally coordinated as in the olivine phase.…”

Probing the local structural changes across the amorphous to crystalline transition in iron phosphate glass: Positron annihilation and micro Raman studies

“…Most stable FePO 4 has hexagonal structure [31] and has been converted to the equivalent orthorhombic structure with a = 8. 6429 Å, b = 4.99 Å, c=11.…”