Atomistic simulation of polyanion cathode materials for lithium batteries

Abstract

Rechargeable lithium-ion batteries are attractive candidates for implementation into new large-scale energy storage applications, such as hybrid and electric vehicles, due to their high energy density. Modern atomistic modelling techniques can provide valuable insights into the fundamental defect and ion transport properties of electrode materials at the atomic scale, which are essential for a full understanding of lithium battery function. In this thesis, three types of polyanion materials, for use as alternative cathodes in lithium batteries, are examined using such computational techniques. Firstly, the mixed-metal phosphate material LiFe0:5Mn0:5PO4 is investigated.The intrinsic defect type in this olivine-structured material with the lowestenergy is the cation antisite defect, in which Li+ and Fe2+/Mn2+ ions exchange positions. Lithium ion diusion occurs down one-dimensional b-axis channels following a curved path in accord with experiment. Migration energies for Fe2+ and Mn2+ antisite cations on Li+ sites suggest that such defects will impede bulk Li+ mobility in LiFe0:5Mn0:5PO4. Secondly, ion conduction paths through the tavorite structures of recently discovered LiFeSO4F and NaFeSO4F are investigated by a combination of static lattice and molecular dynamics simulation techniques. The results indicate that LiFeSO4F is eectively a three-dimensional (3D) lithium-ion conductor with an activation energy of 0:4 eV for long-range diusion, which involves a combination of zigzag paths through [100], [010], and [111] tunnels in the open tavorite lattice. In contrast, for Na+ migration in NaFeSO4F, only one direction ([101]) is found to have a relatively low activation energy (0.6 eV). This leads to a diffiusion coecient that is more than six orders of magnitude lower than in any other direction, suggestingthat NaFeSO4F is a one-dimensional (1D) Na-ion conductor. Finally, the defect and diffiusion properties of LiFe0:5Mn0:5SO4F, which exhibits a complex triplite structure in which the cation (M) sites are shared by Li+, Fe2+ and Mn2+ ions, are examined.Low activation energies (6 0:45 eV) are found for several nearest-neighbourjumps between lithium sites which make up a 3D network of long-range migration pathways. However, due to cation site sharing, coherent long-range diusion may be blocked by Fe2+ and Mn2+ ions which would aect the rate capability of this material.

Date of Award	5 Dec 2012
Original language	English
Awarding Institution	University of Bath
Supervisor	Muhammed Islam (Supervisor)