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Title: Synthesis and analysis of new lithium-ion battery cathode materials
Author: Taylor, Z.
ISNI:       0000 0004 7428 7438
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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The work presented in this thesis focuses on the synthesis and structural characterisation of functional materials for lithium-ion battery cathode applications, with the most promising class of lithium-ion battery cathodes were determined to be lithium-rich rock-salt superstructures. In addition to oxidation of transition metal cations during charging (cationic redox), the anionic redox process involves partial oxidation of lattice oxide (O2- ) to form intermediate peroxo-type species (O2 n- ) to enable additional lithium extraction to maintain electroneutrality in the delithiated structure. Suppression of full oxidation of lattice oxide to molecular oxygen is a key requirement to utilising the lattice oxygen to reversibly extract additional capacity, without compromising the safety and longevity of the battery materials. The mechanisms of lithium ion extraction and insertion into cathode materials are discussed in relation to their structures. Chapter 3 focuses on the structural solution of new lithium-rich rock-salt superstructures of the form Li4+xNi1-xWO6 (x = 0, 0.1, 0.15), by combined Rietveld refinement of high-resolution synchrotron and neutron powder diffraction data. Li4NiWO6 was found to crystallise in the C2/c space group, determined to be a monoclinic distortion of the Fddd Li3Ni2TaO6 structural archetype. Li4.1Ni0.9WO6 and Li4.15Ni0.85WO6 were found to crystallise into the non-centrosymmetric Cm space group, comparable to the layered C2/m Li5ReO6 archetype. Chapter 4 assesses the electrochemical behaviour of Li4.15Ni0.85WO6, obtaining a specific discharge capacity of 200-210 mA·h g-1 , with a reversible capacity of 173 mA·h g-1 when cycled between 1-5 V; attributed to cumulative cationic and anionic redox reactions. Ex-situ X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation states of the cations and oxygen species, showing the reversibility of the redox reaction between lattice oxide and peroxo-type species during the first two electrochemical cycles. By comparison to the observed nickel oxidation states from XPS and X-ray near-edge structure (XANES) spectroscopic analysis, the reversible anionic redox was determined to be responsible for ~2/3 of the observed discharge capacity. Chapter 5 details the solid state synthesis of an unreported solid solution (1-x)LiCoO2·xLi4WO5, with successful doping of small amounts of rock-salt Li4WO5 (P1¯) into the layered LiCoO2 (R3¯m) structure determined by analysis of powder X-ray diffraction (PXRD) data. Electrochemical testing was performed on x = 0.010 and x = 0.025, exhibiting initial discharge capacities of 124 and 128 mA·h g-1 , respectively. The analogously prepared parent phase, LiCoO2, displayed an initial discharge capacity of 121 mA·h g-1 . A considerably greater proportion of the available capacity could be extracted from x = 0.010 and x = 0.025 at fast discharge rates by comparison to LiCoO2; with the doped materials exhibiting a greater retention of the higher discharge capacity during continued cycling. Chapter 6 assesses the synthetic method of flame spray pyrolysis (FSP) for known lithium-ion battery cathodes, using a simple laboratory setup. This technique is used for the production of nanoparticulate products from liquid precursors. The intention was to improve the diffusion kinetics through a reduction in particle size. The merits and limitations of the technique are discussed in relation to the purity, crystallinity and morphology of simple lithium metal oxides assessed by PXRD, scanning electron microscopy and ICP-OES. In addition, due to slow diffusion kinetics being a limiting factor in the performance of polyanionic cathode materials, the possibility of synthesis of such materials by FSP is assessed.
Supervisor: Rosseinsky, M. J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral