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Title: Synthesis and characterisation of Li rare earth-based oxide materials
Author: Chen, Yao-Chang
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2011
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Green and renewable energy materials have attracted great interest because of the soaring price of fossil energy and global warming. Among them, lithium secondary batteries are the key components of portable electronic devices as well as hybrid and electric cars and have been studied for over 30 years. Although in current commercial Li rechargeable batteries liquid and polymer electrolyte are the common electrolyte materials, safety issues are still of concern which therefore leads to an increasing interest in technological improvement and a demand for new materials for all solid state Li rechargeable batteries. In this project, various Li rare earth-oxide based compositions containing Nb, transition metal or trivalent cations were studied in order to find materials for possible Li-ion secondary battery applications. Stoichiometric phase-pure garnet, LisLa3Nb2012, was found to be difficult to synthesise due to the loss of volatile LhO. The covering blanket method involving careful heat profiles to reduce the possibilities of lithia loss during heating has been used in the present study of selected compositions in the Li20-La203-Nb20s system; a solid solution, Lis+xLa3Nbz0I2-osx, has been found. Electrical properties were measured by impedance spectroscopy and samples showed intrinsic Li ionic conductivity, - 10-6 Scm', at room temperature. Several new lithium Nd-based ternary oxides containing also Fe or Rh have been reported, but there was considerable disagreement over the possible stoichiometry of these phases. A detailed study of the ternary phase diagram of the LhO-Nd203-Fe203 system has been carried out with the initial objective to determine the stoichiometry of the previously reported ternary phase. A revised stoichiometry, LillNdlSFe4039, which differs from all previously claimed formulae, is proposed along with a revised phase diagram of the LizO-Nd203-Fe203 system. The crystal structure of this revised ternary phase has been studied using a combination of X- ray and neutron diffraction methods. It is primitive cubic, a = - 12 A, space group Pm "3 n. The structure is complex and can be described as comprising four < III > face-sharing polyhedra with alternating Li06 trigonal prisms and Fe06 octahedra intercepting at the origin and body-centre of the unit cell within an Nd-O framework. Face-sharing trigonal prismatic sites occupied by u: form octagon-like polyhedral coordination; with partially occupancy in one of those two trigonal prismatic sites these face-sharing polyhedra provide a possible short distance Li ion conduction pathway. Structural studies indicate that because the formula unit contains 39 oxygens, Fe is present in a mixture of 3+ and 4+ oxidation states. However, the valence state of Fe is variable, from Fe2+ only to a mixture of Fe 3+/4+, depending on different heat profiles; electrical properties of samples also reveal a change from semiconducting to insulating behaviours using impedance spectroscopy (IS). Using the same methodology, four new phases, Li11NdlsC04039, LiIINdlsAI40385, Li\JSmlSFe4039 and LiIISmISC04039, have been found in a survey of compound formation, Li\JRE1sM4039. Two ternary phase diagrams, LbO-Nd203-Ah03 and Li-Nd-Co-O system, are presented here; the Li20-Nd203-Ah03 system reveals a similar ternary phase diagram to that of Fe analogue system but a new ternary phase, LiCoNd4O8, and a solid solution, Lill-xNdlsC04039-o.5x have been found in the Co system. The crystal structures of those four new phases have been determined and are similar to that of Li11NdlsFe4039. Apart from LilINdlsAl403S5, which shows Li ionic conductivity, all the phases show modest electronic conductivity by IS. Two new phases, LiCoNd4O8 and LiNiNd40g, have been found in a survey of compound formation, LiMRE40s, after heating under suitable conditions. Rietveld refinement of the LiNiNd40s crystal structure was carried out using neutron powder diffraction; it shows that the crystal structure might have a supercell based upon the K2NiF 4 structural model. The electrical properties of these two new phases reveal very similar electrical characteristics, with electronic conductivity and constriction effect in the ceramics. During the study of compound formation, LiMRE40s, an X-ray pattern similar to that of the previously reported phase, LioI4Cuo86Nd204-o, appeared in the attempted synthesis of several LiCuRE40s compositions. In the present study, two new phases, Lio.14Cuos6Pr204-o and Lio.14Cuos6Sm204-8, were prepared. Rietveld refinements of the crystal structures are in agreement with the reported structural model. Electrical properties of samples revealed a very conducting behaviour which can be varied depending on heating conditions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available