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Layered Materials with Metal Site Vacancies

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Part of the Springer Theses book series (Springer Theses)

Abstract

Layered Li–Mn–Ni–O materials were analyzed in a region of the phase diagram where a strange bump was seen in the boundary of the layered region. These structures were found to contain a significant amount of metal site vacancies. The maximum vacancy content was found to result in highly ordered monoclinic structures where manganese occupies two of the \(\sqrt{3} \times \sqrt{3}\) superlattices on the transition metal layers while the third was randomly filled with nickel, lithium and vacancies. The resulting ordering predicted by a Monte Carlo simulation was consistent with the sharp ordering peaks seen in the XRD patterns. The vacancy concentrations were confirmed by Rietveld refinement, density measurements and redox titration; all of which were in good agreement. The role of the vacancies during electrochemical cycling, if there is one, remains unclear. The material with the greatest possible vacancy concentration, Li[Ni\(_{1/6}\Box_{1/6}\)Mn\(_{2/3}\)]O2, showed electrochemical behavior consistent with lithium-rich layered materials, namely high irreversible capacity associated with the high voltage plateau and voltage fade associated with conversion to spinel. However, this material was not, in fact, lithium-rich given that the vacancies result in lithium occupying only 50 % of metal sites. The vacancy results also demonstrated that there were roughly 1 % vacancies in a stoichiometric lithium-rich material lying along the line from Li2MnO3 and LiNi0.5Mn0.5O2. The Monte Carlo simulation suggested that this allows Ni\(^{3+}\) to substitute for Mn\(^{4+}\) on two of the superlattices. This has never been recognized before and a complete understanding of the starting material is crucial to fully understand the complex electrochemical behavior of the lithium-rich positive electrode materials. The exact shape of the other side of the bump (to the left of Li[Ni\(_{1/6}\Box_{1/6}\)Mn\(_{2/3}\)]O2 in the Gibbs triangles) has not been determined. It is also unclear as to why the bump would be so sharp on both sides and this is worthwhile for further study.

Keywords

Rietveld Refinement Vacancy Concentration Metal Site Redox Titration Average Oxidation State 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Copyright information

© Springer International Publishing Switzerland 2014

Authors and Affiliations

  1. 1.Physics and Atmospheric Sciences Dept.Dalhousie UniversityHalifax, Nova ScotiaCanada

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