Structural Study of Layered Oxides and Oxysulfides as Positive Electrode Materials for Rechargeable Lithium Ion Batteries

Abstract

As one of the most advanced energy storage systems, lithium ion batteries have become a necessity of today's information-rich mobile society. Layered oxide LiCoO2 is the positive electrode material used in most of the present commercial rechargeable lithium ion batteries, but the high cost, toxicity and structural limitations of LiCoO2 limit its applications, especially for large-scale appliances, therefore alternative materials are needed.The work described in this dissertation focuses on studies of the electrochemistry and structures of layered oxides and oxysulfides as the positive electrode materials in rechargeable lithium ion batteries. A combination of techniques has been applied to characterize the structures, including the X-ray/neutron diffraction (XRD/ND), X-ray Absorption Near Edge Spectroscopy (XANES), Pair Distribution Function (PDF) analysis along with the reverse Monte Carlo (RMC) simulations, and solid state 6/7Li Magic angle spinning (MAS) Nuclear Magnetic Resonance (NMR).The series of layered oxides Li[NiyMnyCo(1-2y)]O2 (0 < y &#8804; 1/2), with Ni and Mn substitution of partial or all of the Co in LiCoO2, have shown promising electrochemical behavior and triggered a large amount of research. Nonetheless, there is still lack of understanding from the fundamental point of view of the structure-property relationships. Therefore, a systematic study has been performed on this series to investigate their structures and cation orderings. The XRD results confirm the presence of the layered &#61537;-NaFeO2-type structure while XANES experiments verify the presence of Ni2+, Mn4+ and Co3+. The 6Li MAS NMR spectra of compounds with low Ni/Mn contents (x &#8804; 0.10) show several well resolved resonances, which start to merge when the amount of Ni and Mn increases, finally forming a broad resonance at high Ni/Mn contents. Analysis of both the 6Li MAS NMR 6Li[Ni0.02Mn0.02Co0.96]O2 spectrum and neutron PDF data of Li[Ni1/3Mn1/3Co1/3]O2 reveals a non-random distribution of the transition metal (TM) cations in the TM layers, where Ni and Mn have a strong tendency to associate and form Ni/Mn clusters. Following the studies of the pristine materials, a systematic study has been performed to investigate the structural changes of Li[Ni0.05Mn0.05Co0.90]O2 (the y = 0.05 member) upon electrochemical lithium deintercalation. By using a combination of techniques, including XRD, X-ray absorption near edge spectroscopy and solid state NMR, a complete picture of the whole delithiation processes has been revealed. The results show that the non-random cation distribution has large effects on the order of Li removal. The ions located closest to Mn4+ are extracted first, and the oxidation of not only Ni2+ but also some Co3+, is seen in the beginning of Li extraction (less than 0.15 mole is removed). Further deintercalation (additional 0.2 mole of Li removal) induces an insulator to metal transition that is similar to that reported for LiCoO2. When half of the Li ions are extracted, the electrochemical signature for lithium vacancy ordering in the host framework is observed. The NMR results for deintercalation of more than 50% Li were compared to those for LixCoO2 at similar stages of charge, which are reported here for the first time; they indicate that the behavior of these two phases at these potentials is very similar. When the batteries are charged to voltages higher than 4.6 V, very few lithium ions remain in the structure and the O3 to O1 phase transition occurs.Layered Sr2MnO2Cu2m-&#948;Sm+1 (m = 1, 2 and 3, &#948; ~ 0.5) are a novel family of oxysulfides that have been synthesized recently. They consist of alternating Perovskite-type [Sr2MnO2] layer and various thicknesses of antifluorite-type [Cu2S] layers. They operate by a displacement mechanism of their electrochemical reactions towards Li, where most of the inserted Li replaces Cu and forms Li2S-like environment and extrudes Cu out of the framework as metallic particles. The reversible (de)lithiation processes of the m = 2 member have been investigated intensively. The results show that the Li insertion and deintercalation occur via different processes, and the involved active redox species are both Cu from the sulfide layer and Mn from the Perovskite oxide layer. Variations of the sulfur framework are observed upon the insertion/removal of Li, and the results indicate that the properties of the sulfur framework strongly affect the electrochemical behavior and cycling performance of the studied material.