Date

2018

Author

Pişkin, Berke

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In the present study, the effects of Ni:Mn:Co mole ratio and doping elements, i.e. Mo, W, Ag and Cu, in Li(NixMnyCo1−x−y-zMz)O2 (NMC) were systematically investigated as cathode materials for Li-ion batteries. Li(Ni1/3Mn1/3Co1/3)O2 (NMC-111), Li(Ni0.2Mn0.2Co0.6)O2 (NMC-226), Li(Ni0.6Mn0.2Co0.2)O2 (NMC-622), Li(Ni0.2Mn0.6Co0.2)O2 (NMC-262) and their doped compositions were synthesized via spray pyrolysis. Spray pyrolysis method was used to attain a spherical fine-sized morphology in the powders synthesized. This procedure was followed by a heat-treatment to provide well-defined splitting of (006)/(102) and (108)/(110) diffraction peaks in XRD spectra as an indicator for layered structure and good hexagonal ordering. SEM and TEM studies were carried out to investigate the structural properties of the powders synthesized. XRD analyses were performed to reveal the present phases in the structure, while XPS analyses were carried out to identify surface chemistry. Moreover, ICP-MS was employed so as to quantify the contents of elements in the powders. Galvanostatic tests and EIS were carried out to examine the electrochemical performance of NMC cathode materials. Of the undoped NMC cathode materials, NMC-111 performed relatively lower cation mixing in the structure leading highest discharge capacity. In the doped compositions, Mo-doped cathode materials in all Ni:Mn:Co mole ratio exhibited relatively lower cation mixing together with the wider Li-gap. This was also valid for all 622 compositions independent from the doping elements, except W. The cathode materials which had a low cation mixing, while having a wide Li-gap were performed better discharge capacity and capacity retention up to 140 mAhg-1 and ~80%, respectively in the 2.7-4.2 V potential window. The morphology of the powder might be considered as another factor affecting the electrochemical performance. In the structural investigations, two type of particle formation was observed; one was spherical aggregates having large and loose primary particles and the second was spherical aggregates having small and relatively denser particles. The morphology of loose aggregates with relatively larger particles was determined in the cathode materials, e.g. 622-Mo, that were performed higher discharge capacity with a lower internal resistance. This structure provided a higher effective surface area to participate in the electrochemical reactions. This high surface area allowed the intercalation/deintercalation of Li-ions took place at a shorter diffusion time. Present findings revealed the how the doping elements and the Ni:Mn:Co content affect the electrochemical performance and the structure stability of the NMC cathode materials.

Subject Keywords

Lithium ion batteries., Storage batteries., Cathodes., Pyrolysis.

URI

http://etd.lib.metu.edu.tr/upload/12622050/index.pdf
https://hdl.handle.net/11511/27264

Collections

Graduate School of Natural and Applied Sciences, Thesis