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Study of Lithium-transition Metals-orthosilicates as Cathode Materials for Li-ion Batteries

Kalantarian, Mohammad Mahdi | 2014

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45753 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science & Engineering
  6. Advisor(s): Asgari, Sirous
  7. Abstract:
  8. In this study, orthosilicate materials with chemical formula of Li2MSiO4 (M one or two transition metal/metals) were investigated experimentally and theoretically as cathode material of Li-ion batteries. The most important material in this category i.e. Li2FeSiO4 was synthesized by three methods including nitrate-based sol-gel, oxalate-based solid state reaction and oxide-based solid state reaction. Based on XRD and SEM evaluations, grain size of synthesized powders was estimated to be between 15 to 100 nanometers. Of these three methods, oxide-based solid state reaction was employed in this study for the first time. This syntheses method is very important due to the significant lower cost of precursor materials as well as the synthesis process itself. The influences of the amount of carbon in the precursor materials, temperature and time of calcination process and also the heating rate on the final product were investigated and the optimum condition for each parameter was determined. Charge-discharge measurements were performed for assembled batteries using different samples at different conditions. The results established appropriate properties for the investigated materials. In addition, experimentally observed voltage-time (or voltage-capacity that is the same) behaviours were theoretically evaluated. Accordingly, by considering the possible concentration gradients, a model was suggested for voltage-time behaviour and the other relevant quantities including particle size, current rate and reaction energy. Using theoretical calculations and the available experimental data, the proposed mathematical model was verified under assumed conditions. Consequently, two strategies, i.e. particle size modification and usage of additives, were proposed to modify voltage-capacity behavior. One of the most important challenges of orthosilicate cathode materials (as well as polyanion cathode materials such as phosphates and borates) is indeed their low rate capability in charge-discharge process. Here, a very simple approach was suggested to evaluate the rate capability of cathode materials by considering their density of states (DOS) calculated via several density functional theory (DFT) methods, in both the lithiated and the delithiated case. We suggested that these structures could be interpreted as n- or p- type semiconductors, depending on their DOS configuration. On this basis, if the lithiated structure acts as an n-type and the delithiated one as a p-type semiconductor, the cathode is expected to be only “low rate” capable. If the contrary happens, the cathode would sustain high current rates. A qualitative comparison was performed between the most important known cathode materials. On the basis of the proposed model, we suggested some general strategies, related to the synthetic approach, to improve the cathode rate capability; including (i) coating the material with carbon, (ii) an appropriate doping, (iii) coating cathode particles by another material that acts as n- or p-type semiconductor and (iv) coating the cathode particles with a layer of solid material containing lithium ions. In order to perform a more comprehensive study on Li2FeSiO4, three known polymorphs of this material were evaluated using DFT calculations. Relative thermodynamic stability, structural properties, structural stability after lithium ions extraction, theoretical voltage and electrical properties of the polymorphs were investigated. One of the experimentally observed behaviour of this material is a decrease in the voltage value after the first charge cycle. Here, an alternative mechanism was suggested for this phenomenon based on a transition in ferromagnetic ground state of the transition metal (Fe) after the first cycle. The other important cathode material in orthosilicate category is Li2MnSiO4. Three known polymorphs of this material were investigated by DFT calculations. Relative thermodynamic stability, structural properties, structural stability after lithium ions extraction, theoretical voltage, electric and magnetic properties of the polymorphs were evaluated. To justify the fading of Li2MnSiO4 properties as a cathode material, a mechanism based on phase transition under cycling was proposed. Consequently, it is suggested that to evade fading of the properties, particle size should be optimized (about 300 nm), ultra milling should be avoided and Pmn21 structure should be stabilized. Other orthosilicate cathode materials that were investigated by DFT calculations were Li2CoSiO4, Li2NiSiO4 and Li2M0.5N0.5SiO4 (M and N is Mn, Fe, Co and Ni). Structural properties, segregation, structural stability after lithium ions extraction, theoretical voltage, electric and magnetic properties of these materials were evaluated. Their performance as cathode materials for Li-ion batteries was theoretically predicted and the observed differences in their behaviors were justified.
  9. Keywords:
  10. Lithium Ion Batteries ; Cathodes ; Nanomaterials ; Density Functional Theory (DFT) ; Orthosilicate

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