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- Title
- SYNTHESIS OF HIGH PERFORMANCE SILICON ANODES AND SURFACE-MODIFIED NMC CATHODES FOR LI-ION BATTERIES
- Creator
- Ashuri, Maziar
- Date
- 2019
- Description
-
Lithium-ion batteries (LIBs) have revolutionized the portable electronic devices and electric vehicles (EV) and because of this huge demand,...
Show moreLithium-ion batteries (LIBs) have revolutionized the portable electronic devices and electric vehicles (EV) and because of this huge demand, it is important to meet high power, high specific energy, and long cycle life. All mentioned characteristics are directly related to the choice of anode and cathode electrodes. Currently, graphite is used as anode, while lithium cobalt oxide serves as cathode dominantly. Although graphite can deliver ~ 370 mA h g-1 capacity without significant capacity decay for several cycles, however it is not enough to fulfill the requirements for many applications. Silicon with the theoretical capacity of about 10 times higher than graphite is a promising anode. However, this material suffers from huge volume expansion during cycling in addition to its intrinsic low conductivity. From the cathode viewpoint, the need for materials with less cobalt content is necessary. The resources for cobalt element is very limited while the price of cobalt increasing. Furthermore, cobalt is known as toxic element. Therefore, substitution of cobalt with other elements such as manganese and nickel is necessary. Lithium nickel manganese cobalt oxide (NMC) cathode family materials are introduced following this idea.Here in this thesis, two different approaches are introduced to harness the problems associated with silicon anodes. The first approach is the core/shell design and the second one is the silicon/graphite nanocomposite with tailored structure and engineered voids. Both of these designs can be synthesized easily without complicatedsynthesis steps and harmful chemicals. They have the potential of being commercialized and they do not need expensive equipment. The silicon anodes have been tested successfully in the half-cell coin cells.As for the cathode side, two different members of NMC family materials (NMC333 and NMC532) have been tested. To enhance their electrochemical properties and rate capabilities, a facile surface modification using phosphoric acid was employed. This technique resulted in the formation of thin lithium phosphate coating around the particle. The electrodes performed very well in half-cell configuration. It is expected by utilizing the proposed cathode and anode materials in full-cell set up, a high performance battery with fast charge ability is obtained.
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- Title
- NANOMATERIALS FOR ADVANCED BATTERY CATHODES
- Creator
- Moazzen, Elahe
- Date
- 2020
- Description
-
Cathode materials are key components that directly determine the power density of a battery. One of the most effective ways of developing high...
Show moreCathode materials are key components that directly determine the power density of a battery. One of the most effective ways of developing high power density cathodes is bringing them into the nano-scale world, which results in many expected and unexpected properties. Some of the desired characteristics include faster charge/discharge kinetics, improved capacity retention and structural stability due to the higher surface to volume ratio and shorter ion diffusion paths. In this dissertation a number of uniquely designed nano-sized cathode materials and nanocomposites are developed and investigated for alkaline aqueous and lithium ion battery applications. Nickel hydroxide (Ni(OH)2), which is one of the most important cathode materials in alkaline batteries, suffers from low conductivity, which usually leads to inefficient discharge and incomplete utilization of the material. A series of Ni(OH)2/Co(OH)2 core/shell nanoplatelets were synthesized and systematically investigated as cathode materials. Structure-property correlations revealed that electrochemical behavior and reversibility of Co(OH)2 redox conversion depended non-linearly on the average shell thickness, with the best performance (99.6% of theoretical capacity of the composite material) achieved at shell thickness of 1.9 ± 0.3 nm. Two fundamental phenomena were suggested to be responsible for the superior performance: templated shell deposition and galvanic coupling of core and shell materials.Manganese (IV) oxide (MnO2), which is another practical cathode that has a great potential to be utilized for a variety of energy storage systems, still has some major challenges including reversible cycling in rechargeable batteries. One of the most crucial challenges is the fact that polymorphs of MnO2 have different electrochemical activities as aqueous and Li-ion battery cathodes. However, most synthetic samples contain a mixture of polymorphs, which makes the structure-property correlations more complicated. This dissertation reports on systematic studies correlating synthesis, thermal and mechanical processing, and composite formation with polymorph composition, electrochemical performance and ion intercalation mechanisms. Among all the results, several main conclusions were reached: 1) Through control of the synthesis parameters and post-processing, desired phase compositions and nanoparticle morphologies, which optimize MnO2 performance in aqueous alkaline electrolyte, can be achieved. Nanoparticles with higher fraction of the akhtenskite polymorph showed higher reversible capacities in LiOH electrolyte (~210 mAh g-1), with stable performance for over 50 cycles. The effects of sub-nanoparticle organization of MnO2 polymorphs by thermal treatment without any morphology change on cycling performance, phase activation, and charge/discharge mechanisms in LiOH electrolyte as well as the detailed mechanism of the polymorph conversion during annealing were studied and for the first time, demonstrating that the electrochemical activity of MnO2 material strongly depends not only on the lattice structure of individual polymorphs but also on the sub-nanoparticle polymorph architecture and interphases.2) Several processing strategies, including thermal and mechanical processing, and composite fabrication were utilized to develop functional MnO2 cathodes for Li-ion batteries. Improvements in capacity and cycling performance were correlated to the presence of the pyrolusite phase of MnO2 and the crystallite size. Composite fabrication by graphene oxide wrapping also provided significant performance improvements through polymorph composition control and improved conductivity.
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- Title
- A Functionalized 2D Boron Nitride Electrode for Rechargeable Batteries
- Creator
- Tatagari, Vignyatha Reddy
- Date
- 2021
- Description
-
Motivated by the great performance of the graphene oxide battery and its poor safety, in the present work, an attempt is made to fabricate an...
Show moreMotivated by the great performance of the graphene oxide battery and its poor safety, in the present work, an attempt is made to fabricate an alternative battery from functionalized 2-dimensional (2D) boron nitride. The expectation is that functionalized boron nitride can exhibit the same great electrochemical performance as graphene oxide while it would be much more thermally stable. Toward this goal, synthetic opportunities were explored to realize -OBF3 functionalized hexagonal boron nitride. Both top-down and bottom-up synthetic approaches were considered and implemented. In the top-down methods, commercially available bulk hexagonal boron nitride (h-BN) is reacted with functionalization agents such as LiOBF3 and LiOH.BF3. Synthesis of these functionalization agents and their reactions with h-BN were carried out in several different ways. Bottom-up synthetic approach using Boric Acid and Urea was utilized to synthesize turbostratic boron nitride (t-BN), which is an intermedier in the commercial synthesis of hexagonal boron nitride. Turbostratic boron nitride contains exfoliated and -OH functionalized monolayers of boron nitride. An attempt is made to esterify the -OH groups of turbostratic boron nitride to obtain the desired -OBF3 functionalized monolayers of h-BN. Initial electrochemical tests on turbostratic boron nitride and its esterified form are carried out along with ionic conductivity measurements. Only a very limited electrochemical activity was observed due to a low degree of functionalization in these materials, indicating the need for improved synthetic procedures to achieve the desired target materials.
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