Functionalization of particles by atomic layer deposition for energy storage applications

Sarai García García
Abstract & Cover

Powders are essential ingredients for many processes and applications. They are usually classified with relation to their particle sizes and functionalities. While particles in the millimeter size range are intensely used in alimentary, pharmaceutical, cleaning and construction sectors, micrometer and nanometer sized particles are commonly used in energy storage applications, catalysis and electronics. Recent research has focused its attention on micrometer and nanometer sized particles due to the special properties arising from their high surface area to particle size ratio. Functionalization of those particles can greatly improve their performance. In this way, providing added value, like protecting and activating them, or changing their performance. Among the most promising ways of functionalization is the generation of core-shell particles through coating, or the deposition of islands or clusters on the surface of the particles. Nowadays, a wide variety of coating technologies are applied for this purpose. Among those coating technologies, chemical vapor deposition (CVD) became attractive in the recent years thanks to its great thickness control over the deposited coating. However, more recently, atomic layer deposition (ALD) was developed, allowing for ultimate thickness and compositional control of the deposited film in a large variety of geometries. The application of ALD in different fields, including microelectronics, energy storage systems or bioapplications, pushed the application of this technology to materials with diverse geometries, among those being particles. The need for coating powders resulted in the modification of reactors for carrying out ALD processes on such materials. The various reactors are distinct in the way of handling particles; namely, static particle reactors and fluidized bed reactors. While static particle reactors are used to coat small amounts of particles, fluidized bed reactors (FBR) can be used to coat large amounts of particles, allowing the scale-up of the technology for its use in industrial applications. The application of ALD to fabricate or coat materials and components for energy storage systems is intensely investigated and it is beginning to deliver breakthroughs. Batteries belong to the most important energy storage systems thanks to their excellent energy density and energy release rate. Lithium-ion batteries (LIB) are currently the most common battery types for a large variety of applications. In fact, they offer a theoretical energy density of around 300 Wh: ke ' However, their limited specific capacity and the precious cathode materials made researchers looking into other kinds of battery systems as alternatives. Lithium-sulfur (Li-S) batteries became a promising alternative due to their better handling and extraordinary properties of sulfur as cathodic material. Namely, it shows a theoretical energy density of 2600 Wh: kg', higher than that of Li-ion batteries. Moreover, sulfur is environmentally friendly and one of the most abundant elements in the world. However, Li-S batteries suffer from several drawbacks that affect their application and have driven researchers to develop solutions to enable the practical use of lithium-sulfur batteries and in this was increase the energy density and long-term stability. The application of ALD in energy storage systems has shown many improvements by now. In fact, the deposition of certain materials at the nanometric scale has many unique benefits for improving the behavior of Li-S batteries. The objective of this thesis is the study and understanding of ALD coatings on powders, paying special attention to materials that can be used for energy storage devices. Micrometer and nanometer sized particles have been coated with metal oxides, which showed promising alterations and functionalities of powders that have not been observed before. In the first part of the thesis, an initial study of coating particles by ALD is done. For this aim, iron oxide nanoparticles (y-Fe:O3) are coated with titanium dioxide (TiO2), generating FeO,-TiO» core-shell nanoparticles. This study shows that the application of ALD not only coats the particles, but also, depending on the reactants (precursors) used, can also reduce them to form core-shell nanoparticles of Fe;0.-TiO2. This study demonstrates that choosing an appropriate ligand of the metal source can unveil a novel approach to concertedly coat and reduce y-Fe.O3 nanoparticles. Moreover, it is found that the more electronegative the cation of the precursor is, the more energy is necessary to release the ligands, which is conditional for their recombination. Thus, the appropriate design of precursors and selection of substrates will pave the way for numerous new compositions with more and improved functionalities. In the second part of the thesis, the study of ALD on energy storage devices, specifically on lithium-sulfur batteries, is carried out. The modification of the cathode material of lithium-sulfur batteries is done by ALD in a conventional static ALD reactor. The optimal parameters for the ALD application to sulfurbased electrodes are evaluated. Aluminum oxide (AlL,O3) is deposited on prefabricated cathodes, improving the capacity of the systems. In fact, applying only 2 ALD cycles at 85 °C increases the capacity of a lithium-sulfur battery by 13 % for low current densities and by 50 % for high current densities. Finally, a promising powder coating technology is applied in order to coat sulfurcarbon composite powders of cathodes of lithium-sulfur batteries by ALD and in this way considerably improving the performance of those batteries. For this aim, a fluidized bed reactor was constructed. The best results are obtained after applying 5 ALD cycles of Al,Os, sufficient to increase the capacity of the system by 30% at low current densities and by 50% at high current densities, with respect to a standard battery system. Besides, the sulfur loading in the cathodes can be doubled thanks to the morphological improvement provided by the aluminum oxide. After coating, uniform and crack-free electrodes can be fabricated, which significantly exceed the performance of standard electrodes increasing the capacity of lithium-sulfur batteries by 60 %. 

(Donostia, Spain)
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