Text

theses

Applications of lithium ion batteries (LIBs) are expanding rapidly from electronic devices to electric vehicles and many other markets. To further improve their energy storage capacities and durability, scientists have dedicated much efforts on investigating the least understood part of the batteries, the solid electrolyte interphase (SEI). SEI is the reductive decomposition product of the electrolyte of LIBs ( the solvent and Li ion salts), deposited on the surface of the electrodes, during the charge/discharge process. The properties of SEI dramatically influences or even determines the capacity, charging/discharging rate, and cycle life time of a battery. In the case of graphite-based LIBs, one of the most important causes of the capacity loss is due to the irreversible co-intercalation of the solvent into the graphite layers. This unwanted co-intercalation of the solvent causes blister formation and exfoliation in the graphite electrode. To prevent this co-inetercalation. scientists have been trying to alter the commercially used battery electrolyte or to introduce additives to grow a stable SEI that prevents the co-intercaaltion of the solvent.
In chapter 2, for the first time, without altering the most commonly used commercial LIB electrolyte (LiPF6 in ethylene carbonate and dimethyl carbonate) or introduction of any additives, we aimed to prevent the co-intercalation of the solvent by only applying a new electrochemical method as a pre-treatment of charge/discharge processes. Different from the commonly used constant current charging method, which always associates with a large voltage jump applied to the electrodes, this pre-treatment applied a slow changing voltage, which gradually decreases from OCV to 0.2 V vs Li/Li+. The voltage pattern instigates a different double layer structure near the electrode, which in turn leads to different SEI structures. The unique structures efficiently prevent blister formation and graphite exfoliation, therefore can be used to largely minimize the irreversible capacity loss of graphite based LIB electrodes.
In Chapter 3, we focus on the most promising alternative anode materials for the LIBs, silicon (Si). Si as an anode has 10 times higher the maximum theoretical capacity of the-state-of-the-art graphite anode materials. Si is also the second-most abundant element in the earth's crust and is environmentally benign. Large (>10 μm) Si microparticles (SiMP) are especially attractive, due to a low production cost. However, the dramatic volume expansion of silicon (~280%) during battery operation leads to mechanical fracture, inducing loss of the interparticle electrical contact and exposure of the highly reactive fresh Si surface to electrolyte, which leads to continuous (SEI) growth, electrolyte consumption resulting in a low Coulombic efficiency (CE). The SEI formed from the currently used carbonate electrolytes can tolerate only a small volume change (~12%) of graphite but is not robust enough to accommodate 280% of volume change of SiMP. Hence, SiMP anodes exhibit an extremely fast capacity drop to less than 60% of the initial value in the first 20 charge/discharge cycles in contrast to the microsized graphite anodes that achieve cycling CE (CE) >99.9% after 10 cycles, ensuring 1000 cycle life. With collaboration with Dr. Wang and Dr. Chen from University of Maryland we demonstrate that the rationally designed electrolyte provides a simple and practical solution to current battery technology without any modification of binder and fabrication methods. A LiF SEI with high interfacial energy in contact with SiMP is key for accommodating deformation of the lithiated Si during cycling. The LiF layer thickness was directly measured for the first time and compared to the commercially available LIB electrolyte SEI layer thickness. Also, the thickness and the roughness of the soft organic SEI layer was measured and compared to the commercially available electrolyte after charge/discharge to find that the new electrolyte enhances the homogeneity of the SEI layer and decreases the thickness of both inorganic and organic sublayers’ thickness.
In chapter 4, we established a standard protocol for measuring the mechanical properties of complicated samples, such as SEI layers, by using AFM technique. We addressed the details and factors impacting on measuring the mechanical properties of SEI via AFM both by calculations and by using standard samples with known mechanical properties. This protocol which is based on contact mechanics, solves the issues related to discrepancies that are observed in the scientific reports about the mechanical property measurement of SEI.
In the following chapters, we explored the correlation between the intrinsic physical properties, in particular, the work function of nanomaterials with their catalytic activities (both as catalysts themselves/or as catalyst supports) in chemical catalysis and electrochemical-catalysis. There are many reported cases on enhancing/altering the catalytic activity of materials by heteroatom doping. However, physical properties of the material such as work function before and after doping has not been well-studied in many cases. The research efforts have been devoted to shed more light on this fundamentally/practically important aspect of the catalysts in areas, taking advantage of our AFM-based work function measurement (Kelvin Probe Force Microscopy, KPFM).
In chapter 5, in a joint experimental and computational study, we aimed at shedding light on the electronic properties of the most industrially used catalyst support, γ- Al2O3. Combined with a variety of surface characterization techniques, as well as simulations based on Density Functional Theory (DFT) by Prof. Pavanello’s group, we studied and compared the structure and electronic properties of phosphorus doped γ-Al2O3 and non-doped γ-Al2O3 nanoparticles to fundamentally understand how phosphorus doping could fine tune the surface structure and electronic properties of γ-Al2O3 nanoparticles. Our experimental results combined with the theoretical calculations agree in finding that P doping of γ-Al2O3 leads to a significant decrease in its work function. The computational models show that this decrease is due to the formation of a new surface dipole, providing a clear picture of the effect of P doping at the surface of γ- Al2O3. In this study, we uncover a paradigm for tuning support-catalyst interaction that departs from details of the chemistry and intimately involves the electrostatic properties of the doped γ- Al2O3 surface specifically, the surface dipole. Our findings open a new pathway for engineering the electronic properties of metal oxides’ surfaces.
In chapter 6 , inspired by our findings about the correlation between the work function and the catalytic activities of our newly synthesized P-doped graphitic carbon (PGC), we aimed at using this compound to explore its possible catalytic activities towards the electrochemical reduction of CO2. CO2 is the major cause of global warming and its lifetime in the atmosphere is between 100 to 800 years. The amount of CO2 in the atmosphere is at its highest ever (over 400 ppm) and it is likely to continue to rise in the future. The most sustainable image that scientists have for removing the CO2 is to recycle it through renewable energy via electrochemical reactions. The combination of our designed electrochemical cell and our catalyst, resulted in the selective production of iso- propanol with the highest Faradaic efficiency reported (by 70%). Phosphorus configuration in the catalyst was identified through XANES characterization with collaboration with Dr. Lockard’s lab. Troubleshooting the reproducibility issue is undergoing. Inductively coupled plasma - optical emission spectrometry (ICP-OES) measurement shows a large concentration of Iron in the catalyst’s raw material. Iron has a low catalytic activity towards CO2 reduction and if any, it can produce the gas products and not the iso-propanol.

ETD_10212

Ph.D.

Includes bibliographical references

doi:10.7282/t3-t73g-za27

ETD doctoral

The author owns the copyright to this work.