Date of Award
Doctor of Philosophy in Chemistry
Brett L. Lucht
Lithium ion batteries have become the most widely used and state of the art energy storage device in the current century, due to its high volumetric and gravimetric capacity, which makes portable electronic devices a possibility. The new and improved battery material have further increased the energy density of lithium ion batteries which made it possible to be used in high power applications such as electric and hybrid electric vehicles, aerospace applications as well as grid energy storage.
Many problems still exist with the use of Li-ion batteries, such as safety, cost, material availability, environmental impact of used material, and etc. With the gain of the popularity of these devices, the need for even higher energy density keep on arising. Researchers have constantly been working on understanding the reactions taking place in the batteries as well as developing materials to improve the inherent problems of these systems.
One major concern is the inability of li-ion batteries to work in a wide temperature window. Batteries tend to freeze at low temperatures during winter. Although this does not destroy the battery, it makes it harder for use and reduces the reliability. Also, at higher temperatures, due to the use of organic solvents, batteries have the tendency to catch fire. One approach to overcome this problem is to use a solvent like propylene carbonate, which has a working temperature range from -42 to 240 °C. But propylene carbonate tends to co-intercalate into graphite anode and reduce in-between the graphite layers leading it to exfoliate and destroy the cell. In this study alkali metal ions were used as additives to understand the effect of them on the solid electrolyte interphase, that can improve the overall cell performance and use of propylene carbonate as a co-solvent.
Another major problem is the low gravimetric capacity of the commercially used graphite anode. Many scientists have been working on using high energy density material such as silicon, which has ten times the capacity of graphite, as an anode material. But silicon has a propensity to change its volume up to ~300% in the charge discharge cycle which leads to cracking of the silicon particles and continuous consumption of the electrolyte.
Binders play a large role in the performance of silicon-based electrodes. The binder not only helps in the direct mechanical properties of the electrode, but also participate in the solid electrolyte interphase (SEI) formation. Most studies conducted by the research community show the use of polymers as binders. This study shows that the use of single molecules with similar functional groups, can still act as good binders as they react on the surface of silicon to form a better SEI.
One study shows that citric acid, which is a tri carboxylic acid, when used as a binder gives similar performance to polyacrylic acid, which is a polycarboxylic acid binder. When looking at the surface, lithium citrate was observed on the SEI. This gave an insight to functionalize the surface of silicon nanoparticles with citric acid, in order to form an artificial SEI containing lithium citrate. These surface modified nanoparticles have shown better performance with the conventional binders compared to unmodified particles consistent with the findings in the previous study.
Many natural polymeric gums such as chitosan, guar gum, xantham gum, pine resins etc, have been studied as binders. This led us into using a single molecular natural glue, casein, a cheap, alternative binder material for silicon based electrodes. This has shown better performance compared to the conventional PVDF binder.
Koggala Wellalage, Dilni Kaveendi Chandrasiri, "THE EFFECT OF ADDITIVES AND SURFACE MODIFYING AGENTS ON THE SOLID ELECTROLYTE INTERFACE IN LITHIUM ION BATTERIES" (2019). Open Access Dissertations. Paper 863.