Silicon Anodes for The Next Generation Ultrahigh Capacity Lithium-ion Batteries

Abstract

The potential for transforming lithium-ion batteries and the capabilities of future transport vehicles is a huge one given that the capacity of the battery is key in determining the range the vehicle can travel to prior to charging. Lithium ion battery technology is a promising field for the present time and the next few decades. Its value lies in 100s of millions of portable devices, EV and other form of energy storage systems dependant on this technology. Alternative materials such as silicon are being investigated to be part of the lithium ion battery anode composition. That is due to its high capacity compared to other materials such as graphite. Developing more stable lithium ion batteries involves investigating; the different part of the battery structure. This thesis investigates the development of silicon anodes for lithium ion batteries, using various techniques and procedures; and provides a series of results and contributions to the scientific community. This thesis starts with an in-depth literature review for battery technology and the characterisation techniques used in the latest cutting-edge battery research. It explains a general yet comprehensive overview of battery technology including; the history of this technology, the different types of batteries and materials used. The thesis then investigates the optimisation of anode synthesis, followed by detailed studies of synthesised anode composites and characterisation techniques used generally in this field. The prime focus in this thesis is on silicon anodes due to their ultrahigh capacity and potential for future applications. This is explained via a comprehensive study of silicon, silicon/graphite, and silicon/graphite/BTO using two different electrolytes. Using various testing and characterisation techniques including battery cycling system, XPS, SEM, Raman and others; this work then investigates the development and transformation of materials on the surface of the anode before and after cycling, and correlates these findings with the battery performance. Amongst the findings of this work is the benefit of using BTO in the silicon composite which significantly enhances the performance of the battery compared to pure silicon anode. This is observed through the cycling performances of the synthesised cells. The XPS scans for the enhanced silicon/BTO anode at different cycles provide a novel set of results showing slower degradation of the electrolyte, a more stable cycling performance on the longer cycles, and slower irreversible degree of permanent lithium intercalation on the anode compared to the pure silicon anode. The Raman spectrum on the other hand shows a strong direct relation with the degree of crystallinity of the silicon nano powders and the capacity of the battery. The higher the degree of crystallinity the better the specific capacity and the columbic efficiency of the battery. The SEM images also provides an insight on the degree of accumulation of the electrolyte on the surface of the anode after cycling. An in-depth XPS mapping study revealed that within the same anode, regions with very different properties can be found with various degrees of crystallinity (for silicon) and various materials species. Within this mapping observation, the volume expansion of silicon nano powders is recorded and correlated with elemental species observed via long XPS scans. Finally, graphite is added to the silicon/BTO composite and recorded a more stable battery performance mainly due to the high level of conductivity graphite particles provide to the silicon particles. The thesis concludes with detailed list of recommendations and observations, and provides a vision for a future research and development led by in-operando XPS scans and mapping for lithium-ion batteries to observe the development of elemental species on the surface of the anode as they occur. This would help boost the effectiveness of XPS studies for battery research and would provide a better understanding of the changes that happen inside the battery, which would help the development of batteries with higher capacity, more stable and more promising battery technology.