High-density silicon electrode for lithium-ion batteries

Abstract

Lithium-ion batteries are considered to be one of the most promising energy storage devices due to its high energy density. Recently, investigation of Si-based anode materials gets much interest due to its exceptionally high capacity. High-capacity Si materials (4200 mAh/g) is highly desirable to replace the conventional graph-ite (<400 mAh/g) for large-scale applications such as in electric vehicles. However, Si-based anodes suffer from poor cycling stability due to its large volume change during repeated Li (de)insertion. Therefore, exten-sive efforts have been made to fabricate nanostructured Si by using variety synthesis method, such as chemi-cal vapor deposition, atomic layer deposition, etching process and solution reduction, and shown highly re-versible capacities using diverse Si nanostructure. However, Current fabrication techniques for Si nanostruc-ture hinder the commercialization of Si electrode due to its cost-effectiveness and low production throughput. Thus, the development of a more effective synthesis method that can produce Si in a cost-effective way is essential for the success of Si anodes in large-scale batteries. In this thesis, scalable and low-cost fabrication strategies for Si nanostructure are developed by using, modifying, or combining nanofabrication techniques such as magnesiothermic reduction, elecrospinning, surface sol-gel. Furthermore, new polymeric binder, which plays a critical role in battery performance, was fabricated and investigated for realizing high-density Si electrode. The use of sand as an almost infinite and extremely low-cost source for the fabrication of nanostructured Si electrodes for Li-ion batteries is demonstrated using a facile magnesiothermic reduction. The adoption of mild vacuum conditions during the reduction allows the achievement of an unprecedentedly high conversion yield (~ 95%). Furthermore, reaction mechanism of magensiothermic reduction is experimentally identified by using ex-situ X-ray diffraction analysis and ex-situ TEM analysis. The excellent cycle stability of the hierarchical Si nanostructure suggests that this facile synthesis strategy from ultra-low-cost sand particles provides outstanding cost-effectiveness and scalability for commercialization of Si electrodes for energy-storage applications. Simple way to fabricate one-dimensional silicon nanostructures by electrospinning combine with chemical reduction was proposed. Silicon nanowires with high porosity were fabricated from electrospun silica nan-owires and showed excellent cycle stability with negligible capacity degradation, compared to carbon-coated silicon nanoparticles. Silicon nanotubes synthesized using pyridine-like nanowires template were evaluated as Li-ion battery anodes. Hybrid pyridine/silica core-shell nanowires prepared by surface sol-gel reaction were converted to silicon nanotubes by pyrolysis in air

this was followed by the reduction to silicon nanotubes via Mg reduction. The silicon nanotubes electrode showed high capacity with excellent capacity retention It shows excellent high performance in term of cycleability compared to spherical Si nanopowder indicating that controlling the morphology as well as nanoscale dimension is important on battery performance. A highly cross-liked polymeric binder with enhanced mechanical properties was used for high-capacity Si electrode. The dependency of the degree of crosslinking on the pH value and the differences of strength of interaction between binder and Si active materials were investigated. The results demonstrate that the degree of crosslinking and side chain of polymeric binder has a profound effect on the cell performance. To summarize, nanofabrication techniques with low-cost and high yield are widely investigated to fabricate Si nanostructure, such as hierarchical structure, nanowire and nanotube. The electrode with the fabricated nanostructured Si and highly cross-liked polymeric binder show excellent battery performance. These results indicate that the strategies for nanofabrication suggested in this thesis will attract considerable interest in various application fields that require the use of mass-scale and high loading.