Nanostructured silicon anode and sulfur cathode for lithium rechargeable batteries

Abstract

Silicon and sulfur are both attractive electrode materials for next-generation rechargeable lithium batteries because of their abundance, high specific capacity, and low cost. This thesis is mainly focused on the research progress I made on silicon-based anode materials and sulfur-based cathode materials for rechargeable lithium batteries with improved energy density, power density, and cycle life. ? In Chapter 1, the working mechanism of lithium-ion battery, and commercial anode/cathode materials are introduced, which serve as the background information for the following chapters. Then in Chapter 2, the research progress I made in silicon-based anode is presented. To tackle with the problems of silicon anode, such as poor cyclability and early capacity fading due to significant volume change during lithiation and delithiation process, I fabricated a coaxial silicon / anodic titanium oxide / silicon (Si-ATO-Si) nanotube array structure grown on titanium substrate demonstrating excellent electrochemical cyclability. The ATO nanotube scaffold used for Si deposition has many desired features, such as rough surface for enhanced Si adhesion, and direct contact with the Ti substrate working as current collector. More importantly, the ATO scaffold provides a rather unique advantage that Si can be loaded on both the inner and outer surfaces, and an inner pore can be maintained to provide room for Si volume expansion. This coaxial structure shows a capacity above 1500mAh/g after 100 cycles, with less than 0.05% decay per cycle. ? In Chapter 3 and Chapter 4, two kinds of sulfur-based cathode materials are reported. In Chapter 3, a generic and facile method of coating graphene oxide (GO) on sulfur particles is presented. The applications of sulfur/GO core-shell particles as Li-S battery cathode materials are further investigated and the results show that sulfur/GO exhibit significant improvements over bare sulfur particles without coating. Galvanic charge-discharge test using GO/sulfur particles shows a specific capacity of 800 mAh/g is retained after 1000 cycles at 1 A/g current rate if only the mass of sulfur is taken into calculation, and 400 mAh/g if the total mass of sulfur/GO is considered. Most importantly, the capacity decay over 1000 cycles is less than 0.02% per cycle. The coating method developed in this study is facile, robust, and versatile, and is expected to have wide range of applications in improving the properties of particle materials. ? In Chapter 4, in order to further improve the power density of sulfur-based cathode material, a method of fabricating graphene oxide (GO) wrapped porous carbon/sulfur composite (porous C-S) for high performance lithium-sulfur (Li-S) battery cathode material is reported. A porous C-S composite using conductive porous carbon as framework and sulfur within its channels as filler is synthesized to generate essential electrical contact to the insulating sulfur, thus achieving high specific capacity. Further graphene oxide wrapping over porous C-S is used to mitigate the problem of intermediate polysulphide dissolution into electrolyte during lithiation/delithiation. The electrochemical performance of GO wrapped porous C-S (porous C-S/GO) is investigated, and the results show that porous C-S/GO exhibits significant improvement over bare sulfur and porous C-S without GO wrapping in terms of specific capacity and cycling stability, respectively. Galvanic charge-discharge test using porous C-S/GO shows that a specific capacity of 600 mAh/g is retained after 600 cycles at 1 C (=1673 mA/g) current rate if the total mass of porous C-S/GO is considered. Our study proves that graphene oxide can provide a chemically stable interface between active material and binder to keep active materials attached to electrodes during cycling. This could serve as an important guideline for future sulfur-based cathode materials design.