Multifunctional Natural Polysaccharides for Energy Storage Applications

Abstract

An agarose is a polysaccharide material, generally extracted from seaweeds. Agarose is a linear polymer made up of the repeating unit of agarobiose, which is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Many applications of agarose are described in the literature from biology to energy field, such as gel electrophoresis, protein purification, solid culture media, motility assays, template for the fabrication of porous structures, binder, separator membrane, and carbon-coating material for lithium-ion batteries (LIBs). Silicon (Si) has attracted much attention as promising anode material due to its high theoretical capacity (3579 mA h g-1 with composition of Li15Si4 at room temperature), relatively low working potential (< 0.4 versus Li/Li+), its abundance in nature, and low cost.7-10 However, the large volume change of Si (> 300% with composition of Li3.75Si at room temperature) during lithiation/delithiation leads to a serious aggregation of Si, the formation of unstable thick solid-electrolyte-interface (SEI) layers and depletion of electrolyte, which will make critical capacity fading. Furthermore, Si has low electrical conductivity and sluggish lithium-ion diffusivity. These fatal flaws prevent the commercialization of Si anode. There are several solutions to overcome these drawbacks, including Si composites with metal oxide, inactive metals, or carbon materials and Si nanostructuring (e.g., nanoparticles, nanowires, and nanotubes). Another attempt has been tried to develop functional polymeric binders which can alleviate severe volume change of Si anodes. Actually, overall quality of batteries depends on performance of binders, because polymeric binders give adhesion between electrode and current collector, allowing long-term cycling stability. Natural polysaccharide was used as polymeric binder for Si anodes, because it contains many functional groups, which are expected to generate strong adhesion between binder and active material. In chapter II, we demonstrate eco-friendly, abundant natural polysaccharide as a binder for Si-based anode, Si/C composite materials consisting of the Si foam dispersed in hard carbon (HC) synthesized by using agarose, and LiMn2O4 cathodes. Si foam@HC@C was successfully synthesized by a simple carbonization method. The nanostructured Si foam and agarose binder containing many functional groups enables to strong adhesion between Si foam and current collector, leading to enhanced electrochemical properties, including a high specific capacity of 1028 mAh g-1 (60% retention compared to 2nd cycle) and outstanding cycling performance after 200 cycles. Si foam@HC@C electrode showed first discharge capacity and discharge capacity were 654 and 513 mAh g-1 with enhanced initial coulombic efficiency of 78.4%, compared to HC@C with initial coulombic efficiency of 71.6%. LiMn2O4 cathode with agarose binder exhibited high initial coulombic efficiency of 96.2% and stable cycling performance with nearly 100% coulombic efficiency. These results indicate agarose binder can be used for both of anode and cathode due to good electrochemical stability in wide operating voltage. In chapter III, Si/Al2O3 foam particles were simply synthesized by the chemical etching of the Al?Si alloy and a subsequent selective thermal oxidation process. The Si/Al2O3 electrodes with tunable Al2O3 thickness exhibited highly stable cycling performance, excellent rate capability, and suppressed volume expansion. This strategy opens up an effective way to introduce various protecting layers on the surface of other inorganic materials.