Modification of Polysaccharides through Sulfonation and Graft Copolymerization as Multiple Functional Binders for Anodes and Alginate Coated Core-shell Si-based Compound as Anode in Lithium-ion Batteries

Abstract

The investigation on active Lithium-ion battery materials (such as Silicon-based anode, NCM cathode and so on) with ever-increasing energy density draw much more attention, whereas the limits of conventional auxiliary materials, such as binders and conducting additives are being researched. Binders adhere active substances to active substances, active substances to current collectors as well, yielding an interconnected electrode structure that ensures mechanical integrity during the lithiation/delithiation process. Even though the battery binder only accounts for a fraction of battery weight and cost, it is a bottleneck technology in the development of high energy density active materials that experience significant volume variation and side-reactions. In the other hand, most electrodes are using poly(vinilydene fluoride) (PVDF) as binder, but this fluorinated polymer is expensive and requires the use of a volatile and toxic organic solvent such as N-methyl-pyrrolidone (NMP) in the processing. The toxic NMP solvent not only damage the environment, but also harm the health of operators when manufacture the lithium-ion batteries. Hence, the development of novel eco-friendly, low cost and water-soluble binders which recently have gained increasing attention as a promising performance booster for lithium ion batteries with high energy density. Such water soluble polymer binders are either natural, or modified, or synthesized, and they were observed with profoundly enhanced chemical/physical interactions with the electrode materials, stronger mechanical adhesion and evidently improved volume variation durability, leading to dramatic improvements in the electrochemical performances of Si-based anodes, spinel/layered oxide cathodes and S cathodes. In our first research, the water-soluble polysaccharides binders were modified to improve the ionic conductivity. Various polysaccharides have been much attention to potential binder candidates for lithium-ion batteries due to their strong adhesion through number of hydroxyl and carboxyl functional groups as well as water soluble characteristic. On the contrary, the endeavor to improve lithium ion transport through polysaccharide binders has been hardly done. For this purpose, the sulfonation of traditional sodium alginate (Alg) and carboxymethyl cellulose (CMC) binders are performed to introduce sulfo (SO3H) functional groups to the backbone of the polysaccharides. The sulfonation increases the ionic conductivity of the polysaccharide at least 2 mS/cm higher than, compared to unsulfonated polysaccharides in the solution state. Such an increase in the ionic conductivity of binder results in the enhancement of cyclic performance of Li4Ti5O12 (LTO) electrodes. For instance, the sulfonated CMC-containing LTO electrode shows 153.2 mAh g-1 at the 100th cycle, whereas the unsulfonated CMC-containing LTO electrode has 117.6 mAh g-1. The difference becomes more significant as the charge/discharge current rates increase. Besides, various characterization demonstrates that the sulfonated polysaccharides own superior electrochemical properties when used to LIB binder. In our second research, water-soluble conductive binder system would be designed to enhance the electrochemical performance for pure Si anode. Alginate is a kind of natural polysaccharides binder which possess much amount of hydroxyl and carboxyl groups, leading to a good adhesion strength for active materials, especially for Si-based anode. Herein, the adhesive property of alginate was boosted by graft copolymerization with acrylic acid (AA) and butyl acrylate (BA) forming Alg-ABAA binder. AA monomer provides abundant hydroxyl groups which can improved the adhesion strength between Si anode and copper foil, in addition, monomer BA could endow the new binder flexibility. After then, the conductive two dimensional (2D) layered material MXene was connected to the binder Alg-ABAA, the water-soluble conductive binder Alg-AABA-Mx was well synthesized. When employed the binders Alg-AABA and Alg-AABA-Mx to the pure nano Si particles (SiNPs) anode, the capacity of electrodes was dramatically increased. In the case of Alg-ABAA-Mx/SiNPs electrode, even the capacity is lower than Alg-ABAA/SiNPs electrode, it shows an excellent performance in high rate cycling. In the last research, a type of a core-shell Si-based compound was well synthesized. It is well known that Silicon was supposed to the next generation anode material for lithium-ion batteries due to (1) its specific capacity of 4,200 mAhg-1 and volume capacity of 9,786 mAh cm-3, the highest known for a LIB anode; (2) relatively low working potential (0.5 V vs. Li/Li+); and (3) the natural abundance of element Si and its environmental benignity. However, the practical implementation of Si anodes is still blocked due to three major problems. First, poor cycle-life of silicon materials results from pulverization during the huge volumetric fluctuations (>300%) which accompany lithium ion intercalation and deintercalation. Second, drastic irreversible capacity loss and low coulombic efficiency is caused by mechanical fracture of Si anodes during the alloying/dealloying process. Finally, the solid electrolyte interphase (SEI) breaks as the nanostructure shrinks during delithiation. This results in the exposure of the fresh silicon surface to the electrolyte and the reformation of the SEI, resulting in the SEI growing thicker with each charge/discharge cycle. Our purpose is to prolong the cycle life of electrode when using the Si-based material as anode. The polysaccharide alginate was coated on the surface of SiNPs through chemical bond by amidation reaction, therefore the formed core-shell Si-polymer material was used to the lithium-ion batteries as anode. During the charge and discharge process, the core-shell Si-based anode could alleviate the pulverization and keep a long cycle life, maintain a stable and relatively high capacity. Even the inner resistance of the Si-Alg electrode is increased compare to the pure SiNPs electrode, the sacrifice got the stable and long cycle in return.