| dc.description.abstract | Graphite anode has become the mainstream of negative electrode for lithium battery recently. However, because of its low theoretical capacity of 372 mAh/g, and the low capacity retention rate during charge and discharge under high current density, scientists are actively pursuing a new type of material - silicon, which has a higher theoretical capacity than that of carbon negative electrode. Silicon is a high-profile material, because its theoretical capacity (about 3579 mAh/g) is about ten times higher than that of traditional carbon negative electrodes, however it suffers from continuous volume expansion and contraction and the continuous decomposition of the electrolyte during charging and discharging process. The continuous formation of the SEI layer eventually embrittles the material, resulting in lower electronic conductivity and less-than-ideal battery life. This study will explore a novel photopolymerization method to cure electrode, using photocrosslinking reaction to form a network structure to protect silicon anode materials. It is expected to overcome the aforementioned difficulties, and evaluate the feasibility of this technique for high-energy lithium-ion batteries in the future.
In the first part of this study, we explore best water based binder formula documented in literature via thermal polymerization using polyethylene oxide (PEO) as well as polyvinyl alcohol (PVA) binded with polyacrylic acid (PAA) . This binder is well-coated on the silicon electrode, and shows good adhesion with peeling tests and fair electrochemical performance. In the second part, we compared results by several different photo-crosslinkers formulations: G3E(glycerol propoxylate triacrylate, G3POTA),YA(aliphatic urethane acrylate), U-60(aromatic urethane acrylate) and we tried only PAA to photopolymerize to compare as the electrode binder, and tested their long-term cycle life stability under 0.2C. In terms of electrochemical performance, PAA directly photopolymerized maintains 450 mAh/g after 100 cycles at 0.2C, which is the best among all formulas. However, after 3M tape peeling test, it was found that the active material and the adhesion between copper foil is still poor. After adding UV binders, although the adhesion can be enhanced, they do not exhibit substantial improved cycle life performance, in fact the performance is even worse than that of PAA under UV curing , which may be related to the pore size of lithium ions transport between the active material. Here we speculate that adhesion is not the only factor determining battery life.
Further, we use scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for in-depth investigation. Experimental data confirmed that the photocrosslinker is indeed involved in the formation of the SEI layer, and can slow down the consumption and decomposition of the electrolyte and lithium salt. We believe that UV binders can tightly cover the electrode, preventing the continuous capacity fading caused by the continuous reaction of the silicon negative electrode with the electrolyte.
Finally, we improve the cycle life performance of the photopolymerization by adding electrolyte additive FEC. In this study, we choose 4% G3E, which shows good adhesion between the active material and the copper foil and it also shows better cycle life performance. In the cycle life, it can be found that after 100 cycles of charging and discharging at 0.5C and 1C, the capacity can be maintained at 904 mAh/g and 785.6 mAh/g, and after 0.5C 200 cycles the capacity maintained at around 340 mAh/g. Compared with no addition of FEC, the capacity remains only 404 mAh/g after 100 cycles at 0.2C. This experimental prove that the electrolyte additive can not only further reduce the consumption of electrolyte and lithium salt but can prolong the cycle life of the silicon anode.
Concluding from these results, although photopolymerization does not yield superior capacity retention over water based thermal polymerization, it alleviated the time consuming and high energy issues associated with the drying process in traditional water based electrode fabrication. | en_US |