| 摘要: | 石墨碳負極已成為目前鋰電池負極的主流，然而其理論電容量約為372 mAh/g左右，在高電流密度下充放電的電容保持率也較低。因此，科學家正積極開發一種理論電容量更高的新型材料-矽。矽擁有備受矚目的理論電容量（約3579 mAh/g），高出傳統石墨負極約十倍以上。然而，矽在充放電過程中體積不斷膨脹、收縮以及電解液分解生成的固體電解質層（SEI），最終使材料脆化，導致電子導電度較低，影響電池壽命。
本研究旨在探索一種新型光聚合方法來固化電極，利用光交聯反應形成網狀結構以保護矽負極材料。這種方法有望克服前述困難，並評估其在未來高能量鋰離子電池中的可行性。
研究的第一部分，我們將通過熱聚合反應，利用聚乙二醇（PEO）和聚乙烯醇（PVA）與聚丙烯酸（PAA）結合，探索文獻中記載最佳水性粘合劑配方。這種粘合劑能有效地覆蓋在矽電極上，通過剝離測試顯示良好的附著性，並呈現出令人滿意的電化學性能。電性表現也相當出色（PAA-PVA配方經0.2C 100個循環後電容量達764 mAh/g）。
其後，我們比較了幾種不同的光交聯劑配方，包括甘油丙氧基三丙烯酸酯（G3E）、脂肪族聚氨酯丙烯酸酯（YA）、芳香族聚氨酯丙烯酸酯（U-60），以及僅使用PAA進行光聚合以進行電極粘合。並測試了它們在0.2C下的長期循環穩定性。令人驚訝的是，直接光聚合的PAA在0.2C下經過100次循環後仍然保持著最高的電容量（450 mAh/g）。然而，在進行3M膠帶剝離測試後，發現活性材料和銅箔之間的附著性仍然不夠。添加紫外光交聯劑可以增強附著性，但令人失望的是，在紫外光固化下，性能甚至比PAA更差，這可能是交聯劑阻滯了鋰離子在活性材料之間傳輸的。這裡可以推測，附著性並非唯一影響電池壽命的因素。
為深入研究，我們使用掃描電子顯微鏡（SEM）、X射線光電子能譜（XPS）、循環伏安法（CV）和電化學阻抗譜（EIS）。實驗數據證實光交聯劑確實參與SEI層的形成，能夠減緩電解液和鋰鹽的消耗和分解。這表明紫外光交聯劑能夠緊密覆蓋電極，防止矽負極與電解液不斷反應而導致的持續容量衰減。
最後，我們通過添加電解液添加劑FEC來改善光聚合的循環壽命性能。在本研究中，我們選擇了4% G3E作為最佳粘合劑，它顯示出良好的活性材料與銅箔之間的附著性，同時具有較好的循環壽命性能。在0.5C和1C下經過100次充放電循環後，電池容量保持在904 mAh/g和785.6 mAh/g左右。在0.5C下經過200次循環後，容量保持在約340 mAh/g。相比於無添加FEC的情況，在0.2C下經過100次循環後，容量僅保持在404 mAh/g。這些實驗結果證明電解液添加劑不僅可以進一步減少電解液和鋰鹽的消耗，還能延長矽負極的循環壽命。
綜上所述，雖然光聚合並未在電容保持率方面超越水性熱聚合，但它消除了傳統水性電極製備過程中耗時且高能耗的乾燥步驟。通過解決矽負極的挑戰，光聚合展示了其作為提升高能量鋰離子電池性能和壽命的可行技術。
;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. |
