鋰離子電池負極材料表面改質製備與研究

Abstract

首先，本研究提出製備一多功能混合高分子的人造-固相電解液介質(A-SEI)的合理設計，應用於鋰離子電池，在電極中實現良好的固相/電解質界面性質的一個新概念。其中含有兩種高分子，即聚乙二醇辛基苯基醚(C14H22O(C2H4O)n) (PEGPE)和聚(烯丙基胺) ((C3H5NH2)n)，對石墨和石墨-Si複合材料負極而言，伴隨著機械堅固的高性能高分子電極塗層，有四個量身定做的官能基。因為塗層的醚和胺類官能基能螯合鋰離子，在A-SEI塗層可以充當一個緩衝器區，在電解質和石墨之間提供鋰離子配位狀態逐漸變化的離子傳送。這種變化降低了界面能量勢壘，以及在溶劑化/去溶劑化過程，從而加速離子傳輸速率。此外，該兩種高分子組成之間的氫鍵和PEGPE帶有的芳族結構，提供和石墨表面足夠的機械強度和界面粘合性，以及芳族結構與石墨之間的pi-pi相互作用，進而在電池循環過程中可以保護石墨負極的剝離現象，或避免石墨/矽複合材料顆粒的分離。因此，開發A-SEI大大提高了電容量可逆性、循環穩定性，並且特別是高速率的所有負極材料，包括天然石墨、人造石墨、和矽納米顆粒-石墨複合材料的性能。 其次，開發一基材誘導凝聚法(SIC)和靜電自組裝(ESA)的高分子鍍膜的組合式製程，可以獲得導電性添加劑分散良好的鋰離子電池負極材料的結構，這是由輕便實驗實施所製和可擴展的流程。此提升分散穩定性的導電性添加劑，例如炭黑(CB)或碳納米管(CNT)，形成連續的三維網絡結構，導致活性材料之間的電子傳導速率提高。此外，此均勻的分散結構的電極孔隙率形成完整的網絡，方便鋰離子在電解質中的傳輸。電化學實驗表明，該電極的放電功率和循環性能和導電性的添加劑的分散性是一致的。此外，鋰的嵌入和嵌出的接觸電阻和電荷遷移電阻是由於導電性顆粒的均勻分佈所至，導致了較少顯著電極極化。 最後，矽/石墨複合材料，由於能量密度的提高，為新一代鋰離子電池理想的負極材料。商業產品中採用含有少量的改性的矽或矽氧化物來實現高容量矽/石墨複合材料負極。在本研究中，我們研發了靜電自組裝分散技術和混合式表面改質方法的簡便和可擴展的製程，製備矽納米顆粒均勻地被包覆在石墨、碳和高分子之間，形成微米級尺寸的複合材料。此外，所合成的複合材料是非常理想的優異負極，因為它不僅方便了電子的傳導和鋰離子的擴散，也使得彈性和機械緩衝矽納米顆粒於鋰插入和提取過程過程中的體積變化。

Firstly, this research presents a new concept for forming an artificial solid-electrolyte-interphase (A-SEI) for Li-ion battery (LIB) electrodes based on the rational design of multifunctional polymer blend to achieve favorable solid/electrolyte interfacial properties. It leads to the successful development of a mechanically robust and high-performance polymeric electrode coating containing two polymers, namely polyethylene glycol tert-octylphenyl ether (C14H22O(C2H4O)n) (PEGPE) and poly(allyl amine) ((C3H5NH2)n ), with four tailored functional groups for graphite and graphite-Si composite anodes. Because the ether and amine functional groups of the coating can chelate Li ions, the A-SEI coating can act as a buffer zone providing a gradual change in the coordination state of Li ions transferring between electrolytes and graphite. This change lowers the energy barrier to the solvation/de-solvation processes and thus accelerates the ion transfer rate. Moreover, the hydrogen bridge bonding between the two polymer components and the pi-pi interaction between the aromatic structure of PEGPE and the graphite surface yield sufficient mechanical strength and interfacial adhesion to protect graphite anodes from exfoliation and graphite/Si composite particles from disconnection during cycling. Therefore, the developed A-SEI considerably enhanced the capacity reversibility, cycle stability, and, in particular, high-rate performance of all of the studied anode materials, including natural graphite, artificial graphite, and a Si nanoparticle-graphite composite. Secondly, a structure with well-dispersed conductive additives as Li-ion battery anode material is manufactured by the combination processes of substrate-induced coagulation (SIC) and polymer coating by electrostatic self-assembly (ESA), which are implemented experimentally by the facile and scalable processes. This results in the enhancement of the dispersion stability of conductive additives such as carbon black (CB) or carbon nanotube (CNT) and the improvement of interconnection between active materials due to a continuous three-dimensional network for electron conduction. Moreover, the electrode porosity in this uniform dispersion structure forms the complementary network for Li-ion transport in the electrolyte as compared to those distributed with agglomeration. Electrochemical experiments indicated that the rate capabilities and cycle performance of the electrodes are consistent with dispersion properties of conductive additives. Furthermore, the contact resistance and charge transfer resistance for Li-insertion and Li-extraction are smaller due to the homogeneous distribution of conductive particles, which leads to a less significant electrode polarization. Finally, Si/graphite composites are ideal anode materials for LIBs due to the enhancement of energy density for next generation LIBs. Advanced commercial products comprising graphite anodes modified with a few percent of Si or Si oxide to achieve high capacity have been announced. In this research, we demonstrate an electrostatic self-assembly dispersion technique and a hybrid surface modification method as the facile and scalable processes to prepare hierarchical micro-sized composites of Si nanoparticles encapsulated in natural graphite and layers of carbon and polymer. In addition, the as-synthesized composite is remarkably desirable for a superior anode because it not only facilitates the conduction of electron and the diffusion of Li ion but also renders both elastic and mechanical buffer to accommodate the large volume change of Si nanoparticles during Li insertion and extraction process.