Analysis and Modification of Al Current Collector Surface for High Performance Lithium Batteries

高功率型鋰電池的開發主要著手於降低其在電化學系統中的阻抗。在一般電池結構中，電流收集板擔任著在活性材料和外部線路之間聯繫電子傳輸的角色。然而鋁箔作為在鋰電池正極中最常被使用的電流收集板，其表面原生氧化層和較差活物的貼合性卻阻礙了其與活性材料之間的電子傳輸，造成表面電阻過高，進而造成電極的極化和降低其功率性能。為了降低此表面電阻，數種表面改質方式將會在本論文中被討論。 首先，我們將鋁箔表面進行電漿處理，希望透過使用改質過的電流收集板來提高鋰離子電池的電化學性能。在電漿處理中，來自於電漿槍的高能離子束能對基板表面進行轟擊也能透過電漿體輔助化學氣相沉積法對基板進行碳披覆。為了增進此碳層的結晶性，我們也對其進行550℃的退火處理，最後得到一奈米級高貼合性及高導電性碳披覆的鋁箔電流收集板。改質過鋁箔的表面特性，包括形態，化學成分，粗糙度和導電率分佈將被X-ray射線光電子能譜（XPS），拉曼，接觸角和導電式原子力顯微鏡（C-AFM）加以分析。結果顯示改質過的鋁箔表面疏水性和粗糙度都有增加，因此推測其能有較佳活物貼合性。更重要的是部分導電碳能穿透原來阻礙電子傳輸的原生氧化層，進而提升了鋁箔表面導電性。最後在電化學測試下，使用此改質過碳披覆的鋁箔電流收集板的鋰鐵磷(LiFPO4)電極有著較低的表面電阻，且能在高速充放電下(10C)保有較高的容量和更少的極化，以及較佳的循環穩定性。 再者，我們直接使用導電顆粒塗佈加上輾壓對在鋁箔表面進行改質，並研究其對表面電阻的影響。對於此導電塗層中使用不同的粘合劑和導電粒子，我們將以基板的貼合性，顆粒附著於在Al表面表面的外型，和電極的表面電阻進行比較。由於跟Al有著優異的貼合性，海藻酸鈉被選為最佳的導電層黏著劑。石墨片（GF）的塗佈主要增加了鋁箔導電均勻性，導致的所搭配的LiFePO4電極能有更佳的高速充放電電容量。然而，高電位下PF6-會嵌入其石墨結構中，限制其穩定電位操作區間小於4.5 V。碳黑（CB）和TiC粒子則在輾壓後具有更好的氧化鋁層穿透能力，從而導致所組成的LiFePO4電極有著更小的表面電阻。此外，其穩定電位操作區間可高達5V，能滿配合高電壓鋰鎳錳氧(LMNO)電極的電位範圍，並能明顯提高了其的高功率性能。 最後，我們將表面改質電流收集板的應用擴展到鋰硫電池領域。先透過分析電池漏電流和利用TXM觀察S的形貌改面來對其反應機制進行研究。漏電流的大小隨電壓變化可以用來分析正極和電解質的化學電位狀態。而利用TXM同步觀察脫離電極的硫顆粒的形貌隨著電池充放電的變化，我們發現到其在電解液中隨著放電而溶解和隨著充電而再沉積的現象。這個現象可以由多硫化鋰在電解液中的歸中反應及歧化反應加以解釋。由此推導出鋰硫電池的一條新反應路徑，並指出電極中的導電結構會直接對其反應速度造成影響。因此我們透過使用表面改質的鋁箔電流收集板，提升電極的導電均勻性，並顯著的提升了鋰硫電池的高功率使用下的電性。The development of Li ion batteries (LIBs) for high power performance relies on the decrement of overall resistance in an electrochemical system. An important part of the battery configuration is the current collector which plays an essential role on transporting the electron between active materials and external circuit. Al foil is the most commonly use current collector for cathodes of LIBs, but the unavoidable Al oxide layer and the poor contact with the active material hinder the electron transport at the interface. The later high interface resistance gives rise to a large polarization at the electrode and decreases the power performance of LIBs. In this context, several methods to reduce this surface resistance are discussed in detail in this thesis. Firstly, the plasma-surface-treated Al-current collector is studied for enhancing the electro-performance of Li-ion battery. The high energy plasma process by ion beam not only bombards the substrate surface but also produces plasma assisted CVD carbon coating. Thermal annealing at 550 oC is carried out in order to change the crystallinity of this carbon layer. This novel synthesis process leads to the formation a nano-scaled, strongly adhered robust conductive graphitic layer. The surface properties of the treated Al foil including morphology, chemical composition, roughness, and conductivity distribution have been characterized with several surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS), Raman, contact angle, and conductive atomic force microscopy (C-AFM). The result indicates that during this surface treatment the surface changes to hydrophobic along with increasing roughness. Most importantly, the resulting conductive layer penetrates into the native oxide layer which hinders the electron transport. LiFePO4 electrode using this C-coated Al current collector exhibits remarkably reduced charge-transfer resistance, significantly enhanced capacity and less polarization at high C rate (10 C), along with improvement in cycle stability. Moreover, a direct conductive particles coating on Al foil by pressing was studied to change the surface resistance. The contact force, morphology at Al surface and the surface resistance of the electrodes are compared with different binders and conductive particles in this conductive coating layer. It was found that alginate is the suitable binder because of its superior binding force on Al foil. The graphite flake (GF) coating increases the conductive homogeneity at Al surface, resulting in a better rate capability of LiFePO4 electrode. However, the PF6- insertion limits its stability at higher potential than 4.5 V. Carbon black (CB) and TiC particles might have better ability to penetration the Al2O3 layer after pressing, thus resulting in less surface resistance of LiFePO4 electrode. Besides, the high stability up to 5 V of the CB and TiC in the battery can meet the demand of lithium manganese nickel oxide (LMNO) electrode and dramatically improve their high power performance. Finally, to extend the application of surface modified Al current collector to Li-S battery, the reaction mechanism of the Li-S battery is first studied by leakage current analysis and TXM observation. The leakage current change at different voltage presents the change of chemical potential state at the cathode and electrolyte. By in-operando TXM observation on the electron isolated S particles, we can observe dissolution during the discharge process and re-deposition during the charge process. This phenomena can be inferred as a proportionation/ disproportionation reaction of polysulfide in the electrolyte. Thus, a new reaction path is proposed based on this study, and it is remarked that the conductivity at the electrode matrix is essential for determining the reaction rate. By using the surface modified Al current collector, we increase the conductive homogeneity of the conductive matrix in the electrode, thus the rate performance of Li-S battery obtains a dramatic enhancement