Analysis and Modification of Al Current Collector Surface for High Performance Lithium Batteries
Date Issued
2015
Date
2015
Author(s)
Lee, Rung-Chuan
Abstract
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.
Subjects
Al foil
current collector
plasma
carbon coating
surface resistance
Li-S battery
Type
thesis
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