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Title: LixSi anode materials for rechargeable Li-ion thin film batteries
Other Titles: Ke chong fang li li zi bo mo dian chi li gui he jin yang ji cai liao
Authors: Deng, Haixia (鄧海霞)
Department: Dept. of Physics and Materials Science
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Anodes
Fuel cells -- Materials
Storage batteries -- Materials
Notes: CityU Call Number: TK2941.D46 2007
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2007
xix, 173 leaves : ill. (some col.) ; 30 cm.
Type: Thesis
Abstract: This work aims at the investigation of LixSi anode materials for rechargeable Li-ion thin film batteries. Two types of LixSi anode materials are studied: 1) Amorphous silicon (a-Si) thin film, which is believed to be alloyed to LixSi during the electrochemical reactions; 2) Lithiated silicon wafer, where LixSi alloys are chemically synthesized on silicon wafer. The former study focused on the investigation of electrode-substrate interface and the later concentrated at the study of synthesis method of lithiated silicon wafers and the evaluation of the related electrochemical performance. The conditions of the substrate surface significantly affect the performance of the a-Si thin film anode. Two commercial copper (Cu) foils with different smoothness were investigated in this work. Cu foil substrate was modified by Plasma immersion ion implantation with lanthanum (LaPIII). It was found effective in enhancing the adhesion of the electrode to the substrate and had provided more desirable nucleation centers for the growth of thin film electrode. An a-Si film of about 350 nm thick was deposited to the LaPIII surface treated Cu foil. The a-Si electrode gave a high lithium capacity of up to ~4000 mAh/g, which is close to the theoretical capacity of Si (~4200 mAh/g). In a capacity-limited lithiation/delithiation process, near 100% Coulombic efficiency was obtained and kept for 16 cycles at ~0.3C rate (100 μA/cm2). For thinner a-Si film of ~80 nm, a smoother substrate surface was found more desirable for a better electrochemical performance. The kinetic processes of Li-Si reaction were investigated using the electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) techniques on an a-Si film. An equivalent circuit R(QR)(QR)(C(RW)) model was found to have described the kinetic mechanism of the Li-Si reactions quite accurately. Upon lithiation, the Li-Si alloying reaction is limited by the solid diffusion of Li in the a-Si structure; whereas, during delithiation, the Li-Si dealloying reaction is controlled by a slow charger transfer process. The hindered dealloying process was partly attributed to the large internal resistance during delithiation causing polarization of the electrode and capacity loss. The EIS study of the effect of interface conditions between the a-Si film and the Cu foils revealed that improved electrochemical performance by LaPIII technique has stronger correlation with surface characteristics such as the formation of an O/Cu-La-O-Si surface chemical bridge. The surface morphology was also found important, while the increased surface area due to increase in roughness during PIII was believed less important. A novel method for turning silicon wafers into practical Li-Si anode was developed. The LixSi alloy anode was synthesized by evaporating Li metal on Si wafer first and then followed by a heat treatment at 350 ℃ for an appropriate time in a glove box. The lithiated silicon layer after the heat treatment was found about 1 μm thick for a deposited ~169nm thick Li film on silicon wafer. In the 0.08 V- 1.0 V range and at a lithiation/delithiation rate of ~0.5C (100 μA/cm2), this cell attained up to ~ 180 μAh/cm2 capacity. In the first 100 cycles, no initial capacity loss was observed and near 100% Coulombic efficiency was recorded. Lithiated silicon wafer was found a promising anode material for Li-ion thin film batteries. The lithiated silicon wafer was used to assemble with a ~250 nm LiMn2O4 film obtained by RF magnetron sputtering. The electrochemical performance of the full cells (cathode-limited) in both liquid electrolyte (1 M LiClO4 in propylene carbonate) and solid polymer electrolyte (mixture of polyethylene oxide and LiClO4) was evaluated. In liquid electrolyte, ~8 μAh/cm2 lithium capacity was obtained and the capacity maintained at such level for more than 150 cycles at a charge/discharge rate of ~ 8C (100 μA/cm2) between 3.3 V and 4.4 V. When the solid polymer electrolyte was used, the cell showed an average open potential of ~3.6 V. The cell was successfully cycled for 100 times at ~ 1.6C rate (20 μA/cm2). A discharge capacity of up to 2.9 μAh/cm2 was achieved in the first discharge process. About 50% capacity was retained after 100 galvanostatic cycles. Lower ionic conductivity of electrolyte and the degradation of the anode material are believed to be responsible for the smaller capacity and faster capacity decay in the solid state lithium ion cells.
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