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Preparation And Electrochemical Performance Of Core-Shell Nanocomposites And Vacuum Deposited Si Film

Posted on:2008-11-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:T ZhangFull Text:PDF
GTID:1101360215484289Subject:Physical chemistry
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Lithium-ion batteries are one of the great successes of the application of advanced materials and electrochemistry science in the modern energy storage devices. Nowadays, owing to the rapid advancement of electronic technologies, lithium-ion batteries are required to improve their performance on capacity, power and safety. This paper is focused on the silicon materials with high capacity, trying to improve their cycleability and safety by nano-technology, composite-technology and film-technology.The electrochemical performance and capacity-fading mechanism of the polycrystalline Si nanoparticles prepared by laser vapor deposition method were investigated in detail. Silicon has a theoretical capacity of 4200 mAh/g in its fully lithiation composition, Li4.4Si, much higher than that of the commercialized graphite (372 mAh/g). However, the cycling performance of silicon is poor, owing to its severe volume expansion and shrinkage during the insertion and extraction of lithium ions, which results in pulverization of Si particles and eventual loss of Li+ storage ability. In this work, the Si nanoparticles exhibit a spherical morphology and the average particle size is 30 nanometers. Its initial lithiation capacity reaches 2700 mAh/g, showing a better cycleability than micrometer-scale silicon particles. It can be ascribed to the small absolute volume changes occurring for nanomaterials due to their small particle size and uniform particle distribution, which mitigate the pulverization and exfoliation of the particles, improving the stability of the electrodes. HRTEM and SAED results indicate that the crystal structure of Si can be transformed into the amorphous state by lithiation/de-lithiation process, accompanying with the intense aggregation of nanoparticles to reduce there high surface energy, eventually leading to their mergence to larger particles, which is proved by SEM observation, too. The severe mergence results in the irreversible accumulation of lithium in Si, the drop of conductivity and the decrease of lithium diffraction coefficients, even the exfoliation of electrode materials, which is the main reason of capacity fading for the silicon nanoparticles.The core-shell nanocomposite concept represents an efficient solution to this problem. Using the emulsion polymerization method, the Si nanoparticles were coated by polymer to form a core-shell shaped silicon/polyacrylonitrile (PAN) precursor. The precursor was heat-treated at 800°C to carbonize the PAN to form hard carbon shell. The initial lithiation and de-lithiation of this core-shell Si/C nanocomposites are 1750 mAh/g and 1137 mAh/g, respectively. After 20 cycles, the reversible capacity retains 594 mAh/g, 54% of the initial capacity, showing better capacity retention than the pristine Si nanoparticles. It is found that the coating polyacrylonitrile can be transform into stable "hard carbon" shell under low temperature (<1200°C, relative to the graphitization temperature). The hard carbon consists of a lot of micropores, which can definitely be passages for lithium ions, keeping the conductivity and electrochemical activity of the composites. During lithiation and de-lithiation process, the hard shell can act as a barrier to protect the inner Si core from aggregating. Moreover, it can also prevent the nanocomposites themselves from aggregation and mergence.The sol-gel method was also used to prepare core-shell Si/SiOx nanocomposite. Using tetraethoxysilane (TEOS) as the precursor, through a process of hydrolyzation and condensation, the core-shell polysilicone/Si nanocomposite was obtained. Then, the polysilicone can be transformed into the SiOx shell by heat-treatment. Similar with the hard carbon, the SiOx shell, which has a stable crystal structure, can protect the inner Si core from aggregation and mergence. Especially, SiOx can also absorb and retain a large quantity of lithium ions. During lithiation process, the partial SiOx can be reduced to Li2O and lithium silicates, which expand only half as much as Li-Si alloys, serving as a buffer to alleviate volume expansion of the Si nanoparticles, mitigating the destruction of the Si crystalline structure. The initial lithiation and de-lithiation of this core-shell Si/SiOx nanocomposites are 1072 mAh/g and 827 mAh/g. After 20 cycles, the reversible capacity retains 539 mAh/g, which is 65% of the initial capacity, showing better capacity retention than the pristine Si nanoparticles.The modified natural graphite (SSG)/Si nanoparticle composite was prepared by sonicated dispersion and following heat-treatment process. SEM observation indicates that the nanometer-scale Si particles were uniformly and completely coated on the surface of SSG particles, forming an analogous core-shell structure. The initial lithium intercalation and de-intercalation capacity of the 10% SSG/Si composite are 890 mAh/g and 567 mAh/g. The 10% SSG/Si composite shows a very slow capacity fading and retains 96.4% of the original capacity after 20 cycles, respectively, showing good capacity retention. It is mainly due to the heat-treatment that increases the combination force between Si nanoparticles and SSG. This combination force can prevent the aggregation and separation of Si nanoparticles during lithiation and delithiation process, keeping the stability of the Si-coated SSG material. Simultaneously, as the matrix materials, the SSG can keep stable during cycling, and its volume change is small, which increases the stability of the electrode materials.The film-technology is also one of the efficient solutions to improve the cycleability of the silicon negative materials. Using the vacuum vapor deposition method, the Si film was deposited on the Ni and Cu foil roughed by pre-treatments. The original lithiation capacity of 1.8 jim thickness Si film on the Ni foil even reaches the theoretical capacity 4200 mAh/g of silicon, and its original de-lithiation capacity is 3100 mAh/g, retaining over 1000 mAh/g after 200 cycles. Similar with it, the initial lithiation/de-lithiation capacity of 3.6 thickness Si film on the Cu foil are 3110 mAh/g and 2390 mAh/g. Its reversible capacity retains over 1250 mAh/g after 200 cycles. The ratio of surface area to thickness of the vacuum deposited Si film is biggish, which can suppress the volume expansion accompanied with alloying. SEM observation indicates that the surface of the roughed Ni foil has many micro-holes and cracks, and the deposited Si film on it exhibits half spherical. In the case of Cu foil, its surface has many protuberances with ridges, and the deposited Si film on it shows a hill-like structure. Both of the two structures provide spaces for the volume changes. Moreover, the Si film consists of nano-scale particles, which have small volume changes during cycling. HRTEM and SAED results indicate that the nano-sized face-centered Si crystallites are formed among the amorphous Si film after several cycles, and the strain derived from volume changes can be accommodated by slippage at the nanodomain boundaries, so as to improve the stability of the Si film, leading to the good capacity retention with high capacity.
Keywords/Search Tags:Lithium-ion batteries, Anode materials, Core-shell, Silicon nanoparticles, Silicon/carbon nanocomposite, Silicon/SiO_x nanocomposite, Modified natural graphite, Vacuum evaporation deposited silicon film, Emulsion polymerization, Sol-gel
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