学位授与大学：京都大学 ; 取得学位: 博士(工学) ; 学位授与年月日: 2007-09-25 ; 学位の種類: 新制・課程博士 ; 学位記番号: 工博第2866号 ; 請求記号: 新制/工/1421 ; 整理番号: 25551 / The developments in microstructure and phase structure of Sn-based materials gave stable cyclability under the full charge and discharge conditions without the initial large irreversible capacity. Some important structures discovered and their mechanisms for improving the cyclability were discussed as follows. In Chapter 1, the electrode structure of Sn electrodes was discussed, where the theoretical capacity of Li4.4Sn was attained. Weak adhesion of Sn to the Cu foil prevented the powdered Sn electrodes prepared by a conventional slurry-coating process from providing the theoretical capacity of Sn metal from the initial cycle, where the Sn powder peeled off the Cu foil to be disconnected from the foil in electronic conductivity. The electrodeposition process improved the interface adhesion between Sn and Cu foil. The electrodeposited Sn film sticks tightly to the Cu foil after forming the intermediate Cu6Sn5 layer, and it reacts with lithium to the theoretical capacity of Li4.4Sn under the full charge-discharge condition with small irreversible capacity. The initial discharge capacity was 2.5 times as large as that of the graphite. In Chapter 2, the advanced phase structure of the electrodeposited Sn film was discussed, which improved the film in cyclability without a large loss of capacity. The Sn film-Cu foil interface adhesion was still weaker to be delaminated from the Cu foil. Also, the Sn film was pulverized and passivated by electrolyte reduction products. The film was disconnected from the Cu foil in electronic conductivity during the initial cycle and could not continue to react with lithium. The stepwise composition-graded phase structure improved the film in the interface adhesion and active material chemistry to provide better cyclability under the full charge-discharge condition without large sacrifice of their volumetric capacity. The phase structure consists of the Cu6Sn5 and Cu3Sn phases, where Cu6Sn5 phase is the active material with less volume change and electrolyte-reactivity and the Cu3Sn phase enhances the adhesion between the Cu6Sn5 phase and Cu foil. The annealing process formed the stepwise composition-graded phase structure, and the annealing condition controlled the structure to be optimized for better cyclability. In Chapter 3, the advanced microstructure of the electrodeposited Sn film was discussed, which improved the film with the stepwise composition-graded phase structure in cyclability. The annealed rough-surfaced Sn electrode showed better cyclability than the annealed flat-surfaced Sn electrode, even though they had the same stepwise composition-graded phase structure. The micro-columnar structure of the film improved the film in the interface adhesion. The structure self-organizes on the rough surface of the Cu foil during the first charge-discharge cycle. The film is divided into columns of about 10μm square by gaps to accommodate swelled film with much reduced internal stress and strain during the following charge, resulting in less pulverization and enhanced adhesion of the film to the foil. The wavy surface profile of the foil and moderate ductility of the film are critical factors to self-organize the micro-columnar structure of the film during charge and discharge. Close cyclability to typical graphite-anode cells was available for the small cell using the annealed rough-surfaced Sn electrode for 20 cycles. In Chapter 4, the electro co-deposited 79.8Sn-20.2Co alloy film was examined for electrochemical and structural properties, and the feasibility of formation of the microstructure was discussed. The micro-island structure was self-organized on the rough surface of the Cu foil during the first charge-discharge cycle, which is similar to the micro-columnar structure in mechanism for improving the cyclability as well as in the shape. As a result, the Sn-Co alloy electrode offered the same capacity at the 20th cycle as the initial capacity of about 60% of the theoretical capacity of Li4.4Sn. The mechanical properties of the film such as ductility and brittleness give another key for formation of the microstructure, in addition to the rough surface of the current collector foil. In Chapter 5, the anomalous cycle performance of the 79.8Sn-20.2Co alloy electrode was discussed in terms of morphology and phase structure of the film. The film showed four-step change in discharge capacity, which depends on the two-stage phase transformation and morphological change of the film. The film is transformed from the amorphous phase to a new phase with forming fcc-Co particles. The new phase is a key to stable reaction of the Sn-Co film with lithium. Although the new phase initially shows smaller capacity than the amorphous phase, the enlarged surface area of the film activates a new reaction of the new phase to increase capacity. As a result, the new phase provides as large capacity as the amorphous phase. In Chapter 6, the electro co-deposited 92.1Sn-7.9Co alloy film with the microstructure was examined for electrochemical and structural properties, which comprised the new phase of the 79.8Sn-20.2Co alloy film as electrodeposited, and the mechanism of the reaction stability of the new phase was discussed. The 92.1Sn-7.9Co alloy film had the nano-composite phase structure, where the less lithium-active crystalline phase surrounds the amorphous phase to accommodate volume change of the amorphous phase and prevent it from deteriorating, resulting in stable reaction of the amorphous phase with lithium to provide constant capacity. On the other hand, the morphological and phase structural changes of the film improve Li+ diffusion through the film-electrolyte interface, in the film, and in the amorphous phase to increase capacity. As a result, under the full charge-discharge condition, the 92.1Sn-7.9Co alloy film showed monotonous increase in discharge capacity from 663 mAh/g at the 1st cycle to 769 mAh/g at the 20th cycle, which is 77% of the theoretical capacity of Li4.4Sn. Since the phase transformation of the 79.8Sn-20.2Co alloy film caused deterioration of the adhesion between the film and current collector to degrade the cyclability of the film, the phase structure of the 92.1Sn-7.9Co film gives better cyclability when it is prepared in advance of the reaction. There should be still room for further stable cyclability in Sn-Co alloy films. For example, adding a third atom to the film composition may enhance the stability of the reaction of the amorphous phase. Not only electro co-deposition but various thin-filming processes will be proposed to control a three-or-more-component system. However, the adhesion of the film and Cu foil is the crucial factor for the stable cyclability, and the microstructure such as the micro-columnar structure and micro-island structure is the most fundamental and effective in enhancing the adhesion. The micro-columnar structure has been applied for Si and Ge systems and examined for its self-organization mechanism, cyclability, and other electrochemical properties. Some of the results have already reported [67-68]. The advanced lithium ion batteries of group 14 elements with the microstructure may become commercially available in near future. Also, the microstructure should be formed on the flat-surfaced foil with holes in the same stress-concentration mechanism as the rough-surfaced foil. Since the rough-surfaced Cu foil is formed by Cu electrodeposition, there are limitations to control the surface profile. However, the holes can be punched at intervals of a-micrometer-order by using physical processes such as a femto-second laser, resulting in formation of further size-controlled microstructure. Issues of the laser process are beam concentration to make holes of less than 10μm in diameter and long work time. A suitable grating should be a key to overcome the issues and find another potential of the microstructure. / Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第13395号 / 工博第2866号 / 新制||工||1421(附属図書館) / 25551 / UT51-2007-Q796 / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 平尾 一之, 教授 横尾 俊信, 教授 田中 勝久 / 学位規則第4条第1項該当
| Identifer | oai:union.ndltd.org:kyoto-u.ac.jp/oai:repository.kulib.kyoto-u.ac.jp:2433/49148 |
| Date | 25 September 2007 |
| Creators | Tamura, Noriyuki |
| Contributors | 平尾, 一之, 横尾, 俊信, 田中, 勝久, 田村, 宜之, タムラ, ノリユキ |
| Publisher | 京都大学 (Kyoto University), 京都大学 |
| Source Sets | Kyoto University |
| Language | English |
| Detected Language | English |
| Type | DFAM, Thesis or Dissertation |
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