Nano scale surface modification and nanostructure fabrication for energy storage device = 나노 스케일의 표면 개질과 구조체 연구 및 에너지 저장 장치로의 응용

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The performance of energy storage devices are nowadays being significantly improved due to the introduction of nanotechnology and this attributes to an advanced capacity, power and life cycle of materials. Nanotechnology reported so far is developed usually by solid phase method using mechanical mixing and liquid phase such as sol-gel, co-precipitation and hydrothermal. In this thesis, we developed another nanotechnology using ALD (Atomic layer deposition) which is the one of a gas phase chemical process. ALD enables the manufacturing of atomic level of ultrathin film and allows a precise control of the desired thickness and composition. We, with this technology, overcame the limitations that supercapacitor and lithium ion battery could not conquest so far. Supercapacitor is a typical high power energy storage device; however, the scope of application is narrow because of a relatively lower energy density. Operating voltage enhancement is an effective route for high energy density supercapacitor. Unfortunately, widely used activated carbon electrode generally suffers from poor electrochemical stability over 2.5 V. Here we present atomic layer deposition (ALD) encapsulation of activated carbons for high voltage stable supercapacitor. Two-nanometer-thick $Al_2O_3$ dielectric layers are conformally coated at activated carbon surface by ALD, well-maintaining microporous morphology. Resultant electrodes exhibit excellent stability at 3 V operation with 39 % energy density enhancement from 2.5 V operation. Because of the protection of surface functional groups and reduction of electrolyte degradation, 74 % of initial voltage was maintained 50 hr after full charge, and 88 % of capacitance was retained after 5000 cycles at $70^\circ C$ accelerated test, which correspond to 31 and 17 % improvements from bare activated carbon, respectively. This ALD-based surface modification offers a general method to enhance electrochemical stability of carbon materials for diverse energy and environmental applications. The scope of application for lithium ion battery with high energy density is very diverse from small size-IT devices to large sized xEV / ESS devices. However, the lack of safety requires the development of new alternative to replace existing materials. In that respect, $TiO_2$ is often mentioned as the one of replacements for graphite, but is yet to be sure because of a lower capacity which may impose restrictions for application. In this study, we introduce mesoporous $TiO_2$ nanocrystalline which enhanced its reversible capacity. The precise control of ultra-thin thickness of ALD (atomic layer deposition) layers combined to carbon black used as sacrificial template permitted to introduce novel nanostructured $TiO_2$ with significantly high specific surface area. The carbon black not only plays a role as sacrificial template but also induces a well-developed mesoporous structure. Our mesoporous structure with high specific surface area ($260 m^2 g^{-1}$) are very favorable for Li intercalation, which shorten the Li diffusion path in the solid phase and enable electrolyte ions to be transported easily. ALD assisted $TiO_2$ nanocrystalline exhibits remarkably specific capacity enhancements ($280 mAh g^{-1}$), high $1^{st}$ coulomb efficiency (85 %) and excellent cycle ability (80 % @ 100 cycles at 1 C). This ALD assisted template serves a promising novel strategy to synthesize nanostructured electrode material.
Kim, Sang Oukresearcher김상욱researcher
한국과학기술원 :신소재공학과,
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학위논문(박사) - 한국과학기술원 : 신소재공학과, 2015.8,[vii, 78 p. :]


ALD(Atomic Layer Deposition); Supercapcitor; LIB; activated carbon; $TiO_2$ nanocrystalline; 원자증착법; 슈퍼커패시터; 리튬이차전지; 활성탄; 이산화티탄

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