As the elevating fossil energy crisis and environmental issues, developing next generation energy storage devices with high performances is urgent for the operation of emerging devices and systems such as electric vehicles and smart grids. Lithium ion batteries (LIBs), supercapacitors(SCs) and their hybrid systems with the advantage of high performances have attracted great attention in recent years. While many innovative energy storage systems have been developed, the transition metal oxides and their composites have been considered as promising electrode materials due to their high energy density. However, metal oxides had been suffered from poor electrical conductivity and ionic diffusivity leading to low power density. To figure out these problems, nano-sized materials, heterogenous doping method and mesoporous structures are attracted as potential solution to enhance the performances of energy storage devices. In this thesis, we introduce the nano materials such as porous TiO2 and doped-graphene as active electrode materials for enhancing performances of energy storages. These unique nano-structures also offer stable cycles as well as fast kinetics of electronic and ionic transfer.
In chapter II, a hierarchical architecture fabricated by integrating ultrafine titanium dioxide (TiO2) nanocrystals with the binder-free macroporous graphene (PG) network foam for high-performance energy storage is demonstrated, where mesoporous open channels connected to the PG facilitate rapid ionic transfer during the Li-ion insertion/extraction process. Moreover, the binder-free conductive PG network in direct contact with a current collector provides ultrafast electronic transfer. This structure leads to unprecedented cycle stability, with the capacity preserved with nearly 100% Coulombic efficiency over 10,000 Li-ion insertion/extraction cycles. Moreover, it is proven to be very stable while cycling longer than typical electrode structures for batteries. This facilitates ultrafast charge/discharge rate capability even at a high current rate giving a very short charge/discharge time of 40 s. Density functional theory calculations also clarify that Li ions migrate into the TiO2–PG interface then stabilizing its binder-free interface and that the Li ion diffusion occurs via a concerted mechanism, thus resulting in the ultrafast discharge/charge rate capability of the Li ions into ultrafine nanocrystals.
In chapter III, nitrogen-doped mesoporous titanium dioxide (NMTiO2) structures are synthesized via the controlled pyrolysis of metal–organic frameworks. They exhibit interconnected open mesopores allowing fast ion transport and robust cycle stability with nearly 100% coulombic efficiency, along with rich redox-reactive sites allowing high capacity even at a high rate of ≈90 C. Moreover, assembling the NMTiO2 anode with the nitrogen-doped graphene (NG) cathode in an asymmetric full cell shows a high energy density exceeding its counterpart symmetric cell by more than threefold as well as robust cycle stability over 10 000 cycles. Additionally, it gives a high-power density close to 26 000 W kg−1 outperforming that of a conventional sodium-ion battery by several hundred fold, so that full cells can be charged within a few tens of seconds by the flexible photovoltaic charging and universal serial bus charging modules.
In chapter IV, we demonstrate the high-performance electrochemical storages can be realized by control the factors such as porosity, heterogeneousity, surface area, and particle size affecting the performances using the multicomponents and mesoporous structures synthesized through the solvothermal and pyrolysis of a multivalent MOF at a low temperature. This NiCoZnOx allowed high charging-discharging capacity with stable cycles, and the multi-metal exhibited distinct an adavantages comparing with those of single and bi-metals. Moreover, the n-doped carobon nanorribon were also operated by half-cells with totally capacitive hehaviors as like EDLCs. In addition, the hybrid NiCoZnOx//NGNR full-cells was demonstrated to deliver the high energy density, and robust cycle stability over 10,000 cycles with excellent capacity retention. These results support that the hybrid full-cell systems with the optimized structures of activa materials of the electrodes provide rich active sites and fast diffusion paths for charges during the reactions, so that they show remarkably high performances of energy storages.
환경오염과 기후 변화 그리고 에너지 자원 고갈 등의 관한 관심이 높아지게 됨에 따라, 이에 대한 방안으로서 활발히 개발되고 있는 전기자동차 및 스마트 그리드의 응용 분야 등의 발전을 위해, 지속가능한 에너지 자원을 수집하고 변환하는 기술뿐 아니라, 효율적으로 전달하고 저장하며 장치에 전기를 제공하는 기술들에 대한 필요성이 증가하고 있다. 이러한 이유로, 재생 가능한 에너지원으로부터 나오는 전기를 저장하고 활용하기 위해서 전기화학적 기기들이 차세대 기술로서 주목 받고 있다. 그 중, 배터리 시스템은 휴대용 전자기기와 전기자동차 등에 있어서 반드시 필요한 기술 중 하나로서 그 중요성이 점점 부각되고 있는 추세이다. 본 논문에서는, 상기 이유로 요구되어지는 차세대 에너지 저장장치 개발에 관한 연구를 진행하였으며, 에너지 저장 성능에 영향을 미치는 복합적인 요인들인 전극의 구조와 시스템 설계 등의 방법을 통하여 종합적으로 개선을 하기 위한 기술을 연구하였다. 이를 위해, 메조 다공성 구조의 금속산화물 및 그래핀 나노 구조체 류의 전극 재료들을 활용하였으며, 각각 리튬 이온 배터리, 소듐 이온 하이브리드 커패시터 그리고 수계 하이브리드 커패시터 시스템에 적용하여 고성능 에너지 저장 장치를 증명 및 구현 하였다.
