(A) study on synthesis of mesoporous metal oxide with graphene nano-structures and their utilization to enable high performances in energy storages

Abstract

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.