Carbon based nanostructures for electrochemical energy storage

Nowadays, the decreasing availability of fossil fuels has made the demand for efficient storage and usage of energy urgent than ever. Recently, two types of electrochemical energy storage devices have caught extensive attention, namely lithium ion batteries (LIBs) and electrochemical capacitors (ECs). LIBs are now serving as the most important power sources for mobile electronics, while ECs are rising rapidly as an important complement to LIBs, because of their capability to deliver much higher power density as well as longer life time. However, almost all of the advanced materials for anodes in LIBs or electrodes in ECs face the problems of structural failure and hence severe energy density fading, which have dramatically hindered the further development of these systems. In this project, fabricating these materials into nanostructures and combining them with advanced carbon materials, such as carbon nanotubes (CNTs) and ultrathin graphite (UG), are employed as an effective solution. The obtained carbon based nanostructures have been successfully demonstrated as promising candidates for high performance anode materials in LIBs and/or electrode materials for ECs. The work starts with the growth of CNT network on stainless steel (SS) substrate by chemical vapor deposition (CVD). Molybdenum disulfide (MoS2), a typical type of TMDs, is then decorated onto the CNT network through hydrothermal reaction and annealing treatment. The rationally designed MoS2/CNT composite can be directly tested as an anode material in LIBs, without the necessity for any binders. Encouragingly, a high and reversible capacity over 1300 mAh•g−1 is retained for 50 cycles at the current density of 200 mA•g−1, while excellent rate performance is also delivered because of the enhanced structural stability and reaction kinetics in the material. Secondly, vertical aligned arrays of CNTs are fabricated through plasma enhanced CVD (PECVD). The CNT array can be a good template for Si, which is of the highest theoretical capacity among alloy-type anodes in LIBs. By introducing rough Ni layers in between the CNT cores and the Si shells, enhanced adherence, accelerated charge transport and more effective strain relaxation can be achieved. The core-shell Si/Ni/CNT anode manages to provide a high capacity of over 2500 mAh•g−1 with a low fading rate of merely 0.2% per cycle over 110 cycles. The CNT arrays are also used to support NiCo2O4, a novel type of TMOs, with advantages in electrical conductivity and ionic reactivity. The coating of NiCo2O4 is realized by a facile electrochemical deposition method followed by subsequent annealing in air. The NiCo2O4/CNT arrays obtained demonstrate outstanding performances as anodes in LIBs, including a high capacity (1147.6 mAh•g−1 at 100 mA•g−1), excellent cycling retention (no capacity fading over 200 cycles) and outstanding rate capability (712.9 mAh•g−1 at 1000 mA•g−1). The applications of NiCo2O4/CNT core-shell structures are not limited to LIBs but also suitable for ECs. When tested in a three-electrode configuration, the as-prepared NiCo2O4/CNT structures exhibit a specific capacitance of 695 F•g−1 at the current density of 1 A•g−1 and 576 F•g−1 at 20 A•g−1. Hence, the capacitance of the NiCo2O4/CNT electrode remains 91% of its initial value after 1500 cycles at the current density of 4 A•g−1. Furthermore, this structure is enhanced by using the substrate of 3D UG-coated Ni foams, resulting in the architecture of NiCo2O4/CNT/UG on Ni foams. The fabrication processes consist of sequential CVD growth of 3D UG on Ni foam and CNT network on the surface of UG, respectively and hydrothermal and annealing preparation of NiCo2O4 nanorods. This architecture brings advantages including huge surface area, enhanced active material loading and facile electrolyte access. A high capacitance of 973.0 F•g−1 at the current density of 1 A•g−1, outstanding rate performance (capacitance of 715.9 F•g−1 at 15 A•g−1) and also stable cycling at 5 A•g−1 have been successfully demonstrated.DOCTOR OF PHILOSOPHY (EEE