Li-ion batteries (LIBs) and supercapacitors (SC) have been widely employed in a myriad of portable electronics. Their successful applications to power electric vehicles is dependent on improvement of the electrochemical performance of the electrode materials, especially the power/energy densities and the cyclic life. This thesis focuses mainly on developing new metal oxide/sulfide-graphene nanocomposites as the electrode materials with high capacities and excellent cycleability in LIBs and supercapacitors.

Metal oxides have much higher theoretical capacities than conventional graphite base anodes in LIBs, but they suffer from poor electrical conductivities and cycleability. Herein, graphene-NiFe₂O₄ (G-NF) nanocomposites are synthesized as a high capacity anode material fo...[ Read more ]

Li-ion batteries (LIBs) and supercapacitors (SC) have been widely employed in a myriad of portable electronics. Their successful applications to power electric vehicles is dependent on improvement of the electrochemical performance of the electrode materials, especially the power/energy densities and the cyclic life. This thesis focuses mainly on developing new metal oxide/sulfide-graphene nanocomposites as the electrode materials with high capacities and excellent cycleability in LIBs and supercapacitors.

Metal oxides have much higher theoretical capacities than conventional graphite base anodes in LIBs, but they suffer from poor electrical conductivities and cycleability. Herein, graphene-NiFe₂O₄ (G-NF) nanocomposites are synthesized as a high capacity anode material for LIBs. The electrochemical measurements reveal that G-NF nanocomposites have a higher capacity than pure NF. For further improvement of the cycleabilty, a thin layer of carbon is coated on G-NF nanocomposites through a hydrothermal route, resulting in a sandwich-structured G-NF-C anode with exceptional electrochemical performance. G-NF-C nanocomposites show high cycle stability of 1195 mAh g^-1 after 200 cycles measured at 500 mA g^-1. This value is among the highest reported so far for the anodes containing similar NF nanoparticles. The synergy arising from the conductive graphene substrate, well-dispersed NiFe₂O₄ nanoparticles and the protective carbon layer sustaining the sandwich together is responsible for the trait.

G-NF nanocomposites are also studied as electrode in supercapacitors. Electrochemical measurements revealed that the nanocomposite synthesized at 180 °C and 10h has the highest specific capacity. This electrode also shows a promising cycleability by maintaining 90% of the initial capacity after 1500 cycles at a current density of 10 A g^-1.

As the flexibility and high power density are key points for the development of wearable electronic devices, we put an effort to produce 3D flexible supercapacitors based on Ni-G foam and metal sulfides. A facile hydrothermal route is employed to grow Ni₃S₂ hollow rod arrays on a 3D graphene foam which is produced by chemical vapor deposition (CVD) on a Ni foam. The electrochemical measurements show that the NGF/NS-12h electrode delivers an excellent specific capacity of 1900 F g^-1 at a current density of 1 Ag^-1. This electrode also shows a very promising cyclic stability with a specific capacity of 950 Fg^-1 after 2000 cycles at 10 Ag^-1. Its promising cycleabilty is future proved, by maintaining 96% of original capacity, in the 2001^st cycle, while the current density is decreased to 5 Ag^-1. Symmetric cells, fabricated using GNF-NS-12h, can work in a very wide potential window of 1.6 V, and deliver a high specific capacity of 190.5 Fg^-1 at a current density of 4.5 mA cm^-1. This specifc capacity corresponds to maximum specific energy of 67.7 Wh kg^-1 which is among the highest in the symmetric cells reported by now. This finding suggests that the symmetric cells have a great potential for application as high performance flexible supercapacitor devices.