Driven by ever-increasing demands for sustainable energy technologies, such as electric vehicles (EVs), significant research has been directed towards developing alternative materials with higher capacities to replace the existing electrode materials in Li-ion batteries (LIBs) and supercapacitors. These new electrodes should have enhanced energy/power densities and cyclic life for these energy storage devices. This thesis focuses mainly on exploring and developing novel nanostructured metal oxide/carbon composites, including (Ni)-Co-based oxides and SnO
2, as the electrode materials for LIBs and supercapacitors with improved capacities and cyclic life.
Nanostructured cobalt oxide (Co
3O
4) has shown high theoretical capacities in LIBs and pseudocapacitors, and the combination of these materials with a conductive matrix can efficiently increase the performance of energy storage devices. This work reports a facile electrospinning route combined with a well-designed heat treatment process to produce Co
3O
4 nanoparticles encapsulated in porous carbon nanofibers (Co
3O
4@PCNF). The Co
3O
4 nanoparticles of 5-10 nm in diameter are encapsulated in the porous CNFs to offer an excellent capacity of ~1000 mAh g
-1 after the 100
th cycles at 0.1 A g
-1 and high rate performance in LIBs. Moreover, electrochemical measurements show that the Co
3O
4/CNF electrode with a Co
3O
4 mass loading of 68 wt.% has a superior capacity of 586 F g
-1 at a current density of 1 A g
-1. When the current density is increased to 50 A g
-1, about 66.4 % of the original capacity is retained. These superior properties are attributed to the uniform dispersion of fine Co
3O
4 nanoparticles in the CNF matrix which can act as a conducting support for these active nanoparticles. In addition, the formation of onion-like graphitic layers around the Co
3O
4 nanoparticles are found to improve the electrochemical performance. Considering the low-cost and simple preparation method, the synthesized material can make a promising electrode for high performance energy storage devices.
We also report the in-situ synthesis of NiCo
2O
4 nanoparticles chemically bonded to multi-walled carbon nanotubes (NiCo
2O
4/CNT) based on a low-temperature, one-pot hydrothermal approach. The NiCo
2O
4/CNT composite electrodes present excellent bi-functionality for applications in both LIBs and supercapacitors. They deliver a high Li-storage capacity of ~1020 mAh g
-1 after 200 cycles at 300 mA g
-1 while a high pseudocapacitance of 680 F g
-1 is delivered when discharged at 1 A g
-1. Electrodes are also prepared by ‘physical mixture’ of the same constituents at the same concentrations. The physically mixed electrodes present much poorer Li-ion storage of ~370 mAh g
-1 after 100 cycles at 100 mA g
-1 while their psedocapacitances are at least 130-300 F g
-1 lower than those of the ‘composite’ counterparts at different current densities. Several morphological and functional characteristics are responsible for the excellent energy storage behavior of the ‘composite’ in comparison with the ‘physical mixture’ electrodes, including (i) the strong attachment of uniformly dispersed NiCo
2O
4 nanoparticles on the functionalized CNTs with (ii) large surface areas and sites for efficient electrochemical reactions. The CNT substrates also function both (iii) as a conductive network for fast ion/electron transfer and (iv) as a cushion to accommodate the volume expansion during the charge/discharge cycles.
The ordered mesoporous carbon (OMC) framework containing ultrafine tin oxide (SnO
2) nanoparticles encapsulated in the hollow nanochannels of OMC (SnO
2@OMC) has been prepared based on a simple infiltration method. The synthesized material is used as the anode electrode in LIB, showing an excellent cyclic stability and a reversible specific capacity of ~ 1000 mAh g
-1 after 100 cycles at 100 mA g
-1.The electrodes also show a good rate performance with a specific capacity of 680 mA g
-1 at 500 mA g
-1. Careful investigations of the electrochemical characteristics of the SnO
2@OMC electrodes compared with the neat SnO
2 nanoparticle counterparts reveal the positive effects of encapsulation within OMC on Li-storage behavior and stability of the electrode. The reversibility of the conversion reactions, which is largely responsible for the enhanced capacity of the LIBs, is specifically discussed. The unique functional feature of the SnO
2@OMC electrodes is also shown to be an ameliorating factor for the superior performance of the LIBs.
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