| Lithium-ion rechargeable batteries (LIBs) have been widely used in portable electronic devices and electric vehicles,owing to their advantages of higher energy density, higher operational voltage etc. Thus, eco-friendly, high specific capacity and long cycle life of LIBs is the main task of the researchers. Among them, iron-based and cobalt-based transition metal oxides have high capacity, high natural abundance, low cost and corrosion-resistance, which promote the development of LIBs. However, cobalt-based oxides are limited by their high cost and toxicity. Thus, extensive research efforts have been made to fabricate novel ternary cobalt oxides (MCo2O4) by partially substituting Co with less expensive and eco-friendly metal element. Although these two materials have many advantages, the large volume expansion during charge-discharge process and the inherent low conductivity lead to the poor long cycle performances, which seriously affect the development of their commercial applications. For example, Fe2O3 materials faced serious polarization and loss of electrical connectivity upon electrochemical cycling with the conversion reaction mechanism, especially at high rates with drastic volume change (over 200%). This phenomena seriously affected the long cycle stability of LIBs. In addition, the possible formation of a thick solid electrolyte inter-phase (SEI) films on the surfaces of iron-based and cobalt-based transition metal oxide materials as anodes cause consumption of abundant Lithium ions, resulting in a large irreversible capacity loss and low coulombic efficiency. Furthermore, the unstable SEI films may decompose completely, which was catalyzed by transition metals during the Li+ extraction processes. The repeated formation/decomposition of the SEI films result in capacity fading and safety problems. Therefore, there is a great challenge to fabricate stable anodes with high reversible capacity and excellent rate capability. The nano/microstructures could reduce polarization, decrease the diffusion length. α-FeOOH has stable chemical properties and high theoretical specific capacity (905 mAh g-1). Nevertheless, the long cycle life of FeOOH-based anodes is limited because of the low conductivity and large volume change during the charge-discharge process. In addition, these nano-sized FeOOH-based particles are easily aggregated together, which could affect their application in energy storage devices.To improve boost the electrochemical performances, several key strategies have been applied by us. Firstly, one of the most promising strategies is to design nano/microstructures with different dimensions that containing unique features and structural stability, which might effectively accelerate the diffusion of Li+ and to accommodate the mechanical stress during cycling; Secondly, appropriate elements doping could improve the intrinsic conductivity and charge transfer capability for iron-based and cobalt-based transition metal oxides. Doping would modulate the unit cell parameters, increasing the number of free electrons, improving the electrode reaction kinetics; The doped ions as a separate phase also increased the grain-boundary density. The grain-boundary atoms are not in regular crystallographic sites allowing for a more effective Li+ transport along the narrowly-spaced interface. Thirdly, surface modification and coating are proposed to reduce the undesired side reactions and obtain the improved electrochemical performances.In addition, the limited global lithium resources greatly restricted the sustainable development of LIBs. Recently, compared to lithium, the global sodium resources abundant, inexpensive etc.,sodium-ion batteries (SIBs) have the similar electrochemical behavior with LIBs. Therefore, SIBs may become the next-generation energy devices. Currently, the development of SIBs is still in its infancy stage, while the key to the development of SIBs is finding the suitable new electrode materials. So far, many positive electrodes have been proposed, whereas only few negative electrode materials have been investigated and even less candidates as viable anodes for SIBs. Graphite and Si-based materials are the most promising anode materials for LIBs but not for SIBs systems, due to the radius of Na+(～0.102 nm) is larger than that of Li+(～0.076 nm). The interlayer distance of graphite could not match with Na+; Si can not incorporate Na+ ions like in the case of Li+ ions. Challenges are still remaining, especially for anodes before the commercialization of SIBs. The larger radius of Na+ results in sluggish diffusion kinetics of Na+ in electrode materials, leading to lower capacities or electrochemical inactivity. Recently, more and more transition metal oxides/hydroxides have been studied as anodes for SIBs. Iron-based oxides/hydroxides materials with conversion reaction mechanisms used for SIBs have been reported for many times. These anodes show high capacities and energy densities due to the multi-electron reactions. Unfortunately, iron oxides/hydroxides usually suffer a main drawback related to large volume change during alkaline ion insertion or extraction. The large volume change leads to a rapid agglomeration of metal oxides/hydroxides particles and pulverization of the electrode materials, finally causing capacity loss and cycling decay.Supercapacitor has advantages of fast charge-discharge rate, high efficiency, long cycle life, etc. Transition metal oxides with high specific capacity have aroused the interest of many researchers. MCo2O4 has received tremendous interest in supercapacitor applications due to its better electronic conductivity (two orders of magnitude higher than conventional transition metal oxides), higher electrochemical activity, low-cost and environmental friendliness. Ternary metal oxides with two different metal cations exhibit high electrochemical activities because of their complex chemical composition and the synergic effects of multiple metal species. To improve the cobalt-based transition metal oxide materials capacitance properties, main improvement measures are:(1) Composites. Two or more kinds of transition metal oxides compound, transition metal oxides-carbon material compound or transition metal oxides-conductive material composites. In the process of circulation, different materials play their respective advantages, improving the cycle stability and high specific capacity. (2) Design of micro-nano multistage structured materials: Core-shell structure, one-dimensional coaxial structure, three-dimensional multi-level structure, etc. These special structures have large specific surface area and abundant pore structure, promoting better internal seep into the electrode materials, reducing the transmission distance of ions/electrons, decreasing impedance and increasing the utilization of active materials.Based on the above discussions, as a basic research laboratory, we explored the electrochemical properties of doped iron hydroxide and the special structure of cobalt-based transition metal oxides composite materials. The main research contents are listed as follows:(1) One-dimensional Mn-doped a-FeOOH nanorods (diameters of 70～200 nm and lengths of 2～4 μm) and pure α-FeOOH nanorods (diameters of 25～50 nm and lengths of 100～200 nm) have been synthesized through a hydrothermal process combined with a subsequent acid-treatment process. Importantly, compared with their structure characterization and electrochemical performance test, the Mn-doped α-FeOOH nanorods display batter electrochemical properties, showing a discharge capacity of 882.8 mAh g-1 at 1000 mA g-1 after 300 cycles. The nanostructure and doping might be responsible for the enhanced electrochemical performance. Furthermore, the porous Mn-doped α-Fe203 nanorods (with lengths in the range of 12μm and diameters of 20～80 nm) were prepared by annealing the Mn-doped α-FeOOH nanorods in air at 300℃ for 4 h. The electrode outputs a capacity of 860 mAh g-1 at 1000 mA g-1 up to 600 cycles.(2) Doping with large-radius ions is an effective strategy to modify the structural stability and electronic properties of active materials. Mn doping 玖-FeOOH has been prepared by using the similar synthesis method, and different doping contents of Ce-doped α-FeOOH nanorods (mean diameter:70 nm and mean length:270 nm) and pure α-FeOOH nanorods have been synthesized. After the electrochemical performance tests, α-FeOOH,0.5 wt% Ce-doped α-FeOOH and 2 wt% Ce-doped α-FeOOH display the discharge capacity of 254,550 and 388 mAh g-1. This result suggests that 0.5 wt% Ce-doped α-FeOOH electrode has an excellent electrochemical property. And then, a 0.5 wt% Ce-doped α-FeOOH//LiFePO4 full cell is assembled. The first discharge capacity is 580 mAh g-1 at 1000 mA g-1 with a high coulombic efficiency of 97.4%. Additionally, using bigger atoms to replace small atoms can provide larger space for the movement of ions.0.5 wt% Ce-doped α-FeOOH is also examined as an anode material for SIBs, displaying the initial discharge capacity of 587 mAh g-1 at 100 mA g-1.(3) Metal oxides with porous and multicomponent hierarchical nanostructure features as LIBs anode materials could buffer volume variation, address the low bulk density and take full advantages of different components during the charge-discharge process. Uniform 3D hierarchical porous rose-like NiCo2O4/MnCo2O4 and rose-like MCo2O4 is controllably fabricated through a facile hydrothermal process followed by a subsequent heat treatment. The formation process was proposed to expalain the growth of rose-like NiCo2O4 by change the hydrothermal reaction time and the content of triethanolamine. When evaluated as LIBs anode materials, NiCo2O4/MnCo2O4 and NiCo2O4 exhibit high discharge capacity of 1009 and 400 mAh g-1 at 1000 mA g-1 after 600 cycles. In addition, the NiCo2O4/MnCo2O4 composite used as supercapacitor displays an initial specific capacitance of 911.3 F g-1 at 5 A g-1.(4) One-dimensional coaxial core-shell structured MCo204@SiO2@TiO2 (M= Ni, Cu, Mn, Co, Zn) has been obtained via a facile method, which has been used as supercapacitor electrode materials, NiCo204@Si02@TiO2 has the specific capacitance of 12000 F g-1 at the current density of 10 A g-1 after 10000 cycles. The excellent performance may be attributed to the unique one-dimensional coaxial core-shell structures that greatly facilitate to the faster ion and electron transfer rate. |
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