| Lithium-ion batteries (LIBs) are considered as the most promising rechargeable energy storage technology, which can be applied in electronic devices and hybrid electric vehicles. However, the practical application of LIBs is still hampered by the poor electrochemical performance of the electrode materials, such as the low specific capacity, poor cycling stability and rate capability. In these researches, LiFePO4and S (cathode materials), NiO and MnO (anode materials) are selected as research objects, we designed the hierarchical porous micro-/nanostructure as an ideal model for electrode materials, and used novel strategies by using various biomaterials as both templates and carbon sources to fabricate electrode materials. We have made great efforts to understand the key factors of using biotemplates to tune the microstructure and morphology of electrode materials. Moreover, the relationships between the hierarchical porous micro-/nanostructure and electrochemical performances also have been studied. The main contents and results are as follows:1. A new kind of hierarchical spindle-like LiFePO4was successfully synthesized by a facile hydrothermal method for the first time. Reaction time and pH value played multifold roles in controlling the microstructure of LiFePO4. Spindle-like LiFePO4particles with a hierarchical porous structure can be obtained after20h reaction with pH=10. A growth model of "Nucleation-Aggregation-Self-assembly&Rearrangment-Further growth" was proposed to illustrate the formation of spindle-like LiFePO4samples. In order to investigate the growth orientations and grain boundaries of these nanocrystals, spindle-like LiFePO4sample was prepared by slicing it embedded in expoxy, and studied by transmission electron microscopy (TEM). The electrochemical performances clearly demonstrated that this hierarchical porous structure has the good structural stability, and also can increase the reaction surface between the electrolyte and the electrode materials and promote lithium ion diffusion. At a0.1C rate, the initial discharge capacity was163.57mAh/g, and it could remain157.32mAh/g after50cycles, suggesting the excellent cycling stability; Even at a5C rate, the reversible capacity was still as high as110mAh/g, implying the good rate capability.2. The live spirulina was used as both the biotemplate and carbon source for the fabrication of spiral hierarchical porous LiFePO4/C. Owing to the biological activities of algae, spirulina can adsorb and take up metal ions. Therefore, we can precisely replicate the morphology and microstructure of spirulina. The X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Raman results show that the obtained LiFePO4/C sample was composed of abundant LiFePO4grains connected and wrapped with "carbon networks", and attached at the surface of the spiral "carbon backbone". Such unique microstructure could enhance the reaction area, and is also beneficial to increase the electronic/ionic conductivity. As a result, spirulina-templated LiFePO4/C sample exhibited high reversible capacity (at a0.1C, the specific capacity was over140mAh/g after50cycles), excellent cycling performance (the capacity retention rate is nearly100%), good rate capability (at a5C rate, the reversible capacity was over100mAh/g).3. The facile strategy was developed for the fabrication of hierarchical hollow porous NiO/C microspheres deriving from the pollen grains by combining the biotemplating technique with a chemical bath deposition (CBD) method. The structural analysis revealed that NiO nanowalls directly grew on the surfaces of pollen grains and formed a hierarchical porous structure. Compared with porous NiO particles without biotemplates, pollen-templated NiO/C sample has a large specific surface area, multiple pore size distribution and good electronic conductivity. The initial irreversible capacity loss was only284mAh/g with a high coulombic efficiency of74.5%, which was much smaller than pure NiO sample with porous structure. After80cycles with the multiple-rate test, the pollen-templated NiO/C sample still delivered a high reversible capacity of618mAh/g. These enhanced electrochemical performances can be attributed to the following reasons. Firstly, NiO nanowalls can provide short pathways for fast lithium ions intercalation/de-intercalation. Secondly, the multiple porous structures can provide a larger surface area and allow better penetration of the electrolyte, as well as accommodate the strain induced by the volume changes during the charge/discharge cycling process. Thirdly, NiO nanowalls grew directly on the surface of the porous carbon support can form good adhesion and better electrical contact between NiO and carbon, which is beneficial for reducing charge transfer resistance.4. A facile and general approach combined the biotemplating technique with the immersion method was used to synthesize a unique microstructure of MnO/C microspheres. Morphology and microstructure of the hollow porous MnO/C microspheres have been investigated by XRD, TEM, HRTEM and STEM. The results demonstrate that MnO nanoparticles were tightly embedded into the porous carbon matrix and formed penetrative shells, suggesting a hollow feature. It was found that the biotemplates of Nannochloropsis oculata (N. oculata) played multiple roles, which acted as biologic templates to adsorb and take up metal ions via the electrostatic interaction, as well as carbon sources to form porous carbon matrices by the decomposition of carbonaceous organics in cells. As the anode materials in lithium ion batteries, N. oculata-templated MnO/C composites can not only deliver a high reversible capacity of700mAh/g at a low current density of0.1A/g, but also exhibit remarkable rate performance (over230mAh/g at3A/g). Such outstanding morphology and microstructure have the following advantages for achieving excellent electrochemical performance. Firstly, nano-sized MnO particles can effectively reduce the length of Li+solid state transport paths to give high Li+diffusion rate. Secondly, the pores in the MnO/C composite can well contact with the electrolyte, and achieve large reaction area. Thirdly, the amorphous carbon matrix can keep the structural integrity of the electrode, and provide good electronic contacts to reduce the contact resistance between MnO nanocrystals and electrolyte. Fourthly, hollow structure can enlarge the reaction surface area, and reversibly accommodate the drastic volume variation during the charge/discharge cycling process.5. Hollow porous carbon microspheres were synthesized by using Schizochytrium sp. cells as both biotempates and carbon sources. Combined with a facile and novel solvothermal methode, a new sulfur-carbon (S/C) composite was fabricated by confining sulfur in the as-prepared hollow porous carbon microspheres. The microstructure and morphology characterizations demonstrate that Schizochytrium sp. could remain its original morphology and size during the carbonization process. The as-prepared carbon microspheres have a unique hollow porous structure with a diameter of1-2μm, and possess excellent structural stability and good electronic conductivity, which can be considered as an ideal carbon support material for sulfur-carbon electrodes. After the solvothermal reaction, the elemental sulfur was highly dispersed inside the interior void space and micropores of carbon microspheres, and formed strong bondings between S and C. When evaluated as a cathode material for Li-S batteries, hollow porous S/C composites show remarkable electrochemical performances with high specific capacity, good cycling stability, and superior rate capability. In particular, S/C composite electrodes exhibit a high reversible capacity of697.2mAh/g with the capacity retention of95%after50cycles at a current density of0.1A/g. Moreover, even at a high current density of5A/g, the S/C composite still can deliver fascinating capacity of452.8mAh/g. |
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