| Recently, lithium ion batteries (LIBs) have been widely used in portable electronic devices, hybrid electric or all electric vehicles owing to the advantages of high specific capacity, long cycle life and large energy density. As the development of the society, the requirement of LIBs with high performance has to be improved to satisfy people's needs. However, the main commercial graphite as anode material possesses low theoretical specific capacity (372 mAh g-1) and poor ratio performance, which are far below the needs of large-scale energy application and limit the development of LIBs. Thus, exploring new-style anode materials have become the main research orientation for people. Cobalt oxide (Co3O4) has been the focus of scientific workers for LIBs due to its large specific capacity (890 mAh g-1), abundant resources and low cost. But Co3O4 is suffered from weak electric conductivity and the large volume change in the process of lithium ion (Li+) insertion/extraction, which cause the declined cycle life, poor rate capability and restrict its commercial application in LIBs. In order to improve the electrochemical performance and clean the environment, the plants or waste plants are used as the bio-templates and reactants, due to their large specific surface area and porous structure, which can control the morphology and composite of Co3O4, further improve the conductivity and buffer the volume change of products. The main research contents of this thesis are as follows: 1. The novel porous tubular Co3O4 was successfully prepared by a simple, low-cost and eco-friendly process using waste napkin paper as template and organizer. It is very noteworthy that the formation and self-assembly of Co3O4 nanoparticles occur simultaneously. The as-synthesized porous tubular structure with average outer diameter of 2.2 ?m is orderly self-assembled by numerous Co3O4 nanoparticles with diameter of 50-150 run. The specific surface area of typical product (sample 1) is 24.6 m2 g-1 by the BET method, and the majority diameter of pores is about 67 nm. In addition, the effects of different Co2+ concentration on the morphology and electrochemical performance of the products were explored. As anode materials for LIBs, the typical sample shows a high reversible specific capacity (1053 mAh g'1 after 100 cycles at a current density of 100 mA g-1), remarkable cycling performance and a good rate capability of 727 mAh g-1 after 100 cycles at a high specific current density of 500 mA g-1. The excellent electrochemical performance is attributed to the unique porous tubular structure. With these outstanding performances, the as-prepared Co3O4 may be an outstanding candidate anode material for LIBs.2. The waste bamboo leaves were simply treated in ammonia solutions by hydrothermal process, which led abundant cellulose fibers to separate and disperse spontaneously. Then the cellulose fibers were used as templates and reactants. After freeze-drying, followed by facile two-step calcination processes, the porous N-C/Co3O4 composite material was prepared successfully. The Co3O4 nanoparticles with diameter of 100-200 nm were distributed on the surface of N-C fibers with average diameter of 2 ?m. This method exhibits recycling of resource and turning waste into treasure in an eco-friendly way. And the doping N element in C material can enhance the conductivity of the product, which is benefit for improving the electrochemical performance. The discharge specific capacity of the N-C/Co3O4 composite is 887 mAh g-1 after 100 cycles at the current density of 100 mA g'1 and also exhibited a capability of 415 mAh g-1 at a high specific currentdensity of 1 A g-1, showing a high reversible specific capacity and good rate capability.3. The nori was used as sources of C, N and P with double hydrolysis reaction of CoCl2 and NaF for the synthesis of porous N,P-C/Co3O4 composite under the water bath at constant temperature, followed by two-step calcining process. The results of electrochemical test show that the N,P-C/Co3O4 composite exhibits a high reversible specific capacity of 927 mAh g-1 after 100 cycles at the current density of 100 mA g-1, excellent cycling performance and a good rate capability of 454 mAh g-1 at the current density of 1 A g-1. The favorable lithium storage performance is caused by the high specific capacity of Co3O4, the remarkable cycle performance of C, as well as the doping of N and P. Additionally, pyrolysis of the abundant sustainable nori resource provides a new route to prepare N,P-C/Co3O4 composite through a simple and eco-friendly pathway. What's more, this work demonstrated a good example for the development of other anode materials for LIBs. |
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