| Lithium-ion batteries (LIBs) are a new kind of energy-storage devices with high energy density, high output voltage and friendly environment performance, and have been wildly investigated and used in portable electrical appliances such as the mobile phone and laptop, power station and electric vehicles. The anode materials in LIBs with high reversible capacity, long-term cycle stability and high-rate performance can not only meet the demands of the potable devices, but also, most significantly, can provide a new type of promising negative electrode candidates for electric vehicles and hybrid electric vehicles.In this dissertation the structural design, preparation and lithium-storage properties of a novel anode material, hierarchical porous carbon nanosheets (HPCS), were investigated using phenolic formaldehyde resin (PF) as the carbon source and copper nitrate as the template precursor. The morphologies, porous structures and electrochemical performances were studied in detail by SEM, TEM, XRD, IR, BET adsorption, Raman and the electrochemical measurement tools, and the relationship between pore structure and lithium-storage performance was clarified. Firstly, a complex compound of copper nitrate and pyridine was formed, which induces the orientated arrangement of PF resin to generate the nano-layered carbon structure. The effects of copper nitrate addition and carbonization temperature on the morphologies, structures and electrochemical properties of HPCS electrodes were investigated. Then, by further graphitization in2800℃, novel porous graphene nanosheets with low specific surface area and onion-like hollow pores were prepared. Afterwards, nano-sized porous carbon nanosheets and spherical porous carbon were synthesized in a W/O emusion system, respectively, which show both high specific capacities and good high-rate performance for LIBs. Finally, macroporous and slit-typed porous carbon materials were synthesized using SiO2nanospheres with the diameter of200nm and LDHs as the templates, and PF resin and mesophase pitch as the carbon sources, respectively.HPCS were formed from PF resin by the inducement of complex compound of copper nitrate and pyridine, and at the same time Cu ions were reduced into Cu nanoparticles as template to occupy the space in PF resin. The HPCS, possessing the thickness of about30-40nm, width of about several micros and specific surface area of496m2g-1, exhibited high specific capacity and favorable high-rate performance when used as anode material for LIBs. The reversible capacities were748mAh g-1at a current density of20mA g-1and460mAh g-1even at1A g-1, respectively, which were much higher than those of traditional porous carbon materials reported. It also showed superior cyclical stability for only0.3%capacity loss per cycle under high rate charge-discharge process. The high electrochemical performance can be ascribed to the carbon layered porous structure of HPCS, in which the carbon layer is beneficial for the infiltration of electrolyte ions and the mesopores can effectively shorten the ion diffusion path and improve the ion diffusion rate.The copper nitrate loading content has great influence on the morphology, pore structure and lithium-storage performance for HPCS. It was found that, with the addition of Cu(NO3)2, both the specific surface area and mesopore percentage increase. Correspondingly, the electrochemical performances of HPCS electrodes in terms of the specific capacity and rate performance improve for LIBs. The HPCS with the Cu/C atomic ratio of1:2possessed the thickness of1-10nm, specific surface area of560m g-1and mesopore size in the range of30-80nm, and showed the reversible capacity of671mAh g-1at500mA g-1.The morphology, porous structure and electrochemical performance of HPCS were also effected by the carbonization temperature. With the increase of heat treatment temperature, the specific surface area shows the tendency of first increase and then decrease. The carbon nanosheets shrinked to dense structure, pore size also became smaller, and the lithium-storage mechanism is changed from the hard-carbon type to graphite-intercalation mechanism with high reversible capacity in low voltage (-0.2V). By graphitization at2800℃, a new kind of porous graphene nanosheets containing hollow onion-like nanopores (PGN) was obtained from HPCS. The PGN with the thickness of about5-10nm exhibited a high reversible capacity of430mA h g"1at50mA g-1and high-rate Li-storage performance of258mAh g-1at1A g-1and105mAh g-1at10A g-1after700cycles when used as an anode material for lithium ion batteries.When PF resin was cross-linked in a silicone oil dispersion phase by the mechanical stirring of400r min-1, the small-sized HPCS containing micro-, meso-and macropores were formed. This carbon nanosheets displayed the width of100-2000nm, thickness of2-20nm, and specific surface area of516.7m2g-1, and exhibited as high as1193.9mAh g-1reversible capacity at50mA g"1, and587.2mAh g"1after100cycles at1A g-1, respectively. The carbon nanosheets with small width endow the abundant lithium-storage sites in si da/edge/pores of HPCS and high-rate ability.Spherical porous carbon (SPC) with the diameter of5-20μm were prepared from PF resin using copper nitrate as the template precursor in the dispersion phase of peanut oil at certain stirring rate. The effects of stirring rate and curing temperature on the morphology and porous structure were investigated, and it was found that with the increase of stirring rate, the SPC diameter became larger; the higher the curing temperature is, the larger the SPC diameter is. The SPC with small size showed higher reversible capacity and coulumbic efficiency when used as the anode material for LIBs.PF-based porous carbon monoliths (PCM) with uniform macropores of about200nm were prepared using spherical Si2as the template. Compared with the glassy carbon from direct carbonization of PF resin, the PCM exhibited the higher reversible capacity of388mAh g-1at50mA g-1. Using layered double hydroxide (LDH) as template, the PF-based slit-type porous carbon materials with the specific surface areas of650-890m2g-1were synthesized, and it was found that this carbon material exhibited the higher reversible capacity of994mAh g-1at50mA g-1and good high-rate performance with capacity of281.9mAh g-1at1A g-1. Using SiO2and copper nitrate as the bi-templates, novel carbon nanosheets containing both macropores and mesopores were obtained. The lithium-storage capacity can attain more than830mAh g-1when used as the anode material for LIBs. Although more Li-storage sites were introduced by macropores, too many macropores undoubtedly damage the structural stability of carbon sheets and further lead to the decrease of rate performance.A new pitch-based porous carbon monolith was prepared from a mesophase pitch using SiO2as the template. Although this porous carbon possessed low specific surface area of only25.8m2g-, it had high reversible capacity and good high-rate performance when used as the anode material for LIBs:778.9mAh g-1at the current density of50mA g-1and263.9mAh g-1at1A g-1, respectively. Compared with the PF-based porous carbon materials, the pitch-based porous carbons generally have higher graphitization degree and electrical conductivity, which make them possess higher first columbic efficiency of above70%at charge/discharge processes.The effect mechanism of pore size in carbon anode materials on the lithium-storage performance was deduced from the above results as following. Macropores and some part of mesopores play the function of electrolyte ion "reservoir", which is beneficial for the rapid transport of lithium-ions and the improvement of high-rate property. Some part of mesopores and micropores can act as the active sites to store lithium-ions and enhance the specific capacity. However porous carbon anode materials for LIBs usually present the disadvantages of lower first columbic efficiency, which inhibits the practical applications in LIBs. Further explorations including the modification and decoration of surface and the optimization of porous structure for carbon materials should be carried out. |
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