Controlled Fabrication And Property Study On Transitional Metal Compounds

Lithium-ion batteries(LIBs) as green and novel energy-storage devices have been widely applied in electronic products. Currently, the demand for LIBs is increasing due to the rapid development of electric vehicles. In addition, high performances of LIBs are required, such as long-term cycling life, high security and high power density, and therefore which researchers are trying to develop new electrode materials. However, the lower specific energy and the charge/discharge rate of the present commercial graphite severely limit their applications. In view of these disadvantages, we have synthesized various transitional metal compounds with low-cost, excellent electrochemical properties and characterized their structure and morphology. Moreover, the lithium-storage performances have also been studied systematically. The research work mainly contained the following six aspects:(1) A hydrothermal-calcined method is demonstrated to fabricate Mo O2/N-doped graphene(Mo O2/N-GNS) hybrid, in which Mo O2 nanoparticles(NPs) are uniformly dispersed on N-GNS sheets by Mo-N chemical bond formed between Mo O2 and N-GNS. As an anode material for LIBs, the Mo O2/N-GNS hybrid possesses superior rate capability, excellent cycling performance as well as high reversible capacity. The superior electrochemical performance of the Mo O2/N-GNS hybrid can be attributed to the synergistic effects of nitrogen-doping, flexible and conductive GNS, and Mo O2 NPs.(2) We developed a dual strategy, the nanostructure engineering and the proper choice of binder, to achieve excellent lithium storage performance of of α-Fe2O3. α-Fe2O3 nanoellipses with a mean size of 180-230 nm(edge length) and 140-170 nm(edge width) were fabricated by a hydrothermal method in the presence of glycine. Due to the chemical interaction between the α-Fe2O3 nanoellipse and sodium alginate binder, the α-Fe2O3 nanoellipse electrode with sodium alginate binder exhibits greatly enhanced lithium storage performance. The capacity could be retained as high as 1164 m A h g-1 at a current density of 100 m A g-1 for over 60 cycles, whereas the electrode with the conventional poly(vinylidene fluoride)(PVDF) binder suffers from rapid capacity decay under the same test conditions.(3) Significant challenge of transitional metal phosphides as anode materials for LIBs, such as poor cycle stability, must be addressed for their practical applications. Herein, a novel three-dimensional porous molybdenum phosphide@carbon hybrid(3D porous Mo P@C hybrid) has been fabricated by a versatile strategy, using a sol-gel method and Si O2 as a template. This new material possesses 3D interconnected hierarchically porous structure and the carbon coating, which could effectively accommodate the volume variation during the discharge/charge process and offer short pathways for the ions as well as electrons. Consequently, 3D porous Mo P@C hybrid presents stable cycling performance with high reversible capacity. At last, the Li-storage mechanism was investigated systematically.(4) A novel hierarchically porous heteronanorods consisiting of Mo P, Mo3 P and carbon(hierarchically porous Mo P/Mo3P@C heteronanorods) have been fabricated by a reduction/phosphidation strategy. The heteronanorods possess hierarchically porous structure with meso- and macro-pores. This novel material is a promising electrode material for LIBs, presenting stable cycling performance with high reversible capacity of up to 787 m A h g-1 at a current density of 100 m A g-1 after 100 cycles, which originates from the meso- and macro-porous structure, Mo P/Mo3 P heterostructure and the carbon coating.(5) Co9S8/C composite was prepared under hydrothermal conditions with pomelo peel as carbon source. The composite presents excellent lithium storage performance. The reversible capacity is as high as 1007.6 m A h g-1 at a current density of 100 m A g-1 after 100 cycles, which is better than those of pure Co9S8 and pure carbon. The excellent performance can be attributed to several factors, including high conductivity resulting from N and S co-doping, carbon matrix as a buffer layer and C-S-Co chemical bond preventing Co9S8 NPs from aggregation. Because the pomelo peel is waste, the transformation of the pomelo peel into carbon not only protects the environment, but also effectively improves the cycling stability of metal sulfides.(6) Hematite(α-Fe2O3) has emerged as a promising photocatalyst for catalyzing the photo-Fenton reaction. In order to enhance the catalytic activity of α-Fe2O3, tremendous effort has been made to engineer α-Fe2O3 catalysts with controllable facets. Different morphologies of α-Fe2O3 crystals, such as polyhedrons with exposed {101} and {001} facets, rods with predominant {001} facets, ellipses and cylinders, were selectively synthesized via a hydrothermal method. The studies of their catalytic performance have indicated that polyhedrons exhibit much higher catalytic activity than the rods, ellipses and cylinders for the visible-light photocatalytic degradation of methylene blue(MB). The possible reason may be attributed to the higher surface energy of {101} facets. So the catalytic property of α-Fe2O3 nanomaterials is closely related to the surface atomic configurations, which depend on their morphology.