Electrochemical Lithium Storage Performance Of Grephene Composites Materials For Lithium-Ion Batteries

S-based and transition metal oxide electrode materials have high specific capacity and abundant resources. In recent years, they have attracted considerable interests as the secondary lithium battery electrode materials. However, their large-scale commercial application is limited by many problems:sulfur and most transition metal oxide have poor conductivity, which is not favorable to rapid electrochemical reactions; Severe volume expansion of metal oxide occurs in the charge-discharge process, which leads to the destruction of material structure or pulverization of electrode. The "shuttle" effect could happen during the electrochemical reaction of S-based electrode and make the loss of active substances. Above all, the cycle stability and high rate performance can not meet the needs of the practical application. In this paper, the graphene material with high conductivity and structure stability is introduced in the electrode material. The unique sheet-like stacking structure and cladding structure of graphene/sulfur composite materials have been successfully synthesized by different methods. The graphene/Fe2O3composites with "bird’s nest" structure have been synthesized by a facile ethanol-water cosolvent method. The electrochemical performances of the as-synthesized materials have been improved significantly. The main results are listed as follows:A simple and fast electrochemical method was used to exfoliate graphite into graphene sheets. The sulfur were impregnated into the graphene host through thermal diffusion method to obtain the graphene/sulfur composites with high electrochemical performance. The initial discharge capacity of the as-formed composite can reach650mAh g-1, the reversible capacity can keep as high as450mAh g-1over200cycles. Compared with pristine sulfur electrode, the graphene/sulfur cathode displays enhanced electrochemical performances, which can be attributed to the conductive matrix and the unique lamellar structural.This paper reports a facile and novel synthesis of a graphene-sulfur composite material by a rational combination of electrolytic exfoliation of graphite into graphene sheets and the electrodeposition of sulfur in situ as a one-step reaction. The formed sulfur particles are well enfolded by graphene aggregates, showing uniform size distribution with the average size of～1μm. Unlike other methods of synthesis that rely on diffusion of the sulfur into the graphene framework, our method can keep sulfur particles with intimate interaction to the conducting graphene matrix, which are important for retarding the shuttling of soluble polysulfides and rendering the sulfur particles electrically conducting. The resulting graphene-sulfur (70.2wt.%) composite delivers an initial discharge of1180mA h g-1and retains above700mAh g-1after50cycles, representing a promising cathode material for rechargeable lithium batteries with high energy density.The pure Fe2O3and graphene/Fe2O3composites have been synthesized by a facile hydrotherrmal method in ethanol-water cosolvent system. The as-synthesized pure Fe2O3microcubes have a uniform size distribution with the edge length of about1.5μm, while the graphene/Fe2O3composite has "eggs" in the graphene nest structure to improve the structural stability. Compared with pure Fe2O3, graphene/Fe2O3composites show enhanced electrochemical performances in terms of long-term cycling (500mAh g-1,50cycles). These remarkable electrochemical properties can be attributed to the unique nest microstructure, which could remain structural stability, relieve stress and increase reaction areas.