Synthesis And Electrochemistry Performance Studies Of Graphene Anode Material For Lithium-ion Batteries

Lithium-ion batteries feature high energy density, high average output voltage and has no memory effect. The promising application areas of lithium-ion batteries include mobile telecommunication, information technology, consumer electronics, electric vehicles, military and aviation aerospace. Up to now, research in anode material of Li-ion batteries become more and more popular. Majority of the Li-ion batteries use graphite as anode material, since graphite has high conductivity, high reversible specific capacity and unique layer structure. This layer structure is beneficial to extraction and insertion of Li+ and lead to the formation of Li-GIC. The charge and discharge specific capacity of graphite can reach over 300 mAh·g-1, the coulombic efficiency up to 90% and the irreversible capacity lower than 50 mAh·g-1 However the theoretical capacity of graphite is not very high and the graphite material can not compatible with organic solvent, in recent years many research works were carried out on modification of graphite.Since its discovery in the pioneering work of Geim, graphene was considered a potential alternative to graphite in LIB. Graphene is composed of two-dimensional layers with one-atomic thickness and a high surface area of 2630 m2 g-1. It has superior electrical conductivity, outstanding electronic behaviors, remarkable mechanical properties, and broad electrochemical stability window. This paper is a result of research work in the area of electrochemical energy storage devices and Li-ion batteries, and it is believed that the subject under study is a frontier of current research topics. Graphene was prepared by three different methods, the influence of various synthesis conditions were investigated in this work through a series of experiments. The morphology and structure of the samples were investigated by scanning electron microscopy, transmission electron microscope and IR. And following conclusions were drawn:1. Graphene materials were prepared by solution-based reduction of graphene oxide, the effects of ultrasonic dispersion time and refluxing time on the electrochemical behavior of graphene were studied. The results indicated that the graphene material prepared by ultrasonic dispersion for 3 h and refluxing for 24 h under 100℃is better in performance. The first discharge capacity of the graphene electrode is 1030 mAh·g-1 at the current density of 50 mAh·g-1, and the discharge capacity is 546 mAh·g-1 after 20 cycles. The first cycle of the graphene electrode shows a solid electrolyte interface (SEI) formation, which causes the loss of capacity that is irreversible. However the irreversible capacity does not exit from the second cycle.2. Graphene materials were prepared via annealing under nitrogen atmosphere, the effects of three different calcination temperature and time on the electrochemical behavior of graphene were studied. The results indicated that the garphene prepared by three reaction conditions exhibited the electrochemistry performance almost the same to graphite. The graphene calcinated at 900℃for 2 h exhibit a first discharge capacity of 1513 mAh·g-1 at the current density of 50 mAh·g-1, and the discharge capacity is 450 mAh·g-1 after 50 cycles.3. Graphene anode materials for Lithium-ion battery were prepared by H2 thermal reduction of graphite oxide, the effects of reaction temperature and time on the electrochemical behavior of graphene were studied. As compared with traditional way, this method is simple and practicable. The results indicated that the graphene calcinated at 300℃for 2 h under hydrogen atmosphere exhibit a first discharge capacity of 2274 mAh·g-1 at the current density of 50 mAh·g-1, and the discharge capacity is 1648 mAh·g-1 during the second cycle. The discharge capacity is 1283 mAh·g-1 after 50 cycles. After H2 reduction, the carbon/oxygen ratio of graphene is increased from that of graphite oxide due to the removal of oxygen-containing functional groups as it is demonstrated from IR spectra. The d-spacing of resulting graphene nano-sheets is increased to 0.37 nm which facilitates Li intercalation.