Graphene Loaded Metal Oxides As High-performance Lithium-ion Battery Anode Materials

Graphene has been extensively studied as carbon matrix for secondary battery electrode materials due to its large specific surface area, excellent electrical conductivity, good mechanical strength and chemical stability. Transition metal oxides and sulphides have been extensively studied for lithium-ion battery anode materials due to their high capacities. During the charge-discharge process, the pulverization problem induced by large volume changes leads to loss of electrical contact and subsequent rapid capacity fading. In the present study, we use the thermal exfoliation graphene nanosheets (GNS) with large specific surface area and porous structure as matrix for amorphous mesoporous Fe2O3nanofilm, mesoporous NiO nanosheet, MnO nanoparticle, and SnS2nanoparticle. By the combination of these lithium-ion battery active materials and GNS, the volume change during charging and discharging was controlled effectively, thus improving significantly the cycling performance of the active materials.By the ethanol evaporation method, iron nitrate nanofilm was coated on GNS surface. Mesoporous iron oxide/GNS composite was prepared via in situ thermal decomposition at low temperature. The electrochemical test results show that the mesoporous iron oxide/GNS composite possesses excellent cycling performance, and there is no capacity fading after400cycles at1000mA/g. Furthermore, the battery has a large capacity of1000mAh/g at100mA/g, and shows a good rate capability. The existence of covalent chemical bonding formed through oxygen-containing defect sites on GNS surface provides an opportunity to tightly anchor iron oxide on GNS, allowing GNS to constrain the volume change of the MIO nanofilm. Under the constraint effect of the GNS, the volume expansion/contraction of the MIO progresses along the vertical direction of the GNS. Therefore, the lithiation-induced strain is easily relaxed by increasing/decreasing the nanofilm thickness. Furthermore, the mesoporous structure of MIO also partially accommodates the lithiation-induced strain because its high porosity provides free space. These two features can effectively prevent electrode pulverization upon lithium-ion insertion/extraction, thus enhancing the cycling performance. The present results provide a new insight into the accommodation of the large lithiation-induced strain for volume changes. Furthermore, the mesoporous NiO nanoplate/GNS composite was prepared through the growth of Ni(OH)2nanoplate on GNS surface by using the hydrothermal method and subsequently heat treatment at a low temperature. The composite showed a high capacity of about700mAh/g at100mA/g, and good cycle stability and rate capability.This thesis developed one-step method to obtain the composite structure of the metal oxide and graphene. Ethanol evaporation method was used to prepare manganese nitrate/GNS composites. By further decomposition of manganese nitrate and carbothermal reduction under high temperature, MnO nanoparticles were obtained and bound strongly to GNS. The electrochemical test results showed that the MnO/GNS composite possessed a capacity of about700mAh/g at100mA/g, excellent cyclic stability, and good rate capability. During the charge-discharge process, graphene nanosheets served as a three-dimensional conductive network for MnO nanoparticles. Furthermore, the detachment and agglomeration of MnO nanoparticles were effectively prevented due to the tight combination of MnO nanoparticles and graphene. In addition, compared with other metal oxides, MnO electrode showed a lower charging voltage, which is favorable for increasing the operation voltage and energy density when the electrode is used as an anode in full batteries. GNS-SnS2nanocomposite was prepared via a solvothermal method with different loading of SnS2. SEM and TEM results indicated that SnS2particles distributed homogeneously on GNS. The electrochemical properties of the samples as active anode materials for lithium-ion batteries were examined by constant current charge-discharge cycling. The composite with weight ratio between GNS and SnS2of1:4had the highest rate capability among all the samples and its reversible capacity after50cycles was351mAh/g, which was much higher than that of the pure SnS2(23mAh/g). With GNS as conductive matrix, homogeneous distribution of SnS2nanoparticles can be ensured and volume changes of the nanoparticles during the charge and discharge processes can be accomodated effectively, which results in good electrochemical performance of the composites.