Research On Modified Oxide Materials Of Compositing Li Storage Mechanism As Anode For Lithium Ion Batteries

As an energy storage device,an electrochemical power source can achieve the conversion between chemical energy and electrical energy,which can effectively make use of new energy,such as solar energy,wind energy and nuclear energy.Lithium ions batteries(LIBs),a kind of electrochemical power source with advantages such as high energy density,long cycle life,no memory effect and environmentally benign is extensively used and widely used for portable devices.Recently,with decrease in the oil resources and the growing concerns for the deterioration of the climatic conditions of our planet,the replacement of internal combustion cars which uses fossil fuels with more efficient,controlled-emission vehicles has become a focus of the whole society.These applications promote the research and commercialization process of new electrode materials for LIBs with a higher energy density and power density,rapid charging and discharging ability and good cyclability.Due to its easy availability,low cost,environment friendliness,and abundance in nature,transition metal oxide materials,have attracted a lot of research attention as a promising candidate for next generation anode materials.These metal oxides are associated with three kinds of mechanisms when used as an anode materials for LIBs:intercalation-deintercalation reactions,conversion(redox)reactions and alloying-dealloying reactions.Materials undergo different mechanisms have different characteristics:metal oxides react with Li in intercalation reactions have a good cycle stability due to small volume change,however,their specific capacity are fairly low;metal oxides associated with conversion reaction have a higher specific capacity,however,a much worse cyclability,because of pulverization resulting from the larger volume change in its discharge-charge process,its low electric and ionic conductivity;metal oxides react with Li in alloying reactions usually convert to metal first and then react with Li in to form alloys and the giant volume change and slow reaction kinetics make the first reaction irreversible,leading to poor cyclability and much lower capacities than the theoretic specific capacities.These drawbacks above hinder the applications of metal oxide as anode materials for LIBs.A lot of efforts were made to ameliorate the electrochemical performances of metal oxides by using strategies such as nanocrystallization,morphology design and making composite materials with carbon based materials.Aiming at the shortcoming of metal oxide materials,this dissertation focuses on modifications of metal oxides by combining materials with intercalationdeintercalation mechanism and materials with conversion mechanism/alloying-dealloying mechanism.The synergistic effect in mesoscopic scale between the mechanism with not only small volume change but also good cyclability as well as rate performance and mechanism with large volume change but high specific capaciy,can help augment the conductivity of electron and lithium ions,ease the internal stress arise from volume change,thus we obtain a composite with outstanding performance as LIBs anode materials.Ex-situ experiments were used to investigate the electrochemical processes of the composite materials and the results give a strong support to our strategy,suggesting this approach can be extended to other metal oxides.1.A simple and effective carbon-free strategy is carried out to prepare molybdenum oxide with mixed valence as an advanced anode material for lithium-ion batteries.The new material shows a high specific capacity up to 930.6 mAh·g-1,long cycle-life(>200 cycles)and high rate capability.1D and 2D solid-state NMR spectra,as well as XRD data on lithiated sample(after discharge)show that the material is associated both insertion/extraction and conversion reaction mechanisms for lithium storage.The well mixed molybdenum oxides with different valence states and the involvement of both mechanisms are considered as the key to the better electrochemical properties.The strategy can be applied to other transition metal oxides to enhance their performance as electrode materials.2.A facile method was used to modify ?-Fe2O3 to produce ?-Fe2O3/TiO2 pompon-like hollow sphere by using self-sacrificing carbonaceous microspheres as the template.The composition,structure,valence state as well as the lithium storage mechanism of the composite have been carefully studied with ICP elemental analysis,XRF,XRD,electron microscopy,XPS and solid-state NMR spectroscopy.As a new anode material for lithium ion battery,it shows enhanced electrochemical properties with a specific capacity staying above 970 mAh/g for 120 cycles at a current rate of 100 mA/g,which is much better than ?-Fe2O3.The significant improvement can be ascribed to the incorporation of a small amount of TiO2(?5%)in the composite and unique morphology of the heterostructure.This new strategy is expected to be extended to improve the electrochemical properties of other metal oxides for lithium ion battery applications.3.SnO2@Carbon hollow nanospheres were prepared via saturating carbonaceous nanospheres with a Sn2+solution followed by thermal treatment in air.The sample shows a homogeneous distribution of SnO2 nanoparticles and exhibits outstanding energy storage performance.The electrochemical measurement shows that the specific capacity of the composite stays above 1016 mAh/g after 130 cycles at a current rate of 100mA/g and very stable cycling performances.The significant improvement can be ascribed to hollow-shell hierarchical structures with shorter lithium transfer path and better structure-maintaining ability.The present work provides an insight into the design of hierarchical structures and use of mesoscopic synergy to control the particle sizes and increase the reactivities,as well as prevent aggregation,can be applied to prepare other electrode materials for improved energy storage properties.