Fabrication And Electrochemical Lithium/sodium Storasge Properties Of Novel Nanostructured Transition Metal(Fe,Co,Ni)Oxides

With the rapid development of the global economy,the fast depletion of fossil fuels and the increasing environmental concerns,there is an urgent need for developing clean sustainable energy sources and efficient energy storage devices.Rechargeable lithium-ion batteries(LIBs)have become the focus of research because of the advantages of high theoretical capacities,no memory effect,long lifespan and environmental benignity.However,the low electrical/ionic conductivities and poor structural stability of transition metal oxides lead to their poor electrochemical performance,especially at high rate.The major approaches towards remission of these encountered issues are to reduce the size of active materials to nanoscale,design andfabricate carbon-coated materials or mix/dope the active materials with high conductive materials.In addition,owing to the similar electrochemical behavior and abundant natural resource,sodium-ion batteries(SIBs)are considered to be an alternative to Li-ion batteries for use in electric vehicles and large-scale energy storage.In view of the above analysis,this paper has designed and prepared transition metal oxide powder with different microstructures,transition metal oxide film electrodes directly grown on the current collector and the metal ion doped film electrode.Meanwhile,their lithium/sodium storage properties and mechanisms have been investigated intensively.The main contents and results are as follows.(1)FeC2O4·2H2O microrods,tube-like FeC2O4·2H2O microflowers and rod-like FeC2O4·2H2O microflowers were prepared by using chemical precipitation methods at room temperature and controlling the solvent composition,with FeSO4·7H2O and H2C2O4/Na2C2O4 as raw materials.After the heat treatment in Ar,the porous Fe3O4 microrods,tube-like Fe3O4 microflowers and rod-like Fe3O4 microflowers were obtained,respectively.Compared to the commercial anode graphite(372 mAh g-1),the as-prepared three Fe3O4 samples showed better lithium storage performance.a)Porous Fe3O4 microrods delivered a high capacity of 604.7 mAh g-1 after 100 cycles at 100 mA g-1.b)Tube-like Fe3O4 microflowers were self-assembled from the microtubes with a diameter of?1 ?m and porous structure,and the microtubes were composed of many nanoparticles.This material exhibited a revesible capacity of 799.3 mAh g-1 after 100 cycles at 100 mA g-1.c)Rod-like Fe3O4 microflowers self-assembled by microrods maintained a capacity of 464.7 mAh g-1 after 100 cycles at 100 mA g-1.In order to further improve their electrochemical performance,rod-like Fe3O4/C microflower composites were prepared by using gelatin as carbon source.Compared to pristine Fe3O4,Fe3O4/C material showed better cycle performance(836.8 mAh g-1 after 100 cycles at 100 mA g-1)and improved rate capability.(2)Different Co(OH)F precusor films on copper foil were fabricated by a low-temperature solvothermal/hydrothermal method using CoSO4·7H2O,NH4F and urea as raw materials with water/isopropanol or water as solvent.After annealing treatment in Ar atmosphere,CoO microflower/microsphere films were obtained.When employed directly as the electrodes for lithium ion batteries(LIBs)and sodium ion batteries(SIBs),the as-prepared CoO films both showed excellent electrochemical properties.a)For lithium storage,the CoO microflower films delivered a reversible capacity of 1297.9 mAh g-1 after 500 cycles at 454.5 mA g-1,and the CoO microsphere films exhibited a high capacity of 950 mAh g-1 after 120 cycles at 200 mA g-1.b)For sodium storage,the CoO microflower film electrode showed a capacity of 277.8 mAh g-1 after 100 cycles at 90.9 mA g-1,the CoO microsphere film delivered a capacity of 172 mAh g-1 after 100 cycles at 100 mA g-1.Further,the sodium storage mechanism of CoO was investigated by ex situ XRD,ex situ XPS and ex situ TEM.(3)The synthesis method for the 3D foam-like CoNiO2 film and CoNiO2 nanosheet array film on Cu foil was similar to that for the CoO films,except for adding NiSO4·6H2O into the raw materials.The growth mechanisms of 3D foam-like CoNiO2 film and CoNiO2 nanosheet array film were investigated by the SEM images at different reaction times.The sodium storage properties of the CoNiO2 films and the corresponding scraped powders were compared and analyzed.a)The 3D foam-like CoNiO2 film delivered a reversible capacity of 323.7 mAh g-1 after 100 cycles at 100 mA g-1,while the scraped powders only held a capacity of 126 mAh g-1 after 100 cycles.b)The CoNiO2 nanosheet array film exhibted an excellent sodium storage performance,with a capacity of 746.6 mAh g-1 after 100 cycles at 200 mA g-1.In contrast,the specific capacity of the scraped powders was only 122.3 mAh g-1 after 100 cycles.The remarkable electrochemical performance of the film electrodes may be attributed to their unique nanostructures and binder-free feature,which effectively improved the ionic and electron conductivities of the electrodes.