Phase Transition Mechanism And Electrochemical Performance Of Transition Metal Compound Composite Carbon Submicron Fiber

With the development and progress of science and technology,the demand for the energy storage devices is increasing.Lithium-ion batteries(LIBs)are considered as a promising energy storage device because of their high energy density and long cycle life.As an important part of LIBs,anode materials play a decisive role in the performance of batteries.Owing to the advantages of abundant resource,low cost and excellent electrochemical properties,transition metal compounds(for example,metal oxides and intermetallic compounds)are widely used as anode materials for LIBs.In this case,metal oxides can store more lithium ions due to higher theoretical specific capacity,and intermetallic compounds can catalyze the reversible formation and decomposition of solid electrolyte interface films.However,metal compounds suffer from the large volume changes during the charge and discharge processes,leading to structural collapse and poor cycling stability.Varoius methods have been proposed to improve their cycle stability,i.e.,compositing metal compounds with carbon submicron fibers.Nowadays,metal compounds/carbon submicron fibers have been prepared by using acetylacetonate as a precursor of metal compound,and great progress in the optimization of their electrochemical properties has been achieved.However,the chemical reactions and phase transformation of acetylacetonate/polymer fibers in the low-temperature pre-oxidation process have not been clarified.In addition,the phase transformation of two or more acetylacetonates in carbon submicron fibers during high temperature annealing process is rarely reported.Furthermore,the correlation between microstructure and electrochemical properties of metal compounds still lacks a deep understanding.Herein,polyacrylonitrile(PAN)was used as carbon source.The chemical reactions and structural changes were investigated for the iron acetylacetonate composited with PAN fibers in the pre-oxidation process.The composites of carbon submicron fibers and two acetylacetonates were prepared,and their phase transformation behavior during the annealing process was carefully studied.In addition,the effect of microstructure on electrochemical properties of the products obtained at different temperatures was explored in detail.The main research contents and results are listed as follows.(1)Iron(III)acetylacetonate/PAN fibers membranes were prepared by electrospinning,and the chemical reactions and phase transformation occurring in the pre-oxidation process were investigated by Fourier transform infrared spectroscopy,high-resolution transmission electron microscopy and electron energy-loss spectroscopy.It is found that after the pre-oxidation treatment,cyclization of PAN takes place,and iron acetylacetonate in the PAN fibers transforms into amorphous metal oxides,i.e.,Fe O and Fe2O3.(2)Nickel(II)acetylacetonate/iron(III)acetylacetonate/PAN fibers membranes were prepared by electrospinning.The composites of Fe Ni-based compounds and carbon submicron fibers were prepared through subsequent pre-oxidation and annealing processes,and their phase transformation mechanism during annealing process and electrochemical properties were investigated.A binary metal oxide,Ni Fe2O4,forms in carbon submicron fibers at 342 oC.With the increase of annealing temperature,part of Ni Fe2O4 is reduced to intermetallic compound,Fe0.64Ni0.36.When the temperature reaches 676 oC,Ni Fe2O4 is completely transformed into Fe0.64Ni0.36.As anode materials for LIBs,carbon submicron fibers loaded with Ni Fe2O4/Fe0.64Ni0.36 exhibit the best rate performance and cycling stability.In addition,the lithium storage behavior of carbon submicron fibers loaded with Ni Fe2O4 is mainly controlled by a diffusion-controlled process,while it is dominated by a pseudocapacitive process for the carbon submicron fibers loaded with Fe0.64Ni0.36.(3)Manganese(III)acetylacetonate/iron(III)acetylacetonate/PAN fibers membranes were prepared by electrospinning.The composites of Fe/Mn-based compounds and carbon submicron fibers were prepared through subsequent pre-oxidation and annealing processes,whose phase transformation behavior and electrochemical properties were investigated in detail.At low annealing temperatures(400-600 oC),a single metal oxide Fe3O4 and the other single metal oxide Mn O form in sequence in the carbon submicron fibers.With the increase of annealing temperature(700 oC),part of Fe3O4 is reduced to intermetallic compound Fe3 C,and the carbon submicron fibers are loaded with Fe3O4,Fe3 C and Mn O.When the annealing temperature reaches 800 oC,Fe3O4 completely converts into Fe3 C,and the carbon submicron fibers are loaded with Fe3 C and Mn O.As anode materials for LIBs,carbon submicron fibers loaded Fe3O4/Fe3C/Mn O exhibit the best rate performance and cycling stability.In addition,the anode materials obtained at higher annealing temperature possess lower charge transfer impedance and higher Li+ diffusion ability.