Nitrogen-doped Carbon Decorated Metal Oxides Composites For Lithium Ion Storage With Enhanced Performance

Lithium ion batteries?LIBs?have been widely applied in portable electronics due to their available high voltage,high energy density and long life-time.As the most common used anode of LIBs,traditional carbon materials have some disadvantage,such as the low theoretical special capacity and unsafe operation in electric vehicle application,which have difficulty to meet the increasing demand for LIBs.The anode performance of carbon materials could be significantly improved by doping with nitrogen.This is mainly due to the net positive charge stemming from the higher electronegativity of N??= 3.04?than carbon??=2.55?and the stronger interactions between the N-doped carbon structure and the Li ions.On the other hand,considerable efforts have been devoted to seeking alternative anode materials with better performance.Hematite iron oxide??-Fe2O3?is considered to be a prospective anode material for LIBs because of its high theoretical capacity(800-1000 mAh g^-1).Mn3O4 have been investigated as the alternative anodes for LIBs due to their high theoretical capacity(937 mAh g^-1)and the low voltage platform?-0.2 V?.However,the poor cycling performance resulted from large volume expansion/shrinkage and agglomeration during the lithiation/delithiation and the low rate performance still challenge metal oxides serving as LIBs anode materials.To improve the cyclic performances of the metal oxides anodes,one successful strategy is to integrate carbon materials with metal oxide.In this study,metal oxide/carbon materials composites were successfully synthesized by one-step hydrothermal reaction followed with thermal annealing.The morphology and composition of samples were characterized via scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?and thermal analysis?TGA?,and their electrochemical performances as anode materials for LIBs were investigated using galvanostatic charge-discharge and cyclic voltammetry methods.1.Nitrogen-doped carbon fibers?NC?were prepared by thermal annealing in Ar atmospheres with polypyrrole nanofibers as carbon and nitrogen source.NC/Fe2O3 and NC/Mn3O4 were further successfully synthesized by one-step hydrothermal reaction.The samples were characterized via scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?and thermal analysis?TGA?.Electrochemical performances of NC/Fe2O3and NC/Mn3O4 as anode materials for LIBs were investigated using galvanostatic charge-discharge and cyclic voltammetry methods.It was found that the prepared composites anode deliver the high initial lithiation special capacity(948 mAh g^-1 for NC/Fe2o3 and 964 mAh g^-1 for NC/Mn3O4 at 100mAg^-1 with the cutoff voltage 0.01?3.0 V).After 100 cycles,the lithiation special capacity still remained at 628 mAh g^-1 and 807 mAh g^-1 for NC/Fe2O3and NC/Mn3O4,respectively.Even at 2000 mA g^-1,the lithiation special capacity of NC/Fe2O3 and NC/Mn3O4 were both still higher than 400 mAh g^-12.Highly nitrogen-doped carbon plate?CP?was prepared by one-step hydrothermal reaction followed with thermal annealing in Ar atmospheres.Metal oxide/carbon plate composites were further successfully obtained by one-step hydrothermal reaction.Scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Fourier transform infrared spectroscopy?FTIR?and thermal analysis?TGA?was employed to characterize the morphology and composition of the samples.Electrochemical performances of CP/Fe2O3 and CP/Mn3O4 as anode materials for lithium ion batteries were investigated using galvanostatic charge-discharge and cyclic voltammetry methods.It was found that the lithiation special capacity of the prepared composites anode was 631 mAh g^-1 for CP/Fe2O3 and 726 mAh g^-1 for CP/Mn3O4 at 100 mA g^-1 after 100 cycles.But at 2000 mA g^-1,the lithiation special capacity reduced to 303 mAh g^-1 and 203 mAh g^-1 for CP/Fe2O3 and CP/Mn3O4,respectively.Therefore,it is demonstrated that the prepared metal oxide/carbon materials composites are promising anode material candidates for LIBs.These excellent electrochemical performances of composite anode for LIBs could be attributed to the robust and high-conducting interconnected carbon materials embedded with a mass of small metal oxide nanocrystals,which not only can offer large quantity of accessible active sites for lithium-ion reaction as well as good conductivity and short diffusion length for electron and ion transport,but also can effectively suppress the aggregation and buffer the volume expansion of metal oxide nanocrystals during the repeated lithiation and delithiation processes.