Iron-Based Composites As Freestanding Electrodes For Energy Storage Devices

As one of the most abundant elements on earth,iron and its compounds are closely bonded to our daily life.Due to the advantages of iron compounds such as easy access,low price,nontoxic and easy processing et al.Researches about iron compounds has always been a hot topic in human history.Due to the development of industrial world,the demand for energy storage equipment is getting higher and higher in recent years.In search for efficient and clean energy storage materials.Iron-based materials found its position in electrochemical energy storage devices and made great contributions to it.But the intrinsic low conductivity hindered its boarder application,though weaving iron-based materials into freestanding conductive electrode is an effective way to solve this problem.In this work,we constructed freestanding iron-based electrodes though electrospinning technique and graphene aerogel loading process to make the best use of iron-based compounds in energy storage devices.The main achievements in this work include the following three parts:1.When used as the anode material for lithium ion(Li-ion)batteries,iron oxides(Fe2O3)exhibit high theoretical capacity of 1007 mAh g-1.However,Fe2O3 as an anode material still suffers from significant capacity irreversibility,short life span,and poor performance at higher current density due to its low conductivity and severe volume fluctuation during repeated lithium insertion and extraction.Here a porous ?-Fe2O3/C fiber(PFC)electrode was fabricated by electrospinning technique,in which poly(methyl methacrylate)(PMMA)was added to promote the uniform distribution of ?-Fe2O3 nanoparticles in polyacrylonitrile(PAN)fiber and create pores in the fibers during the heating process.The porous carbon fibers improved the electric conductivity of PFC electrodes and alleviated the volume expansion of ?-Fe2O3 during the charge/discharge process.In-situ XRD reveals the improved reaction kinetics of PFC.The optimized PFC2 electrode was able to maintain a reversible capacity of 1088 mAh g-1 after 300 cycles at 0.2 C,and a stable cycling performance at 2 C for 1000 cycles.2.Sodium is more abundant and widely available compared with lithium,which makes sodium ion(Na-ion)batteries a promising candidate in replacing Li-ion batteries to be applied for large-scale energy storage devices.Therefore,based our previous work,we further used iron acetylacetonate(FeAA)to replace ?-Fe2O3 nanoparticles in electrospinning,expecting the in-situ formed ?-Fe2O3 embedded porous carbon fiber(?-Fe2O3/PCF)electrode with more uniform dispersed Fe2O3 can yield a better performance in Na-ion batteries.The BET results proved the ?-Fe2O3/PCF electrode has a higher specific surface area of 68.2 m2 g-1.In-situ formed ?-Fe2O3 nanoparticles uniformly dispersed in the porous carbon fibers.This structure effectively reduces the volume expansion during the cycling process.Similar to PFC electrodes,the porous carbon fibers enhance the wettability of the electrolyte and the conductivity of the electrode.The ?-Fe2O3/C composite fiber electrode gives a high discharge capacity of 437 mAh g-1 and a high Coulomb efficiency of 88%in the first cycle.3.The development of electric vehicles requires high energy density and high-power density batteries which promoted the development of lithium sulfur batteries.In this study,a hybrid rGO/CNTs aerogel embedding FeP nanocubes was fabricated by instantly freezing and phosphorizing metal organic frameworks(MOF)-containing precursors,and used as a lithium-sulfur battery cathode for hosting polysulfides.A high specific capacity of 1312.3 mAh g-1 at 0.2 C and prolonged cycling with only 0.037%decay per cycle at 1 C were achieved for over 500 cycles,together with an exceptional high areal capacity up to 8.5 mAh cm-2 at the sulfur loading of 9.6 mg cm-2.This remarkable performalce is ascribed to the strong immobilization of polysulfides by the FeP anchoring material,the conductive and microporous carbon scaffolds in providing adequate interfaces for charge transfer,the homogeneous catholyte distribution for promoting sulfur utilization,as well as the catalytic effect of FeP on expediting the redox conversion of polysulfides.In conclusion,we fully take advantage of high-performance iron-based electrodes in different electrochemical systems.Through fabricating freestanding electrodes,remarkable electrochemical performances of iron-based materials have been achieved in Li-ion,Na-ion and Li-S batteries.