Synthesis Of Iron-based Oxides And Properties Of Electrode/Electrolyte Interfaces For Lithium-ion Batteries

Simple transition metal oxides (such as MnO2、α-Fe2O3、Fe3O4、Cr2O3、Co3O4、MnO、Cu2O) have long been intensively investigated as possible candidates for thenext generation anode materials in lithium ion batteries (LIBs), because of their highreversible capacities (700mAh/g). Among the transition metal oxides, iron oxides(α-Fe2O3、Fe3O4) are extensively investigated as anode materials for LIBs due to theirhigh theoretical capacity, inexpensive materials and environment friendly, and so on.However, the conductivity of the iron oxides is low, and the lithiation of iron oxidesusually leads to huge volume changes, and consequently, resulting in poor cyclingstability and rate capability, which hamper the applications of iron oxides as the anodematerials. To circumvent these obstacles, the most common and effective method is tosynthesis of iron oxides/carbon composites and iron oxides with special morphologies.Based on the above two points, the different types of carbon materials were mixedwith iron oxides, and iron oxides with special morphologies were synthesized in thispaper, in order to search for anode materials of high density. The electrochemicalimpedance spectroscopy (EIS) techniques were used to explore the electrode kineticprocesses and the electrode interface performance. The main research content andresults are as follows:(1) The α-Fe2O3/C composites were prepared by high-temperature solid-statereaction. The structure and electrochemical performance of the composites werecharacterized by X-ray diffraction (XRD), scanning electron miscroscopy (SEM),charge/discharge test and electrochemical impedance spectroscopy (EIS). Theelectrochemical test results indicated that the α-Fe2O3/C composites showed areversible charge capacity of935.3mAh/g after50cycles, and had better cycleperformance compared with commercial α-Fe2O3. Electrochemical impedancespectroscopy test indicated that there appeared three semicircles respectivelyrepresenting the Li-ion migration in solid electrolyte interface film (SEI film),electrical conductivity and charge transfer in the first lithiation, and their evolutiveprinciples were also investigated.(2) The α-Fe2O3/GNS、α-Fe2O3/CNTs hybrid materials and α-Fe2O3microparticleswere synthesized by a facile hydrothermal method, respectively, and the carbon motifefforts on the morphology, structure and electrochemical performance were studiedsystematically. The results showed that the α-Fe2O3/GNS、α-Fe2O3/CNTs electrodes exhibited a large reversible capacity and rate capability, especially excellent long-lifecycling performance at a high current. The improvements can be due to the CNTs inthe3D network, which several functions, including1) alleviating the mechanicalstress caused by the severe volume change and preventing the aggregation betweenthe active materials;2) providing large reaction surface and favoring the efficientelectrode/electrolyte interface contact;3) increasing the electronic conductivity ofelectrodes by forming3D conductive network.(3) The α-Fe2O3hollow nanostructures were prepared by a facile hydrothermalmethod. As the reaction time increases, α-Fe2O3underwent an evolution fromspindlelike precursors to nanotubes. Based on evidence from the abovetime-dependent morphology evolution evidence, the formation process of thenanotubes can be proposed as taking place by “dissolution” of the spindle-likeprecursors from the tips toward the interior along the axis, resulting in rod-likecrystals, semi-nanotubes and eventually hollow nanotubes, which follows apreferential dissolution along the [001] direction of nanotubes (C axis). Furthermore,the experiments were conducted with a fixed mass of sulfate ions and ferric ions butvarious quantities of phosphate ions (PO43-), and a series of nanostructure includingshort nanotubes, very short nanotubes, and nanorings were obtained. The resultsshowed that the roles that phosphate and sulfate ions played in the formation of thehollow nanostructure should be different, namely, phosphate ions played a moreimportant role than sulfate ions in the formation of the precursors in the early stage ofα-Fe2O3formation process, while sulfate ions favored the dissolution of α-Fe2O3dueto their coordination effect with ferric ions, resulting in the formation of ananostructure with hollow interior. Electrochemical measurements indicated that theα-Fe2O3nanotubes showed the best electrochemical performance, that the α-Fe2O3nanotubes displayed a large reversible capacity of1131mAh/g at100mA/g after65cycles, which was83%retention of the first charge capacity. Addition, the α-Fe2O3nanotubes presented the highest lithium storage capacity and best rate capacity atvarious rates, due to its special structure.(4) The Fe@Fe2O3core-shell nanoparticles anchored on graphene or CNTs hadbeen firstly synthesized by using a facile hydrothermal reaction. The galvanostaticcycling test showed that the Fe@Fe2O3/graphene electrode displayed a reversiblecharge capacity of959.3mAh/g up to90cycles at a current density of100mA/g,which was86.4%retention of the first charge capacity. At high current of5C, the Fe@Fe2O3/graphene electrode remained at515mAh/g after280cycles. Furthermore,the first lithiation process of Fe@Fe2O3/graphene electrode was studied byelectrochemical impedance spectroscopy (EIS) at different potentials. There appearedthree semicircles respectively representing the Li-ion migration in solid electrolyteinterface film (SEI film) and contact problems, electrical conductivity and chargetransfer in the first discharge process, and the change of kinetic parameters forlithiation process of Fe@Fe2O3/graphene electrode as a function of potential wasdiscussed in detail.The results showed that the Fe@Fe2O3/CNTs electrode exhibited a reversiblecapacity of702.7mAh/g up to60cycles at a current density of100mA/g, whichdisplayed much rate capability, especially, a large reversible capacity at high current.EIS tests showed that the Fe@Fe2O3/CNTs electrode had much smaller SEI resistanceand charge-transfer resistance due to the CNTs and Fe metal in the Fe@Fe2O3/CNTscomposites, resulting in the improvement of the electrochemical performance of theFe@Fe2O3/CNTs composites.(5) The Fe3O4-HSs and Fe3O4-HSs/CNTs hybrid materials were synthesized bysolvothermal method, respectively. After70cycles, the Fe3O4-HSs/CNTs electrodeexhibited a reversible capacity of1153.8mAh/g, which was87.8%retention of thefirst reversible capacity. Even at10.0A/g, the reversible capacity of Fe3O4-HSs/CNTselectrode remained552.7mAh/g after350cycles.The Fe3O4/CNTs hybrid material was synthesized by hydrothermal reaction.Charge-discharge tests showed that the initial discharge was1421mAh/g forFe3O4/CNTs composites,1651mAh/g for Fe3O4/C composites and2194mAh/g forcommercial Fe3O4, and the reversible capacity was1030mAh/g for Fe3O4/CNTscomposites,513mAh/g for Fe3O4/C composites and280mAh/g for commercialFe3O4after55cycles. The main Nyquist characteristic of Fe3O4/CNTs electroderecorded by EIS showed both high and middle frequency region, and an arc in thelow-frequency region, which respectively representing the Li-ion migration in solidelectrolyte interface film (SEI film), charge transfer and phase transformation in thefirst lithiation.