1.Introduction

The rapid development in the portable electronics and electric vehicles desperately requires high-performance energy storage con figuration[1].On account of high capacity,fast charge-discharge rates,long lifespan,and high stability,rechargeable lithium-ion battery(LIB)is a very promising energy storage devices that can satisfy the constant rise energy storage demand[2].Owing to cost,limited Li-ion diffusion,potential operational risks,and environmental impact,the adoption of the current LIBs in strategically momentous applications has become much slower[3,4].The key point to enhance LIBs performance and reduce cost is mainly based on the developmentofthe electrode materials.Tillnow,commercialized graphite is commonly utilized as the anode material due to its good conductivity,plentiful resource,and low cost[5].However,graphite materials have hampered the application of LIBs in desirable fields owing to the limited theoretical capacity(372 mAh g-1)and poor rate performance[2,6,7].To achieve superior LIBs performance,tremendous researches have been triggered to exploring appropriate anode materials with high capacity,outstanding rate performance and high cycling stabilities,such as silicon-based materials,tinbased materials,transition metal sulfides and transition metal oxides[8-10].Transition metal oxides(e.g.,Fe2O3,Fe3O4,Co3O4,NiO,MnO2and SnO2)are one category of alternative potential anodes materials for energy storage in LIBs due to the high theoretical capacities(2-3 times that of graphite materials),high power density,low cost,and wide availability[11-13].

Among the myriad transition metal oxides,Fe2O3have been investigated intensively as a forward-looking anode material toward LIBs on account of the high theoretical capacity(1005 mAh g-1),high corrosion resistance,environmental benignity,abundant availability,and low cost[14-19].The high capacity of Fe2O3is primarily achieved by the reversible conversion reaction between Fe2O3and Li+[15,20].In addition,the volumetric capacity of Fe2O3is approximately six times higher than that of graphite owing to the high inherent density(5.24 g cm-3for Fe2O3vs.2.16 g cm-3for graphite)[16].Despite of these distinct features,similar to all high-capacity transitional metal oxides,Fe2O3inevitably suffer from low rate capability and inferior cycle ability due to slow reaction kinetics,poor electronic conductivity,severe volume expansions,pulverization,and agglomeration during the charge-discharge processes[11,21,22].Two main strategies have been summarized to overcome the above deterioration.One of the strategies is that Fe2O3can be composited with various carbonaceous materials(e.g.,amorphous carbons,graphite,graphene,carbon nanotubes,and hollow carbon sphere)to enhance electronic conductivity and buffer the volume expansion[23].Unfortunately,the capacity of the Fe2O3/carbon composites can be reduced because carbonaceous materials usually offer lower capacity and density than Fe2O3[12].In this regard,the other effective strategy is to carefully design different dimensions,hierarchical,and hollow nanostructures [24].The well-designed efficient nano structures can reduce Li-ion diffusion pathway,accelerate Liion transfer,and restrain volume expansion,resulting in improved reaction kinetics and cycle stability[16].

Herein,we report a high-performance anode for LIBs based on α-Fe2O3nanoplates which have exceeded its theoretical capacity.The α-Fe2O3nanoplates can be synthesized by a simple iron ion-based ionic liquid(1-octyl-3-methyl imidazolium tetrachloride ferrate,[Omim]FeCl4)assisted solvothermal route.For comparison,the α-Fe2O3solid microspheres and α-Fe2O3 irregular nanoparticles are also prepared by solvothermal processwithC8H17)2(CH3)2N]FeCl4and FeCl3as precursor,respectively.Scheme 1 exhibits the proposed formation process of α-Fe2O3nanoplates.The iron ion-based ionic liquid has been designed for the formation of α-Fe2O3nanoplates.The iron ionbased ionic liquid can play role as iron source and template for formation of the α-Fe2O3nanoplates[25,26].On account of unique nanoplate structures and gum arabic as binder,the α-Fe2O3nanoplates as the anode of LIBs can display high rate capability and outstanding cycling property.The α-Fe2O3 nanoplates electrode maintains the high capacity of 1211 mAh g-1(≈1.2 times of theoretical capacity)in 0.5 A g-1 after 100 cycles.

2.Experimental section

2.1.Fabrication of α-Fe2O3nanomaterials

Iron ion-based ionic liquids([Omim]FeCl4and[(C8H17)2(CH3)2N]FeCl4)were prepared with conventional ionic liquids(1-octyl-3-methylimidazolium chloride, [Omim]Cl and[(C8H17)2(CH3)2N]Cl)and FeCl3⋅6H2O[25,27].The α-Fe2O3 nanoplates were synthesized by a simple iron ion-based ionic liquid assisted solvothermal method.16 mmol of[Omim]FeCl4were dissolved in 80 mL of absolute ethanol with vigorous stirring.24 mmol of NaOH was added into the above solution which was changed to red immediately.The red solution was continuously stirred(30 min)and then placed into a 100 mL Teflon-sealed autoclave.The Teflon-sealed autoclave was maintained at 160°C for 12 h.After that,the products were collected by centrifugation and washed with absolute ethanol and distilled water for four times,and continuously dried in an oven at 60 °C for 12 h.For comparison,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles were synthesized with[(C8H17)2(CH3)2N]FeCl4 and FeCl3⋅6H2O(16 mmol)as precursors via the same process,respectively.

3）在体育舞蹈传授过程中，让学生懂得体育舞蹈赛事组织，学习体育舞蹈基本礼仪，促使学生相互沟通，增进友谊，增强自信心，丰富大学生的社会文化生活。

2.2.Characterization

Scheme 1.Schematic of the proposed formation process of α-Fe2O3nanoplates.

Available online 2 February 2018

2.3.Electrochemical evaluation

The crystalline structure of the α-Fe2O3nanomaterials was examined by XRD(Fig.2).The XRD patterns of all α-Fe2O3 nanomaterials match well with the hexagonal structure of α-Fe2O3(JCPDS No.33-0664).No other iron-containing compounds are observed,such as Fe3O4,γ-Fe2O3,β-FeOOH,and so on.The results of the SEM and the XRD patterns illustrate that different precursor can only change the morphology of the as-obtained nanomaterials and cannot result in the change of the crystalline phase.It further demonstrates that the ironbased ion liquid[Omim]FeCl4is not only used as iron source but working as the surfactants which control the formation of the nanoplate structure of α-Fe2O3[25,26].Fig.S2 illustrates the nitrogen absorption/desorption analysis of the α-Fe2O3nanomaterials.The BET specific surface area of the α-Fe2O3nanoplates(24.4270 m2g-1)is larger than that of the α-Fe2O3solid microspheres(13.7406 m2g-1)and the α-Fe2O3 irregular nanoparticles(10.6128 m2g-1).The nanoplates structure with larger specific surface area can hinder the volume expansion,amplify contact area between electrode and electrolyte,permit the penetration of electrolyte,offering extra-capacity through a pseudo-capacitance progress with Li storage,which eventually cause the enhancement of electrochemical performance of α-Fe2O3[28].

（3）培育发展新的经济增长极。推进水务科技跨越发展。全力打造楚禹公司专业技术、产品开发和市场拓展团队。强化资质建设和产品研发，提高公司核心竞争力，寻求向其他相关行业延伸发展。完成公司股份制改造，积极推动资产（本）向证券化方向发展，推进楚禹公司跨越发展，使其成为经济发展的引擎。培育发展新的增长极。对国家鼓励支持发展且发展空间大、符合湖北省漳河工程管理局未来发展方向的项目，从政策、体制、机制、资金、管理、技术等方面给予重点支持，形成未来经济发展新的增长极。同时，鼓励支持个人创业和投资参股企业升级改造。

3.Results and discussion

SEM intuitively reveals the information with regard to the morphology of the α-Fe2O3nanomaterials prepared by solvothermal process.Figs.1(a and b)show the morphology of the α-Fe2O3nanoplates.As shown in Figs.1(a and b),the nanoplate structures can be formed by solvothermal process when[Omim]FeCl4is utilized as precursor.The average diameter of the nanoplate is approximately 300-500 nm.In order to investigate the microstructures of the α-Fe2O3nanoplates,TEM has been employed(Fig.1(c)).The α-Fe2O3 nanoplates are self-assembled by numerous nanoparticles.From high-magnification TEM image of the α-Fe2O3nanoplates(Fig.1(d)),the crystal lattice spacing of the α-Fe2O3 nanoplates is ca.0.27 nm corresponding to the(104)crystal plane of the α-Fe2O3.As shown in the inset in Fig.1(d),the SAED pattern of the α-Fe2O3nanoplates reveals the single crystalline diffraction pattern for the α-Fe2O3,which is consistent with the reported result[25].Figs.S1(a and b)clearly exhibit that the α-Fe2O3 prepared with[(C8H17)2(CH3)2N]FeCl4is solid microsphere structure with diameter of approximately 1 μm.It can be seen from Fig.S1(b)that the surface of the solid microspheres is relatively rough.Figs.S1(c and d)show that the morphology of the α-Fe2O3prepared with FeCl3⋅6H2O is irregular and not uniform nanoparticle.

To fabricate the α-Fe2O3electrodes,a conventional slurrycoating process was applied.The aqueous slurry,containing α-Fe2O3,carbon black conductive additive and gum arabic binder,was mixed with a defined amount to meet the desired weight ratios at 7:1.5:1.5 in aqueous solution.The homogeneous slurry was uniformly pasted onto the copper foil with an area of 1 cm2.All the pasted Cu foils were dried at 80°C to assume the complete removal of H2O.The areal mass loading was typically ca.1-2 mg cm-2based on the calculation of α-Fe2O3.The LIBs performance of α-Fe2O3electrodes were tested in CR2032 coin-type cells by LANDCT 2001A battery tester(Wuhan,China)in a voltage range(0.001-2.5 V vs.Li/Li+).CR2032 coin-type cells were installed in a glove box(MBRAUN),which used polypropylene microporous film as the separator,lithium foil as the counter electrode,and 1 M LiPF6in ethylene carbonate/dimethyl carbonate(1:1 by volume)as the electrolyte.An electrochemical workstation(CHI 660E,CHI Instrument,InC.)was applied to estimate the cyclic voltammetry(CV)curve at 0.1 mV s-1and a potential range from 0.001 to 2.5 V.

（3）撰写技术指导原则规定，“在某些情况下有理由限制适应症，例如，建议药品不作为某种感染的一线治疗”应予描述。中国说明书没有这种描述，而美国说明书则有“作为不能用一线治疗方案个体的一线治疗替代方案”的描述。

This work has been financially supported by the National Natural Science Foundation of China (No.21506081,21506077),Jiangsu University Scientific Research Funding(15JDG048),Chinese Postdoctoral Foundation(2016M590420)and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Fig.1.(a,b)SEM images of the α-Fe2O3nanoplates;(c)TEM image of the α-Fe2O3nanoplates;(d)high-magnification TEM image of the α-Fe2O3nanoplates,the inset in this figure shows the SAED image of the α-Fe2O3nanoplates.

Fig.2.XRD of the α-Fe2O3nanoplates(a),the α-Fe2O3solid microspheres(b)and the α-Fe2O3irregular nanoparticles(c).

The authors declare no conflict of interests.

Galvanostatic charge-discharge pro files of the α-Fe2O3 nanoplates electrode for the first three cycles between 0.001 and 2.5 V vs.Li at 0.5 A g-1are presented in Fig.3(b).The initial discharge and charge capacity of the α-Fe2O3nanoplates are 3037 and 1954 mAh g-1,that are higher than the theoreticalcapacity of α-Fe2O3 (6 molLiinsertion,1005 mAh g-1).It is clearly seen that the initial capacity is far exceeding the theoretical capacity of α-Fe2O3,which can be explained by the formation of SEI film and the further Li storage via a pseudo-capacitance progress[32,33].The initial corresponding Coulombic efficiency is 64%.In the following cycles,～100%of the Coulombic efficiency can be obtained,indicating that the SEI film is very stable formed on surface of electrodes.These results indicate that the α-Fe2O3nanoplates exhibit the outstanding electrochemical performance.

Fig.3.Cyclic voltammetry curves(a)and voltage capacity pro files(b)of the α-Fe2O3nanoplates.

The rate capabilities of the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3 irregular nanoparticles electrode are illustrated in Fig.4(a).Encouragingly,the α-Fe2O3nanoplates electrode exhibits an outstanding reversible capacity of approximate 1950 mAh g-1 at 0.5 A g-1.The α-Fe2O3nanoplates electrode still delivers highest specific capacities.It also exhibits high specific capacitance of 1300,970,790,600,462 and 370 mAh g-1at a current density of 1,2,5,10,15,20 A g-1,respectively.97%of the original capacity(970 mAh g-1)recovered when the current density back to 2 A g-1.The α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode only deliver low reversible capacity of approximate 790 and 680 mAh g-1at 0.5 A g-1,respectively.Compared with the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode,the α-Fe2O3nanoplates electrode exhibits outstanding rate performances,indicating fast reaction kinetics and outstanding electrode integrity of the α-Fe2O3nanoplates electrode.

Fig.4.(a)Rate performance of the α-Fe2O3nanoplates,the α-Fe2O3solid microspheres and the α-Fe2O3 irregular nanoparticles; (b) the charge-discharge curves of the α-Fe2O3nanoplates at various rates.

To evaluate the interface kinetic and Li+diffusion rate of the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode,electrochemical impedance spectra(EIS)has been analyzed.The Nyquist plots of three fresh cells using the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode as anode for LIBs are investigated at an open circuit voltage state and shown in Fig.5.Each Nyquist plot comprises of a semicircle and a sloping line,which attribute to the charge transfer resistance and the Li-ion diffusion impedance,respectively[34].The EIS results con firm that the α-Fe2O3nanoplates electrode exhibits the lowest internal resistance,indicating high conductivity and high charge transfercapability.Compared with the fresh cell,the diameter of the semicircle for the three electrodes after cycling is diminished(Fig.S4).It indicates that the charge-transfer resistance is decreased after 10 cycles,which attributed to forming Fe nanoparticles via irreversible reactions[30].The α-Fe2O3nanoplates electrode demonstrates lower resistance than the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode.It indicates a great reduced charge-transfer resistance of the α-Fe2O3nanoplates at the electrode/electrolyte interface,suggesting the fast kinetics for Li+ insertion/extraction during the electrochemical reaction process.

Fig.5.Nyquist plots of the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode.

The cycling performances of the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode are measured at 0.5 A g-1,as shown in Fig.S5.The α-Fe2O3nanoplates electrode stilldeliversan excellentreversible capacity of 1211 mAh g-1after 100 cycles,which is about 1.2 times of theoretical capacity(1005 mAh g-1)and much higher than that oftheα-Fe2O3solidmicrosphereselectrode(554mAhg-1)and the α-Fe2O3irregular nanoparticles electrode(550 mAh g-1).The long-life cycling performances of the α-Fe2O3nanoplates electrode,the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode are also measured at 10 A g-1.As shown in Fig.6,the α-Fe2O3nanoplates electrode delivers initial charge and discharge capacity of 966 and 732 mAh g-1,respectively.After 1000 cycle,it still exhibitsthe high reversible capacity of approximately 520 mAh g-1which can be continued to maintain after 1000 cycles with remarkable Coulombic efficiency(approximately 100%).In contrast,the reversible discharge capacities of the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode are unstable and fade very fast with the following cycles(Fig.S6 and Fig.S7).The α-Fe2O3solid microspheres electrodeandtheα-Fe2O3irregular nanoparticles electrode only deliver the reversible discharge capacity of approximately 250 and135mAhg-1after1000cycles,respectively.Fig.S8reveals SEM images of the Fe2O3-based electrode before and after1000 cycles.Compared with the α-Fe2O3solid microspheres electrode and the α-Fe2O3irregular nanoparticles electrode,the surfacemor phologyoftheα-Fe2O3nanoplateselectrodeisvery well retained after 1000 cycles,which to a certain extent explains the stable cycling.Several recent works related to α-Fe2O3-based anodes for LIBs are summarized in Fig.7[16,30,35-42].In low current density,the α-Fe2O3nanoplates possess high reversible capacity compared with other α-Fe2O3 with different structures and α-Fe2O3-based composites.specifically,the α-Fe2O3nanoplates in this work also possess excellent cycling performance in high current density.A systematic comparison has been shown in Table S1.The outstanding electrochemical property of theα-Fe2O3nanoplates is owed to the uniform nanoplate structures.The nanoplate structures can offer a small size and large surface area,which efficiently promotes contact between electrode and electrolyte[24,43].Furthermore,the poly saccharide and glycoproteins in gumarabic are indispensable to enhance the toughness of the α-Fe2O3electrode,which can contribute to improve stability of the α-Fe2O3electrode[44].

Fig.6.Cyclic performance(a)and voltage capacity pro files(b)of the α-Fe2O3 nanoplates electrode at a current density of 10 A g-1.

Fig.7.specific capacity versus cycle number for the Fe2O3-based electrode for LIBs.

4.Conclusions

Supplementary data related to this article can be found at https://doi.org/10.1016/j.gee.2018.01.005.

Conflict of interest

For the sake of thoroughly illuminating the electrochemical performance of α-Fe2O3as the anode materials for LIBs,α-Fe2O3electrode was prepared with a green gum arabic as binder and water as solvent(Fig.S3).Gum arabic can contribute to unifor mdis persion and good contact ofα-Fe2O3and conductive additives,which enhance the charge transfer and the rate performance of LIBs[7,29].The electrochemical properties of α-Fe2O3electrode are first evaluated by cyclic voltammograms(CV)measurement for the first three cycles in 0.001-2.5 V(vs.Li/Li+)at 0.1 mV s-1(Fig.3).Fig.3(a)exhibitsthe initialthree CV curves of the α-Fe2O3nanoplates electrode.In the first cathodic cycle,a weak cathodic peak at about 1.45 V is appeared,which is associated with Li-ion insertion into crystal structures of α-Fe2O3to form α-Li2Fe2O3(2Li+ + α-Fe2O3+2e-→α-Li2Fe2O3[15]).Thepeakataround0.80Vis caused by the further reduction of the α-Li2Fe2O3to form Fe0 and Li2O(α-Li2Fe2O3+4Li++4e- → 2Fe0+3Li2O)[30].The strong cathodic peak at 0.27 Vis assigned to the irreversibleformation of solid electrolyte interface (SEI) film and some sidereactions between the electrode materials and electrolyte[31]. The anodic peak at 1.74Vis associated with the oxidation of Fe00(Fe0→Fe3+)[21].In the subsequent scan cycle,the cathodic peak and anodic peak shift respectively to 0.73 V and 1.76 V with a decreasing peak intensity which is attributed to irreversible processes of the electrode due to forming SEI film.The reversible electrochemical reaction about α-Fe2O3in LIBs can be described as[15]:Fe2O3+6Li↔2Fe+3Li2O.

Acknowledgements

图5展示了载波相位恢复后（时长0.6 s）的信号幅度及相位，去掉了1.772 GHz的微波调制信号，仅包括多普勒频率随时间的变化，图中的相位是解缠绕之后的结果。

Appendix A.Supplementary data

In summary,the α-Fe2O3nanoplates were synthesized by iron ion-based ionic liquid assisted solvothermal route.Iron ion-based ionic liquid can be applied as reactant and template in the reactive process.Three samples with different structure are evaluated as anode materials for LIBs.Compared with the α-Fe2O3solid microspheres and the α-Fe2O3irregular nanoparticles,the α-Fe2O3nanoplates exhibit outstanding cycling property and high rate capability.The α-Fe2O3nanoplates electrode can display a high reversible capacity of around 1950 mAh g-1at 0.5 A g-1,and maintain the high reversible capacity of 1211 mAh g-1(≈1.2 times of the theoretical capacity)after 100 cycles.Even,the high reversible capacity of 520 mAh g-1can still remain after 1000 cycles at 10 A g-1.The outstanding performance indicates the immense potential of the α-Fe2O3nanoplates as anode material in the LIBs.

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