α-Fe2O3 Nanorods as Anode Material for Lithium Ion Batteries

LETTER

https://www.360docs.net/doc/e011470061.html,/JPCL α-Fe2O3Nanorods as Anode Material for Lithium Ion Batteries

Yong-Mao Lin,Paul R.Abel,Adam Heller,and C.Buddie Mullins*

Departments of Chemical Engineering and Chemistry and Biochemistry,Center for Electrochemistry,Texas Materials Institute,and Center for Nano-and Molecular Science,University of Texas at Austin,1University Station,C0400Austin,Texas78712-0231, United States

b Supporting Information

H ematite iron oxide(α-Fe2O3)has been studied as a water-

splitting photoelectrode,1à3as a gas sensor,4and as the active anode material in lithium ion batteries.5In the latter,it is potentially superior to the presently used graphite.5Because the intercalation potential of lithium in graphite is near the reversible potential of the Li/Li+half-cell,unintentional electrodeposition of lithium dendrites could short the cell and poses an unaccep-table safety hazard in large vehicular power systems.6,7The higher electromotive force(emf)of lithium in iron oxide,1.63V versus Li/Li+,8assures that no metallic lithium dendrites will grow. Although this inherent safety is achieved at the cost of a lower cell voltage,the lower cell voltage need not translate to a lesser energy density:with6mols of lithium being consumed in the stoichio-metric reduction ofα-Fe2O3to Fe,9the theoretical gravimetric and volumetric lithium storage capacities for hematite are 1006mAh/g and5331Ah/L,exceeding the372mAh/g and837 Ah/L capacities of graphite.The capacity ofα-Fe2O3is,however, not well-retained upon cycling.Unlike graphite,which only experiences a～10%volume change when it is fully lithated,10 the volume change ofα-Fe2O3is much larger,～96%during lithium insertion/deinsertion,leading to the disintegration of the anode within a small number of cycles.11Reducing the inserted lithium to less than one Li atom perα-Fe2O3molecule improves the cyclability because of the limited cell volume change,but it also reduces the capacity to one-sixth of the theoretical value.12,13To approach the high intrinsic theoretical capacity ofα-Fe2O3,it is necessary to design the anode such that it will better accommodate the strain associated with lithium insertion/deinsertion.

It is well-recognized that nanostructured materials not only better accommodate large strains but also provide short di?usion paths for Li+insertion/deinsertion.14,15Consequently,nanostructured α-Fe2O3particle anodes have been engineered from nano-particles,16à18nanosheets,16nanodiscs,19nanotubes,4,20nano-wires,11nanocapsules,21nano?akes,22,23nano?owers,24,25and nano-rods.26à29Among these studies,α-Fe2O3with a nanorod structure as an anode material for lithium-ion batteries was?rst reported by Wu et al.in2006,who also found that the?rst charge/discharge capacities are highly dependent on the morphology of the hema-tite nanorods.26Although long-term cyclability results were not reported in the study by Wu and coworkers,they demonstrated that α-Fe2O3nanorods could be a promising material for lithium storage.Liu and coworkers also synthesized and tested single-crystallineα-Fe2O3nanorods with diameters in the range of60à80nm.27They measured an initial reversible capacity as high as 955mAh gà1with electrodes made of theirα-Fe2O3nanorods,and the capacity retention was also promising:763mAh gà1after30 cycles at0.1C rate.Recently,Song and coworkers successfully synthesizedα-Fe2O3nanorods on titanium foil via a facile hydro-thermal method.28The hematite nanorod array electrode exhibited good electrochemical performance at relatively high charge/dis-charge rate and retained reversible capacities of562mAh gà1at 0.2C and444mAh gà1at0.5C,respectively,after50cycles.On the basis of these studies,it is apparent that by introducing nanostruc-turedα-Fe2O3the electrochemical behavior of the anode can be signi?cantly improved in terms of cyclability and capacity retention. Moreover,Dahn and coworkers reported the choice of binder can have a major impact on the electrochemical performance of α-Fe2O3powder electrode.30Better cycling performance was

Received:October10,2011

Accepted:October28,2011

Published:October28,2011

achieved,～800mAh g à1for 100cycles at a 0.2C rate,by using sodium carboxymethyl cellulose (CMC)binder instead of con-ventional poly(vinylidene ?uoride)(PVDF)binder on electro-des made of submicrometer-sized α-Fe 2O 3.

Here we build upon this important previous research in optimizing the performance of iron oxide anodes and describe a promising negative electrode for lithium ion batteries that is composed of potentially easy to manufacture,narrowly dis-persed,single-crystalline hematite nanorods.The α-Fe 2O 3nano-rod electrode not only exhibited high initial reversible capacities of 908mAh g à1at a 0.2C rate and 837mAh g à1at a 0.5C rate,respectively,but also fully retained these capacities after numer-ous cycles.

The synthesis method used to produce the α-Fe 2O 3nanorods employed in this study was motivated by the work of Wang and coworkers,31who used hydrothermal synthesis with 1,2-diami-nopropane as a shape controlling agent.The details of our synthetic procedure,which was slightly di ?erent than that employed by Wang et al.,31is discussed in the Experimental Methods section.SEM images (Figure 1)show that the average diameter of the prepared α-Fe 2O 3nanorods is ～40nm,with an average length of ～400nm.Figure 1b shows an end-view of the α-Fe 2O 3nanorods with hexagonal structure,indicating that each nanorod grew along the [001]direction of rhombohedral hematite.32To investigate the particle size e ?ect of α-Fe 2O 3on the electro-chemical performance,micrometer-sized and submicrometer-sized hematite particles were employed as references for com-parison with the nanorods.Micron-sized α-Fe 2O 3particles were prepared by the same hydrothermal synthesis but without the addition of 1,2-diaminopropane.The submicrometer-size α-Fe 2O 3particles (product number:310050,Aldrich)were purchased without any further treatment.Submicrometer-size α-Fe 2O 3particles were purchased from Aldrich and had a particle size distribution ranging from 50nm to ～1μm,with the diameters of typical particles being 100à500nm (Figure S1of the Supporting Information).The micrometer-sized α-Fe 2O 3particles synthe-sized in our laboratory by the same hydrothermal process but without adding the shape-controlling agent 1,2-diaminopropane ranged in diameter from 1to 6μm (Figure S2of the Supporting Information).

A TEM image of a typical nanorod synthesized by the hydrothermal method is shown in Figure 2a.As is evident from the periodic lattice fringes across the entire nanorod (Figure 2b),each of the nanorods is a monocrystal.The measured lattice spacing is 3.65?(Figure 2c),corresponding to the (012)plane of α-Fe 2O 3.(The d spacing =3.68?for the (012)plane

according to JCPDS no.00-033-0664.)The angle between the parallel fringes and the long axis direction of the nanorod was determined to be 32.4°,which is in good agreement with the theoretical angle between the (012)plane and the [001]direction.31The X-ray powder di ?raction pattern con ?rms the hematite structure (Figure S3of the Supporting Information).Figure 3a shows the initial discharge and charge curves of the α-Fe 2O 3nanorod-based electrode for the ?rst ?ve cycles at 0.2C rate.During the ?rst discharge,the ?rst plateau appears at 1.6V versus Li/Li +,followed by a smooth voltage drop to ～1V.This has been reported as a feature for lithium insertion in nanosized α-Fe 2O 3to form Li x Fe 2O 3.12,33The second plateau at ～0.87V re ?ects the reduction of Fe 3+to Fe 0.The ?rst discharge and charge capacities are 1191and 908mAh g à1,respectively.The initial irreversible capacity loss of the α-Fe 2O 3nanorod electrode

Figure 1.SEM images of α-Fe 2O 3nanorods (a)at a low magni ?cation and (b)end-view at a high magni ?cation.The yellow dashed line in panel b outline the hexagonal structure of a single nanorod.

Figure 2.TEM images of a single α-Fe 2O 3nanorod (a)at a low magni ?cation (b)at a medium magni ?cation,and (c)at a high magni ?ca-tion.The white arrows and solid lines in panel c indicate two consecutive lattice fringes.

Figure 3.Voltage pro ?les of electrodes made with (a)α-Fe 2O 3nano-rods,(b)α-Fe 2O 3submicrometer particles,and (c)α-Fe 2O 3micro-meter-sized particles.All electrodes cycled at

0.2C

rate (201mA g à1).

is 282mAh g à1,much smaller than that of iron oxide nanopar-ticles with diameters of ～10nm,for which Wu et al.reported an irreversible capacity loss greater than 1000mAh g à1.16After the ?rst cycle,the capacity of the α-Fe 2O 3nanorod electrode studied here stabilizes at ～930mAh g à1,and no capacity fading is seen during the ?rst ?ve cycles.The initial irreversible capacity loss could result from the formation of a solid electrolyte interface (SEI)on the iron oxide surface during the ?rst lithium insertion process,which is a disadvantage of using nanostructure materials because the high electrode/electrolyte interfacial area may lead to more irreversible side reactions.34However,1D nanostruc-tures like nanorods can o ?er a small diameter to enhance lithium di ?usion and yet still provide a limited surface area to prevent excessive side reactions.Therefore,the α-Fe 2O 3nanorod elec-trode can achieve both high reversible capacity and a low initial capacity loss.

The initial voltage pro ?les of the electrode made with α-Fe 2O 3submicrometer particles (purchased from Aldrich)are shown in Figure 3b.During the ?rst discharge,only a long voltage plateau appears at 0.8V,which is lower than that of the α-Fe 2O 3nanorod electrode (0.87V).The higher overpotential of the electrode made with α-Fe 2O 3submicrometer particles compared with the electrode composed of nanorods means the smaller-sized α-Fe 2O 3nanorod is kinetically more favorable for lithium di ?usion.The initial irreversible capacity loss of the electrode made with the α-Fe 2O 3submicrometer particles was slightly higher than 300mAh g à1,and their capacity declines slowly after the ?rst cycle (Figure 3b).The electrode made with hydrothermally synthesized micrometer-sized α-Fe 2O 3particles without adding a morphology controlling agent shows a voltage plateau at 0.7V in the initial discharge curve (Figure 3c).The high overpotential was caused by the large α-Fe 2O 3particle size,which hinders lithium di ?usion.The electrode made with micrometer-sized α-Fe 2O 3particles has a particularly high initial irreversible capacity loss greater than 500mAh g à1(Figure 3c).The micrometer-sized α-Fe 2O 3particles should have less speci ?c surface area than the hematite nanorods and the submicrometer-size particles,hence the major capacity loss was likely not only caused by irreversible SEI formation but also due to the loss of electro-nic continuity upon disintegration of the α-Fe 2O 3particles due to volume expansion and contraction during lithiation

and delithiation steps.The capacity of the electrode with micro-meter sized α-Fe 2O 3particles fades much faster than that of the α-Fe 2O 3nanorods and the submicrometer-sized particles.

Reversible capacities of α-Fe 2O 3electrodes made with nano-rods,submicrometer particles,and micrometer-sized particles cycled at 0.2C rate up to 30cycles are shown in Figure 4.The initial capacity of the electrode made with the submicrometer particles is near 750mAh g à1,dropping to ～700mAh g à1after 20cycles.Note that the electrochemical performance of our submicrometer particles is comparable in performance to elec-trodes with similar sizes of hematite particles and the same CMC binder,as reported by Dahn and coworkers.30The initial rever-sible capacity of the electrode made with micrometer-sized α-Fe 2O 3is merely 500mAh g à1,and its reversible capacity drops within 10cycles to 354mAh g à1,a value below the theoretical capacity of graphite (372mAh g à1),showing that the large volume change upon cycling can be accommodated by small but not by large α-Fe 2O 3particles.Overall,the electrode made with 40nm ?400nm α-Fe 2O 3nanorods outperforms both the electrodes made with micrometer-sized α-Fe 2O 3particles and with submicrometer α-Fe 2O 3particles,retaining a reversible capacity of ～900mAh g à1after 30cycles.

Electrodes made of α-Fe 2O 3nanorods,submicrometer parti-cles,and micrometer-sized particles were also cycled with a higher current density,0.5C rate (503mA g à1)for 100cycles.The initial voltage pro ?les of the ?rst ?ve cycles of these electrodes are shown in Figure S4of the Supporting Information,and the capacity retention for each electrode is plotted in Figure https://www.360docs.net/doc/e011470061.html,pared with the result for electrodes cycled at 0.2C rate,the initial capacity of each electrode was lower when cycled at a 0.5C rate.This is due to concentration polarization of Li ions in α-Fe 2O 3resulting from a di ?usion-limited process,which is especially more obvious at the higher C rate.Cycling at higher current density not only results in a lower speci ?c capacity,it can also induce larger localized strain because of the concentration polarization.For these reasons,the capacities of electrodes made of α-Fe 2O 3submicro-meter particles and micrometer-sized particles dropped to around 500and 200mAh g à1,separately,after 30cycles at a 0.5C rate (Figure 5).However,the electrode made of α-Fe 2O 3nanorods can maintain a capacity as high as 800mAh g à1within the ?rst 30cycles,more than twice the theoretical capacity of graphite.

Figure 4.Reversible capacities of α-Fe 2O 3electrodes made with nanorods,submicrometer particles,and micrometer-sized particles.All electrodes cycled at 0.2C rate (201mA g à1).Figure 5.Reversible capacities of α-Fe 2O 3electrodes made with nanorods,submicrometer particles,and micrometer-sized particles.All electrodes cycled at

0.5

C rate (503mA g à1).

The hematite nanorods can provide particularly short Li +di ?u-sion paths,accommodate the strain,and thereby avoid the capacity loss upon cycling.What is interesting is that after 30cycles the capacity of the α-Fe 2O 3nanorod electrode kept increasing before leveling o ?at ～970mAh g à1at the 90th cycle.Similar behavior for an α-Fe 2O 3electrode with CMC as the binder has been reported by Dahn and coworkers.30Although the root cause of this pheno-menon has yet to be determined,the electrochemical performance of α-Fe 2O 3nanorods is obviously higher than α-Fe 2O 3submic-rometer particles and micrometer particles.

To better understand the failure mechanism of electrodes with large particle sizes,electrodes made of α-Fe 2O 3micrometer-sized particles have been investigated by SEM before and after cycling.The cycled electrode was disassembled from the coin cell and soaked into acetonitrile for 24h,then rinsed with ethanol to remove the residual electrolyte,followed by drying under vacuum at 80°C overnight before SEM imaging.Figure 6a shows the image of a pristine electrode with micrometer-sized α-Fe 2O 3particles which are well-surrounded by conductive carbon parti-cles.After 100cycles at 0.5C rate,the micrometer-sized α-Fe 2O 3particles were pulverized and showed poor electrical continuity on both the interior and exterior of the hematite particle (Figure 6b).The changes of morphology for micrometer-sized α-Fe 2O 3particles before and after cycling can explain why the electrodes with large α-Fe 2O 3particles performed poorly.Elec-trodes made of α-Fe 2O 3nanorods with/without cycling were also investigated by the same method.Figure 6c shows a pristine electrode composed of α-Fe 2O 3nanorods before performing any electrochemical testing.After cycling,it can be seen that the surface of the α-Fe 2O 3nanorod electrode was covered by a layer of material,which is possibly the SEI formed during the lithiation process (Figure 6d).Unlike the micrometer-sized α-Fe 2O 3particles,which pulverize after cycling,the α-Fe 2O 3nanorods maintain their morphology after 100cycles at 0.5C rate.It is clear that the strain generated during the lithiation/delithiation pro-cess can be accommodated by the α-Fe 2O 3nanorods but not by

the large α-Fe 2O 3particles which disintegrated and resulted in low reversible capacity.

The electrochemical impedance behavior of the various α-Fe 2O 3electrodes was investigated to examine the kinetics of lithium ion transfer by using an electrochemical analyzer (CHI 604D,CHI).The impedance of the electrochemical system was interpreted in terms of Nyquist plots,which describe the gain and phase of the frequency response in polar coordinates.Figure 7shows the Nyquist plots for cells containing electrodes with di ?erent size α-Fe 2O 3particles,which were acquired individually under their open circuit voltage state after 100cycles at 0.5C rate.Electrochemical impedance spectroscopy (EIS)was carried out over a wide frequency range from 100kHz to 0.01Hz with an ac perturbation voltage of 5mV.Each plot consists of a semicircle in the high-frequency region,which was attributed to the charge transfer process,and a sloping line in the low-frequency region that related to the mass transfer of lithium ions.The electrochemical system can be simply modeled by a Randles equivalent circuit,as shown in the inset of Figure 7,35where R Ωis the ohmic resistance,C D is the double-layer capacitance,R CT is the charge transfer resistance,and Z W is the Warburg impedance describing the solid-state di ?usion of Li +in α-Fe 2O 3.The results from ?tting the model to performance data are summarized in Table 1.At very high frequencies (above 10kHz),only the ohmic resistance can be observed,which is mainly due to (i)external connections,(ii)contact resistance,and (iii)ionic conduction within the electrolyte.36Because the same electrolyte and identical cell con ?gurations were employed,all three electrodes have a similar value of R Ωfor the ohmic resistance (～18Ω)obtained from the ?tting result or which can be easily determined from the high

Figure 6.SEM images of (a)pristine α-Fe 2O 3micrometer particles electrode,(b)α-Fe 2O 3micrometer particles electrode after 100cycles at 0.5C rate,(c)pristine α-Fe 2O 3nanorod electrode,and (d)α-Fe 2O 3nanorod electrode after 100cycles at 0.5C rate.

Figure 7.Electrochemical impedance spectroscopy of electrodes with α-Fe 2O 3nanorods,submicrometer particles α-Fe 2O 3,and micrometer-sized particles α-Fe 2O 3.All measured after 100cycles at 0.5C rate.

Table 1.Electrochemical Impedance Spectroscopy Fitting Results Data for Electrodes with α-Fe 2O 3Nanorods,Sub-micrometer Particles α-Fe 2O 3and Micrometer-Sized Parti-cles α-Fe 2O 3

electrode materials R Ω[Ω]R CT [Ω]α-Fe 2O 3microparticles 18.630.2α-Fe 2O 3submicrometer particles

19.2

13.0α-Fe 2O 3nanorods

18.4

12.4

frequency intercept with the real axis (x axis)in the Nyquist plots.In the medium-to high-frequency region,from 10Hz to 10kHz,a semicircle appears for each cell that is attributed to an interfacial charge transfer coupled with a double-layer capaci-tance.The charge transfer resistance can also be determined by ?tting the Randles equivalent circuit or directly measuring the diameter of the semicircle in the Nyquist plot.The electrode made of micrometer-sized particles has a large charge transfer resistance,30.2Ω,signi ?cantly higher than that of the electrodes made of submicrometer particles (13.0Ω)and nanorods (12.4Ω).This result suggests that after cycling the electron transfer is more di ?cult in the electrode made of micrometer-sized parti-cles,which may have resulted from the pulverization of α-Fe 2O 3during the lithiation/delithiation process,as seen by SEM.At frequencies lower than 10Hz,an inclined line is displayed for each spectrum,which represents the di ?usion of Li +in α-Fe 2O 3.The slope of the line for the α-Fe 2O 3nanorod electrode is ～45°for frequencies ranging from 1to 10Hz,which can be well-described by the Warburg impedance as a semi-in ?nite linear di ?usion process.At very low frequencies (lower than 1Hz),the slope for the hematite nanorod electrode gets steeper,which implies that the transport behavior has shifted from semi-in ?nite di ?usion to ?nite di ?usion.37Compared with the electrodes made of α-Fe 2O 3submicrometer particles and α-Fe 2O 3micrometer particles,the α-Fe 2O 3nanorod electrode has a steeper slope in the lower frequency regions.This is attributed to the smaller average feature size of the α-Fe 2O 3nanorods;hence both the bulk and surface of the α-Fe 2O 3nanorods are more accessible to lithium-ions.

To test the limit of the high rate capability of the α-Fe 2O 3nanorod electrode,we assembled and charged/discharged step-wise an additional cell from 0.2to 5C (0.2,0.5,1,2,and 5C),with each C rate for 10cycles (Figure 8).There was always a capacity drop immediately after switching from a lower C rate to a higher C rate,which can be explained by the concentration polarization of Li ions in the α-Fe 2O 3nanorods resulting from a di ?usion limited process.The α-Fe 2O 3nanorod electrode held its capacity for 0.2and 0.5C but started decaying slightly at 1C.Nevertheless,a reversible capacity as high as ～800mAh g à1can still be held at 1C for 10cycles.As the C rate increased from 1to 2C and then to 5C,the capacity retention of the hematite nanorod electrode became worse,only exhibiting a capacity of

～300mAh g à1at 5C for 10cycles.The cell after cycling at various C rates up to 5C was disassembled to investigate the issue associated with the capacity fading.The SEM image in Figure S5of the Supporting Information shows that even cycled at a C rate as high as 5C,corresponding to a current density of 5A g à1,the nanorods can still be observed.Although the nanorods were distorted after cycling,which could be from the strain generated by the fast lithium insertion/deinsertion,they did not shatter and lose their continuity.Hence,there might be other reasons leading to the fast capacity fading at higher C rates rather than the disintegration of the hematite nanorods.The failure of the CMC binder could be one of the reasons because the adhesion between the electrode materials to the current collector was not good after cycling at high C rates when compared with the electrode cycled at lower C rates.Dahn et al.has noted the importance of binder choice for materials with substantial volume change upon lithiation.30Employing elas-tomeric polymers as the binder could possibly further improve the high rate performance of the α-Fe 2O 3nanorod electrode.

In summary,hydrothermally synthesized monocrystalline hematite nanorods were investigated as an anode material for lithium ion batteries.Electrodes made of α-Fe 2O 3nanorods outperformed electrodes fabricated from submicrometer-and micrometer-sized particles in terms of reversible capacity,cy-clability,and rate capability toward lithium storage.High initial reversible capacities of 908mAh g à1at 0.2C rate and 837mAh g à1at 0.5C rate were achieved for α-Fe 2O 3nanorod electrode,and these capacities were fully retained after numerous cycles.The excellent performance of the hematite nanorod electrode can be attributed to the small diameter elongated nanostructure that provides a short di ?usion path for lithium-ion di ?usion and also accommodates the strain generated during the lithiation/delithiation process.

’EXPERIMENTAL METHODS

Materials and Synthesis .α-Fe 2O 3nanorods were prepared by adding iron chloride hexahydrate (FeCl 336H 2O,Alfa Aesar)to deionized water to form a 0.5M ferric chloride solution and then stirred for 15min.After stirring,the 1,2-diaminopropane (C 3H 10N 2,Alfa Aesar)was added at a 1:1volume ratio,and stirring was continued for another 15min.The solution was then transferred to a Te ?on-lined stainless steel autoclave and heated to 220°C for 20h.After the mixture cooled to room tempera-ture,the precipitate was collected by centrifugation,washed twice with deionized water,and dried under vacuum at 80°C.Characterization .The α-Fe 2O 3nanorods were characterized by scanning electron microscopy (SEM)using a Zeiss Supra 40VP scanning electron microscope,by transmission electron microscopy (TEM)using a JEOL 2010F transmission electron microscope,and by X-ray di ?raction (XRD)using a Bruker-Nonius D8Advance powder di ?ractometer.

Electrochemical Measurements .To evaluate the electrochemical performance,we prepared the iron oxide electrodes by mixing the α-Fe 2O 3(60wt %)with conductive carbon black (Super P àLi,Timcal)(20wt %)and with 90kDa CMC (Aldrich)(20wt %)using water as the dispersion medium.The concentration of conductive additive and binder in the α-Fe 2O 3nanorod electrode was optimized,and details can be found in the Supporting Information.It has been shown that electrodes made with CMC perform better than electrodes made with the traditional binder àPVDF.30,38The mixture was slurry-cast on 10μm thick Cu foil using an automatic applicator and a notch bar with

Figure 8.Reversible capacities of electrode made with α-Fe 2

O 3nano-rods cycled at various C rates.

100μm clearance,then dried under vacuum at120°C for12h. After that,the as-preparedα-Fe2O3slurry-cast?lm was punched into disk working electrodes.All electrochemical measurements were carried out using2032type coin cells with the as-prepared working electrode.Cells were assembled in an argon atmosphere glovebox using lithium foil as the counter/reference electrode and 1M LiPF6in EC/DMC(1:1vol/vol)(LP30,EMD Chemicals) as the electrolyte.A polypropylene membrane(Celgard2400, Celgard)was used as a separator.These assembled coin cells were cycled with a multichannel battery test system(BT2143,Arbin) between5mV and3V at a0.2C rate(201mA gà1)and at a0.5C rate(503mA gà1),corresponding to the rates of fully charging or discharging the cell within5h(for0.2C)and2h(for0.5C), separately.

’ASSOCIATED CONTENT

b Supporting Information.Additional SEM images;X-ray powder di?raction pattern;voltage pro?les ofα-Fe2O3cells;and electrochemical results employing various amount of conductive additive and binder.This material is available free of charge via the Internet at https://www.360docs.net/doc/e011470061.html,.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:mullins@https://www.360docs.net/doc/e011470061.html,.

’ACKNOWLEDGMENT

The Welch Foundation supported Y.-M.L.and A.H.(through grant F-1131)and C.B.M.(through grant F-1436).Additionally, this work has been partially supported by the U.S.Army Research O?ce(for CBM under contract/grant number W911NF-09-1-0130).Paul R.Abel gratefully acknowledges the Fannie and John Hertz Foundation for a graduate fellowship.

’REFERENCES

(1)Kay,A.;Cesar,I.;Gratzel,M.New Benchmark for Water Photo-oxidation by Nanostructuredα-Fe2O3Films.J.Am.Chem.Soc.2006, 128,15714–15721.

(2)Hahn,N.T.;Ye,H.C.;Flaherty,D.W.;Bard,A.J.;Mullins,C.B. Reactive Ballistic Deposition ofα-Fe2O3Thin Films for Photoelec-trochemical Water Oxidation.ACS Nano2010,4,1977–1986.

(3)Hahn,N.T.;Mullins,C.B.Photoelectrochemical Performance of Nanostructured Ti-and Sn-Dopedα-Fe2O3Photoanodes.Chem. Mater.2010,22,6474–6482.

(4)Chen,J.;Xu,L.N.;Li,W.Y.;Gou,X.L.α-Fe2O3Nnanotubes in Gas Sensor and Lithium-Ion Battery Applications.Adv.Mater.(Weinheim,Ger.) 2005,17,582–586.

(5)Hassoun,J.;Croce,F.;Hong,I.;Scrosati,B.Lithium-Iron Battery:Fe2O3Anode Versus https://www.360docs.net/doc/e011470061.html,mun. 2011,13,228–231.

(6)Manthiram, A.Materials Challenges and Opportunities of Lithium Ion Batteries.J.Phys.Chem.Lett.2011,2,176–184.

(7)Goodenough,J.B.;Kim,Y.Challenges for Rechargeable Li Batteries.Chem.Mater.2010,22,587–603.

(8)Li,H.;Balaya,P.;Maier,J.Li-Storage via Heterogeneous Reaction in Selected Binary Metal Fluorides and Oxides.J.Electrochem. Soc.2004,151,A1878–A1885.

(9)Abraham,K.M.;Pasquariello,D.M.;Willstaedt,E.B.Prepara-tion and Characterization of Some Lithium Insertion Anodes for Secondary Lithium Batteries.J.Electrochem.Soc.1990,137,743–749.

(10)Zou,L.;Kang,F.Y.;Li,X.H.;Zheng,Y.P.;Shen,W.C.;Zhang, J.Investigations on the Modi?ed Natural Graphite as Anode Materials in Lithium Ion Battery.J.Phys.Chem.Solids2008,69,1265–1271. (11)Nuli,Y.N.;Zhang,P.;Guo,Z.P.;Liu,H.K.Shape Evolution of α-Fe2O3and Its Size-Dependent Electrochemical Properties for Lithium-Ion Batteries.J.Electrochem.Soc.2008,155,A196–A200. (12)Larcher,D.;Masquelier,C.;Bonnin,D.;Chabre,Y.;Masson, V.;Leriche,J.B.;Tarascon,J.M.E?ect of Particle Size on Lithium Intercalation intoα-Fe2O3.J.Electrochem.Soc.2003,150,A133–A139.

(13)Kitaura,H.;Takahashi,K.;Mizuno,F.;Hayashi,A.;Tadanaga, K.;Tatsumisago,M.Preparation ofα-Fe2O3Electrode Materials via Solution Process and Their Electrochemical Properties in All-Solid-State Lithium Batteries.J.Electrochem.Soc.2007,154,A725–A729. (14)Arico,A.S.;Bruce,P.;Scrosati,B.;Tarascon,J.M.;Van Schalkwijk,W.Nanostructured Materials for Advanced Energy Conver-sion and Storage Devices.Nat.Mater.2005,4,366–377.

(15)Wang,Y.;Cao,G.Z.Developments in Nanostructured Cath-ode Materials for High-Performance Lithium-Ion Batteries.Adv.Mater. (Weinheim,Ger.)2008,20,2251–2269.

(16)Wu,M.S.;Ou,Y.H.;Lin,P.Iron Oxide Nanosheets and Nanoparticles Synthesized by a Facile Single-Step Coprecipitation Method for Lithium-Ion Batteries.J.Electrochem.Soc.2011,158,A231–A236. (17)Wu,X.L.;Guo,Y.G.;Wan,L.J.;Hu,C.W.α-Fe2O3 Nanostructures:Inorganic Salt-Controlled Synthesis and Their Electro-chemical Performance toward Lithium Storage.J.Phys.Chem.C2008, 112,16824–16829.

(18)Ma,J.M.;Lian,J.B.;Duan,X.C.;Liu,X.D.;Zheng,W.J.α-Fe2O3:Hydrothermal Synthesis,Magnetic and Electrochemical Proper-ties.J.Phys.Chem.C2010,114,10671–10676.

(19)Chen,J.S.;Zhu,T.;Yang,X.H.;Yang,H.G.;Lou,X.W.Top-Down Fabrication of alpha-Fe2O3Single-Crystal Nanodiscs and Micro-particles with Tunable Porosity for Largely Improved Lithium Storage Properties.J.Am.Chem.Soc.2010,132,13162–13164.

(20)Liu,J.P.;Li,Y.Y.;Fan,H.J.;Zhu,Z.H.;Jiang,J.;Ding,R.M.; Hu,Y.Y.;Huang,X.T.Iron Oxide-Based Nanotube Arrays Derived from Sacri?cial Template-Accelerated Hydrolysis:Large-Area Design and Reversible Lithium Storage.Chem.Mater.2010,22,212–217. (21)Kim,H.S.;Piao,Y.;Kang,S.H.;Hyeon,T.;Sung,Y.E. Uniform Hematite Nanocapsules Based on an Anode Material for Lithium Ion https://www.360docs.net/doc/e011470061.html,mun.2010,12,382–385.

(22)Reddy,M.V.;Yu,T.;Sow,C.H.;Shen,Z.X.;Lim,C.T.;Rao,

G.V.S.;Chowdari,B.V.R.α-Fe2O3Nano?akes as an Anode Material for Li-Ion Batteries.Adv.Funct.Mater.2007,17,2792–2799. (23)Chun,L.;Wu,X.Z.;Lou,X.M.;Zhang,Y.X.Hematite Nano?akes as Anode Electrode Materials for Rechargeable Lithium-Ion Batteries.Electrochim.Acta2010,55,3089–3092.

(24)Zeng,S.Y.;Tang,K.B.;Li,T.W.;Liang,Z.H.;Wang,D.; Wang,Y.K.;Qi,Y.X.;Zhou,W.W.Facile Route for the Fabrication of Porous Hematite Nano?owers:Its Synthesis,Growth Mechanism, Application in the Lithium Ion Battery,and Magnetic and Photocatalytic Properties.J.Phys.Chem.C2008,112,4836–4843.

(25)Zhou,W.;Lin,L.J.;Wang,W.J.;Zhang,L.L.;Wu,Q.O.;Li, J.H.;Guo,L.Hierarchial Mesoporous Hematite with“Electron-Trans-port Channels”and Its Improved Performances in Photocatalysis and Lithium Ion Batteries.J.Phys.Chem.C2011,115,7126–7133. (26)Wu,C.Z.;Yin,P.;Zhu,X.;OuYang,C.Z.;Xie,Y.Synthesis of Hematite(α-Fe2O3)Nanorods:Diameter-Size and Shape E?ects on Their Applications in Magnetism,Lithium Ion Battery,and Gas Sensors. J.Phys.Chem.B2006,110,17806–17812.

(27)Liu,H.;Wang,G.X.;Park,J.;Wang,J.;Zhang,C.Electro-chemical Performance ofα-Fe2O3Nanorods as Anode Material for Lithium-Ion Cells.Electrochim.Acta2009,54,1733–1736.

(28)Song,Y.Q.;Qin,S.S.;Zhang,Y.W.;Gao,W.Q.;Liu,J.P. Large-Scale Porous Hematite Nanorod Arrays:Direct Growth on Titanium Foil and Reversible Lithium Storage.J.Phys.Chem.C2010, 114,21158–21164.

(29)Tartaj,P.;Amarilla,J.M.Iron Oxide Porous Nanorods with Di?erent Textural Properties and Surface Composition Preparation,

Characterization and Electrochemical Lithium Storage Capabilities. J.Power Sources2011,196,2164–2170.

(30)Li,J.;Dahn,H.M.;Krause,L.J.;Le,D.B.;Dahn,J.R.Impact of Binder Choice on the Performance ofα-Fe2O3as a Negative Electrode. J.Electrochem.Soc.2008,155,A812–A816.

(31)Li,Z.M.;Lai,X.Y.;Wang,H.;Mao,D.;Xing,C.J.;Wang,D. Direct Hydrothermal Synthesis of Single-Crystalline Hematite Nano-rods Assisted by1,2-Propanediamine.Nanotechnology2009,20,245603.

(32)Eggleston,C.M.;Hochella,M.F.The Structure of Hematite (001)Surfaces by Scanning Tunneling Microscopy-Image Interpretation, Surface Rexation,and Step Structure.Am.Mineral.1992,77,911–922.

(33)Larcher,D.;Bonnin,D.;Cortes,R.;Rivals,I.;Personnaz,L.; Tarascon,https://www.360docs.net/doc/e011470061.html,bined XRD,EXAFS,and Mossbauer Studies of the Reduction by Lithium ofα-Fe2O3with Various Particle Sizes.J.Electro-chem.Soc.2003,150,A1643–A1650.

(34)Bruce,P.G.;Scrosati,B.;Tarascon,J.M.Nanomaterials for Rechargeable Lithium Batteries.Angew.Chem.,Int.Ed.2008,47,2930–2946.

(35)Bard,A.J.;Faulkner,L.R.Electrochemical Methods:Funda-mentals and Applications,2nd ed.;Wiley:New York,2001.

(36)Ru?o,R.;Hong,S.S.;Chan,C.K.;Huggins,R.A.;Cui,Y. Impedance Analysis of Silicon Nanowire Lithium Ion Battery Anodes. J.Phys.Chem.C2009,113,11390–11398.

(37)Yu,P.;Ritter,J.A.;White,R.E.;Popov,B.N.Ni-Composite Microencapsulated Graphite as the Negative Electrode in Lithium-Ion Batteries-II.Electrochemical Impedance and Self-Discharge Studies. J.Electrochem.Soc.2000,147,2081–2085.

(38)Beattie,S.D.;Larcher,D.;Morcrette,M.;Simon,B.;Tarascon, J.M.Si Electrodes for Li-Ion Batteries-A New Way to Look at an Old Problem.J.Electrochem.Soc.2008,155,A158–A163.