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Solid State Sciences 36 (2014) 1e7

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Solid State Sciences

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Co1�xFe2þxO4 (x ¼ 0.1, 0.2) anode materials for rechargeablelithium-ion batteries

Alok Kumar Rai, Trang Vu Thi, Jihyeon Gim, Vinod Mathew, Jaekook Kim*

Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Bukgu, Gwangju 500-757, Republic of Korea

a r t i c l e i n f o

Article history:Received 9 April 2014Received in revised form30 June 2014Accepted 3 July 2014Available online 15 July 2014

Keywords:CoFe2O4

AnodeRate capabilityLithium-ion battery

* Corresponding author. Tel.: þ82 62 530 1703; faxE-mail address: jaekook@chonnam.ac.kr (J. Kim).

http://dx.doi.org/10.1016/j.solidstatesciences.2014.07.01293-2558/© 2014 Elsevier Masson SAS. All rights res

a b s t r a c t

A cobalt-poor or iron rich bicomponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 anodematerials have been successfully prepared using simple, cost-effective, and scalable urea-assisted auto-combustion synthesis. The threshold limit of lower cobalt stoichiometry in CoFe2O4 that leads toimpressive electrochemical performance was identified. The electrochemical performance shows thatthe Co0.9Fe2.1O4/Fe2O3 electrode exhibits high capacity and rate capability in comparison to aCo0.8Fe2.2O4/Fe2O3 electrode, and the obtained data is comparable with that reported for cobalt-richCoFe2O4. The better rate performance of the Co0.9Fe2.1O4/Fe2O3 electrode is ascribed to its unique stoi-chiometry, which intimately prefers the combination of Fe2O3 with Co1�xFe2þxO4 and the high electricalconductivity. Further, the high reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodesis most likely attributed to the synergistic electrochemical activity of both the nanostructured materials(Co1�xFe2þxO4 and Fe2O3), reaching beyond the well-established mechanisms of charge storage in thesetwo phases.

© 2014 Elsevier Masson SAS. All rights reserved.

1. Introduction

Recently, new candidates for anode materials using iron-basedspinel oxides with a general formula of AFe2O4 (A ¼ Co, Zn, Ni,Mn) have been reported for use in lithium ion batteries (LIBs) due totheir high gravimetric specific capacities [1]. Spinel transition-metal oxides with two metal elements make it feasible to tunethe energy density and working voltage by varying the metalcontent [2]. Among these, cobalt ferrite (CoFe2O4) has receivedparticular attention due to its high theoretical capacity of916 mAh g�1, which is two times higher than that of graphite(372 mAh g�1). In general, the lithium storage mechanism inCoFe2O4 is attributed to a redox conversion reaction(CoFe2O4 þ 8Liþ þ 8e� 4 Co þ 2Fe þ 4Li2O), wherein it is reducedto metals nanograins dispersed in to Li2O matrix upon lithiationand then reversibly restored to their initial oxidation states duringdelithiation. On the other hand, among the other alternative anodematerials, Fe2O3 has always been regarded as very appealingtransition metal oxide because of its much higher capacity(1007 mAh g�1) than that of conventional graphite (372 mAh g�1),as well as nontoxicity, high corrosion resistance and low processing

: þ82 62 530 1699.

02erved.

cost. The lithium storage mechanism of Fe2O3 is based on theassumption of the reversible reduction of the oxide into metallic Fe(Fe2O3 þ 6Liþ þ 6e� 4 2Fe þ 3Li2O). However, similar to otheranodes, both the above said materials suffer from common issuessuch as poor cycling performance, a large initial irreversible ca-pacity loss, and poor rate capability due to material pulverizationand slow charge diffusion during the charging and dischargingprocesses [3e5].

The present study attempts to focus on improving the perfor-mance of spinel based CoFe2O4 oxides. In fact, great efforts havebeen made to improve the electrochemical performance of theseiron-based oxides [6,7]. The most commonly used strategy is todesign hybrid nanocomposites such as metal oxide/carbon com-posite [8,9]. However, carbon containing hybrids generally exhibitrelatively low specific capacity compared to pure metal oxidesowing to the presence of carbon, as well as its low theoretical ca-pacity. On the other hand, optimizing the architecture of mixedtransition-metal oxides at the nano-scale to improve the kineticshas been also one of the main approaches. Due to the small di-mensions, the nanoparticles can tolerate the strain associated withexpansion/contraction much better. In addition, recently, M. Jiaet al. (2013) and M. Zhang et al. (2013) designed new cobalt-richCoFe2O4-Co rods, which showed better electrochemical perfor-mance than pure CoFe2O4 electrodes [3,10]. Further, synthesizingbicomponent metal oxides has been also proven to be an efficient

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e72

route to improve capacity and rate capability in comparison to eachindividual component since bicomponent metal oxides often in-tegrates two types of functional materials for a synergistic effectthat can improve the intrinsic properties of each component suchas electrical/ionic conductivity, electrochemical reactivity, andmechanical stability [11e13]. Therefore, it is still a great challengeto develop a simple and low cost method for synthesizing novelnanostructured bicomponent electrode architectures withimproved electrochemical performances.

Herein, we report on an interesting strategy to combat theeffects of a drastic volume change more effectively by combiningtwo phases that react with lithium efficiently and improve thecapacity. It is also believed that during the charge or dischargeprocess in such a bicomponent mixture electrode, the volumeexpansion or contraction in the two phases is expected tohappen sequentially, thus reducing the strain and improving thestability [14]. It is well-known that Fe2O3 has higher theoreticalcapacity than that of spinel-type CoFe2O4. However, to utilize thehigh capacity and minimize the structure instability of Fe2O3, wedescribe the preparation of bicomponent architecture comprisedof nanostructured CoFe2O4 and Fe2O3 and their electrochemicalperformances were also tested as anode materials for secondaryLIBs. The threshold limit of lower cobalt stoichiometry inCoFe2O4 that leads to impressive electrochemical performancewas also identified. The cobalt-poor Co1�xFe2þxO4 (x ¼ 0.1)electrode demonstrated better properties among the preparedstoichiometric compositions in Co1�xFe2þxO4 (x ¼ 0.1, 0.2). Thehigh reversible capacity of the obtained bicomponent mixturesof Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 can be attributed tothe synergistic electrochemical activity of both the nano-structured material (Co1�xFe2þxO4 and Fe2O3), reaching beyondthe well-established mechanisms of charge storage in these twophases.

2. Experimental

2.1. Materials synthesis

Nanosized cobalt-poor or iron rich mixtures of Co1�xFe2þxO4(x ¼ 0.1, 0.2) were synthesized by facile and cost-effective urea-assisted auto-combustion synthesis [15,16]. Urea-assisted auto-combustion synthesis is an efficient and convenient method toprepare metal oxide nanoparticles at relatively low temperature.This process produces sub-nanometer-size of metal oxide nano-particles by self-generated heat of reaction within a very short re-action time. The advantage of urea is that it can form stablecomplexes with metal ions to increase solubility and prevent se-lective precipitation of the metal ions during water removal. Inaddition, the resultant oxide ash after combustion is generallycomposed of very fine particles with the desired stoichiometrylinked together in a network structure [15,16]. The calculatedamounts of cobalt nitrate [CoN2O6$6H2O, 98% Aldrich] and ironnitrate [Fe(NO3)3$9H2O, 98% Junsei extra pure] were carefully dis-solved in deionized water separately under continuous stirring atroom temperature. The obtained aqueous nitrate solutions weremixed together and subsequently an aqueous solution of urea((NH2CONH2, 99%, Aldrich) was added. The urea-to-nitrate molarratio (urea : cobalt nitrate¼ 10:6 and urea : iron nitrate¼ 15:6) wasmaintained to facilitate controlled combustion [15,16]. The ob-tained turbid solution was evaporated on a hot plate using amagnetic stirrer at 300 �C, which finally turned into gels and burnton its own. In order to eliminate possible organic residues and tostabilize the microstructure of the samples, the as-synthesizedpowders were subsequently annealed at 700 �C for 6 h in airatmosphere.

2.2. Materials characterization

The crystal structure and morphology of the obtained productswere investigated using a Shimadzu X-ray diffractometer with CuKa radiation (l ¼ 1.5406 Å), as well as field-emission scanningelectron microscopy (FE-SEM, S-4700 Hitachi) and field-emissiontransmission electron microscopy (FE-TEM, Philips Tecnai F20 at200 kV). For FE-TEM characterization, the samples were first dip-ped in ethanol and dispersed by ultrasonic vibration before coatingonto copper grids. The chemical compositions of transition metalsin the annealed bicomponent mixtures were analyzed by aninductively coupled plasma (ICP) emission spectroscopy (PerkinElmer OPTIMA 4300 DV).

2.3. Electrochemical measurements

The synthesized products were uniformly mixed with super-Pand PVDF in a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone solvent to form a slurry. The resulting slurry wascoated onto a copper foil current collector and dried under vacuumat 80 �C overnight. The slurry was punched into circular electrodesafter pressing between stainless steel twin rollers, in order toimprove the contact between the active material and copper foil.The 2032 coin-type cells were assembled in a glove box under a dryargon atmosphere using lithiummetal as a reference electrode, anda polymer membrane together with glass fiber as a separator. Theelectrolyte used was 1 M LiPF6 dissolved in a 1:1 (volume ratio)mixture of ethylene carbonate and dimethyl carbonate. The cell wasassembled in a glove box filled with argon gas. The cells were cycledbetween 0.01 and 3.0 V at different C-rates (BTS-2004H, Nagano,Japan). Cyclic voltammetry (CV) measurements were carried out ona Bio Logic Science Instrument (VSP 1075) over the potential range0.0e3.0 V at a scanning rate of 0.1 mV s�1. Electrochemicalimpedance spectroscopy (EIS) measurements of the electrodeswere also carried out using a Bio Logic Science Instrument (VSP1075) after the cell was cycled for 5 cycles.

3. Results and discussion

3.1. Structural and morphological analysis

XRD patterns of Co1�xFe2þxO4 (x ¼ 0.1, 0.2) powders annealed at700 �C for 6 h are shown in Fig. 1(a) and (b). Both the samples showa major phase of spinel CoFe2O4 (JCPDS No. 22-1086, Fd-3m (227))along with the expected Fe2O3 (JCPDS No. 89e0599) as a secondaryphase. It can be clearly observed that the intensity of Fe2O3 peaksincreases with the decrease of the amount of cobalt. The sharpdiffraction peaks and high intensity indicate good crystallinity inboth samples. No peaks of any other phases or any other impuritieswere detected. To synthesize the bicomponent mixture electrode,the authors have reduced the amount of Co or increased theamount of Fe during the experiment in spite of the 1:2 ratio to formthe targeted compound Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3.It is believed that the extra amount of Fe present in the form ofFe2O3 as an extra phase can be helpful to enhance the electro-chemical performance of host material. However, to know the exactchemical composition of Co and Fe in both the obtained bicom-ponent mixtures, ICP-AES analysis was performed and the resultsare displayed in Table 1. The results showed that the chemicalcompositions of both the samples are nearly equal to the stoi-chiometric ratio as targeted. In addition, ICP data was also takeninto consideration to roughly calculate the phase fraction ratio ofCoFe2O4 and Fe2O3 in both the obtainedmixtures assuming that theCoFe2O4 and Fe2O3 are perfectly stoichiometric and the detailedresults are displayed in Table 1. The phase fraction ratio of CoFe2O4

Fig. 1. XRD patterns of Co1�xFe2þxO4 (a) x ¼ 0.1 and (b) x ¼ 0.2.

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e7 3

and Fe2O3 is 89% and 11% for Co0.9Fe2.1O4/Fe2O3 and 79% and 21% forCo0.8Fe2.2O4/Fe2O3 samples, respectively.

Fig. 2(a) and (b) shows FE-SEM images of the annealed bicom-ponent mixture of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 sam-ples, respectively. The FE-SEM image displays that the powdersconsist of spherical nanoparticles with faceted and slightlyagglomerated grain-packed morphology. Inhomogeneous particlegrowth was also observed. The image also demonstrates that thecomposition variation process has very little influence on themorphology, but has a major influence on the electrochemicalperformance (discussed in the later section). Fig. 2(c) and (d) il-lustrates the FE-TEM images of annealed bicomponent mixtures ofCo0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3. In both the samples, theparticles have spherical shape with different distributions ofdiameter. The particles sizes are in the range of 50e100 nm. Itappears that the nanoparticles were reunited together during the

Table 1Chemical composition and phase fraction analysis of spinel type Co0.9Fe2.1O4/Fe2O3

and Co0.8Fe2.2O4/Fe2O3 materials.

Target composition ICP concentrations (wt%) Practical mole composition(normalized to Fe)

Co Fe

Co0.9Fe2.1O4 25.4 53.9 Co0.93Fe2.1O4

Co0.8Fe2.2O4 23.1 55.7 Co0.87Fe2.2O4

Phase fraction calculation of CoFe2O4 and Fe2O3

Targetcomposition

ICP calculation Phase calculation

Element ICP value(wt%)

Practicalmolar ratio(normalizedto Co as 1)

Extraremainedmol

Composition Phasefraction

Co0.9Fe2.1O4 Co 25.4 1 e CoFe2O4 0.89Fe 53.9 2.239397 0.239397 Fe2O3 0.11

Co0.8Fe2.2O4 Co 23.1 1 e CoFe2O4 0.79Fe 55.7 2.544599 0.544599 Fe2O3 0.21

synthesis probably due to their magnetic behavior. In addition,Fig. 2(e) and (f) shows typical HR-TEM images of Co0.9Fe2.1O4/Fe2O3and Co0.8Fe2.2O4/Fe2O3 samples, revealing that the particles arestructurally uniform with a lattice spacing of about 2.5 Å, whichcorresponds to the (311) lattice plane of the CoFe2O4. Furthermore,the insets of Fig. 2(e) and (f) shows the corresponding Fast-Fouriertransform (FFT) patterns of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. The FFT images were taken from themarked red square regions in the HR-TEM images. The measuredlattice spacings in the FFT patterns for both the samples also tend tosupport the formation of CoFe2O4.

3.2. Electrochemical properties

Fig. 3(a) and (b) presents the CV curves of the Co0.9Fe2.1O4/Fe2O3and Co0.8Fe2.2O4/Fe2O3 electrodes, for the initial three cycles at ascan rate of 0.1 mV s�1 between 0.0 and 3.0 V (vs. Li/Liþ), respec-tively. It can be obviously seen that the first CV curve differs fromthe subsequent CV curves. In the first scan, both the electrodesshow a small cathodic peak at 0.84 V, which could be assigned tothe formation of stable intermediate LinCo1�xFe2þxO4 (x ¼ 0.1, 0.2)phase [17]. Furthermore, the following large and sharp cathodicpeak at 0.56 V corresponds to the reduction of Fe3þ and Co2þ totheir metallic states and the formation of Li2O, accompanied by thedecomposition of organic electrolyte to form a solid electrolyteinterphase (SEI) layer [17,18]. In the charge process of both theelectrodes, the anodic peaks at 1.67 V and 1.92 V are ascribed to theoxidation of the metallic iron and cobalt to Fe3þ and Co2þ,respectively. In the second scan, the shift in the cathodic and anodicpeaks to 0.94 V and 1.50 V and 1.72 V and 1.89 V for both theelectrodes can be attributed to the reversible reduction andoxidation reaction of Fe2O3 and CoO to Fe and Co metals and viceversa, respectively. The decrease of the redox peak intensity impliesthat the capacity is decreased after the first cycle. In addition, thecathodic and anodic peaks positions for both the electrodes in thefollowing third cycle are also almost at same position corre-sponding to second cycle such as 0.94 V and 1.49 V and 1.72 V and1.90 V, respectively, which indicate good structural stability andelectrochemical reversibility.

Fig. 4(a) and (b) shows the charge/discharge curves of thebicomponent electrodes, namely, Co0.9Fe2.1O4/Fe2O3 andCo0.8Fe2.2O4/Fe2O3 for the 1st, 2nd, and 5th cycles at a current rate of0.1C (1C¼ 916mAg�1). It can be clearly observed that the curve areaand voltage plateau regions in the discharge and charge profiles ofboth the electrodes are almost in the same position, indicating thatsimilar redox reactions take place in the electrodes. In addition, theelectrodes also demonstrate typical chargeedischarge behavior fortransition-metal anode materials [19]. The first discharge curve ofboth electrodes exhibited a distinctly long flat potential plateau atabout 0.85 V followed by a gradual sloping until the deep dischargelimit of 0.01 V. This behavior is explained well by the conversionreactions of Fe3þ and Co2þ to their metallic states and the formationof Li2O (CoFe2O4þ 8Liþþ 8e�4 Coþ 2Feþ 4Li2O), respectively [3].One additional small voltage plateau at ~1.0 V can be also seen in thefirst discharge of both the samples. The voltage of this plateau iscloser to that of cobalt reduction in Co3O4 than to that of ironreduction [20]. The first charge curves show a steady and smoothvoltage increase, indicating a different electrochemical mechanismfrom the first discharge such as oxidation reactions of metallic Feand Co (Co þ 2Fe þ 4Li2O 4 CoO þ Fe2O3 þ 8Li) [3]. The potentialplateau is shifted upward at around 1.0 V in the subsequentdischarge curves with a more sloping profile accompanied by agradual increase of Coulombic efficiency. The discharge and chargecapacities in the first run are 1273.1 and 910.8 mAh g�1 and 1157.8and 830.9 mAh g�1 for the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/

Fig. 2. FE-SEM image (a, b); FE-TEM images (c, d); and HR-TEM images (e, f) of the Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 samples, respectively. Inset of (e) and (f) is theircorresponding indexed FFT patterns.

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e74

Fe2O3 electrodes respectively, with similar Coulombic efficiencies(~71%). The irreversible capacity loss for the first cycle is probablydue to the incomplete conversion reaction and the solid electrolyteinterface (SEI) layer formation at the electrode/electrolyte interfacecaused by the reduction of electrolyte in the voltage range of0.05e0.8 V [20]. The specific capacities obtained for theCo0.9Fe2.1O4/Fe2O3 composition is higher than that of Co0.8Fe2.2O4/Fe2O3 composition over all the measured cycle numbers (Fig. 4c)and even all the investigated current rates (Fig. 4d), probably due tothe higher electrical conductivity in the former electrode and thepreferable stoichiometric composition. For example, the 2nd and5th reversible discharge capacities obtained for the Co0.9Fe2.1O4/Fe2O3 electrode (917.8 and 862.5 mAh g�1) are higher than those ofthe Co0.8Fe2.2O4/Fe2O3 electrode (827.9 and 768.8 mAh g�1).

Fig. 4(c) shows the cycling stability of both the bicomponentelectrodes (Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) at a constantcurrent rate of 0.1C. It is interesting to observe that the capacity ofboth the electrodes decreases in the first 30 cycles, and thengradually increases as the cycle number increases, which could beattributed to the activation process in cobalt ferrite electrodes[21,22]. Two aspects can be taken into account for the presentobservation: (i) during first discharge, a large amount of lithiumions inserted into the active materials layers induces a suddenvolume expansion and simultaneously may also block furthertransfer of Liþ ions from electrode to electrolyte. Therefore, it ispossible that the conversion reaction may not be fully reversibleafter the first charge and thereby result in capacity loss duringextended cycling. This factor may also be responsible for the low

Fig. 3. Cyclic voltammetry (CV) plots of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrodes.

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e7 5

coulombic efficiency in the first cycle. (ii) After repeated Liþ ionsinsertion/de-insertion, both the bicomponent mixture electrodesbecome more expanded and porous to some extent, as demon-strated by Ex-situ FE-TEM images (Fig. 6). The porous feature couldeffectively aid the electrolyte to access the inner part of the activematerials and thus, the conversion reaction tend to become morereversible and release Liþ ions. Meanwhile, the enhanced electrode/electrolyte contact area may also contribute to improve the storagecapacities [21]. The Co0.9Fe2.1O4/Fe2O3 electrode delivers an initialcharge capacity of 910.8 mAh g�1 and retains a capacity of758.2 mAh g�1 after 80 cycles, corresponding to ~83% of the initialcharge capacity, while the Co0.8Fe2.2O4/Fe2O3 electrode decaysquite rapidly from the initial charge capacity of 830.9 to635.8 mAh g�1 after same number of cycles (corresponding to 77%

Fig. 4. Discharge/charge profiles of (a) Co0.9Fe2.1O4/Fe2O3 and (b) Co0.8Fe2.2O4/Fe2O3 electrocurrent rates between 0.1 and 6.4C.

of the initial charge capacity). The initial Coulombic efficiency is71%, and in the following cycles, the Coulombic efficiency of boththe electrodes gradually increases along with cycling number,keeping above ~97% in the whole cycling, indicating good revers-ibility of the electrodes. Clearly, the Co0.9Fe2.1O4/Fe2O3 nanoparticleelectrode has better cycling performance. However, the obtainedvalues of electrochemical performance of both the bicomponentelectrodes are comparable to those reported for nano-sizedCoFe2O4 and Fe2O3 electrodes, but the synthesis strategy adoptedin the present study is cost-effective, simple, and scalable unlikepreviously reported syntheses [3,10,23e25].

To evaluate the rate capability, both the bicomponent electrodes(Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) were cycled at variouscurrent rates, and the results are shown in Fig. 4(d). Both the

des. (c) Cycling performance at constant rate of 0.1C and (d) rate capability at different

Fig. 5. EIS plots of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes.

Fig. 6. Ex-situ FE-TEM images of cycled electrodes (a) Co0.9Fe2.1O4/Fe2O3 and (b)Co0.8Fe2.2O4/Fe2O3.

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e76

electrodes exhibit good rate performance; Co0.9Fe2.1O4/Fe2O3showing the highest specific capacity at high current rate. Precisely,the Co0.9Fe2.1O4/Fe2O3 electrode demonstrates better rate perfor-mance with a charge capacity of 328.9 mAh g�1 at a high currentrate of 6.4C, whereas only 229.3 mAh g�1 is delivered by theCo0.8Fe2.2O4/Fe2O3 electrode at the same current rate. Furthermore,between 80 and 90% of the charge capacity can be recovered whenthe current is returned back to 0.1C, which is indicative of the betterstructural stability. However, the obtained bicomponent mixtureelectrodes exhibit capacities comparable to that of original singleCoFe2O4 component; the enhancement is not simply as a result ofintroducing of a higher capacity component. Instead, it more likelyoriginates from the unique heterostructure of the mixture

electrode, which is elaborated as follows. It is noted that reversibleformation and decomposition of the Li2O nanomatrix could beelectrochemically driven by the metal nanoparticles formed in-situ. Precisely, the nanoparticles of Fe can probably make the extraLi2O reversibly convert to lithium ion if there is any extra Li2Opresent. The irreversibility during the initial electrochemical reac-tion indicates that the CoFe2O4 nanoparticles may facilitate extraLi2O formation during discharge. The presence of Fe nanoparticlesmay thus make the extra Li2O (provided by CoFe2O4) reversiblyconvert to lithium ion, giving the electrodes higher reversible ca-pacity [14,26,27]. Hence, the presence of Fe nanoparticles at theinterface between Co1�xFe2þxO4 and Fe2O3 may improve thereversibility of the reaction and further result in a higher reversiblecapacity. In addition, it is highly possible that the high surface areaand the nanoparticles size of both the samples enable better con-tact between active materials and the electrolyte and therebyreduce the traverse time for both electrons and lithium ions.

Fig. 5 presents typical EIS plots of both the bicomponentmixture(Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3) electrodes. Both elec-trodes were investigated after 5 charge/discharge cycles anddisplay similar behavior. In both the cases, it can be seen that theimpedance curves consist of two semicircles in the high and me-dium frequency regions and an approximately 45� inclined line inthe low frequency region. Generally, the semicircle in the high-frequency region is related to Liþ-ion migration through the SEIfilm covered on the surface of the electrodes, the semicircle in themiddle-frequency region is attributed to charge transfer throughthe electrode/electrolyte interface, and the steep sloping line isassigned to solid-state diffusion of the Liþ-ions into the bulk of the

A.K. Rai et al. / Solid State Sciences 36 (2014) 1e7 7

electrode material matrix [15,16]. The impedance fitting was per-formed using EC-lab software and the corresponding equivalentcircuits are also shown in the insets of Fig. 5. The circuit parametersR1, R2, R3 and Zw correspond to the electrolyte resistance, thediffusion resistance of Li ions through SEI layer, the charge transferresistance and the Warburg impedance, respectively. From thefitting results, it can be seen that the charge transfer (R3) value ofCo0.9Fe2.1O4/Fe2O3 electrode is slightly smaller than that ofCo0.8Fe2.2O4/Fe2O3 electrode, indicating better electronic conduc-tivity in the former electrode. Furthermore, the values of R1, R2 andZw of the Co0.9Fe2.1O4/Fe2O3 electrode are also smaller than those ofthe Co0.8Fe2.2O4/Fe2O3 electrode.

Fig. 6(a) and (b) show the ex-situ FE-TEM images of Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes after being cycled. In brief,the cycled electrode was initially dissociated from the cell in anargon-filled glove box. The electrode was then washed withdimethyl carbonate solvent, followed by sonication treatment, andthen dried overnight before FE-TEM observation. Both the elec-trodes exhibited similar morphology. It can be observed that theCo0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 nanoparticles revealdeteriorated crystallinity probably due to the repeated lithiuminsertion/extraction process being accompanied by volumeexpansion/contraction of the Co1�xFe2þxO4 structure. In addition, itis also observed that the nanoparticles are being embedded on thecarbon shell from the ketjen black used for electrode fabrication.However, the expansion and contraction of the nanoparticlesassociated with lithium ion insertion and extraction may be con-tained by carbon acting as an alternative buffer and thereby pre-serving the structural integrity of the electrode during cycling [28].More importantly, it is seen that the nanoparticles anchored ontothe carbon shell and did not drop out even after the ultrasonicationtreatment before performing TEM measurement.

4. Conclusions

In summary, cobalt-poor or iron rich bicomponent mixtures ofCo0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 have been fabricated asan anode material for LIBs via simple urea-assisted auto-combus-tion synthesis. The threshold limit of lower cobalt stoichiometry inCoFe2O4 that leads to the impressive electrochemical performancewas identified. As compared to Co0.8Fe2.2O4/Fe2O3 electrode, theCo0.9Fe2.1O4/Fe2O3 electrode exhibited higher lithium storage ca-pacity, better cyclic stability, and good rate capability, which can beascribed to the high electrical conductivity or the preferable stoi-chiometric composition in the latter electrode. Furthermore, thehigh reversible capacity in Co0.9Fe2.1O4/Fe2O3 and Co0.8Fe2.2O4/Fe2O3 electrodes is most likely attributed to the synergistic elec-trochemical activity of both the nanostructured materials(Co1�xFe2þxO4 and Fe2O3), reaching beyond the well-establishedmechanisms of charge storage in these two phases.

Acknowledgments

This work was supported by the Global Frontier R&D Program(2013e073298) on Center for Hybrid Interface Materials (HIM)funded by the Ministry of Science, ICT & Future Planning. Thisresearch was also supported by the Ministry of Science, ICT &Future Planning (MSIP), Korea, under the Convergence InformationTechnology Research Center (C-ITRC) support program (NIPA-2013-H0301-13-1009) supervised by the National IT Industry PromotionAgency (NIPA).

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