Synthesis of nano-sized ZnCo2O4 anchored with graphene nanosheets

Synthesis of nano-sized ZnCo 2O 4anchored with graphene nanosheets as an anode material for secondary lithium ion batteries

Alok Kumar Rai,Trang Vu Thi,Baboo Joseph Paul,Jaekook Kim *

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

A R T I C L E I N F O Article history:

Received 26June 2014

Received in revised form 2September 2014Accepted 2September 2014

Available online 22September 2014Keywords:ZnCo 2O 4Graphene Composite

Anode material

Lithium-ion batteries

A B S T R A C T

ZnCo 2O 4/graphene and pure ZnCo 2O 4nanoparticles were synthesized by simple and low temperature urea-assisted auto-combustion method and annealed at 400 C for 5h in air atmosphere.The electron microscopy image of the obtained ZnCo 2O 4/graphene nanocomposite revealed that the ZnCo 2O 4nanoparticles were randomly distributed and anchored on the surface of reduced graphene nanosheets.The graphene nanosheets signi ?cantly reduced the particles size and suppressed the aggregation of ZnCo 2O 4nanoparticles as well as maintaining the good electrical conductivity of the overall sample.The size of the nanoparticles was in the range of 50–100nm and the size of nanoparticles in the nanocomposite sample in the range of 25–50nm.The electrochemical results showed that the ZnCo 2O 4/graphene nanocomposite electrode had greatly improved cycling stability with high reversible capacity of 755.6mAh g à1after 70cycles,and better rate capability of 378.1mAh g à1at 4.5C than pure ZnCo 2O 4nanoparticles (299.8mAh g à1after 70cycles and 302.4mAh g à1at 4.5C).The enhanced electrochemical performance of the nanocomposite could be ascribed to the positive synergistic effect of the combination of the ZnCo 2O 4nanoparticles and conducting graphene nanosheets.

?2014Elsevier Ltd.All rights reserved.

1.Introduction

In recent years,rechargeable lithium ion batteries (LIBs)have been successful for portable electronic devices,but their low energy density,high cost and poor cycling stability still prevent their applications in next-generation wireless communication devices,electric vehicles,hybrid electric vehicles,power tools,uninterrupted power sources,stationary storage batteries,and microchips.In addition,the graphite anode with low theoretical capacity (372mAh g à1)is also one of the major factors,lowering the present demands of LIBs.It is therefore necessary to ?nd better alternative anode materials.Among the new high-performance anode materials for next-generation LIBs,transition metal oxides have attracted much attention recently because of their advantage in high speci ?c capacity [1].Particularly,cobalt oxide (Co 3O 4)has been proposed as one of the most promising anode candidate because of its high capacity and excellent cycle life [1].However,the major limiting factors of Co 3O 4are its large volume expansion/contraction and severe particle aggregation associated with the Li +-ion insertion and extraction process,which lead to electrode pulverization and the loss of interparticle contact.However,many

efforts have been made to replace cobalt in Co 3O 4partially with environment friendly and less-expensive alternative elements (Zn,Cu,Ni,Mg and Fe),which undergo simultaneously conversion and alloying/de-alloying reactions together [2,3].As a result,among the Co-based spinels,ZnCo 2O 4is quite attractive because of its various advantages such as low toxicity,lower cost,and high thermal stability.More importantly,after Zn 2+is reduced to Zn during the discharge process,LiZn alloy is formed under further reduction of the corresponding metal [4].Therefore,an extra discharge capacity has been obtained from the formation of the Li –Zn alloy [4,5].In addition,by mixing two transition metal oxides,it is possible to combine their competitive advantages such as the ?exibility to alter the theoretical capacity and control the working voltages [2].Moreover,the incorporation of Zn can improve the cycling performance through the synergetic interac-tion between Co and Zn:both Zn and Co are electrochemically active with respect to Li +ion;and the presence of Zn in the cobalt oxide matrix and the presence of cobalt oxide in the ZnO matrix can be mutually bene ?cial for each other [6–8].However,like other transition metal oxides,ZnCo 2O 4also suffers from poor electrical conductivity and electrode pulverization,which is induced by its huge volume changes during the charge/discharge processes,resulting in fast capacity decrease and poor rate capability.Therefore,low conductivity and large volume changes during the cycle process need to be further improved.Two effective

*Corresponding author.Tel.:+82625301703;fax:+82625301699.E-mail address:jaekook@chonnam.ac.kr (J.Kim).https://www.360docs.net/doc/9c13096351.html,/10.1016/j.electacta.2014.09.0790013-4686/?2014Elsevier Ltd.All rights reserved.

Electrochimica Acta 146(2014)577–584

Contents lists available at ScienceDirect

Electrochimica Acta

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t

a

strategies can solve these problems:making nanostructures to withstand the volume changes and making a graphene-based nanocomposite to enhance the electrical conductivity.However,to improve the electrochemical performance of the ZnCo2O4material, taking advantage of nanostructures is a popular method[3,6,8–11]. The electrochemical performance has been improved to an extent by manipulating nanostructures that resulted in increased active sites for Li+ion insertion/deinsertion reactions and short diffusion lengths for both lithium(Li+)ion and electrons(eà).On the other hand,graphene nanosheets,a single-atom-thick sheet of honey-comb carbon lattice,exhibits a number of intriguing unique properties,such as superior electronic conductivity,high surface area,large surface-to-volume ratio,good mechanical properties [1,12–14].Therefore,recently several metal oxide-graphene nano-composites have been reported as high performance electrode materials for rechargeable lithium ion batteries[1].

Therefore,by combining these two strategies,we have synthesized an anode material for high performance LIBs in which nano-sized ZnCo2O4particles are anchored with graphene nano-sheets.To the best of our knowledge,there is no report about the electrochemical behavior of the ZnCo2O4/graphene nanocompo-site as an anode synthesized by simple,cost-effective and facile approach at a relatively low temperature.However,in the present work,the ZnCo2O4/graphene nanocomposite was synthesized by a facile urea-assisted auto-combustion synthesis method combined with subsequent annealing treatment at low temperature of400 C for5h.Urea-assisted auto-combustion synthesis is an ef?cient and convenient method for preparing metal oxide nanoparticles at relatively low temperatures[1,15,16].This process produces sub-nanometer-size metal oxide nanoparticles by self-generated heat of reaction with very short reaction time.The advantage of urea is that it can form stable complexes with metal ions that increase the solubility and prevent the selective precipitation of the metal ions during water removal[1,15,16].In addition,the oxides that form after combustion are generally composed of very?ne particles with the desired stoichiometry linked together in a network structure.The sub-nanometer particles were randomly dispersed and anchored on the reduced graphene nanosheets during combustion.More importantly,it is also believed that urea-assisted auto-combustion synthesis is very effective for the fabrication of various mixed metal oxide nanoparticles composed of different metal cations[1,15,17].It is possible that the existence of secondary(impurity)metal cations may accelerate the electrochemical reaction by acting as a catalyst or act as a pillar to accommodate the strain induced by volume changes or it may also participate in the electrochemical reaction if it is electro-chemically active[1,15,17].The obtained nanocomposite electrode exhibited higher reversible capacity,better cycling stability and improved rate capability than pure ZnCo2O4nanoparticles.

2.Experimental

2.1.Preparation of Graphene Oxide and Graphene Nanosheets

In a typical synthesis method,graphene oxide(GO)is?rst synthesized by the modi?ed Hummers method[18],and the obtained GO colloidal suspension is kept at room temperature for a long time.Second,the obtained GO is reduced to graphene nanosheets by the polyol-based reduction method.Detailed preparation procedure for the reduction of graphene nanosheets can be found in our previous papers[13,14].

2.2.Materials Synthesis

The ZnCo2O4/graphene nanocomposite and pure ZnCo2O4 nanoparticle samples were synthesized by urea-assisted auto-combustion method and the detailed preparation procedure can be seen in the Scheme1.All the chemical reagents utilized in the study were of analytical grade and used without further puri?cation.Brie?y,zinc nitrate hexahydrate[(Zn(NO3)2.6H2O, 98%,Sigma Aldrich)and cobalt nitrate hexahydrate[(Co (NO3)2.6H2O,98%,Sigma Aldrich]with the stoichiometric ratio of1:2were dissolved in distilled water separately under continuous stirring at room temperature to obtain transparent solutions.After that,both solutions were mixed together with a prepared aqueous solution of urea(NH2CONH2,99%.Sigma Aldrich),and the ratio between urea and nitrates was maintained at20:12to allow for controlled combustion(urea:zinc nitrate=10: 6and urea:cobalt nitrate=10:6)[15].At the same time,10wt%of the reduced graphene nanosheets were also added to the mixed solution to allow the nanoparticles to anchor on the reduced graphene nanosheets.The obtained ternary mixed solution was evaporated on a hot plate using a magnetic stirrer at350 C under continuous stirring to remove excess water.During the evapora-tion,the homogeneously mixed solution turned viscous,eventu-ally becoming a gel.The formed gel slowly foamed,swelled,and ?nally burned on its own.In order to eliminate possible organic residues and to stabilize the microstructure of the ZnCo2O4/ graphene nanocomposite powder,the as-synthesized powder was subsequently annealed at400 C for5h in air atmosphere.For comparison,pure ZnCo2O4nanoparticles were also synthesized under the same condition without addition of graphene nano-sheets.The overall combustion reactions are represented,respec-tively as follows[17]:

Scheme1.Schematic illustration of the preparation of ZnCo2O4/graphene nanocomposite. 578 A.K.Rai et al./Electrochimica Acta146(2014)577–584

3Zn eNO 3T2t3Co eNO 3T2t10NH 2CONH 2tGraphene

àà!1=2O 2

ZnCO 2O 4=graphene t2ZnO tCoO t16N 2t20H 2O t10CO 2

(i)

3Zn eNO 3T2t6Co eNO 3T2t15NH 2CONH 2!30

3ZnCo 2O 4t24N 2t30H 2O t15CO 2

(ii)

The presence of N 2and O 2from air was not been taken into account in the reactions described.In the urea-assisted auto-combustion synthesis,the nitrate ions act as the oxidizer,while the urea acts as the fuel.The reaction products are ?nely divided into the metal oxide and the evolved gases N 2,CO 2,and H 2O.The excess urea also decomposes into ammonia and other gases.2.3.Materials Characterization

The crystalline phase of the samples were recorded by X-ray powder diffraction (XRD)on a Shimadzu X-ray diffractometer equipped with Cu K a radiation (l =1.5406?).The morphologies of the synthesized samples were examined using ?eld-emission scanning electron microscopy (FE-SEM,Hitachi S-4700)and ?eld-emission transmission electron microscopy (FE-TEM,Philips Tecnai F20,KBSI,Chonnam National University)operated at an accelerat-ing voltage of 200kV.The carbon content in both the annealed samples of ZnCo 2O 4/graphene nanocomposite and pure ZnCo 2O 4nanoparticles were determined by CHN element analysis on a Flash-2000Thermo Fisher.

2.4.Electrochemical Measurements

Electrochemical performances of the obtained samples were measured by assembling half-cells.The working electrode was prepared by mixing the active materials (ZnCo 2O 4/graphene nano-composite and pure ZnCo 2O 4),Super –P and polyvinylidene ?uoride binder in a weight ratio of 70:20:10in N-methyl-2-pyrrolidinone to form a homogenous slurry.The slurry was uniformly pasted on a copper foil current collector and then dried in a vacuum oven at 100 C for 12h.After that,the slurry was pressed between stainless steeltwin rollers and punched into circulardiscs.In a glove box under argon atmosphere,the half-cells were assembled as CR-2032coin-type cells.Metallic lithiumwas used as the counter electrode and the Celgard 2400membrane with glass ?ber was used as the separator.For the electrolyte,1M LiPF 6was dissolved in a mixture of ethylene carbonate and dimethyl carbonate at the volume ratio of 1:1.Cyclic voltammetry (CV)measurement of the electrodes were performed between 0to 3V (versus Li +/Li)using a Bio Logic Science Instrument (VSP 1075)at a scan rate of 0.1mV s à1.Galvanostatic testing of the coin cells was conducted using a programmable battery tester over the potential range 0.01–3.0V vs Li +/Li (BTS-2004H,Nagano,Japan).Electrochemical impedance spectroscopy (EIS)measurement of the electrodes were also carried out on same Bio Logic Science Instrument (VSP 1075).Before the measurements,the electrode was cycled for 10cycles and then measured in the frequency range from 0.01Hz to 1.0MHz.A small ac signal of 5mV in amplitude was used as the perturbation of the system throughout the tests.EIS was used to measure the electronic conductivities of the assembled cell using lithiumfoil acting as both the counter and reference electrodes.3.Results and Discussion

3.1.Crystal Structure and Morphology

Fig.1(a)and (b)shows the XRD patterns of the ZnCo 2O 4/graphene nanocomposite and pure ZnCo 2O 4samples,respectively

annealed at 400 C for 5h in air atmosphere.The major diffraction peaks in the XRD pattern of the annealed ZnCo 2O 4/graphene nanocomposite sample are well indexed with the cubic ZnCo 2O 4spinel phase [JCPDS card no.23–1390,space group Fd3m (227)].On the other hand,the nanocomposite sample also shows additional peaks in the pattern,which are well matched with ZnO (JCPDS card no.79–0205)and CoO (JCPDS card no.75–0533).The presence of these two metal oxides peaks are due to the decomposition of ZnCo 2O 4,which may have occurred due to the consumption of the reduced graphene nanosheets at elevated temperatures [1].An annealing temperature of 400 C was chosen as the carbonation temperature,because a carbon-rich polysaccharide tends to be carbonize at temperatures as low as 400 C [19].The XRD pattern of the annealed pure ZnCo 2O 4sample (Fig.1b)can be fully indexed as the standard cubic ZnCo 2O 4spinel phase.No signi ?cant impurities or other phases were observed,indicating the high purity of the product.

The surface morphologies and structures of the pure ZnCo 2O 4nanoparticles and ZnCo 2O 4/graphene nanocomposite samples are evaluated from the FE-SEM and FE-TEM images shown in Fig.2.As shown in Fig.2(a),the ZnCo 2O 4nanoparticles with different diameters tend to severely aggregate with each other.The agglomerated nanostructures of pure ZnCo 2O 4nanoparticles consist of spherical morphology of primary particles and found to be in the range of 50–100nm.It is well-known that aggregated large particles as an anode material lead to poor rate performance

Fig.1.X-ray diffraction patterns of (a)ZnCo 2O 4/graphene nanocomposite and (b)pure ZnCo 2O 4nanoparticles.

A.K.Rai et al./Electrochimica Acta 146(2014)577–584579

due to the long diffusion path for both electrons(eà)and lithium (Li+)ions transport during the Li+ion insertion/extraction process. After the introduction of reduced graphene nanosheets,the aggregation of ZnCo2O4nanoparticles was reduced and clearly anchored on the surface of reduced graphene nanosheets,as shown in Fig.2(b).The primary particles size was much smaller in the range of25–50nm than the size of the pure ZnCo2O4 nanoparticles.Therefore,graphene can prevent the aggregation of ZnCo2O4nanoparticles to a certain extent,possibly due to the partition effect of graphene nanosheets[1].The small nano-particles can provide short path lengths for both eàand Li+ions transport during the Li-ion insertion/extraction process,resulting in better rate capability.It is also reasonable to suggest that during the urea-assisted auto-combustion process,the nanoparticles are strongly anchored on the surface of the reduced graphene nanosheets at high density to facilitate rapid electron transport between the underlying graphene nanosheets[1].The morphol-ogies and structures of both samples are evaluated further using the FE-TEM images.Fig.2(c)shows the FE-TEM image of the pure ZnCo2O4nanoparticles,which con?rmed the packed structure and the aggregation of primary nanoparticles.Fig.2(d)shows the FE-TEM image of the ZnCo2O4/graphene nanocomposite sample.The ZnCo2O4nanoparticles are randomly dispersed on the surface and edges of the graphene nanosheets and the aggregation of these nanoparticles were prevented effectively.More importantly,there was a strong interfacial interaction between the ZnCo2O4nano-particles and the reduced graphene nanosheets because ZnCo2O4 nanoparticles were still anchored on the surface and edge of the

Fig.2.FE-SEM and FE-TEM images of pure ZnCo2O4nanoparticles(a and c)and ZnCo2O4/graphene nanocomposite(b and d)respectively.

Fig.3.Cyclic voltammograms of(a)ZnCo2O4/graphene nanocomposite and(b)pure ZnCo2O4nanoparticles electrodes.

580 A.K.Rai et al./Electrochimica Acta146(2014)577–584

reduced graphene nanosheets,and no obvious bare ZnCo2O4 nanoparticles were dispersed outside the reduced graphene nanosheets,even after a long time of sonication during the preparation of the TEM specimen.

In addition,to know the accurate percentage of carbon in the annealed samples,CHN analysis has been also performed.In this method,the samples are combusted and the amounts of carbon, hydrogen and nitrogen are determined by a gas chromatographic analysis of the combustion products.The percentage of carbon in the ZnCo2O4/graphene nanocomposite and pure ZnCo2O4nano-particles samples were found to be0.23%and0.09%,respectively. We believed that carbon easily reacted with some of oxygens in the material during heat treatment in air atmosphere and evaporated as CO or CO2[1].

3.2.Electrochemical Performance

Fig.3(a)and(b)shows the?rst three cyclic voltammogram(CV) curves of the electrodes made from ZnCo2O4/graphene nano-composite and pure ZnCo2O4nanoparticles,respectively,at a scan rate of0.1mV sà1in the potential range of0–3V.The?rst cycle is substantially different from the second one,which clearly shows an irreversible reduction in the?rst discharge.In the?rst cathodic scans of both electrodes,two peaks can be observed located at$ 0.43V and$0.63V for the ZnCo2O4/graphene nanocomposite electrode and$0.39V and0.74V for the pure ZnCo2O4nano-particles electrode.However,the?rst small cathodic peak can be assigned to the reduction of Zn2+to metal,accompanied by an irreversible reaction related to the decomposition of the organic electrolyte to form a solid electrolyte interphase(SEI)layer[4,20]. This SEI layer plays a major role in the irreversible capacity loss and obstruct the movement of lithium ions.The second strong peak can be attributed to the reduction of Co3+to Co[4,20].Two broad oxidation peaks are also observed at$1.69V and$2.09V for the ZnCo2O4/graphene nanocomposite electrode and$1.71V and 2.08V for the pure ZnCo2O4nanoparticles electrode,correspond-ing to the oxidation of Zn0to Zn2+and Co0to Co3+,respectively[3]. In the second and third cycles,the cathodic peak shifted to a higher potential of about$1.08V for both electrodes,indicative of different electrochemical reactions governing the two processes [6].From the second cycle onward,the reduction peaks in the cathodic scan and the oxidation peaks in the anodic scan overlap very well in the nanocomposite electrode than nanoparticles electrode,indicating that the ZnCo2O4/graphene nanocomposite electrode exhibits good stability and cyclability for the insertion and extraction of lithium ions.

The discharge/charge curves for the1st,2nd and10th cycles were recorded at a constant current rate of0.1C[1C=903mA gà1, corresponding to8.33mol of recyclable Li per mole of ZnCo2O4as per equations(iv)to(vii)]in a potential range between0.01–3.0V vs Li+/Li.Fig.4(a)and(b)shows the voltage-capacity pro?les of the ZnCo2O4/graphene nanocomposite and pure ZnCo2O4nanoparticle electrodes,respectively.From the voltage pro?les,the1st discharge curves of both electrodes exhibit large distinct plateaus between$0.9–1.0V,followed by a gradual voltage decrease to the cutoff potential of0.01V,which corresponds to the reduction of ZnCo2O4to Zn and Co,followed by the alloying of Zn with Li to LiZn [6,21].Most importantly,the ZnCo2O4/graphene nanocomposite

Fig.4.Discharge/charge pro?les of(a)ZnCo2O4/graphene nanocomposite and(b)pure ZnCo2O4nanoparticles electrodes.(c)Cycling performance at constant current rate of

0.1C and(d)comparison of the rate capability at various current rates between0.1C to4.5C.

A.K.Rai et al./Electrochimica Acta146(2014)577–584581

electrode also shows the additional reduction of ZnO to Zn at around0.68V along with the generation of amorphous Li2O [21,22].The large discharge plateau disappears in the later cycles, indicating irreversible reactions took place,such as electrolyte decomposition,or the formation of an SEI layer on the surface of the particles.In addition,from the second cycle onwards,the long discharge potential plateau is replaced by a long slope between 1.25and0.50V,well consistent with the previous reports[6,10]. Conversely,during charging,both Zn and Co metal can reversibly react with Li2O to form metal oxides at potentials above1.0V[21]. The discharge/charge electrochemical process can be clari?ed as follows:

ZnCo2O4+8Li++8eà!Zn+2Co+4Li2O(iii) Zn+Li++eà$LiZn(iv) Zn+Li2O$ZnO+2Li++2eà(v) 2Co+2Li2O$2CoO+4Li++4eà(vi)

2CoOt2

3

Li2O$

2

3

Co3O4t

4

3

Litt

4

3

eà(vii)

CHN analysis already con?rmed that the percentage of carbon is only0.23%and0.09%in the designed nanocomposite and pure ZnCo2O4nanoparticles samples respectively.However,the theo-retical capacity(C)of the ZnCo2O4/graphene nanocomposite can be calculated as follows[23]:

C theoretical?C ZnCo2O4?%mass ofZnCo2O4tC carbon

?%mass of carbon

?903mAh gà1?99:77%t744mAh gà1?0:23%

?902:6mAh gà1

The initial discharge and charge capacities of the ZnCo2O4/ graphene nanocomposite and pure ZnCo2O4nanoparticle electro-des are1211.5mAh gà1and861.4mAh gà1and1106.2mAh gà1and 784.7mAh gà1,respectively.In the case of nanocomposite,only the weight of the ZnCo2O4active material was taken into consideration in the calculation of speci?c capacity.The initial Coulombic ef?ciency was71%for both the electrodes.The irreversible capacity loss in the?rst cycle may be attributed to the formation of an SEI layer and the reduction of the metal oxide to metal with the formation of Li2O.After the1st cycle,the following charge/ discharge curves of both the electrodes tended to be stable and the Coulombic ef?ciency gradually increased with the cycle number, remaining almost$97%throughout the whole cycling process. Reversible discharge and charge capacities of871.1and831.5mAh gà1and867.2and848.3mAh gà1were achieved in the2nd and 10th cycles,respectively for ZnCo2O4/graphene nanocomposite electrode,which are much higher than those of the pure ZnCo2O4 nanoparticles electrode(795.4and767.6mAh gà1for the2nd cycle and784.1and771.1mAh gà1for the10th cycle).Author's believed that the less capacity of pure ZnCo2O4electrode is possibly because of its low electronic conductivity and its pulverization induced by huge volume changes during the charge/discharge processes, which could have led to the loss of electrical contact between the ZnCo2O4nanoparticles and the current collector,resulting in its fast capacity fading after few cycles,as observed in Fig.4(c).More importantly,the obtained capacities of the nanocomposite electrode is almost more than two times higher than the theoretical capacity of the commercially used graphite anode (372mAh gà1).In addition,the improved electrochemical performance of the nanocomposite electrode can be attributed to the synergistic effect between the ZnCo2O4nanoparticles and the conducting graphene nanosheets.The decreased size of the ZnCo2O4nanoparticles in the nanocomposite sample provided a large electrode/electrolyte interface area and shortened the Li+ ions diffusion path.More importantly,the anchoring of nano-particles to the graphene nanosheets in the nanocomposite prevented nanoparticles aggregation and also electrode cracking during continuous cycling.The graphene nanosheets served as reliable conductive channels between individual components of the active material and the current collectors.

Fig.4(c)shows the cycling performances of the ZnCo2O4/ graphene nanocomposite and pure ZnCo2O4electrodes at a constant current rate of0.1C for70cycles.Obviously,the nanocomposite electrode exhibits much higher reversible capacity and better cycling performance than the pure electrode.The reversible charge capacity of the pure ZnCo2O4nanoparticles electrode decreasing from784.7mAh gà1to only299.8mAh gà1with capacity retention of38%after70cycles.However,the ZnCo2O4/graphene nanocomposite electrode retained a reversible charge capacity of755.6mAh gà1from its initial reversibility charge capacity of861.4mAh gà1and showed88%of excellent capacity retention after same number of cycles.The high performance of the ZnCo2O4/graphene nanocomposite may be attributed to the large electrochemical active surface area and high electrical conductivity of the graphene nanosheets,which decreases the internal resistance in the lithium cells.In addition, the reduced graphene nanosheets also act as a buffer medium to overcome the problem associated with volume expansion/ contraction in lithium ion cells when ZnCo2O4nanoparticles react with lithium during lithium ion insertion/extraction.In the present case,it should be especially noted that10wt%of the added graphene nanosheets(even CHN analysis determines it as being very less such as0.23wt%after heat treatment at400 C)in the ZnCo2O4/graphene nanocomposite can provide improved electro-chemical performance and is the most appropriate for practical application to LIBs.

Fig.4(d)represents the rate capability of the ZnCo2O4/graphene nanocomposite and pure ZnCo2O4electrodes between current rates of0.1C to 4.5C after3cycles at each current rate.The

Fig. 5.Nyquist plots of ZnCo2O4/graphene nanocomposite and pure ZnCo2O4

nanoparticles electrodes measured in the frequency range between0.01Hz and

1.0MHz.

582 A.K.Rai et al./Electrochimica Acta146(2014)577–584

ZnCo2O4/graphene nanocomposite electrode illustrates better rate capability than the pure ZnCo2O4electrode at all investigated current rates.For example,the ZnCo2O4/graphene nanocomposite and pure ZnCo2O4nanoparticle electrodes exhibit reversible charge capacities of861.4mAh gà1and784.7mAh gà1at0.1C, 878.2mAh gà1and757.1mAh gà1at0.2C,822.8mAh gà1and707.9 mAh gà1at0.4C,738.6mAh gà1and650.5mAh gà1at0.8C,623.1 mAh gà1and554.3mAh gà1at1.6C,469.5mAh gà1and402.3mAh gà1at 3.2C and378.1mAh gà1and302.4mAh gà1at 4.5C, respectively.More importantly,when the current rate returns back to the initial C-rate of0.1C,the charge capacity can be recovered to the same initial value,indicating that the structure of both electrodes was stable during cycling.However,the ZnCo2O4/ graphene hetero-architecture not only suppresses the aggregation of ZnCo2O4nanoparticles but also prevents the restacking of the graphene nanosheets,resulting in a large electrode/electrolyte interface area.The large interface area not only provides more Li+ insertion/extraction sites,but also facilitates fast Li+ion transfer between the electrode and the electrolyte,thus leading to a large reversible capacity of the nanocomposite electrode.Most impor-tantly,we believe that among the above said reasons about the improvement of the electrochemical performance of the ZnCo2O4/ graphene nanocomposite sample,the synergistic effect between ZnCo2O4and ZnO and CoO components in the nanocomposite is also one of the major factors and needs to be considered in terms of capacity retention[21].In fact,many reports have shown that the outer material in a heterostructure can suppress the pulverization of the inner material during cycling[1,21,24].On the other hand, the presence of Co nanoparticles in the ZnCo2O4can provide an extra Li2O nanomatrix according to Eqs.(vi)and(vii),which improves the reversibility of the reaction corresponding to Eq.(v) [21,25],allowing the ZnO to be?xed at the heterojunction during lithiation and delithiation.In addition,the obtained values of electrochemical performances for both the electrodes are compa-rable than those reported for ZnCo2O4nanoparticles,but the synthesis adopted in the present study is very cost-effective and simple compared to that in the previously reported counterparts [7–9,21].

To understand the better electrochemical performance of the ZnCo2O4/graphene nanocomposite electrode,EIS measurements were also carried out on both the electrodes after10charge/ discharge cycles,as shown in Fig.5.It can be clearly seen that the impedance spectra are almost similar in shape for both the electrodes.The plots consist of a depressed semicircle in the high and middle frequency regions and a straight line in the low-frequency region.In the impedance spectroscopy,the high-frequency semicircle is attributed to the SEI?lm and/or contact resistance,while the semicircle in the medium-frequency region is assigned to the charge-transfer impedance at the electrode/ electrolyte interface.The inclined line at an approximate angle of45 to the real axis corresponds to the lithium-diffusion process within the electrodes(Warburg impedance)[1].For the lithium-ion batteries,the charge transfer resistance is a measure of the charge transfer kinetics[26],and the charge transfer process determines the rate of transfer reaction.The smaller the charge transfer resistance,the smaller the diameter of the semicircle. Obviously,the diameter of the semicircle of ZnCo2O4/graphene electrode is much smaller than that of pure ZnCo2O4nanoparticle electrode,demonstrating that the charge-transfer resistance is lower for ZnCo2O4/graphene nanocomposite material,which are in good agreement with results of the electrochemical performance, suggesting that the good conductivity network forms due to the introduction of graphene nanosheets.

To further understand the effects of the graphene nanosheets on the electrode integrity,the electrode surface of ZnCo2O4/ graphene nanocomposite and pure ZnCo2O4nanoparticles after 70cycles was observed.The ex-situ results are illustrated in Fig.6. As shown in Fig.6(a),the ZnCo2O4/graphene nanocomposite electrode maintains good geometric integrity without cracking. This also indicates that the active materials remain intact during cycling and thereby an excellent cycling stability of ZnCo2O4/ graphene nanocomposite electrode.On the contrary,the pure ZnCo2O4nanoparticles electrode cracks severely(Fig.6b),and even the ZnCo2O4nanoparticles are pulverized after70cycles.The structure destruction of pure ZnCo2O4nanoparticles due to the cycling is responsible for its gradually decrease capacity.By contrast,the introduction of graphene nanosheets provides a buffer for the volume change of the ZnCo2O4nanoparticles and enhances its structure stability.

4.Conclusions

A facile way to synthesize ZnCo2O4/graphene nanocomposite and pure ZnCo2O4nanoparticle by the use of urea-assisted auto combustion synthesis at low temperature of400 C is developed. The ZnCo2O4nanoparticles anchored on the surface of the reduced graphene nanosheets.In addition,the graphene nanosheets signi?cantly reduced the particles size and suppressed the aggregation of ZnCo2O4nanoparticles.The particles size was in the range of50–100nm for pure ZnCo2O4sample and the nanoparticles size in the nanocomposite sample was25–50nm. The ZnCo2O4/graphene nanocomposite electrode showed higher reversible capacity,excellent cycleability and higher rate capability than the pure ZnCo2O4nanoparticles electrode.The

Fig.6.Ex-situ FE-SEM images of ZnCo2O4/graphene nanocomposite(a)and pure ZnCo2O4nanoparticles(b)electrodes after70cycles of charging/discharging at current rate of0.1C.

A.K.Rai et al./Electrochimica Acta146(2014)577–584583

electrochemical performance of the nanocomposite sample was enhanced by the reduced graphene nanosheets,which can buffer the volume expansion and contraction as a?exible constraint and facilitate both the electron and lithium ion transfers in the ZnCo2O4/graphene nanocomposite sample as well as maintaining the good electrical conductivity of the overall electrode during the discharge/charge process.Additionally,we believe that the improved electrochemical performance of the ZnCo2O4/graphene nanocomposite sample is due to the synergistic effects not only between ZnCo2O4and graphene nanosheets but also between ZnCo2O4and ZnO and CoO.The electrochemical test results indicated that the ZnCo2O4/graphene nanocomposite prepared by the urea-assisted auto combustion method is a promising anode material with high capacity and better rate performance for lithium-ion batteries.

Acknowledgments

This work was supported by the Global Frontier R&D Program (2013-073298)on Center for Hybrid Interface Materials(HIM) funded by the Ministry of Science,ICT&Future Planning. References

[1]A.K.Rai,J.Gim,L.T.Anh,J.Kim,Partially reduced Co3O4/graphene nano-

composite as an anode material for secondary lithium ion battery,Electro-chim.Acta100(2013)63–71.

[2]J.Cabana,L.Monconduit,https://www.360docs.net/doc/9c13096351.html,rcher,M.R.Palacin,Beyond intercalation-based

Li-ion batteries:the state of the art and challenges of electrode materials reacting through conversion reactions,Adv.Mater.22(2010)E170–E192. [3]W.Luo,X.Hu,Y.Sun,Y.Huang,Electrospun porous ZnCo2O4nanotubes as a

high performance anode material for lithium-ion batteries,J.Mater.Chem.22 (2012)8916–8921.

[4]H.Liu,J.Wang,One-pot synthesis of ZnCo2O4nanorod anodes for high power

Lithium ions batteries,Electrochim.Acta92(2013)371–375.

[5]L.Zhou,H.B.Wu,T.Zhu,X.W.Lou,Facile preparation of ZnMn2O4hollow

microspheres as high-capacity anodes for lithium-ion batteries,J.Mater.

Chem.22(2012)827–829.

[6]Y.Sharma,N.Sharma,G.V.Subba Rao,B.V.R.Chowdari,Nanophase ZnCo2O4as

a high performance anode material for Li-ion batteries,Adv.Funct.Mater.17

(2007)2855–2861.

[7]C.C.Ai,M.C.Yin,C.W.Wang,J.Sun,Synthesis and characterization of spinel

type ZnCo2O4as a novel anode material for lithium ion batteries,J.Mater.Sci.

39(2004)1077–1079.

[8]Y.Qiu,S.Yang,H.Deng,L.Jin,W.Li,A novel nanostructured spinel ZnCo2O4

electrode material:morphology conserved transformation from a hexagonal shaped nanodisk precursor and application in lithium ion batteries,J.Mater.

Chem.20(2010)4439–4444.

[9]D.Deng,J.Y.Lee,Linker-free3D assembly of nanocrystals with tunable unit

size for reversible lithium ion storage,Nanotechnology22(2011)355401.[10]N.Du,Y.Xu,H.Zhang,J.Yu,C.Zhai,D.Yang,Porous ZnCo2O4nanowires

synthesis via sacri?cial templates:high-performance anode materials of Li-ion batteries,Inorg,Chem.50(2011)3320–3324.

[11]B.Liu,J.Zhang,X.Wang,G.Chen,D.Chen,C.Zhou,G.Shen,Hierarchical three-

dimensional ZnCo2O4nanowire arrays/carbon cloth anodes for a novel class of high-performance?exible lithium-ion batteries,Nano Lett.12(2012) 3005–3011.

[12]A.K.Rai,L.T.Anh,J.Gim,V.Mathew,J.Kang,B.J.Paul,N.K.Singh,J.Song,J.Kim,

Facile approach to synthesize CuO/reduced graphene oxide nanocomposite as anode materials for lithium-ion battery,J.Power Sources244(2013)435–441.

[13]A.K.Rai,J.Gim,S.Kang,V.Mathew,L.T.Anh,J.Kang,J.Song,B.J.Paul,J.Kim,

Improved electrochemical performance of Li4Ti5O12with a variable amount of graphene as a conductive agent for rechargeable lithium-ion batteries by solvothermal method,Mater.Chem.Phys.136(2012)1044–1051.

[14]A.K.Rai,J.Gim,J.Song,V.Mathew,L.T.Anh,J.Kim,Electrochemical and safety

characteristics of TiP2O7–graphene nanocomposite anode for rechargeable lithium-ion batteries,Electrochim.Acta75(2012)247–253.

[15]A.K.Rai,L.T.Anh,J.Gim,J.Kim,One-step synthesis of CoO anode material for

rechargeable lithium-ion batteries,Ceram.Int.39(2013)9325–9330. [16]A.K.Rai,L.T.Anh,J.Gim,V.Mathew,J.Kim,Low temperature synthesis of

porous tin oxide anode for high-performance lithium-ion battery, Electrochim.Acta109(2013)461–467.

[17]A.K.Rai,J.Gim,T.V.Thi,D.Ahn,S.J.Cho,J.Kim,High rate capability and long

cycle stability of Co3O4/CoFe2O4nanocomposite as an anode material for high-performance secondary lithium ion batteries,J.Phys.Chem.C118(2014) 11234–11243.

[18]W.S.Hummers,R.E.Offeman,Preparation of graphitic oxide,J.Am.Chem.Soc.

80(1958)1339.

[19]X.W.Lou,J.S.Chen,P.Chen,L.A.Archer,One-pot synthesis of carbon-coated

SnO2Nanocolloids with improved reversible lithium storage properties,Chem.

Mater.21(2009)2868–2874.

[20]M.V.Reddy,K.Y.H.Kenrick,T.Y.Wei,G.Y.Chong,G.H.Leong,B.V.R.Chowdari,

Nano-ZnCo2O4material preparation by molten salt method and its electrochemical properties for lithium batteries,J.Electrochem.Soc.158 (2011)A1423–A1430.

[21]C.W.Lee,S.Seo,D.W.Kim,S.Park,K.Jin,D.Kim,K.S.Hong,Heteroepitaxial

growth of ZnO nanosheet bands on ZnCo2O4submicron rods toward high-performance Li ion battery electrodes,Nano Res.6(2013)348–355.

[22]M.Ahmad,S.Yingying,A.Nisar,H.Sun,W.Shen,M.Wei,J.Zhu,Synthesis of

hierarchical?ower-like ZnO nanostructures and their functionalization by Au nanoparticles for improved photocatalytic and high performance Li-ion battery anodes,J.Mater.Chem.21(2011)7723–7729.

[23]H.Yang,T.Song,S.Lee,H.Han,F.Xia,A.Devadoss,W.Sigmund,U.Paik,Tin

indium oxide/graphene nanosheet nanocomposite as an anode material for lithium ion batteries with enhanced lithium storage capacity and rate capability,Electrochim.Acta91(2013)275–281.

[24]W.Zhou,C.Cheng,J.Liu,Y.Y.Tay,J.Jiang,X.Jia,J.Zhang,H.Gong,H.H.Hng,T.

Yu,Epitaxial growth of branched a-Fe2O3/SnO2nano-heterostructures with improved lithium-ion battery performance,Adv.Funct.Mater.21(2011) 2439–2445.

[25]X.Xue,Z.Chen,L.Xing,S.Yuan,Y.Chen,SnO2/a-MoO3core-shell nanobelts

and their extraordinarily high reversible capacity as lithium-ion battery anodes,https://www.360docs.net/doc/9c13096351.html,mun.47(2011)5205–5207.

[26]M.Zhang,M.Jia,High rate capability and long cycle stability Fe3O4–graphene

nanocomposite as anode material for lithium ion batteries,J.Alloys and Comp.

551(2013)53–60.

584 A.K.Rai et al./Electrochimica Acta146(2014)577–584