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Electrochimica Acta 52 (2007) 4853–4857

A novel micro-spherical CoSn2/Sn alloy composite as high capacityanode materials for Li-ion rechargeable batteries

Hong Guo a,b, Hailei Zhao a,∗, Xidi Jia a, Xue Li a, Weihua Qiu a

a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, Chinab Department of Chemistry and Life Science, Qujing Normal University, Qujing 655000, China

Received 13 October 2006; received in revised form 18 January 2007; accepted 20 January 2007Available online 2 February 2007

bstract

Micro-scaled spherical CoSn2/Sn alloy powders synthesized from oxides of Sn and Co via carbothermal reduction at 800 ◦C were examined forse as anode materials in Li-ion battery. The phase composition and particle morphology of the CoSn2/Sn alloy composite powders were investigatedy XRD, SEM and TEM. The prepared CoSn2/Sn alloy composite electrode exhibits a low initial irreversible capacity of ca. 140 mAh g−1, a highpecific capacity of ca. 600 mAh g−1 at constant current density of 50 mA g−1, and a good rate capability. The stable discharge capacities of

00–515 mAh g−1 and the columbic efficiencies of 95.8–98.1% were obtained at current density of 500 mA g−1. The relatively large particle sizef CoSn2/Sn alloy composite powder is apparently favorable for the lowering of initial capacity loss of electrode, while the loose particle structuralharacteristic and the Co addition in Sn matrix should be responsible for the improvement of cycling stability of CoSn2/Sn electrode.

2007 Elsevier Ltd. All rights reserved.

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eywords: CoSn2 alloy composite; Anode materials; Electrochemical property

. Introduction

Recently, the effort to develop and to promote Li-ion batter-es has intensively increased owing to environmental and energyoncerns [1]. Among the varieties of anode materials, Sn-basedaterials have been projected as one of the most promising

andidate in substitute of the already commercialized graphite372 mAh g−1), due to their high capacity, high packing densitynd safe thermodynamic potentials compared to carbonaceousaterials for lithium-ion secondary batteries [2,3]. Even this, its

ommercial use is still held back. The most critical problem forhis material is the drastic volume change during Li insertionnd extraction reactions, which easily causes the electrode toail by pulverization, and thus resulting in rapid capacity degra-ation of Sn-based electrode [4,5]. Many attempts have beenade recently to solve these problems by use of nano-sized Sn

lloy particles as active material, or by use of intermetallic alloysith a composite structure containing an active or inactive hostatrix [6,7]. It is revealed that nano alloys would not effectively

∗ Corresponding author. Tel.: +86 10 62334863; fax: +86 10 62332570.E-mail address: hlzhao@mater.ustb.edu.cn (H. Zhao).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.01.058

othermal reduction; Li-ion batteries

mprove the durability of alloy electrode, because of the unex-ected syndromes, such as electrochemical agglomeration of theano-sized metal particles, large irreversible capacity and unsta-le SEI film [8,9]. In contrast, the introduction of intermetallicsr multiphase active/inactive material seems to be a more effec-ive way to control the volume changes of the alloy electrode7,10], where the inactive component can buffer the volumehange caused by the active component and thus increase theycling stability of the electrode, such as SnCu [9,11] and SnNi4,12,13]. Though the electrochemistry performance of thesen-based alloys has been improved obviously, the cycleabilitynd the fabrication cost are still unacceptable for practical use.herefore, intensive attentions are still paid to enhance the elec-

rochemical performance and reduce the cost of Sn-based anodeaterials.Our previous work revealed that by carbothermal reduction

pproach, binary alloy SnxSb powder could be successfully pre-ared from its corresponding oxides [14,15]. In present research,his method is used to prepare a microcrystalline CoSn2/Sn

lloy composite. Co has high hardness and excellent ductibil-ty. Addition of Co to metal matrix usually can increase theuctibility and reduce the disintegration of alloy powers [16,17].herefore, it is expected that the introduction of Co into Sn

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854 H. Guo et al. / Electrochim

atrix would increase its ductibility to buffer volume changend thus improve the electrochemical cycling stability of Sn-ase electrode. Sn0.6Co0.4 thin film electrode prepared viaagnetron sputtering by Dahn et al. [18] shows a capacity of

50 mAh g−1,and electro-co-deposited Sn–Co alloy electroden a copper foil by Tamura et al. [19] shows specific capac-ty density over 586 mAh g−1. These methods are either highost or not suitable for economical production in large-scale.o our best knowledge, there are no reports so far availablebout CoSn2/Sn alloy composite synthesized by carbothermaleduction method and used as anode for lithium ion batteries.n this work, CoSn2/Sn alloy powder was prepared via car-othermal reduction method from tin and cobalt oxides, withhe intention of reducing the production cost and obtaining the

icro-sized particles to decrease the initial irreversible capacityoss. The electrochemical performance of synthesized CoSn2/Snlloy composite was investigated primarily.

. Thermodynamic calculation

For the systems of SnO2–C and Co3O4–C, there are followingeactions:

nO2 + 2C → Sn + 2CO ↑ (1)

o3O4 + 4C → 3Co + 4CO ↑ (2)

ccording to the thermodynamics data [20], the Gibbs freenergies �G◦

T of each reaction at different temperatures arealculated and plotted as a function of temperature, as illus-rated in Fig. 1. It is revealed that the reactions (1) and (2) willccur theoretically when the temperature is higher than 650 and40 ◦C, respectively. Considering the kinetics effects, the syn-hesis temperature of Sn–Co alloy was selected to be 800 ◦C. At00 ◦C the Gibbs free energy �G◦

800 ◦C of reactions (1) and (2)re −61.37 and −338.22 kJ, respectively. Once Sn is reduced

rom their oxides, Sn will exist in a liquid form due to its lowelting point of 231.9 ◦C. This will make it easy to combineith Co to form Co–Sn alloy.

ig. 1. Gibbs free energies of reactions (1) and (2) as a function of temperature.

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cta 52 (2007) 4853–4857

. Experimental

SnO2 (99.9%, STREM Chemicals), Co3O4 (99.9%, Merck)nd carbon powder (>99%, STREM Chemicals) were used asaw materials. The mixtures of SnO2, Co3O4 and C in molaratio 9:1:22 were prepared according to the following reaction:

SnO2 + Co3O4 + 22C → 3CoSn2 + 3Sn + 22CO ↑ (3)

After mixed and ground, the sample was calcined at 800 ◦Cor 2 h in argon atmosphere at the heating rate of 5 ◦C/min in aube furnace, and then allowed to cool down naturally to roomemperature inside the off-powered furnace. X-ray diffractionXRD) was carried out to identify the phase composition ofynthesized powders over the 2θ range from 10◦ to 90◦ usingRigaku D/max-A diffractometer with Cu K� radiation. Mor-hologies of the synthesized alloy powders were observed with aitachi S-3500N scanning electron microscope (SEM), and theicrostructural characteristics of CoSn2/Sn were observed by

ransmission electron microscope (TEM, JEM-100CX) and theattice structure was identified by selected area electron diffrac-ion (SAED) technique. TEM sample were dispersed in ethanolnd collected on a holey micro grid supported on a copper mesh.

For electrochemical performance evaluation, half-cell stud-es were performed. In the experimental CoSn2/Sn electrode,arbon (acetylene black) powder and polyvinylidene fluoridePVDF) were used as conductive additive and binder, respec-ively. The synthesized CoSn2/Sn powders were mixed withcetylene black and PVDF dissolved in N-methyl-pyrrolidinonen the weight ratio of 80:10:10 to form slurry, which was paintedn a copper foil used as current collector. After solvent evapora-ion, the electrode was pressed and dried at 120 ◦C under vacuumor 24 h. The cells were assembled in argon filled glove-box.

etallic lithium foil was used as counter electrode. The elec-rolyte was 1 M LiPF6 (Merck, battery grade) in a mixture ofthyl carbonate (EC) and dimethyl carbonate (DMC) (1:1 inol. ratio). Celgard 2400 polyethylene was used as the separa-or. Cycling tests were carried out at the charge and dischargeurrent density of 50 and 500 mA g−1, respectively, in the volt-ge range of 0.01–1.2 V versus Li/Li+ by LAND BT-10 testerWuhan, China).

. Results and discussion

Fig. 2 gives the XRD pattern of the synthesized CoSn2/Snlloy composite powders. Compared with standard powder X-ay diffraction data of metal Sn and Sn–Co intermetallics, theRD lines of CoSn2/Sn powders can be indexed to be CoSn2

nd Sn. No peaks assignable to SnO2 and Co3O4 were identi-ed, indicating that all oxides have been reduced completely.he low intensity of diffraction peaks suggests the poor crys-

allinity of CoSn2 and Sn, while the relatively high backgroundndicates the coexistence of amorphous phase in the synthe-

ized alloy powders. According to the infrared carbon analysis,.08 wt.% carbon was remained in the prepared powder com-osite. Therefore, the amorphous phase should come from theemained carbon.

H. Guo et al. / Electrochimica Acta 52 (2007) 4853–4857 4855

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Fig. 4. TEM micrograph of Co–Sn alloy powders with two different structures:multicrystalline (a) and single crystal (b). The inset in (a) is the selected areaeCm

ig. 2. XRD patterns of CoSn2/Sn alloy composite powders and raw materialsf SnO2 and Co3O4.

SEM observation as shown in Fig. 3 reveals that the synthe-ized CoSn2/Sn powders have spheroid particle shape, rangedrom 10 to 20 �m. The close inspection on the large particleeveals that the single particle is, in fact, an aggregate con-aining lot of small grains, as shown in the inset of Fig. 3. Itas a loose structure with obvious small hole among differentrains. The holes and the quite amount of grain boundary inhe relatively large particle will surely help to buffer the volumehanges of CoSn2/Sn electrode during electrochemical reaction,nd consequently contribute greatly to the good cycle stability.DS mapping analysis discloses that the synthesized alloy pow-ers have a Co:Sn atomic percent ratio of 23.11:76.89, which islose to the starting Co/Sn ratio in SnO2–Co3O4–C mixtures, i.e.o:Sn = 1:3 (in atom) before calcinations. The little tin-richedomposite is considered to be resulted from the accuracy limitf the equipment. EDS spot analysis indicate that CoSn and Sn

2isperses uniformly in the matrix, while the remained carbonarticles exist between large aggregates.

ig. 3. SEM image of CoSn2/Sn alloy composite powders. The inset is the singlearticle of CoSn2/Sn.

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lectron diffraction of CoSn2 particle, showing multicrystalline characteristic ofoSn2 65-2697. The inset in (b) is the selected area electron diffraction of Snetal particle, showing the single lattice structure of Sn 04-0673.

Fig. 4(a) and (b) show the TEM images of Co–Sn alloy pow-er prepared by carbothermal reduction. It shows a clear grainhape, and the crystal edges can be observed. A multicrystaltructure with clear crystal loops of diffraction and a distinct sin-le crystalloid lattice structure of Sn can be detected by SAED,s illustrated in Fig. 7. This indicates again that the single spheri-al particles observed in SEM are practically aggregates of metaln and intermetallic CoSn2, which is in good agreement withRD results.The cycling performance profiles of CoSn2/Sn alloy compos-

te electrode at different current densities are shown in Fig. 5.he initial irreversible capacity loss is about 140 mAh g−1 at

he current density of 50 mA g−1. The relatively large particleize and low specific surface area of CoSn2/Sn alloy compos-te powders should be responsible for the lower initial capacityoss, because large particle size usually corresponds to lowerurface impurity, especially oxides, which is considered to bene of the main origins of the initial capacity loss. Besides, theower surface area and thus interface area with the electrolyteill result in less SEI film, which is another reason for the low

nitial irreversible capacity loss of the synthesized CoSn2/Snlectrode.

When charged and discharged at 50 mA g−1, the CoSn2/Snlectrode displays a specific capacity of ca. 600 mAh g−1 for

4856 H. Guo et al. / Electrochimica Acta 52 (2007) 4853–4857

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tecapacity profile becomes more stable with cycle number, sug-gesting that the synthesized CoSn2/Co electrode has a stableelectrochemical reaction performance.

ig. 5. Cycling performance of CoSn2/Sn alloy composite electrode at differenturrent densities.

he first 15 cycles. After that, the capacity degrades gradually,05 mAh g−1 can be kept after 38 cycles. In order to investigatehe rate capability, electrochemical test was set at the currentensity of 500 mA g−1. It is interesting to note that the syn-hesized CoSn2/Sn alloy composite electrode exhibits a goodycling stability at the current density of 500 mA g−1 until 35ycles. The even better cycling performance of CoSn2/Sn elec-rode at 500 mA g−1, compared to that at 50 mA g−1, may be dueo the possible hermetic seal problem of the test cells. Due tohe high specific capacity of the synthesized Sn–Co alloy elec-rode, the cycling process usually takes a long time at low currentensity. Thus some electrolyte evaporation during the long-timeycling may occur, which will certainly deteriorate the electro-hemical properties of the electrode. Nevertheless, both of themhow a fast capacity fade after 38 cycles. The coulombic effi-iencies are 96.1–98.6% at the current density of 50 mA g−1 and5.8–98.1% at the current density of 500 mA g−1 except of thest one, respectively.

The high reversible capacity of CoSn2/Sn composite can bettributed to the high theoretical capacity of Sn component. Theower initial capacity loss also contributes in certain extent tohe high reversible capacity of CoSn2/Sn electrode. The holesnd the grain boundaries existing in CoSn2/Sn single particlesill favor to buffer the volume change caused by the uptake

nd removal of Li ions from Sn component, and relieve theechanical stress on the active particles and eventually increase

he cycling stability of CoSn2/Sn electrode. Though the pres-nce of the inactive component cobalt results in the reductionf the theoretical capacity compared with the pure metal tin, thentroduction of inert Co with excellent ductibility can prevent thective Sn from aggregation and buffer in certain extent the unde-irable volume change during cycling. Besides, the small amountf remained carbon is surely favorable for the improvement oflectrical conductivity and geometrical stability of CoSn2/Snlectrode. As a result, the cycleability of CoSn2/Sn electrode ismproved.

The charge/discharge curves for the 1st, 2nd and 15th cycles

re shown in Fig. 6. Lithium ions insertion into and extractionrom CoSn2/Sn alloy composite in our experimental half cells defined as discharge and charge processes, respectively. In

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ig. 6. Discharge/charge curves of a CoSn2/Sn composite electrode for thest, 2nd and 15th cycles. Electrode potential range is 0–1.2 V vs. Li/Li+;harge–discharge current density is 50 mA g−1.

he initial discharge, the potential drops rapidly to a plateauf 0.38 V and then decreases gradually to 0.01 V. There islittle step slope ranged from 1.0 to 0.85 V, corresponding

o the formation of SEI film. From the second cycle, thislope disappears, indicating that a stable SEI film has beenormed. The plateau at 0.38 V and the gentle slope from that to.01 V corresponds to the lithiation and delithiation reaction ofarious forms of Sn. From the 2nd cycle, the potential plateaut 0.38 increases to 0.42 V. This voltage fluctuation may beesulted from the irreversible decomposition reaction of CoSn2o Co and Sn [21]. The lithiation potential of CoSn2 shouldiffer from that of pure Sn according to the thermodynamiconsideration.

The differential capacity versus potential curves indicated byhe different lines in Fig. 7 are the characteristic of CoSn2/Snlectrode prepared by carbothermal method. The differential

ig. 7. Differential capacity vs. potential for three cycles of CoSn2/Sn preparedy carbothermal method. The thin lines show the first cycle of the electrode andhe thick lines show the cycles 5 and 21.

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. Conclusion

By properly controlling the synthesis conditions, micro-caled spherical CoSn2/Sn alloy composites were synthesizedia carbothermal reduction from corresponding oxides of Snnd Co directly. The synthesized CoSn2/Sn composite electrodehows a low initial irreversible capacity of ca. 140 mAh g−1

nd a high specific capacity of ca. 600 mAh g−1. The rateapability of CoSn2/Sn is excellent, the stable dischargeapacities of ca. 500 mAh g−1 and the columbic efficien-ies of 95.8–98.1% were obtained at the current density of00 mA g−1. Its relatively larger particle size and the lowerpecific surface area of CoSn2/Sn powders should be respon-ible to the improvement of initial capacity loss. The loosend porous structure characteristics of CoSn2/Sn particles willelp to buffer the volume change of electrode during chargend discharge process and thus to be most favorable to theycling stability of electrode. The introduction of Co inton matrix can prevent the aggregation of Sn particles anduffer the volume change of electrode in certain extent dur-ng cycling, which are also benefit to the cycling stability oflectrode.

cknowledgement

The authors would like to acknowledge financial supportrovided by National Natural Science Foundation of China (no.0371007).

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