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Electrochemistry Communications 9 (2007) 2207–2211

Spherical Sn–Ni–C alloy anode material with submicro/microcomplex particle structure for lithium secondary batteries

Hong Guo, Hailei Zhao *, Xidi Jia

School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Received 25 April 2007; received in revised form 9 June 2007; accepted 11 June 2007Available online 22 June 2007

Abstract

Submicro/micro-scaled spherical Sn–Ni–C alloy powders synthesized from oxides of Sn and Ni via carbothermal reduction at 900 �Cwere examined for use as anode materials in Li-ion battery. The synthesized spherical Sn–Ni–C particles show a loose micro-sized struc-ture and a multi-phase composition. The reaction product carbon oxide gases yielded in the carbothermal reduction process should beresponsible to the loose structure characteristics of Sn–Ni–C particles. The prepared Sn–Ni–C alloy composite electrode exhibits a stablereversible capacity of 310 mA h g�1 at constant current density of 100 mA g�1, and can be retained at 290 mA h g�1 after 25 cycles. Thespace existing in loose particle can accommodate the large volume changes during charge/discharge cycling. The ductile component Niplays as a buffer to relieve the mechanical stress induced by the large volume changes upon cycling. The remained carbon can prevent theaggregation between small alloy particles. All these factors contribute greatly to the excellent cycling stability of Sn–Ni–C alloy electrode.This carbothermal reduction method is simple, cheap and mass-productive, thus suitable to large scale production of alloy anode pow-ders used for lithium ion batteries.� 2007 Elsevier B.V. All rights reserved.

Keywords: Sn–Ni–C alloy composite; Anode materials; Carbothermal reduction; Li-ion rechargeable batteries

1. Introduction

Numerous research groups in both academia and indus-try around the world have made many efforts to design anddevelop advanced materials for Li rechargeable batteries[1–3]. Among the varieties of anode materials, Sn-basedmaterial with high theoretical specific capacity(994 mA h g�1) has been projected as one of the mostpromising candidates in substitute of the already-commer-cialized graphite (372 mA h g�1), due to its high capacity,high packing density and safe thermodynamic potentialscompared to carbonaceous materials for lithium secondarybatteries [4–6]. However, Sn-based alloy anode electrodessuffer severe mechanical disintegration caused by the dras-tic volume changes during lithium insertion and extraction,which easily results in the degradation of capacity, and

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.06.021

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

therefore greatly limits the commercialization [7–9]. Forthe enhancement of performance, large works have beenfocused on controlling the volume change of alloy anodesby using nano-sized particles as active materials [10–12].Even though nano-sized can alleviate the volume changeto a certain extent, large evidential reports have declaredthat reducing only the size of active particles cannot effec-tively improve the cycleability of the alloy materials for thecase of Sn, because the nano-particles are apt to aggregateand form inactive dense blocks after several cycles, eventu-ally losing the activity [11,13–15]. The core/shell structuredcomposite anode materials [16,17] were proposed to over-come the aggregation problem of nano-particles, however,the shell material usually decreases the specific capacity ofelectrodes.

In the present work, we propose a new strategy to solvethe problems: to prepare spherical particle with submicro/micrometer complex structure where the microsized parti-cles are composed of large amounts of submicro-sized

Fig. 1. The structure model of synthesized Sn–Ni–C alloy particle. The microsized ball (a) is practical composed of many sub-micro particles shown as (b).

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grains, as illustrated in the structural model Fig. 1. The lowspecific surface area of the micro-sized particles can preventthe aggregation of active particles, while the submicro-sizedgrains inside the large particles can homogenize the expan-sion/contraction strains, and the space between the submi-cro-sized grains can relieve the mechanical stress and bufferthe volume change of the particles caused by the lithiationand delithiation processes.

Sn–Ni–C composite powders with the expected structurewere prepared via a carbothermal reduction method from tinand nickel oxides. The thermodynamic calculation indicatesthat SnO2 and NiO can be reduced by carbon at tempera-tures higher than 650 �C. Because a quite amount of carbonoxide gases will be produced during the reaction process, it ispossible to produce alloy particle with loose structure. Addi-tionally, the lower melting point of component Sn wouldmake the particle have spherical shape, which will contributegreatly to a high packing density in practical Li-ion batteriesand allow a high volumetric capacity density. Though Nishows inert with Li, its excellent flexibility and electrical con-ductive character should be favorable for the improvementof structural stability and rate capability of alloy electrode.During the carbothermal reduction process, a little amountof carbon may be remained, which can act as a barrier to pre-vent the aggregation between Sn–Ni particles. Therefore,this structure takes both the advantages of sub-micro-parti-cles and micro-particles, the former allows the alloy particleto undergo a homogeneous stress distribution during lithiumuptaking and removal process, while the latter could help toavoid the electrochemical congregation of the active parti-cles during cycling process. This is apparently beneficial tothe structural and thus the cycling stability of alloy electrode.

2. Experimental

The preparation procedure of Sn–Ni–C alloy compositeis described as follows: SnO2 (99.9%, STREM Chemicals),NiO (99.9%, Merck), and carbon powder (>99%, STREM

Chemicals) were used as raw materials. The average parti-cle size of raw materials of NiO, SnO2 and C are 50 nm,100 nm and 40 lm, respectively, according to SEM obser-vation and laser particle size analysis. The mixtures ofSnO2, NiO, and C, according to reaction (1), were mixedand ground in agate pestle for 2 h.

2SnO2 þNiOþ 5C! NiSn2 þ 5CO " ð1ÞThe mixture was calcined at 900 �C for 2 h in argon

atmosphere at the heating rate of 5 �C/min in a tube fur-nace, and then allowed to cool down naturally to roomtemperature inside the off-powered furnace. X-ray diffrac-tion (XRD) was carried out to identify the phase composi-tion of synthesized powders over the 2h range from 10� to90� using a Rigaku D/max-A diffractometer with CuKaradiation. Morphologies of the synthesized alloy powderswere observed with a Hitachi S-3500N scanning electronmicroscope (SEM), and the microstructural characteristicsof CoSn2/Sn were observed by transmission electronmicroscope (TEM, JEM-100CX) and the lattice structurewas identified by selected area electron diffraction (SAED)technique. TEM samples were dispersed in ethanol and col-lected on a holey micro grid supported on a copper mesh.

For electrochemical performance evaluation, half-cellstudies were performed. In the experimental Sn–Ni–C elec-trode, Carbon (acetylene black) powder and polyvinylidenefluoride (PVDF) were used as conductive additive and bin-der, respectively. The synthesized Sn–Ni–C powders weremixed with acetylene black and PVDF dissolved in N-methyl-pyrrolidinone in the weight ratio of 80:10:10 toform slurry, which was painted on a copper foil used ascurrent collector. After solvent evaporation, the electrodewas pressed and dried at 120 �C under vacuum for 24 h.The cells were assembled in argon filled glove-box. Metalliclithium foil was used as counter electrode. The electrolytewas 1 M LiPF6 (Merck, battery grade) in a mixture of ethylcarbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol.ratio). Celgard 2400 polyethylene was used as the separa-tor. Cycling tests were carried out at the charge and dis-

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charge current density of 100 mA g�1, in the voltage rangeof 0.01–1.5 V versus Li/Li+ by LAND BT-10 tester(Wuhan, China).

3. Results and discussion

The XRD pattern of the synthesized Sn–Ni–C alloycomposite powders, shown in Fig. 2, declares that theproduct is composed of Sn, Ni3Sn4 and a little amount ofNi component. No peaks assignable to SnO2 and NiO wereidentified, indicating that all oxides have been reducedcompletely. In addition, the sharp peak suggests the highcrystallinity of the synthesized metal and intermetallicscomponents. The relatively high background implies theco-existence of amorphous component. EDS analysis ofthe synthesized alloy powders indicates the existence of alittle amount of carbon, which should be the origin of theamorphous phase. The remained carbon comes from theraw material. For reaction (1), there exists equilibriumbetween CO and CO2 at a certain temperature accordingto the thermodynamic concern, therefore the addition ofcarbon based on reaction (1) is excessive and a certainamount of carbon should be remained. The remained car-bon can suppress the aggregation between the alloy parti-cles, and hence could make contribution to the structuralstability of electrode. The presence of a little amount ofNi is surely favorable to the improvement of electronicconductivity of Sn–Ni–C composites. Besides, the presenceof little amount of Ni and carbon can also accommodatethe volume change caused by alloying and de-alloying ofSn with Li and thus prevent the degradation of the Sn–Ni–C electrode.

The SEM image of synthesized Sn–Ni–C alloy compos-ite powders is shown in Fig. 3a. The synthesized powdersshow mainly spherical shape, ranging from 0.1 to 50 lm.SEM observation on fracture of a single particle, as shownin Fig. 3b, indicates that the single particle of the synthe-

Fig. 2. XRD pattern of NiSn2 alloy composite powders synthesized bycarbothermal reduction.

sized Sn–Ni–C alloy powders is not a solid ball but has aloose microstructure characteristic. It can be obviouslynoticed that the relatively large particles are made up ofsubmicro-sized small particles. This structure is almostthe same as the expected model, as shown in Fig. 1.Fig. 3c shows the TEM image of the synthesized alloy pow-der. SAED analysis indicates the poly-crystalline character-istics of micro-sized Sn–Ni alloy particles, as evidenced bythe inserted loops of diffraction. This is another evidence toconfirm that the large particles observed by SEM are prac-tically the large aggregates of sub-micro crystal grains.

According to the thermodynamic calculation (notshown here), solid metal Ni with high melting point of1453 �C will be reduced preferentially from NiO than Snfrom SnO2. The reduced Sn will exist in the liquid formdue to its lower melting point (231.9 �C) and thus has ahigh reactivity at synthesis temperature, which will lead iteasy to alloy with the existent metal Ni to yield intermetal-lic phases. According to the XRD results (Fig. 2), a quiteamount of Sn still exists in the resultant alloy composite.The liquid form of Sn allows it to connect the small inter-metallic particles together to form big particles. Mean-while, the surface tension of Sn liquid will favor theformation of a ball-shape particle to reduce the surfaceenergy. On the other hand, a large amount of carbon oxidegases will be produced during carbothermal reaction. Thesegases can prevent the firm contact between the intermetallicsmall particles and at the same time generate porous in thelarge particle. As a result, a relatively large particle with aloose inner structure and composed of a lot of small parti-cles was formed. This structure cannot only keep the smallparticle effect of electrode but also prevent further aggrega-tion of particles from forming inactive dense blocks due toits lower specific surface area. Moreover, the loose struc-ture of particle offers a sufficient void space, in which Sn–Ni–C alloy particles can experience a volume changewithout a collapse of shell. The space inside the large par-ticles can help to buffer the volume changes of Sn–Ni–Celectrode during electrochemical reaction and consequentlyimprove the cycling stability of Sn–Ni–C electrode.

The cycling performance profile of Sn–Ni–C alloy com-posite electrode at constant current density of 100 mA g�1

is shown in Fig. 4. The stable reversible capacity of elec-trode is 310 mA h g�1, and can be retained at 290 mA h g�1

after 25 cycles. The capacity retention keeps over 97.5% inthe following 25 cycles. The initial capacity loss is relativelyhigh, ca. 150 mA h g�1, which is considered to be mostlyresulted from surface impurity, especially oxides, whichare believed to be one of the main origins of the irreversiblecapacity [18]. In the first lithiation phase, the oxides will bereduced to yield Li2O, resulting in the irreversible capacity,as Li cannot extract from Li2O during the electrochemistryreaction [19]. Moreover, the formation of SEI film on theelectrode surface may also make contribution to the highinitial capacity of alloy electrode [13]. The initial delithia-tion capacity is slightly lower than that on the second cycle,probably due to the activation process of Sn–Ni system,

Fig. 3. SEM (a), (b) and TEM (c) of the submicro/micro complex structure of Sn–Ni–C alloy particles. The inset in (c) is the selected area electrondiffraction of alloy particle, showing the multi-crystalline characteristic of Sn3Ni4 04-0845.

Fig. 4. Cycling performance of Sn–Ni–C alloy composite electrode atconstant current density of 100 mA g�1.

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which was often observed in Ni-based anode [20–22]. Fromthe second cycle, Sn–Ni–C electrode shows an excellentcycleability. The special microstructural characteristics ofSn–Ni–C alloy composite particles make the volumechange of electrode occur more homogeneously and

smoothly, and eventually result in a good structural andcyclic stability of electrode.

4. Conclusion

Spherical Sn–Ni–C alloy composite powders with sub-micro/micro complex particle structure were fabricatedby carbothermal reduction from the corresponding oxidesof Sn and Ni. The synthesized spherical Sn–Ni–C particlehas a loose structure and is composed of fine grains withdifferent compositions. The stable discharge capacities ofca. 310 mA h g�1 and the columbic efficiencies of 96.7–99.3% were obtained at the current density of 100 mA g�1.The space existing in loose particle can accommodate thevolume change of electrode during cycling. The ductilecomponent Ni will play as a buffer to relieve the volumechange stress of electrode. The remained carbon can pre-vent the aggregation between small alloy particles. Allthese factors contribute greatly to the excellent cycling sta-bility of Sn–Ni–C alloy electrode. The good electrochemi-cal performance of the spherical Sn–Ni–C composite willserve them as promising anode materials for the next gen-eration of Li-ion batteries. This method is simple, cheapand mass-productive. This strategy should be viable toother alloy materials system used as anode for Li recharge-able batteries.

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Acknowledgments

The authors acknowledge financial support provided byNational Natural Science Foundation of China (No.50371007) and 863 Program of National High TechnologyResearch Development Project of China (No.2006AA03Z231).

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