Synthesis and electrochemical characteristics of Sn–Sb–Ni

alloy composite anode for Li-ion rechargeable batteries

Hong Guo a,b, Hailei Zhao a,*, Xidi Jia a, Weihua Qiu a, Fenge Cui a

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

Received 24 May 2006; received in revised form 6 August 2006; accepted 28 August 2006

Available online 29 September 2006

Abstract

Micro-scaled Sn–Sb–Ni alloy composite was synthesized from oxides of Sn, Sb and Ni via carbothermal reduction. The phase

composition and electrochemical properties of the Sn–Sb–Ni alloy composite anode material were studied. The prepared alloy

composite electrode exhibits a high specific capacity and a good cycling stability. The lithiation capacity was 530 mAh g�1 in the

first cycle and maintained at 370–380 mAh g�1 in the following cycles. The good electrochemical performance may be attributed to

its relatively large particle size and multi-phase characteristics. The former reason leads to the lower surface impurity and thus the

lower initial capacity loss, while the latter results in a stepwise lithiation/delithiation behavior and a smooth volume change of

electrode in cycles. The Sn–Sb–Ni alloy composite material shows a good candidate anode material for the rechargeable lithium ion

batteries.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Alloy; B. Chemical synthesis; D. Electrochemical properties

1. Introduction

Li rechargeable batteries get wider and wider application in the world and has become one of the most developing

potential power sources in the 21st century [1]. In order to meet the increasing need for larger gravimetric and

volumetric capacity, many efforts have been paid to develop new electrode materials [2,3]. Sn-based materials have

attracted great attentions as replacement for graphite anodes in Li rechargeable batteries because of their superior

lithium storage capacity (994 mAh g�1 for Li22Sn4). The main problem to limit the commercialization of Sn-based

materials lies in their relatively large volume expansion upon lithiation/delithiation that results in mechanical

disintegration of the electrode and consequent capacity fade. Many researches have been tried to alleviate the

volumetric change of alloy anodes by use of nano-sized Sn alloy particles as active material, or by use of intermetallic

alloys with a composite structure containing an active or inactive host matrix and well-dispersed active materials. It is

revealed that reducing the size of active particles only would not effectively improve the cycleability of the alloy

materials, because the nano-particles are apt to aggregate, forming inactive dense blocks after several cycles [4,5]. The

combination of active and inactive components is an effective way to overcome the big volume change problem of

alloy electrode, where the inactive component can buffer the volume change caused by the active component, and thus

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Materials Research Bulletin 42 (2007) 836–843

* Corresponding author. Tel.: +86 10 62334863; fax: +86 10 62332570.

E-mail address: hlzhao@mater.ustb.edu.cn (H. Zhao).

0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2006.08.032

increase the cycling stability of electrode, such as SnCu [6–8] and SnNi [9–11]. Another promising effective approach

is to employ the binary active/active alloy or intermetallics, in which both of the active components can react with Li

but at different potentials in the charging/discharging process, such as SnSb [12–15], SnAg [16,17]. In this case, the

volume change takes place in a step-wise manner rather than at a certain fixed potential, thus the unreacted component

can accommodate the strain yielded by the reacted phase. As a result, the geometric integration and cycling stability of

electrode are improved [18,19].

In present work, a strategy is developed to take the advantages of active/inactive and active/active alloys to prepare

a Sn–Sb–Ni multi-phase alloy composite electrode. In this ternary system, the lithiation/delithiation feature of active/

active alloy is kept and meanwhile the resistance to the volume change is enhanced by the inactive component Ni.

Though, the Ni shows inert with Li, its excellent flexibility and electric character will contribute greatly to the cycling

stability of electrode. Additionally, the two active components Sn and Sb in this ternary system may allow the alloy

have a high specific capacity. To our best knowledge, there are no reports available on Sn–Sb–Ni alloy composite used

as anode for lithium ion batteries. In this work, Sn–Sb–Ni alloy powder was prepared via carbothermal reduction

method from tin, antimony and nickel oxides with the intention of reducing the production cost and obtaining the

micro-sized particles to decrease the initial irreversible capacity loss. The electrochemical performance of the

synthesized Sn–Sb–Ni alloy composite was investigated primarily.

2. Thermodynamic calculation

For the systems of SnO2–C, Sb2O3–C and NiO–C, there are the following reactions:

SnO2þ 2C ! Sn þ 2CO " (1)

Sb2O3þ 3C ! 2Sb þ 3CO " (2)

NiO þ C ! Ni þ CO " (3)

According to the thermodynamics data, the Gibbs free energy DG�T of each reaction at different temperatures is

calculated and plotted as a function of temperature, as illustrated in Fig. 1. It is revealed that the reactions (1)–(3) will

occur theoretically when the temperature is higher than 650 8C, 430 8C and 440 8C, respectively. Considering the

kinetics effects, the synthesis temperature of Sn–Sb–Ni alloy was selected to be 900 8C. At 900 8C the Gibbs free

energies DG�900�C of reactions (1)–(3) are �99.546, �231.911 and �81.815 kJ, respectively. Once Sn and Sb are

reduced from their oxides, they will exist in a liquid form due to their low melting point (231.9 8C for Sn and 631 8Cfor Sb). This will make them easy to alloy each other and/or with Ni element.

H. Guo et al. / Materials Research Bulletin 42 (2007) 836–843 837

Fig. 1. Gibbs free energies of reactions (1)–(3) as a function of temperature.

3. Experimental

SnO2 (99.9%, STREM Chemicals), Sb2O3 (99.9%, Merck), NiO (99.9% STREM Chemicals) and carbon powder

(>99%, STREM Chemicals) were used as raw materials. The mixtures of SnO2, Sb2O3, NiO and C were prepared

according to reaction (4).

2SnO2þ Sb2O3þ 2NiO þ 9C ! 2SnSbNi þ 9CO " (4)

After mixed and ground, the sample was calcined at 900 8C for 2 h in argon atmosphere at the heating rate of 5 8C/

min in a tube furnace, and then allowed to cool down naturally to room temperature inside the off-powered furnace. X-

ray diffraction (XRD) was carried out to identify the phase composition of synthesized powders over the 2u range from

108 to 908 using a Rigaku D/max-A diffractometer with Cu Ka radiation. Morphologies of the synthesized alloy

powders were observed with a Hitachi S-3500N scanning electron microscope (SEM).

For electrochemical performance evaluation, half-cell studies were performed. In the experimental Sn–Sb–Ni

electrode, C (acetylene black) powder and polyvinylidene fluoride (PVDF) were used as conductive additive and

binder, respectively. The synthesized Sn–Sb–Ni powders were mixed with acetylene black and PVDF dissolved in N-

methyl-pyrrolidinone in the weight ratio of 80:10:10 to form slurry, which was painted on a copper foil used as current

collector. After solvent evaporation, the electrode was pressed and dried at 120 8C under vacuum for 24 h.

The cells were assembled in argon filled glove-box. Metallic lithium foil was used as counter electrode. The

electrolyte was 1 M LiPF6 (Merck, battery grade) in a mixture of ethyl carbonate (EC) and dimethyl carbonate (DMC)

(1:1 in vol. ratio). Celgard 2400 polyethylene was used as the separator. Cycling tests were carried out at the charge and

discharge current density of 100, 150 and 200 mAg�1, respectively, in the voltage range of 0.01–1.5 V versus Li/Li+ by

LAND BT-10 tester (Wuhan, China). Cyclic voltammetry was performed between 0.01 V and 1.5 V with scan rate of

0.05 mV s�1.

4. Results and discussion

The XRD pattern of the synthesized Sn–Sb–Ni alloy composite powders, shown in Fig. 2, declares that the product

is composed of SnSb, Ni3Sn4 and NiSb intermetallics and a certain amount of Sn and Ni components. No peaks

assignable to SnO2, Sb2O3 and NiO were identified, indicating that all oxides have been reduced completely. In

addition, the sharp peak suggests the high crystallinity of the synthesized intemetallics components, while the

relatively high background implies the co-existence of amorphous components. A 5 wt.% carbon was remained in the

composite according to the infrared carbon analysis. EDS analysis of the synthesized alloy powders gave a Sn:Sb:Ni

atomic percent ratio of 43:33:24, showing seemingly that tin-riched composite is obtained. The chemical composition

of synthesized material is somewhat deviating from the starting ratio of Sn:Sb:Ni, i.e. 1:1:1 (in atom) before

calcinations. This is considered to be resulted from the big difference in melting point between Sn, Sb and Ni. Due to

H. Guo et al. / Materials Research Bulletin 42 (2007) 836–843838

Fig. 2. XRD pattern of Sn–Sb–Ni alloy composite powders.

its low melting point, Sn will have low viscosity and high fluidity at 900 8C and thus easily coat on the surface of nickel

contained particles which have a relatively high melting point (1726 8C). It is difficult for EDS to detect the inner part

of a coated particle; as a consequence, EDS gave a high content of Sn for the synthesized Sn–Sb–Ni powders.

The SEM images of synthesized Sn–Sb–Ni composite powders are shown in Figs. 3 and 4. The synthesized

powders show mainly two different shapes, spherical particles range from 1 to 30 mm and polygonal particles with an

average size of 20 mm, such as dots A and B, respectively. EDS analysis, as shown in Figs. 5 and 6, reveals that the

main components of sphere A are Sn, Sb, Ni with a small quantity of carbon, while that of particle B is carbon with a

little amount of Sn, Sb and Ni. The presence of a little amount of carbon is surely favorable to the improvement of

electronic conductivity of Sn–Sb–Ni composite. The carbon particle can also accommodate the volume change caused

by alloying and de-alloying of Sn and Sb with Li and thus prevent, in certain extent, the degradation of the Sn–Sb–Ni

electrode.

The cycling performance profiles of Sn–Sb–Ni alloy composite electrode at different current densities are shown in

Fig. 7. It exhibits good stability with the discharge capacities of 370–380 mAh g�1 and the columbic efficiencies of

98–99% at the current density of 100 and 150 mAg�1, and discharge capacities of 300–330 mAh g�1 with columbic

efficiencies of 96–97% at the current density of 200 mAg�1. The specific capacity difference at different current

densities is mainly due to the increased kinetic polarization at high current rate. The high reversible capacity of Sn–

Sb–Ni alloy composite electrode can be ascribed to the high lithium insertion capacity of components Sn and Sb.

H. Guo et al. / Materials Research Bulletin 42 (2007) 836–843 839

Fig. 3. Macrocosmic SEM image of Sn–Sb–Ni alloy composite powders.

Fig. 4. Integrant SEM image of Sn–Sb–Ni alloy composite powders.

Multi-element characteristics allow the lithiation and delithiation of Sn–Sb–Ni electrode to take place at different

potentials, i.e. in a stepwise behavior, thus make the volume change of electrode occur more smoothly, and

eventually result in a good structural and cyclic stability of electrode. Compared to our previous results of SnSb

electrode [13], the inactive element Ni contributes greatly to the good cycleability of Sn–Sb–Ni electrode. Due to its

good ductility, Ni can buffer the volume changes of Sn and Sb during the process of electrochemical reaction and

thus prevent the cracking and pulverization of electrode. The good durability of Sn–Sb–Ni alloy composites is also

from the dispersion of carbon in Sn–Sb–Ni particles, which can prevent the aggregation and degradation of the alloy

particles.

The initial irreversible capacity is about 150 mAh g�1, which is lower than most reported alloy anode

electrodes [4,17,18,20,21]. The relatively large particle size should be responsible for the improvement in initial

irreversible capacity. The low specific surface area resulted from the large-sized particles of Sn–Sb–Ni will lead

to low surface impurity, especially oxides, which is believed to be one of the main origins of the irreversible

H. Guo et al. / Materials Research Bulletin 42 (2007) 836–843840

Fig. 5. EDS analysis of dot A in Fig. 4.

Fig. 6. EDS analysis of dot B in Fig. 4.

capacity [19]. Furthermore, the lower surface area and thus interface area with the electrolyte will result in less

SEI film, which is another reason for the low initial irreversible capacity loss of the synthesized Sn–Sb–Ni

electrode.

The charge/discharge curves for the first and second cycles are shown in Fig. 8. Lithium ions inserting into and

extracting from Sn–Sb–Ni alloy composite in our tested half-cell are defined as discharge and charge processes,

respectively. In the initial discharge, the potential drops rapidly to a plateau of 0.65 V and then decreases gradually to

0.01 V, there is a little gradual slope ranged from 1.0 V to 0.65 V corresponding to the formation of SEI film. From the

second cycle, the initial potential plateau increases from 0.65 V to ca. 0.8 V. This voltage fluctuation may be due to the

decomposition of some intermetallics during the first discharge process.

The cyclic voltammetry plot of Sn–Sb–Ni alloy composite electrode are shown in Fig. 9. The CV plots show

four pairs of peaks, corresponding to the four potential plateaus in charge/discharge curves. Peak A around 0.8 V

corresponds to the formation of Li3Sb, while the peaks B, C and D represents the potential dependent formation of

various Li–Sn alloys, because Sn can form different Li–Sn phases with Li [12]. For delithiation process, these

insertion processes are reversed. Li will first be extracted from the Li–Sn alloy (D0, C0 and B0), then from Li–Sb

alloy (A0). Except the formation of the SEI film, the Sn–Sb–Ni alloy composite will be reformed when the cell is

recharged to 1.2 V. The CV results reveal that the charge/discharge mechanism is very like to that of SnSb

alloy previously synthesized by the same method [13], indicating that SnSb intermetallic is the main contributor

to the capacity of electrode while the other phases play mainly as buffer to accommodate the volume change

H. Guo et al. / Materials Research Bulletin 42 (2007) 836–843 841

Fig. 7. Cycling performance of Sn–Sb–Ni alloy composite electrode at different current densities.

Fig. 8. Charge/discharge curves of Sn–Sb–Ni composite electrode: the first and second cycle.

of electrode during charging and discharging process, thus leading to a good cycling stability of Sn–Sb–Ni

electrode.

5. Conclusion

Micro-scaled Sn–Sb–Ni alloy composites were synthesized via carbothermal reduction from corresponding oxides

of Sn, Sb and Ni directly. The Sn–Sb–Ni composite electrode shows a low initial irreversible capacity of 150 mAh g�1

and a stable specific capacity of 380 mAh g�1. The relatively larger particle size and the lower specific surface area of

Sn–Sb–Ni powders should be responsible to the improvement in initial capacity loss. The multiphase characteristics of

Sn–Sb–Ni alloy composite allows the lithiation/delithiation to occur in a stepwise behavior and thus the volume of

alloy to change in a smooth manner, which is considered to be most favorable to the cycling stability of electrode. The

ductile Ni can buffer the big volume change of electrode and thus make great contribution to the cycling stability of

electrode. The excellent electrochemical performance of the synthesized Sn–Sb–Ni alloy composite suggests that it

stands as a promising anode material for secondary Li-ion batteries.

Acknowledgement

The authors would like to acknowledge the financial support provided by National Natural Science Foundation of

China (No. 50371007).

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