目录结构

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参考文献

1 Introduction▲
2 Experimental▲
2.1 Preparation and characterization
2.2 Electrochemical measurement
3 Results and discussion▲
3.1 Structure characterization of Sn-Sb-Cu nanospheres
3.2 Electrochemical characteristics of Sn-Sb-Cu alloys nanoparticles
4 Conclusion▲
图表▲

Rare Metals2020年第10期

Improved electrochemical performance of ternary Sn-Sb-Cu nanospheres as anode materials for lithium-ion batteries

Rong Yang Xiang-Jun Zhang Teng-Fei Fan Dan-Ping Jiang Qi Wang

Research & Development Centre for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals

作者简介：*Rong Yang,e-mail:roger_yangr@sina.com;

收稿日期：14 July 2013

基金：financially supported by the National Natural Science Foundation of China (No.51202014);

Improved electrochemical performance of ternary Sn-Sb-Cu nanospheres as anode materials for lithium-ion batteries

Rong Yang Xiang-Jun Zhang Teng-Fei Fan Dan-Ping Jiang Qi Wang

Research & Development Centre for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals

Abstract：

Ternary Sn-Sb-Cu alloy nanoparticles were successfully synthesized via co-reduction of metal chlorides in aqueous alkaline solution.The results of the transmission electron microscopy(TEM) show that the as prepared Sn-Sb-Cu nanoparticles have a specific hollow structure with a uniform particle size of 10-20 nm.As there are not any hard templates in the synthesis system,a galvanic displacement reaction mechanism is proposed to account for the formation of the hollow nanostructures.When the alloy powders are used as anode materials for lithium-ion batteries,they exhibit relatively high electrochemical capacity and good cyclic retention.The good electrochemical performance can be attributed to the inactive Cu species.During electrochemical reactions,the inactive copper phase in the hollow structure serves as a soft and ductile matrix,which alleviates the mechanical stresses caused by the severe volume change during lithium insertion and extraction.With their high reversible capacities,the Sn-Sb-Cu alloys are a promising candidate as the anode material of rechargeable lithium-ion batteries.

Keyword：

Sn-Sb-Cu alloy; Hollow nanospheres; Anode materials; Lithium-ion batteries;

Received： 14 July 2013

1 Introduction

Lithium-ion batteries are extremely important power sources for the modern information age because of their wide applications in various portable electronic devices and hybrid electric vehicles [ 1] .Advanced electrode materials of lithium batteries attracted many research attentions in pursuing higher power density and longer lifetime.Carbon based materials were widely used as anode materials in commercial lithium-ion batteries,owing to their low potential plateau,acceptable capacity,stable cycling ability and low cost [ 2] .Since Fujiphoto Film Celltec reported that some tin-based oxides exhibited high capacity and stable cycling performance [ 3] ,new anode materials based on lithium alloys were widely studied [ 4] .In order to overcome the irreversible capacity caused by the formation of Li₂O during electrochemical reaction in the first cycle [ 5, 6, 7] ,intermetallic compounds instead of oxides were taken into account.Owning to their superior lithium storage capacities,metals such as Sn,Sb,Al,Si,and Ag can be good substitutes for the traditional carbon based anode materials [ 4] .However,these materials suffer from severe volume change during Li insertion and extraction that often causes electrode disintegration and rapid capacity fading [ 8, 9] .In recent years,considerable effort has been made to overcome this problem using composite materials [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] ,including SnCo [ 10, 11, 12, 13] ,Sn-Cu [ 14, 15, 16, 17, 18, 19] and Sn-Ni [ 20, 21] composites.The inactive phase serves as a buffer which partly alleviates mechanical stress caused by the volume change of the active phase.Another promising approach is to use active phases to buffer the volume change,such as Sn-Sb [ 22, 23, 24] ,Sn-Ag [ 25] and Al-Sb [ 26] composites.In this case,the components react at different potentials thus the volume change takes place step by step,so that the electrode integration and cycling stability are improved.

According to the previous research,the binary inactive/active alloys or composites have relatively good cycling abilities,but poor reversible capacities due to the large content of the inactive phase.On contrary,the binary active/active alloys or composites have high capacities,but bad capacity retention.In this work,it was studied the preparation and the electrochemical characteristics of Sn-Sb-Cu ternary alloy nanoparticles,which would take the advantages of both inactive/active and active/active alloys.In this ternary system,the lithiation and delithiation features of the Sn-Sb alloys are kept,and the inactive Cu phase effectively alleviates the volume change during discharge/charge process.The synthesis was performed in aqueous solution using NaBH₄ to reduce the metal chlorides.Surprisingly,the introduction of Cu led to the formation of hollow nanostrutctures.When the as-prepared samples were used as anode materials for lithium-ion batteries,they exhibited relatively high electrochemical capacity and good cyclic retention,because the inactive Cu phase and the hollow nanostructrures both alleviated the severe volume change which would usually cause capacity fading during discharge/charge process [ 27, 28, 29] .

2 Experimental

2.1 Preparation and characterization

In a typical procedure,nanoparticles of the Sn-Sb-Cu alloy were prepared by reductive precipitation of the corresponding metal chlorides in NaBH₄ aqueous solution.All reagents employed were commercially available and used as supplied without further purification.To produce the Sn-Sb-Cu alloy powder,Solution 1 contained metal chlorides:SnCl₂-2H₂O (0.09 mol-L^-1),SbCl₃(0.03 mol·L^-1),CuCl₂-2H₂O (0,0.06 and 0.15 mol·L^-1)and 1.00 mol·L^-1 sodium citrate (Na₃C₆H₅O₇.2H₂O) as a complexant.Solution 2 contained 0.5 mol·L^-1 NaBH₄ as reducing agent and NaOH was added to adjust the pH of the solution to 12.The two separate aqueous solutions were cooled in iced water respectively and Solution 1 was added drop-wise to Solution 2 under strong magnetic stirring,and then continued stirring for additional 1 h.During the whole process,the reaction vessel was cooled in the iced water.The resultant suspension was separated by centrifuge and rinsed several times using de-ionized water and acetone.Finally,the precipitate was dried under vacuum at room temperature overnight.

The compositions of the product were analyzed by inductively coupled plasma spectroscopy (ICP,LEEMAN LABS PROFILE SPEC).The crystal structure of the product was characterized by X-ray diffraction (XRD,Rigaku D-max 200,Cu Kα).The size and morphology of the as prepared samples were investigated using scanning electron microscopy (SEM,Hitachi S4800) and transmission electron microscopy (TEM,JEOL 200CX and Tecnai F30).

2.2 Electrochemical measurement

In order to measure the electrochemical performance,the electrodes were prepared by coating the slurry of the active material powders (80 wt%),acetylene black (10 wt%) and polyvinylidene fluoride (PVDF)(10 wt%) dissolved in n-methyl pyrrolidinone on to a Cu foil substrate.After coating,the electrodes were pressed at 10 MPa and dried at120℃under vacuum for 24 h.Li metal foil was utilized as the counter electrode,1 mol·L^-1 LiPF₆ in ethylene carbonate (EC) and dimethyl carbonate (DMC)(1:1 by volume) was used as the electrolyte,and Celgard 2400 was used as the separator.Half-cells were assembled in an argon-filled glove-box.The cell performance was estimated galvanostatically at a current density of 100 mA·g^-1for both charge (Li extraction) and discharge (Li insertion)at room temperature.The cells were cycled in the voltage range of 0.005-1.500 V (vs.Li/Li⁺).Cyclic voltammetric measurements were performed to examine the cathode reaction and anode reaction using the above-mentioned cell in the voltage range of 2.5-0 V (vs.Li/Li⁺) at a sweep rate of 0.1 mV·s^-1.

3 Results and discussion

3.1 Structure characterization of Sn-Sb-Cu nanospheres

The compositions of the as-prepared samples are complied in Table 1.For convenience,hereafter the alloy composites are referred to SnSb,SnSbCu-1 and SnSbCu-2 samples.The XRD patterns of the as prepared composite powders are shown in Fig.1.For Sn-Sb alloy sample,the nanoparticles are composed of tetragonal Sn (JCPDS card No.02-0709) and SnSb phases (JCPDS card No.33-0118).As to Sn-Sb-Cu samples,with the addition of copper,the SnCu binary alloy (Cu₆Sn₅)(JCPDS card No.45-1488)appears instead of the Sn phase.The SnSb phase still remains.With the continuous addition of copper,no dissociative or oxidized copper phase can be observed in the XRD pattern,because the additional copper reduced by NaBH₄ is amorphous.Compared with the Sn-Sb alloy composite,the diffraction peaks of the Sn-Sb-Cu samples are broader,indicating the poor crystallization of the samples,which suggests that the introduction/incorporation of copper would evidently reduce the crystallinity of the samples.

下载原图

Table 1 Inductively coupled plasma spectroscopy (ICP) results for as-prepared product

Fig.1 XRD patterns of as-prepared alloy powders

Figure 2 shows the SEM images of the as-prepared composite powders.From the SEM images,it is apparent that the particle size has a uniform distribution and is estimated to be 10-20 nm.To further investigate the insight structure of the as-prepared products,TEM was used.Compared with that of the SnSb sample (Fig.2b),the TEM images (Fig.2d,f) of the SnSbCu samples reveal that these products consist of spherical hollow spheres.The shell thickness of hollow spheres is detected to be about5 nm by high resolution TEM.In order to investigate the formation mechanism,other binary alloy powders (Sn-Cu,Sb-Cu) were also acquired via the similar synthetic procedure.But the hollow spheres could not be obtained.On the basis of the above observation,we proposed that during the initial stage of reaction,Sn-Sb nanospheres are preferentially formed by the reduction of metal chlorides,owning to the kinetic effects.As there is excessive NaBH₄in the solution,any Sn or Sb that is oxidized to metal cat ion during Cu deposition will be combined with Cu²⁺and co-reduced near the surface of the nanosphere,which leads to hollow nanoparticles.The SnSb phase plays a significant role during the formation of the hollow spheres,which would reduce the diffusion rate of the faster-diffusing species (Cu²⁺) to prevent the collapse of the spheres [ 30] .Consequently,the hollow nanospheres of the binary alloy could not be obtained via the similar procedure.It should be mentioned that many different hollow spheres,such as Pt [ 31] ,Pt-Au [ 32] Co-Pt [ 33] and Fe-Co [ 34] hollow spheres,formed via the similar galvanic displacement reactions were reported.

3.2 Electrochemical characteristics of Sn-Sb-Cu alloys nanoparticles

The discharge/charge curves of the as prepared sample for the 1st,5th,10th and 20th cycles are shown in Fig.3.The curves of the three samples are quite similar,because they share the same active materials (Sn,Sb) in composition.The slopes above 0.8 V at the first discharge are observed for all the three samples,which correspond to the formation of the solid electrolyte interphase (SEI) layer [ 22] and lithium storage in the defect sites.The Sn-Sb-Cu samples consume more charge in this potential region due to their larger surface area which would contain more defect sites and consume more electrolyte to form the SEI layer compared with the binary alloy sample [ 22] .The voltage region between 0.8and 0.7 V is related to the reaction of Li-Sb alloy reactions,Li inserts SnSb to form Li₃Sb and Sn [ 35] .The voltage region below 0.7 V is attributed to the multi-step Li-Sn alloy reactions.In this voltage area,the element Li insert into the alloy continuously and different Li-Sn alloys (Li₂Sn₅,LiSn,Li₇Sn₃,Li₅Sn₂,Li₂₂Sn₅)are formed along with the voltage stepping down [ 36] .The corresponding regions in the charge curves which are attributed to the delithiation reaction can also be observed,respectively.Concerning about the capacity change,the capacities of the Sn-Sb-Cu hollow nanosphere samples reach almost the same values after the first charge-discharge cycle,while that of the Sn-Sb alloy composite are significantly shifted to lower values,which suggests that the ternary alloy hollow nanosphere samples have better capacity retention compared with the binary alloy nanoparticle sample.

Fig.2 FESEM images of a SnSb,c SnSbCu-1,e SnSbCu-2,and TEM images of b SnSb,d SnSbCu-1,f SnSbCu-2

Fig.3 Discharge/charge profile of a SnSb,b SnSbCu-1,and c SnSbCu-2 at 1st,5th,10th and 20th cycles

Fig.4 Cyclic voltammogram of as-prepared a SnSb,b SnSbCu-1 and c SnSbCu-2 at 1st,2nd and 5th cycles

The electrode reaction processes are distinguished more clearly from the cyclic voltammogram as shown in Fig.4.Compared to the Sn-Sb alloy composites,additional reduction peaks from the Sn-Sb-Cu samples at the potential about 1.7 V (vs.Li/Li⁺) are observed during the first cycle,and no longer appear in the later cycles,which are attributed to the irreversible lithium insertion into the defect sites.Compared with the Sn-Sb alloy composites,the Sn-Sb-Cu samples exhibit lower crystallinity,which implies more defects on the particles.As a result,large irreversible capacities are observed during the first cycle.The SEI films are formed between voltage values of 1.2-1.6 V [ 22] .Accordingly,the broad peaks during the first cycle can be observed in the cyclic voltammogram for all three samples.The two peaks below 1.0 V are associated with the Li-Sb and Li-Sn alloying process,respectively,which demonstrates that Li insertion reactions act in a step-wise process [ 35] .The Li-Sn alloys are formed at lower voltage after the Li-Sb alloying process takes place at higher voltage region.The corresponding oxidation peaks also prove that the Li extraction acts in a similar process.Moreover,the peaks become smoother as the cycle numbers increase,indicating that the reversibility of lithium insertion/extraction ability of the alloys decline gradually.

Fig.5 Discharge capacities of as-prepared samples

Figure 5 compares the discharge capacities of the SnSb-Cu hollow nanospheres and that of the Sn-Sb alloys nanocomposites under the same condition.With the increase of the copper content in the Sn-Sb-Cu alloys,the theoretical capacities decrease,because Cu is regarded as an inactive phase which cannot make any contribution to the discharge and charge capacities [ 4] .The discharge capacities of the Sn-Sb alloys nanocomposites show significant fading during cycling,since the discharge/charge process is accompanied by severe volume change.However,except the first cycles,Sn-Sb-Cu hollow nanospheres exhibit good cycling abilities,high reversible specific capacities and high columbic efficiency.SnSbCu-1 and SnSbCu-2 samples still show 380and 320 mAh.g-1 reversible capacities after 30 cycles,respectively,with 82.6%and 77.4%capacity retention.These favorable cycling abilities are partially attributed to the hollow structures,which provides large interspace to accommodate the severe volume expansion caused by the lithium insertion during discharge process.The inactive Cu phase serves as a matrix to alleviate the volume change and prevent the aggregation of the nanoparticles.Figure 6 shows the columbic efficiency of the as prepared samples.Compared with the binary SnSb alloys composites,the Sn-Sb-Cu alloys composites have evidently lower columbic efficiency in the first cycle due to its poorer crystallinity,which causes nanoparticles to be favored to form a passivating film(SEI) [ 22] .The lithium insertion into the defect sites is another important factor for the low columbic efficiency in the first cycle.In the following cycles,the Sn-Sb-Cu hollow nanospheres exhibit higher columbic efficiencies,above 95%of which are observed,than that of the SnSb alloys composites.This would also be attributed to the good cycling performance of the Sn-Sb-Cu hollow nanospheres due to the buffer effects of the hollow structure and the Cu phase.

Fig.6 Columbic efficiency of as-prepared samples

As mentioned above,the introduction of copper into the SnSb alloy system leads to the formation of the hollow nanostructure in the ternary alloy system.Since the inactive Cu is well dispersed in the nanoparticles to buffer the severe volume change during lithiation and delithiation process,the cycling performance of the alloy powders is improved.Compared with the inactive/active alloys,such as Sn-Cu [ 14, 15, 16, 17] and Sb-Cu [ 37] alloys,the Sn-Sb-Cu ternary alloy exhibits higher capacity.According to the good electrochemical performance of the Sn-Sb-Cu ternary alloy,this type of anode materials which are composed by the inactive/active/active ternary alloys should be promising anode materials of lithium-ion batteries.

4 Conclusion

In summary,the Sn-Sb-Cu hollow nanospheres were successfully prepared by co-reducing the metal chloride in alkaline solution.The results of the TEM show that the as prepared Sn-Sb-Cu nanoparticles have a specific hollow structure with a uniform particle size of 10-20 nm.When the as-prepared ternary alloys are used as anode material for lithium-ion batteries,they exhibit higher reversible lithium-ion storage capacities and better cycling performance compared with the simple binary alloy powders.The excellent electrochemical performances of the Sn-SbCu ternary alloys suggest that they would be potential candidate as anode materials for practical use.Many other ternary alloys can be obtained from the similar method by simply using different metal chlorides as the precursors.The successful preparation of the Sn-Sb-Cu ternary alloys will lead to the strategy of preparing a new type of anode materials for lithium-ion batteries.