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Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

Nano Energy (]]]]) ], ]]]–]]]

http://dx.doi.org/12211-2855/& 2014 P

E-mail address: l

Please cite this arlithium-ion batteri

RAPID COMMUNICATION

SnLi4.4 nanoparticles encapsulated in carbonmatrix as high performance anode materialfor lithium-ion batteries

Xiulin Fan, Jie Shao, Xuezhang Xiao, Xinhua Wang,Shouquan Li, Hongwei Ge, Lixin Chen

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Application forBatteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University,Hangzhou 310027, PR China

Received 23 April 2014; received in revised form 27 June 2014; accepted 23 July 2014

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KEYWORDSLithium-ion battery;Anode;SnLi4.4;Tin

0.1016/j.nanoen.2ublished by Elsevi

xchen@zju.edu.cn

ticle as: X. Fan, ees, Nano Energy (

AbstractInduction melting associated with simple ball-milling is utilized to synthesize a SnLi4.4@C core–shell hierarchical composite in which nanometer-sized SnLi4.4 particles are uniformly dispersedand encapsulated by carbon matrix. When evaluated as anode materials for lithium ionbatteries, the composite exhibits a reversible capacity of 680 mA h g�1 after 200 cycles at200 mA g�1. A capacity of 310 mA h g�1 is obtained even at a high rate of 5000 mA g�1. Thesuperior electrochemical performance is ascribed to the fact that the prelithiated SnLi4.4 willnot exert any expansion stress on the carbon matrix during the subsequent delithiation andlithiation processes, therefore guarantee the sustainable integrity of the composite in theprolonged cycling. The carbon matrix offers continuous transport paths for Li+ ions andelectrons inside the composite. Meanwhile the carbon can sufficiently prevent the disintegra-tion and aggregation of Sn nanoparticles upon prolonged cycling. The present study effectivelycircumvents the low initial Coulombic efficiency of the Sn-related nanocomposites and providesa protocol for pairing lithium-free cathodes to make the next-generation high energy lithiumion batteries.& 2014 Published by Elsevier Ltd.

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(L. Chen).

t al., SnLi4.4 nanoparticles encap2014), http://dx.doi.org/10.1016/

Introduction

High capacity alloying anode materials for lithium ionbatteries, such as silicon and tin, have stimulated greatresearch interest in the last 10 years [1]. Tin is a promising

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anode active material due to its extremely high volumetriccapacity (7313 mA h mL�1) and high gravimetric capacity(993 mA h g�1), which is about 9 and 3 times that of thecommercial graphite, respectively [2]. However, one of itsprimary disadvantages is that large volume variations (260%)will lead to rapid pulverization of Sn particles and loss ofcapacity during cycling, and thereby hinders its practicalapplications [3].

Common approaches to improving the cycling perfor-mance of tin-based anode are to use nanostructured tin[4], and Sn composites [5]. Quite recently, significantlyimproved electrochemical performance was achieved forthe embedded/confined Sn anodes [6]. These nanocompo-sites can better accommodate the volume change due tothe faster stress relaxations which enhance the cyclingperformance of the materials. In addition, nanoparticlesof anode provide a much shorter diffusion path for thelithium ions, which also improves the rate capability of theelectrodes [7]. Unfortunately, nanoscale geometries requireadvanced processing methods with carefully designed pro-cedures, especially for the low melting point of Sn (232 1C),which results in increased anode powder costs. Although thecycling performance and rate capability were enhanced forthe nanostructured tin composites, however, a drawback tothis approach is that the high surface area of nanostruc-tured materials significantly increases solid electrolyteinterphase (SEI) formation during the first electrochemicalcycle, which causes a high irreversible capacity loss andleads to low Coulombic efficiency values such as 25–60%[4c,6i,8], depending on the structure of tin and thecomposition of the anode composite. The relatively lowvalues of Coulombic efficiency need excess cathodematerials to offset the irreversible lithium ions and preventhigh specific capacity anodes from being viable when pairedwith cathodes in a “full cell”. In addition, due to the largesurface to volume ratio of nanoparticles, a low tap densityis obtained, which reduces the volumetric capacity of theelectrode [4d].

Recently, many cathode materials, such as V2O5 [9], MnO2

[10], sulfur [11], and metal fluorides [12] have shown thepotential to have much higher Li storage capacity comparedwith the commercial LiCoO2 and LiFePO4 cathode materials.To further increase the energy of the batteries, it is desir-able to pair Sn or Si anodes with high capacity cathodes[13]. Although these cathode materials have many advan-tages (e.g. low cost, abundance, and high energy); however,neither anode nor any of these cathodes contains lithium.Therefore, for this type of combination, either the cathodeor the anode needs to be prelithiated [13].

To circumvent the low initial Coulombic efficiency ofSn-anodes and even provide an anode candidate for thelithium-free cathode materials, we synthesized the hier-archical SnLi4.4@C composite. By cost-effective ball millingthe pre-lithiated Sn anode materials (SnLi4.4) with car-bon black, a facile and scalable method is demonstratedto make Sn anodes viable in practical batteries. Previously,ball-milling has been utilized as an effective way tosynthesize the composites used for lithium electrode mate-rials [14]. However, for the Sn-related anode, some rigidabrasives should be added [15] or some special devices areutilized during ball-milling [16] due to the excellent ducti-lity of Sn. After alloying with Li, the material becomes

Please cite this article as: X. Fan, et al., SnLi4.4 nanoparticles encaplithium-ion batteries, Nano Energy (2014), http://dx.doi.org/10.1016

brittle and fragile [17], which can be easily smashed intonanoparticles during ball milling. When further ball milledwith carbon, the nanometer-sized SnLi4.4 particles becomeuniformly dispersed in the flexible matrix of carbon, forminga composite with size of microns or sub-microns. Within thishierarchy, SnLi4.4 nanoparticles are well confined by thecarbon matrix, and will not exert any expansion stress onthe carbon matrix during cycling, therefore resulting in anexcellent electrochemical performance in terms of capa-city, Coulombic efficiency, and cycling performance. Beingin the fully lithiated state, SnLi4.4 overcomes the low initialCoulombic efficiency of as-mentioned Sn nanocomposites andallows the use of these mentioned lithium-free cathodes.

Experimental section

Materials

Tin foil (99.5%), lithium (99.9%), were purchased fromSinopharm Chemical Reagent Co., Ltd. Materials handlingand sample preparation were performed in an argon-filledglovebox, where the oxygen and water concentration werekept below 1 ppm. SnLi4.4 was synthesized by inductionmelting stoichiometric mixtures of Tin foil and lithiummetals in argon atmosphere. The working voltage of induc-tion melting apparatus is 380 V, with a working current ofabout 10 A. The as-prepared ingots were smashed and thenmechanically milled for 40 h under 0.1 MPa of pure argonatmosphere by the Planetary mill (QM-3SP4J, Nanjing) at400 rpm to prepare SnLi4.4 powder. Then, the SnLi4.4 powderwith carbon black (7:3 in weight) was further milled for 40 hto prepare SnLi4.4@C composite. Tens grams of compositecan be prepared in a vial.

Characterization

X-ray diffraction experiments of the samples were per-formed on an X'Pert Pro X-ray diffractometer (PANalytical,the Netherlands) with Cu Kα radiation at 40 kV and 40 mA.XPS was carried out on a VG ESCALAB MARK II system withMg Kα radiation (1253.6 eV) at a base pressure of 1� 10�8

Torr. All binding energy values were referenced to the C 1speak of carbon at 284.6 eV with an uncertainty of 70.2 eV.Transmission electron microscopy (TEM, Tecnai G2 F30) andSEM (Hitachi SU-70) equipped with energy dispersive spec-trometer (EDS) were performed to examine the morphologyand microstructure of the products. During the sampletransfer and measurements, a lab-built argon filled containerwas used to protect the samples from air and moisture.

Electrochemical investigation

The electrochemical tests were performed using a coin-typehalf cell (CR 2025). Metallic lithium was used as the negativeelectrode. The as-prepared SnLi4.4@C composite was uniformlyspread onto a nickel foam current collector, and then pressedunder a pressure of about 10 MPa. The typical mass loading ofthe active material was E4 mg/cm2. The electrolyte solutioncomprised 1 M LiPF6/ethylene carbonate (EC)/dimethyl carbo-nate (DMC)/ethylmethyl carbonate (EMC) (1:1:1 by volume).

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3SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries

The cells were assembled with a polypropylene (PP) micro-porous film (Celgard 2300) as the separator. Charge–dischargeexperiments were performed at different current densitiesbetween 0.005 and 3 V using a LAND CT2001A Battery Cycler atroom temperature. Cyclic voltammetry (CV) was performed onthe CHI660E electrochemical workstation in the potentialrange 0–3.0 V (vs Li+/Li) at a scan rate of 0.1 mV/s.

Results and discussion

Material preparation and characterization

Figure 1 shows the XRD patterns of as-cast SnLi4.4, ball milledSnLi4.4 and SnLi4.4@C composite. For comparison, the patternof carbon black is also compiled. All the main peaks of as-castSnLi4.4 could be indexed to the cubic SnLi4.4 (JCPDS no. 65-0296, space group: F23, 196), while no peaks of Sn or Li weredetected. The diffraction peaks of SnLi4.4 after ball millingappeared much weaker and broader compared to the XRDpatterns of the as-cast SnLi4.4 sample, implying a significantdecrease in size and crystalline correlation length throughball milling. After ball milled with carbon black, the peaks ofSnLi4.4 were further broadened, with no peaks of carbondetected. Therefore, it is reasonable to suggest that thelong-range ordered graphite structure is mostly destroyedduring ball milling with SnLi4.4. The fragile SnLi4.4 would benanocrystalline and even disordered after the prolonged ballmilling process, resulting in the broadening and weakening ofthe XRD patterns. Using the Scherrer equation (d=0.9λ/βcosθ), the average crystalline size of SnLi4.4 was estimatedto be 9.5 and 6 nm from 2θ and λ values of the highest peak(822) of SnLi4.4 for the ball milled SnLi4.4 and SnLi4.4@Ccomposite, respectively. These results indicate that ballmilling is a good approach for reducing the as-cast SnLi4.4to nanometer-sized particles, due to the evident fragilityof the material. The dramatic decrease of the grain sizewill promote the facile diffusion of the lithium ion duringelectrochemical cycling.

The morphology of the as-prepared composite was studiedusing transmission electron microscopy (TEM, Figure 2a–d)and scanning electron microscopy (SEM, Figures S1 and S2).Figure 2a shows a typical irregular shaped as-preparedSnLi4.4@C particle with dimension at �1.5 mm. In the magni-fied TEM image (Figure 2b), it can be clearly seen that the

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Please cite this article as: X. Fan, et al., SnLi4.4 nanoparticles encaplithium-ion batteries, Nano Energy (2014), http://dx.doi.org/10.1016/

particles have a hierarchical structure: small primary island-like SnLi4.4 nanoparticles are uniformly distributed andenclosed in the large secondary carbon particles. Figure 2cdisplays a set of parallel fringes with the space of 0.34 nm inthe shell of the secondary particles, corresponding to theundestroyed (002) planes of carbon. Apart from this set offringes, few lattices are observed in the surface layer,indicating the amorphization of carbon black during ballmilling, in line with the XRD results. In the HADDF-STEMmode, it is more obvious to observe the distribution ofdifferent elements, as shown in Figure 2d. The darker colorin the composites represents the light element, which iscarbon here, and the much brighter color represents theSnLi4.4 alloy particles. Nanoparticles of SnLi4.4 are homoge-neously dispersed and wrapped in the carbon matrix. The ele-ment distribution is further evidenced by the cross-sectionalcompositional line profiles of the composite (Figure 2e),which clearly shows that the outer shell of the composite iscomposed of carbon and the SnLi4.4 nanoparticles are dis-persed in the carbon matrix. A typical composite particlegenerally contains thousands of SnLi4.4 nanoparticles with sizefrom several nanometers to tens nanometers. The SEMelemental mapping image of SnLi4.4@C composite confirmsthat all of the particles are encapsulated by carbon, as shownin Figure S2. In order to further verify if our synthesizedSnLi4.4@C composite has a high-quality encapsulation struc-ture, a surface-sensitive XPS experiment was carried out toexamine the chemical characteristics of the surface of thecomposites. For comparison, pure SnLi4.4 was also tested.Figure 2f shows the full XPS spectrums of SnLi4.4@C compositeand pure SnLi4.4, in which the photoelectron lines at a bindingenergy of about 285 and 532 eV are attributed to C 1s and O1s, respectively [18]. Intensive peaks of Sn 3d5/2 and 3d3/2 at486 and 494 eV for the pure SnLi4.4 can be observed, but thereis almost an absence of the signals of Sn 3d5/2 and 3d3/2 forSnLi4.4@C composite. Because the atomic sensitivity factor ofSn is much higher than that of C [19], absence of its peaksimplies that, in our synthesized SnLi4.4@C composite, theSnLi4.4 particles appear to be completely sealed inside thecarbon layers. The formation of the hierarchical nanostruc-ture for the SnLi4.4@C composite is hypothesized to arise fromthe intrinsic physical properties of the SnLi4.4 and carbonblack, and the scheme is shown in Figure 2g. The SnLi4.4 isfragile and brittle while the carbon black is much ductile andflexible. In the initial stages of milling, the ductile carbonblack can easily be deformed into lamellae along the (002)planes by ball-powder-ball collisions, while the brittle SnLi4.4particles can be readily fragmented and comminuted intonanometer size. These fragmented brittle nanoparticlestend to become occluded by the ductile constituents andtrapped in the ductile particles [20]. With further milling, theductile lamellae get convoluted and refined, and meanwhilethe brittle nanoparticles get uniformly dispersed among theductile matrix [20b].

It is believed that the entire core–shell structure willbe more favorable to keep the architecture integrityduring electrochemical discharge and charge processes com-pared with the bare and pure SnLi4.4 nanoparticles. In thisembedded hierarchical architecture, the carbon matrix offersa robust framework that provides good conductivity and alsoacts as a mechanical support to prevent the agglomerationand pulverization of the individual Sn nanoparticles during

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Figure 2 (a) and (b) TEM images of SnLi4.4@C; (c) HR-TEM image of SnLi4.4@C; (d) and (e) HADDF-STEM image of SnLi4.4@C and thecorresponding cross-sectional compositional line profiles of SnLi4.4@C. (f) XPS spectra of bare SnLi4.4 without carbon black andSnLi4.4@C; (g) schematic illustration of the preparation process of the SnLi4.4@C composite, IM and BM mean induction melting andball milling, respectively.

X. Fan et al.4

electrochemical lithiation/delithiation process. As Sn is in thelithiated state (SnLi4.4) initially, it will not exert any expan-sion stress on the carbon matrix during the subsequentlithiation/delithiation process, therefore the carbon matrixcan keep much better stability compared with other confinedcomposites [14a]. The uniform dispersion of nano-SnLi4.4particles in the composite can effectively reduce the distancethat Li-ions and electrons must travel during cycling in thecomposite.

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Electrochemical performance

Figure 3a shows the charge–discharge profiles of the SnLi4.4@Ccomposite at a current rate of 200 mA g�1 between 0.005 and

Please cite this article as: X. Fan, et al., SnLi4.4 nanoparticles encaplithium-ion batteries, Nano Energy (2014), http://dx.doi.org/10.1016

3 V. The initial discharge and charge capacities of SnLi4.4@Ccomposite are 313 and 743 mA h g�1 with an apparentCoulombic efficiency of 237%. Because the main active speciesof Sn has been prelithiated, the limited first dischargecapacity can be mainly ascribed to the lithiation of carbon,the formation of SEI films and the electrolyte decomposition.The first delithiation capacity is close to the theoreticalcapacity (775 mA h g�1) of the SnLi4.4@C composite (SnLi4.4:C=7:3 by weight), based on the theoretical capacity of carbon(372 mA h g�1) and metallic tin (993 mA h g�1) (the theore-tical capacity was calculated by assuming the mixture ofSnLi4.4 and carbon according to Ctheoretical=CSn�%mass of Sn+Ccarbon�%mass of carbon=992� 0.65+372� 0.35=775 mAh g�1. In the equation, the pre-inserted Li has been deducted,therefore the weight of Sn and carbon accounts for 65% and

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5SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries

35% in the composite, respectively). For the electrochemicalcycles, the voltage plateaus occurring near 0.6 V are apparentin both discharge (lithiation) and charge (delithiation) profiles,reflecting the process of Li+ insertion and deinsertion. Theseplateaus are typical characteristics of Sn electrodes [6h,21].

The cycling performance of SnLi4.4 and SnLi4.4@C wasevaluated under the same conditions, as shown in Figure 3b.The SnLi4.4 electrode shows an initial charge capacity of906 mA h g�1. Then, the capacity continuously decreases

Please cite this article as: X. Fan, et al., SnLi4.4 nanoparticles encaplithium-ion batteries, Nano Energy (2014), http://dx.doi.org/10.1016/

within the following 40 cycles and exhibits a capacity ofabout 330 mA h g�1 in the 41th cycle with only 36% capacityretention. For comparison, the SnLi4.4@C composite exhibitsan excellent cycling performance, although slight decreaseof capacity was also observed in the first several cycles.The Coulombic efficiency approaches 99% after the 6thcycle, suggesting its high reversibility. Interestingly, thereversible capacity of the SnLi4.4@C composite graduallyincreases to 700 mA h g�1 from 40th cycle to the 100thcycle. The behavior of capacity increase has been reportedin the Sn/C nanocomposite [6g,22], SnO2 nanocomposite[23] and other metal oxide systems [24]. After 200th cycle,a capacity of 680 mA h g�1 can be obtained. These compar-isons indicate that the unique architecture with activeSnLi4.4 nanoparticles dispersed within carbon matrix andencapsulated by carbon layer can provide high reversiblecapacity for alloying/dealloying reactions, realizing analmost completely reversible alloying reaction at a widevoltage window. Considering the SnLi4.4 content in thecomposite, it is clear that the material maintains capacitiesclose to the theoretical values for Sn even after hundreds ofcharge–discharge cycles, and exhibits minimal irreversiblelosses and excellent Coulombic efficiency. Such uniquenanostructure endows the composite with excellent elec-trochemical performance as an anode material for lithiumion batteries. To the best of our knowledge, this is the firsttime to directly utilize the lithiated Sn as anode materials,which exhibits a superior electrochemical performance.Induction melting and ball milling are widely used prepara-tion methods in industry. Therefore, it is believed that as-prepared anode material can be easily extended to largescale production.

Figure 3c shows the cyclic voltammograms (CVs) of theSnLi4.4@C electrode after 200th cycling at a scan rate of0.1 mV/s between 0 and 3 V. The reduction peaks at 0.2,0.35 and 0.55 V are overlapped and formed an evidentcathodic peak, corresponding to the lithiation of Sn. Oxida-tion peaks at 0.52, 0.67, and 0.78 V are assigned todelithiation reaction of the LixSn alloy. The unchanged peakposition and current intensities imply excellent electroche-mical cycling stability of the SnLi4.4@C composite. Thesuperior stability is attributed to the favorable SnLi4.4@Celectrode structure mentioned above.

The SnLi4.4@C composite also shows favorable high-ratecapability, as illustrated in Figure 4. The hierarchical com-posite can deliver a capacity of �605 mA h g�1 at 500 mA g�1,�545 mA h g�1 at 1000 mA g�1, and �425 mA h g�1 at 2000mA g�1; even at a current density of as high as 5000 mA g�1, itcan still exhibit a charge–discharge capacity of about 310 mA hg�1. More importantly, when the current rate gradually returnsto 200 mA g�1, a stable capacity of 650 mA g�1 can revert.Carbon black is networked carbon with excellent electricalconnectivity [25]. Under dry ball-milling conditions, the carbonblack becomes strongly bonded to the SnLi4.4 nanoparticles,which can provide the SnLi4.4@C composite anode withimproved current collection capability, thus an excellent ratecapability.

The significantly improved cycling performance and ratecapability could be attributed to the following three mainreasons. First, the unique hierarchy with island SnLi4.4embedded in the carbon matrix can form stable and rapidlithium and ion channels during electrochemical cycling.

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Second, pulverization of the composite incurred by volumeexpansion would be alleviated, because there is no need tocreate new volumes in the composite for the incoming Li, as inthe case of traditional Sn/C composite. Third, the carbonmatrix can effectively prevent the Sn nanoparticles fromaggregation and disintegration during electrochemical cycling.

To further understand the reaction mechanism and cyclingstability of the SnLi4.4@C electrode, we performed SEM, EDS,TEM and SAED analysis to characterize the structural andmorphological changes of the electrode after 200th dealloyingreaction. Figure 5a and b presents SEM images of the SnLi4.4@Ccomposite after 200th delithiation reaction. No pulverizationand disintegration of the electrode can be observed, whichcan explain the excellent cycling stability of the SnLi4.4@Ccomposite. As previously mentioned, the encapsulated SnLi4.4nanoparticles will not exert tensile stress on the carbon matrix

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Figure 5 (a) and (b) SEM images of SnLi4.4@C after 200th electroch200th electrochemical cycles. (d) and (e) TEM images of SnLi4.4@C aSAED patterns of the composite and EDS results of A and B areas incycles.

Please cite this article as: X. Fan, et al., SnLi4.4 nanoparticles encaplithium-ion batteries, Nano Energy (2014), http://dx.doi.org/10.1016

during the cycling, thereby circumvent the inevitable fractureand pulverization of the composite. We also performed theelement mapping (C, Sn and F) on the selected area ofthe composite, as shown in Figure 5c. The C-map shows thecarbon matrix. It clearly indicates that the Sn-map exhibits avery homogenous distribution and coincides well with theC-map, demonstrating Sn is highly dispersed in the carbonmatrix. This homogeneity of dispersed Sn distribution withoutany detectable aggregation is favorable for prolonged electro-chemical cycling. The signals of F element come from theLiPF6 in the electrolyte.

Detailed structure of the SnLi4.4@C composite aftercycling is also characterized by the TEM, SAED, EDS andHRTEM. The images (Figure 5d and e) clearly show that mostof the fine grains are with a diameter of less than 8 nm(although some bigger ones with size of about 20 nm canalso be observed) inside the carbon matrix. SAED, EDS andthe HRTEM confirmed that these ultra-small particles arenano-Sn particles. The corresponding diffuse ring-like SAEDpatterns indicate the dramatic reduction of Sn crystallitesafter cycling. In addition, magnified TEM image of cycledcomposite shows an outer carbon layer, which is furtherevidenced by EDS results (inset of Figure 5e). The high-resolution TEM image shown in Figure 5f reveals that latticefringes with a basal distance of 0.29 nm can be observedfrom the locally magnified image of the Sn nanoparticles(the upper left inset of Figure 5f), which is in goodagreement with the (200) lattice spacing of Sn (JCPDS no.65-0296). No graphite lattice is observed in the carbonmatrix, which again confirms the amorphization of carbonblack. Based on the above analysis, it is concluded thatduring repeated electrochemical cycling, the SnLi4.4 willcollapse into ultra-small nanoparticles because of the

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emical cycles. (c) EDS mapping of Sn, C and F for SnLi4.4@C afterfter 200th electrochemical cycles, the insets of (d) and (e) are(e); (f) HRTEM image of SnLi4.4@C after 200th electrochemical

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7SnLi4.4 nanoparticles encapsulated in carbon matrix as high performance anode material for lithium-ion batteries

tremendous volume variation during delithiation and lithia-tion, while the composite remains constant. The in situformed ultra-small Sn nanoparticles are stably embedded inthe carbon matrix, which can effectively alleviate the largevolume change and curb the pulverization and aggregationof Sn nanoparticles in the subsequent cycling. This canexplain why the SnLi4.4@C composite could have theamazing cycling performance and rate capability, while thebare SnLi4.4 particles exhibit a rather poor cycling performance.

Conclusions

Hierarchical SnLi4.4@C composite with SnLi4.4 nanoparticlesuniformly dispersed in the carbon matrix was simply andsuccessfully fabricated by induction melting of Sn and Li andsubsequent ball milling with carbon black. Using as an anodematerial for Li ion batteries, the as-prepared SnLi4.4@Ccomposite exhibits a charge–discharge capacity of 680 mA hg�1 at 200 mA g�1 after 200th cycle. A capacity of 310 mAh g�1 was obtained even at a high current density of 5000mA g�1. The excellent electrochemical performance of theSnLi4.4@C composite is due to (1) nanoscaled particles ofSnLi4.4 distributed in the carbon matrix, which can not onlyserve as the favorable electron and lithium ion transportchannels, but also effectively curb the pulverization andaggregation of Sn nanoparticles in the prolonged cycling;(2) because the active material of SnLi4.4 have beenprelithiated, the pulverization incurred by volume expan-sion would be alleviated and therefore, much more stabilitycan be achieved for the composite. The simplified prepara-tion process and the high electrochemical performance ofthe SnLi4.4@C composite make it promising in the applica-tion as the anode materials for lithium-ion batteries,especially for paring with the lithium-free cathode materi-als for the next generation lithium ion batteries.

Acknowledgments

The authors gratefully acknowledge the support of theNational Natural Science Foundation of China (21303161),Program for Innovative Research Team in University ofMinistry of Education of China (IRT13037), the Key Scienceand Technology Innovation Team of Zhejiang Province(2010R50013), the China Postdoctoral Science Foundation(2012M521167) and the International Postdoctoral ExchangeProgram (2013).

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.07.020.

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