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Electrochimica Acta 144 (2014) 376–382

Contents lists available at ScienceDirect

Electrochimica Acta

j our na l ho me pa g e: www.elsev ier .com/ locate /e lec tac ta

nhanced Electrochemical Stability of Sn-Carbon Nanotubeanocapsules as Lithium-Ion Battery Anode

hun-jing Liua, Hao Huanga,∗, Guo-zhong Caoa,b, Fang-hong Xuea,amon Alberto Paredes Camachoa, Xing-long Donga,∗

School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, People’s Republic of ChinaDepartment of Materials Science and Engineering, University of Washington, Seattle, WA 98195, United States

r t i c l e i n f o

rticle history:eceived 7 May 2014eceived in revised form 3 July 2014ccepted 16 July 2014vailable online 20 August 2014

a b s t r a c t

Direct current (DC) arc-discharge method is used to fabricate Sn-carbon nanotube nanocapsules (Sn-CNTNCs). The Sn is partially-filled into multi-walled CNTs, as an ideal configuration for the active materialsas lithium-ion battery anode; Sn nanoparticles provide large storage capacity and CNTs confine andaccommodate the volume expansion of Sn as well as provide the conductive network and contributetheir own capacity. Large initial specific capacity and stable cyclic performance are identified in the

eywords:ithium-ion batteryarbon nanotubeirect current arc-discharge methodyclic voltammetry

electrochemical tests. Such novel nanostructure provides a solution to the volume expansion issue of ahigh-capacity electrode.

© 2014 Elsevier Ltd. All rights reserved.

lectrochemical impedance spectroscopy

. Introduction

Lithium-ion batteries (LIBs) have been attracting considerablettention because of their wide applications as the main powerource in portable electronic products and electric vehicles [1].ne of the critical factors to obtain a high-performance LIB lies on

he anode materials that can work with the desirable properties ofppropriate operating voltage, large specific capacity and structuraltability. Among all the candidates, carbon has been utilized as theommercial anode material due to its low cost and good cyclic sta-ility. However, the low theoretical and practical capacity retains

ts limitation for high-power requirements [2–4]. In recent works,ome active metals (e.g., Si [5,6], Al [7] and Sn [8,9]) are set as theegative electrode materials and find owning high specific capaci-ies (4200 mAh g−1, 2234 mAh g−1 and 993 mAh g−1, respectively)hrough forming alloys with lithium. Sn, therein the metals, haseen considered as one of the most promising substitutes for thearbon due to its electric conductivity and chemical stability. How-

ver, the severe expansion up to 300% volume change [2,10] thatn suffers during the Li-ion insertion/desertion process results ineteriorative structures and poor cycling performances [11,12].

∗ Corresponding author. Tel.: +86 411 84706130; fax: +86 411 84709284.E-mail addresses: huanghao@dlut.edu.cn (H. Huang), dongxl@dlut.edu.cn

X.-l. Dong).

ttp://dx.doi.org/10.1016/j.electacta.2014.07.068013-4686/© 2014 Elsevier Ltd. All rights reserved.

To avoid the degradation of Sn electrodes, one active approachis to create free space for the volume expansion by integratingultra-small size Sn components into carbon structures, i.e., Sn-Cnanocomposite. Several positive features can be expected: (1) theultra-small size of Sn nanoparticles; small particle size can signif-icantly reduce the strain and improve the diffusion ability of Liions in the electrode materials; (2) the carbon matrix; the flexi-ble carbon frame will release the strain from the Sn nanoparticlesand accommodate the excessive volume change. Further, it canprevent Sn nanoparticles from heavy aggregation over cyclic dis-charging/charging; (3) both Sn and C can independently store Liions, and thus provide high specific capacity. In such a context,the Sn-C composites with various hybrid morphologies have beenachieved by different preparation methods, such as aerosol spraypyrolysis [9], chemical vapor deposition [13], ball milling [14],hydrothermal reaction [15], electrochemical deposition [16], andetc. However, it remains a significant challenge to obtain highlyuniform Sn-C nanocomposites with controlled structure and purity.

In this work, we report the fabrication of Sn-CNT nanocapsules(NCs) as a LIB anode by means of DC arc-discharge method. Thesynthesized nanosized products have a uniform morphology ofrod-like Sn nanograins partially-filled in the CNTs. In this hybrid

nanostructure, CNTs are expected to bond the Sn grains and alsoprovide adequate room for the volume expansion. Moreover, thewell-crystallized walls of the CNTs help to transfer the electronsbetween the electrode and Sn during Li ions insertion/extraction

ica A

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C.-j. Liu et al. / Electrochim

rocess. A typical kinetic mode of barrier-layer diffusion is furtherbserved in the electrochemical impedance measurement of Sn-NT NCs electrode, which forms the positive effects of diffusionuffering and stabilization of electrode reactions.

. Experimental

.1. Synthesis of Sn-CNT NCs

The schematic diagram of the modified arc-discharge equip-ent is shown in Fig. 1 [17]. To begin with, micron-sized Sn bulk

99.99% purity) was set as the anode on a copper stage and car-on rod served as the cathode. After the chamber was evacuated to0−2 Pa, a mixture gas of argon (0.03 MPa) and methane (0.01 MPa)as introduced in as the working atmosphere. The methane wassually used as the carbon source. Later, the arc discharge was

gnited and the Sn bulk was evaporated for 10 minutes at a steadyurrent of 90 A. At last, after being passivated for 12 hrs., the Sn-CNTCs were collected and removed from the water-cooled chamberall. As the counterpart, the Sn nanoparticles (NPs) were prepared

n the same routine, however with a tungsten cathode and in antmosphere of argon (0.03 MPa) and hydrogen (0.01 MPa).

.2. Materials characterizations

The microstructure and morphology were determined by trans-ission electron microscopy (TEM, Tecnai220 S-TWIM). X-ray

hotoelectron spectroscopy (XPS) measurements were carriedut with an ESCALAB 250Xi spectrometer, using a focused andonochromatized Al K� radiation (hv = 1486.6 eV). The sample was

laced in the analysis chamber with a pressure of less than 10−8 Pa.arbon structure was characterized by Raman spectroscopy withhe Red Light laser line at 632.8 nm (InVia). The content of carbonlement was measured by Elemental Analyzer (vario EL III).

.3. Electrochemical Measurements

The working electrodes consisted of Sn-CNT NCs (90 wt.%) andolyvinylidene fluoride (PVDF, 10 wt.%) dissolved in N-methylyrrolidinone (NMP), and these two components were mixed

ogether thoroughly to form slurry, then coated onto copper foilubstrates and dried at 120 ◦C under vacuum for 24 h. Electro-hemical responses of the anodes were investigated directly usingR2025 coin-type half-cells assembled in an ultra-pure argon-filled

Fig. 1. The schematic diagram of the modified arc-discharge equipment.

cta 144 (2014) 376–382 377

glove box. Lithium metals and polypropylene film were used ascounter anode and separator, respectively. The electrolyte was 1 MLiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbon-ate (1:1, vol.%). Electrochemical measurements were performed ona Land CT2001A test system. The cells were cycled between cutoffvoltages of 0.01 and 2.00 V (vs. Li/Li+) at a constant current den-sity of 100 mA g−1 at room temperature. Cyclic voltammetry (CV)was performed on a CHI660D electrochemical workstation in thevoltage range of 0.01-2.00 V (vs. Li/Li+) at a scan rate of 0.1 mV s−1.Electrochemical impedance spectroscopy (EIS) was conducted in a100 KHz-0.01 Hz frequency range with a perturbation amplitude of5 mV.

3. Results and discussion

3.1. Microstructure and Morphology of Sn-CNT NCs

The high-resolution TEM images showing the morphologies ofthe Sn-CNT NCs and Sn NPs are presented in Fig. 2. The Sn NPs are inspherical shape with the mean diameter of 50-80 nm (Fig. 2(a)). Inthe close observation of a particle (Fig. 2(b)), it is found that Sn NPhas a clear core/shell structure, i.e., the uniform thin tin-oxide shellencapsulating the Sn core. Such nanostructure of a metallic coreand congenerous metal oxide shell exists commonly in pure metalNPs due to the experience of passivation after the arc-dischargeprocess, it doesn’t change with the processing condition variation[17]. Fig. 2(c) presents a uniform structure of CNTs, with the averagesize of about 40 nm in diameter and 200-300 nm in length. The CNTshave multi walls of 5-7 graphene layers and are partially-filled withmetal tin. The electron diffraction pattern of the as-synthesized Sn-CNT NCs is shown in the inset of Fig. 2(d), which exhibits a pattern ofpoly-crystalline rings assigning to the (200), (220), (301) planes ofSn. The results from the Elemental Analyzer suggest that the carbonweights ∼17% of the Sn-CNT NCs.

In order to identify the chemical composition, the XPS studiesare carried out on the Ar-ion cleaned surface of the Sn-CNT NCsspecimen. Fig. 2(e) and (f) illustrate the spectra for C and Sn ele-ments respectively. The binding energy of 284.57 eV for C1s mainlycorresponds to the carbon atoms (Fig. 2(e)). The peaks in Fig. 2(f)correspond to the Sn spectrum with different valance state. Peakslocated at around 484.98 and 493.38 eV could be assigned to Sn3d5/2 and Sn 3d3/2, respectively; peaks at 486.82 and 495.30 eV areassigned to Sn Oxide [18]. The XPS results demonstrate the exist-ence of the core-shell structure in the Sn-CNT NCs nanoparticles,with the presence of SnO2 on the surface◦

The structures of the carbon shells are analyzed by Raman spec-tra, as shown in Fig. 3. Two intensive peaks are denoted as D-bandand G-band allocate at about 1326 cm−1 and 1593 cm−1, corre-sponding respectively to the A1g and E2g carbon vibration modes[11,13]. These two peaks are related to the sp2 electronic config-uration containing an electron in the � orbital and dominate inthe most Raman spectra of graphite materials with small crystal-lite size (or the so-called semicrystalline graphite). Strong D-peak,normally aroused by the presence of in-plane substitutional het-eroatoms, vacancies, grain boundaries or other defects, indicates alarge amount of disordered carbon existing in the carbon shells, as itis observed in Fig. 2(d). The G-band is typical for graphite and carbonblacks, originated from the stretching vibration of any C pairs of sp2

sites. In amorphous carbonaceous materials, the large and diffuse Dand G bands located at 1200 and 1550 cm−1 correspond to broadenand overlapped sp2 and sp3 bonded carbon [11]. The graphite grainsize in the expression of the in-plane correlation length, La, can

be determined by the Tuinstra-Koenig equation, La = c� (ID/IG)−1,where c� is about 4.4 nm [19,20]. ID/IG can be obtained from aLorentzian function fitting with the Raman curve and the corre-sponding La is calculated as 3.10 nm in this case. In comparison with

378 C.-j. Liu et al. / Electrochimica Acta 144 (2014) 376–382

inset

oCw

3m

Satd

Fig. 2. TEM and HRTEM images of Sn NPs (a),(b), Sn-CNT NCs (c) (d) and the

ur previous work (the La is 4.73) [21], this value implied that theNTs were immature, mainly due to the gas evaporation processas too short to form well-developed CNTs by the DC arc method.

.2. Electrochemical performance of Sn-CNT NCs electrodeaterials

In order to investigate the cycling performance of the pure

n NPs and Sn-CNT NCs anode electrode, typical sloping volt-ge profiles of Sn NPs and Sn-CNT NCs electrode are obtained inhe potential range of 0.01 V to 2.00 V and at a constant currentensity of 100 mA g−1 (0.1 C), as shown in Fig. 4(a) and (b). In

1000 150 0 200 0 250 0

ID/IG=0.93

.u.a/ytisnetnI

Rama n S hift / c m-1

D G

ID/IG=1.42Sn-CNT NCs

CNTs

Fig. 3. Raman spectra of Sn-CNT NCs and CNTs.

diffraction pattern, high-resolution XPS of (e) C1s and (f) Sn in Sn-CNT NCs.

the case of Sn-CNT NCs electrode, the 1st discharge and chargecurves own coupled plane voltage regions at 0.80 V, 0.51 V/0.80 Vand 0.32 V/0.62 V, attributed to the Faradaic response of lithia-tion/delithiation reactions [14]. The first voltage region at 0.80 Vthat is absent in the 2nd discharge curve, corresponds to the forma-tion of the solid electrolyte interface (SEI) [14]. The initial specificcapacity of Sn NPs anode electrodes is 935 mAh g−1, close to thetheoretical capacity of Sn (CP∗

Sn = 993 mAh g−1) [9]. However, itdecreases rapidly to 380 mAh g−1 in the 2nd discharge and fully failsat the 4th cycle, as shown in Fig. 4(c). The Sn-CNT NCs electrode hasthe initial specific capacity of 850 mAh g−1. Although having lessinitial capacity than Sn NPs, the Sn-CNT NCs electrode only experi-enced a slight decrease of capacity within the first 3 cycles, and thenbecame stable at about 600 mAh g−1. The Coulombic efficiency, aratio of charge capacity and discharge capacity, approaches 90%from the 2nd cycle, further showing a good reversibility of the Sn-CNT NCs electrode. It was reported that the pure multi-walled CNTssynthesized by arc-discharge method can maintain its reversiblecapacity to 165 mAh g−1 after 10 cycles [22]. With known carboncontent (∼17 wt.% CNT) in Sn-CNT, the total capacity (536 mAh g−1)of the electrode, and the pure CNT’s capacity (165 mAh g−1, refer-ence 22), the capacity of Sn in Sn-CNT NCs electrode was calculatedto be 612 mAh g−1 after 10 cycles. This result clearly indicates thecapacity of Sn component can be well maintained with the protec-tion from CNT, however, the capacity of Sn rapidly decayed to 9mAh g−1 after 10 cycles without the protection of CNT. The excel-lent retention of capacity is attributed to the well-tailored structureof the Sn-CNT NCs, which has enough room to buffer the volumeexpansion of Sn during the insertion of Li ions.

Fig. 5 shows the cyclic voltammetry curves in the range of 0.01-2.00 V at a scanning rate of 0.10 mV s−1 for Sn-CNT NCs anodeelectrode. The different peaks in the curves are the indications ofphase changes during the redox reaction with the Li opposite. A

C.-j. Liu et al. / Electrochimica Acta 144 (2014) 376–382 379

Fig. 4. Discharge/charge curves for pure Sn (a) and Sn-CNT NCs anode electrode(b), (c) Cycling performance of Sn-CNT NCs and pure Sn NPs anode electrodes. Them

vci[ai(

0.0 0.5 1.0 1.5 2.0-0.002 0

-0.001 5

-0.001 0

-0.000 5

0.000 0

0.000 5

0.001 0

0.001 5

Cur

rent

/A

Poten tial/V

with a huge increase of volume [8,26]. When the potential reduces

easurement was carried out at a current density of 100 mA g-1.

isible reduction peak at around 0.70-0.93 V (peak S) in the firstathodic scan disappears in the following cycles, which is accred-ted to the decomposition of electrolyte and formation of SEI film14]. The peaks D, E in the voltage range of 0.18-0.25 V vs. Li/Li+ are

+

ssociated with the reaction between Li and C, which is writtenn Eq. (1) [23]. The coupled peaks in the higher voltage ranges of A0.82-0.89 V)/A’ (0.53-0.60 V), B (0.70-0.79 V) and C (0.53-0.60 V)/C’

Fig. 5. Cyclic voltammogram curves of Sn-CNT NCs scanned between 0.01-2.00 V ata rate of 0.1 mV s-1.

(0.23-0.26 V) correspond to the stepwise redox reactions betweenLi-Sn, respectively shown in the Eqs. (2-4) [13–15,24].

6C + yL+ + ye− � LiyC6 (1)

5Sn + 2Li+ + 2e− � Li2Sn5 (2)

Li2Sn5 + 3Li+ + 3e− � 5LiSn (3)

LiSn + 17Li+ + 17e− � Li22Sn5 (4)

Thus, the atomic mass ratio of Li in Li-Sn alloy phase (from Li2Sn5to Li22Sn5) increases from 2.29% to 20.46% during the lithium inter-calation process. After the first cycle, the lithiation peaks (Peaks A’and C’) of Sn enhance greatly, which implies more amount of Sninvolved in the reaction as the Li-ion active matter. Two delithiumcarbon peaks(Peaks D and E) emerge in the second anode scan,corresponding to the oxidation reaction of C with Li ions. However,the reduction peaks of carbon intercalation cannot be located inthe CV curves, mainly due to the potential range (0.0∼0.2 V [25])overlapping with that of the lithium-ion insertion of LiSn. The over-all enhancement of lithium peaks are comprehensively denoted bythe reasons as the fully infiltration of the active materials in theelectrolyte, well-building of the conductive networks by the CNTshells, and the more expression of the electronic loss through theelectrode in the electrochemical reactions.

Fig. 6 shows the mechanism diagram of the anode electrode inthe process of discharging and charging. Actually in the prepara-tion of the electrode, we did not add any conductive agent, e.g.,carbon black, considering the presence of the conductive shells ofthe Sn-CNT NCs. The carbon walls, hence, become the only path forthe electron transport. In addition, the interspace between the car-bon layers is ∼0.340 nm, which is much larger than the diameterof the lithium ion (0.152 nm). Such large space between adjacentlayers would allow the lithium ions infiltrate easily through thecarbon walls and accessible to Sn during the insertion and extrac-tion process. Large proportion of defects in CNTs, as observed in theHR-TEM and Raman results, will further fascinate the diffusion ofthe Li ions. During the cathodic (lithiation) process, metal Sn andLi+ form Li2Sn5 and Li22Sn5 phases at the potentials of 0.56 V and0.25 V, respectively. The electrons are transferred through the CNTwalls and get to the Sn core/CNT shells interface. The incorporationof Li+ leads to the formation of Sn-Li alloy, however accompanied

to the intercalation potential of carbon (0.0∼0.2 V [25]), LiC6 is alsoformed to accommodate extra lithium ions, also contributing tocapacity. The anodic process (delithiation) presents a reverse of

380 C.-j. Liu et al. / Electrochimica Acta 144 (2014) 376–382

Fig. 6. The mechanism figures of Sn-CNT NCs anode elec

Table 1Equivalent circuit parameters of Sn NPs electrode and Sn-CNT NCs electrode.

Sample R1(�) R2(�) R3(�) CPE1(F) IF(mA cm−2) D0(cm2 s−1)

Sn 24.6 540.8 - - 0.03 9.5E-14Sn/C 9.2 174.9 172.8 3. 4E-5 0.09 2.2E-12after 1st cycle

of Sn/C10.2 - 15. 6 - 1.06 1.1E-11

after 8th cycleof Sn/C

8.9 - 79.5 - 0.21 2.5E-12

after 14th cycleof Sn/C

10.1 - 118.9 - 0.14 3.5E-13

tSc

3

eslietrieaCi

rtibad

layer, ω is the angular frequency. Deriving from the fitting resultsin Table 1, R and C of Sn-CNT NCs electrode are 174.9 � and

he cathodic process. Therefore, for the Sn-CNTs electrode, bothn and CNTs participate in the lithiation/delithiation process andontribute to the lithium ions storage capacity.

.3. EIS measurements of Sn(C) electrode

In order to study on the interfacial properties of both Sn NPslectrode and Sn-CNT NCs electrode, the EIS are measured on bothamples under the same conditions. The corresponding equiva-ent circuit modeling and fitting plots are also shown in Fig. 7. Its seen that the fitting plots are well consistent with the EIS of theirlectrodes, respectively. Among them, the EIS of Sn NPs anode elec-rode is composed of a depressed semicircle in the high frequencyange (reflected at Sn/electrolyte interface (S2)) and the Warburgmpedance at the low frequencies [27,28]. The EIS of Sn-CNT NCslectrode is composed of two depressed semicircles in the highnd middle frequency ranges (reflected at S2 and the interface ofNT/electrolyte (S1), respectively) [29] and the Warburg impedance

n the low frequency range.The fitting data of the EIS, including the electrolyte contact

esistance (R1, Interface between electrolyte and electrode), the Li+

ransfer resistance at S2 (R2, Solid state diffusion resistance of Li ionsn electrode), the charge transfer resistance in S1 (R3, the interfaceetween carbon layer and electrolyte), Faradic current density (IF)

nd the diffusion coefficient (D0) of Li+, are recorded in Table 1. Theiffusion coefficient D0of Li+ diffused in Sn particles and Faradic

trode during the discharging and charging process.

current density IF of CNT/electrolyte interface can be calculatedfrom Eq. (5) and (6) [27]:

D0 = 12

×(

RT

AF2�wCS

)(5)

IF = RT/nFR2 (6)

where R is the gas constant (8.314 Jmol−1 K−1), T is the room tem-perature (298 K), A is the area of the electrode nominal surface areaand F is the Faraday constant (9.6485 × 104 C mol−1). �w is the War-burg coefficient, Cs is the molar concentration of Li+ and n is thenumber of electrons transferred per molecule during the intercala-tion. Comparing the data in the table, we find that the Sn-CNT NCselectrode has much smaller R2 value, nearly one-third of the Sn NPselectrode. The oxide layer on the Sn NPs surface greatly increasesthe Li-ion transfer resistance at S2, while the Li+ diffusion ability ofthe Sn-CNT NCs is fascinated by the less-passivated surface of the Sncore that is protected by the CNT shells. Further, large improvementof IF and D0 values in the Sn-CNT NCs electrode are attributed tothe enhanced conductivity [29] and a shorter diffusion path withinthe walls of the CNT shell [18].

Having the character of barrier-layer diffusion in the case ofplanar electrode, the diffusion impedance (Zd) of the Sn-CNT NCselectrode can be written as Eq. (7) [30].

Zd = R + 1jωC

= Z0F � |IF |

nFCS

√D0

(l

3√

D0

− j

√D0

ωl

)(7)

R = Z0F � |IF |l

3nFCSD0(8)

C = nFCSl

Z0F � |IF | (9)

where R and C are the equivalent resistance and capacitance,respectively, Z0

F is the Faraday impedance out of consideringdiffusion impedance, � is the reaction series of the reactants inthe electrode, l is the effective thickness of the diffusion barrier

3.3754 × 10−5 F, when the diffusion mode of S1 is considered. Ifmultiplying Eq. (8) and (9), l can be determined by R and C, that

C.-j. Liu et al. / Electrochimica Acta 144 (2014) 376–382 381

0 50 0 100 0 150 0 200 0

-Z'' /

Ohm

Z' / Ohm

0 10 0 20 0 30 0 40 0 50 0 60 0 70 0 80 0

0

200

400

600

800

1000

-Z''

/ Ohm

Z' / O hm

Fig. 7. Nyquist plots of (a) Sn NPs anode electrode and (b) Sn-CNT NCs anode electrode and their equivalent circuit fitting and the corresponding modelings. R1, the electrolytecontact resistance (including the bulk resistance in the electrolyte, separator, and electrodtransfer resistance in the carbon/electrolyte interface (S1). W1, the Warburg charge diffuspace charge capacitance in the Sn/electrolyte interface. CPE2, the space charge capacitan

Fig. 8. Nyquist plots and the fits for: Cycling after 1st, 8th and 14th cycles of theSn-CNT NCs electrodes at the 2 V ((a) and (b) is the detail view). R4, the SEI resis-tance, CPE3, the capacitance of the electrode/electrolyte interface (S3) for Sn-CNTNCs electrode.

e). R2, the Li+ transfer resistance in the Sn/electrolyte interface (S2). R3, the chargesion processes of lithium ions in the Sn electrode or Sn-CNTs electrode. CPE1, thece in carbon/electrolyte interface.

is, l =√

3RCD0 = 1.967nm. As it is described, the thickness of theCNT wall is about 2.38 nm (∼7 graphene layers). Hence we canconclude that the Li+ diffusion at S2 is limited to happen withinthe thickness of CNT walls.

Fig. 8 shows the Nyquist plots and fits of the Sn-CNT NCs elec-trode after 1st, 8th and 14th cycles ((a) and (b)), with the inset ofcorresponding equivalent circuit modeling. The parameters fromthe equivalent circuit are also listed in Table 1. The first depressedsemicircle in the high frequency, as dot-circled in the inset ofFig. 8(b), is caused by the presence of SEI [29] that appears at elec-trode/electrolyte interface (S3) [31]. After the first cycle, the chargetransfer resistance R3 drops from 172.8 � to 15.6 � rapidly due tothe better contact between the carbon layers and electrolyte [32].Then it increases to 118.9 � after the 14th cycle, due to the capac-ity fading during the cycling process [33]. The diffusion coefficientand the corresponding exchange current density at CNT/electrolyteinterface have the contrary trend according to the Eqs. (5) and (6),i.e., increase from the open circuit to the 1st cycle, and then reducegradually. All the data variations commonly hint the physical evo-lution in the electrode, i.e., fully infiltration of the active materials inthe electrolyte after the first cycle, well-building of the conductivenetworks through the CNT shells, and the enhanced charge transferbetween CNT/electrolyte interfaces.

4. Conclusions

As a new strategy toward the large-volume-change issue ofhigh-capacity anode materials, we displayed a novel structureof CNT-coated Sn nanocapsules fabricated by the arc-dischargemethod. Nanocrystallized Sn partially-filled into the CNTs. Largeroom inside of the CNTs remains for the volume expansion of Snduring the Li-ion insertion. CNT shells can bond Sn and also provideconductive network for the electron delivery between the metalelectrode and the Sn surface. Due to the specific nanostructure, theSn-CNT NCs anode displays a high initial specific capacity, near toits theoretic capacity. CNTs demonstrated positive effects on thebuilding of new interfaces, enhanced charge transfer, and Li-iondiffusion.

Acknowledgments

We acknowledge financial support by the National Key BasicResearch and Development Program (Grant No. 2011CB936002),

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