Enhanced Electrochemical Stability of Sn-Carbon Nanotube Nanocapsules as Lithium-Ion Battery Anode

Accepted Manuscript Title: Enhanced Electrochemical Stability of Sn-Carbon Nanotube Nanocapsules as Lithium-Ion Battery Anode Author: Chun-jing Liu Hao Huang Guo-zhong Cao Fang-hong Xue Ramon Alberto Paredes Camacho Xing-long Dong PII: DOI: Reference:

S0013-4686(14)01472-8 http://dx.doi.org/doi:10.1016/j.electacta.2014.07.068 EA 23106

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

7-5-2014 3-7-2014 16-7-2014

Please cite this article as: C.-j. Liu, H. Huang, G.-z. Cao, F.-h. Xue, R.A.P. Camacho, X.-l. Dong, Enhanced Electrochemical Stability of Sn-Carbon Nanotube Nanocapsules as Lithium-Ion Battery Anode, Electrochimica Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.07.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced Electrochemical Stability of Sn-Carbon Nanotube Nanocapsules as Lithium-Ion Battery Anode Chun-jing Liu1, Hao Huang1,*, Guo-zhong Cao1,2, Fang-hong Xue1, Ramon Alberto Paredes Camacho1,

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Xing-long Dong1,* School of Materials Science and Engineering, Dalian University of Technology，Dalian 116023，People’s Republic of China

2

Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, United States

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ABSTRACT

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Direct current (DC)arc-discharge method is used to fabricate Sn-carbon nanotube nanocapsules (Sn-CNT NCs). The Sn is partially-filled into multi-walled CNTs, as an ideal configuration for the active materials as lithium-ion

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battery anode; Sn nanoparticles provide large storage capacity and CNTs confine and accommodate the volume expansion of Sn as well as provide the conductive network and contribute their own capacity. Large initial specific

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capacity and stable cyclic performance are identified in the electrochemical tests. Such novel nanostructure provides

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a solution to the volume expansion issue of a high-capacity electrode.

* Corresponding author: Tel.: +86 411 84706130. Fax: +86 411 84709284. E-mail addresses: [email protected] (H. Huang), [email protected] (X.L. Dong).

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1.

1. Introduction Lithium-ion batteries (LIBs) have been attracting considerable attention because of their wide applications as the

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main power source in portable electronic products and electric vehicles [1]. One of the critical factors to obtain a

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high-performance LIB lies on the anode materials that can work with the desirable properties of appropriate

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operating voltage, large specific capacity and structural stability. Among all the candidates, carbon has been utilized as the commercial anode material due to its low cost and good cyclic stability. However, the low theoretical and

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practical capacity retains its limitation for high-power requirements [2-4]. In recent works, some active metals (e.g., Si [5, 6], Al [7] and Sn [8,9]) are set as the negative electrode materials and find owning high specific capacities

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(4200 mAh g-1, 2234 mAh g-1 and 993 mAh g-1, respectively) through forming alloys with lithium. Sn, therein the

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metals, has been considered as one of the most promising substitutes for the carbon due to its electric conductivity and chemical stability. However, the severe expansion up to 300% volume change [2,10] that Sn suffers during the

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Li-ion insertion/desertion process results in deteriorative structures and poor cycling performances [11,12].

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To avoid the degradation of Sn electrodes, one active approach is to create free space for the volume expansion by integrating ultra-small size Sn components into carbon structures, i.e., Sn-C nanocomposite. Several positive features can be expected: (1) the ultra-small size of Sn nanoparticles; small particle size can significantly reduce the strain and improve the diffusion ability of Li ions in the electrode materials; (2) the carbon matrix; the flexible carbon frame will release the strain from the Sn nanoparticles and accommodate the excessive volume change. Further, it can prevent Sn nanoparticles from heavy aggregation over cyclic discharging/charging; (3) both Sn and C can independently store Li ions, and thus provide high specific capacity. In such a context, the Sn-C composites with various hybrid morphologies have been achieved by different preparation methods, such as aerosol spray pyrolysis

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[9], chemical vapor deposition [13], ball milling [14], hydrothermal reaction [15], electrochemical deposition [16], and etc.. However, it remains a significant challenge to obtain highly uniform Sn-C nanocomposites with controlled structure and purity.

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In this work, we report the fabrication of Sn-CNT nanocapsules (NCs) as a LIB anode by means of DC arc-discharge method. The synthesized nanosized products have a uniform morphology of rod-like Sn nanograins

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partially-filled in the CNTs. In this hybrid nanostructure, CNTs are expected to bond the Sn grains and also provide

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adequate room for the volume expansion. Moreover, the well-crystallized walls of the CNTs help to transfer the electrons between the electrode and Sn during Li ions insertion/extraction process. A typical kinetic mode of

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barrier-layer diffusion is further observed in the electrochemical impedance measurement of Sn-CNT NCs electrode,

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which forms the positive effects of diffusion buffering and stabilization of electrode reactions.

2.1. Synthesis of Sn-CNT NCs

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2. Experimental

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The schematic diagram of the modified arc-discharge equipment is shown in Fig.1 [17]. To begin with,

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micron-sized Sn bulk (99.99% purity) was set as the anode on a copper stage and carbon rod served as the cathode. After the chamber was evacuated to 10-2 Pa, a mixture gas of argon (0.03 MPa) and methane (0.01 MPa) was introduced in as the working atmosphere. The methane was usually used as the carbon source. Later, the arc discharge was ignited and the Sn bulk was evaporated for 10 minutes at a steady current of 90 A. At last, after being passivated for 12 hrs., the Sn-CNT NCs were collected and removed from the water-cooled chamber wall. As the counterpart, the Sn nanoparticles (NPs) were prepared in the same routine, however with a tungsten cathode and in an atmosphere of argon (0.03 MPa) and hydrogen (0.01 MPa).

2.2. Materials characterizations The microstructure and morphology were determined by transmission electron microscopy (TEM, Tecnai220 3

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S-TWIM). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an ESCALAB 250Xi spectrometer, using a focused and monochromatized Al Kα radiation (hv=1486.6 eV). The sample was placed in the analysis chamber with a pressure of less than 10-8 Pa. Carbon structure was characterized by Raman spectroscopy

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with the Red Light laser line at 632.8 nm (InVia). The content of carbon element was measured by Elemental Analyzer (vario EL
).

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2.3. Electrochemical Measurements

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The working electrodes consisted of Sn-CNT NCs (90 wt.%) and polyvinylidene fluoride (PVDF, 10 wt.%) dissolved in N-methyl pyrrolidinone (NMP), and these two components were mixed together thoroughly to form

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slurry, then coated onto copper foil substrates and dried at 120 ℃ under vacuum for 24 h. Electrochemical

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responses of the anodes were investigated directly using CR2025 coin-type half-cells assembled in an ultra-pure argon-filled glove box. Lithium metals and polypropylene film were used as counter anode and separator,

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respectively. The electrolyte was 1M LiPF6 dissolved in a mixture of ethylene carbonate/diethyl carbonate (1:1, vol.%). Electrochemical measurements were performed on a Land CT2001A test system. The cells were cycled

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between cutoff voltages of 0.01 and 2.00 V (vs. Li/Li+) at a constant current density of 100 mA g-1 at room

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temperature. Cyclic voltammetry (CV) was performed on a CHI660D electrochemical workstation in the voltage 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 a 100 KHz-0.01 Hz frequency range with a perturbation amplitude of 5 mV.

3. Results and discussion

3.1 Microstructure and Morphology of Sn-CNT NCs The high-resolution TEM images showing the morphologies of the Sn-CNT NCs and Sn NPs are presented in Fig. 2. The Sn NPs are in spherical shape with the mean diameter of 50-80 nm (Fig. 2(a)). In the close observation of a

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particle (Fig. 2(b)), it is found that Sn NP has a clear core/shell structure, i.e., the uniform thin tin-oxide shell encapsulating the Sn core. Such nanostructure of a metallic core and congenerous metal oxide shell exists commonly in pure metal NPs due to the experience of passivation after the arc-discharge process, it doesn’t change

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with the processing condition variation [17]. Fig. 2(c) presents a uniform structure of CNTs, with the average size of about 40 nm in diameter and 200-300 nm in length. The CNTs have multi walls of 5-7 graphene layers and are

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partially-filled with metal tin. The electron diffraction pattern of the as-synthesized Sn-CNT NCs is shown in the

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inset of Fig. 2(d), which exhibits a pattern of poly-crystalline rings assigning to the (200), (220), (301) planes of Sn. The results from the Elemental Analyzer suggest that the carbon weights ~17% of the Sn-CNT NCs.

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In order to identify the chemical composition, the XPS studies are carried out on the Ar-ion cleaned surface of the

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Sn-CNT NCs specimen. Fig. 2(e) and (f) illustrate the spectra for C and Sn elements respectively. The binding energy of 284.57 eV for C1s mainly corresponds to the carbon atoms (Fig. 2(e)). The peaks in Fig. 2(f) correspond

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to the Sn spectrum with different valance state. Peaks located at around 484.98 and 493.38 eV could be assigned to Sn 3d5/2 and Sn 3d3/2, respectively; peaks at 486.82 and 495.30 eV are assigned to Sn Oxide [18]. The XPS results

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the surface。

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demonstrate the existence of the core-shell structure in the Sn-CNT NCs nanoparticles, with the presence of SnO2 on

The structures of the carbon shells are analyzed by Raman spectra, as shown in Fig. 3. Two intensive peaks are denoted as D-band and G-band allocate at about 1326 cm-1 and 1593 cm-1, corresponding respectively to the A1g and E2g carbon vibration modes [11,13]. These two peaks are related to the sp2 electronic configuration containing an electron in the π orbital and dominate in the most Raman spectra of graphite materials with small crystallite size (or the so-called semicrystalline graphite). Strong D-peak, normally aroused by the presence of in-plane substitutional heteroatoms, vacancies, grain boundaries or other defects, indicates a large amount of disordered carbon existing in the carbon shells, as it is observed in Fig. 2(d). The G-band is typical for graphite and carbon blacks, originated from 5

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the stretching vibration of any C pairs of sp2 sites. In amorphous carbonaceous materials, the large and diffuse D and G bands located at 1200 and 1550 cm-1 correspond to broaden and overlapped sp2 and sp3 bonded carbon [11]. The graphite grain size in the expression of the in-plane correlation length, La, can be determined by the Tuinstra-Koenig

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equation, La=cλ (ID/IG)-1, where cλ is about 4.4 nm [19,20]. ID/IG can be obtained from a Lorentzian function fitting with the Raman curve and the corresponding La is calculated as 3.10 nm in this case. In comparison with our

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previous work (the La is 4.73) [21], this value implied that the CNTs were immature, mainly due to the gas

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evaporation process was too short to form well-developed CNTs by the DC arc method.

3.2 Electrochemical performance of Sn-CNT NCs electrode materials

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In order to investigate the cycling performance of the pure Sn NPs and Sn-CNT NCs anode electrode, typical

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sloping voltage profiles of Sn NPs and Sn-CNT NCs electrode are obtained in the potential range of 0.01 V to 2.00 V and at a constant current density of 100 mA g-1 (0.1C), as shown in Fig. 4(a) and (b). In the case of Sn-CNT NCs

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electrode, the 1st discharge and charge curves own coupled plane voltage regions at 0.80 V, 0.51 V/0.80 V and 0.32 V/0.62 V, attributed to the Faradaic response of lithiation/delithiation reactions [14]. The first voltage region at 0.80

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V that is absent in the 2nd discharge curve, corresponds to the formation of the solid electrolyte interface (SEI) [14].

(

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The initial specific capacity of Sn NPs anode electrodes is 935 mAh g-1, close to the theoretical capacity of Sn =993 mAh g-1) [9]. However, it decreases rapidly to 380 mAh g-1 in the 2nd discharge and fully fails at the 4th

cycle, as shown in Fig. 4(c). The Sn-CNT NCs electrode has the initial specific capacity of 850 mAh g-1. Although having less initial capacity than Sn NPs, the Sn-CNT NCs electrode only experienced a slight decrease of capacity within the first 3 cycles, and then became stable at about 600 mAh g-1. The Coulombic efficiency, a ratio 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 CNTs synthesized by arc-discharge method can maintain its reversible capacity to 165 mAh g-1 after 10 cycles [22]. With known carbon content (~17 wt.% CNT) in 6

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Sn-CNT, the total capacity (536 mAh g-1) of the electrode, and the pure CNT’s capacity (165 mAh g-1, reference 22), the capacity of Sn in Sn-CNT NCs electrode was calculated to be 612 mAh g-1 after 10 cycles. This result clearly indicates the capacity of Sn component can be well maintained with the protection from CNT, however, the capacity

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of Sn rapidly decayed to 9 mAh g-1 after 10 cycles without the protection of CNT. The excellent retention of capacity is attributed to the well-tailored structure of the Sn-CNT NCs, which has enough room to buffer the volume

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expansion of Sn during the insertion of Li ions.

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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 anode electrode. The different peaks in the curves are the indications of phase changes during the

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redox reaction with the Li opposite. A visible reduction peak at around 0.70-0.93 V (peak S) in the first cathodic

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scan disappears in the following cycles, which is accredited to the decomposition of electrolyte and formation of SEI film [14]. The peaks D, E in the voltage range of 0.18-0.25 V vs. Li/Li+ are associated with the reaction between Li+

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and C, which is written in Eq. (1) [23]. The coupled peaks in the higher voltage ranges of A (0.82-0.89 V)/A’ (0.53-0.60 V), B (0.70-0.79V) and C (0.53-0.60 V)/C’ (0.23-0.26 V) correspond to the stepwise redox reactions

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between Li-Sn, respectively shown in the Eqs. (2-4) [24,13-15].

Thus, the atomic mass ratio of Li in Li-Sn alloy phase (from Li2Sn5 to Li22Sn5) increases from 2.29% to 20.46% during the lithium intercalation process. After the first cycle, the lithiation peaks (Peaks A’ and C’ ) of Sn enhance greatly, which implies more amount of Sn involved in the reaction as the Li-ion active matter. Two delithium carbon peaks（Peaks D and E）emerge in the second anode scan, corresponding to the oxidation reaction of C with Li ions. 7

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However, the reduction peaks of carbon intercalation cannot be located in the CV curves, mainly due to the potential range (0.0～0.2V [25]) overlapping with that of the lithium-ion insertion of LiSn. The overall enhancement of lithium peaks are comprehensively denoted by the reasons as the fully infiltration of the active materials in the

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electrolyte, well-building of the conductive networks by the CNT shells, and the more expression of the electronic loss through the electrode in the electrochemical reactions.

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Fig. 6 shows the mechanism diagram of the anode electrode in the process of discharging and charging. Actually

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in the preparation of the electrode, we did not add any conductive agent, e.g., carbon black, considering the presence of the conductive shells of the Sn-CNT NCs. The carbon walls, hence, become the only path for the electron

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transport. In addition, the interspace between the carbon layers is ~0.340 nm, which is much larger than the diameter

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of the lithium ion (0.152 nm). Such large space between adjacent layers would allow the lithium ions infiltrate easily through the carbon walls and accessible to Sn during the insertion and extraction process. Large proportion of

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defects in CNTs, as observed in the HR-TEM and Raman results, will further fascinate the diffusion of the Li ions. During the cathodic (lithiation) process, metal Sn and Li+ form Li2Sn5 and Li22Sn5 phases at the potentials of 0.56 V

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and 0.25 V, respectively. The electrons are transferred through the CNT walls and get to the Sn core/CNT shells

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interface. The incorporation of Li+ leads to the formation of Sn-Li alloy, however accompanied with a huge increase of volume [8,26]. When the potential reduces to the intercalation potential of carbon (0.0～0.2 V [25]), LiC6 is also formed to accommodate extra lithium ions, also contributing to capacity. The anodic process (delithiation) presents a reverse of the cathodic process. Therefore, for the Sn-CNTs electrode, both Sn and CNTs participate in the lithiation/delithiation process and contribute to the lithium ions storage capacity.

3.3 EIS measurements of Sn(C) electrode In order to study on the interfacial properties of both Sn NPs electrode and Sn-CNT NCs electrode, the EIS are measured on both samples under the same conditions. The corresponding equivalent circuit modeling and fitting 8

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plots are also shown in Fig. 7. It is seen that the fitting plots are well consistent with the EIS of their electrodes, respectively. Among them, the EIS of Sn NPs anode electrode is composed of a depressed semicircle in the high frequency range (reflected at Sn/electrolyte interface (S2)) and the Warburg impedance at the low frequencies [27,

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28]. The EIS of Sn-CNT NCs electrode is composed of two depressed semicircles in the high and middle frequency ranges (reflected at S2 and the interface of CNT/electrolyte (S1), respectively) [29] and the Warburg impedance in the

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low frequency range.

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The fitting data of the EIS, including the electrolyte contact resistance (R1, Interface between electrolyte and electrode), the Li+ transfer resistance at S2 (R2 , Solid state diffusion resistance of Li ions in electrode), the charge

and Faradic current density

of CNT/electrolyte interface can be calculated from Eq. (5) and (6) [27] :

(5)

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(6)

is the gas constant (8.314 Jmol-1 K-1),

is the room temperature (298 K),

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where

nominal surface area and

of Li+ diffused in Sn particles

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) of Li+, are recorded in Table 1. The diffusion coefficient

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diffusion coefficient (

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transfer resistance in S1 (R3, the interface between carbon layer and electrolyte), Faradic current density ( ) and the

is the Faraday constant (9.6485×104 C mol-1).

molar concentration of Li+ and

is the area of the electrode

is the Warburg coefficient,

is the

is the number of electrons transferred per molecule during the intercalation.

Comparing the data in the table, we find that the Sn-CNT NCs electrode has much smaller R2 value, nearly one-third of the Sn NPs electrode. The oxide layer on the Sn NPs surface greatly increases the Li-ion transfer resistance at S2, while the Li+ diffusion ability of the Sn-CNT NCs is fascinated by the less-passivated surface of the Sn core that is protected by the CNT shells. Further, large improvement of IF and

values in the Sn-CNT NCs electrode are

attributed to the enhanced conductivity [29] and a shorter diffusion path within the walls of the CNT shell [18]. Having the character of barrier-layer diffusion in the case of planar electrode, the diffusion impedance (

) of the

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Sn-CNT NCs electrode can be written as Eq.(7) [30].

(7)

(9)

are the equivalent resistance and capacitance, respectively,

is the reaction series of the reactants in the electrode,

thickness of the diffusion barrier layer, and

is the Faraday impedance out of

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considering diffusion impedance,

is the effective

is the angular frequency. Deriving from the fitting results in Table 1,

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and

of Sn-CNT NCs electrode are 174.9 Ω and 3.3754×10-5 F, when the diffusion mode of S1 is considered. If

multiplying Eq. (8) and (9),

can be determined by

and

, that is,

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where

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(8)

. As it is described,

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the thickness of the CNT wall is about 2.38 nm (~7 graphene layers). Hence we can conclude that the Li+ diffusion at S2 is limited to happen within the thickness of CNT walls.

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Fig. 8 shows the Nyquist plots and fits of the Sn-CNT NCs electrode after 1st, 8th and 14th cycles ((a) and (b)),

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with the inset of corresponding equivalent circuit modeling. The parameters from the equivalent circuit are also listed in Table 1. The first depressed semicircle in the high frequency, as dot-circled in the inset of Fig. 8(b), is caused by the presence of SEI [29] that appears at electrode/electrolyte interface (S3) [31]. After the first cycle, the charge transfer resistance

drops from 172.8 Ω to 15.6 Ω rapidly due to the better contact between the carbon

layers and electrolyte [32]. Then it increases to 118.9 Ω after the 14th cycle, due to the capacity fading during the cycling process [33]. The diffusion coefficient and the corresponding exchange current density at CNT/electrolyte interface have the contrary trend according to the Eqs. (5) and (6), i.e., increase from the open circuit to the 1st cycle, and then reduce gradually. All the data variations commonly hint the physical evolution in the electrode, i.e., fully 10

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infiltration of the active materials in the electrolyte after the first cycle, well-building of the conductive networks through the CNT shells, and the enhanced charge transfer between CNT/electrolyte interfaces.

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4. Conclusions

As a new strategy toward the large-volume-change issue of high-capacity anode materials, we displayed a novel

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structure of CNT-coated Sn nanocapsules fabricated by the arc-discharge method. Nanocrystallized Sn

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partially-filled into the CNTs. Large room inside of the CNTs remains for the volume expansion of Sn during the Li-ion insertion. CNT shells can bond Sn and also provide conductive network for the electron delivery between the

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metal electrode and the Sn surface. Due to the specific nanostructure, the Sn-CNT NCs anode displays a high initial

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specific capacity, near to its theoretic capacity. CNTs demonstrated positive effects on the building of new interfaces, enhanced charge transfer, and Li-ion diffusion.

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ACKNOWLEDGMENTS

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We acknowledge financial support by the National Key Basic Research and Development Program (Grant No. 2011CB936002), the National Natural Science Foundation of China (Grant No. 51171033, 11004019, 51271044),

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and the Fundamental Research Funds for the Central Universities (2012DUT12RC(3)101).

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nanocrystals for high-rate lithium-ion battery, Nano lett. 13 (2013) 5289. [29] M.S. Kim, D. Bhattacharjya, B.Z. Fang, D.S. Yang, T.S. Bae, J.S. Yu, Morphology-dependent Li storage

pt

performance of ordered mesoporous carbon as anode material, Langmuir 29 (2013) 6754.

Ac ce

[30] C.N. Cao, J.Q. Zhang, An introduction to electrochemical impedance spectroscopy, Beijing, 2002, pp. 95-98. [31] K.F. Chen, D.F. Xue, Room-temperature chemical transformation route to CuO nanowires toward high-performance electrode materials, J. Phys. Chem. C 117 (2013) 22576. [32] Y. Xia, Z. Xiao, X. Dou, H. Huang, X.H. Lu, R.J. Yan, Y.P. Gan, W.J. Zhu, J.P. Tu, W.K. Zhang, X.Y. Tao, Green and facile fabrication of hollow porous MnO/C microspheres from microalgaes for lithium-ion batteries, ACS Nano 7 (2013) 7083. [33] Q.S. Xie, F. Li, H.Z. Guo, L.S. Wang, Y.Z. Chen, G.H. Yue, D.L. Peng, Template-free synthesis of amorphous double-shelled zinc−cobalt citrate hollow microspheres and their transformation to crystalline ZnCo2O4 14

Page 14 of 25

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pt

ed

M

an

us

cr

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microspheres, ACS Appl. Mater. Interfaces 5 (2013) 5508.

15

Page 15 of 25

Table(s)

Table 1 Equivalent circuit parameters of Sn NPs electrode and Sn-CNT NCs electrode. (mA cm-2 )

（cm2 s -1 ）

R1 （Ω）

R2

R3 （Ω）

CPE1(F)

Sn

24.6

540.8

-

-

0.03

9.5E-14

Sn/C

9.2

174.9

172.8

3. 4E-5

0.09

2.2E-12

after 1st cycle

10.2

-

15. 6

-

1.06

after 8th cycle

8.9

-

79.5

-

0.21

after 14th cycle

10.1

-

118.9

-

ip t

Sample

1.1E-11

cr

2.5E-12 3.5E-13

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pt

ed

M

an

us

0.14

Page 16 of 25

us

cr

ip t

Figure(s)

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pt

ed

M

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

Page 17 of 25

M

an

us

cr

ip t

Figure(s)

ed

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

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

Page 18 of 25

Figure(s)

D

CNTs

1500

ID/IG=0.93

2000 -1

2500

an

1000

ID/IG=1.42

us

Sn-CNT NCs

cr

ip t

Intensity / a.u.

G

Raman Shift / cm

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pt

ed

M

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

Page 19 of 25

Figure(s)

(a)

Voltage vs. (Li/Li+) / V

2.0

1.6

Sn NPs st

nd

discharge

nd

charge

nd

charge

2 1 0.8

2

ip t

1 discharge

1.2

0.4

0.0 200

400

600

800

Capacity / mAh g

1000

cr

0

-1

(b)

us

1.6

an

Sn-CNT NCs 1.2

st 1 discharge nd 2 discharge nd 1 charge nd 2 charge

0.8

M

Voltage vs. (Li/Li+)/ V

2.0

0.4

0.0 200

400

600

ed

0

800

1000

-1

Capacity / mAh g

1400

(c) 100

Sn-CNT NCs

pt 800

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-1

Capacity / mAh g

80

1000

60

Sn-CNT NCs

600

40

400

200

20

Sn NPs

Coulombic Efficiency (%)

1200

0

0

1

0 2

3

4

5

6

7

8

9

10

11

Cycle number

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. -1

The measurement was carried out at a current density of 100 mA g .

Page 20 of 25

Figure(s)

0.0015

B

A: 0.82-0.89 V B: 0.70-0.79 V C: 0.53-0.60 V D: 0.25 V E: 0.18 V

A 0.0010

C ED

0.0000

st

1 cycle

-0.0005

S

S : 0.70-0.93 V A': 0.53-0.60 V C': 0.23-0.26 V

A'

-0.0010 2

nd

cycle

ip t

Current/A

0.0005

rd

3 cycle

-0.0015

C' 0.0

0.5

1.0

1.5

Potential/V

cr

-0.0020 2.0

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

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pt

ed

M

an

us

rd

Page 21 of 25

us

cr

ip t

Figure(s)

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M

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Fig. 6 The mechanism figures of Sn-CNT NCs anode electrode during the discharging and charging process.

Page 22 of 25

Figure(s)

(b)

(a) 1000

1600

Equivalent Circuit Modeling of Sn-CNT NCs

Equivalent Circuit Modeling of Sn NPs 800

CPE 1

R1

R2

CPE 1

W1

-Z'' / Ohm

-Z'' / Ohm

600 800 initial curve fit curve

400

R2

R3

S2

S1

200

0 500

1000

1500

2000

0

100

200

300

400

500

600

700

800

cr

0

W1

initial curve fit curve

400

0

CPE 2

ip t

R1

1200

Z' / Ohm

Z' / Ohm

circuit fitting and the corresponding modelings.

, the electrolyte contact resistance (including the bulk , the Li+ transfer resistance in the Sn/electrolyte

an

resistance in the electrolyte, separator, and electrode). interface (S2).

us

Fig. 7 Nyquist plots of (a) Sn NPs anode electrode and (b) Sn-CNT NCs anode electrode and their equivalent

, the charge transfer resistance in the carbon/electrolyte interface (S1).

charge diffusion processes of lithium ions in the Sn electrode or Sn-CNTs electrode.

, the space charge

M

, the space charge capacitance in carbon/electrolyte interface.

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pt

ed

capacitance in the Sn/electrolyte interface.

, the Warburg

Page 23 of 25

Figure(s)

1000

(a) Equivalent Circuit Modeling of Sn-CNT NGs after cycles R1

800

CPE3

CPE1

R4

R3

CPE2 R2

W1

th

14 cycle SEI interface Interface Interface between between Sn/electrolyte Sn/carbon

400 st

th

1 cycle

8 cycle

ip t

-Z'' / Ohm

600

fitting curves

200 

0

200

400

600

cr

0 800

1000

Z' /Ohm 250

(b)

-Z'' / Ohm

10

5

st 1

0

10

100

20

30

Z' / Ohm

0 0

ed

50

50

100

150

th

8

40

M

-Z'' / Ohm

150

The first semicircle in high frequency

an

200

us

15

th

14

200

250

Z' / Ohm

pt

Fig. 8 Nyquist plots and the fits for: Cycling after 1st, 8th and 14th cycles of the Sn-CNT NCs

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electrodes at the 2V ((a) and (b) is the detail view).

, the SEI resistance, CPE3, the

capacitance of the electrode/ electrolyte interface (S3) for Sn-CNT NCs electrode.

Page 24 of 25

Research Highlights

A simple arc-discharge approach for the synthesis of Sn-carbon nanotube nanocapsules, Sn nanorods semi-filled into the multi-walled CNTs are presented. Provides a conductive network and a solution to the volume expansion issue of a high-capacity

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electrode. According to the following figure. Charging and discharging mechanism of lithium ions were

Ac

ce pt

ed

M

an

us

cr

reflected in experiment and theory analysis.

Page 25 of 25