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MWCNT nanocomposite anodes by pulse electrodeposition for Li-ion batteries
Applied Surface Science 290 (2014) 6–12
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Production of Sn/MWCNT nanocomposite anodes by pulse electrodeposition for Li-ion batteries Mehmet Uysal ∗ , Tugrul Cetinkaya, Ahmet Alp, Hatem Akbulut Sakarya University Engineering Faculty, Department of Metallurgical & Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
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Article history: Received 20 June 2013 Received in revised form 18 October 2013 Accepted 28 October 2013 Available online 5 November 2013 Keywords: Electro co-deposition Sn/MWCNTs nanocomposite Pulse electrodeposition Electrochemical behaviour Li-ion batteries
a b s t r a c t A tin/multi-walled carbon nanotube (Sn/MWCNT) composite was prepared by ultrasonic-pulse electrodeposition on a copper substrate in a chloride bath containing different concentrations of multi-walled carbon nanotubes. The morphology and structure of the Sn/MWCNTs composites were characterised by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectrometry. The electrochemical performance of Sn/MWCNTs nanocomposites has been investigated by charge/discharge tests, cyclic voltammetric experiments and the ac impedance technique. The additive amount of MWCNTs was shown to be a vital factor to enhance Sn/MWCNTs composite anodes for cyclability and discharge capacity. Volume changes and morphological changes in Sn can be reduced by the addition of a MWCNTs that has sufficient flexibility to act as a buffer. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion secondary batteries have the highest energy density in commercialised batteries, and the demand for these batteries as a high-power source has been steadily increasing due to the advancement of portable electronic devices [1–5]. The most common anode material used in Li-ion batteries is graphite due to its low cost, availability and durability [5–7]. In graphitic anodes, the Li+ insertion mechanism corresponds to the reversible, progressive intercalation of Li+ ions between graphene layers to reach a theoretical capacity of 372 mAh g−1 if LiC6 is formed, compared with a practical capacity of 350 mAh g−1 [6,7]. Alternatively, Sn has received much attention as an anode material for lithium-ion batteries due to its low voltage platform and large theoretical specific capacity (Li4.4 Sn, 992 mAh g−1 ) [8,9]. The theoretical capacity of pure tin is 994 mAh g−1 , which is three times that of the graphite anode (372 mAh g−1 ), based on the end lithiated phase Li4.4 Sn. Crystallographic studies suggest that the realistic form of this end phase could be Li17 Sn4 (thus, 4.25 Li per Sn). Therefore, its maximum gravimetric capacity could be 959.5 mAh g−1 , which is still much higher than most common graphite anodes [10]. However, a pure Sn electrode suffers severely from its poor cycle performance due to its abrupt volume expansion (up to approximately 300%) during the charge/discharge process [10]. This morphological
∗ Corresponding author. Tel.: +90 264 295 57 95; fax: +90 264 295 56 01. E-mail address: [email protected] (M. Uysal). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.162
change results in the cracking and crumbling of the electrode with the concomitant loss of electrical contact between interparticles, particles and the current collector, which results in poor cycling performance [1,11]. To overcome this problem, we attempted to fabricate Sn electrodes by co-electrodeposition with MWCNTs, which are active in the potential and act as a buffer zone for preventing the crumbling of the Sn electrodes [12,13]. The composition of the Sn/MWCNT composite electrodes fabricated by pulse coelectrodeposition is affected by controlling the particle size, the MWCNTs concentration, the stirring speed, and the current density. Recently, electrodeposited Sn-based thin films [11–13] were extensively evaluated regarding their ability to overcome the above problems during the charge–discharge reaction. The pulse electrodepositing method is simpler than the currently used powder preparation method because there is no need for any binder or conductive additives [1]. Numerous studies on the DC electrodeposition of tin and tin-alloys have been performed. However, very few studies have focused on the pulse electrodeposition of Sn/MWCNT composite coatings. In this study, to improve the electrochemical property of a Sn electrode, we prepared the Sn/MWCNT film electrode by a pulse electrodeposition method, which is effective for improving the adhesion strength between the active material and the substrate. The main aim in introducing MWCNTs into the Sn layer is to increase electrode conductivity and accommodate the mechanical stress caused by a large-volume expansion during cycling. We also propose a low-cost and flexible process using the simple electrodeposition method, equipped with pulse capability.
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Table 1 Overview of pulse electrodeposition parameters for preparation of Sn/MWCNTs coatings. Bath composition SnCl2 ·2H2 O K4 P2 O7 Gelatin
25 g/L 170 g/L 0.5 g/L
Pulse electrodeposition parameters Peak current density pH Duty cycle % Frequency MWCNTs concentrations Plating time Temperature
20 mA/cm2 8.0–9.0 25% 100 Hz 1.0, 2.0, 5.0 g/L 3 min 25–30 ◦ C Fig. 1. Schematic diagram of the wave shape of pulse deposition.
2. Experimental procedure 2.1. MWCNTs functionalisation The MWCNTs are quasi-one-dimensional nanomaterials that are primarily available in bundled form because of strong van der Waals interactions between the tube walls [14]. To obtain successful deposition of the carbon nanotubes, MWCNTs should be uniformly dispersed in the plating solution, and the suspension must be stable. One of the major issues with the fabrication of MWCNT-reinforced composite coatings is related to the nonuniform dispersion of MWCNTs in the composite matrices because the MWCNTs easily coalesced in the aqueous solution due to their high surface energy. In this investigation, surface treatment of MWCNTs was performed using a nitric acid/sulphuric acid solution to improve the dispersion of the MWCNTs into the electrodeposited Sn/MWCNT coating. The acid treatment not only removes most of the metal, but also produces carboxyl, aldehyde, and other oxygen containing functional groups [15] on the surface of the MWCNTs that help in the uniform dispersion of MWCNTs without any additional dispersing additives. In the present investigation, 2 g of MWCNTs was added into a 2:1 solution of nitric acid to sulphuric acid in a glass bottle. The mixture was subsequently magnetic stirred at 100 ◦ C for 1 h. Then, the MWCNTs were further collected on a 0.2 m filter, rinsed with distilled water, and dried at 100 ◦ C for 4 h. Next, the obtained MWCNTs were dispersed in the electrolyte without addition of any dispersing agent. 2.2. Pulse electrodeposition of Sn/MWCNT composite coatings Sn/MWCNT composite coatings were deposited from a chloride bath onto the copper foils. All of the chemicals were of analytical reagent grade, and analytic/chemical standards were used for the preparation of the bath solutions. The components as well as some experimental conditions of the optimised Sn/MWCNT composite electrodeposition bath are summarised in Table 1. The multi-walled CNT were supplied by Arry Nano and had a diameter of 50–60 nm and a length of 10 m. Functionalised MWCNTs were added to the electrolytes, containing 0 g/L, 1 g/L, 2 g/L and 5 g/L MWCNTs, and sonication was applied during the composite coating and the mixing of the electrolytes. A plating cell containing 250 ml of the solution was immersed in a thermostatically controlled largevolume water bath that was kept at a room temperature. The pretreated MWCNTs were dispersed in the electrolyte via the aid of ultrasonic agitation. A high-purity (99.99%) electrolytic tin sheet was used as the soluble anode. The copper substrates were mechanically polished using a sequence of 400, 600 and 1200 mesh emery papers, followed by a sequence of cleanings (acetone, ethanol and deionised water) to prepare the substrate surface for electrodeposition. The copper substrate was then activated in a 25% H2 SO4
solution for approximately 2 min. This activated copper substrate was placed parallel to the vertically oriented tin plate in the plating bath at a distance of 5 cm. Continuous stirring of the electrolyte was performed using a magnetic stirrer, and sonication was conducted using an Ultrasonic Processor (UP400S) at a frequency of 20 kHz and a power of 60 W. The substrate was submerged at a fixed position within the plating solution to prevent agglomeration of MWCNTs in the electrolyte suspension. The duty cycle is defined as ton /(ton + toff ), where ton is the pulseon period and toff is the relaxation period. A current waveform for the pulsed electrodeposition is illustrated in Fig. 1. The conditions of the pulse method were as follows: a bath at room temperature, a peak current density of 20 mA cm−2 , a frequency of 100 Hz, a pulse of the duty ratio of 0.25, and a deposition time of 3.0 min. After co-electrodeposition, each Sn and Sn/MWCNT composite electrode was cut into cylindrical coupons of approximately 16 mm in diameter and dried. 2.3. Characterizations The crystal structure and surface morphology of the composite coatings were examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Raman spectrometry was employed to verify the composite deposition of the Sn/MWCNT. Coin type CR2016 test cells were assembled in an argon filled glove box. The co-deposited Sn/MWCNT nanocomposites were used as the working electrode, Li foil was used as the counter electrode, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in volume) was used as the electrolyte. The working and counter electrodes were separated with a polypropylene (PP) separator. Charge–discharge characteristics were obtained at 293 K (room temperature) between 0.02 V and 1.5 V at a constant current of 200 mA g−1 . Moreover, EIS of assembled CR2016 coin cells were measured in frequency range 1000 kHz to 0.1 Hz with AC amplitude of 10 mV using Gamry Instrument Version 6.4. 3. Results and discussion 3.1. Microstructural characterisation XRD patterns of the Sn/MWCNT coatings with increasing MWCNT volume fraction are shown in Fig. 2. The diffraction peaks indexed to Sn clearly exist for all of the electrodes, but no peaks belonging to MWCNT exist. The examined patterns of the Sn/MWCNT coatings exhibited a reduced peak intensity compared to the pure Sn electrodeposit, but the relative magnitude of the peaks remains the same. In addition, increased peak broadening was observed with increasing MWCNTs content. The average
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Fig. 3. Raman spectrum of nanostructure Sn/MWCNTs composite coating. Fig. 2. XRD pattern of the pulse electrodeposited Sn and Sn/MWCNTs nanocomposite electrodes.
grain size of the coatings was calculated from the diffraction peak width using the Scherrer equation. As it is well known, grain size was found to decrease with increased amounts of MWCNTs in the electrodeposited composite materials [16,17]. The Sn/MWCNT composite coating has an average grain size of 100 nm, which is significantly smaller than the 175 nm obtained from the unreinforced Sn coating. The presence of MWCNTs in a metal deposit may induce smaller grains due to a large increase of nucleation sites. Namely, the growth of the electrodeposited layer is a competition between nucleation and crystal growth. The carbon nanotubes provide more nucleation sites and, hence, slow up the crystal growth; subsequently, the corresponding Sn matrix in the composite coating has a smaller crystal size [17]. The defects on the MWCNTs provide active
nucleation sites for the Sn and produce core-shell-like structures, which were reported to be beneficial for accommodating stresses arising from volume increase during Li intercalation [18]. The Raman spectra contained the characteristic signatures that indicate the presence of MWCNTs in all of the deposited tin films. The Raman spectrum of the nanostructure of the Sn/MWCNT coatings (Fig. 3) showed sharp features at ∼1345 cm−1 (D-band) and ∼1580 cm−1 (G-band). The D-band (∼1350 cm−1 ) is generally attributed to defects in the curved graphite sheet and tube ends [19]. This band also corresponds to either disordered or small crystallites of sp2 networks and could be typical of MWCNTs finite size effects. Additionally, the peak at ∼2650 cm−1 completely disappears with the incorporation of tin nanoparticles [20]. This outcome would mean that the defect-induced effects in curved graphite
Fig. 4. Surface SEM images of Sn/MWCNTs composite films electrodeposited at various MWCNT concentrations in the plating baths of (a) 1 g/L, (b) 2 g/L, and (c) 5 g/L.
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absorbed MWCNTs on the Sn layer were either buried within the Sn grains or entrapped at the triple points and grain boundaries of the Sn grains during successive electrodeposition of the Sn layer, resulting in the formation of the Sn/MWCNT composite. Our experimental findings also support the idea that Sn grains were deposited on the surfaces MWCNTs and tended to accumulate at triple grain junctions. In the literature, it is typical to increase the MWCNTs co-deposition by increasing the MWCNTs concentration in the electrolyte. Increasing concentration of MWCNTs in the electrolyte, the amount of MWCNTs co-deposition can be decreased due to MWCNTs agglomeration [23]. However, in our experimental results, very few of MWCNTs agglomerates was detected to be deposited in the coatings, further at 5 g/L concentration of MWCNTs (see Fig. 4c). 3.2. Electrochemical testing of Sn and Sn/MWCNT composite electrodes Fig. 5. Relationship between MWCNT concentration in the electrolyte and MWCNT content in the composite.
sheets and at tube ends would tend to modulate the bonding environment with the incorporation of tin particles. These results proved that composite electrodeposition was a practicable method for preparing a Sn/MWCNT composite. Fig. 4 shows surface SEM images of Sn/MWCNT composite films electrodeposited under various MWCNTs concentrations in the plating bath. These coatings were produced with constant duty cycles (50%), a pulse frequency of 100 Hz and peak current density of 20 mA cm−2 . Fig. 4 shows that the MWCNTs penetrated into the Sn particles and were uniformly distributed. The MWCNTs appear to be well dispersed and are embedded in the tin matrix. The homogenous distribution and dispersion ability of carbon nanotubes is significant because the MWCNT are expected to bear the stresses during electrochemical cycling. Because of the high-volume increase in tin based electrodes, the electrodes are pulverised after a certain number of electrochemical cycles. As MWCNTs are very strong and flexible, they can carry the load, which is transferred from the Sn matrix materials. Therefore, it can be concluded that MWCNTs can also act as stress buffering components. Moreover, the MWCNTs act as bridges for electrons [22] and can provide a path for lithium-ion insertion. With increasing concentration of MWCNTs, as shown in Fig. 5, the amount of MWCNTs embedded on the Sn matrix increased. The MWCNTs content of the composite film was about 1.2 vol.%, 1.6%, 2.5% for Sn/MWCNT(1 g/L) Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L), respectively. Choi et al. [21] showed that
Fig. 6 presents the cyclic voltammetry (CV) curves of Sn and Sn/MWCNT(5 g/L) film anodes in the 1st and 2nd cycles, scanned at 0.5 mV s−1 between 0.02 V and 1.5 V. During the 1st and 2nd cycles, the Sn/MWCNT anodes have curves in which four anodic peaks and three cathodic peaks can be observed. This finding demonstrates that the alloying and de-alloying of Sn changed in the 2nd cycle. Fig. 6a shows the first differential discharge profile, which shows three reduction peaks at approximately 0.22 V, 0.39 V and 0.65 V that are derived from Li alloying from the Sn/MWCNTs composites. The irreversible peak of electrolyte decomposition in the first cycle above 1.2 V can barely be observed, which implies a small irreversible capacity during the first cycle for the Sn/MWCNT composite electrode. Four other anodic peaks can be clearly observed at approximately 0.45, 0.65, 0.75 and 0.82 V vs. Li/Li+ . These peaks are related to the Li de-alloying from Sn in the Sn/MWCNT nanocomposite. This conclusion has been verified by the charge/discharge curves below. Fig. 6b shows that for the pure Sn electrode, there is an irreversible peak of electrolyte decomposition in the first cycle above 1.1 V. These peaks may be attributed to solid electrolyte interphase (SEI) film formation and a small amount of oxide metal irreversible Li+ insertion [24]. Approximately three pairs of other cathodic/anodic peaks can be observed [25]. Since the low amount of MWCNTs in the co-deposited Sn layer and the surface coverage of the MWCNTs by Sn, the electrochemical contribution of MWCNTs was omitted in this work. Fig. 7 illustrates the galvanostatic 1st and 2nd charge-discharge curve of the Sn and the Sn/MWCNT composite film anodes in the range of 0.02–1.5 V at a current density of 200 mA g−1 , used for investigating the effect of MWCNTs on the electrochemical behaviour of composite electrodes. The alloying into and the
Fig. 6. Cyclic voltammograms of (a) Sn/MWCNTs (5 g/L) electrode and (b) pure Sn electrode.
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Fig. 7. The first two-cycle’s discharge/charge curves of the Sn and Sn/MWCNTs electrodes.
de-alloying of lithium ions from the Sn/MWCNT electrodes are defined as the discharge and charge of the anode, respectively. The first discharge capacities for the pure Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes are 754, 788, 800 and 830 mAh g−1 , respectively, within the potential range from 0.02 to 1.5 V. All of the Sn/MWCNT composites show higher capacities in the 1st discharge than the pure Sn electrodes. As shown in Fig. 7a, there are several plateaus in the voltage profiles for the alloying and de-alloying of the Sn/MWCNT composites. It also can be seen in Fig. 7a–c that the Sn/MWCNT(5 g/L) composite electrode showed better capacity retention than the Sn/MWCNT(1 g/L) composite electrode. In good agreement with the literature, the formation of Li–Sn alloy induces a large-volume (up to approximately 300%) increase, which could create microcracks and destroy the integrity of the electrode, result in high irreversible capacity, the electrical isolation of the Sn electrode from the Cu substrate and poor cycle performance in the pure tin electrode [12,26,27]. Moreover, this capacity fading might be due to repeated tin particle aggregation and pulverisation during cycling [12,26,27]. To overcome this problem of the Sn electrodes, we attempted to fabricate Sn/MWCNT composite electrodes using pulse co-electrodeposition, introducing different MWCNTs concentrations in the electrolyte of 1.0, 2.0 and 5.0 g/L. Fig. 8 displays the cycling stability of the pure Sn, Sn–MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes at a current density of 200 mA g−1 and potential between 0.02 V and 1.5 V. As shown in Fig. 8, the pure tin electrodes exhibit poorer cycle performance than the Sn/MWCNT composite electrodes. The capacity retention for the first 10 cycles is maintained at 43%, 56%, 72%, and 75% of the discharge capacity for the pure Sn,
Fig. 8. Cycle performance of the Sn and Sn/MWCNTs electrodes pulse electrodeposited with different concentrations.
Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes, respectively. After employing 30 cycles, the discharge capacities are still 185, 231, 361, and 418 mAh g−1 for the pure Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes, respectively. In terms of the ratio of the specific capacity retained after 30 cycles to the second discharge capacity, the capacity retention for the bare Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) samples are 25%, 30%, 45%, and 53%, respectively. The pure nanocrystalline Sn anode has the highest first discharge capacity (754 mAh g−1 ); however, its capacity fades
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Fig. 9. Impedance response of the cells containing the pure Sn and Sn/MWCNTs electrodes (a) before charge–discharge and (b) after 30 cycles.
rapidly upon cycling. Furthermore, this value is still much lower than the theoretical capacity (990 mAh g−1 ) of the pure tin electrode. Therefore, using nanocrystalline tin alone as an electrode in Li-ion battery is not suitable. These results show that the cycling stability of Sn–MWCNT(5 g/L) is the best of all of the composites. For composites, the capacity retention of Sn/MWCNT(5 g/L) is better than that of Sn/MWCNT(1 g/L) and Sn/MWCNT(2 g/L). This outcome indicates that the MWCNTs surrounded by the nano-Sn particles improves the stability of tin particles against agglomeration and provides good buffering against the local volume changes in the Li–Sn alloying and de-alloying reactions. Moreover, the good electrical conductivity of MWCNTs is beneficial for keeping the Sn particles electrically connected during the entire alloy and alloying process [28,29]. Jhan et al. [30] studied Sn/C C (MWCNTs) composite anode materials by carbothermal reduction. They found that the added MWCNTs not only serve as the separator, but also mitigate the conductivity loss and reduce the electrode impedance. Park et al. studied that the electrochemical performances of Sn C composite anode prepared by electrodeposition at different thickness, indicating a reversible capacity of about 200 mAh g−1 over 20 cycles [12]. They suggested that the incorporation of carbon improves the cycling stability of Sn C electrodes. Moreover, Sn/CNT composite anodes were synthesised by Chang et al., demonstrating a capacity of 380 mAh g−1 within 50 cycles [22]. Guo et al. fabricated Sn/MWCNT composites having a capacity of 400 mAh g−1 within 20 cycles by the chemical reduction method [25]. The comparison of our results in the light of the some results from literature showed that we have more satisfactory discharge capacity. To verify the effect of MWCNTs on the electronic conductivity of the nanocomposites and pure Sn electrodes, electrochemical impedance spectroscopy (EIS) measurements were performed on the Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn–MWCNT(5 g/L) samples using a sine wave of 5 mV amplitude over a frequency range 1000 kHz to 0.1 Hz. Fig. 9 shows the Nyquist plots obtained from the Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) before and after the charge–discharge cycles. The shapes of these two spectra are rather similar. The highfrequency arc in the spectra is attributed to the charge-transfer reaction at the interface of the electrolyte and the electrode. The subsequent inclined line is attributed to the diffusion of Li+ ions at the Sn electrodes. The resistance of the Sn/MWCNT(5 g/L) electrode is lower than that of the pure Sn electrode. It is found that the size of the depressed semicircle in the middle frequency range for the Sn/MWCNT(5 g/L) sample before and after charge–discharge cycles (after 30 cycles) is smaller when compared with the pure Sn sample, revealing the lower charge transfer resistance of the Sn/MWCNT(5 g/L). This result indicates that the electronic
conductivity of the Sn/MWCNT(5 g/L) sample was improved due to the good electrical conductivity of the MWCNTs in the composite materials, where they served as both an active material and a conductor in the anode composite [2,25]. 4. Conclusions Sn/MWCNT nanocomposite coatings were deposited from a plating tin bath using pulse electrodeposition with a homogenous distribution of MWCNTs. The co-deposition of MWCNTs in a tin electrolytic coating depends on the MWCNTs concentration in the bath. MWCNTs content in the deposit increased by an increase in the MWCNTs concentration in the bath. A high reversible capacity, and fairly good cyclability were achieved for Sn/MWCNT(5 g/L) electrodes. Improvement of the anode performance due to the MWCNTs loading includes a remarkable increase of the cyclability; effective suppression of the stress caused from volume change during charge/discharge; the electroconductivity was enhanced. Therefore, it is concluded that the electrochemical performance of electrodes can be enhanced by controlling the morphology of the electrode materials and MWCNTs concentration. The pulse electro co-deposition of Sn/MWCNT on the copper current collector seems very flexible and practically simple method to obtain high discharge capacities in the Li-ion batteries. The discharge capacity obtained more than 400 mAh g−1 when MWCNTs concentration has increased to 5 g/L in the electrolyte. These predict that Sn/MWCNT nanocomposite electrodes can be good candidates if the MWCNTs content increased beyond 5 g/L without agglomeration. Acknowledgements This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under the contract number 109M464. The authors thank the TUBITAK MAG workers for their financial support. References [1] K. Uia, S. Kikuchi, Y. Kadoma, N. Kumagai, S. Itob, Electrochemical characteristics of Sn film prepared by pulse electrodeposition method as negative electrode for lithium secondary batteries, J. Power Sources 189 (2009) 224–229. [2] S. Nam, S. Kim, S. Wi, H. Choi, S. Byun, S.M. Choi, S.I. Yoo, K. Tae Lee, B. Park, The role of carbon incorporation in SnO2 nanoparticles for Li rechargeable batteries, J. Power Sources 211 (2012) 154–160. [3] G.W. Wang, J.H. Ahn, M.J. Lindsay, L. Sun, Graphite–Sn composite as anode materials for Li ion batteries, J. Power Sources 97–98 (2001) 211–215. [4] H. Zhao, C. Jiang, X. Hea, J. Rena, C. Wan, Advanced structures in electrodeposited tin base anodes for lithium ion batteries, Electrochim. Acta 52 (2007) 7820–7826.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Production of Sn/MWCNT nanocomposite anodes by pulse electrodeposition for Li-ion batteries Mehmet Uysal ∗ , Tugrul Cetinkaya, Ahmet Alp, Hatem Akbulut Sakarya University Engineering Faculty, Department of Metallurgical & Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
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Article history: Received 20 June 2013 Received in revised form 18 October 2013 Accepted 28 October 2013 Available online 5 November 2013 Keywords: Electro co-deposition Sn/MWCNTs nanocomposite Pulse electrodeposition Electrochemical behaviour Li-ion batteries
a b s t r a c t A tin/multi-walled carbon nanotube (Sn/MWCNT) composite was prepared by ultrasonic-pulse electrodeposition on a copper substrate in a chloride bath containing different concentrations of multi-walled carbon nanotubes. The morphology and structure of the Sn/MWCNTs composites were characterised by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectrometry. The electrochemical performance of Sn/MWCNTs nanocomposites has been investigated by charge/discharge tests, cyclic voltammetric experiments and the ac impedance technique. The additive amount of MWCNTs was shown to be a vital factor to enhance Sn/MWCNTs composite anodes for cyclability and discharge capacity. Volume changes and morphological changes in Sn can be reduced by the addition of a MWCNTs that has sufficient flexibility to act as a buffer. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Lithium ion secondary batteries have the highest energy density in commercialised batteries, and the demand for these batteries as a high-power source has been steadily increasing due to the advancement of portable electronic devices [1–5]. The most common anode material used in Li-ion batteries is graphite due to its low cost, availability and durability [5–7]. In graphitic anodes, the Li+ insertion mechanism corresponds to the reversible, progressive intercalation of Li+ ions between graphene layers to reach a theoretical capacity of 372 mAh g−1 if LiC6 is formed, compared with a practical capacity of 350 mAh g−1 [6,7]. Alternatively, Sn has received much attention as an anode material for lithium-ion batteries due to its low voltage platform and large theoretical specific capacity (Li4.4 Sn, 992 mAh g−1 ) [8,9]. The theoretical capacity of pure tin is 994 mAh g−1 , which is three times that of the graphite anode (372 mAh g−1 ), based on the end lithiated phase Li4.4 Sn. Crystallographic studies suggest that the realistic form of this end phase could be Li17 Sn4 (thus, 4.25 Li per Sn). Therefore, its maximum gravimetric capacity could be 959.5 mAh g−1 , which is still much higher than most common graphite anodes [10]. However, a pure Sn electrode suffers severely from its poor cycle performance due to its abrupt volume expansion (up to approximately 300%) during the charge/discharge process [10]. This morphological
∗ Corresponding author. Tel.: +90 264 295 57 95; fax: +90 264 295 56 01. E-mail address: [email protected] (M. Uysal). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.162
change results in the cracking and crumbling of the electrode with the concomitant loss of electrical contact between interparticles, particles and the current collector, which results in poor cycling performance [1,11]. To overcome this problem, we attempted to fabricate Sn electrodes by co-electrodeposition with MWCNTs, which are active in the potential and act as a buffer zone for preventing the crumbling of the Sn electrodes [12,13]. The composition of the Sn/MWCNT composite electrodes fabricated by pulse coelectrodeposition is affected by controlling the particle size, the MWCNTs concentration, the stirring speed, and the current density. Recently, electrodeposited Sn-based thin films [11–13] were extensively evaluated regarding their ability to overcome the above problems during the charge–discharge reaction. The pulse electrodepositing method is simpler than the currently used powder preparation method because there is no need for any binder or conductive additives [1]. Numerous studies on the DC electrodeposition of tin and tin-alloys have been performed. However, very few studies have focused on the pulse electrodeposition of Sn/MWCNT composite coatings. In this study, to improve the electrochemical property of a Sn electrode, we prepared the Sn/MWCNT film electrode by a pulse electrodeposition method, which is effective for improving the adhesion strength between the active material and the substrate. The main aim in introducing MWCNTs into the Sn layer is to increase electrode conductivity and accommodate the mechanical stress caused by a large-volume expansion during cycling. We also propose a low-cost and flexible process using the simple electrodeposition method, equipped with pulse capability.
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Table 1 Overview of pulse electrodeposition parameters for preparation of Sn/MWCNTs coatings. Bath composition SnCl2 ·2H2 O K4 P2 O7 Gelatin
25 g/L 170 g/L 0.5 g/L
Pulse electrodeposition parameters Peak current density pH Duty cycle % Frequency MWCNTs concentrations Plating time Temperature
20 mA/cm2 8.0–9.0 25% 100 Hz 1.0, 2.0, 5.0 g/L 3 min 25–30 ◦ C Fig. 1. Schematic diagram of the wave shape of pulse deposition.
2. Experimental procedure 2.1. MWCNTs functionalisation The MWCNTs are quasi-one-dimensional nanomaterials that are primarily available in bundled form because of strong van der Waals interactions between the tube walls [14]. To obtain successful deposition of the carbon nanotubes, MWCNTs should be uniformly dispersed in the plating solution, and the suspension must be stable. One of the major issues with the fabrication of MWCNT-reinforced composite coatings is related to the nonuniform dispersion of MWCNTs in the composite matrices because the MWCNTs easily coalesced in the aqueous solution due to their high surface energy. In this investigation, surface treatment of MWCNTs was performed using a nitric acid/sulphuric acid solution to improve the dispersion of the MWCNTs into the electrodeposited Sn/MWCNT coating. The acid treatment not only removes most of the metal, but also produces carboxyl, aldehyde, and other oxygen containing functional groups [15] on the surface of the MWCNTs that help in the uniform dispersion of MWCNTs without any additional dispersing additives. In the present investigation, 2 g of MWCNTs was added into a 2:1 solution of nitric acid to sulphuric acid in a glass bottle. The mixture was subsequently magnetic stirred at 100 ◦ C for 1 h. Then, the MWCNTs were further collected on a 0.2 m filter, rinsed with distilled water, and dried at 100 ◦ C for 4 h. Next, the obtained MWCNTs were dispersed in the electrolyte without addition of any dispersing agent. 2.2. Pulse electrodeposition of Sn/MWCNT composite coatings Sn/MWCNT composite coatings were deposited from a chloride bath onto the copper foils. All of the chemicals were of analytical reagent grade, and analytic/chemical standards were used for the preparation of the bath solutions. The components as well as some experimental conditions of the optimised Sn/MWCNT composite electrodeposition bath are summarised in Table 1. The multi-walled CNT were supplied by Arry Nano and had a diameter of 50–60 nm and a length of 10 m. Functionalised MWCNTs were added to the electrolytes, containing 0 g/L, 1 g/L, 2 g/L and 5 g/L MWCNTs, and sonication was applied during the composite coating and the mixing of the electrolytes. A plating cell containing 250 ml of the solution was immersed in a thermostatically controlled largevolume water bath that was kept at a room temperature. The pretreated MWCNTs were dispersed in the electrolyte via the aid of ultrasonic agitation. A high-purity (99.99%) electrolytic tin sheet was used as the soluble anode. The copper substrates were mechanically polished using a sequence of 400, 600 and 1200 mesh emery papers, followed by a sequence of cleanings (acetone, ethanol and deionised water) to prepare the substrate surface for electrodeposition. The copper substrate was then activated in a 25% H2 SO4
solution for approximately 2 min. This activated copper substrate was placed parallel to the vertically oriented tin plate in the plating bath at a distance of 5 cm. Continuous stirring of the electrolyte was performed using a magnetic stirrer, and sonication was conducted using an Ultrasonic Processor (UP400S) at a frequency of 20 kHz and a power of 60 W. The substrate was submerged at a fixed position within the plating solution to prevent agglomeration of MWCNTs in the electrolyte suspension. The duty cycle is defined as ton /(ton + toff ), where ton is the pulseon period and toff is the relaxation period. A current waveform for the pulsed electrodeposition is illustrated in Fig. 1. The conditions of the pulse method were as follows: a bath at room temperature, a peak current density of 20 mA cm−2 , a frequency of 100 Hz, a pulse of the duty ratio of 0.25, and a deposition time of 3.0 min. After co-electrodeposition, each Sn and Sn/MWCNT composite electrode was cut into cylindrical coupons of approximately 16 mm in diameter and dried. 2.3. Characterizations The crystal structure and surface morphology of the composite coatings were examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Raman spectrometry was employed to verify the composite deposition of the Sn/MWCNT. Coin type CR2016 test cells were assembled in an argon filled glove box. The co-deposited Sn/MWCNT nanocomposites were used as the working electrode, Li foil was used as the counter electrode, and 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 in volume) was used as the electrolyte. The working and counter electrodes were separated with a polypropylene (PP) separator. Charge–discharge characteristics were obtained at 293 K (room temperature) between 0.02 V and 1.5 V at a constant current of 200 mA g−1 . Moreover, EIS of assembled CR2016 coin cells were measured in frequency range 1000 kHz to 0.1 Hz with AC amplitude of 10 mV using Gamry Instrument Version 6.4. 3. Results and discussion 3.1. Microstructural characterisation XRD patterns of the Sn/MWCNT coatings with increasing MWCNT volume fraction are shown in Fig. 2. The diffraction peaks indexed to Sn clearly exist for all of the electrodes, but no peaks belonging to MWCNT exist. The examined patterns of the Sn/MWCNT coatings exhibited a reduced peak intensity compared to the pure Sn electrodeposit, but the relative magnitude of the peaks remains the same. In addition, increased peak broadening was observed with increasing MWCNTs content. The average
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Fig. 3. Raman spectrum of nanostructure Sn/MWCNTs composite coating. Fig. 2. XRD pattern of the pulse electrodeposited Sn and Sn/MWCNTs nanocomposite electrodes.
grain size of the coatings was calculated from the diffraction peak width using the Scherrer equation. As it is well known, grain size was found to decrease with increased amounts of MWCNTs in the electrodeposited composite materials [16,17]. The Sn/MWCNT composite coating has an average grain size of 100 nm, which is significantly smaller than the 175 nm obtained from the unreinforced Sn coating. The presence of MWCNTs in a metal deposit may induce smaller grains due to a large increase of nucleation sites. Namely, the growth of the electrodeposited layer is a competition between nucleation and crystal growth. The carbon nanotubes provide more nucleation sites and, hence, slow up the crystal growth; subsequently, the corresponding Sn matrix in the composite coating has a smaller crystal size [17]. The defects on the MWCNTs provide active
nucleation sites for the Sn and produce core-shell-like structures, which were reported to be beneficial for accommodating stresses arising from volume increase during Li intercalation [18]. The Raman spectra contained the characteristic signatures that indicate the presence of MWCNTs in all of the deposited tin films. The Raman spectrum of the nanostructure of the Sn/MWCNT coatings (Fig. 3) showed sharp features at ∼1345 cm−1 (D-band) and ∼1580 cm−1 (G-band). The D-band (∼1350 cm−1 ) is generally attributed to defects in the curved graphite sheet and tube ends [19]. This band also corresponds to either disordered or small crystallites of sp2 networks and could be typical of MWCNTs finite size effects. Additionally, the peak at ∼2650 cm−1 completely disappears with the incorporation of tin nanoparticles [20]. This outcome would mean that the defect-induced effects in curved graphite
Fig. 4. Surface SEM images of Sn/MWCNTs composite films electrodeposited at various MWCNT concentrations in the plating baths of (a) 1 g/L, (b) 2 g/L, and (c) 5 g/L.
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absorbed MWCNTs on the Sn layer were either buried within the Sn grains or entrapped at the triple points and grain boundaries of the Sn grains during successive electrodeposition of the Sn layer, resulting in the formation of the Sn/MWCNT composite. Our experimental findings also support the idea that Sn grains were deposited on the surfaces MWCNTs and tended to accumulate at triple grain junctions. In the literature, it is typical to increase the MWCNTs co-deposition by increasing the MWCNTs concentration in the electrolyte. Increasing concentration of MWCNTs in the electrolyte, the amount of MWCNTs co-deposition can be decreased due to MWCNTs agglomeration [23]. However, in our experimental results, very few of MWCNTs agglomerates was detected to be deposited in the coatings, further at 5 g/L concentration of MWCNTs (see Fig. 4c). 3.2. Electrochemical testing of Sn and Sn/MWCNT composite electrodes Fig. 5. Relationship between MWCNT concentration in the electrolyte and MWCNT content in the composite.
sheets and at tube ends would tend to modulate the bonding environment with the incorporation of tin particles. These results proved that composite electrodeposition was a practicable method for preparing a Sn/MWCNT composite. Fig. 4 shows surface SEM images of Sn/MWCNT composite films electrodeposited under various MWCNTs concentrations in the plating bath. These coatings were produced with constant duty cycles (50%), a pulse frequency of 100 Hz and peak current density of 20 mA cm−2 . Fig. 4 shows that the MWCNTs penetrated into the Sn particles and were uniformly distributed. The MWCNTs appear to be well dispersed and are embedded in the tin matrix. The homogenous distribution and dispersion ability of carbon nanotubes is significant because the MWCNT are expected to bear the stresses during electrochemical cycling. Because of the high-volume increase in tin based electrodes, the electrodes are pulverised after a certain number of electrochemical cycles. As MWCNTs are very strong and flexible, they can carry the load, which is transferred from the Sn matrix materials. Therefore, it can be concluded that MWCNTs can also act as stress buffering components. Moreover, the MWCNTs act as bridges for electrons [22] and can provide a path for lithium-ion insertion. With increasing concentration of MWCNTs, as shown in Fig. 5, the amount of MWCNTs embedded on the Sn matrix increased. The MWCNTs content of the composite film was about 1.2 vol.%, 1.6%, 2.5% for Sn/MWCNT(1 g/L) Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L), respectively. Choi et al. [21] showed that
Fig. 6 presents the cyclic voltammetry (CV) curves of Sn and Sn/MWCNT(5 g/L) film anodes in the 1st and 2nd cycles, scanned at 0.5 mV s−1 between 0.02 V and 1.5 V. During the 1st and 2nd cycles, the Sn/MWCNT anodes have curves in which four anodic peaks and three cathodic peaks can be observed. This finding demonstrates that the alloying and de-alloying of Sn changed in the 2nd cycle. Fig. 6a shows the first differential discharge profile, which shows three reduction peaks at approximately 0.22 V, 0.39 V and 0.65 V that are derived from Li alloying from the Sn/MWCNTs composites. The irreversible peak of electrolyte decomposition in the first cycle above 1.2 V can barely be observed, which implies a small irreversible capacity during the first cycle for the Sn/MWCNT composite electrode. Four other anodic peaks can be clearly observed at approximately 0.45, 0.65, 0.75 and 0.82 V vs. Li/Li+ . These peaks are related to the Li de-alloying from Sn in the Sn/MWCNT nanocomposite. This conclusion has been verified by the charge/discharge curves below. Fig. 6b shows that for the pure Sn electrode, there is an irreversible peak of electrolyte decomposition in the first cycle above 1.1 V. These peaks may be attributed to solid electrolyte interphase (SEI) film formation and a small amount of oxide metal irreversible Li+ insertion [24]. Approximately three pairs of other cathodic/anodic peaks can be observed [25]. Since the low amount of MWCNTs in the co-deposited Sn layer and the surface coverage of the MWCNTs by Sn, the electrochemical contribution of MWCNTs was omitted in this work. Fig. 7 illustrates the galvanostatic 1st and 2nd charge-discharge curve of the Sn and the Sn/MWCNT composite film anodes in the range of 0.02–1.5 V at a current density of 200 mA g−1 , used for investigating the effect of MWCNTs on the electrochemical behaviour of composite electrodes. The alloying into and the
Fig. 6. Cyclic voltammograms of (a) Sn/MWCNTs (5 g/L) electrode and (b) pure Sn electrode.
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Fig. 7. The first two-cycle’s discharge/charge curves of the Sn and Sn/MWCNTs electrodes.
de-alloying of lithium ions from the Sn/MWCNT electrodes are defined as the discharge and charge of the anode, respectively. The first discharge capacities for the pure Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes are 754, 788, 800 and 830 mAh g−1 , respectively, within the potential range from 0.02 to 1.5 V. All of the Sn/MWCNT composites show higher capacities in the 1st discharge than the pure Sn electrodes. As shown in Fig. 7a, there are several plateaus in the voltage profiles for the alloying and de-alloying of the Sn/MWCNT composites. It also can be seen in Fig. 7a–c that the Sn/MWCNT(5 g/L) composite electrode showed better capacity retention than the Sn/MWCNT(1 g/L) composite electrode. In good agreement with the literature, the formation of Li–Sn alloy induces a large-volume (up to approximately 300%) increase, which could create microcracks and destroy the integrity of the electrode, result in high irreversible capacity, the electrical isolation of the Sn electrode from the Cu substrate and poor cycle performance in the pure tin electrode [12,26,27]. Moreover, this capacity fading might be due to repeated tin particle aggregation and pulverisation during cycling [12,26,27]. To overcome this problem of the Sn electrodes, we attempted to fabricate Sn/MWCNT composite electrodes using pulse co-electrodeposition, introducing different MWCNTs concentrations in the electrolyte of 1.0, 2.0 and 5.0 g/L. Fig. 8 displays the cycling stability of the pure Sn, Sn–MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes at a current density of 200 mA g−1 and potential between 0.02 V and 1.5 V. As shown in Fig. 8, the pure tin electrodes exhibit poorer cycle performance than the Sn/MWCNT composite electrodes. The capacity retention for the first 10 cycles is maintained at 43%, 56%, 72%, and 75% of the discharge capacity for the pure Sn,
Fig. 8. Cycle performance of the Sn and Sn/MWCNTs electrodes pulse electrodeposited with different concentrations.
Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes, respectively. After employing 30 cycles, the discharge capacities are still 185, 231, 361, and 418 mAh g−1 for the pure Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) electrodes, respectively. In terms of the ratio of the specific capacity retained after 30 cycles to the second discharge capacity, the capacity retention for the bare Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) samples are 25%, 30%, 45%, and 53%, respectively. The pure nanocrystalline Sn anode has the highest first discharge capacity (754 mAh g−1 ); however, its capacity fades
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Fig. 9. Impedance response of the cells containing the pure Sn and Sn/MWCNTs electrodes (a) before charge–discharge and (b) after 30 cycles.
rapidly upon cycling. Furthermore, this value is still much lower than the theoretical capacity (990 mAh g−1 ) of the pure tin electrode. Therefore, using nanocrystalline tin alone as an electrode in Li-ion battery is not suitable. These results show that the cycling stability of Sn–MWCNT(5 g/L) is the best of all of the composites. For composites, the capacity retention of Sn/MWCNT(5 g/L) is better than that of Sn/MWCNT(1 g/L) and Sn/MWCNT(2 g/L). This outcome indicates that the MWCNTs surrounded by the nano-Sn particles improves the stability of tin particles against agglomeration and provides good buffering against the local volume changes in the Li–Sn alloying and de-alloying reactions. Moreover, the good electrical conductivity of MWCNTs is beneficial for keeping the Sn particles electrically connected during the entire alloy and alloying process [28,29]. Jhan et al. [30] studied Sn/C C (MWCNTs) composite anode materials by carbothermal reduction. They found that the added MWCNTs not only serve as the separator, but also mitigate the conductivity loss and reduce the electrode impedance. Park et al. studied that the electrochemical performances of Sn C composite anode prepared by electrodeposition at different thickness, indicating a reversible capacity of about 200 mAh g−1 over 20 cycles [12]. They suggested that the incorporation of carbon improves the cycling stability of Sn C electrodes. Moreover, Sn/CNT composite anodes were synthesised by Chang et al., demonstrating a capacity of 380 mAh g−1 within 50 cycles [22]. Guo et al. fabricated Sn/MWCNT composites having a capacity of 400 mAh g−1 within 20 cycles by the chemical reduction method [25]. The comparison of our results in the light of the some results from literature showed that we have more satisfactory discharge capacity. To verify the effect of MWCNTs on the electronic conductivity of the nanocomposites and pure Sn electrodes, electrochemical impedance spectroscopy (EIS) measurements were performed on the Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn–MWCNT(5 g/L) samples using a sine wave of 5 mV amplitude over a frequency range 1000 kHz to 0.1 Hz. Fig. 9 shows the Nyquist plots obtained from the Sn, Sn/MWCNT(1 g/L), Sn/MWCNT(2 g/L) and Sn/MWCNT(5 g/L) before and after the charge–discharge cycles. The shapes of these two spectra are rather similar. The highfrequency arc in the spectra is attributed to the charge-transfer reaction at the interface of the electrolyte and the electrode. The subsequent inclined line is attributed to the diffusion of Li+ ions at the Sn electrodes. The resistance of the Sn/MWCNT(5 g/L) electrode is lower than that of the pure Sn electrode. It is found that the size of the depressed semicircle in the middle frequency range for the Sn/MWCNT(5 g/L) sample before and after charge–discharge cycles (after 30 cycles) is smaller when compared with the pure Sn sample, revealing the lower charge transfer resistance of the Sn/MWCNT(5 g/L). This result indicates that the electronic
conductivity of the Sn/MWCNT(5 g/L) sample was improved due to the good electrical conductivity of the MWCNTs in the composite materials, where they served as both an active material and a conductor in the anode composite [2,25]. 4. Conclusions Sn/MWCNT nanocomposite coatings were deposited from a plating tin bath using pulse electrodeposition with a homogenous distribution of MWCNTs. The co-deposition of MWCNTs in a tin electrolytic coating depends on the MWCNTs concentration in the bath. MWCNTs content in the deposit increased by an increase in the MWCNTs concentration in the bath. A high reversible capacity, and fairly good cyclability were achieved for Sn/MWCNT(5 g/L) electrodes. Improvement of the anode performance due to the MWCNTs loading includes a remarkable increase of the cyclability; effective suppression of the stress caused from volume change during charge/discharge; the electroconductivity was enhanced. Therefore, it is concluded that the electrochemical performance of electrodes can be enhanced by controlling the morphology of the electrode materials and MWCNTs concentration. The pulse electro co-deposition of Sn/MWCNT on the copper current collector seems very flexible and practically simple method to obtain high discharge capacities in the Li-ion batteries. The discharge capacity obtained more than 400 mAh g−1 when MWCNTs concentration has increased to 5 g/L in the electrolyte. These predict that Sn/MWCNT nanocomposite electrodes can be good candidates if the MWCNTs content increased beyond 5 g/L without agglomeration. Acknowledgements This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under the contract number 109M464. The authors thank the TUBITAK MAG workers for their financial support. References [1] K. Uia, S. Kikuchi, Y. Kadoma, N. Kumagai, S. Itob, Electrochemical characteristics of Sn film prepared by pulse electrodeposition method as negative electrode for lithium secondary batteries, J. Power Sources 189 (2009) 224–229. [2] S. Nam, S. Kim, S. Wi, H. Choi, S. Byun, S.M. Choi, S.I. Yoo, K. Tae Lee, B. Park, The role of carbon incorporation in SnO2 nanoparticles for Li rechargeable batteries, J. Power Sources 211 (2012) 154–160. [3] G.W. Wang, J.H. Ahn, M.J. Lindsay, L. Sun, Graphite–Sn composite as anode materials for Li ion batteries, J. Power Sources 97–98 (2001) 211–215. [4] H. Zhao, C. Jiang, X. Hea, J. Rena, C. Wan, Advanced structures in electrodeposited tin base anodes for lithium ion batteries, Electrochim. Acta 52 (2007) 7820–7826.
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