- Email: [email protected]
MWCNT nanocomposite negative electrodes for lithium microbatteries
Microelectronic Engineering 126 (2014) 143–147
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Double phase tinoxide/tin/MWCNT nanocomposite negative electrodes for lithium microbatteries Mirac Alaf ⇑, Deniz Gultekin, Hatem Akbulut Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
a r t i c l e
i n f o
Article history: Received 17 October 2013 Received in revised form 14 May 2014 Accepted 30 June 2014 Available online 8 July 2014 Keywords: Tin oxide Carbon nanotube Lithium microbatteries Areal capacity
a b s t r a c t As anode material for lithium microbatteries, SnO2/Sn/MWCNT nanocomposites were produced by two steps; thermal evaporation and subsequent plasma oxidation. Metallic tin was first evaporated onto free-standing MWCNT buckypapers having controlled porosity and subsequent RF plasma oxidized in Ar:O2(1:1) gas mixture. The ratio between metallic tin (Sn) and tinoxide (SnO2) was controlled with plasma oxidation time. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to determine the structure and morphology of the obtained nanocomposites. The discharge/charge tests, electrochemical impedance spectroscopy (EIS) and rate capability tests were carried out to characterize the electrochemical properties of the composite. The flexible and free-standing SnO2/Sn/MWCNT nanocomposite electrode showed core–shell and functionally gradient composite structure. SnO2/Sn electrode was failed after 30 cycles and MWCNT paper electrode displays only 75 mAh g1 capacity value between 10 and 100 cycles. SnO2/Sn/MWCNT double phase electrode, exhibited 610 mAh g1 specific discharge capacity even after 100 cycles. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Energy storage technology is becoming an important research area to provide needs of today’s wireless and mobile applications. Rechargeable Li-ion batteries are aroused a lot of interest because of their high voltage, high energy density, stable cycling, and environmental-friendly properties. Nowadays, the continuous miniaturization of electronic devices such as implantable medical devices, smartcards, microelectromechanical systems (MEMS), remote sensors, and RFID tags is led to the growing demands of micropower sources [1]. Rechargeable microbatteries are recently become the topic of widespread research for use in low power applications and energy storage systems for electronic devices. The general requirements of the microbatteries for these applications are high specific energy, wide range of temperature stability, low self-discharge rate and flexibility of cell design [2]. Tin, tinoxide and tin based composites as candidate of electrodes of Li-ion batteries are very attractive because of their characteristic properties such as high specific lithium storage capacities, thermal and chemical stability and environmentally friendly [1,3,4]. However, during the charge–discharge process, tin based electrodes show large volume changes, which cause pulverization of the electrodes after a few cycles [5]. In order to ⇑ Corresponding author. Tel.: +90 264 295 6772; fax: +90 264 295 5601. E-mail address: [email protected] (M. Alaf). http://dx.doi.org/10.1016/j.mee.2014.06.029 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
improve cycling performances of tin based electrodes, more than one solution was offered and studied in present study. Among the solutions that improve cycling performances of the batteries, production of electrodes in nanostructure is widely studied [6]. High energy density and high power density (the same as rate capabilities) can be achieved from electrodes composed of nanostructured materials. Nanostructured electrodes provide large electrode–electrolyte interfacial area coupled with short diffusion distances within the electrodes themselves [7]. Beside nanostructured electrode, using of SnO2/Sn composites could be a solution in realizing increased reversible capacity as well as reduced irreversible capacity and capacity fade upon cycling, as this could increase the Sn:Li2O ratio in the anode matrix [8]. Another impressive way to obviate volume change problems is to disperse tin based materials in a carbon matrix or encapsulate them with carbon especially carbon nano tubes to accommodate the strain of volume change during the alloying and dealloying processes. Use of carbon as matrix support on which tin based nanoparticles are able to good accommodate the mechanical stress experienced during volume changes [9]. As carbon matrix, MWCNT buckypapers are popular because they offer unique features such as flexibility, ability to be free-standing, having porous structure and good mechanical stability. Carbon nano tube paper, also called buckypapers, are self-supporting networks of entangled CNT assemblies arranged in a random fashion and held together by van der Waals interactions at the tube–tube junctions [10].
144
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
In this study; SnO2/Sn double phase structure formed on the MWCNT buckypapers via thermal evaporation and subsequent plasma oxidation. It was targeted to produce free-standing, flexible electrodes that have good cycling performances. The electrode was produced as compositional gradient materials with changing Sn in the MWCNT buckypaper pores gradually to provide stress relaxation for high cycle electrochemical applications. The electrochemical performance of this electrode was compared to a SnO2/Sn coating deposited onto stainless steel and bare MWCNT buckypaper. 2. Experimental details The MWCNTs supplied from Arry Nano (Germany) was employed in this work. Purification and chemical oxidation of MWCNTs was carried out with different oxidation agents [10]. Aqueous MWCNTs suspension was sonicated for 1 h to form a well dispersed suspension, which subsequently vacuum filtered through PVDF membrane filters of 220 nm pore size to form buckypapers. After drying at room temperature in a vacuum oven for 24 h the MWCNT buckypapers were peeled off from the filtration membrane. For the two stages electrode production, high purity metallic tin (99.999%) was firstly evaporated onto MWCNT buckypaper using a multifunctional PVD unit. During the thermal evaporation, chamber was filled with argon (Ar) a pressure of 1 Pa. The evaporation was carried out at 100 A current and from a tungsten (W) boat. In the second step, Sn/MWCNT nanocomposite was subjected to RF plasma oxidation process using oxygen (O2) and argon (Ar) gas mixture in the ratio of 1:1 for 1 h. The detailed experimental work, effects of evaporation time and plasma oxidation times on structural and electrochemical properties were provided in our previous studies [11,12]. An XRD (Rigaku D/MAX 2000) with Cu-Ka radiation has been used to determine the composition, relative phase amounts and structure of the nanocomposites. Transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN) and field emission gun scanning electron microscopy (FEG-SEM, JEOL 6335F) were used for examine surface morphologies. TEM samples of the anode paper were prepared by removing a small piece from the paper and mounting it on a folding copper mesh oyster grid. Coin-type (CR2016) test cells were assembled in an argon-filled glove box and the details of the CR2016 button type cell assembling can be found in our previous work [12]. The cells were cyclically tested on a MTI BST8-MA Battery Analyzer using constant current density over a voltage range of 0.1–2 V for SnO2/Sn on stainless steel electrodes and 0.05–3 V for SnO2/Sn on MWCNT buckypapers. Electrochemical impedance spectroscopy (EIS) measurements were carried out on the samples using a sine wave of 10 mV amplitude over a frequency range of 100 kHz–0.01 Hz using Gamry Instrument Version 5.67. The mass of active material in the SnO2/Sn/ MWCNT nanocomposite electrode was calculated as:
M active ¼ M SnO2 =Sn þ M C
ð1Þ
where MSnO2 =Sn was the SnO2/Sn coating’s mass and MC was mass of the SnO2/Sn coated carbon nano tubes participating in the lithiation/delithiation process. Depth of SnO2/Sn coated carbon nano tubes were measured with the aid of dot-map EDS analyzes and mass of CNT was calculated. 3. Results and discussion MWCNT buckypaper was produced as flexible, uniform, smooth and crack-free disks using chemically oxidized MWCNTs via vacuum filtration method. After vacuum drying, paper was easily
peeled off from membrane. Fig. 1 presents SEM-EDS dot mapping analysis result after thermal evaporation of metallic tin onto MWCNT buckypaper from cross-section area. Evaporated metallic tin is mainly located in between MWCNT buckypaper pores and Sn rich layer depth is about 5 lm. Along the red line, elemental composition was analyzed from 10 points with EDS. According to EDS results, the variation of atomic percent of tin (Sn) is shown in the graph. It can be concluded from the dot-map analysis that Sn shows a gradient composition through the center of buckypaper. It is well known the gradient phase distribution in the composites is beneficial for decreasing crack initiation and therefore, failure [13]. This implies that this functionally gradient structure will cause stress relaxation during the electrochemical reactions. After the thermal evaporation process, Sn/MWCNT sample was subjected to a RF plasma oxidation process at oxygen (O2) and argon (Ar) gas mixture in the ratio of 1:1 to produce SnO2/Sn/ MWCNT nanocomposite. Fig. 2 shows SEM image and XRD result. No obvious reflection peaks from impurities are detected, providing evidence of the high purity of the product as shown in Fig. 2a. The nanocomposites have Sn, SnO2 and C peaks, which agrees well with the standard data files (Sn: JCPDS No. 01-0892958 and SnO2: JPDS No. 00-041-1445), and all of the nanocomposites have a crystalline structure. It should be noted that a diffraction peak at around 26°, which is the main peak of tetragonal SnO2 (1 1 0) almost overlaps with the main peaks of hexagonal C (0 0 2) which confirmed with works in the literature [14]. Sn based phases were deposited around the MWCNTs forming a core–shell structure. As can be seen from the high magnification micrograph located at the upper-right corner of the Fig. 2b, Sn was transformed to SnO2 as very fine nanoparticles on the MWCNT surfaces showing core–shell structure. SnO2/Sn/MWCNT nanocomposites were further analyzed by TEM in order to confirm the presence of Sn and SnO2 nanoparticles onto MWCNT surfaces. TEM sample was prepared by removing a small piece of the anode paper and mounting it onto a folding copper mesh oyster grid. Fig. 3a presents image of uncoated MWCNT and coated MWCNT with SnO2/Sn. Fig. 3b implies that the crystal size of the deposited SnO2/Sn is very fine and shows the lattice fringes of Sn and SnO2 nanoparticles. As can be seen from Fig. 3b, 0.29 nm d-spacing is assigned to the (2 0 0) plane of tin (Sn) and 0.33 nm d-spacing is ascribed to the (1 1 0) plane of cassiterite (SnO2). The similar Tem observations and related diffraction pattern were studied in the work of Rath et al. and Thune et al. [15,16]. For comparing a SnO2/Sn coating deposited onto stainless steel and bare MWCNT buckypaper were investigated in electrochemically. Fig. 4 presents the discharge capacities per gram and cm2 (geometric area) vs. cycle number for cells made from SnO2/Sn/ MWCNT free-standing electrode, MWCNT buckypaper and SnO2/ Sn thin film electrode. The cell assembled from SnO2/Sn thin film on the stainless steel is failed after approximate 30 cycles. Initial discharge capacity of SnO2/Sn thin film is 1100 mAh g1 and capacity fading rate is very fast after 10 cycles. It is believed this capacity decrease is because of Sn based electrode disintegration caused by a high-volume increase [5]. Free-standing MWCNT paper electrode displays 415 mAh g1 initial discharge capacity and this value is fixed at approximately 75 mAh g1 after 10 cycles. The initial discharge capacity of SnO2/Sn/MWCNT free standing electrode is 1544 mAh g1. The capacity retention is still very high as 610 mAh g1 even after 100 cycles electrochemical test. Compared with the SnO2/Sn double phase structures deposited on the stainless steel coin electrodes, the SnO2/Sn/MWCNT nanocomposite showed outstanding performance with high capacity and satisfactory cycling stability. The mechanisms of electrode operation during lithium insertion and extraction are shown in Fig. 5. The large volume variation leads to the pulverization of the SnO2/Sn thin film electrode after cycling. However, SnO2/Sn/MWCNT nanocomposite
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
145
Fig. 1. SEM-EDS dot mapping analysis result of composite from cross-section and atomic percent of tin (Sn) along with red line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD pattern and (b) FESEM image of SnO2/Sn/MWCNT nanocomposite.
provided a solution to dimensional instability problems by limiting the changes in volume of electrode. The MWCNT buckypaper acts as both mechanical support and electrical conductor. The buffering effect of SnO2/Sn/MWCNT nanocomposite gives the battery dimensional stability and can prevent the repeated aggregation and pulverization of the dispersed tin nanoparticles during charging and discharging. In our design the pulverization of electrode material was thought to be tolerated by producing firstly a porous current collector and then coating the MWCNT surfaces with a thin shell
Fig. 3. TEM images of SnO2/Sn coated MWCNT at (a) lower magnification and (b) higher magnification.
of active tin based materials. In the case of tin based coatings on the stainless steel coins the only stress relaxation mechanism was the stress accommodation between the nanograins and
146
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
Fig. 4. Discharge capacities of electrodes per gram and cm2 (geometric area) vs. cycle number.
mesoporous structure, which is not well enough to accommodate the stresses arised from the volume increase during Li intercalation. However, designing a highly porous buckypaper with flexible and very high elastic modulus MWCNTs expected to behave as a buffer materials to provide bearing of the stresses. Electrochemical results are the unique evidences that our imagined model works very well and very high capacity retention values were obtained after 100 cycle test [17,18]. Electrochemical Impedance Spectroscopic (EIS) measurements were carried out to confirm the effect of MWCNTs on changing the charge transfer resistance in SnO2/Sn and SnO2/Sn/MWCNT nanocomposite electrodes using a sine wave of 10 mV amplitude over a frequency range of 100 kHz to 0.1 Hz. Fig. 6 shows the Nyquist plots obtained in the Li-ion cells before and after the electrochemical performance tests. The obtained impedance data were analyzed using the ZsimpWin 3.21 program which allowed the chisquare (v2) value to judge the quality of the equivalent circuit fitting [19]. Resistance values were calculated using the electrical circuit models R(CR)(QR)(CR) and R(Q(RW)). It can be seen from Fig. 6a, which shows EIS spectra of SnO2/Sn electrode that diameter of EIS spectra increases even after 5 cycles electrochemical tests due to pulverization of electrodes after cycling, and this leads to loss of electronic contact between current collector and active materials [20]. Pulverization of the electrode in the SnO2/Sn coated on the stainless steel resulted in decreasing diffusion since the Warburg diffusion curve tend to become flattened after 5 cycles. In the case of SnO2/Sn/MWCNT nanocomposite electrode shown in Fig. 6b, the charge transfer resistances are smaller than that of
Fig. 6. Electrochemical impedance spectra of (a) SnO2/Sn and (b) SnO2/Sn/MWCNT nanocomposite electrodes.
obtained in the SnO2/Sn electrodes for both before and after cycles. When the Warburg diffusion curve analyzed the diffusion of the Li ions are more active in the SnO2/Sn/MWCNT nanocomposite electrode and after 100 cycles there is no any slope change, which can be attributed excellent conductivity and stability of the electrode. The EIS data are also summarized in Table 1, which shows parameters of the equivalent circuit for the SnO2/Sn and SnO2/Sn/ MWCNT nanocomposite electrodes after fitting the diameter of
Fig. 5. The mechanisms of electrode operation before and after cycling.
147
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147 Table 1 Impedance parameters of calculated from equivalent circuit models. Electrodes
Circuit models
Sn/SnO2
[R(CR)(QR)(CR)]
Sn/SnO2/MWCNT
[R(Q(RW))]
R s (O)
R ct (O)
Before cycling After 5 cycles
5.55 1.4
397 610
Before cycling After 100 cycles
5.2 5.8
257 589
4. Conclusions Double phase SnO2/Sn/MWCNT nanocomposite electrode was successfully produced using MWCNT buckypaper as flexible and free-standing by thermal evaporation and subsequent plasma oxidation. The double phase structure was achieved by plasma oxidation. Thermally evaporated tin was mainly inserted between MWCNT buckypaper pores leading to form functionally gradient structure. Depositing Sn on the MWCNTs and subsequent plasma oxidation caused to create a core–shell structure. This core–shell structure accompanied with compositional gradient structure led to obtain large interfacial area and increase stress relaxation arisen from volume increase. Electrochemical performance of SnO2/Sn/ MWCNT nanocomposite electrode was comprised both with SnO2/Sn thin film onto stainless steel substrates and MWCNT buckypaper. SnO2/Sn/MWCNT nanocomposite showed outstanding performance with high capacity and satisfactory cycling stability. Even at 100 cycles no cell failure was detected for all the studied cell assemblies. Extremely high discharge capacity of nanocomposite electrodes makes them good candidates for possible microbattery applications. Acknowledgments The authors would like to acknowledge the financial support of Scientific and Technical Research council of Turkey (TUBITAK) under the contract number 109M464 and Sakarya University, Coordination of Scientific Research Project (BAPK) under the contract number 2010-50-02-017. References
Fig. 7. Rate capacity of SnO2/Sn/MWCNT nanocomposite at different C rates.
the semicircular curve. From the Table 1, it can be concluded that nanocomposite SnO2/Sn/MWCNT electrode has lower charge transfer resistance and stable solid electrolyte interface compared with SnO2/Sn electrode. This is attributed to the excellent contribution of MWCNT buckypapers leading high electronic conductivity, superior load bearing nature and stress distribution to prevent electrode pulverization. Fig. 7 shows the rate performance of the electrodes made from SnO2/Sn/MWCNT nanocomposite at various C-rates ranging from 0.1 to 4 C. In Fig. 7a, as the current density increases from 0.2 to 0.5 C, the discharge capacity decreases slightly from 980 to 820 mAh g–1. When the current density decreases from 2 to 0.2 C the discharge capacity yields the same capacity of approximately to 800 mAh g–1. In Fig. 7b, decreasing the current density from 4 to 0.1 C, the discharge capacity increases slightly from 420 to 810 mAh g–1, revealing the superior reversibility of the SnO2/Sn/ MWCNT gradient nanocomposite and their suitability as a high rate anode material.
[1] J. Song, M.-Z. Cai, Q.-F. Dong, M.-S. Zheng, Q.-H. Wu, S.-T. Wu, Electrochim. Acta 54 (2009) 2748–2753. [2] J. Mosa, J.F. Velez, I. Lorite, N. Arconada, M. Aparicio, J. Power Sources 205 (2012) 491–494. [3] S.H. Lee, S.H. Jee, K.S. Lee, S.C. Nam, Y.S. Yoon, Electrochim. Acta 87 (2013) 905– 911. [4] D. Ahn, X. Xiao, Y. Li, A.K. Sachdev, H.W. Park, A. Yu, Z. Chen, J. Power Sources 212 (2012) 66–72. [5] Y. Fu, R. Ma, Y. Shu, Z. Cao, X. Ma, Mater. Lett. 63 (2009) 1946–1948. [6] K.T. Lee, J. Cho, Nano Today 6 (2011) 28–41. [7] E.R. Leite, Nanostructured Materials Electrochemical for Energy Production and Storage, Springer, USA, 2009. [8] Y. Kim, Y. Yoon, D. Shin, J. Anal. Appl. Pyrol. 85 (2009) 557–560. [9] J. Ren, J. Yang, A. Abouimrane, D. Wang, K. Amine, J. Power Sources 196 (2011) 8701–8705. [10] U. Tocoglu, M. Alaf, O. Cevher, M.O. Guler, H. Akbulut, J. Nanosci. Nanotechnol. 12 (2012) 9169–9174. [11] M. Alaf, D. Gultekin, H. Akbulut, J. Nanosci. Nanotechnol. 12 (2012) 9058– 9066. [12] M. Alaf, H. Akbulut, J. Power Sources 247 (2014) 692–702. [13] M.T. Tilbrook, R.J. Moon, M. Hoffman, Compos. Sci. Technol. 65 (2005) 201– 220. [14] W. Han, A. Zettl, Nano Lett. 3 (2003) 681–683. [15] K. Vijayarangamuthu, S. Rath, J. Alloy. Compd. 616 (2014) 706–712, http:// dx.doi.org/10.1016/j.jallcom.2014.04.187. [16] E. Thune et al., Thin Solid Films (2014), http://dx.doi.org/10.1016/ j.tsf.2014.04.055. [17] J.H. Lee, B.-S. Kong, S.B. Yang, H.-T. Jung, J. Power Sources 194 (2009) 520–525. [18] M.-K. Song, S. Park, F.M. Alamgir, J. Cho, M. Liu, Mater. Sci. Eng. R 72 (2011) 203–252. [19] H. Gerengi, Y. Kocak, A. Jazdzewska, M. Kurtay, H. Durgun, Constr. Build. Mater. 49 (2013) 471–477. [20] T. Cetinkaya, M.O. Guler, H. Akbulut, Microelectron. Eng. 108 (2013) 169–176.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Double phase tinoxide/tin/MWCNT nanocomposite negative electrodes for lithium microbatteries Mirac Alaf ⇑, Deniz Gultekin, Hatem Akbulut Sakarya University, Engineering Faculty, Dept. of Metallurgy and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
a r t i c l e
i n f o
Article history: Received 17 October 2013 Received in revised form 14 May 2014 Accepted 30 June 2014 Available online 8 July 2014 Keywords: Tin oxide Carbon nanotube Lithium microbatteries Areal capacity
a b s t r a c t As anode material for lithium microbatteries, SnO2/Sn/MWCNT nanocomposites were produced by two steps; thermal evaporation and subsequent plasma oxidation. Metallic tin was first evaporated onto free-standing MWCNT buckypapers having controlled porosity and subsequent RF plasma oxidized in Ar:O2(1:1) gas mixture. The ratio between metallic tin (Sn) and tinoxide (SnO2) was controlled with plasma oxidation time. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to determine the structure and morphology of the obtained nanocomposites. The discharge/charge tests, electrochemical impedance spectroscopy (EIS) and rate capability tests were carried out to characterize the electrochemical properties of the composite. The flexible and free-standing SnO2/Sn/MWCNT nanocomposite electrode showed core–shell and functionally gradient composite structure. SnO2/Sn electrode was failed after 30 cycles and MWCNT paper electrode displays only 75 mAh g1 capacity value between 10 and 100 cycles. SnO2/Sn/MWCNT double phase electrode, exhibited 610 mAh g1 specific discharge capacity even after 100 cycles. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Energy storage technology is becoming an important research area to provide needs of today’s wireless and mobile applications. Rechargeable Li-ion batteries are aroused a lot of interest because of their high voltage, high energy density, stable cycling, and environmental-friendly properties. Nowadays, the continuous miniaturization of electronic devices such as implantable medical devices, smartcards, microelectromechanical systems (MEMS), remote sensors, and RFID tags is led to the growing demands of micropower sources [1]. Rechargeable microbatteries are recently become the topic of widespread research for use in low power applications and energy storage systems for electronic devices. The general requirements of the microbatteries for these applications are high specific energy, wide range of temperature stability, low self-discharge rate and flexibility of cell design [2]. Tin, tinoxide and tin based composites as candidate of electrodes of Li-ion batteries are very attractive because of their characteristic properties such as high specific lithium storage capacities, thermal and chemical stability and environmentally friendly [1,3,4]. However, during the charge–discharge process, tin based electrodes show large volume changes, which cause pulverization of the electrodes after a few cycles [5]. In order to ⇑ Corresponding author. Tel.: +90 264 295 6772; fax: +90 264 295 5601. E-mail address: [email protected] (M. Alaf). http://dx.doi.org/10.1016/j.mee.2014.06.029 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.
improve cycling performances of tin based electrodes, more than one solution was offered and studied in present study. Among the solutions that improve cycling performances of the batteries, production of electrodes in nanostructure is widely studied [6]. High energy density and high power density (the same as rate capabilities) can be achieved from electrodes composed of nanostructured materials. Nanostructured electrodes provide large electrode–electrolyte interfacial area coupled with short diffusion distances within the electrodes themselves [7]. Beside nanostructured electrode, using of SnO2/Sn composites could be a solution in realizing increased reversible capacity as well as reduced irreversible capacity and capacity fade upon cycling, as this could increase the Sn:Li2O ratio in the anode matrix [8]. Another impressive way to obviate volume change problems is to disperse tin based materials in a carbon matrix or encapsulate them with carbon especially carbon nano tubes to accommodate the strain of volume change during the alloying and dealloying processes. Use of carbon as matrix support on which tin based nanoparticles are able to good accommodate the mechanical stress experienced during volume changes [9]. As carbon matrix, MWCNT buckypapers are popular because they offer unique features such as flexibility, ability to be free-standing, having porous structure and good mechanical stability. Carbon nano tube paper, also called buckypapers, are self-supporting networks of entangled CNT assemblies arranged in a random fashion and held together by van der Waals interactions at the tube–tube junctions [10].
144
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
In this study; SnO2/Sn double phase structure formed on the MWCNT buckypapers via thermal evaporation and subsequent plasma oxidation. It was targeted to produce free-standing, flexible electrodes that have good cycling performances. The electrode was produced as compositional gradient materials with changing Sn in the MWCNT buckypaper pores gradually to provide stress relaxation for high cycle electrochemical applications. The electrochemical performance of this electrode was compared to a SnO2/Sn coating deposited onto stainless steel and bare MWCNT buckypaper. 2. Experimental details The MWCNTs supplied from Arry Nano (Germany) was employed in this work. Purification and chemical oxidation of MWCNTs was carried out with different oxidation agents [10]. Aqueous MWCNTs suspension was sonicated for 1 h to form a well dispersed suspension, which subsequently vacuum filtered through PVDF membrane filters of 220 nm pore size to form buckypapers. After drying at room temperature in a vacuum oven for 24 h the MWCNT buckypapers were peeled off from the filtration membrane. For the two stages electrode production, high purity metallic tin (99.999%) was firstly evaporated onto MWCNT buckypaper using a multifunctional PVD unit. During the thermal evaporation, chamber was filled with argon (Ar) a pressure of 1 Pa. The evaporation was carried out at 100 A current and from a tungsten (W) boat. In the second step, Sn/MWCNT nanocomposite was subjected to RF plasma oxidation process using oxygen (O2) and argon (Ar) gas mixture in the ratio of 1:1 for 1 h. The detailed experimental work, effects of evaporation time and plasma oxidation times on structural and electrochemical properties were provided in our previous studies [11,12]. An XRD (Rigaku D/MAX 2000) with Cu-Ka radiation has been used to determine the composition, relative phase amounts and structure of the nanocomposites. Transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN) and field emission gun scanning electron microscopy (FEG-SEM, JEOL 6335F) were used for examine surface morphologies. TEM samples of the anode paper were prepared by removing a small piece from the paper and mounting it on a folding copper mesh oyster grid. Coin-type (CR2016) test cells were assembled in an argon-filled glove box and the details of the CR2016 button type cell assembling can be found in our previous work [12]. The cells were cyclically tested on a MTI BST8-MA Battery Analyzer using constant current density over a voltage range of 0.1–2 V for SnO2/Sn on stainless steel electrodes and 0.05–3 V for SnO2/Sn on MWCNT buckypapers. Electrochemical impedance spectroscopy (EIS) measurements were carried out on the samples using a sine wave of 10 mV amplitude over a frequency range of 100 kHz–0.01 Hz using Gamry Instrument Version 5.67. The mass of active material in the SnO2/Sn/ MWCNT nanocomposite electrode was calculated as:
M active ¼ M SnO2 =Sn þ M C
ð1Þ
where MSnO2 =Sn was the SnO2/Sn coating’s mass and MC was mass of the SnO2/Sn coated carbon nano tubes participating in the lithiation/delithiation process. Depth of SnO2/Sn coated carbon nano tubes were measured with the aid of dot-map EDS analyzes and mass of CNT was calculated. 3. Results and discussion MWCNT buckypaper was produced as flexible, uniform, smooth and crack-free disks using chemically oxidized MWCNTs via vacuum filtration method. After vacuum drying, paper was easily
peeled off from membrane. Fig. 1 presents SEM-EDS dot mapping analysis result after thermal evaporation of metallic tin onto MWCNT buckypaper from cross-section area. Evaporated metallic tin is mainly located in between MWCNT buckypaper pores and Sn rich layer depth is about 5 lm. Along the red line, elemental composition was analyzed from 10 points with EDS. According to EDS results, the variation of atomic percent of tin (Sn) is shown in the graph. It can be concluded from the dot-map analysis that Sn shows a gradient composition through the center of buckypaper. It is well known the gradient phase distribution in the composites is beneficial for decreasing crack initiation and therefore, failure [13]. This implies that this functionally gradient structure will cause stress relaxation during the electrochemical reactions. After the thermal evaporation process, Sn/MWCNT sample was subjected to a RF plasma oxidation process at oxygen (O2) and argon (Ar) gas mixture in the ratio of 1:1 to produce SnO2/Sn/ MWCNT nanocomposite. Fig. 2 shows SEM image and XRD result. No obvious reflection peaks from impurities are detected, providing evidence of the high purity of the product as shown in Fig. 2a. The nanocomposites have Sn, SnO2 and C peaks, which agrees well with the standard data files (Sn: JCPDS No. 01-0892958 and SnO2: JPDS No. 00-041-1445), and all of the nanocomposites have a crystalline structure. It should be noted that a diffraction peak at around 26°, which is the main peak of tetragonal SnO2 (1 1 0) almost overlaps with the main peaks of hexagonal C (0 0 2) which confirmed with works in the literature [14]. Sn based phases were deposited around the MWCNTs forming a core–shell structure. As can be seen from the high magnification micrograph located at the upper-right corner of the Fig. 2b, Sn was transformed to SnO2 as very fine nanoparticles on the MWCNT surfaces showing core–shell structure. SnO2/Sn/MWCNT nanocomposites were further analyzed by TEM in order to confirm the presence of Sn and SnO2 nanoparticles onto MWCNT surfaces. TEM sample was prepared by removing a small piece of the anode paper and mounting it onto a folding copper mesh oyster grid. Fig. 3a presents image of uncoated MWCNT and coated MWCNT with SnO2/Sn. Fig. 3b implies that the crystal size of the deposited SnO2/Sn is very fine and shows the lattice fringes of Sn and SnO2 nanoparticles. As can be seen from Fig. 3b, 0.29 nm d-spacing is assigned to the (2 0 0) plane of tin (Sn) and 0.33 nm d-spacing is ascribed to the (1 1 0) plane of cassiterite (SnO2). The similar Tem observations and related diffraction pattern were studied in the work of Rath et al. and Thune et al. [15,16]. For comparing a SnO2/Sn coating deposited onto stainless steel and bare MWCNT buckypaper were investigated in electrochemically. Fig. 4 presents the discharge capacities per gram and cm2 (geometric area) vs. cycle number for cells made from SnO2/Sn/ MWCNT free-standing electrode, MWCNT buckypaper and SnO2/ Sn thin film electrode. The cell assembled from SnO2/Sn thin film on the stainless steel is failed after approximate 30 cycles. Initial discharge capacity of SnO2/Sn thin film is 1100 mAh g1 and capacity fading rate is very fast after 10 cycles. It is believed this capacity decrease is because of Sn based electrode disintegration caused by a high-volume increase [5]. Free-standing MWCNT paper electrode displays 415 mAh g1 initial discharge capacity and this value is fixed at approximately 75 mAh g1 after 10 cycles. The initial discharge capacity of SnO2/Sn/MWCNT free standing electrode is 1544 mAh g1. The capacity retention is still very high as 610 mAh g1 even after 100 cycles electrochemical test. Compared with the SnO2/Sn double phase structures deposited on the stainless steel coin electrodes, the SnO2/Sn/MWCNT nanocomposite showed outstanding performance with high capacity and satisfactory cycling stability. The mechanisms of electrode operation during lithium insertion and extraction are shown in Fig. 5. The large volume variation leads to the pulverization of the SnO2/Sn thin film electrode after cycling. However, SnO2/Sn/MWCNT nanocomposite
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
145
Fig. 1. SEM-EDS dot mapping analysis result of composite from cross-section and atomic percent of tin (Sn) along with red line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. (a) XRD pattern and (b) FESEM image of SnO2/Sn/MWCNT nanocomposite.
provided a solution to dimensional instability problems by limiting the changes in volume of electrode. The MWCNT buckypaper acts as both mechanical support and electrical conductor. The buffering effect of SnO2/Sn/MWCNT nanocomposite gives the battery dimensional stability and can prevent the repeated aggregation and pulverization of the dispersed tin nanoparticles during charging and discharging. In our design the pulverization of electrode material was thought to be tolerated by producing firstly a porous current collector and then coating the MWCNT surfaces with a thin shell
Fig. 3. TEM images of SnO2/Sn coated MWCNT at (a) lower magnification and (b) higher magnification.
of active tin based materials. In the case of tin based coatings on the stainless steel coins the only stress relaxation mechanism was the stress accommodation between the nanograins and
146
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147
Fig. 4. Discharge capacities of electrodes per gram and cm2 (geometric area) vs. cycle number.
mesoporous structure, which is not well enough to accommodate the stresses arised from the volume increase during Li intercalation. However, designing a highly porous buckypaper with flexible and very high elastic modulus MWCNTs expected to behave as a buffer materials to provide bearing of the stresses. Electrochemical results are the unique evidences that our imagined model works very well and very high capacity retention values were obtained after 100 cycle test [17,18]. Electrochemical Impedance Spectroscopic (EIS) measurements were carried out to confirm the effect of MWCNTs on changing the charge transfer resistance in SnO2/Sn and SnO2/Sn/MWCNT nanocomposite electrodes using a sine wave of 10 mV amplitude over a frequency range of 100 kHz to 0.1 Hz. Fig. 6 shows the Nyquist plots obtained in the Li-ion cells before and after the electrochemical performance tests. The obtained impedance data were analyzed using the ZsimpWin 3.21 program which allowed the chisquare (v2) value to judge the quality of the equivalent circuit fitting [19]. Resistance values were calculated using the electrical circuit models R(CR)(QR)(CR) and R(Q(RW)). It can be seen from Fig. 6a, which shows EIS spectra of SnO2/Sn electrode that diameter of EIS spectra increases even after 5 cycles electrochemical tests due to pulverization of electrodes after cycling, and this leads to loss of electronic contact between current collector and active materials [20]. Pulverization of the electrode in the SnO2/Sn coated on the stainless steel resulted in decreasing diffusion since the Warburg diffusion curve tend to become flattened after 5 cycles. In the case of SnO2/Sn/MWCNT nanocomposite electrode shown in Fig. 6b, the charge transfer resistances are smaller than that of
Fig. 6. Electrochemical impedance spectra of (a) SnO2/Sn and (b) SnO2/Sn/MWCNT nanocomposite electrodes.
obtained in the SnO2/Sn electrodes for both before and after cycles. When the Warburg diffusion curve analyzed the diffusion of the Li ions are more active in the SnO2/Sn/MWCNT nanocomposite electrode and after 100 cycles there is no any slope change, which can be attributed excellent conductivity and stability of the electrode. The EIS data are also summarized in Table 1, which shows parameters of the equivalent circuit for the SnO2/Sn and SnO2/Sn/ MWCNT nanocomposite electrodes after fitting the diameter of
Fig. 5. The mechanisms of electrode operation before and after cycling.
147
M. Alaf et al. / Microelectronic Engineering 126 (2014) 143–147 Table 1 Impedance parameters of calculated from equivalent circuit models. Electrodes
Circuit models
Sn/SnO2
[R(CR)(QR)(CR)]
Sn/SnO2/MWCNT
[R(Q(RW))]
R s (O)
R ct (O)
Before cycling After 5 cycles
5.55 1.4
397 610
Before cycling After 100 cycles
5.2 5.8
257 589
4. Conclusions Double phase SnO2/Sn/MWCNT nanocomposite electrode was successfully produced using MWCNT buckypaper as flexible and free-standing by thermal evaporation and subsequent plasma oxidation. The double phase structure was achieved by plasma oxidation. Thermally evaporated tin was mainly inserted between MWCNT buckypaper pores leading to form functionally gradient structure. Depositing Sn on the MWCNTs and subsequent plasma oxidation caused to create a core–shell structure. This core–shell structure accompanied with compositional gradient structure led to obtain large interfacial area and increase stress relaxation arisen from volume increase. Electrochemical performance of SnO2/Sn/ MWCNT nanocomposite electrode was comprised both with SnO2/Sn thin film onto stainless steel substrates and MWCNT buckypaper. SnO2/Sn/MWCNT nanocomposite showed outstanding performance with high capacity and satisfactory cycling stability. Even at 100 cycles no cell failure was detected for all the studied cell assemblies. Extremely high discharge capacity of nanocomposite electrodes makes them good candidates for possible microbattery applications. Acknowledgments The authors would like to acknowledge the financial support of Scientific and Technical Research council of Turkey (TUBITAK) under the contract number 109M464 and Sakarya University, Coordination of Scientific Research Project (BAPK) under the contract number 2010-50-02-017. References
Fig. 7. Rate capacity of SnO2/Sn/MWCNT nanocomposite at different C rates.
the semicircular curve. From the Table 1, it can be concluded that nanocomposite SnO2/Sn/MWCNT electrode has lower charge transfer resistance and stable solid electrolyte interface compared with SnO2/Sn electrode. This is attributed to the excellent contribution of MWCNT buckypapers leading high electronic conductivity, superior load bearing nature and stress distribution to prevent electrode pulverization. Fig. 7 shows the rate performance of the electrodes made from SnO2/Sn/MWCNT nanocomposite at various C-rates ranging from 0.1 to 4 C. In Fig. 7a, as the current density increases from 0.2 to 0.5 C, the discharge capacity decreases slightly from 980 to 820 mAh g–1. When the current density decreases from 2 to 0.2 C the discharge capacity yields the same capacity of approximately to 800 mAh g–1. In Fig. 7b, decreasing the current density from 4 to 0.1 C, the discharge capacity increases slightly from 420 to 810 mAh g–1, revealing the superior reversibility of the SnO2/Sn/ MWCNT gradient nanocomposite and their suitability as a high rate anode material.
[1] J. Song, M.-Z. Cai, Q.-F. Dong, M.-S. Zheng, Q.-H. Wu, S.-T. Wu, Electrochim. Acta 54 (2009) 2748–2753. [2] J. Mosa, J.F. Velez, I. Lorite, N. Arconada, M. Aparicio, J. Power Sources 205 (2012) 491–494. [3] S.H. Lee, S.H. Jee, K.S. Lee, S.C. Nam, Y.S. Yoon, Electrochim. Acta 87 (2013) 905– 911. [4] D. Ahn, X. Xiao, Y. Li, A.K. Sachdev, H.W. Park, A. Yu, Z. Chen, J. Power Sources 212 (2012) 66–72. [5] Y. Fu, R. Ma, Y. Shu, Z. Cao, X. Ma, Mater. Lett. 63 (2009) 1946–1948. [6] K.T. Lee, J. Cho, Nano Today 6 (2011) 28–41. [7] E.R. Leite, Nanostructured Materials Electrochemical for Energy Production and Storage, Springer, USA, 2009. [8] Y. Kim, Y. Yoon, D. Shin, J. Anal. Appl. Pyrol. 85 (2009) 557–560. [9] J. Ren, J. Yang, A. Abouimrane, D. Wang, K. Amine, J. Power Sources 196 (2011) 8701–8705. [10] U. Tocoglu, M. Alaf, O. Cevher, M.O. Guler, H. Akbulut, J. Nanosci. Nanotechnol. 12 (2012) 9169–9174. [11] M. Alaf, D. Gultekin, H. Akbulut, J. Nanosci. Nanotechnol. 12 (2012) 9058– 9066. [12] M. Alaf, H. Akbulut, J. Power Sources 247 (2014) 692–702. [13] M.T. Tilbrook, R.J. Moon, M. Hoffman, Compos. Sci. Technol. 65 (2005) 201– 220. [14] W. Han, A. Zettl, Nano Lett. 3 (2003) 681–683. [15] K. Vijayarangamuthu, S. Rath, J. Alloy. Compd. 616 (2014) 706–712, http:// dx.doi.org/10.1016/j.jallcom.2014.04.187. [16] E. Thune et al., Thin Solid Films (2014), http://dx.doi.org/10.1016/ j.tsf.2014.04.055. [17] J.H. Lee, B.-S. Kong, S.B. Yang, H.-T. Jung, J. Power Sources 194 (2009) 520–525. [18] M.-K. Song, S. Park, F.M. Alamgir, J. Cho, M. Liu, Mater. Sci. Eng. R 72 (2011) 203–252. [19] H. Gerengi, Y. Kocak, A. Jazdzewska, M. Kurtay, H. Durgun, Constr. Build. Mater. 49 (2013) 471–477. [20] T. Cetinkaya, M.O. Guler, H. Akbulut, Microelectron. Eng. 108 (2013) 169–176.