- Email: [email protected]
Ni thick film as high performance anode for lithium-ion batteries
Journal of Power Sources 272 (2014) 794e799
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrodeposited three-dimensional porous SieOeC/Ni thick film as high performance anode for lithium-ion batteries Xin Qian a, Tao Hang a, *, Hiroki Nara b, Tokihiko Yokoshima b, Ming Li a, Tetsuya Osaka b, ** a
State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, China b Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A 3D porous SieOeC/Ni thick film anode is electrodeposited from organic electrolyte. The 3D porous structure enhances adhesive force between SieOeC composite and substrate. The highly porous structure acts as buffer matrix to accommodate volume change of Si. This anode exhibits high capacity and excellent cyclability at 0.5 C-rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2014 Received in revised form 3 September 2014 Accepted 5 September 2014 Available online 16 September 2014
A novel 3D porous SieOeC/Ni thick film anode is successfully prepared by electrodeposition of porous Ni on Cu substrate and galvanostatical electrodeposition of SieOeC composite on porous Ni substrate. The 3D porous SieOeC/Ni thick film is electrochemically activated at a current density of 50 mA cm2 for the first cycle and 200 mA cm2 (0.5 C) for the subsequent cycles, it displays superior electrochemical performance with discharge capacity of 706.3 mAh g1 of Si after 100 cycles. The properties of this thick film is analyzed by field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy with energy dispersive X-ray analyzer (STEM-EDX). The results show that SieOeC composite not only covers the surface area of porous Ni but also attaches to the highly porous dendritic walls, along with the porous structure of Ni which provides proper accommodation for the volume change of silicon during the lithiation/delithiation processes, are believed to result in the high capacity and excellent cyclability. © 2014 Elsevier B.V. All rights reserved.
Keywords: SieOeC/Ni thick film 3D porous anode Electrodeposition Organic electrolyte Lithium-ion battery
1. Introduction Lithium-ion batteries with high power density and outstanding cyclability have been regarded as one of the most promising energy storage devices [1,2]. Recently, the rapid development of portable
* Corresponding author. Tel./fax: þ86 21 3420 2748. ** Corresponding author. Tel.: þ81 3 5286 3202; fax: þ81 3 3205 2074. E-mail addresses: [email protected] (T. Hang), [email protected] (T. Osaka). http://dx.doi.org/10.1016/j.jpowsour.2014.09.042 0378-7753/© 2014 Elsevier B.V. All rights reserved.
electronic devices and battery-operated electric vehicles urgently promote the research of next generation lithium-ion batteries [3]. Silicon has attracted much attention due to its low discharge potential and extremely high theoretical capacity of 4200 mAh g1 (Li4.4Si alloy), which is more than ten times higher than that of graphite electrode with the theoretical capacity of 372 mAh g1 [4]. However, silicon suffers great volume change (~300%) in the charge (lithiation)/discharge (delithiation) processes, which induces serious structural stability problems of the electrode materials and loss of electrical contact with the current collector, as the result, the
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
795
electrode shows poor electrochemical performance including rate capability and cycle life. In order to accommodate the great volume change of active material silicon during the charge/discharge cycles, much attention has been paid on the exploration of silicon-based nanomaterials, such as silicon nanowires, nanotubes, coreeshell structures and thin films [5e12]. Recently, a novel SieOeC composite thin film [9e12] has been synthesized on copper substrate by electrodeposition of Si in organic electrolyte, which delivers high discharge capacity of 842 mAh g1 after 7200 cycles with excellent cycling stability. However, the SieOeC composite thin film is easy to peel off from flat copper current collector during charge/discharge cycles which impedes its usage. While silicon composite electrodeposited on nickel micro-nanocones hierarchical structured current collector really reinforces adhesion strength with the substrate during cycling to some extent [11]. Nevertheless, the average size of micro-nanocones is only 500 nm in height, which limits the reinforcing effect to several hundreds of namometers in height from the bottom. However, more thickness is needed to develop high capacity lithium-ion battery using the SieOeC film. Buffer of volume change of the thickness during the charge and discharge process is needed. Moreover, ion pass in the thick SieOeC film and high conductivity of the film are needed to use thick films. Thus, the substrate with 3D macrostructure realizing good adhesion for deposit, ion pass and conductivity, is good technique to apply the thick SieOeC film to anode of lithium secondary battery. In this paper, we report a three-dimensional porous SieOeC/Ni thick film using electrodeposition of porous nano-Ni on copper substrate [13,14]. The thick film can be directly used as anode without adding binders or conductive additives. The 3D porous nano-metal (Ni or Cu) composed of homogeneously distributed pores and dendritic walls have drawn broad attention since which can provide high surface area, short diffusion length of ions and strong mechanical support [15e17]. In the meanwhile, we also focus on the improvement of galvanostatical electrodeposition parameters for the purpose to fabricate SieOeC composite thick film which is both time-consuming and high-cost for magnetron sputtering, physical vapor deposition (PVD) or chemical vapor deposition (CVD) method [6]. Fig. 1 shows a schematic of typical fabrication procedure for 3D porous SieOeC/Ni thick film and lithiation process in the first activation cycle. The design contains size-fitted pores as good buffer to accommodate the volume change of silicon as well as rough surface area to improve adhesion strength between the SieOeC composite and current collector. Then the SieOeC composite is electrodeposited on porous Ni as anode for energy storage purpose, the diameter of these pores decreases obviously and many pores are almost filled, these changes are due to great volume expansion of active material silicon during first lithiaiton process.
fabrication of porous Ni was conducted in a glass container with a Pt counter electrode and the Cu foil as a working electrode, the working area of Cu foil was precisely controlled at 1.00 cm2 with the other surface areas covered by insulting tapes, the electrolyte contained 2 mol L1 NH4Cl (Sinopharm Chemical Reagent, 99.5%) and 0.1 mol L1 NiCl2$6H2O (Sinopharm Chemical Reagent, 98.0%) at a pH value of 3.5 [14]. Before electrodeposition, the Cu foil was immersed in 20 v/v% H2SO4 for 15 s, rinsed with deionized water, and dried by the heater blower. Electrodeposition of porous Ni was performed at a current density of 3.0 A cm2 for 60 s with violently stirring of the electrolyte at room temperature. Then the specimen was rinsed with deionized water, dried in vacuum chamber overnight and transferred to glove box filled with Ar atmosphere. In the second step, the organic electrolyte containing 0.5 mol L1 SiCl4 (Tokyo Chemical, 98.0%) and 0.5 mol L1 tetrabuthylammonium perchlorate in propylene carbonate (TBAP, Energy Chemical, 98%/PC, ACROS Organics, 99.5%) were prepared in glove box with dew point below 100 C. The galvanostatical electrodeposition was conducted in a three-electrode glass cell equipped with a Pt quasi-reference electrode, a Pt counter electrode and 3D porous Ni on Cu foil as a working electrode, the whole experiment was performed in Ar atmosphere with dew point below 100 C. A constant cathodic current of 1.0 mA cm2 was applied to pass a charge of 10 C cm2 for electrodeposition of SieOeC composite. The 3D porous SieOeC/Ni thick film was transferred into another three-electrode glass cell containing 1 mol L1 LiPF6 electrolyte dissolved in EC:DMC (1:1 v/v) (Dongguan Shanshan Battery Material, water content less than 20 ppm), lithium metal foil as reference and counter electrodes. Electrochemical performance of the thick film as anode for lithium-ion battery was measured with a current density of 50 mA cm2 for activation in the first lithiation/ delithiation cycle and 200 mA cm2 (0.5 C) in the subsequent cycles in the potential range between 0.01 V and 1.20 V vs. Li/Liþ. Cyclic voltammetry (CV) was carried out on this anode in a potential window of 0.01e1.50 V vs. Li/Liþ at a scan rate of 0.1 mV s1. The galvanostatic charge/discharge tests were performed on a battery testing system (Kejing, Shenzhen) and CV was conducted on an electrochemical workstation (CHI660E, Chenhua, Shanghai) at room temperature. The surface morphology of the 3D porous SieOeC/Ni film was characterized with field emission electron microscopy (FESEM, Siron 200), the FESEM also provided energy dispersive X-ray analyzer (EDX) which enabled elemental analysis of SieOeC composite thick film. The crystallographic structure of the film was analyzed by means of scanning transmission electron microscopy with EDX (STEM-EDX, JEM 2010).
2. Experimental
The SEM images of electrodeposited 3D porous Ni film are shown in Fig. 2(a)e(c). A top view of porous Ni film in Fig. 2(a) clearly shows the pores are homogeneously distributed. The thickness of the 3D porous Ni film is approximately 15 mm which was measured from the cross-sectional SEM image (not shown here), both the thickness and morphology of cross-sectional porous Ni film are consistent with the result shown in cross-sectional SEM image of porous Ni film in Ref. [14]. Fig. 2(b) demonstrates that the structure is composed of not only highly porous dendritic walls but also numerous pores with feature size approximately 8 mm in diameter. Fig. 2(c) shows magnified SEM image of Ni particles at nanometer scale with feature size ranging from several hundreds of nanometers to about 1 mm in diameter. After electrodeposition of SieOeC composite film on the porous Ni substrate, the SEM images of SieOeC composite film are shown
The preparation of 3D porous SieOeC/Ni thick film included two-steps electrodeposition. All solvents and chemicals were of reagent quality without further purification. In the first step,
Fig. 1. Schematic of fabrication process of the electrodeposited three-dimensional porous SieOeC composite thick film and lithiation process.
3. Results and discussion
796
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
Fig. 2. Top view SEM images of surface morphologies of electrodeposited 3D porous Ni (a), (b), (c) and 3D porous SieOeC/Ni thick film (d), (e), (f).
in Fig. 2(d)e(f). Fig. 2(d) shows that the SieOeC composite are homogenously distributed, Fig. 2(e) demonstrates that the SieOeC/ Ni thick film maintains highly porous structure, the dendritic walls become much more compact and inside the pores are filled with SieOeC composite to different extent, thus the average diameter of these pores decreases to approximately 5 mm Fig. 2(f) shows the image of a Ni particle covered by SieOeC composite at nanometer scale. Compared with Fig. 2(c), it is obvious that SieOeC composites can homogenously attach themselves to the rough surface of Ni particles by electrodeposition method in organic electrolyte. The elemental composition and crystallographic structure of the SieOeC composite thick film was investigated by STEM. The existence of elemental Si, O and C was confirmed by STEM-EDX, Fig. 3(a) shows EDX result that Si, O, C and Cl were detected, Fig. 3(b) shows dark field image of part of SieOeC composite and elemental mappings of Si, O and C, which demonstrates that these three elements were homogeneously distributed at nanometric scale in the composite. TEM image shown in Fig. 3(c) demonstrates that the SieOeC composite film has a highly porous structure with nanometric interparticle spacing, which is consistent with the cross-sectional SEM analysis of 3D porous SieOeC composite film. Fig. 3(d) shows the selected area electron diffraction (SAED) pattern (inset in Fig. 3(c)) of circle area highlighted with white solid line in Fig. 3(c), the diffraction ring in the SAED pattern proves the amorphous characteristics of silicon in the composite. Fig. 3(e) shows the high resolution transmission electron microscopy (HRTEM) image of the rectangle area highlighted with white solid line in Fig. 3(c), no lattice fringe images can be observed from the view, which corresponds quite well to the result that elemental silicon is amorphous. And the elemental C and O are decomposition products of organic electrolytes, which act as good buffer to accommodate great volume change during charge/discharge cycling. Considering the XPS analysis of SieOeC composite in our
previous report [9], it is suggested that the SieOeC composite thick film is composed of Si existing in an oxidized state and decomposition products of the organic electrolyte. The cycling performance and coulombic efficiency of the 3D porous SieOeC/Ni thick film anode are shown in Fig. 4(a), the porous SieOeC/Ni thick film anode was electrochemically activated at a current density of 50 mA cm2 for the first cycle, then charged/discharged at a current density of 200 mA cm2 (0.5 C) for 100 cycles. All charge/discharge capacities are calculated based on the assumption that the SieOeC composite maintains its maximum mass of 0.5 mg with a passed charge of 10 C during electrodeposition and silicon achieves its content of 60% (maximum mass of Si is 0.3 mg), which have been measured with a passed charge of 2 C in previous work [9]. The lithiation/delithiation processes during first activation cycle deliver a charge/ discharge capacity of 5315 mAh g1 of Si (1.5945 mAh cm2) and 1530 mAh g1 of Si (0.4588 mAh cm2), respectively. The extremely high irreversible capacity loss (~70%) is mainly attributable to the reduction of SiOx and formation of solid electrolyte interphase (SEI) film [9,10,18e20]. It is important to note that we did not consider the activation cycle in all schematics plotting processes for different current density reasons. The charge/ discharge capacity in the first cycle with a current density of 200 mA cm2 (0.5 C) are 1408 mAh g1 of Si and 1105 mAh g1 of Si, respectively, the coulombic efficiency rapidly rises to 92% in the fourth cycle and keeps steady from 5th to 100th cycle. The discharge capacity maintains 1025.3 mAh g1 of Si in 5th cycle and 706.3 mAh g1 of Si in 100th cycle, respectively, leaving an average capacity retention of 98.8% per cycle. The excellent cycling performance is mainly attributable to the highly porous structure of SieOeC/Ni thick film which serves as compact buffering matrix to accommodate the volume change of silicon during charge/ discharge processes.
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
797
Fig. 3. STEM image of the SieOeC composite thick film: (a) EDX analysis; (b) dark field image and elemental mappings of Si, O, C; (c) TEM image of SieOeC composite film; (d) SAED pattern of circle area in (c); (e) HRTEM image of rectangle area in (c).
The charge/discharge potential profiles of the 3D porous SieOeC/Ni thick film anode in the 1st, 2nd, 10th, 50th, 100th cycles between 0.01 V and 1.20 V at a current density of 200 mA cm2 (0.5 C) are shown in Fig. 4(b) (the activation cycle excluded). From 1st to 10th cycle, the first pseudo plateau during charge process gradually increases from 0.28 V to 0.34 V, and the pseudo plateau remain at 0.34 V in the subsequent cycles, the second pseudo plateau at about 0.14 V is also shown in the profiles, and the following potential gradually drops to 0.01 V which is the typical charge behavior of amorphous silicon [21e23]. The two pseudo plateaus during charge process indicate insertion of Liþ into amorphous silicon to transition to a series of LixSi alloys. Furthermore, inset graph in Fig. 4(b) shows the CV profile of 3D porous SieOeC/Ni thick film anode after activation and fifteen charge/ discharge cycles. During the charge process, the reduction peak near 0.14 V in the cathodic branch is observed which is probably attributable to the transformation of amorphous silicon to a series of LixSi phase, the result of which is consistent with the charge/ discharge potential profiles analysis. While in the discharge process, two oxidation peaks near 0.35 V and 0.5 V in the anodic branch are observed, respectively. It is entirely possible due to a two-step transformation from LixSi phase to amorphous silicon [21]. The rate performance of 3D porous SieOeC/Ni thick film anode was also investigated by applying a series of discharge current
varying from 0.5 C to 10 C, which is shown in Fig. 5(a). The electrochemical cell was activated with a current density of 50 mA cm2 for the first cycle, then galvanostatically charged/discharged with current density of 200 mA cm2, 400 mA cm2, 600 mA cm2, 800 mA cm2, 1000 mA cm2, 400 mA cm2 (correspond to 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C respectively) for 10 cycles, respectively. The 3D porous SieOeC/Ni thick film anode maintained discharge capacity of 992.7 mAh g1, 766 mAh g1, 637.7 mAh g1, 465 mAh g1, 255.3 mAh g1 and 737 mAh g1, respectively, at 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C. The capacity retention increasingly decreases as current rises, but the coulombic efficiency keeps rising as current increases, upwards 99.2%, and keeps a steady profile at each rate in the meanwhile. Moreover, when the current returns to 1.5 C under condition that the anode was cycled at higher rates for many cycles, the discharge capacity could be recovered. The highly porous structure, rough surface with strong adhesion force and excellent accommodation ability of silicon volume change are responsible for the remarkable rate performance of the 3D porous SieOeC/Ni thick film anode. The charge/discharge potential profiles of the 3D porous SieOeC/Ni thick film anode at 5th, 15th, 25th, 35th, 45th, 55th cycle which corresponds to 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C-rate are shown in Fig. 5(b). From 0.5 C-rate to 10 C-rate, the plateau during charge process decreases from 0.34 V to 0.14 V apparently, the charge capacity decreases obviously as C-rate increases. However, the
798
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
Fig. 4. (a) Cycling performance of 3D porous SieOeC/Ni thick film anode with a current density of 200 mA cm2 (0.5 C); (b) Potential profiles between 0.01 V and 1.20 V, inset graph shows CV profile at a scan rate of 0.1 mV s1 after activation and fifteen charge/discharge cycles.
charge plateau returns to 0.29 V at the 55th cycle when the current returns to 1.5 C-rate, which corresponds quite well to the charge plateau of the 15th cycle at 1.5 C-rate. On the other hand, the plateau at about 0.35 V and 0.5 V are observed in the 5th discharge process at 1.5 C-rate, while the first plateau at about 0.35 V disappears before the 15th discharge process and it increases to 0.38 V (15th cycle, 1.5 C), 0.46 V (25th cycle, 3 C), 0.53 V (35th cycle, 5 C), 0.62 V (45th cycle, 10 C). The second plateau at about 0.5 V gradually increases to 0.55 V (15th cycle, 1.5 C), 0.62 V (25th cycle, 3 C), 0.68 V (35th cycle, 5 C) and 0.72 V (45th cycle, 10 C). However, the plateau returns to 0.38 V and 0.55 V in the 55th discharge process at 1.5 C-rate when the current returns to 1.5 C-rate. These characteristics of the charge/discharge profiles during cycling at various current rates obviously demonstrate that electrochemical performance of the SieOeC/Ni thick film anode will strongly decrease under high-rate cycling, while the performance recovers when the current returns to 1.5 C strongly indicates electrochemical stability of the anode material. To further understand how the morphology of SieOeC composite thick film changes and how the excellent cycling performance is maintained during electrochemical cycling, the 3D porous SieOeC/Ni thick film anode was retained for SEM investigation at first fully lithiated state (Fig. 6(a)) in the activation cycle with a current density of 50 mA cm2, the other 3D porous SieOeC/Ni thick film anode was also retained for SEM analysis after activation cycle at first fully delithiated state (Fig. 6(b)). Fig. 6(a) shows obvious volume expansion of SieOeC composite thick film, dramatic decrease of both amount and average diameter of these pores and many small cracks formed on the surface area. These features are consistent with the fully lithiated state of silicon with over 300% volume expansion in the first electrochemical cycle. Fig. 6(b) shows
Fig. 5. (a) Rate performance of 3D porous SieOeC/Ni thick film anode at various currents of 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C; (b) Potential profiles between 0.01 V and 1.20 V of 5th (0.5 C), 15th (1.5 C), 25th (3 C), 35th (5 C), 45th (10 C), 55th (1.5 C) cycles at various current rates.
that the volume expansion phenomenon of SieOeC composite thick film almost disappeared, while the average diameter of these pores remained almost unchanged compared with Fig. 6(a). The existence of thick solid electrolyte interphase on the surface of 3D porous SieOeC/Ni architecture can provide straightforward evidence for the extremely high irreversible capacity loss in the first charge/discharge cycle. Fig. 6(c) and (d) show the SEM image of 3D porous SieOeC/Ni thick film anode at discharged state after 100 cycles with a current density of 200 mA cm2 (0.5 C) and its magnified image. Both the number and average diameter of the pores decrease obviously, which is attributable to the great volume expansion of silicon during lithiation process. However, only a few small cracks appeared on the surface of the film without further pulverization of the anode material, which can be clearly observed in Fig. 6(d). The cracks provided obvious evidence for capacity loss after 100 cycles, on the other hand, the well maintained structural integrity of the 3D porous SieOeC/Ni thick film reasonably accounted for the excellent electrochemical stability. 4. Conclusions In summary, a 3D porous SieOeC/Ni thick film anode for lithium-ion battery was successfully fabricated by a two-step electrodeposition method. The distinctive highly porous structure offers large surface area, strong adhesion force between active material and Ni dendrites, which not only enables homogenous electrodeposition of SieOeC composite but also acts as buffering matrix to accommodate the great volume change of silicon during charge/discharge (lithiation/delithiation) processes. Moreover, the specially designed 3D porous structure can be well maintained during electrochemical cycling processes. As a result, the 3D porous SieOeC/Ni thick film anode delivers an impressive reversible
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
799
Fig. 6. Top view SEM images of 3D porous SieOeC/Ni thick film (a) at fully lithiated state and (b) at fully delithiated state in the first cycle; (c) low magnification and (d) high magnification image at discharged state after 100 cycles.
capacity of 1105 mAh g1 of Si in the first cycle and 706.3 mAh g1 of Si in the 100th cycle at 0.5 C-rate, leaving an excellent average capacity retention over 98.8% per cycle. Thus, the unique substrate with 3D macrostructure realizing good adhesion for deposit, ion pass and conductivity, is good technique to apply the thick SieOeC film to anode of lithium secondary battery. Furthermore, the 3D porous SieOeC/Ni thick film prepared by electrodeposition method provides a simple approach for scaled-up manufacturing of silicon-based anode materials used in lithium-ion batteries due to their low cost and applicability of manufacturing process. Acknowledgments This work is sponsored by National Natural Science Foundation of China (No. 21303100) and Shanghai Natural Science Foundation (No. 13ZR1420400). References [1] [2] [3] [4]
C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Adv. Mater. 22 (2010) E28eE62. M. Armand, J.-M. Tarascon, Nature 451 (2008) 652e657. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2009) 587e603. W. Weydanz, M. Wohlfahrt-Mehrens, R. Huggins, J. power sources 81 (1999) 237e242.
[5] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31e35. [6] X.L. Chen, K. Gerasopoulos, J.C. Guo, A. Brown, C.S. Wang, R. Ghodssi, J.N. Culver, Adv. Funct. Mater. 21 (2011) 380e387. [7] L. Hu, H. Wu, Y. Gao, A. Cao, H. Li, J. McDough, X. Xie, M. Zhou, Y. Cui, Adv. Energy Mater. 1 (2011) 523e527. [8] L.-F. Cui, R. Ruffo, C.K. Chan, H. Peng, Y. Cui, Nano Lett. 9 (2009) 491e495. [9] H. Nara, T. Yokoshima, T. Momma, T. Osaka, Energy Environ. Sci. 5 (2012) 6500e6505. [10] H. Nara, T. Yokoshima, M. Otaki, T. Momma, T. Osaka, Electrochim. Acta 110 (2013) 403e410. [11] T. Hang, H. Nara, T. Yokoshima, T. Momma, T. Osaka, J. Power Sources 222 (2013) 503e509. [12] T. Osaka, H. Nara, T. Momma, T. Yokoshima, J. Mater. Chem. A 2 (2014) 883e896. [13] X. Xia, J. Tu, Y. Zhang, Y. Mai, X. Wang, C. Gu, X. Zhao, J. Phys. Chem. C 115 (2011) 22662e22668. [14] Y.Q. Zhang, X.H. Xia, X.L. Wang, Y.J. Mai, S.J. Shi, Y.Y. Tang, C.G. Cu, J.P. Tu, J. Power Sources 213 (2012) 106e111. [15] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610e1614. [16] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 16 (2004) 237e240. Kim, D. Wook [17] D. Youba aKim, J. Mater. Chem. A 2 (2014) 2478e2481. [18] A. Agrawal, A. Austin, J. Electrochem. Soc. 128 (1981) 2292e2296. [19] H. Kim, J. Cho, Nano Lett. 8 (2008) 3688e3691. [20] S. Zhang, Z. Du, R. Lin, T. Jiang, G. Liu, X. Wu, D. Weng, Adv. Mater. 22 (2010) 5378e5382. [21] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochem. Commun. 9 (2007) 796e800. [22] W. Wang, P.N. Kumta, Acs Nano 4 (2010) 2233e2241. [23] Y. Fan, Q. Zhang, Q. Xiao, X. Wang, K. Huang, Carbon 59 (2013) 264e269.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Electrodeposited three-dimensional porous SieOeC/Ni thick film as high performance anode for lithium-ion batteries Xin Qian a, Tao Hang a, *, Hiroki Nara b, Tokihiko Yokoshima b, Ming Li a, Tetsuya Osaka b, ** a
State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Rd., Shanghai 200240, China b Research Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A 3D porous SieOeC/Ni thick film anode is electrodeposited from organic electrolyte. The 3D porous structure enhances adhesive force between SieOeC composite and substrate. The highly porous structure acts as buffer matrix to accommodate volume change of Si. This anode exhibits high capacity and excellent cyclability at 0.5 C-rate.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 August 2014 Received in revised form 3 September 2014 Accepted 5 September 2014 Available online 16 September 2014
A novel 3D porous SieOeC/Ni thick film anode is successfully prepared by electrodeposition of porous Ni on Cu substrate and galvanostatical electrodeposition of SieOeC composite on porous Ni substrate. The 3D porous SieOeC/Ni thick film is electrochemically activated at a current density of 50 mA cm2 for the first cycle and 200 mA cm2 (0.5 C) for the subsequent cycles, it displays superior electrochemical performance with discharge capacity of 706.3 mAh g1 of Si after 100 cycles. The properties of this thick film is analyzed by field emission scanning electron microscopy (FESEM) and scanning transmission electron microscopy with energy dispersive X-ray analyzer (STEM-EDX). The results show that SieOeC composite not only covers the surface area of porous Ni but also attaches to the highly porous dendritic walls, along with the porous structure of Ni which provides proper accommodation for the volume change of silicon during the lithiation/delithiation processes, are believed to result in the high capacity and excellent cyclability. © 2014 Elsevier B.V. All rights reserved.
Keywords: SieOeC/Ni thick film 3D porous anode Electrodeposition Organic electrolyte Lithium-ion battery
1. Introduction Lithium-ion batteries with high power density and outstanding cyclability have been regarded as one of the most promising energy storage devices [1,2]. Recently, the rapid development of portable
* Corresponding author. Tel./fax: þ86 21 3420 2748. ** Corresponding author. Tel.: þ81 3 5286 3202; fax: þ81 3 3205 2074. E-mail addresses: [email protected] (T. Hang), [email protected] (T. Osaka). http://dx.doi.org/10.1016/j.jpowsour.2014.09.042 0378-7753/© 2014 Elsevier B.V. All rights reserved.
electronic devices and battery-operated electric vehicles urgently promote the research of next generation lithium-ion batteries [3]. Silicon has attracted much attention due to its low discharge potential and extremely high theoretical capacity of 4200 mAh g1 (Li4.4Si alloy), which is more than ten times higher than that of graphite electrode with the theoretical capacity of 372 mAh g1 [4]. However, silicon suffers great volume change (~300%) in the charge (lithiation)/discharge (delithiation) processes, which induces serious structural stability problems of the electrode materials and loss of electrical contact with the current collector, as the result, the
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
795
electrode shows poor electrochemical performance including rate capability and cycle life. In order to accommodate the great volume change of active material silicon during the charge/discharge cycles, much attention has been paid on the exploration of silicon-based nanomaterials, such as silicon nanowires, nanotubes, coreeshell structures and thin films [5e12]. Recently, a novel SieOeC composite thin film [9e12] has been synthesized on copper substrate by electrodeposition of Si in organic electrolyte, which delivers high discharge capacity of 842 mAh g1 after 7200 cycles with excellent cycling stability. However, the SieOeC composite thin film is easy to peel off from flat copper current collector during charge/discharge cycles which impedes its usage. While silicon composite electrodeposited on nickel micro-nanocones hierarchical structured current collector really reinforces adhesion strength with the substrate during cycling to some extent [11]. Nevertheless, the average size of micro-nanocones is only 500 nm in height, which limits the reinforcing effect to several hundreds of namometers in height from the bottom. However, more thickness is needed to develop high capacity lithium-ion battery using the SieOeC film. Buffer of volume change of the thickness during the charge and discharge process is needed. Moreover, ion pass in the thick SieOeC film and high conductivity of the film are needed to use thick films. Thus, the substrate with 3D macrostructure realizing good adhesion for deposit, ion pass and conductivity, is good technique to apply the thick SieOeC film to anode of lithium secondary battery. In this paper, we report a three-dimensional porous SieOeC/Ni thick film using electrodeposition of porous nano-Ni on copper substrate [13,14]. The thick film can be directly used as anode without adding binders or conductive additives. The 3D porous nano-metal (Ni or Cu) composed of homogeneously distributed pores and dendritic walls have drawn broad attention since which can provide high surface area, short diffusion length of ions and strong mechanical support [15e17]. In the meanwhile, we also focus on the improvement of galvanostatical electrodeposition parameters for the purpose to fabricate SieOeC composite thick film which is both time-consuming and high-cost for magnetron sputtering, physical vapor deposition (PVD) or chemical vapor deposition (CVD) method [6]. Fig. 1 shows a schematic of typical fabrication procedure for 3D porous SieOeC/Ni thick film and lithiation process in the first activation cycle. The design contains size-fitted pores as good buffer to accommodate the volume change of silicon as well as rough surface area to improve adhesion strength between the SieOeC composite and current collector. Then the SieOeC composite is electrodeposited on porous Ni as anode for energy storage purpose, the diameter of these pores decreases obviously and many pores are almost filled, these changes are due to great volume expansion of active material silicon during first lithiaiton process.
fabrication of porous Ni was conducted in a glass container with a Pt counter electrode and the Cu foil as a working electrode, the working area of Cu foil was precisely controlled at 1.00 cm2 with the other surface areas covered by insulting tapes, the electrolyte contained 2 mol L1 NH4Cl (Sinopharm Chemical Reagent, 99.5%) and 0.1 mol L1 NiCl2$6H2O (Sinopharm Chemical Reagent, 98.0%) at a pH value of 3.5 [14]. Before electrodeposition, the Cu foil was immersed in 20 v/v% H2SO4 for 15 s, rinsed with deionized water, and dried by the heater blower. Electrodeposition of porous Ni was performed at a current density of 3.0 A cm2 for 60 s with violently stirring of the electrolyte at room temperature. Then the specimen was rinsed with deionized water, dried in vacuum chamber overnight and transferred to glove box filled with Ar atmosphere. In the second step, the organic electrolyte containing 0.5 mol L1 SiCl4 (Tokyo Chemical, 98.0%) and 0.5 mol L1 tetrabuthylammonium perchlorate in propylene carbonate (TBAP, Energy Chemical, 98%/PC, ACROS Organics, 99.5%) were prepared in glove box with dew point below 100 C. The galvanostatical electrodeposition was conducted in a three-electrode glass cell equipped with a Pt quasi-reference electrode, a Pt counter electrode and 3D porous Ni on Cu foil as a working electrode, the whole experiment was performed in Ar atmosphere with dew point below 100 C. A constant cathodic current of 1.0 mA cm2 was applied to pass a charge of 10 C cm2 for electrodeposition of SieOeC composite. The 3D porous SieOeC/Ni thick film was transferred into another three-electrode glass cell containing 1 mol L1 LiPF6 electrolyte dissolved in EC:DMC (1:1 v/v) (Dongguan Shanshan Battery Material, water content less than 20 ppm), lithium metal foil as reference and counter electrodes. Electrochemical performance of the thick film as anode for lithium-ion battery was measured with a current density of 50 mA cm2 for activation in the first lithiation/ delithiation cycle and 200 mA cm2 (0.5 C) in the subsequent cycles in the potential range between 0.01 V and 1.20 V vs. Li/Liþ. Cyclic voltammetry (CV) was carried out on this anode in a potential window of 0.01e1.50 V vs. Li/Liþ at a scan rate of 0.1 mV s1. The galvanostatic charge/discharge tests were performed on a battery testing system (Kejing, Shenzhen) and CV was conducted on an electrochemical workstation (CHI660E, Chenhua, Shanghai) at room temperature. The surface morphology of the 3D porous SieOeC/Ni film was characterized with field emission electron microscopy (FESEM, Siron 200), the FESEM also provided energy dispersive X-ray analyzer (EDX) which enabled elemental analysis of SieOeC composite thick film. The crystallographic structure of the film was analyzed by means of scanning transmission electron microscopy with EDX (STEM-EDX, JEM 2010).
2. Experimental
The SEM images of electrodeposited 3D porous Ni film are shown in Fig. 2(a)e(c). A top view of porous Ni film in Fig. 2(a) clearly shows the pores are homogeneously distributed. The thickness of the 3D porous Ni film is approximately 15 mm which was measured from the cross-sectional SEM image (not shown here), both the thickness and morphology of cross-sectional porous Ni film are consistent with the result shown in cross-sectional SEM image of porous Ni film in Ref. [14]. Fig. 2(b) demonstrates that the structure is composed of not only highly porous dendritic walls but also numerous pores with feature size approximately 8 mm in diameter. Fig. 2(c) shows magnified SEM image of Ni particles at nanometer scale with feature size ranging from several hundreds of nanometers to about 1 mm in diameter. After electrodeposition of SieOeC composite film on the porous Ni substrate, the SEM images of SieOeC composite film are shown
The preparation of 3D porous SieOeC/Ni thick film included two-steps electrodeposition. All solvents and chemicals were of reagent quality without further purification. In the first step,
Fig. 1. Schematic of fabrication process of the electrodeposited three-dimensional porous SieOeC composite thick film and lithiation process.
3. Results and discussion
796
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
Fig. 2. Top view SEM images of surface morphologies of electrodeposited 3D porous Ni (a), (b), (c) and 3D porous SieOeC/Ni thick film (d), (e), (f).
in Fig. 2(d)e(f). Fig. 2(d) shows that the SieOeC composite are homogenously distributed, Fig. 2(e) demonstrates that the SieOeC/ Ni thick film maintains highly porous structure, the dendritic walls become much more compact and inside the pores are filled with SieOeC composite to different extent, thus the average diameter of these pores decreases to approximately 5 mm Fig. 2(f) shows the image of a Ni particle covered by SieOeC composite at nanometer scale. Compared with Fig. 2(c), it is obvious that SieOeC composites can homogenously attach themselves to the rough surface of Ni particles by electrodeposition method in organic electrolyte. The elemental composition and crystallographic structure of the SieOeC composite thick film was investigated by STEM. The existence of elemental Si, O and C was confirmed by STEM-EDX, Fig. 3(a) shows EDX result that Si, O, C and Cl were detected, Fig. 3(b) shows dark field image of part of SieOeC composite and elemental mappings of Si, O and C, which demonstrates that these three elements were homogeneously distributed at nanometric scale in the composite. TEM image shown in Fig. 3(c) demonstrates that the SieOeC composite film has a highly porous structure with nanometric interparticle spacing, which is consistent with the cross-sectional SEM analysis of 3D porous SieOeC composite film. Fig. 3(d) shows the selected area electron diffraction (SAED) pattern (inset in Fig. 3(c)) of circle area highlighted with white solid line in Fig. 3(c), the diffraction ring in the SAED pattern proves the amorphous characteristics of silicon in the composite. Fig. 3(e) shows the high resolution transmission electron microscopy (HRTEM) image of the rectangle area highlighted with white solid line in Fig. 3(c), no lattice fringe images can be observed from the view, which corresponds quite well to the result that elemental silicon is amorphous. And the elemental C and O are decomposition products of organic electrolytes, which act as good buffer to accommodate great volume change during charge/discharge cycling. Considering the XPS analysis of SieOeC composite in our
previous report [9], it is suggested that the SieOeC composite thick film is composed of Si existing in an oxidized state and decomposition products of the organic electrolyte. The cycling performance and coulombic efficiency of the 3D porous SieOeC/Ni thick film anode are shown in Fig. 4(a), the porous SieOeC/Ni thick film anode was electrochemically activated at a current density of 50 mA cm2 for the first cycle, then charged/discharged at a current density of 200 mA cm2 (0.5 C) for 100 cycles. All charge/discharge capacities are calculated based on the assumption that the SieOeC composite maintains its maximum mass of 0.5 mg with a passed charge of 10 C during electrodeposition and silicon achieves its content of 60% (maximum mass of Si is 0.3 mg), which have been measured with a passed charge of 2 C in previous work [9]. The lithiation/delithiation processes during first activation cycle deliver a charge/ discharge capacity of 5315 mAh g1 of Si (1.5945 mAh cm2) and 1530 mAh g1 of Si (0.4588 mAh cm2), respectively. The extremely high irreversible capacity loss (~70%) is mainly attributable to the reduction of SiOx and formation of solid electrolyte interphase (SEI) film [9,10,18e20]. It is important to note that we did not consider the activation cycle in all schematics plotting processes for different current density reasons. The charge/ discharge capacity in the first cycle with a current density of 200 mA cm2 (0.5 C) are 1408 mAh g1 of Si and 1105 mAh g1 of Si, respectively, the coulombic efficiency rapidly rises to 92% in the fourth cycle and keeps steady from 5th to 100th cycle. The discharge capacity maintains 1025.3 mAh g1 of Si in 5th cycle and 706.3 mAh g1 of Si in 100th cycle, respectively, leaving an average capacity retention of 98.8% per cycle. The excellent cycling performance is mainly attributable to the highly porous structure of SieOeC/Ni thick film which serves as compact buffering matrix to accommodate the volume change of silicon during charge/ discharge processes.
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
797
Fig. 3. STEM image of the SieOeC composite thick film: (a) EDX analysis; (b) dark field image and elemental mappings of Si, O, C; (c) TEM image of SieOeC composite film; (d) SAED pattern of circle area in (c); (e) HRTEM image of rectangle area in (c).
The charge/discharge potential profiles of the 3D porous SieOeC/Ni thick film anode in the 1st, 2nd, 10th, 50th, 100th cycles between 0.01 V and 1.20 V at a current density of 200 mA cm2 (0.5 C) are shown in Fig. 4(b) (the activation cycle excluded). From 1st to 10th cycle, the first pseudo plateau during charge process gradually increases from 0.28 V to 0.34 V, and the pseudo plateau remain at 0.34 V in the subsequent cycles, the second pseudo plateau at about 0.14 V is also shown in the profiles, and the following potential gradually drops to 0.01 V which is the typical charge behavior of amorphous silicon [21e23]. The two pseudo plateaus during charge process indicate insertion of Liþ into amorphous silicon to transition to a series of LixSi alloys. Furthermore, inset graph in Fig. 4(b) shows the CV profile of 3D porous SieOeC/Ni thick film anode after activation and fifteen charge/ discharge cycles. During the charge process, the reduction peak near 0.14 V in the cathodic branch is observed which is probably attributable to the transformation of amorphous silicon to a series of LixSi phase, the result of which is consistent with the charge/ discharge potential profiles analysis. While in the discharge process, two oxidation peaks near 0.35 V and 0.5 V in the anodic branch are observed, respectively. It is entirely possible due to a two-step transformation from LixSi phase to amorphous silicon [21]. The rate performance of 3D porous SieOeC/Ni thick film anode was also investigated by applying a series of discharge current
varying from 0.5 C to 10 C, which is shown in Fig. 5(a). The electrochemical cell was activated with a current density of 50 mA cm2 for the first cycle, then galvanostatically charged/discharged with current density of 200 mA cm2, 400 mA cm2, 600 mA cm2, 800 mA cm2, 1000 mA cm2, 400 mA cm2 (correspond to 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C respectively) for 10 cycles, respectively. The 3D porous SieOeC/Ni thick film anode maintained discharge capacity of 992.7 mAh g1, 766 mAh g1, 637.7 mAh g1, 465 mAh g1, 255.3 mAh g1 and 737 mAh g1, respectively, at 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C. The capacity retention increasingly decreases as current rises, but the coulombic efficiency keeps rising as current increases, upwards 99.2%, and keeps a steady profile at each rate in the meanwhile. Moreover, when the current returns to 1.5 C under condition that the anode was cycled at higher rates for many cycles, the discharge capacity could be recovered. The highly porous structure, rough surface with strong adhesion force and excellent accommodation ability of silicon volume change are responsible for the remarkable rate performance of the 3D porous SieOeC/Ni thick film anode. The charge/discharge potential profiles of the 3D porous SieOeC/Ni thick film anode at 5th, 15th, 25th, 35th, 45th, 55th cycle which corresponds to 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C-rate are shown in Fig. 5(b). From 0.5 C-rate to 10 C-rate, the plateau during charge process decreases from 0.34 V to 0.14 V apparently, the charge capacity decreases obviously as C-rate increases. However, the
798
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
Fig. 4. (a) Cycling performance of 3D porous SieOeC/Ni thick film anode with a current density of 200 mA cm2 (0.5 C); (b) Potential profiles between 0.01 V and 1.20 V, inset graph shows CV profile at a scan rate of 0.1 mV s1 after activation and fifteen charge/discharge cycles.
charge plateau returns to 0.29 V at the 55th cycle when the current returns to 1.5 C-rate, which corresponds quite well to the charge plateau of the 15th cycle at 1.5 C-rate. On the other hand, the plateau at about 0.35 V and 0.5 V are observed in the 5th discharge process at 1.5 C-rate, while the first plateau at about 0.35 V disappears before the 15th discharge process and it increases to 0.38 V (15th cycle, 1.5 C), 0.46 V (25th cycle, 3 C), 0.53 V (35th cycle, 5 C), 0.62 V (45th cycle, 10 C). The second plateau at about 0.5 V gradually increases to 0.55 V (15th cycle, 1.5 C), 0.62 V (25th cycle, 3 C), 0.68 V (35th cycle, 5 C) and 0.72 V (45th cycle, 10 C). However, the plateau returns to 0.38 V and 0.55 V in the 55th discharge process at 1.5 C-rate when the current returns to 1.5 C-rate. These characteristics of the charge/discharge profiles during cycling at various current rates obviously demonstrate that electrochemical performance of the SieOeC/Ni thick film anode will strongly decrease under high-rate cycling, while the performance recovers when the current returns to 1.5 C strongly indicates electrochemical stability of the anode material. To further understand how the morphology of SieOeC composite thick film changes and how the excellent cycling performance is maintained during electrochemical cycling, the 3D porous SieOeC/Ni thick film anode was retained for SEM investigation at first fully lithiated state (Fig. 6(a)) in the activation cycle with a current density of 50 mA cm2, the other 3D porous SieOeC/Ni thick film anode was also retained for SEM analysis after activation cycle at first fully delithiated state (Fig. 6(b)). Fig. 6(a) shows obvious volume expansion of SieOeC composite thick film, dramatic decrease of both amount and average diameter of these pores and many small cracks formed on the surface area. These features are consistent with the fully lithiated state of silicon with over 300% volume expansion in the first electrochemical cycle. Fig. 6(b) shows
Fig. 5. (a) Rate performance of 3D porous SieOeC/Ni thick film anode at various currents of 0.5 C, 1.5 C, 3 C, 5 C, 10 C, 1.5 C; (b) Potential profiles between 0.01 V and 1.20 V of 5th (0.5 C), 15th (1.5 C), 25th (3 C), 35th (5 C), 45th (10 C), 55th (1.5 C) cycles at various current rates.
that the volume expansion phenomenon of SieOeC composite thick film almost disappeared, while the average diameter of these pores remained almost unchanged compared with Fig. 6(a). The existence of thick solid electrolyte interphase on the surface of 3D porous SieOeC/Ni architecture can provide straightforward evidence for the extremely high irreversible capacity loss in the first charge/discharge cycle. Fig. 6(c) and (d) show the SEM image of 3D porous SieOeC/Ni thick film anode at discharged state after 100 cycles with a current density of 200 mA cm2 (0.5 C) and its magnified image. Both the number and average diameter of the pores decrease obviously, which is attributable to the great volume expansion of silicon during lithiation process. However, only a few small cracks appeared on the surface of the film without further pulverization of the anode material, which can be clearly observed in Fig. 6(d). The cracks provided obvious evidence for capacity loss after 100 cycles, on the other hand, the well maintained structural integrity of the 3D porous SieOeC/Ni thick film reasonably accounted for the excellent electrochemical stability. 4. Conclusions In summary, a 3D porous SieOeC/Ni thick film anode for lithium-ion battery was successfully fabricated by a two-step electrodeposition method. The distinctive highly porous structure offers large surface area, strong adhesion force between active material and Ni dendrites, which not only enables homogenous electrodeposition of SieOeC composite but also acts as buffering matrix to accommodate the great volume change of silicon during charge/discharge (lithiation/delithiation) processes. Moreover, the specially designed 3D porous structure can be well maintained during electrochemical cycling processes. As a result, the 3D porous SieOeC/Ni thick film anode delivers an impressive reversible
X. Qian et al. / Journal of Power Sources 272 (2014) 794e799
799
Fig. 6. Top view SEM images of 3D porous SieOeC/Ni thick film (a) at fully lithiated state and (b) at fully delithiated state in the first cycle; (c) low magnification and (d) high magnification image at discharged state after 100 cycles.
capacity of 1105 mAh g1 of Si in the first cycle and 706.3 mAh g1 of Si in the 100th cycle at 0.5 C-rate, leaving an excellent average capacity retention over 98.8% per cycle. Thus, the unique substrate with 3D macrostructure realizing good adhesion for deposit, ion pass and conductivity, is good technique to apply the thick SieOeC film to anode of lithium secondary battery. Furthermore, the 3D porous SieOeC/Ni thick film prepared by electrodeposition method provides a simple approach for scaled-up manufacturing of silicon-based anode materials used in lithium-ion batteries due to their low cost and applicability of manufacturing process. Acknowledgments This work is sponsored by National Natural Science Foundation of China (No. 21303100) and Shanghai Natural Science Foundation (No. 13ZR1420400). References [1] [2] [3] [4]
C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Adv. Mater. 22 (2010) E28eE62. M. Armand, J.-M. Tarascon, Nature 451 (2008) 652e657. J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2009) 587e603. W. Weydanz, M. Wohlfahrt-Mehrens, R. Huggins, J. power sources 81 (1999) 237e242.
[5] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31e35. [6] X.L. Chen, K. Gerasopoulos, J.C. Guo, A. Brown, C.S. Wang, R. Ghodssi, J.N. Culver, Adv. Funct. Mater. 21 (2011) 380e387. [7] L. Hu, H. Wu, Y. Gao, A. Cao, H. Li, J. McDough, X. Xie, M. Zhou, Y. Cui, Adv. Energy Mater. 1 (2011) 523e527. [8] L.-F. Cui, R. Ruffo, C.K. Chan, H. Peng, Y. Cui, Nano Lett. 9 (2009) 491e495. [9] H. Nara, T. Yokoshima, T. Momma, T. Osaka, Energy Environ. Sci. 5 (2012) 6500e6505. [10] H. Nara, T. Yokoshima, M. Otaki, T. Momma, T. Osaka, Electrochim. Acta 110 (2013) 403e410. [11] T. Hang, H. Nara, T. Yokoshima, T. Momma, T. Osaka, J. Power Sources 222 (2013) 503e509. [12] T. Osaka, H. Nara, T. Momma, T. Yokoshima, J. Mater. Chem. A 2 (2014) 883e896. [13] X. Xia, J. Tu, Y. Zhang, Y. Mai, X. Wang, C. Gu, X. Zhao, J. Phys. Chem. C 115 (2011) 22662e22668. [14] Y.Q. Zhang, X.H. Xia, X.L. Wang, Y.J. Mai, S.J. Shi, Y.Y. Tang, C.G. Cu, J.P. Tu, J. Power Sources 213 (2012) 106e111. [15] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 15 (2003) 1610e1614. [16] H.C. Shin, J. Dong, M. Liu, Adv. Mater. 16 (2004) 237e240. Kim, D. Wook [17] D. Youba aKim, J. Mater. Chem. A 2 (2014) 2478e2481. [18] A. Agrawal, A. Austin, J. Electrochem. Soc. 128 (1981) 2292e2296. [19] H. Kim, J. Cho, Nano Lett. 8 (2008) 3688e3691. [20] S. Zhang, Z. Du, R. Lin, T. Jiang, G. Liu, X. Wu, D. Weng, Adv. Mater. 22 (2010) 5378e5382. [21] V. Baranchugov, E. Markevich, E. Pollak, G. Salitra, D. Aurbach, Electrochem. Commun. 9 (2007) 796e800. [22] W. Wang, P.N. Kumta, Acs Nano 4 (2010) 2233e2241. [23] Y. Fan, Q. Zhang, Q. Xiao, X. Wang, K. Huang, Carbon 59 (2013) 264e269.