Letter pubs.acs.org/NanoLett
Three-Dimensional Ni/TiO2 Nanowire Network for High Areal Capacity Lithium Ion Microbattery Applications Wei Wang,†,∥ Miao Tian,†,∥ Aziz Abdulagatov,‡,∥ Steven M. George,‡,§,∥ Yung-Cheng Lee,†,∥ and Ronggui Yang*,†,∥ †
Department of Mechanical Engineering, ‡Department of Chemical and Biological Engineering, and §Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ∥ DARPA Center for Integrated Micro/Nano-Electromechanical Transducers (iMINT), University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: The areal capacity of nanowire-based microbatteries can be potentially increased by increasing the length of nanowires. However, agglomeration of high aspect ratio nanowire arrays could greatly degrade the performance of nanowires for lithium ion (Li-ion) battery applications. In this work, a three-dimensional (3-D) Ni/TiO2 nanowire network was successfully fabricated using a 3-D porous anodic alumina (PAA) template-assisted electrodeposition of Ni followed by TiO2 coating using atomic layer deposition. Compared to the straight Ni/TiO2 nanowire arrays fabricated using conventional PAA templates, the 3-D Ni/TiO2 nanowire network shows higher areal discharging capacity. The areal capacity increases proportionally with the length of nanowires. With a stable Ni/TiO2 nanowire network structure, 100% capacity is retained after 600 cycles. This work paves the way to build reliable 3-D nanostructured electrodes for high areal capacity microbatteries. KEYWORDS: Nanowire network, templated fabrication, lithium ion battery, atomic layer deposition
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structures have been realized in some very special material systems. For example, 3-D connection was formed in gold nanobelts by utilizing the cross-linking structure in the precursor14 and the t-Se nanowire network was formed due to the spontaneous organization during the crystallization and coalescence processes.15 Besides the nanowire network formed in a matrix, branched nanowires are grown to connect neighboring nanowires as a 3-D nanowire network.16−18 However, these 3-D nanowire networks are not suitable for applications in Li-ion microbatteries because they are either not battery materials or not able to form a continuous film due to the nature of the micro-size units. Templated synthesis is considered to be an effective approach for fabricating nanowires of a variety of materials. In particular, porous anodic alumina (PAA) template has been widely used for fabricating nanowire arrays of metals, metal alloys, oxides, and semiconductors because of its simplicity, versatility, and high degree of process control.19 Compared to conventional 2-D thin film batteries, superior battery performance with improved areal capacity and rate capability has been
ith the rapid advances in integrated microsystems, it is of great importance to develop high-capacity lithium ion (Li-ion) microbatteries to meet the increasing power requirement.1−3 Compared to the conventional two-dimensional (2D) thin-film batteries, nanowire arrays exhibit great advantages to be used as electrodes for high-capacity and high-power microbatteries: (1) The three-dimensional (3-D) architecture leads to a much larger surface area and a reduced Li-ion diffusion length, (2) liquid electrolyte can reach a nanostructured surface easily through the inter-wire spacing, and (3) the space between nanowires can accommodate a large volume change during the charging/discharging processes.4−8 The areal capacity of the nanowire-based microbatteries can potentially be increased by increasing the length of nanowires. However, the nanowire arrays with high aspect ratio are known to agglomerate with the formation of nanowire clusters,9 which leads to significant performance degradation because of the increase of supplementary interface between active materials, the block of Li-ion flow, and the decrease of a specific surface area of nanowire arrays.5,10 Three-dimensional nanowire network structures, which have stable structural support and conducting network due to multiple interconnections across nanowires, would be desirable for various energy conversion and storage applications.5,11−13 To date only a very limited number of 3-D nanowire network © 2011 American Chemical Society
Received: October 1, 2011 Revised: December 16, 2011 Published: December 30, 2011 655
dx.doi.org/10.1021/nl203434g | Nano Lett. 2012, 12, 655−660
Nano Letters
Letter
Figure 1. Schematic fabrication process flow for straight Ni/TiO2 nanowire arrays (top) and 3-D Ni/TiO2 nanowire network (bottom), which uses Al foils with different impurity. The side holes in the 3-D PAA template result in interconnections in 3-D nanowire network.
nanowire network after electroplating. After the PAA templates were fabricated, a ∼50 nm thin layer of gold was sputtered on the bottom of the PAA templates to serve as the conducting seed layer for electroplating. After the electrodeposition process in NiSO4 and NiCl2 based electrolyte solution and subsequent removal of the PAA templates in a 5 M NaOH solution, the Ni nanowires were synthesized as the inverted structures of the PAA templates. The length of the Ni nanowire samples could be controlled by adjusting the electroplating time, while the diameter of the nanowires is controlled by the anodization parameters during the template fabrication process (anodization voltage and the wet-etching time in H3PO4 acid). The short nanowires with a length of about 5 μm were obtained after 30 min electrodeposition, and the long nanowires with a length of about 30 μm were obtained after 3 h electrodeposition. ALD was then employed to coat the all Ni nanowire surfaces with a layer of active material TiO2. The as-obtained Ni nanowire samples were cleaned with 5 wt % H2SO4 and dried in vacuum oven before the ALD coating to minimize the influence of native oxide. ALD technique is chosen because ALD has unique capability to deposit highly conformal thin films on high aspect ratio micro- or nanostructures.25,26 Fifty cycles of Al2O3 are ALD coated before ALD TiO2 coating as the seed layer for better nucleation and growth of the TiO2 layer.21 We note that the as-deposited Al2O3 and TiO2 are both in amorphous phase (Figure S1, Supporting Information), and the 50 cycles seed-layer ALD Al2O3 coating is conductive and would not block the electrons transport (Figure S2, Supporting Information). More detailed fabrication information can be found in the Supporting Information. Figure 2a shows the optical image of an 8 × 8 cm2 3-D PAA template, which is anodized with 1000 series Al foil (McMaster 9060K16) at 160 V in 0.3 M H3PO4 at −3 °C and followed by a wet-etching process in 3 wt % H3PO4 solution at 45 °C for 3 h. Figure 2b shows the field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7401F) image of the crosssectional structure of this 3-D PAA template. Side-holes are densely distributed on the wall of parallel nanochannels, which is due to the impurity in the Al foil.23 The distance between the vertical nanochannels and the diameter of the side holes in the PAA templates are controllable by changing the anodization voltage and the wet-etching time of the template in H3PO4 acid.
demonstrated in straight nanowire arrays of Fe3O4/Cu, Ni−Sn/ Cu, TiO2/Al, and LiCoO2/Al fabricated using PAA templates.20−22 However, all these studies used very short nanowires with a length of about 2−3 μm. Further enhancement is challenging since longer straight nanowires tend to agglomerate when they are released from the conventional PAA templates with straight nanochannels.9 The conventional PAA templates are fabricated by anodizing high-purity (99.999%) Al foil. When the Al foils used for anodization is not pure (purity 97% for most cycles at the discharging rates of C/10 and C/5 and almost 100% for the testing cycles at the discharging rate of 5C, except the initial cycle and the first cycles after the current density is changed. Furthermore, the 3-D nanowire network structure is very stable during the cycling process. Figure 5b shows the cross-sectional SEM image of the 3-D Ni/TiO2 nanowire network after 72 cycles of testing. Compared to the SEM image before testing in Figure 3b, no obvious structural change can be observed, which suggests an ultrastable structure of the 3-D nanowire network. Figure 6 shows the rate capability of 3-D Ni/TiO2 nanowire network with different ALD TiO2 coating thickness. The initial discharging for both samples was conducted at a rate of C/11− C/12. The current density was then doubled after every four cycles. The areal capacity gradually decreases as the discharging rate increases. The sample with 16 nm thick ALD TiO2 coating retains 40% of the initial capacity at a high discharging rate of 7C after the current density was increased 32 times. Compared to the previous reported TiO2 coated short Al nanorods,21 the 3-D Ni/TiO2 exhibits a similar rate capability but with more than 10 times higher areal capacity. The rate capability can be further improved by reducing the thickness of TiO2 coating. As decreasing TiO2 thickness from 16 to 8 nm, the capacity at the high discharging rate of 4C remains 68% of the initial capacity after the current density was increased 32 times. The rate capability is strongly related to the diffusion time of Li-ions in the nanostructures.34 The characteristic time for Li diffusion in electrodes is given by t = L2/D, where t is the diffusion time, L
Figure 6. Rate capabilities of 32 μm long 3-D Ni/TiO2 nanowire network structure with different TiO2 coating thicknesses.
is the diffusion length, and D is the diffusion constant. As the coating thickness (diffusion length) decreases, the diffusion time decreases and the rate capability is improved accordingly. In summary, we have successfully fabricated 3-D Ni/TiO2 nanowire network based on a 3-D PAA template-assisted electrodeposition and ALD processes. Due to the formation of multiple interconnections in 3-D nanowire network, agglomeration in its counterpart straight nanowire arrays has been be eliminated, which results in a much higher areal discharging capacity compared to the straight Ni/TiO2 nanowire arrays using conventional PAA template. The areal discharging capacity of 3-D Ni/TiO2 nanowire network increases proportionally to the length of the nanowires. The 3-D Ni/TiO2 nanowire network anode also exhibits excellent rate capability with enhanced areal capacity. The versatile approach for building 3-D nanowire network presented in this work can be extended to a variety of energy conversion and storage applications.
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ASSOCIATED CONTENT
S Supporting Information *
The detailed fabrication processes and microstructure analysis of Ni/TiO2 nanowire arrays. This material is available free of charge via the Internet at http://pubs.acs.org. 659
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(23) Molchan, I. S.; Molchan, T. V.; Gaponenko, N. V.; Skeldon, P.; Thompson, G. E. Electrochem. Commun. 2010, 12, 693. (24) Wang, W.; Zhang, G. Q.; Li, X. G. J. Phys. Chem. C 2008, 112, 15190. (25) George, S. M. Chem. Rev. 2010, 110, 111. (26) Elam, J. W.; Routkevitch, D.; Mardilovich, P. P.; George, S. M. Chem. Mater. 2003, 15, 3507. (27) Kim, H.; Lee, H. B. R.; Maeng, W. J. Thin Solid Films 2009, 517, 2563. (28) Xu, J. W.; Jia, C. H.; Cao, B.; Zhang, W. F. Electrochim. Acta 2007, 52, 8044. (29) Sun, X. D.; Ma, C. L.; Wang, Y. D.; Li, H. D. Nanotechnology 2004, 15, 1535. (30) Ortiz, G. F.; Hanzu, I.; Djenizian, T.; Lavela, P.; Tirado, J. L.; Knauth, P. Chem. Mater. 2009, 21, 63. (31) Gerasopoulos, K.; Chen, X. L.; Culver, J.; Wang, C. S.; Ghodssi, R. Chem. Commun. 2010, 46, 7349. (32) Fang, H. T.; Liu, M.; Wang, D. W.; Sun, T.; Guan, D. S.; Li, F.; Zhou, J. G.; Sham, T. K.; Cheng, H. M. Nanotechnology 2009, 20, 225701. (33) Hibino, M.; Abe, K.; Mochizuki, M.; Miyayama, M. J. Power Sources 2004, 126, 139. (34) Bruce, P. G.; Scrosati, B.; Tarascon, J. −M. Angew. Chem., Int. Ed. 2008, 47, 2930.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: +1-303-7351003.
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ACKNOWLEDGMENTS This work is supported by an University of Colorado Innovative Seed Grant and the DARPA Center on Nanoscale Science and Technology for Integrated Micro/Nano-Electromechanical Transducers (iMINT), supported by the Defense Advanced Research Projects Agency (DARPA) N/MEMS S&T Fundamentals program under grant no. N66001-10-1-4007 issued by the Space and Naval Warfare Systems Center Pacific (SPAWAR). Some of the micro/nanofabrication work was conducted in the Colorado Nanofabrication Laboratories, supported in part by the NNIN and the National Science Foundation under grant no. ECS-0335765.
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