3D Nanoporous Nanowire Current Collectors for Thin Film

Feb 7, 2012 - Hence, current collectors based on 3D architectures and nanoscale roughness has ... to build 3D nanostructured thin film batteries.7,13â...
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3D Nanoporous Nanowire Current Collectors for Thin Film Microbatteries Sanketh R. Gowda,† Arava Leela Mohana Reddy,‡ Xiaobo Zhan,‡ Huma R. Jafry,‡ and Pulickel M. Ajayan*,†,‡,§ †

Department of Chemical and Biomolecular Engineering, ‡Department of Mechanical Engineering and Materials Science, and Department of Chemistry, Rice University, Houston, Texas 77005, United States

§

S Supporting Information *

ABSTRACT: Conventional thin film batteries are fabricated based on planar current collector designs where the high contact resistance between the current collector and electrodes impedes overall battery performance. Hence, current collectors based on 3D architectures and nanoscale roughness has been proposed to dramatically increase the electrode-current collector surface contact areas and hence significantly reduce interfacial resistance. The nanorod-based current collector configuration is one of several 3D designs which has shown high potential for the development of high energy and high power microbatteries in this regard. Herein we fabricate a nanoporous nanorod based current collector, which provides increased surface area for electrode deposition arising from the porosity of each nanorods, yet keeping an ordered spacing between nanorods for the deposition of subsequent electrolyte and electrode layers. The new nanostructured 3D current collector is demonstrated with a polyaniline (PANI)-based electrode system and is shown to deliver improved rate capability characteristics compared to planar configurations. We have been able to achieve stable capacities of ∼32 μAh/cm2 up to 75 cycles of charge/discharge even at a current rate of ∼0.04 mA/cm2 and have observed good rate capability even at high current rates of ∼0.8 mA/cm2. KEYWORDS: Nanoporous, nanorod, lithium battery, three dimensional, current collector

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Aperiodic foamlike structures with pores size less than 10 nm show very high surface area which in turn enhances the active mass of electrode accommodated per footprint area. Prior demonstrations of nanoporous (with pore size less than 10 nm) Li battery design have been shown for bulk systems where individual electrodes (cathode, anode) are fabricated in 3D format separated by a conventional thick separator.8−12 Because of the complexity in the aperiodic porous structure, it is very challenging to fabricate consecutive electrode and electrolyte layers without the presence of electrical shorts to realize an interpenetrating 3D battery. Herein we demonstrate the use of Au nanoporous nanorods as Li battery current collectors which exhibits two levels of porosity, where (i) each nanorod exhibits high surface (pore size less than 10 nm throughout the volume of the nanorod) for better mass accommodation and (ii) the space between individual nanorods (pore size ∼50−100 nm; alumina template controlled) allows for the assembly of the electrolyte and electrode layers to fabricate the full 3D energy storage device. Polyaniline17 cathode material has been used to avoid any low-voltage Li ion insertion into the gold nanostructure. The novel electrode/current collector config-

anostructured materials for Li ion battery electrodes have been shown to exhibit faster ion and electron kinetics compared to conventional microsized particle electrodes.1−4 Nanoporous architectures for Li ion battery current collectors have led to significant improvement in charge/discharge rates4−8 of thin film battery electrodes. Single electrodes have been fabricated on periodic and aperiodic nanoporous current collectors to improve ion and electron kinetics.6−12 But what remains challenging to this day is the realization of a fully functional 3D nanostructured Li ion battery where the three essential components of a battery (anode, cathode and electrolyte), are conformally coated over each other. The fabrication of an optimized 3D nanostructured device could lead to unprecedented improvement in the Li ion diffusion kinetics in the entire device, within and between the electrodes. Particularly, the periodic nanorod-based current collector proposed by Simon and co-workers serves as a great substrate to build 3D nanostructured thin film batteries.7,13−16 The template grown nanorods provide for good control of spacing between individual nanorods for the electrode/electrolyte layers and improve surface area (mass of active electrode per footprint area) for assembling high rate 3D Li ion batteries. It is important to note that the pore size (space between nanorods) in these structures is ∼50−100 nm, typical of the interpore distance in commercial WHATMAN alumina membranes. © 2012 American Chemical Society

Received: October 3, 2011 Revised: January 3, 2012 Published: February 7, 2012 1198

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Figure 1. (a) Schematic diagram of the vertically aligned array of nanotubes and the subsequent electrodeposition of the PANI layer onto the nanotube current collector. (b) Schematic diagram of the vertically aligned array of nanoporous nanorods and the subsequent electrodeposition of the PANI electrode layer onto the nanorod current collector.

Figure 2. (a) SEM image of aligned array of gold nanotube current collector. (b) Polyaniline-coated gold nanotube array. (c) Cyclic voltammetry of the PANI coated Au nanotube electrode (d) Voltage vs discharge capacity profile of the PANI coated Au nanotube array.

the recent past.4,7,23−25 Figure 1 shows the schematic representation of advanced 3D nanostructured configurations for current collectors around which thin film electrodes have been be fabricated. Nanotube based current collectors are expected to exhibit higher surface area for electrode deposition in comparison to conventional nanorod7 based current collectors due to the added inner wall surface. Whereas

uration has shown improvement in the discharge capacity per footprint area and rate capability of the polyaniline electrode material. Conventional thin film battery electrodes have been fabricated on planar current collectors.19−22 But because of energy per unit area restrictions in planar thin film designs, the search for rough 3D structures have gained more importance in 1199

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Figure 3. (a) Low-magnification SEM image of the PANI coated nanoporous Au nanorod. (b) High-magnification SEM image of PANI-coated nanoporous Au nanorod clearly showing the PANI and Au components. (c) Voltage vs discharge capacity graphs for the galvanostatic measurements conducted on the nanoporous nanorod current collector configuration. (d) Discharge capacity vs cycle number plot showing reversible capacities for the three different configurations: planar, nanotube, and nanoporous nanorod up to 75 cycles.

attached onto a stainless steel substrate (Figure 2a). From the Scanning Electron Microscopy (SEM) image in Figure 2a, it is clear that the nanotubes have ∼20 nm wall thickness and an inside diameter of ∼180 nm. The nanotubes are separated by an average distance of around ∼200 nm. The nanotubes are ∼500 nm in height and have higher surface area compared to the conventionally used planar or nanorod7 substrates due to the available inner wall surface for electrode deposition. A PANI film was electropolymerized onto the gold nanotube array to form a conformal coating around each nanotube, and SEM was used to characterize the resulting PANI/Au nanostructure. The SEM image in Figure 2b clearly shows that the PANI coating around the nanotubes is conformal after electropolymerization on the Au nanotubes. We have also fabricated the conventional planar design for thin film batteries by electrodepositing a thin film of Au onto a stainless steel substrate and then polyaniline electrode film onto it to compare its performance with that of the nanotube based configuration. The nanotube based configuration shows improved electrode surface area and hence the electrode mass distribution per footprint area compared to that of the thin film PANI electrode. The cyclic voltammetry scans for the Au nanotube/ PANI electrode is shown in Figure 2c. The CV scans were conducted between 2 and 3.6 V at a scan rate of ∼0.5 mV/s. The first anodic scan was started from the open circuit voltage of the cell until 2 V vs Li/Li+, and an anodic peak was observed around ∼2.9 V, which indicates the discharge process of the cell where the PANI electrode is reduced. The first cathodic scan shows a peak at around 3.4 V, showing the charge process where the PANI electrode is oxidized. The subsequent anodic and cathodic scans show good reversibility of the PANI redox

nanoporous nanorod current collectors are expected to show even higher surface area due to the internal pore surface area within each individual nanorod.26 Because of the difference in available surface areas for the three current collector substrates (planar, nanotube, nanoporous nanorod) used in this study, electrode distributions (electrode thickness) varies for each of the cases (planar, nanotubes, and nanoporous nanorods). When the total charge passed during electrodeposition of an electrode film is kept constant, the thickness of the conformal electrode film deposited onto the current collectors is expected to reduce with the increase in total surface area of different current collectors. Hence, the rate capability of the electrode is expected to improve upon increasing surface area of the current collectors. To demonstrate this important concept, we choose gold as our current collector material and PANI as our electrode material. We fabricate the nanostructured gold current collector using hard template assisted electrodeposition technique (Suppporting Information) followed by polyaniline electropolymerization on the nanostructured Au. Polyaniline (PANI) is chosen as our cathode material due to the high voltage with respect to Li/Li+ and the ease of fabrication. Also, PANI is known to change redox states between charge/ discharge processes, thereby exhibiting a large change in electronic conductivity between charge/discharge.19 Hence, a nanostructured current collector morphology is expected to have a significant effect on the rate capability of the PANI electrode material. First we have looked at the vertically aligned nanotube based current collector as a substrate for Li battery electrodes. The vertically aligned gold nanotube array was fabricated using a hard template assisted electrodeposition technique and 1200

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determining the overall rate capability for electrodes having lower electronic conductivity, it is reflected in the difference in rate capabilities of the three configurations. Hence, the novel 3D nanoporous architecture for current collectors could also be used for cathode materials such as LiCoO2, LiMn2O4, etc., that suffer from low electronic conductivity to improve rate capability of the electrode materials. We have also conducted high rate galvanostatic charge/ discharge measurements to evaluate the rate capability of the nanoporous nanorod current collector design. Polyanilinecoated nanoporous Au nanorods were tested against Li foil at current rates of 40, 200, 400, and 800 μA/cm2, corresponding to C rates 1C, 5C, 10C, and 20C, respectively, to evaluate the capacity retention of the 3D electrode at high current rates. From the voltage vs discharge capacity profiles (Figure 4a) we

process and the peaks are shifted to 2.8 and 3.3 V, respectively. Figure 2d shows the voltage vs discharge capacity curves for the PANI/Au nanotube configuration. Figure S1 (Supporting Information) shows the voltage profile of the planar configuration of the PANI electrode. From the voltage profile of the planar configuration it can be seen that the discharge plateau is clearer from the second cycle onward as electrolyte accessibility to the electrode is improved after the first cycle. In the case of the nanotube based configuration (Figure 2d) the electrolyte is better accessible to the PANI film due to the intertube spacing. An improvement in the reversible capacity of nanotube based configuration is observed compared to the planar configuration after 75 cycles of charge/discharge. This could be attributed to the improved mass distribution of PANI around Au nanotubes compared to a planar film. We have also fabricated vertically aligned Au nanoporous nanorod array as a current collector to further improve the electrode surface area compared to that of the nanotube and planar based configurations. The Au nanoporous nanorod array was fabricated using a hard template assisted alloying/ dealloying technique (see Supporting Information). The nanoporous nanorods are 500 nm in height and are separated by an inter-rod distance of around ∼200 nm. We have also characterized these nanorods by TEM, and they show a highly porous structure for the nanorod, resulting in an interconnected network of Au nanoparticles with an internal pore size of around ∼5−10 nm (see Supporting Information Figure S2). The PANI electrode was electropolymerized onto the Au nanoporous nanorod substrate, and the resulting structure was characterized by SEM and TEM, as shown in Figure 3a,b and Figure S3. From the high magnification transmission electron microscopy image (Figure S3) it is clear that the polyaniline forms a conformal coating around the nanoporous nanorod substrate and also fills the internal pores within the nanorod. This results in improved (i) mass distribution (thinner layers of PANI due to higher surface for mass to deposit on) compared to the nanotube or planar based design and (ii) very intimate contact of PANI electrode with the interconnected Au current collector. Galvanostatic charge/discharge measurements at current rate 0.04 mA/cm2 were conducted, and the resultant voltage vs capacity curves for the PANI coated nanoporous nanorod array are shown in the Figure 3c. From the voltage vs capacity profiles we can clearly observe the typical plateaus for PANI electrode during charge and discharge processes. A large irreversible capacity loss is observed after the first charge/ discharge cycle for the nanoporous nanorod configuration. A reversible capacity of ∼32 μAh/cm2 was observed after 75 cycles of charge/discharge for the nanoporous nanorod configuration. We have also conducted extended cycling tests on the three configurations at 0.04 mA/cm2, and the discharge capacity vs cycle number is plotted as shown in Figure 3d. As we can observe from Figure 3c, reversible discharge capacities of ∼2, 8, and 30 μAh/cm2 were observed for planar, nanotube, and nanoporous nanorod current collector systems, respectively, after 75 cycles. Hence, we can confirm that the nanoporous nanorods show much better rate capability compared to the planar or nanotube based current collectors. This can be attributed to the increased surface area due to the network of nanopores of size ∼5−10 nm in the nanoporous gold nanorod, hence forming thinner conformal layers of polyaniline electrode around the current collector facilitating faster electron and ion transport. As the electron transport from the electrode to current collector is an important step in

Figure 4. (a) Voltage vs discharge capacity graphs for the galvanostatic measurements conducted on the nanoporous nanorod configuration at currents 40, 80, 200, and 800 μA/cm2. (b) Discharge capacity vs cycle number plot showing reversible capacities at four different currents (40, 80, 200, and 800 μA/cm2) up to 75 cycles.

can observe that the polyaniline is undergoing the redox reaction to exhibit the electrochemical properties. The redox reaction is still observed as the voltage plateau even as the current rate of operation is increased. Figure 4b shows that the capacity of the cell is well retained up to 75 cycles of charge/ discharge for the above current rates of operation. At current rates of operation, 0.2 and 0.4 mA/cm2, it can be observed that the loss in reversible capacity is minimal. It is also observed that the 3D polyaniline electrode shows capacity retention of ∼30% of the nominal capacity even at high current rate of 0.8 mA/ cm2. The half-cells showed stable discharge capacities up to 75 cycles of charge/discharge. The 3D polyaniline electrode has shown good rate capability due to the reduced electron and ion diffusion distances. Shorter ion and electron diffusion distances 1201

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(14) Rhodes, C. P.; Long, J. W.; Pettigrew, K. A.; Stroud, R. M.; Rolison, D. R. Nanoscale 2011, 3, 1731. (15) Cheah, S. K.; Perre, E.; Rooth, M.; Fondell, M.; Harsta, A.; Nyholm, L.; Boman, M.; Gustaffson, T.; Lu, J.; Simon, P.; Edstrom, K. Nano Lett. 2009, 9, 3230. (16) Gowda, S. R.; Reddy, A. L. M.; Manikoth, S. M.; Zhan, X.; Ci, L.; et al. Nano Lett. 2011, 11, 101. (17) Gowda, S. R.; Reddy, A. L. M.; Zhan, X.; Ajayan, P. M. Nano Lett. 2011, 11, 3329. (18) Prieto, A. L.; Mosby, J. M.; Arthur, T. S. US Patent App. 12/ 391,197, 2009. (19) Novak, P.; Müller, K.; Santhanam, K. S. V.; Haas, O. Chem. Rev. 1997, 97, 207. (20) Bates, J. B.; Dudney, N. J.; Neudecker, B.; Ueda, A.; Evans, C. D. Solid State Ionics 2000, 135, 33. (21) Bates, J. B.; Dudney, N. J.; Gruzalski, G. R.; Luck, C. F. US Patent 5338625, 1994 (22) Dudney, N. Mater. Sci. Eng., B 2005, 116, 245. (23) Golodnitsky, D.; Yufit, V.; Nathan, M.; Shechtman, I.; Ripenbein, T. Solid State Ionics 2006, 177, 26. (24) Min, H.-S.; Park, B. Y.; Taherabadi, L.; Wang, C.; Yeh, Y.; et al. J. Power Sources 2008, 178, 795. (25) Nathan, M.; Golodnitzky, D.; Yufit, V.; Strauss, E.; Ripenbein, T.; Shectman, I.; Menkin, S.; Peled, E. J. Microelectromech. Syst. 2005, 14, 879. (26) Ji, C.; Searson, P. C. Appl. Phys. Lett. 2002, 81, 4437.

are a result of reduced PANI electrode thickness due to the high surface current collector design (nanoporous Au nanorod). In conclusion, we find that 3D nanoporous designs for electrodes have resulted in improved rate characteristics than that of conventional planar thin film based designs. Polyaniline electrodes have been electropolymerized onto nanotube and nanoporous nanorod based current collectors. The nanoporous nanorod current collector shows capacity retention of up to 30% of the nominal reversible capacity (32 μAh/cm2) even at high current rates of 0.8 mA/cm2. The nanoporous nanorod current collectors allow for improved mass accommodation per geometric area compared to nanorod or nanotube based designs which could be important for cathode layer depositions. The nanoporous nanorod design proposed could be helpful in increasing electrode mass per unit area and still preserve the space between nanorods for subsequent electrolyte and electrode layer deposition. The nanoporous nanorod design demonstrates an example of combining aperiodicity (within pores of each nanowire) and periodicity (vertically aligned array of nanorods with constant inter-rod distance) for the design of high-energy and high-power 3D microbatteries.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS P.M.A. acknowledges the support from the Hartley Family Foundation, Rice University start-up funds, National Institute of Health, Army Research Office and Multidisciplinary University Research Initiative (MURI) grant for providing funding for this work. S.R.G. acknowledges support from National Institute of Health (RO1CA12842). A.L.M.R. acknowledges the support from Army Research Office. X.Z. acknowledges the support from the MURI grant.

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