In Situ Transmission Electron Microscopy Observation of Pulverization

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LETTER pubs.acs.org/NanoLett

In Situ Transmission Electron Microscopy Observation of Pulverization of Aluminum Nanowires and Evolution of the Thin Surface Al2O3 Layers during Lithiation Delithiation Cycles Yang Liu, Nicholas S. Hudak, Dale L. Huber, Steven J. Limmer, John P. Sullivan, and Jian Yu Huang* Sandia National Laboratories, Albuquerque, New Mexico 87185, United States

bS Supporting Information ABSTRACT: Lithiation delithiation cycles of individual aluminum nanowires (NWs) with naturally oxidized Al2O3 surface layers (thickness 4 5 nm) were conducted in situ in a transmission electron microscope. Surprisingly, the lithiation was always initiated from the surface Al2O3 layer, forming a stable Li Al O glass tube with a thickness of about 6 10 nm wrapping around the NW core. After lithiation of the surface Al2O3 layer, lithiation of the inner Al core took place, which converted the single crystal Al to a polycrystalline LiAl alloy, with a volume expansion of about 100%. The Li Al O glass tube survived the 100% volume expansion, by enlarging through elastic and plastic deformation, acting as a solid electrolyte with exceptional mechanical robustness and ion conduction. Voids were formed in the Al NWs during the initial delithiation step and grew continuously with each subsequent delithiation, leading to pulverization of the Al NWs to isolated nanoparticles confined inside the Li Al O tube. There was a corresponding loss of capacity with each delithiation step when arrays of NWs were galvonostatically cycled. The results provide important insight into the degradation mechanism of lithium alloy electrodes and into recent reports about the performance improvement of lithium ion batteries by atomic layer deposition of Al2O3 onto the active materials or electrodes. KEYWORDS: Pulverization, Al2O3 coating, Al nanowires, lithium ion batteries, atomic layer deposition (ALD)

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igh energy/power density, good cyclability, and low cost are critical for lithium ion batteries (LIBs) in applications such as next generation electric vehicles and stationary power backup for fluctuating energy sources such as wind and solar energy. Nanostructured materials, including nanoparticles and nanowires (NWs), are promising candidates compared to their bulk counterparts due to the short electron and ion transport path and better accommodation of the large stresses incurred during lithiation/delithiation. As a potential anode material for LIBs, aluminum has the following advantages. First, aluminum has relatively high theoretical capacity (∼993 mAh/g for LiAl)1,2 compared with that of commercially used graphite (∼372 mAh/g), while its volume expansion is only about 97%.3 Second, the flat and wide plateaus in the charge discharge curves2 indicate the steady power output of LIBs with aluminum as anode, which is one important factor of high-performance LIBs. Last, aluminum is an inexpensive and environmentally benign element, which is crucial for a more extensive application of LIBs. Aluminum NWs in particular are of interest, as other alloying NW systems have demonstrated significant improvements over their bulk counterparts. There are a number of reasons to expect these improvements, such as the short ion diffusion paths in NWs, improved rate capabilities. Additionally, the small diameter may provide an easy strain relief path and can in some cases improve mechanical robustness during the expansion and contraction that occurs during the alloying and dealloying reactions. The mechanical issue is critical as the primary factor preventing the application of r 2011 American Chemical Society

aluminum and other metals as anode materials is the pulverization of the active materials with lithiation delithiation cycling, which leads to loss of electrical contact and capacity after relatively few cycles.4 12 Such pulverization processes, which involve the nucleation and evolution of voids or crack initiation, are not well understood. Another compelling reason to study the lithium-cycling behavior of Al is that Al always has its native Al2O3 surface layer, which provides an ideal system for studying the effect of atomic layer deposition (ALD) of Al2O3 coatings on active materials and electrodes. The role of these coatings on the cyclability and capacity retention of LIBs is under intensive investigations currently but remains unknown. It has been widely reported that ALD Al2O3 on active electrode materials, such as LiCoO2, LiMn2O4, and MoO3, can improve the durability and rate capability of LIBs.13 24 However, the underlying mechanisms as well as the evolution of these surface coatings of Al2O3 during cycling are not understood. We have prepared a material to investigate the mechanical properties during lithiation delithiation cycles that is thin enough to be electron transparent so it can be used to view structural evolution in an in situ Transmission Electron Microscopy (TEM) experiment. Larger quantities can also be cycled under galvanostatic control in a traditional electrochemical test cell to Received: June 21, 2011 Revised: August 23, 2011 Published: August 29, 2011 4188

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Figure 1. Pulverization of a single aluminum NW upon electrochemical cycling. (a) Schematic illustration of the in situ experimental setup. (b k) Morphology evolution of a single Al NW upon cycling. (b) A pristine Al NW with diameter about 40 nm contacted with the Li2O/Li electrode to form an electrochemical device. (c) An Al NW after the first lithiation, showing the volume expansion in both radial and longitudinal directions. (d) Voids were formed during the first delithiation stage, as pointed out by the red and yellow arrowheads. (e) The voids shown in panel d evolved to larger voids, and many new nanovoids emerged, as pointed to by the blue triangles. (f) After the second lithiation, the voids shown in panel e shrunk and were partly healed. (g) The number and size of voids both increased after the second delithiation. The void marked by a red triangle was newly emerged. (h k) Sequential TEM images showing the structure evolution of the same Al NW as shown in (g) during further two cycles. The voids in the NW increased after each delithiation and appeared along the whole NW. Finally, the pristine straight Al NW pulverized to nanoparticles. (l, m) Close views of the areas marked by blue and red dotted rectangles in (k), respectively, showing clearly the pulverized nanoparticles enwrapped by a glass tube. (n) A typical electron diffraction pattern (EDP) from pristine Al NW, showing a single crystalline structure. (o) A typical EDP of the NW after lithiation, showing single crystalline Al was converted to polycrystalline LiAl alloy. (p) EDP of the Al NW shown in panel k after four cycles, indicating the single crystalline Al was pulverized to many nanoparticles. (q) TEM image of a typical pristine Al NW, showing the amorphous Al2O3 surface layer with a thickness about 4 5 nm. (r) TEM image of a zoom-in area of the NW after four cycles, showing the thickness of the surface layer was increased to about 6 nm.

provide quantitative capacity data. Al NWs, approximately 40 nm in diameter and 15 μm long, have been synthesized by electrodeposition in an anodic aluminum oxide template. The template was then dissolved in an aqueous CrO3/H3PO4 bath to yield surface bound, free-standing Al NWs. The NWs were mechanically removed from the surface to produce free Al NWs. See the Supporting Information for details of the preparation of the aluminum oxide templates and the NW formation and release. Here we use Al NWs with a naturally oxidized surface Al2O3 layer (about 4 5 nm) as a model system to investigate these two unknown questions: what causes the pulverization of Al and what is the role of the surface Al2O3 layer during cycling? We constructed an electrochemical device consisting of an individual Al NW electrode, a layer of Li2O solid electrolyte, and a bulk Li metal counter electrode inside the TEM to conduct real-time observation of the structural evolution of the aluminum during the electrochemical reaction. A schematic illustration of our experimental setup is shown in Figure 1a. A few Al NWs were attached to an Al rod using conducting epoxy, serving as the working

electrode. Bulk lithium metal was scratched with a tungsten wire inside a glovebox and the tungsten wire was loaded into the TEM with an exposure time to the air less than 2 s. The Li metal attached to the tungsten wire served as the counter and reference electrodes. The naturally grown Li2O layer on the surface of Li metal served as the solid electrolyte for Li+ transport. Using the piezomanipulator (Nanofactory TEM-scanning tunneling microscopy (STM) holder), the Li2O/Li electrode was driven to contact one of the selected Al NWs. Lithiation and delithiation experiments were conducted by applying different potentiostatic holds of the working electrode with respect to the counter electrode. In the experiments, potentials of 2 V during lithiation and +4 V during delithiation were typically applied to the Al NW against the Li counter electrode. In order to minimize the effects of electron beam impinging on the samples, the beam was “blanked” during lithiation delithiation processes except for very short exposures (∼1 s) about every several minutes for the purpose of recording images. An extremely low electron beam dose was also used in imaging to minimize the beam effects. 4189

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Figure 2. Morphology evolution of the surface layer during the first cycle, and EELS spectra and maps of Li, Al, and O elements of an Al NW after three electrochemical cycles. (a) A pristine Al NW with a surface Al2O3 layer of about 5 nm. (b) The fully lithiated NW, showing the thickness of surface layer was increased to about 7 nm. (c) The NW after the first delithiation, showing the pulverization of the Al NW and the thickness of the surface layer (about 7 8 nm) did not change obviously from that after first lithiation. (d, e) EELS spectra of Li-K, Al-L, and O-K edges with the pre-edge background subtracted, showing the presence of Li, Al, and O in the cycled NW. (f) Zero loss image showing that the pulverized nanoparticles were confined by a tubelike surface layer, like peas in a pea pod. (g i) EELS maps of Li, Al, and O, respectively, indicating the nanoparticles in the tube were Al nanoparticles and the surface layer consisted of Li, Al, and O. The energy-filtered maps were obtained using a three-window technique.

Figure 1n is the electron diffraction pattern (EDP) of a typical Al NW, showing single crystalline structure. The surface of the Al NW is coated with a naturally oxidized amorphous layer with a typical thickness about 4 5 nm,25 as shown in Figure 1q. Figure 1b shows a pristine Al NW with a diameter about 40 nm after contact with the Li2O/Li electrode, forming an electrochemical device in the TEM. When a potential of 2 V was applied, lithiation of the Al NW occurred. After lithiation, the pristine straight NW became curved and its diameter also increased (Figure 1c), indicating volume expansion in both radial and longitudinal directions. The EDP from typical Al NW after lithiation confirmed that the single crystalline Al NW was converted to polycrystalline LiAl alloy (Figure 1o). The relatively sparse and bright spots in the EDP suggest the existence of large single crystalline grains in the lithiated Al NW. The volume expansion of Al NW after lithiation was estimated to be about 100%, close to the theoretical volume expansion for LiAl phase. The observed crystalline LiAl phase after lithiation indicates that electrochemically driven solid state amorphization (ESA) did not occur in the Li Al system, which may be due to the relatively facile nucleation of LiAl with NaTl structure.26

This is also the first direct visualization of crystalline LiAl alloy formation during lithiation. After 184 s of delithiation, two nanovoids were formed at the left side and in the middle of the NW, as marked by red and yellow arrowheads in Figure 1d, respectively. The size and number of voids increased with further delithiation, as pointed by the blue triangles in Figure 1e. The formation and growth of these nanovoids were caused by the dealloying of lithium from the LiAl alloy. During the lithiation in the second cycle, the voids shrunk and were partly healed (Figure 1f). But the number and size of voids increased after second cycle delithiation (Figure 1g). Panels h k of Figure 1 are the images taken after each lithiation delithiation cycle in the third and fourth cycles. It is very clear that the number and size of the voids increased further upon more cycles. After four cycles, the voids appeared everywhere along the NW, dividing the Al NW into many isolated nanoparticles enclosed by a glass tube (Figure 1k), essentially a pulverization of the metal electrode upon electrochemical cycling. Panels l and m of Figure 1 are higher magnification views of the areas marked by blue and red dotted rectangles in Figure 1k, 4190

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Figure 3. Evolution of the surface Al2O3 layer to the Li Al O glass layer. (a) A pristine Al NW with about 4 nm surface Al2O3 layer contacted with Li2O/Li electrode to establish an electrochemical device. (b) Lithiation of the surface layer, whose thickness was increased to about 5 nm. (c) The Al NW with the lithiated surface layer. (d f) EELS maps of Li, Al, and O, respectively, indicating that the surface Al2O3 layer had evolved to Li Al O layer after lithiation.

respectively, clearly showing the pulverized nanoparticles enwrapped by a glass tube. The EDP of the NW after four lithiation delithiation cycles (Figure 1p) confirmed the formation of nanocrystalline Al. Figure 1r is an enlarged image of an area in Figure 1k, showing the thickness of the surface amorphous layer was increased to about 6 nm. It was found that a stable amorphous layer was formed on the surface of the NW after electrochemical cycling (Figure 1d m). This surface layer served as a closed tube to confine the pulverized Al nanoparticles, preventing them from losing contact with the current collector. The diameter of this tube was larger than that of the original Al2O3 shell, and the thickness of the layer also increased (Figure 1r). Panels a c of Figure 2 show higher magnification views of the morphology changes of the surface layer in the first lithiation delithiation cycle. Figure 2a shows a pristine Al NW with a surface Al2O3 shell of about 5 nm, which increased to about 7 nm after lithiation (Figure 2b). Upon delithiation, the NW was pulverized to nanoparticles, while the surface layer had a thickness about 7 8 nm (Figure 2c), which did not shrink and was similar to that before delithiation, indicating a stable surface layer was formed after the first lithiation. Here it should be noted that the radial expansion from about 40 nm (Figure 2a) to 62 nm (Figure 2b) corresponds a volume expansion much more than 100%, that is because the radial expansion along the Al NW is not uniform. For example, as shown in Figure 1c, the radial expansion on the left side is larger than that in the middle part. But if the whole NW is considered, the volume expansion is estimated to be about 100%, which is consistent with the LiAl phase determined from the EDP from the lithiated Al NW (Figure 1o). We used electron energy loss spectroscopy (EELS) elemental mapping to characterize the composition of this layer. Panels d and e of Figure 2 exhibit the EELS spectra from an Al NW after three cycles. The rise of Li K edge peaks at 58.4 and 62.4 eV, Al-L edge peaks at 94.9 eV (Figure 2d) and O-K edge peaks with an onset about 525 eV

(Figure 2e) from the cycled Al NW confirmed the presence of Li, Al, and O in the NW. Zero loss image and EELS maps of Li, Al, and O are shown in panels f i of Figure 2, respectively. The zero loss image (Figure 2f) clearly shows the pulverized nanoparticles enclosed by a tube formed on the NW surface after three cycles. The Al map (Figure 2h) matches well with the dark contrasted area of the zero loss image (Figure 2f), indicating that the nanoparticles in the tube were Al nanoparticles after delithiation and that the surface layer also contained Al. Combining the Al map with Li (Figure 2g) and O (Figure 2i) maps, the surface layer consists of Li, Al, and O. It is reported that the Li2O Al2O3 glass exhibits high ionic conductivity and low electronic conductivity.27 The lithium ion conductivity of Li Al O glass is up to the order of 10 6 S/cm at room temperature,27 and the Li Al O compound is known to facilitate fast transport of lithium ion.28 32 Therefore, we believe that the surface layer formed a Li Al O glass, which served as a good solid electrolyte for Li ion diffusion along the length of the NW. It has been reported recently that the critical strain for crack propagation in a 5 nm ALD Al2O3 surface coating occurs at about 5.1%,33 while the about 100% volume expansion during lithiation corresponds to a strain of about 26%.18 The intact tube structure of the surface layer that did not crack or fracture after lithiation can be attributed to the Li Al O glass formation from the lithiation of Al2O3 layer, which may have a much larger critical fracture strain than Al2O3. The ALD Al2O3 coating is mostly used on cathode materials, such as LiCoO2 and LiMn2O4, which have very small volume changes (e.g., about 2% for LiCoO2).34 It is expected that in these cathode materials the formed Li Al O glass surface layer can totally accommodate the strain induced during the lithiation delithiation and mechanically confine the active materials to prevent them from breaking off the electrodes. Recently, ultrathin ALD Al2O3 coating was used on amorphous Si thin film for lithium ion batteries,35 which has a much larger volume expansion (about 280% for amorphous Si)36 than Al 4191

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Nano Letters (about 100%), and the Al2O3 coated Si film showed much fewer cracks than the uncoated Si film. It can be concluded that the improvement of the performance of LIBs using Al2O3 coatings on active materials is attributed to the formation of the Li Al O glass layer, which has the following two functions: (1) The Li Al O glass layer can provide a facile Li-ion transport path, relative to that in the usually formed solid electrolyte interface (SEI). (2) The mechanically robust Li Al O glass layer can mitigate mechanical degradation of the active materials to prevent them from breaking off the electrodes (i.e. pulverization). We also used controlled electrochemical lithiation of single Al NW to reveal the evolution of pristine Al2O3 layer to Li Al O glass solid electrolyte. It was found that the surface Al2O3 layer underwent lithiation first, after which the inner Al core was lithiated immediately (movies S2 S4 in the Supporting Information). This phenomenon is different from the lithiation of SnO2 NW coated with carbon, in which the carbon layer and the SnO2 core are lithiated nearly simultaneously along the longitudinal direction.37 In the controlled electrochemical lithiation experiment, the lithiation was stopped before the initiation of lithiation of the inner Al core (movie S5 in the Supporting Information). Figure 3a is the image showing a pristine Al NW contacted with Li2O/Li electrode. The surface Al2O3 layer underwent lithiation first, and the thickness of the layer was increased from about 4 nm to about 5 nm, as shown in Figure 3b. The smaller increase in thickness in Figure 3b compared to Figures 1r and 2b was from the incomplete lithiation of this layer in this experiment. The lithiation was then stopped, and the Li2O/Li electrode was removed from the NW (Figure 3c) to prevent the lithiation of the aluminum core. EELS maps of Li, Al, and O are shown in panels d f of Figure 3, respectively. Obviously, the surface Al2O3 layer evolved to Li Al O glass layer. We did not observe metallic Al nanoparticles in the amorphous layer (Figure 1r) as in the case of the Sn nanoparticles previously observed in the amorphous matrix of SnO2 NWs after lithiation delithiation.38 Furthermore, the Al EELS map (Figure 2d) exhibits smooth contrast at the surface layer, indicating that the Li Al O glass layer, acting as solid electrolyte, was stable. This explains the irreversible capacity observed during initial lithiation of ALD-modified particles and electrodes.17,18 While in situ TEM during lithiation delithiation cycling can give a great deal of insight into structural evolution of the NWs, no quantitative electrochemical information can be extracted. The current levels required to lithiate and delithiate a single NW are too small to be controlled or measured, so a conventional electrochemical cell is discharged and charged with potentiostatic control. It is therefore likely that the cycling rate, which appears to be on the order of 10C (10 full lithiations in 1 h), varies during each individual cycle and from cycle to cycle. Furthermore, high overpotentials are necessary in this system to drive the Li+ diffusion through the solid Li2O electrolyte. These points raise concern as to the degree to which the observed structural evolution mimics the evolution of the NWs in a realistic LIB environment. Thus, we performed standard electrochemical cycling experiments on the NWs while still bound to a stainless steel substrate. A half-cell (aluminum NWs vs lithium metal counter electrode) was constructed with a standard LIB liquid electrolyte, LiPF6, in a mixture of ethylene carbonate and dimethyl carbonate. The voltage profile and specific capacity of galvanostatic cycling at 1C rate (one full theoretical lithiation or delithation per hour) are shown in panels a and b of Figure 4, respectively. The cycling data agree very well with the TEM observations and reinforce the conclusions described above.

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Figure 4. Galvanostatic cycling of an aluminum NW sample in a lithium half-cell. (a) Voltage profile of Al NWs during cycling at 1C rate. The capacity loss begins immediately and occurs rapidly. (b) Charge capacity of Al NW sample as a function of cycle number. Loss of capacity occurs during delithiation, with lithiation steps having approximately the same capacity as the previous delithiation. (c) Voltage profile as a function of charge capacity for the initial cycling of Al NW sample. The initial, sloping high voltage is attributed to the irreversible lithiation of Al2O3, while the relatively stable voltage near 0.2 V corresponds to lithiation of Al metal. Delithiation occurs at approximately 0.5 V and shows an enormous loss of capacity in the first cycle.

As shown in Figure 4c, the first lithiation curve consists of a sloping voltage profile followed by a flat plateau around 0.2 V vs Li/Li+. The initial sloping voltage has been previously attributed, without direct evidence, to a surface reaction, and the flat portion of the curve typically corresponds to LiAl formation close to the theoretical value of 1 Ah/g.2,39 In the in situ TEM experiments, we noted that the first reaction observed was the lithiation of the oxide layer, which proceeded to completion before lithiation of the underlying Al. So, with the addition of the in situ TEM experiments in this study, we can definitively attribute this sloping voltage to the irreversible lithiation of the surface oxide. As shown in Figure 4c, the irreversible oxide lithiation step (the sloping portion of the potential curve) corresponds to a specific capacity of approximately 200 mAh/g. The capacity associated with LiAl formation (the flat portion of the potential curve) in the first cycle is then approximately 600 mAh/g, which is 400 mAh/g less than the theoretical value. These values suggest that the oxide layer has approximately half the specific capacity 4192

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Nano Letters for lithium as pure aluminum. This is not surprising since the lithiation of aluminum to LiAl and of Al2O3 to LiAlO2 both proceed to an Li:Al ratio of 1 and Al2O3 is 53% Al by mass. For these small diameter NWs, the mass of the surface Al2O3 and that of the Al core are estimated to be similar. It should be noted, though, that the lithiation of Al2O3 to LiAlO2 (a known crystal phase) cannot proceed without additional oxygen. Thus, the surface oxide reaction may be nonstoichiometric unless sufficient oxygen can be scavenged to form LiAlO2. Delithiation in the first cycle shows a very significant loss in capacity, reaching only half of the 600 mAh/g attributed to LiAl formation. This is consistent with the loss in electrically conductive pathways noted in the in situ TEM images. The second lithiation step has approximately the same capacity as the first delithiation, as no additional loss of conductivity occurs in the lithiation step. This trend continues throughout the remaining cycles, with capacity loss occurring on the delithiation steps, and each lithiation having a comparable capacity as the preceding delithiation. This is consistent with the structural evolution that is observed in the TEM experiments; pulverization occurs during delithiation leading to loss of connectivity and conductivity in the electrode material. In summary, electrochemical lithiation delithiation cycles of individual aluminum NWs with naturally oxidized surface Al2O3 layers were visualized by in situ TEM. The pulverization process of aluminum NW upon electrochemical cycling was observed. Voids were formed during the initial delithiation stage and continued to expand during delithiation in each subsequent cycle. This void formation and evolution were caused by the dealloying of lithium from LiAl. The surface Al2O3 layer was lithiated first to form a stable Li Al O glass tube, which not only acted as a solid electrolyte but also mechanically confined the pulverized nanoparticles to prevent them from losing contact with the current collector. Electrochemical lithiation delithiation cycling was also conducted in a conventional electrochemical test cell to mimic a more realistic LIB environment. The cycling behavior broadly agrees with the picture developed by the in situ microscopy experiments. The initial irreversible lithiation of the oxide can be observed in the voltage profile and the loss of electrical conduction due to pulverization can be seen in the rapid loss of capacity during delithiation in each subsequent cycle. The excellent agreement between these two methods demonstrates that, although the in situ TEM experiments cannot utilize normal electrochemical testing conventions such as galvanostatic cycling and the use of liquid carbonate electrolytes, the structures observed are relevant to real systems. Finally, the role of the Al2O3 layer has been elucidated. We have definitively shown that the sloping voltage noted in the initial lithiation step that was previously attributed to an unknown surface reaction is the lithiation of the surface oxide. We have also shown that the lithiated oxide is an amorphous shell that is able to deform to accommodate the swelling of the underlying NW during lithiation. The Al2O3 layer is ultimately detrimental in the system studied here since it causes a significant irreversible lithium loss and provides no cycling benefit. Though the oxide layer has good ion conductivity, it does not provide electrical conductivity and when the metal pulverizes, capacity is irreversibly lost. These results do, however, indicate that the improved cycle life of LIBs with ALD/Al2O3 coating on the active materials may be due to the formation of a stable Li Al O glass layer, which allows ion transport while protecting the active materials from breaking off the electrode.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Aluminum NW synthesis procedure and movies of lithiation of individual Al NWs. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Portions of this work were supported by the Center for Integrated Nanotechnologies (CINT), by a Laboratory Directed Research and Development (LDRD) project at Sandia National Laboratories (SNL), and by the Science of Precision Multifunctional Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. The LDRD supported the development and fabrication of platforms. The NEES center supported the development of TEM techniques. CINT supported the Al NW synthesis and TEM capability. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. ’ REFERENCES (1) Lindsay, M. J.; Wang, G. X.; Liu, H. K. J. Power Sources 2003, 119 121, 84–87. (2) Hamon, Y.; Brousse, T.; Jousse, F.; Topart, P.; Buvat, P.; Schleich, D. M. J. Power Sources 2001, 97 98, 185–187. (3) Fauteux, D.; Koksbang, R. J. Appl. Electrochem. 1993, 23 (1), 1–10. (4) Larcher, D.; Beattie, S.; Morcrette, M.; Edstrom, K.; Jumas, J.-C.; Tarascon, J.-M. J. Mater. Chem. 2007, 17 (36), 3759–3772. (5) Besenhard, J. O.; Hess, M.; Komenda, P. Solid State Ionics 1990, 40 41 (Part 2), 525–529. (6) Cui, L.-F.; Hu, L.; Choi, J. W.; Cui, Y. ACS Nano 2010, 4 (7), 3671–3678. (7) Besenhard, J. O.; Yang, J.; Winter, M. J. Power Sources 1997, 68 (1), 87–90. (8) Lei, X.; Ma, J. Mater. Chem. Phys. 2009, 116 (2 3), 383–387. (9) Au, M.; McWhorter, S.; Ajo, H.; Adams, T.; Zhao, Y.; Gibbs, J. J. Power Sources 2010, 195 (10), 3333–3337. (10) Park, C.-M.; Kim, J.-H.; Kim, H.; Sohn, H.-J. Chem. Soc. Rev. 2010, 39 (8), 3115–3141. (11) Morales, J.; Trocoli, R.; Franger, S.; Santos-Pe~ na, J. Electrochim. Acta 2010, 55 (9), 3075–3082. (12) Li, H.; Wang, Z.; Chen, L.; Huang, X. Adv. Mater. 2009, 21 (45), 4593–4607. (13) Li, C.; Zhang, H.; Fu, L.; Liu, H.; Wu, Y.; Rahm, E.; Holze, R.; Wu, H. Electrochim. Acta 2006, 51 (19), 3872–3883. (14) Eftekhari, A. Solid State Ionics 2004, 167 (3 4), 237–242. (15) Miyashiro, H.; Kobayashi, Y.; Seki, S.; Mita, Y.; Usami, A.; Nakayama, M.; Wakihara, M. Chem. Mater. 2005, 17 (23), 5603–5605. (16) Scott, I. D.; Jung, Y. S.; Cavanagh, A. S.; Yan, Y.; Dillon, A. C.; George, S. M.; Lee, S.-H. Nano Lett. 2011, 11 (2), 414–418. 4193

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(17) Jung, Y. S.; Cavanagh, A. S.; Riley, L. A.; Kang, S.-H.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S.-H. Adv. Mater. 2010, 22 (19), 2172–2176. (18) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Jung, Y. S.; Yan, Y.; Lee, S.-H.; Dillon, A. C. ChemPhysChem 2010, 11 (10), 2124–2130. (19) Kim, Y. J.; Kim, T.-J.; Shin, J. W.; Park, B.; Cho, J. J. Electrochem. Soc. 2002, 149 (10), A1337–A1341. (20) Kweon, H.-J.; Park, J.; Seo, J.; Kim, G.; Jung, B.; Lim, H. S. J. Power Sources 2004, 126 (1 2), 156–162. (21) Lee, S.-W.; Kim, K.-S.; Moon, H.-S.; Kim, H.-J.; Cho, B.-W.; Cho, W.-I.; Ju, J.-B.; Park, J.-W. J. Power Sources 2004, 126 (1 2), 150–155. (22) Jung, Y. S.; Cavanagh, A. S.; Dillon, A. C.; Groner, M. D.; George, S. M.; Lee, S.-H. J. Electrochem. Soc. 2010, 157 (1), A75–A81. (23) Riley, L. A.; Cavanagh, A. S.; George, S. M.; Lee, S.-H.; Dillon, A. C. Electrochem. Solid-State Lett. 2011, 14 (3), A29–A31. (24) Kim, Y. J.; Kim, H.; Kim, B.; Ahn, D.; Lee, J.-G.; Kim, T.-J.; Son, D.; Cho, J.; Kim, Y.-W.; Park, B. Chem. Mater. 2003, 15 (7), 1505–1511. (25) Campbell, T.; Kalia, R. K.; Nakano, A.; Vashishta, P.; Ogata, S.; Rodgers, S. Phys. Rev. Lett. 1999, 82 (24), 4866–4869. (26) Limthongkul, P.; Jang, Y.-I.; Dudney, N. J.; Chiang, Y.-M. Acta Mater. 2003, 51 (4), 1103–1113. (27) Glass, A. M.; Nassau, K. J. Appl. Phys. 1980, 51 (7), 3756–3761. (28) Garcia, A.; Torres-Trevino, G.; West, A. R. Solid State Ionics 1990, 40 41 (Part 1), 13–17. (29) Mizuhata, M.; Beleke, A. B.; Watanabe, H.; Harada, Y.; Deki, S. Electrochim. Acta 2007, 53 (1), 71–78. (30) Wen, Z.; Itoh, T.; Ikeda, M.; Hirata, N.; Kubo, M.; Yamamoto, O. J. Power Sources 2000, 90 (1), 20–26. (31) Ulihin, A. S.; Slobodyuk, A. B.; Uvarov, N. F.; Kharlamova, O. A.; Isupov, V. P.; Kavun, V. Y. Solid State Ionics 2008, 179 (27 32), 1740–1744. (32) Wang, Q. Y.; Liu, J.; Murugan, A. V.; Manthiram, A. J. Mater. Chem. 2009, 19 (28), 4965–4972. (33) Miller, D. C.; Foster, R. R.; Zhang, Y.; Jen, S.-H.; Bertrand, J. A.; Lu, Z.; Seghete, D.; O’Patchen, J. L.; Yang, R.; Lee, Y.-C.; George, S. M.; Dunn, M. L. J. Appl. Phys. 2009, 105 (9), 093527. (34) Reimers, J. N.; Dahn, J. R. J. Electrochem. Soc. 1992, 139 (8), 2091–2097. (35) Xiao, X.; Lu, P.; Ahn, D. Adv. Mater. 2011, DOI: 10.1002/ adma.201101915. (36) Krishnan, R.; Lu, T.-M.; Koratkar, N. Nano Lett. 2011, 11 (2), 377–384. (37) Zhang, L. Q.; Liu, X. H.; Liu, Y.; Huang, S.; Zhu, T.; Gui, L.; Mao, S. X.; Ye, Z. Z.; Wang, C. M.; Sullivan, J. P.; Huang, J. Y. ACS Nano 2011, 5 (6), 4800–4809. (38) Huang, J. Y.; Zhong, L.; Wang, C. M.; Sullivan, J. P.; Xu, W.; Zhang, L. Q.; Mao, S. X.; Hudak, N. S.; Liu, X. H.; Subramanian, A.; Fan, H.; Qi, L.; Kushima, A.; Li, J. Science 2010, 330 (6010), 1515–1520. (39) Hudak, N.; Huber, D. ECS Trans. 2011, 33 (24), 1–13.

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dx.doi.org/10.1021/nl202088h |Nano Lett. 2011, 11, 4188–4194