Letter pubs.acs.org/NanoLett
Microstructural Evolution of Tin Nanoparticles during In Situ Sodium Insertion and Extraction Jiang Wei Wang,† Xiao Hua Liu,*,‡ Scott X. Mao,*,† and Jian Yu Huang*,‡ †
Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡ Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *
ABSTRACT: The microstructural changes and phase transformations of tin nanoparticles during electrochemical sodiation were studied with a nanosized sodium ion battery using in situ transmission electron microscopy. It was found that the first sodiation process occurred in two steps; that is, the crystalline Sn nanoparticles were initially sodiated via a two-phase mechanism with a migrating phase boundary to form a Na-poor, amorphous NaxSn alloy (x ∼ 0.5), which was further sodiated to several Na-rich amorphous phases and finally to the crystallized Na15Sn4 (x = 3.75) via a single-phase mechanism. The volumetric expansion was about 60% in the first step and 420% after the second step. However, despite the huge expansion, cracking or fracture was not observed, which is attributed to the second step of the single-phase sodiation that accommodates large portion of the sodiationinduced stress over the entire particle. Excellent cyclability was also observed during the reversible sodiation/desodiation cycles, showing great potential of Sn nanoparticles as a robust electrode material for rechargeable batteries. KEYWORDS: Tin nanoparticles, sodiation, amorphous NaxSn alloy, Na15Sn4, sodium ion battery, in situ transmission electron microscopy
M
processes. Using Sn nanoparticles (NPs) as a model system, we found that the electrochemical sodiation of the crystalline Sn NPs is a two-step reaction at room temperature, characterized by transition from a two-phase mechanism to a single-phase mechanism. In the first step, an amorphous NaxSn (a-NaxSn, x ∼ 0.5) phase is growing by consuming the pristine Sn with a moving phase boundary, and formation of such a Napoor phase accounts for a modest volumetric expansion around 60%. In the second step, continued Na insertion leads to formation of several Na-rich amorphous phases and finally the crystalline Na15Sn4 (c-Na15Sn4) phase. The total volumetric expansion after full sodiation is about 420%. Figure 1a presents a schematic illustration of the solid cell that enables the electrochemical experiments of individual Sn NPs.23,26 The Sn NPs were purchased from Skyspring Nanomaterials Inc., and they were single crystals with diameters of about 80−400 nm and used as received. The nanosized solid cell consisted of a working electrode of such Sn NPs dispersed on a platinum (Pt) rod, a counter-electrode of bulk Na metal
otivated by the success in the development of Li-ion batteries,1−3 there is growing interest in Na-ion batteries for electrical vehicles and power backup applications.4−6 Advantages of Na-ion batteries over Li-ion batteries include the natural abundance and low cost of Na,4,5 especially for future large-scale applications like solar or wind farms. Guided by the chemistry of Li-ion batteries, potential candidate materials for Na-ion batteries include hard carbon,7 other Group-IV elements like Si, Ge, Sn,8,9 and various metallic oxides 10,11 for the negative electrode, and NaCrO2, 12 NaTi2(PO4)3,7 and Na(Ni0.5Mn0.5)O2 for the positive electrode.13 Theoretical investigations have shown that the Na alloys and sodiated hard carbon have significantly lower volumetric energy densities than that of Li alloys and lithiated graphite, due to the increased size of Na atoms as compared to the Li atoms.4,14−16 Therefore, high-energy electrodes are desired in the first place for the development of competitive Na-ion batteries. However, in contrast to the rich literature on Li-ion batteries, the science of Na-ion batteries is much less understood in most material systems. Inspired by our recent success in the in situ transmission electron microscopy (TEM) study of Li-ion battery materials,17−25 we built a nanosized Na-ion battery to study the fundamental science of the Na insertion and extraction © 2012 American Chemical Society
Received: September 5, 2012 Revised: October 12, 2012 Published: October 23, 2012 5897
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using the experimental conditions (accelerating voltage, camera length, etc.) and by decreasing the grain size of the corresponding crystalline phases to 0.2 nm. Figure 1b−g shows typical morphology evolution of several Sn NPs during the first sodiation process. The pristine Sn NPs were single crystals and had a spherical shape with different diameters (Figure 1b). After the front surface of the (Na2O + NaOH) electrolyte touched the Sn NPs, the sodiation process was initiated by applying a negative potential on the Sn NPs against the Na electrode. The sodiated part showed a lighter contrast at the contact point (Figure 1c), and the yellow arrows mark the Na ion insertion direction in the large Sn NP. As a clear phase boundary was moving forward (Figure 1d), more Sn was converted to a-NaxSn alloy, suggesting a “two-phase” mechanism. Such a sharp phase boundary separating the pristine and reacted parts was also observed in the first lithiation (alloying with Li) process of other Group IV elements such as Si.24 However, compared to the Si lithiation, the diameter change in the two-phase sodiation stage of Sn was not dramatic, for example, from 210 nm of the pristine Sn NP (Figure 1b) to 247 nm of the amorphized product (Figure 1e). The volumetric expansion was ∼60%, indicating a low degree of Na insertion. Intriguingly, the two-phase sodiation process continues in the small Sn NPs attached to the large NP in the center (Figure 1e−f). Meanwhile, the volume of the central NaxSn NP continued to grow as more Na ions were inserted (Figure 1e−g); however, no phase boundary was observed. Therefore, the further sodiation appeared to be a single-phase process. Large volumetric expansion was seen during this single-phase sodiation stage, and some dark crystal nuclei formed in the a-NaxSn parent phase (Figure 1g), indicating formation of a new phase at the late stage. The phase transformations in the entire process were studied with electron diffraction, which will be detailed later. The separation of the two-phase and single-phase regimes during sodiation of the Sn NPs is quite unique. Compared to the analogous lithiation process of Si, the first step of two-phase sodiation is similar to the structural evolutions in Si and Ge upon Li insertion,19,24,27 where a sharp interface is migrating during the growth of new phases and depletion of the old ones. However, unlike approaching the final compositions during the initial amorphization of Si24 and SnO2,17 more Na+ ions could be inserted into the Na-poor a-NaxSn particles without an obvious reaction front, and as a result the volume of the aNaxSn NPs continued to grow (Figure 1e−g). In fact, the second single-phase stage accounts for most Na insertion and volumetric expansion and thus plays an important role in the mechano-electrochemical coupling process. Particularly, despite the large volumetric expansion over 400% at full sodiation, no Sn NPs cracked or fractured, which was quite surprising and unexpected. For some of the Sn NPs, a core−shell intermediate state was observed during sodiation (the top Sn NP with the core−shell morphology in Figure 1f), indicating the fast sodium diffusion on the Sn NP surface.24 Lithiation-induced size-dependent fracture has been discovered in Si NPs (i.e., Si NPs with diameters larger than 150 nm always crack and fracture upon initial lithiation);24 however, for the tested Sn NPs with diameters in the range of 80−360 nm, despite larger sizes, no crack or fracture was observed. Most of the volumetric expansion occurs in the second step, that is, single-phase sodiation stage, which avoids the large concentrated stress developed at a sharp phase boundary as is the case for Si NP’s
Figure 1. Experimental setup and the typical morphological evolution of Sn NPs during sodiation. (a) Schematic illustration of the half-cell configuration of the nanosized Na-ion batteries. The working electrode is the Sn NPs dispersed on a Pt rod, and the counter electrode is a piece of Na metal on a W rod. A layer of Na2O and NaOH mixture is the solid electrolyte for Na+ transport. (b−g) Typical morphological evolution of Sn NPs during electrochemical sodiation. The pristine Sn NPs were spherical with different diameters (b). In the first step of sodiation, the reaction front was propagating from one side to the other. The volumetric expansion after this step was about 60% (c−d). In the second step of sodiation, there was no obvious reaction front, but the volume continued to expand (e−g). After the sodiation, the NaxSn NPs were partially crystallized.
on a tungsten (W) rod, and a solid electrolyte of naturally grown sodium oxide and hydroxide (Na2O + NaOH) on the Na metal. In a typical experiment, fresh Na metal was scratched off from a freshly cut surface of Na bulk with the W rod inside the glovebox filled with helium (water and oxygen concentrations both below 1 ppm). The working and counter electrodes were mounted onto the Nanofactory TEM-scanning tunneling microscopy (STM) holder in the glovebox, which was transferred in a homemade sealed plastic bag filled with dry helium and loaded into the TEM column. The Na metal was exposed to the air for about 2 s during the holder loading process, and as a result, a layer of Na2O and NaOH mixture was grown on the Na surface, which served as the solid electrolyte for Na+ ion transport. A typical electron diffraction pattern (EDP) of the (Na2O + NaOH)/Na part is presented in Figure S1 of the Supporting Information. During the in situ experiments, potentials of −0.5 to −2 V were applied to the Sn NPs with respect to the Na metal to drive sodiation (Na insertion) and +2 to +4 V for desodiation (Na extraction). To determine the structure of the intermediate a-NaxSn phases, the experimental EDPs were compared with the simulated ones that were calculated with the commercial package JEMS, by 5898
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lithiation. In the two-phase amorphization stage, the volumetric expansion is only 60% for Na−Sn alloying but 300% for Li−Si alloying, which directly leads to the size-dependent fracture in the latter due to the large stress developed at the sharp phase boundary.24 This has several important consequences, including (1) multiple intermediate Na−Sn phases; (2) different stress development and plastic deformation; and (3) possibly superior mechanical stability of Sn NPs. Therefore, the Sn NPs may be a good anode material for sodium batteries in terms of accommodating the large volumetric expansion enabled by the unique reaction mechanisms. With the capability of phase identification by electron diffraction in the in situ TEM experiments, we focus on the multiple Na−Sn alloy phases formed in the electrochemical sodiation process. Figure 2 shows the microstructural evolution of more crystalline Sn NPs alloyed with Na. Figure 2a−b shows the morphology and EDP of the pristine Sn NPs with different diameters. The EDP of the pristine Sn NPs indicates a tetragonal crystal structure (a = 5.8316 Å, c = 3.1813 Å, space group 141, Figure 2b). Figure 2c−d shows the microstructure of the sodiated Sn NPs in the single-phase stage. The average volumetric expansion was about 280% (Figure 2c), suggesting that the sodiation had proceeded to the second step and the crystalline Sn NPs were converted into a-NaxSn phase (Figure 2d). A thin layer of crystal Na2O was observed on the surface of the a-NaxSn NPs, as indicated by the halos in Figure 2d. Upon further sodiation, more Na+ ions were inserted into the aNaxSn phase, leading to more volumetric expansion (Figure 2e). After the Sn NPs were fully sodiated, the total volumetric expansion of the sodiated Sn NPs was about 445%. The EDP indicated that the a-NaxSn phase was finally converted into the c-Na15Sn4 phase (Figure 2f). Figure 2g presents the simulated EDP of the c-Na15Sn4 phase with the intensity profile (a = 13.140 Å, space group 220),28 which is in excellent agreement with the experimental results. There are multiple Na−Sn alloy phases in the Na−Sn phase diagram.29 Previous studies suggested that the intermediate amorphous a-LixM (M = Si, Ge, or Sn) phases have large solubility ranges for lithium,19,30,31 and apparently different intermediate amorphous phases correspond to different volumetric expansions.4,31 In our experiments, three a-NaxSn phases were identified with electron diffraction by controlling the extent of sodiation, as manifested by the corresponding volumetric changes. Unlike diffraction patterns from crystals, the difference between the amorphous phases was embedded in the subtle discrepancy of the halos in the EDPs (Figure 3). To indentify the composition of the different a-NaxSn phases, the diffraction patterns for the known Na−Sn alloy phases were simulated with the experimental TEM conditions (such as accelerating voltage) and by breaking the long-range order of the corresponding crystalline phases (i.e., by decreasing the grain size to 0.2 nm). The volumetric expansions for different NaxSn phases and their corresponding theoretical values were listed in Table 1. Figure 3a shows the EDP of the a-NaxSn phase right after the two-phase sodiation. The EDP was taken when the reaction front just passed the entire Sn NP. Comparative study indicated that the experimental EDP best matches the NaSn2 phase (a = 13.392 Å, b = 6.854 Å, c = 15.489 Å, space group 12);28 therefore we denote it as the aNaSn2 phase. The experimentally measured average volumetric expansion after the first step sodiation was about 54−63%, which agrees well with the theoretical value of 56% (Table 1).
Figure 2. Microstructural evolution of Sn NPs during sodiation. (a−b) Pristine Sn NPs. The electron diffraction pattern (EDP) indicates that the Sn NPs had the tetragonal structure (white tin). (c−d) Fully amorphized Sn NPs. The EDP shows that the Sn NPs were transformed into a-NaxSn with average volume expansion about 280%. (e−f) Fully sodiated Sn NPs. The EDP indicates that the aNaxSn alloy was crystallized to the c-Na15Sn4 phase with volume expansion about 445%. (g) Simulated EDP of the c-Na15Sn4 phase.
The EDP of the second a-NaxSn phase (Figure 3b) was similar to that shown in Figure 2d. This phase was frequently observed in the second step sodiation where no phase boundary was seen; thus it is probably a metastable intermediate Na−Sn phase. The diffuse rings of this phase fit well to the simulated pattern of Na9Sn4 (a = 5.42 Å, b = 9.39 Å, c = 29.62 Å, space group 63);28 thus it is denoted as a-Na9Sn4, with the volumetric expansion ranging from 210% to 280%. 5899
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Figure 3. Three a-NaxSn phases in the single-phase sodiation. (a) EDP of the first a-NaxSn phase, which was taken when the reaction front just swept the entire Sn NPs. The simulated EDP indicated that the composition of the first a-NaxSn phase was close to the NaSn2 phase. (b) EDP of the second a-NaxSn phase. The amorphous halos match the simulated a-Na9Sn4 phase. (c) EDP of the third a-NaxSn phase, which was identified as aNa3Sn based on volumetric expansion. It is structurally close to the c-Na15Sn4 phase as tiny c-Na15Sn4 crystallites usually nucleate in this phase. (d) Schematic illustration of the structural evolution of Sn NPs during the sodiation.
NPs during the sodiation. The sodiation of Sn NPs was initiated by a two-phase reaction to form a-NaSn2 with a moving reaction front, followed by several single-phase transformations without obvious phase boundaries to the final c-Na15Sn4 phase. Since there are at least eight distinct Na−Sn alloy phases in the Na−Sn phase diagram,29 it is believed that more a-NaxSn phases may be controllably obtained or identified with better instrumentation for the electrochemistry and electron diffraction. Recently, Obrovac et al. reported structural transformations of Sn films during electrochemical sodiation using the in situ Xray diffraction (XRD) technique.32 They observed four distinct two-phase regions during sodiation, which do not match the phases predicted by the equilibrium phase diagram except for the c-Na15Sn4 phase upon full sodiation. Such discrepancy with the equilibrium phase diagram was explained with difficulty in forming thermodynamically favorable but structurally complex phases at low temperatures. A similar discrepancy was previously found in the Li−Si system,33 which led to the discovery of the Li15Si4 phase and suggested that phase transformation in an electrochemical process does not necessarily follow, usually deviating from the equilibrium.23,26 However, our in situ TEM observation of the single-phase sodiation is not consistent with the two-phase plateaus observed in the in situ XRD experiments,32 which could be attributed to the phase behaviors that are highly dependent on kinetics factors such as experimental configuration and sample conditions.34 Nevertheless, these inconsistent observations seem to suggest that systematic study of the electrochemical alloying process is necessary for the Sn-based electrodes. The Sn NPs showed excellent cyclability during the sodiation/desodiation cycles. Figure 4 presents the typical microstructural evolution of Sn NPs during the six sodiation/ desodiation cycles. In the first three cycles (Figure 4a−g), the Sn NPs underwent nearly reversible volumetric expansion and shrinkage. The EDP (Figure 4m) indicated that the Sn NPs were partially sodiated to the a-Na9Sn4 phase. The sodiated aNa9Sn4 phase transformed back to crystallized Sn phase during
Table 1. Statistics of Experimentally Observed Volumetric Expansion for NaxSn Phases volumetric expansion
a-NaSn2
measured
54% 63%
theoreticalb
∼56%
a-Na9Sn4 211% 225% 250% 257% 276% 276% ∼252%
a-Na3Sna 317% 327% 330% 330% 368% ∼336%
c-Na15Sn4 402% 403% 445% 449% 453% 459% ∼420%4
a
The a-Na3Sn was partially crystallized. bThe volumetric expansion for different Na−Sn alloys has a linear relationship with the Na content. After full sodiation (c-Na15Sn4 phase), the total volumetric expansion is about 420%.4 The volumetric expansions for other Na−Sn alloys are calculated based on this linear relationship.
The third a-NaxSn phase was usually partially crystallized (Figure 3c), with the volumetric expansion of 320−370%. The volumetric expansion of the third a-NaxSn phase is close to the theoretical volumetric expansion of Na3Sn phase (336%) (Table 1); thus it is denoted as a-Na3Sn. The crystallized phase was identified to be c-Na15Sn4 (Figure 3c). After fully sodiation, the whole a-Na3Sn NP was converted to the cNa15Sn4 phase with the volumetric expansion of about 402− 459% with respect to the original particle size, which was close to the theoretical value of ∼420%.4 It is noted that the EDPs of the second and third a-NaxSn phases were quite similar to each other, both having two diffraction halos at the similar positions. The major difference is from the inner halos, that is, their relative intensity and broadness. For the a-Na9Sn4 phase, the relative intensity of inner ring was higher (i.e., brighter in the EDP, Figure 3b), while the a-Na3Sn phase has a broader inner halo (Figure 3c). Moreover, partial crystallization usually occurs in the a-Na3Sn phase, and the diffraction spots from the c-Na15Sn4 phase showed up in the EDPs, which can be used to distinguish the aNa9Sn4 and a-Na3Sn phases, along with the different volumetric expansion. Figure 3d summarizes the structure evolution of Sn 5900
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Figure 4. Microstructural evolution of the Sn NPs in sodiation/desodiation cycles. (a−l) Morphological evolution in the first six sodiation/ desodiation cycles. (m−n) EDPs of sodiated phase after the second (m) and sixth sodiation cycles (n), which were a-Na9Sn4 and c-Na15Sn4 phases, respectively. (o) EDP of desodiated phase after the second desodiation and identified as c-Sn.
observed in Li ion batteries using Sn NPs as the anode.37,38 As a low-melting-point (Tm = 232 °C) metal with the reconstruction of Sn after Na extraction to its crystalline state, it seems that Sn may be engineered to make a self-healing electrode yet with high energy density. Conclusions. In conclusion, the microstructural evolution and phase transformation of Sn nanoparticles during Na insertion and extraction are studied with in situ TEM in the nanobattery configuration. Pristine Sn undergoes two-step sodiation to form amorphous NaSn2 in the first step and sequentially to amorphous Na9Sn4, Na3Sn, and crystalline Na15Sn4 phases in the second step. The first step is a two-phase reaction with modest expansion around 60%, while the second step is single-phase reaction to the final volumetric expansion of 420%. These mechanisms offer better accommodation of the sodiation-induced stress, and reversible sodiation/desodiation processes were demonstrated for Sn particles without fracture. Such understanding sheds light on the design of rechargeable batteries with Sn as a promising electrode material, and the methodology can be readily applied to the study of other electrode materials for Na-ion batteries.
the desodiation, as indicated in Figure 4o. It is noted that the shape of the Sn NPs was changed during the desodiation with the volumetric shrinkage (Figures 4c,e). Occasionally, the sodium would come out and grow on the surface of the NPs during the desodiation (Figure 4e,g). In the following sodiation, the sodium grown out of the NPs could be reabsorbed by the Sn NPs to form the Na−Sn alloy (Figure 4e−i). However, the precipitated sodium might otherwise induce a short circuit of the battery,35,36 which should be avoided in real batteries. In the following three sodiation/desodiation cycles, the Sn NPs were fully sodiated to the c-Na15Sn4 phase (Figure 4n), and a larger volumetric expansion was obtained (Figure 4h−j). The shape of desodiated Sn NPs became more and more irregular with increasing cycles. Figure S2 of the Supporting Information presents another Sn NPs undergoing the sodiation/desodiation cycles in the beam blank experiment (i.e., the electron beam was blocked except for imaging after each cycle). Similarly, the Sn NPs showed excellent cyclability. The Sn NPs were also sodiated into the fully crystallized c-Na15Sn4 phase and could be completely desodiated into the c-Sn phase. This is different from the delithiation of LixSi, which leads to the formation of amorphous Si.18,19 However, like the one shown in Figure 4, the shape of this Sn NPs changed significantly after desodiation, from sphere to irregular shape (Figure S2). The shape change of Sn NPs might be induced by the structure reconstruction as 5901
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(17) 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. (18) Liu, X. H.; Zhang, L. Q.; Zhong, L.; Liu, Y.; Zheng, H.; Wang, J. W.; Cho, J.-H.; Dayeh, S. A.; Picraux, S. T.; Sullivan, J. P.; Mao, S. X.; Ye, Z. Z.; Huang, J. Y. Nano Lett. 2011, 11 (6), 2251−2258. (19) Liu, X. H.; Zheng, H.; Zhong, L.; Huang, S.; Karki, K.; Zhang, L. Q.; Liu, Y.; Kushima, A.; Liang, W. T.; Wang, J. W.; Cho, J.-H.; Epstein, E.; Dayeh, S. A.; Picraux, S. T.; Zhu, T.; Li, J.; Sullivan, J. P.; Cumings, J.; Wang, C.; Mao, S. X.; Ye, Z. Z.; Zhang, S.; Huang, J. Y. Nano Lett. 2011, 11 (8), 3312−3318. (20) Liu, Y.; Hudak, N. S.; Huber, D. L.; Limmer, S. J.; Sullivan, J. P.; Huang, J. Y. Nano Lett. 2011, 11 (10), 4188−4194. (21) Liu, X. H.; Wang, J. W.; Liu, Y.; Zheng, H.; Kushima, A.; Huang, S.; Zhu, T.; Mao, S. X.; Li, J.; Zhang, S.; Lu, W.; Tour, J. M.; Huang, J. Y. Carbon 2012, 50 (10), 3836−3844. (22) Liu, Y.; Zheng, H.; Liu, X. H.; Huang, S.; Zhu, T.; Wang, J.; Kushima, A.; Hudak, N. S.; Huang, X.; Zhang, S.; Mao, S. X.; Qian, X.; Li, J.; Huang, J. Y. ACS Nano 2011, 5 (9), 7245−7253. (23) Liu, X. H.; Liu, Y.; Kushima, A.; Zhang, S.; Zhu, T.; Li, J.; Huang, J. Y. Adv. Energy Mater. 2012, 2 (7), 722−741. (24) Liu, X. H.; Zhong, L.; Huang, S.; Mao, S. X.; Zhu, T.; Huang, J. Y. ACS Nano 2012, 6 (2), 1522−1531. (25) Liu, X. H.; Wang, J. W.; Huang, S.; Fan, F.; Huang, X.; Liu, Y.; Krylyuk, S.; Yoo, J.; Dayeh, S. A.; Davydov, A. V.; Mao, S. X.; Picraux, S. T.; Zhang, S.; Li, J.; Zhu, T.; Huang, J. Y. Nat. Nanotechnol. 2012, DOI: 10.1038/nnano.2012.170. (26) Liu, X. H.; Huang, J. Y. Energy Environ. Sci. 2011, 4, 3844−3860. (27) Liu, X. H.; Huang, S.; Picraux, S. T.; Li, J.; Zhu, T.; Huang, J. Y. Nano Lett. 2011, 11 (9), 3991−3997. (28) ICSD. Fachinformationszentrum Karlsruhe: Karlsruhe, Germany, 2006. (29) ASM Handbook Vol. 3: Alloy Phase diagrams; The Materials Information Society: Materials Park, OH, 1992; p 302. (30) Danet, J.; Brousse, T.; Rasim, K.; Guyomard, D.; Moreau, P. Phys. Chem. Chem. Phys. 2010, 12 (1), 220−226. (31) Obrovac, M. N.; Christensen, L.; Ba Le, D.; Dahn, J. R. J. Electrochem. Soc. 2007, 154 (9), A849−A855. (32) Ellis, L. D.; Hatchard, T. D.; Obrovac, M. N. J. Electrochem. Soc. 2012, 159 (11), A1801−A1805. (33) Obrovac, M. N.; Christensen, L. Electrochem. Solid-State Lett. 2004, 7 (5), A93−A96. (34) Beattie, S. D.; Hatchard, T.; Bonakdarpour, A.; Hewitt, K. C.; Dahn, J. R. J. Electrochem. Soc. 2003, 150 (6), A701−A705. (35) Orsini, F.; Du Pasquier, A.; Beaudoin, B.; Tarascon, J. M.; Trentin, M.; Langenhuizen, N.; De Beer, E.; Notten, P. J. Power Sources 1998, 76 (1), 19−29. (36) Liu, X. H.; Zhong, L.; Zhang, L. Q.; Kushima, A.; Mao, S. X.; Li, J.; Ye, Z. Z.; Sullivan, J. P.; Huang, J. Y. Appl. Phys. Lett. 2011, 98 (18), 183107. (37) Chao, S.-C.; Song, Y.-F.; Wang, C.-C.; Sheu, H.-S.; Wu, H.-C.; Wu, N.-L. J. Phys. Chem. C 2011, 115 (44), 22040−22047. (38) Chao, S.-C.; Yen, Y.-C.; Song, Y.-F.; Chen, Y.-M.; Wu, H.-C.; Wu, N.-L. Electrochem. Commun. 2010, 12 (2), 234−237.
ASSOCIATED CONTENT
S Supporting Information *
Figures showing the microstructural of the (Na2O + NaOH)/ Na electrode and the morphological evolution of the Sn NPs during the sodiation/desodiation cycles without electron beam illumination. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]; jyhuang8@ yahoo.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Portions of this work were supported by a Laboratory Directed Research and Development (LDRD) project at Sandia National Laboratories (SNL) and partly by 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 No. DESC0001160. The LDRD supported the development and fabrication of platforms. The NEES center supported the development of TEM techniques. The Sandia-Los Alamos Center for Integrated Nanotechnologies (CINT) supported the TEM capability. 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. S.X.M. acknowledges NSF CMMI 08 010934 through University of Pittsburgh and Sandia National Lab support.
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