Letter pubs.acs.org/NanoLett
Monodisperse Sn Nanocrystals as a Platform for the Study of Mechanical Damage during Electrochemical Reactions with Li Linping Xu,† Chunjoong Kim,† Alpesh K. Shukla,† Angang Dong,‡ Tracy M. Mattox,‡ Delia J. Milliron,‡ and Jordi Cabana*,† †
Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, California 94720, United States ‡ The Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Monodisperse Sn spherical nanocrystals of 10.0 ± 0.2 nm were prepared in dispersible colloidal form. They were used as a model platform to study the impact of size on the accommodation of colossal volume changes during electrochemical lithiation using ex situ transmission electron microscopy (TEM). Significant mechanical damage was observed after full lithiation, indicating that even crystals at these very small dimensions are not sufficient to prevent particle pulverization that compromises electrode durability. KEYWORDS: Monodisperse nanocrystals, energy storage, tin, lithium alloys, mechanical damage
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number of authors.10−12 Cracking in large silicon particles was found to initiate at the surface due to the build-up of tensile stress as a sharp interface between a crystalline Si core and an amorphous LixSi shell with x ≥ 3 forms and propagates.10,12 The exact critical size values were found to differ between studies, most likely because of differences in experimental conditions, but appeared to be well above 100 nm. The amorphization process during reaction with Li could hypothetically dissipate part of the mechanical energy involved in the volume expansion. Sn has a very different phase transformation mechanism, as it transitions between multiple lithiated intermediates that are crystalline.13,14 Elastic softening was predicted to occur with Li content, increasing the chances of mechanical failure when strain accumulates during a twophase transformation.15 The viability of nanosized Sn particles as electrodes has been assessed, with favorable cycling performance in some cases.16 However, modeling of the phase transformation by Wolfenstine et al. led to the hypothesis that the ability of Sn nanoparticles to circumvent cracking is limited, the critical sizes being even smaller than a unit cell.17,18 In a separate report, Dimitrijevic et al. predicted that fracture could be negligible only if the particle size is equal to or lower than 20 nm, and the interparticle half-spacing greater than 30 nm.19 Recent experimental evidence with large nanoparticles suggests that the critical size for fracture is notably smaller for Sn than for Si.20
nergy is stored in a battery through opposed redox reactions at two electrodes, which involve transfer of electrons and ions through the external circuit and the electrolyte, respectively. The energy storage capacity of the device scales with the amount of charge exchanged between electrodes, as does the extent of transformation the electrode undergoes.1 In Li-ion batteries, the highest storage capacities are achieved through electrochemical alloying reactions, such as with Si (4200 mAh/g, theoretical capacity) or Sn (990 mAh/ g),2 as well as through the reduction of transition metal compounds to their metallic state (up to more than 1000 mAh/ g).3 However, these reactions involve colossal volume changes between initial and final states,4 leading to severe mechanical stress and, ultimately, failure. Consequently, commercial Li-ion batteries still rely on intercalation materials such as LiMO2 (M = Ni, Co, Mn, positive electrode) and graphite (negative), at the expense of lower capacities. Unfortunately, the energy density of these devices falls short of large-scale applications such as electric drive vehicles,5 considered a central player in the path toward a society based on renewable energies instead of oil.6 As a result, extensive efforts continue to be directed at technologically enabling alloy- and conversion-based electrodes. Reduction of primary particle size to the nanoscale is one of the most studied approaches toward alleviating the deleterious effects of volume expansion.7 Several studies of the mechanical stability in nanoparticles are available for Si. Experimentally, failure was found to occur during either the lithiation or delithiation step depending on the conditions and material form.8−10 The existence of a critical crystal size below which fracture did not occur upon lithiation was proposed by a © 2013 American Chemical Society
Received: February 1, 2013 Revised: March 2, 2013 Published: March 11, 2013 1800
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interparticle spacing as low as 3 nm was consistent with the presence of a homogeneous surfactant coating (as shown in Figures 1a and S1b,c). The particles were crystalline, as shown by powder X-ray diffraction (XRD, Figure 1c) and supported by high-resolution TEM (HR-TEM, Figure 1b and S1d). The XRD pattern and the fast Fourier transform (FFT) of HR-TEM images (insets in Figure 1b) match the tetragonal structure of β-Sn (tetragonal, I41/amd space group, JCPDS card #04-0673), with no other detectable crystalline impurities. TEM images collected from different regions were used to analyze particle size distribution. Measurements from 300 particles revealed an average diameter of 10.0 ± 0.2 nm (Figure 2a and d). This
There are several examples in the literature of cracking during crystalline phase transitions in nanocrystals. It was observed in CdSe rods of lengths as short as 16 nm after a pressure-induced transition between zinc blende/wurtzite and rock-salt structures,21 despite the lack of significant shape change,22 as well as during cationic exchange reactions in spheres of less than 10 nm diameter.23 Such transformations involve single volume changes below 50%, as opposed to consecutive anisotropic expansions between LixSn phases that add up to ∼350% of the initial volume.24 In previous studies, all of the experimental information on the mechanical stability of nanoparticles during alloying with lithium was gathered from samples with little control of morphological homogeneity. However, such homogeneity would afford the opportunity to perform an accurate analysis of the changes introduced, since any deviation from the initial size/shape distribution can be ascribed to the electrochemical reaction. Tolbert and Alivisatos pioneered the use of highly tailored nanocrystals as a model platform for an accurate assessment of first order phase transitions.25,26 Here, we describe the colloidal synthesis of monodisperse Sn nanocrystals (NCs). They were subsequently used in an ex situ transmission electron microscopy (TEM) study of the changes in morphology during electrochemical cycling. This approach furnished valuable insight into the existence of mechanical damage even in very small spherical particles (10.0 ± 0.2 nm diameter). Sn NCs were synthesized by reducing bis[bis(trimethylsilyl)amino]tin(II) (Sn[N(SiMe3)2]2) under N2 using 1.0 M lithium triethylborohydride in THF at 120 °C, with oleylamine as both solvent and capping agent. TEM revealed that the resulting particles were spherical and found to arrange in a 2-dimensional closed packed way in some areas in the grid (Figure 1a), highlighting their uniformity (see also a representative SEM and TEM images in Figure S1a,b). The existence of regular
Figure 2. TEM micrographs and corresponding size distribution histograms for Sn NCs: (a and d) as prepared, (b and e) after lithiation to 0 V, and (c and f) after delithiation to 2 V (see Figure S5 for electrochemical data). Shadowed bars in e and f show the particle size distribution of pristine Sn NCs for comparison.
value was consistent, within error, with estimates derived from fittings of the XRD peak broadening. The size of the particles was found to be slightly smaller than in a previous report of the colloidal synthesis of Sn NCs by photoreducing a similar Sn precursor.27 The electrochemical properties of Sn NCs were evaluated by building composite electrodes (15 wt % carbon and polyvinyldifluoride binder, respectively). The electrodes were subsequently tested in Li metal half cells. Complex voltage− capacity curves were observed during the first lithiation− delithiation cycle (Figure 3a), with poorly defined plateaus at
Figure 1. (a) TEM micrographs of pristine Sn NCs. (b) HR-TEM images of selected crystals, with the corresponding fast Fourier transform (FFT) in the inset along the [010] zone axis being consistent with the tetragonal structure of β-Sn. (c) XRD pattern with referenced lines indicating the reflection positions for β-Sn (JCPSD card #: 04-0673). 1801
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distributions, ranging from 25 to 150 nm (Figure S3). Despite the lower first cycle specific capacity, indicative of less extensive electrolyte decomposition (note the smaller charge contribution of high voltage peaks in Figure S2b), the Coulombic efficiency of commercial Sn was noticeably worse than Sn NCs upon cycling. Part of this inefficiency is ascribed to loss of material due to mechanical failure during lithiation. Indeed, the Sn NC electrodes were found to retain capacity substantially better than the commercial material. Therefore, reducing the particle size and improving homogeneity is definitely a favorable strategy to improve the electrode performance. However, ultimately, the capacity loss observed after 50 cycles was still too high for practical purposes. To evaluate the contribution of mechanical instabilities to the capacity loss of the Sn NCs, their morphological evolution upon lithiation and delithiation was studied by ex situ TEM, using the methodology proposed by McDowell et al. to study Si nanotubes.36 In brief, the nanoparticles were dropcast onto ∼20 nm thick carbon-coated Cu TEM grids, which were directly used as working electrodes in Li-metal half cells (Figure S4). Cyclic voltammetry (CV) was subsequently performed. Comparison of the corresponding electrochemical signatures with those of a blank grid (Figure S5) revealed a significant increase in charge passed both under reducing and oxidizing conditions when the grids were loaded with Sn NCs. Further, while the CV of the blank was largely featureless, a series of more or less resolved peaks were observed when Sn NCs were present. These electrochemical features were similar to those observed in the thick composite electrodes (see Figures 3a, S2, and S5). All in all, these results prove that loading the Sn NCs on TEM grids was an effective setup for electrochemical lithiation and delithiation. The grids were harvested from the cells and analyzed by TEM. The size distribution after electrochemical cycling was also obtained by measuring the size of 300 selected particles. The corresponding images after a full lithiation (i.e., reduction to 0 V vs Li+/Li0) revealed a decrease in particle size and an increase in inhomogeneity, the particle shapes appearing to be substantially more irregular than in the pristine state (Figure 2a vs b). As a result, the size distribution was broadened (Figure 2d vs e). It should be noted that a bias toward larger particles existed in the histograms of cycled samples because particles showing diameter ≤2 nm could not be accurately measured and, thus, were excluded from the counts. Indeed, the existence of a substantial level of crystalline particle debris, not observed in the pristine state, was clear, with fragments smaller than 5 nm. These observations provide direct evidence of particle cracking during lithiation even for the very small particles used in this work. Further information was gathered by HR-TEM. Figure 4 shows representative examples of the resulting high magnification images for the samples after lithiation and delithiation. A high degree of crystallinity was observed within particles across the grid following a discharge to 0 V. The fast Fourier transformed patterns (see selected examples as insets in Figure 4a) confirmed that lithiation occurred during the experiment, as they could not be matched to any form of pure Sn. However, it was not possible to distinguish among the several lithiated phases that could form during the reaction (e.g., LiSn, Li7Sn3, Li7Sn2, and Li22Sn5)13 due to the insufficient accuracy in the measured d-spacings. The size distribution and the morphology after extraction of Li were similar to the lithiated particles (Figure 2c and f). The resulting fragments were still found to be crystalline by HR-TEM. This preservation of crystallinity
Figure 3. (a) Voltage-specific capacity profiles at selected cycles of Li metal half cells containing Sn NCs as working electrode. (b) Comparison of the performance of Li metal half cells with Sn NCs (green) and commercial Sn nanoparticles (red). Physico-chemical characterization data, including differential capacity plots, for the latter can be found as Supporting Information.
1.35, 1.2, 0.95, 0.6, and 0.35 and 0.25 V (Figure S2a), respectively. While the processes above 0.8 V are typically associated with electrolyte decomposition on the surface of the particles,28 the plateaus below this voltage are indicative of twophase reactions between discrete LixSn alloy phases at different x.13,24 The fact that they were less resolved than in bulk Sn electrodes29 is ascribed to the increased surface area in nanoparticulate electrodes. On one hand, different surface and subsurface sites react at slightly different potentials with respect to each other and the bulk,30,31 so that increased surface-to-bulk ratios lead to a concomitant dispersion of values at which the electrochemical reaction takes place. On the other hand, increased electrode surface area leads to more extensive decomposition of the electrolyte during the lithiation, especially at low potentials, which also tends to wash out the electrochemical profile. The existence of side reactions explains the higher first discharge specific capacity (1550 mAh/g) than the theoretical value based solely on the lithiation of Sn to form Li22Sn5 (990 mAh/g). These reactions are known to be largely irreversible,32,33 as exemplified by the significant disappearance of anodic processes above 0.8 V on subsequent cycles (Figures 3a and S2). The resulting first cycle Coulombic efficiency, ca. 65%, is comparable to other values in the literature for Sn-based electrodes.34,35 Reduced capacity and further signal washout were observed as the cycle number increased (Figure 3a). Figure 3b summarizes the cycling performance of Sn NCs, compared to a commercial Sn sample (Sigma-Aldrich, product #576883) consisting of particles with much wider size 1802
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would be expected following the volume expansion during the reaction; the small size observed in Figure 2 would imply very thick oxide layers whose mass contrast should be visible in a TEM bright field image,41 as shown in previous work.42 Yet no evidence of such oxide shells was found in any of the images collected in this study. While some surface oxidation cannot be precluded, it would not be detectable only if the resulting layers were very thin, simply introducing a small error in the measurement. Further, chemical oxidation cannot explain the notable shape change when going from pristine to lithiated samples. The existence of mechanical damage falls well within the trends observed in other nanocrystal systems of similar dimensions undergoing phase transformations, such as for CdSe during pressure cycling21 or cation exchange.23 However, it is in apparent contrast with the recent report by Wang et al. of the stability of Sn during sodiation.43 In that case, it is unclear whether the particles remained single crystalline after the reaction, as only low resolution imaging was reported. The appearance of very small crystals (∼10 nm) within a larger particle, as well as extensive deformation and roughening upon sodiation, suggest that some mechanical changes indeed took place. At any rate, some of the differences could also be explained by the conditions of the experiment in the two cases. While Wang et al.43 performed the reaction by placing Sn particles in direct contact with Na metal and applying a huge overpotential to overcome the resistivity of a Na2O/Na2CO3 surface layer, the lithiation was performed electrochemically in this study. The resulting kinetics of reaction are probably very different in the in situ experiment than in our ex situ conditions. Finally, sodiation was found to occur mostly through a singlephase, second order transition, in contrast to the multiple twophase processes for lithiation. Disregarding the fact that such mechanism is in contradiction with results in actual electrochemical cells,44 homogeneous, second-order transitions result in lower mechanical strain compared to processes involving boundaries, thus potentially facilitating preservation of particle integrity. It is also possible that, as proposed by Dimitrijevic et al.,19 mechanical failure in the conditions of this study is induced by particles crushing into each other during expansion due to the insufficient distance between them. However, their model for calculation was based on the 3D matrix with periodical occupation of spherical Sn, which is quite different from our experimental setup. Indeed, at the low particle loadings we used, it would seem like the TEM grid can offer enough free volume space to avoid mechanical damage by collision. The sodiation study by Wang et al.43 also supports this assumption; no clear crushing was apparent even in regions where several particles agglomerated. Based on our observations during a carefully controlled electrochemical lithiation, we believe that the mechanical failure in Sn nanoparticles is consistent with Wolfenstine et al.,17,18 who proposed a critical size to avoid particle cracking below the dimensions of the unit cell. The coexistence of two alloy phases with huge lattice misfit leads to strain buildup that cannot be accommodated even at 10 nm particle size, and, possibly, even smaller. In conclusion, 10 nm Sn nanocrystals were prepared in dispersible colloidal form, with narrow size distributions. They were used as a model platform to study the impact of size on the properties of the material as an electrode in Li batteries. Although this highly tailored nanomaterial exhibited better cyclability than commercially available particles, appreciable capacity decay was observed upon cycling in half cells. Ex situ
Figure 4. Selected HR-TEM images of Sn NCs after (a) lithiation to 0 V and (b) delithiation to 2 V (see Figure S5 for electrochemical data). Insets depict the FFT patterns of phases identified in the images. The arrows in (ii) indicate the position of spots in the pattern. Detailed phase identification cannot be done for lithiated Sn phases due to the lack of structural information and similarities in d-spacing among LixSn phases. FFT patterns of delithiated Sn phases showed the formation of α-Sn along the [011] zone axis.
after a full cycle is in agreement with the mechanism of phase transition reported upon electrochemical alloying of Sn with Li.14,37 The corresponding FFT patterns (see insets in Figure 4b) matched α-Sn (cubic, Fd3̅m space group),38 confirming that the lithiation was reversed. Twin boundaries were observed in a few crystals (see Figure 4b for an example). It is noteworthy that the α polymorph was formed upon delithiation when considering that β-Sn was the initial phase and is the stable polymorph at room temperature.38 Similar observations during electrochemical cycling of Sn phases have been reported39 and appear to be correlated with the size of cycled crystals, the cubic phase being favored at smaller sizes. Modifications of the relative stability of different polymorphs of the same compound with crystal size are well-documented in the literature, especially for oxides.40 The possible role of electrochemical potentials and the subsequent electro-mechanical grinding in this particular case cannot be established at this point and should be the focus of follow-up studies. Our ex situ TEM study using a model platform in the form of high-quality Sn NCs shows that this material cannot accommodate the volume expansion introduced during lithiation while retaining morphological integrity. While a small exposure to air during sample transfer was unavoidable (see procedure in Supporting Information), chemical oxidation did not seem to be a major contributor to the observations. This effect would be particularly significant in the lithiated sample. Particles up to three times larger than the pristine state 1803
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While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California.
TEM revealed that extensive mechanical damage during the electrochemical reaction was not avoided even at such small sizes. In the context of the electrode performance, we conclude that cracking subsequently induces decrepitation and electrical isolation of increasingly large regions, as well as exposing fresh surfaces for continued reaction with the electrolyte. This work suggests that approaches based on size reduction may not lead to a practical solution to Sn-based electrodes. Instead, the focus should be placed on dealing with the implications of inevitable particle damage, through the creation of self-healing structures45 and with strategies to passivate the surface of the particles toward reaction with the electrolyte.28,34 It is worthy of notice that this conclusion may not apply to Si, which has been shown to resist cracking during volume expansions at much larger crystal sizes.10 It is possible that the difference between Sn and Si, both Li alloy systems, lies on the atomic mechanism of transformation. Amorphization in the case of Si could be an effective means to relieve the mechanical strain associated with the formation of sharp boundaries between crystalline phases and reduce the interfacial energy penalty that has been predicted to dominate the phase transformations in nanocrystals.46 Our results highlight the importance of assessing sizescaling effects on specific electrode materials and avoiding overgeneralization among materials undergoing large volume expansion upon lithiation. We believe our approach of using monodisperse nanocrystals to study electrochemical phase transformations could be extended to other conditions and systems, thereby uncovering the interplay between thermodynamics and kinetics in a variety of mechanisms. Generally, such knowledge could also be framed in the context of other transformations, such as those induced by temperature47,48 and pressure,26 to expand our knowledge of the physical chemistry of solids.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental section; additional electron microscopy images of pristine Sn NCs and commercial Sn; differential capacity plots of electrochemical data; schematic of the ex situ TEM setup and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
L.X. and C.K. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS L.X., C.K., A.S., and J.C. were supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under the Batteries for Advanced Transportation Technologies (BATT) Program. Portions of this work were carried out at the Molecular Foundry as a user project and DJM was supported by a DOE Early Career Research Program grant, both funded by the DOE Office of Science, Office of Basic Energy Sciences. All funding was provided under Contract No. DE-AC02-05CH11231. This document was prepared as an account of work sponsored by the United States Government. 1804
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