Letter pubs.acs.org/NanoLett
Hollow Iron Oxide Nanoparticles for Application in Lithium Ion Batteries Bonil Koo,*,† Hui Xiong,† Michael D. Slater,‡ Vitali B. Prakapenka,§ Mahalingam Balasubramanian,∥ Paul Podsiadlo,† Christopher S. Johnson,‡ Tijana Rajh,† and Elena V. Shevchenko*,† †
Center for Nanoscale Materials and ‡Chemical Sciences and Engineering, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § Center for Advanced Radiation Sources, University of Chicago, 5640 S. Ellis Avenue, Chicago, Illinois 60637, United States ∥ Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Material design in terms of their morphologies other than solid nanoparticles can lead to more advanced properties. At the example of iron oxide, we explored the electrochemical properties of hollow nanoparticles with an application as a cathode and anode. Such nanoparticles contain very high concentration of cation vacancies that can be efficiently utilized for reversible Li ion intercalation without structural change. Cycling in high voltage range results in high capacity (∼132 mAh/g at 2.5 V), 99.7% Coulombic efficiency, superior rate performance (133 mAh/g at 3000 mA/g) and excellent stability (no fading at fast rate during more than 500 cycles). Cation vacancies in hollow iron oxide nanoparticles are also found to be responsible for the enhanced capacity in the conversion reactions. We monitored in situ structural transformation of hollow iron oxide nanoparticles by synchrotron X-ray absorption and diffraction techniques that provided us clear understanding of the lithium intercalation processes during electrochemical cycling. KEYWORDS: Hollow nanoparticles, cation vacancies, lithium ion battery, in situ study, iron oxide
B
hollow metal oxide NPs is attributed to a coalescence of cation vacancies formed as a result of the difference between metal and oxygen diffusion rates (Dmetal > Doxygen) during the oxidation of metal NPs;13,18 however hollow NPs can still contain uncoalesced vacancies. Nakamura et al. suggested a vacancy diffusion mechanism from the internal pores into the shells of iron oxide NPs at a high temperature (400−500 °C), which can possibly produce a large occupancy of vacancies in the hollow NP shell as compared to the solid NPs.19 This is intriguing since nanoscale materials with defects are of huge importance for the next generation energy storage devices.20 For example, Mn4+ cation vacancies were critical to increase the cell voltage in proton-insertion reactions21 and the protonstabilized cation vacancies in V2O5 increased lithium ion charge capacity.22 Also it was very recently found that Mo-substituted γ-Fe2O3 enhanced lithium ion battery performance by generating additional cation vacancies.5 The lithium ion intercalation into V2O5 with high concentration of cation vacancies have been recently studied by X-ray absorption near edge spectroscopy (XANES) and X-ray photon spectroscopy (XPS) showing significantly slower change of reduction rate of V5+.22−25 These studies motivated us to analyze the hollow iron
atteries are considered as a key method to maximize the efficiency of energy use. Ideal electrodes for battery application should be cheap, nontoxic, have high energy and power densities (meaning high capacity and high rate performance) and last long (meaning 100% Coulombic efficiency and stability of the electrochemical performance versus cycling). Very large capacities (up to 1007 mAh/g), low cost, ease of fabrication and low toxicity of iron oxides make them attractive candidates as electrodes in lithium ion batteries.1−4 The large capacity resulting from reducing iron oxide to iron is promising for lithium battery anodes; however, this conversion during the electrochemical cycling is accompanied by fading of the electrode performance due to material pulverization and subsequent loss of electrical contacts between the active material and current collector. On the other hand, cycling of iron oxide-based electrodes in the high voltage range where only intercalation of Li ions occurs has been recently found to be promising to design cathodes.5−12 High capacities in Li+ uptake/removal were obtained by cycling in the range of 4.5−1.5 V; however, the reported materials demonstrated relatively low capacity (∼40 mAh/g) above 2.5−3 V8,9,12 and low performance at high rate (20 mAh/g5,8,9 or C/56,11,12). The unique nature of hollow metal oxide nanoparticles (NPs)13 such as a thin shell, large internal void, and doubled surface area14 has raised a lot of interest for their applications in drug delivery15 and energy storage.16,17 The formation of © 2012 American Chemical Society
Received: February 1, 2012 Revised: March 15, 2012 Published: April 2, 2012 2429
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oxide NPs for lithium ion storage. In addition to the potentially high concentration of cation vacancies, hollow morphology offers faster diffusion for Li ions uptake/removal as compared to the solid NPs of the same mass1−4 and can better tolerate the volume changes associated with the phase transitions that occur during electrochemical cycling. Even though different forms of iron oxide NPs have been extensively studied for lithium ion batteries within past few years,17,26−31 to the best of our knowledge this is the first detailed study on hollow γ-Fe2O3 NPs. Here, we report on the role of the high concentration of cation vacancies in hollow iron oxide NPs in lithium intercalation reactions. In situ synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) studies were performed to understand the structural and electronic changes of iron oxide NPs during the lithium uptake and removal. At the example of model system such as iron oxide nanoparticles, we analyzed how the structural particularities associated with the mechanism of their formation affect the performance of material as an electrode in Li ion batteries. This report explains significantly higher capacities observed in the case of hollow nanoparticles as compared with their bulk analogues and emphasize the importance of morphology of NPs for targeted applications. The NPs were sealed between two layers of multiwall carbon nanotubes (MWCNTs) without any binder and additive and annealed at 200 °C (Supporting Information Figure S1−S3). HRTEM images (Figure 1a,b) show that as-synthesized 7.2 nm/2.6 nm Fe/Fe3O4 core/shell nanoparticles were transformed into 15.1 nm hollow nanoparticles with ∼3.9 nm shell thickness during the annealing process. XRD patterns (Figure 1c) revealed that while the XRD peaks of as-synthesized core/ shell NPs are in agreement with Fe3O4 peaks, the annealed hollow NPs showed a difference in (422), (511), and (440) peak positions assuming the transformation of Fe3O4 oxide shell into γ-Fe2O3 during the annealing process. In order to provide deeper insights into this transformation, XANES analysis was performed on the annealed hollow particles. Figure 1d indicates that indeed γ-Fe2O3 phase was formed during the annealing. We believe that oxidation of Fe/Fe3O4 core/shells into hollow γ-Fe2O3 shells occurred at the expense of oxygen or water molecules absorbed at the surface of NPs and/or captured inside the CNTs. Since the γ-Fe2O3 (defect spinel) phase contains cation (Fe) vacancies unlike Fe3O4 (spinel), the transformation of Fe3O4 into γ-Fe2O3 can be considered as an evolution of vacancies in the iron oxide system.18,19,32 The hollow γ-Fe 2O3 NPs have a more polycrystalline/amorphous nature as compared with Fe/ Fe3O4 core/shell NPs, as it is evidenced by significantly broader XRD reflections (Figure 1c). Surprisingly, hollow nanoparticles oxidized in solution were less polycrystalline/ amorphous and had lower concentration of vacancies (Supporting Information Figure S4). Annealing of solid NPs also results in their oxidation as it is confirmed by the shift of the XRD reflections (Figure 1g). XANES data also indicates the higher oxidation state of iron in annealed samples of solid NPs as compared with Fe3O4. TEM analysis shows slight (from 13.5 to 14.0 nm) increase in size of solid NPs before and after annealing (Figure 1e,f). Even though both hollow and solid Fe3O4 NPs were transformed into γ-Fe2O3, careful analysis of XRD and EXAFS data revealed a significant difference in vacancy occupancy between annealed hollow and solid NPs. Figure 2a
Figure 1. TEM images of (a,e) as-synthesized and (b,f) annealed for 12 h at 200 °C of hollow and solid iron oxide NPs, respectively. XRD patterns and XANES spectra obtained on (c,d) hollow and (g,h) solid NPs before and after annealing, respectively. Bulk γ-Fe2O3 and Fe3O4 were used as references for XRD and XANES.
shows that the (440) XRD peak intensity of the hollow γ-Fe2O3 NPs (I440/I311 = 0.67) was significantly (∼30%) higher than that of the solid γ-Fe2O3 NPs (I440/I311 = 0.52). Intensities of (440) and (400) reflections in XRD patterns characterize the contribution of the tetrahedral and octahedral sublattices, respectively, and inversely their occupancy and position of cation vacancies.33,34 Simulated XRD pattern of γ-Fe2O3 with higher concentrations of Fe vacancies (Fe3+1.68□1.32O4) in tetrahedral and octahedral positions as compared to standard γFe2O3 (Fe3+2.67□0.33O4) (the squares represent vacancies) showed XRD peak enhancement at (440) position without peak position change (see Supporting Information for details). As a result, in the case of hollow NPs we can conclude higher concentration of cation vacancies in octahedral sites. (Supporting Information Table S4, Figure S5a). EXAFS study confirms this observation. Thus, the Fourier transforms (FT) of k3weighted Fe EXAFS data on both samples reveal that the 2430
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Figure 2. (a) XRD patterns of the hollow and solid γ-Fe2O3 NPs normalized by (311) peak intensities. (b) EXAFS data on the hollow and solid γ-Fe2O3 NPs (bulk γ-Fe2O3 was used as a reference). The Fourier transforms of k3-weighted Fe EXAFS data were plotted in (b).
hollow nanoparticles have significantly low signals at Fe−Fe distance (2−3.5 Å) while the solid nanoparticles have similar intensity comparing to the bulk γ-Fe2O3 (Figure 2b). This observation supports a larger amount of Fe vacant sites in the hollow NPs. The observed extra vacancies in the hollow NPs can be attributed not only to an abundant diffusion source (Fe core) but also to protonated oxygen sites around the cation vacancies, in which protons (H+) compensate the net charge balance similar to the case of MnO221 or V2O522 systems with cation vacancies. We assume that water molecules adsorbed during the oxidation of iron NPs with nondried air and their further handling under ambient conditions of NPs can provide protons to compensate the net charge. The presence of LiOH in discharged hollow NP samples after full conversion reaction can presumably support the presence of protons in the iron oxide structure (Supporting Information Figure S6). Different forms of iron oxide17,26−31 including solid NPs6,9,11,35−37 have been extensively studied for lithium ion storage. Typical conversion reaction of iron oxide generally occurs at a low voltage (with a long voltage plateau at ∼1 V) while intercalation occurs at a relatively high voltage particularly in the case of γ-Fe2O35,38−42 or nanosized other iron oxide.6−12 It has been shown that lithium intercalation occurred through the octahedral iron vacancies.9,38−42 Thus large amount of octahedral cation vacancies in the hollow NPs can be very advantageous for the lithium ion storage. In order to investigate the vacancy effect on lithium ion storage, we conducted in situ XRD and XANES/EXAFS measurements on electrode made from hollow γ-Fe2O3 NPs during the first discharge up to 3.5 Li+ insertion (see Supporting Information for 1 Li+ calculation). In these studies eight data points were selected within the range of 0 to 3.5 insertion of Li+. Figure 3a shows a plot of voltage versus lithium ion insertion at the current density of 150 mA/g. Analysis of the XRD and XANES data allowed us to identify three characteristic stages during Li+ insertion (Figure 3b,c). At
Figure 3. (a) Voltage curve at the first discharge state (up to ∼3.5 Li+ insertion) versus metallic Li counter electrode. (b) XRD data (stars correspond to simulated patterns of lithiated rock salt iron oxide, LixFe1‑x‑y□ yO1). (c) XANES spectra and (d) EXAFS data measured at eight points depicted in the voltage profile shown in (a). The Fourier transforms of k3-weighted Fe EXAFS data were plotted in (d). (e) The Fe oxidation state obtained from linear combinations of XANES plots in (c).
early stages of the discharge (data points 1 to 3 corresponding to 0 to ∼1 Li+ insertion), there is no significant change in the overall positions and intensities and only (440) peaks showed a slight shift to larger d-spacing with intensities decreased. Taking into account that the (440) peak corresponds to the plane containing Fe octahedral sites, this observation suggests that lithium ions are intercalated into octahedral Fe vacancies, leading to slightly enlarged lattice spacing at (440) without affecting overall inverse spinel (oxygen) frames at (311).9,39−42 After 1 Li+ intercalation, a sudden increase in (400) intensity was observed (point 4) while maintaining the overall structure. This phenomenon is attributed to Fe cation transition from tetrahedral to octahedral sites during lithium ion intercalation into inverse spinel iron oxide.9,39,41,42 Further discharge (data points from 5 to 8 corresponding to 1.7 to ∼3.5 Li+ insertion) demonstrates a change of overall crystal structure. The 2431
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intensities of (400) XRD peaks are increased as compared to (311) and (440) peaks and their position are shifted to smaller 2θ. This indicates that further discharging results in the change of the crystal structure from spinel γ-Fe2O3 to rock salt LiFe2O3 that is in agreement with the previously reported data for microsized γ-Fe2O3.39 EXAFS data obtained in the in situ experiments (Figure 3d) indicate that the first Fe−Fe peak at 2−3 Å (indicating Fe octahedral sites) significantly increases during the third stage that confirms the formation of a rock salt phase. Furthermore, increased intensities at (400) positions suggests an octahedral vacancy filling similar to the early stage Li+ intercalation. Detailed atomic configurations and XRD simulations are shown in Supporting Information. The oxidation state change of iron during lithium ion intercalation was monitored in situ by XANES technique (Figure 3c). The plot of oxidation states calculated from linear combinations of XANES spectra (Figure 3e) demonstrates a significantly slower rate of reduction of iron cations as compared to the theoretical one. Moreover, the intercalation up to ∼1 Li+ results in (phase I) show almost no change in the oxidation state of iron. Considering that the theoretical line relies on the assumption that the lithium ion insertion accompanies with reduction of iron, this implies that lithium ions can be inserted, surprisingly, without reducing of iron cations assuming that a large amount of Fe vacancies in the hollow NPs can serve as hosts for lithium ion without change of iron oxidation state. In hollow γ-Fe2O3 NPs, 44% of total iron sites (∼1.32 Fe atoms) are vacant. Typically no change of oxidation state of metal cations or its slower rate of reduction is attributed to the side reaction, for example, formation of solidelectrolyte-interface (SEI) layer. However, in our case we can exclude this process since intercalation of Li ions in phase I occurs at relatively high voltages where the formation of SEI is very unlikely. Capacitive surface reaction such as double-layer capacitance or pseudocapacitance is also ruled out since we, indeed, observed structural changes by XRD. Note that no change of oxidation state of cations associated with capacity beyond theoretical value during Li+ intercalation was previously observed in nonstoichiometric vanadium oxide22−25,43 and amorphous metal oxides.44−47 It was shown that nonsurface hydroxyl groups can serve as charge-storage sites resulting in extra capacity without oxidation state change of cations, e.g. MnO244 and RuO2 cases.46,47 We assume that the protonated oxygen sites around cation vacancies in the hollow iron oxide NPs can accumulate the charges upon Li+ intercalation without affecting the oxidation state of iron until the filling of the vacancies is complete. Thus, in situ studies revealed that a large amount of lithium ions (∼1.35 Li+ per γ-Fe2O3 up to the middle of phase II) can intercalate into hollow iron oxide NPs without structural change and reduction of iron cations due to the extra Fe vacancies surrounded by protonated oxygen groups. The high concentration of vacancies and no change of the oxidation state during discharge at high potentials allowed us to assume that the hollow iron oxide NPs can be used as lithium ion intercalation hosts at high voltage range. To examine this, we cycled the electrodes in the high voltage range (4.5−1.5 V). Strikingly, using this cycling regime we observed reversible capacity (192 mAh/g) corresponding to 1.1 Li+ at the current density of 30 mA/g with 98.7% Coulombic efficiency and excellent capacity retention (Figure 4a). Maintained hollow morphology (insets in Figure 4a) also validates the reversible Li+ intercalation into vacancies without major structural change
Figure 4. Amount of intercalated/deintercalated Li+ ions as a function of cycle numbers at different rates (a) with different individual electrodes and (b) on one same electrode. In order to examine the effect of vacancy amount on the capacity number, Li+ intercalation (4.5−1.5 V) was repeated after 15 conversion reactions (3.0−0.01 V) in (b). (c) Voltage profiles of the hollow NP electrode tested with different current rates (30, 300, and 3000 mAh/g) cycled in the voltage range of 4.5−1.5 V. (d−f) Voltage profiles showing that extensive cycling enhances the working voltages for each rate test.
unlike conversion reactions of transition metal oxides.48 Highresolution TEM images taken on the samples cycled 20 times in the voltage range of 4.5−1.5 V (Supporting Information Figure S9) at 30 mA/g show larger crystalline domain as compared with highly polycrystalline initial hollow NPs. This observation supports lithium ion intercalation through the iron oxide shell and rules out a surface reaction. Moreover, hollow NPs-based electrode has superior rate performance (133 mAh/g at 3000 mA/g) without capacity fading during 500 cycles (Supporting Information Figure S10b). Surprisingly, cycling at slower rate of the electrodes that previously underwent extensive cycling at higher rate (300 cycles at 300 mA/g) resulted in reaching almost theoretical capacity (221 mAh/g) calculated for the hollow iron oxide NPs with 44% (1.32 Fe atoms) of the vacancies (Figure 4b). Extensive cycling at slow rate not only affected the capacity but also resulted in the change of the 2432
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voltage profile (Figure 4d,e,f). We found that higher concentration of lithium ions can intercalate into hollow NPs at higher voltage. Thus, we observed 104 and 63 mAh/g at 2.5 and 3.0 V (30 mA/g), respectively, which are significantly higher values as compared with previously reported data on iron oxide NPs.5−12 These data confirm the idea of the development of more effective intercalation pathway for Li+ ions during cycling in the high voltage range, probably, through the ordering of vacancies into channels.49 Extensive cycling also resulted in the development of the pronounced plateau in the voltage resulting in higher capacity at high voltage (Figure 4d,e,f). TEM data confirm the improving of the short-range order in the cycled samples (Supporting Information Figure S9). In addition, Figure 4b indicates that subsequent increase of the cycling rate brought the capacity down to the same number as prior to the slow rate study confirming the reproducibility of rate performance and stability of the electrodes. It is worth mentioning that previously, solid nanosized iron oxides have been studied for lithium ion intercalation hosts working in the high voltage region as cathodes by several research groups. Thus the nanosized iron oxide materials were reported to have high values of capacity (1 Li+ for α-Fe2O312 and 1.9 Li+ for Fe3O4 NPs9) without overall structural change (hcp for α-Fe2O3 and spinel for Fe3O4). However, this was attributed mainly to the small domain size of NPs that provides the short diffusion length and the high surface area that can better accommodate strains after intercalation process. On the other hand, in contrast to our results it was observed that Li+ ion intercalation resulted in the increase of the unit cell accompanied by the iron transition from tetrahedral to octahedral sites9,39,41,42 and change of the oxidation state of iron.6,10 Also solid NPs were reported to have poor rate performance (generally working at 20 mA/g5,8,9 or slower than C/56,11,12,38,40) and experienced capacity fading.8,9,38 In contrast to the previous reports, the hollow γ-Fe2O3 NPs with extra Fe vacancies showed neither iron oxidation state change nor iron site transition and demonstrated excellent capacity retention and superior rate performance. Also, in control experiments with solid NPs synthesized under similar conditions as hollow NPs we found that electrodes assembled from solid NPs exhibited significantly lower capacity as compared with hollow ones cycled in the voltage range of 4.5−1.5 V (Figure 4b). As a result we can emphasize the importance of the synthetic route of NPs. In order to investigate the effect of structural defects on the capacity of material in full conversion reaction, we cycled electrodes prepared from hollow and solid iron oxide NPs in the voltage range of 3.0−0.01 V. Theoretically, 44% of total vacant iron sites (1.32 Fe sites) in γ-Fe2O3 are supposed to increase capacity of hollow iron oxide NPs up to 1228 mAh/g meaning that vacancies can provide additional 221 mAh/g to normal γ-Fe2O3 capacity (1007 mAh/g). However, we found that the full conversion reaction results in the dissipation of octahedral vacancies. Thus cycling in this voltage regime results in the lower intensities of (440) peaks in the cycled samples (Supporting Information Figure S11). Also the gentle slopes of the hollow NP electrodes in 3.0−1.5 V voltage range characteristic to the intercalation region still exist but were significantly shrunk (Figure 5c). We attribute this observation to the dissipation of the vacancies as a result of oxidation by Li2O during the charging process followed by coalescence of cation vacancies, the phenomenon that takes place during the oxidation of metal particles by O2.13,18 To investigate the effect
Figure 5. The charge/discharge capacities as a function of cycle numbers during full conversion reaction (3.0−0.01 V) of (a) hollow and (b) solid NPs. (c) Voltage profiles of discharged hollow and solid NPs versus cycle number. The plots were normalized by the curve of the third discharged samples. The hollow NP electrode indicated lowered amount of Fe vacancies while the solid NP electrode showed no intercalation plateau. (d) Amount of Li+ intercalated/deintercalated in 3.5−1.5 V voltage range versus cycle numbers with different initial Fe vacancy concentrations (0, 7, and 15 conversion reactions) of the hollow NP electrodes.
of vacancies on the Li+ intercalation into iron oxide NPs, high voltage tests were conducted on the samples that underwent several conversion reactions in which we observed decreased concentration of Fe vacancies (Figure 5d). Also, we observed the decreased capacity of the electrodes cycled in the cathode regime (4.5−1.5 V) after 15 conversion reactions (Figure 4b). Thus systematic capacity drop observed in the case of samples that previously underwent conversion reactions confirms that the lithium ions are intercalated into Fe vacancies, not by surface reaction or pseudocapacitance. We have found that hollow γ-Fe2O3 NPs are enriched with iron vacancies in octahedral sites that can be utilized for lithium ion storage applications. Our results revealed that the cation vacancies in 15.1 nm hollow NPs with 3.9 nm thick shell can serve as hosts for lithium ions at high voltage range (4.5−1.5 V), allowing utilization of γ-Fe2O3 NPs as cathode material with high capacity (up to 219 mAh/g), excellent Coulombic efficiency (99.7%), good capacity retention, and superior rate performance. In such a system, we observed 104 and 63 mAh/g at 2.5 and 3.0 V (30 mA/g), respectively. No structural change occurs until all vacancies are filled. Also no change in the morphology of NPs has been detected by TEM in samples extensively cycled that can provide the excellent mechanical connectivity of the active material in the electrode. Moreover, even though we observed slow dissipation of iron vacancies during conversion reaction, cation vacancies can still boost a conversion reaction capacity up when hollow γ-Fe2O3 NPs are used as an anode. In our study, we purposely focused on relatively large NPs since in situ X-ray studies on smaller hollow nanoparticles were very complicated because of their extremely polycrystalline/ 2433
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amorphous nature. However, electrochemical studies on smaller hollow iron oxide NPs being cycled in high voltage range demonstrated even better performance with the same high stability as larger hollow NPs. Voltage profile of small hollow NPs cycled in high voltage ranges is characterized by pronounced plateau at ∼3.0 V. Thus we found that 132 and 84 mAh/g at 2.5 and 3.0 V, respectively, can be achieved in the case of 5.7 nm hollow iron oxide NPs with less than 2 nm thick shell (Supporting Information Figure S12) that can make them promising candidates for cathode applications. Our results emphasize the importance of the new morphologies for the targeted applications. Experimental results on differently sized hollow NPs and dissipation of vacancies in conversion reactions indicate a strong correlation between concentration of vacancies and intercalation capacities in hollow iron oxide NPs. The higher concentration of iron vacancies in hollow iron oxide NPs as compared with solid NPs results in their high intercalation and conversion capacities. We believe that further optimization of size and composition of hollow NPs can be used to improve their electrochemical performance.
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ASSOCIATED CONTENT
S Supporting Information *
Synthetic routes for hollow and solid iron oxide NPs, electrode fabrication and electrochemical test with lithium ion, X-ray characterization methods, weight analysis of fabricated electrode membranes, extra vacancy analysis, simulations of various iron oxide phases with different vacancy occupancies, XRD of hollow γ-Fe2O3 NP electrode after conversion reactions, 1 Li+ capacity calculation for hollow NPs in a carbon nanotube electrode, high-resolution TEM images of hollow γFe2O3 NPs, rate performance of hollow γ-Fe2O3 NP electrode, and small hollow NP electrode test are included. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +1-630-252-7633. Fax: +1-630-252-5739 E-mail: (B.K.)
[email protected]; (E.V.S.)
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge helpful discussions with Dr. M. M. Thackeray and Dr. D. Kim. This work was supported by the U.S. Department of Energy, U.S. DOE-BES, under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The X-ray diffraction work was performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR-0622171) and Department of Energy - Geosciences (DE-FG02-94ER14466). The XANES and EXAFS studies were performed at sector 20 and research at this facility was supported by the U.S. DOE, NSERC (Canada). Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 2434
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