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Mössbauer effect studies of sputter-deposited tin–cobalt and tin–cobalt–carbon alloys
Journal of Alloys and Compounds 443 (2007) 114–120
M¨ossbauer effect studies of sputter-deposited tin–cobalt and tin–cobalt–carbon alloys A.D.W. Todd a , R.A. Dunlap a,b , J.R. Dahn a,b,∗ a
Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5, Canada b Institute for Research in Materials, Dalhousie University, Halifax, NS, Canada Received 1 June 2007; accepted 6 June 2007 Available online 13 June 2007
Abstract Sn–Co–C alloys are now used as negative electrodes in the latest generation of Li-ion batteries, although not much is known about the phases present in the materials, their micro or nanostructure nor their physical properties. Here, sputtered combinatorial libraries of Sn1−x Cox (0 < x < 0.6) and [Sn0.63 Co0.37 ]1−y Cy (0.1 < y < 0.5) were studied by 119 Sn M¨ossbauer effect spectroscopy and X-ray diffraction to learn their phase content and micro or nanostructure. The sputtered Sn1−x Cox films showed the presence of crystalline -Sn, a new metastable cubic Sn–Co phase, an amorphous Sn1−x Cox (0.26 < x < 0.45) phase and Co3 Sn2 as x, or the Co content, increased. The sputtered [Sn0.63 Co0.37 ]1−y Cy (0.1 < y < 0.5) materials consist of amorphous grains of Sn0.63 Co0.37 separated by a carbon matrix. The impact of this nanostructure on the utility of these materials as negative electrodes in Li-ion batteries is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: M¨ossbauer spectroscopy; Li-ion batteries; Sn–Co; Sn–Co–C
1. Introduction Recent studies have shown that amorphous or nanocrystalline alloy materials have great potential as negative electrodes for Liion batteries [1,2]. M¨ossbauer effect spectroscopy has become an important tool in the study of the structural properties of some of these materials in conjunction with X-ray diffraction (XRD) and electrochemistry [3–6]. Materials that appear to be amorphous by XRD may have small-scale nanocrystalline structure that is not evident using XRD. M¨ossbauer effect spectroscopy has the advantage of being able to probe the structure of these materials on the atomic scale. A cell containing a negative electrode composed of a SnCoC alloy is now commercially available from the Sony Corporation. The supplier claims that the cell is composed of small grains of SnCo in a C matrix [7]. Compositions of this material close to Sn0.34 Co0.29 C0.37 have the important property that Li can be reversibly inserted and removed many times, making this an
∗
Corresponding author at: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5, Canada. E-mail address: [email protected] (J.R. Dahn). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.030
attractive material for negative electrodes [8]. The application of M¨ossbauer effect spectroscopy to these materials would provide detailed information about the microstructure. This method has already been applied to some related Sn-containing materials of interest as electrode materials. For example, the insertion of lithium into SnO and SnO·B2 O3 ·P2 O5 amorphous glasses was studied in situ using this technique by Courtney et al. [9]. M¨ossbauer effect spectra were recorded at various stages of lithiation to determine the oxidation states and the phases that occur during this process. The insertion of Li into SnS and tin composite oxides (TCO) has been studied by Robert et al. [10]. Alcantara et al. [11] have used M¨ossbauer effect spectroscopy to study Co2 SnO4 at different stages of lithiation. Ja´en et al. have studied the structural properties of electrodeposited SnCo, Co3 Sn2 and CoSn2 alloys using M¨ossbauer effect spectroscopy and X-ray diffraction measurements [12,13]. In situ XRD and M¨ossbauer effect measurements have been made on electrodes of CoSn2 at various stages of lithiation [14]. In the present work, we have utilized Sn M¨ossbuer effect spectroscopy in conjunction with X-ray diffraction to investigate sputter-deposited libraries of Sn1−x Cox and [Sn0.63 Co0.37 ]1−y Cy that have not been lithiated. Details of the electrochemistry of libraries similar to these have been studied and have been
A.D.W. Todd et al. / Journal of Alloys and Compounds 443 (2007) 114–120
reported previously [8,15]. Such sputtered materials will not have an identical microstructure as the Sony material, which is presumably manufactured by other methods. Nevertheless, the study of Sn–Co–C materials close to the composition suggested by Sony [7] can provide significant information concerning the chemistry of this phase system and the Li insertion mechanism. 2. Experimental methods Libraries of thin-film Sn1−x Cox (Library 1) and [Sn0.63 Co0.37 ]1−y Cy (Library 2) were produced using a Corona Vacuum Systems V3-T sputter deposition system modified for combinatorial materials science. The deposition chamber is evacuated by a turbo pump backed by a roughing pump and a cryogenic pump is used to remove water vapour. Typical base pressure for deposition is ∼3 × 10−7 Torr. Deposition takes place under Ar gas at a pressure of ∼2.0 mTorr. Details of the sputtering system are available in Ref. [16]. The 50 mm diameter by 6 mm thick Co (99.95% pure) sputtering target was obtained from Williams Advanced Materials [17]. A Sn sputtering target of ∼50 mm diameter by ∼3 mm thick (99.85% pure) was cut from a Sn plate obtained from Alfa Aesar [18]. A target of nominal composition Sn0.6 Co0.4 was prepared by heating a stoichiometric mixture of Co powder (99.9% pure, Sigma–Aldrich) and Sn powder (99.95% pure, Alfa Aesar) to ∼1200 ◦ C in an induction furnace under vacuum. The resultant liquid was poured into a watercooled copper mould in vacuum and allowed to cool. The resulting disc was then machined into a target 50 mm in diameter and 6 mm thick. A carbon-sputtering target (∼50 mm diameter by ∼6 mm thick, 99.999% pure) was obtained from Kurt J. Lesker Co. [19]. All targets were mounted on ∼3-mm-thick copper backing plates using SilverTech PT-1 silver epoxy from Williams Advanced Materials. Libraries were prepared by simultaneously depositing the materials to be studied onto 15–18 pieces of 25 m thick kapton film (each 25 mm × 100 mm) for the M¨ossbauer measurements and two 13 mm × 75 mm Si(1 0 0) wafers,
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one for structural characterization via XRD and the other for compositional characterization via electron microprobe. After deposition, the kapton pieces were carefully aligned, then stacked on top of each other to provide a thickness of deposited material adequate for M¨ossbauer effect spectroscopy. Library 1 was produced by depositing Sn through a “constant” mask and Co through a “linear out” mask yielding the compositions Snl−x Cox (0.02 < x < 0.57) [16]. Library 2 was produced using a “constant” mask on the nominally Sn0.6 Co0.4 target and a “linear out” mask on the C target. This produced the pseudo-binary library [Sn0.63 Co0.37 ]1−y Cy (0.12 < y < 0.50). Compositions of the films were determined as a function of position using a JEOL 8200 electron microprobe. A third constant-composition sample of Sn0.83 Co0.17 was sputtered using Sn and Co targets without masks. This sample was sputtered on a large area of 25 m thick polystyrene film and also onto a Si(1 0 0) wafer piece. The material sputtered on polystyrene was recovered as powder by subsequently dissolving the polystyrene in toluene and rinsing several times. The recovered powder was studied by powder X-ray diffraction using a Siemens D-5000 diffractometer equipped with a Cu-target X-ray tube and a diffracted beam monochromator. X-ray diffraction patterns for Libraries 1 and 2 were obtained using an Inel CPS-120 curved, position sensitive detector coupled to a Cu target X-ray generator equipped with a monochromator limiting the incident X-rays to Cu K␣. A programmable x–y translation stage enabled patterns to be taken every 2 mm along each library. The Si(1 0 0) wafer acts as a zero-background holder for the thin film as the ∼6◦ incident angle of the beam does not satisfy the Bragg condition for the wafer. Room temperature 119 Sn M¨ossbauer effect spectra were collected using a Wissel System II spectrometer operating in the constant acceleration mode. A Ca119m SnO3 source was used and the velocity scale was calibrated relative to CaSnO3 . Data were accumulated using an Ortec multichannel scaler with ACE-MCS software. A 4.5 mm × 25 mm lead aperture was used to select the portion of the film to be measured. Sixteen spectra were acquired by moving the sample in front of the aperture in 4.5 mm steps with a computer-controlled translation stage. A 4.5 mm aperture results in sampling a ∼3% range of compositions at each step. The average composition at each step is quoted. Acquisition times were 12 h per spectrum for Library 1 and 24 h per spectrum for Library
Fig. 1. Room temperature 119 Sn M¨ossbauer effect spectra and X-ray diffraction patterns of Sn1−x Cox (Library 1) as a function of Co content. The velocity scale is measured relative to CaSnO3 . The numbers below the M¨ossbauer spectra are the atomic percentage of Co. Solid lines through the data represent the total fits as described in the text and the dashed lines are the spectral components. The lines to the right of the X-ray diffraction patterns are the ranges for the observed phases: (dot-dashed line) Co3 Sn2 ; (short dashes) unknown phase; (long dashes) Sn. The region with no line is amorphous.
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2. Details of the combinatorial M¨ossbauer apparatus can be found in Refs. [20,21].
3. Results and discussion 3.1. SnCo thin films Room temperature 119 Sn M¨ossbauer effect spectra and Xray diffraction patterns of Sn1−x Cox (Library 1) as a function of Co content are illustrated in Fig. 1. The X-ray diffraction patterns indicate the phases present as a function of Co content as follows: up to 17 at.% Co, crystalline -Sn is present as evidenced by strong Bragg peaks at 2θ = 30.6◦ , 32.0◦ , 43.9◦ and 44.9◦ as expected for tetragonal white tin. For 17–22 at.% Co, an unidentified phase (to be discussed further below) shows a strong diffraction peak at 2θ = 29.24◦ in addition to peaks from crystalline -Sn. For 26–45 at.% Co, the diffraction patterns show broad peaks characteristic of an amorphous phase. For 48–57 at.% Co, Co3 Sn2 is the most abundant phase present. There is the possibility of a small amount of an amorphous Sncontaining phase or more likely, substitutionally placed atoms. As the composition moves away from stoichiometic Co3 Sn2 , it is likely that the excess Sn atoms are introduced substitutionally into Co sites in the Co3 Sn2 structure. The mixing of Sn and Co atoms on the lattice sites would create a greater distribution of Sn environments thereby further smearing out the quadrupole split doublet due to the stoichiometric Co3 Sn2 . There is evidence of this effect in the M¨ossbauer spectra of Fig. 2. The x = 57 spectrum (the closest composition to stoichiometric Co3 Sn2 in the library) is found to have the sharpest peaks for the spectra that contain Co3 Sn2 . Room temperature 119 Sn M¨ossbauer effect spectra as illustrated in Fig. 1 have been analyzed on the basis of models that are most compatible with the phases observed in X-ray patterns. The -Sn phase was fit to a Lorentzian singlet. The unidentified phase was fit to a Lorentzian pattern with the
Fig. 2. Measured room temperature 119 Sn M¨ossbauer center shifts, δ, of Sn1−x Cox (Library 1) as a function of Co content: (solid squares) -Sn phase; (open squares) unknown phase; (triangles) amorphous phase; (solid circles) Co3 Sn2 . The solid line is a linear interpolation from the center shift value for pure -Sn and literature value for dilute Sn in Co [21]. The velocity scale is referenced to CaSnO3 .
possibility of a quadrupole splitting. The amorphous phase was fit to a Voigt-based function [22]. Here a single Gaussian distribution of quadrupole splittings was used with a linear correlation between the center shift, δ, and the quadrupole splitting, Δ, of the form δ = δ0 + αΔ
(1)
In Eq. (1) δ0 and α were fitted parameters. The Co3 Sn2 phase was fit to a Lorentzian doublet [13]. The fitted curves are shown by the solid lines in Fig. 1. The spectra for 12, 17 and 22 at.% Co were fit to patterns for both -Sn and the unidentified phase and these sub-spectra are shown by the broken lines in the figure. For 12 at.% Co the Xray diffraction pattern did not show any clear evidence of the unidentified phase but the inclusion of a second spectral component with parameters appropriate for this phase improved the fit and suggested the presence of a small quantity of this phase. The inclusion of a non-zero quadrupole splitting for the subspectrum of the unidentified phase did not improve the fit and suggests that this component is suitably described as a singlet. The measured room temperature 119 Sn M¨ossbauer center shift, δ, of Sn1−x Cox as a function of Co content is illustrated in Fig. 2. For the amorphous phase, the mean center shift is shown. A linear interpolation between the literature values for the center shift of pure -Sn (+2.56 mm/s) and dilute Sn in Co (+1.76 mm/s) [23] is shown by the solid line in the figure. Room temperature quadrupole splittings, Δ, as a function of Co content are shown in Fig. 3 for compositions that were fit to Lorentzian or Voigt-based doublets. The values of the quadrupole splittings found for 48–57 at.% Co are consistent with value of ∼1.3 mm/s [12,13]. The center shift of the -Sn phase as seen in Fig. 2, is consistent with the value for pure -Sn. The unidentified phase shows a center shift of approximately +1.7 mm/s and an average of the Sn and unidentified phase center shifts weighted by the relative
Fig. 3. Measured room temperature 119 Sn M¨ossbauer quadrupole splittings, Δ, of Sn1−x Cox (Library 1) as a function of Co content; (triangles) amorphous phase and (solid circles) Co3 Sn2 .
A.D.W. Todd et al. / Journal of Alloys and Compounds 443 (2007) 114–120
areas of the sub-spectra is consistent with the interpolated line in the figure. The center shifts for the amorphous phase are also consistent with the interpolated line. The values of the center shift for the Co3 Sn2 phase are consistent with literature value of +1.79 mm/s [12,13]. The properties of the unidentified phase may be considered from the results of the M¨ossbauer studies. The 22 at.% Co sample shows the clearest contribution from the unidentified phase. The value of the -Sn center shift for this composition indicates that the Co resides entirely in the unidentified phase. The intensities of the two components of this spectrum are 74% Sn and 26% unidentified phase. If it is assumed that the recoil-free fractions of -Sn in the two phases are equal, this gives a composition for the unidentified phase of Sn48 Co52 . The actual Sn content may be lower if the Debye temperature of the unidentified phase is higher than that for pure -Sn. The lack of detectable quadrupole splitting in this component of the spectrum suggests a high degree of symmetry of the Sn sites. Fig. 4a shows the powder XRD pattern of the unidentified phase as measured on the constant composition sample. The powder pattern shows a co-existence of an amorphous component and crystalline component having a set of Bragg peaks that were indexed based on a cubic unit cell with a lattice constant ˚ Table 1 shows that the observed and calculated Bragg of 3.06 A. peak positions agree well based on this unit cell. Fig. 4b shows the XRD pattern (measured on the thin film library) of the location in Library 1 having approximately the same composition as the constant composition sample displayed in Fig. 4a. The crystalline Bragg peaks in the two samples agree well. Thus, we believe the features in the Mossbauer spectra of the unidentified
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Fig. 4. XRD spectra of the unknown phase. Top panel: powder recovered from polystyrene showing the six peaks due to a cubic Sn–Co phase. The Miller indices are indicated next to each peak. Bottom panel: XRD pattern of a similar composition from Library 1. Peaks due to the unknown phase are indicated by vertical dashed lines and peaks due to crystalline Sn are indicated by vertical solid lines and the Miller indices of the Sn peaks are indicated. Both samples have average compositions of approximately 84 at.% Sn and 16 at.% Co.
phase correspond to those of a cubic phase with a lattice con˚ There is only one Sn–Co phase, Co3 Sn, reported stant a = 3.06 A. ˚ close to that observed having a cubic lattice constant (2.94 A) [25,26]. This Co3 Sn phase is metastable and has been prepared by splat cooling [25] and by rapid quenching [26]. It is possible that a similar metastable phase exists at the Sn-rich end of the
Fig. 5. Room temperature 119 Sn M¨ossbauer effect spectra and X-ray diffraction patterns of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content. The velocity scale is measured relative to CaSnO3 . The numbers below the M¨ossbauer spectra are the atomic percentage of C. Solid lines through the data represent the total fits as described in the text and the dashed lines are the spectral components corresponding to the amorphous phase and a small amount of Sn oxide.
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Table 1 ˚ The differences are also indicated Observed and calculated Bragg peak positions for the unidentified phase, based on a cubic lattice constant of a = 3.06 A. Index
2θ (Observed)
2θ (Corrected) [24]
2θ (Calculated)
Δ(2θ)
d (Calculated)
100 110 111 200 210 211
29.27 41.75 51.82 60.63 68.58 76.2
29.169 41.652 51.726 60.54 68.494 76.118
29.162 41.172 51.702 60.462 68.516 76.144
0.007 −0.06 0.024 0.077 −0.022 −0.026
3.062 2.165 1.768 1.531 1.369 1.25
composition range and can be accessed by the rapid quenching rate afforded by sputter deposition [27]. Such a phase would be expected to have a larger lattice constant, as observed, because Sn is larger than Co. Further studies are needed to completely characterize this new metastable phase. 3.2. SnCoC thin films Room temperature 119 Sn M¨ossbauer effect spectra and Xray diffraction patterns of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content are illustrated in Fig. 5. The X-ray patterns indicate that all compositions in this library are amorphous or nanostructured. The M¨ossbauer effect spectra are all broadened doublets as was the case for the amorphous compositions from Library 1. This suggests that the addition of carbon does not strongly affect the local environment of the Sn atoms. Thus it is expected that a detailed analysis of the spectra will be consistent with a nanostructured material consisting of small grains of amorphous Sn–Co separated by a “matrix” of carbon. This is not unexpected based on the C–Co, C–Sn and Co–Sn phase diagrams [28] that show C–Co and C–Sn are immiscible systems, while there are numerous Co–Sn intermetallic phases. The spectra in Fig. 5 were fit to a Gaussian distribution of Lorentzian doublets as described for the spectra of the amorphous Sn1−x Cox phase shown in Fig. 1. Spectral fits were improved by the inclusion of a weak singlet near zero velocity, corresponding to the presence of a small amount of Sn oxide. Room temperature M¨ossbauer center shifts, ␦, of the amorphous phase of [Sn0.63 Co0.37 ]1−y Cy as a function of C content are shown in Fig. 6 and the corresponding quadrupole splittings are shown in Fig. 7. These are mean values for the fitted distributions. These compositions are an extension of the composition Sn0.63 Co0.37 from Library 1 and the results from the analysis of this spectrum are shown in Figs. 6 and 7 for comparison. The analysis of the amorphous spectra in Library 1 (in particular, the spectrum for the composition Sn0.63 Co0.37 ) as well as those from Library 2 have been appropriately fit with a Voigtbased function. The analysis indicates a negative value for the coefficient α in Eq. (1). The data in Fig. 6 suggest a Sn center shift that becomes slightly more positive with increasing C content in [Sn0.63 Co0.37 ]1−y Cy . Fig. 7 indicates a weakly decreasing quadrupole splitting with increasing C content and is consistent with the trends suggested by the sign of α. The relatively small change in Sn center shift as a function of C content suggests that there is no direct interaction between the Sn atoms and the C
Fig. 6. Measured room temperature 119 Sn M¨ossbauer center shifts, δ, of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content (solid circles). The solid square is the value for Sn0.63 Co0.37 from Library 1. The solid line is a linear least squares fit to the data. The velocity scale is referenced to CaSnO3 .
atoms. A model that would appropriately describe the observed behavior is based on amorphous Sn–Co grains in a carbon matrix with Co–C interactions at the interface. The preference for carbon to interact with Co at the interface has been proposed in
Fig. 7. Measured room temperature 119 Sn M¨ossbauer quadrupole splittings, Δ, of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content (solid circles). The solid square is the value for Sn0.63 Co0.37 from Library 1. The solid line is a linear least squares fit to the data.
A.D.W. Todd et al. / Journal of Alloys and Compounds 443 (2007) 114–120
Fig. 8. Schematic illustration of Sn–Co grains in a ‘carbon sea’. C shown in grey and the Sn–Co grains in cross-hatch.
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milling cannot reproduce the fast quenching rates that are associated with the sputtering process used here. Therefore, the amorphous phases produced here may not be attainable by alternative methods, which may lead to the formation of more a crystalline-grained structure. While these M¨ossbauer effect studies have been crucial for the understanding of the role of Sn and C in the phase formation in these films, the details of the Co–C interactions cannot be directly probed. Further studies of these libraries by other techniques, such as XPS, are planned and may help to clarify these issues. Acknowledgements
literature [7] for similar materials. With increasing C content, the SnCo grains decrease in size. The increasing surface area of the SnCo grains results in a greater degree of Co–C interaction. Thus there is an increased tendency for Co to reside near the surface of the amorphous grains and a corresponding increase in Sn near the grain center. This would result in a more positive Sn center shift as a consequence of increases in Sn–Sn contacts and decreased Sn–Co contacts within the grain. This would also increase the symmetry of the Sn environments yielding a smaller quadrupole splitting. 4. Conclusions The present studies have illustrated the details of the phase formation in sputtered Sn–Co thin films by the application of X-ray diffraction and 119 Sn M¨ossbauer spectroscopy. These results have been helpful for the interpretation of our studies of Sn–Co–C films. The present M¨ossbauer effect studies show that sputtered [Sn0.63 Co0.37 ]1−y Cy materials can be best thought of as a nanocomposite of grains of amorphous Sn0.63 Co0.37 separated by a carbon matrix. As the carbon content increases, the separation between the grains presumably increases. These results help understand the variation in the stability of differential capacity versus potential as a function of charge–discharge cycle number for sputtered [Sn0.60 C0.40 ]1−y Cy materials (e.g., see Fig. 7 in Ref. [8]). As the carbon content increases, the separation between nanoscale grains of amorphous Sn0.6 Co0.4 presumably increases, thus making it more and more difficult for tin aggregation to occur during charge–discharge cycling. Compositions with y ∼ = 0.4 were found to exhibit stable differential capacity versus potential curves for over 25 cycles, while compositions with y ∼ = 0.2 did not [8]. The latter samples showed evidence for aggregation of Sn into larger clusters, leading to poor charge–discharge cycle life, presumably because the initial Sn0.6 Co0.4 grains were not separated widely enough. It is our opinion that materials with overall compositions of Sn0.36 Co0.24 C0.40 prepared by alternative methods directed to form nanocomposites, such as high-energy ball milling, will also consist of Sn–Co regions separated by a carbon matrix as schematically illustrated in Fig. 8. This is consistent with the report from Inoue [7]. However, alternative synthesis methods, such as high-energy ball
The authors acknowledge funding from NSERC and 3M Canada Co. under the auspices of the Industrial Research Chairs program. References [1] Y. Idota, M. Mishima, Y. Miyaki, T. Kubota, T. Miyasaka, US Patent no. 5,618,640 (1997). [2] R.L. Turner, D.J. McClure, L.J. Krause, M.M. Buckett, J.R. Dahn, O. Mao, US Patent no. 6,203,944 (2001). [3] M.D. Fleischauer, M.N. Obrovac, J.D. McGraw, R.A. Dunlap, J.M. Topple, J.R. Dahn, J. Electrochem. Soc. 153 (2006) A484. [4] R.A. Dunlap, O. Mao, J.R. Dahn, Phys. Rev. B 59 (1999) 3494. [5] R.A. Dunlap, D.A. Small, D.D. MacNeil, M.N. Obrovac, J.R. Dahn, J. Alloys Compd. 289 (1999) 135. [6] J.R. Dahn, R.L. Turner, O. Mao, R.A. Dunlap, A.E. George, M.I. Buckett, D.W. McClure, L.J. Krause, Thin Solid Films 408 (2002) 111. [7] H. Inoue, International Meeting on Lithium Batteries, Biarritz, France, 2006. [8] J.R. Dahn, R.E. Mar, A. Abouzeid, J. Electrochem. Soc. 153 (2006) A361. [9] I.A. Courtney, R.A. Dunlap, J.R. Dahn, Electrochem. Acta 45 (2000) 51. [10] F. Robert, P.-E. Lippens, J. Olivier-Fourcade, J.-C. Jumas, M. Morcette, J. Power Sources 146 (2005) 492. [11] R. Alcantara, G.F. Ortiz, P. Lavela, J.L. Tirado, Electrochem. Commun. 8 (2006) 731. [12] J. Ja´en, M.L. Vars´anyi, E. Kov´acs, I. Czak´o-Nagy, A. Buz´as, A. V´ertes, L. Kiss, Electrochim. Acta 29 (1984) 1119. [13] A. V´ertes, J. Ja´en, M.L. Vars´anyi, J. Radioanal. Nucl. Chem. Art. 90 (1985) 383. [14] C.M. Ionica-Bousquet, P.E. Lippens, L. Aldon, J. Olivier-Fourcade, J.C. Jumas, Chem. Mater. 18 (2006) 6442. [15] A.D.W. Todd, R.E. Mar, J.R. Dahn, J. Electrochem. Soc. 153 (2006) A1998. [16] J.R. Dahn, S. Trussler, T.D. Hatchard, A. Bonakdarpour, J.N. MeullerNeuhaus, K.C. Hewitt, M. Fleischauer, Chem. Mater. 14 (2002) 3519. [17] Williams Advanced Materials, 2978 Main Street, Buffalo, NY 14214, Phone (716) 837 1000 (http://www.williams-adv.com). [18] Alfa Aesar, 26 Parkridge Road, Ward Hill, MA 01835, Phone (978) 521 6300 (http://www.alfa.com). [19] Kurt J. Lesker Co., P.O. Box 10, 1925 Route 51, Clairton, PA 15025-2700, Phone 412 387 9200 (http://www.lesker.com). [20] R.A. Dunlap, N.C. Deschamps, R.E. Mar, S.P. Farrell, J. Phys. Condens. Matter 18 (2006) 4907. [21] J.D. McGraw, M.D. Fleischauer, J.R. Dahn, R.A. Dunlap, Philos. Mag. 86 (2006) 5017. [22] R.A. Dunlap, D.W. Lawther, P. Hargraves, P. Sinclair, J. Phys. F: Metal Phys. 18 (1988) 2479. [23] P.A. Flinn, in: G.K. Shenoy, F.E. Wagner (Eds.), M¨ossbauer Isomer Shifts, North-Holland Publishing Company, Amsterdam, 1978, pp. 609–610, and references therein.
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M¨ossbauer effect studies of sputter-deposited tin–cobalt and tin–cobalt–carbon alloys A.D.W. Todd a , R.A. Dunlap a,b , J.R. Dahn a,b,∗ a
Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5, Canada b Institute for Research in Materials, Dalhousie University, Halifax, NS, Canada Received 1 June 2007; accepted 6 June 2007 Available online 13 June 2007
Abstract Sn–Co–C alloys are now used as negative electrodes in the latest generation of Li-ion batteries, although not much is known about the phases present in the materials, their micro or nanostructure nor their physical properties. Here, sputtered combinatorial libraries of Sn1−x Cox (0 < x < 0.6) and [Sn0.63 Co0.37 ]1−y Cy (0.1 < y < 0.5) were studied by 119 Sn M¨ossbauer effect spectroscopy and X-ray diffraction to learn their phase content and micro or nanostructure. The sputtered Sn1−x Cox films showed the presence of crystalline -Sn, a new metastable cubic Sn–Co phase, an amorphous Sn1−x Cox (0.26 < x < 0.45) phase and Co3 Sn2 as x, or the Co content, increased. The sputtered [Sn0.63 Co0.37 ]1−y Cy (0.1 < y < 0.5) materials consist of amorphous grains of Sn0.63 Co0.37 separated by a carbon matrix. The impact of this nanostructure on the utility of these materials as negative electrodes in Li-ion batteries is discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: M¨ossbauer spectroscopy; Li-ion batteries; Sn–Co; Sn–Co–C
1. Introduction Recent studies have shown that amorphous or nanocrystalline alloy materials have great potential as negative electrodes for Liion batteries [1,2]. M¨ossbauer effect spectroscopy has become an important tool in the study of the structural properties of some of these materials in conjunction with X-ray diffraction (XRD) and electrochemistry [3–6]. Materials that appear to be amorphous by XRD may have small-scale nanocrystalline structure that is not evident using XRD. M¨ossbauer effect spectroscopy has the advantage of being able to probe the structure of these materials on the atomic scale. A cell containing a negative electrode composed of a SnCoC alloy is now commercially available from the Sony Corporation. The supplier claims that the cell is composed of small grains of SnCo in a C matrix [7]. Compositions of this material close to Sn0.34 Co0.29 C0.37 have the important property that Li can be reversibly inserted and removed many times, making this an
∗
Corresponding author at: Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS B3H 3J5, Canada. E-mail address: [email protected] (J.R. Dahn). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.030
attractive material for negative electrodes [8]. The application of M¨ossbauer effect spectroscopy to these materials would provide detailed information about the microstructure. This method has already been applied to some related Sn-containing materials of interest as electrode materials. For example, the insertion of lithium into SnO and SnO·B2 O3 ·P2 O5 amorphous glasses was studied in situ using this technique by Courtney et al. [9]. M¨ossbauer effect spectra were recorded at various stages of lithiation to determine the oxidation states and the phases that occur during this process. The insertion of Li into SnS and tin composite oxides (TCO) has been studied by Robert et al. [10]. Alcantara et al. [11] have used M¨ossbauer effect spectroscopy to study Co2 SnO4 at different stages of lithiation. Ja´en et al. have studied the structural properties of electrodeposited SnCo, Co3 Sn2 and CoSn2 alloys using M¨ossbauer effect spectroscopy and X-ray diffraction measurements [12,13]. In situ XRD and M¨ossbauer effect measurements have been made on electrodes of CoSn2 at various stages of lithiation [14]. In the present work, we have utilized Sn M¨ossbuer effect spectroscopy in conjunction with X-ray diffraction to investigate sputter-deposited libraries of Sn1−x Cox and [Sn0.63 Co0.37 ]1−y Cy that have not been lithiated. Details of the electrochemistry of libraries similar to these have been studied and have been
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reported previously [8,15]. Such sputtered materials will not have an identical microstructure as the Sony material, which is presumably manufactured by other methods. Nevertheless, the study of Sn–Co–C materials close to the composition suggested by Sony [7] can provide significant information concerning the chemistry of this phase system and the Li insertion mechanism. 2. Experimental methods Libraries of thin-film Sn1−x Cox (Library 1) and [Sn0.63 Co0.37 ]1−y Cy (Library 2) were produced using a Corona Vacuum Systems V3-T sputter deposition system modified for combinatorial materials science. The deposition chamber is evacuated by a turbo pump backed by a roughing pump and a cryogenic pump is used to remove water vapour. Typical base pressure for deposition is ∼3 × 10−7 Torr. Deposition takes place under Ar gas at a pressure of ∼2.0 mTorr. Details of the sputtering system are available in Ref. [16]. The 50 mm diameter by 6 mm thick Co (99.95% pure) sputtering target was obtained from Williams Advanced Materials [17]. A Sn sputtering target of ∼50 mm diameter by ∼3 mm thick (99.85% pure) was cut from a Sn plate obtained from Alfa Aesar [18]. A target of nominal composition Sn0.6 Co0.4 was prepared by heating a stoichiometric mixture of Co powder (99.9% pure, Sigma–Aldrich) and Sn powder (99.95% pure, Alfa Aesar) to ∼1200 ◦ C in an induction furnace under vacuum. The resultant liquid was poured into a watercooled copper mould in vacuum and allowed to cool. The resulting disc was then machined into a target 50 mm in diameter and 6 mm thick. A carbon-sputtering target (∼50 mm diameter by ∼6 mm thick, 99.999% pure) was obtained from Kurt J. Lesker Co. [19]. All targets were mounted on ∼3-mm-thick copper backing plates using SilverTech PT-1 silver epoxy from Williams Advanced Materials. Libraries were prepared by simultaneously depositing the materials to be studied onto 15–18 pieces of 25 m thick kapton film (each 25 mm × 100 mm) for the M¨ossbauer measurements and two 13 mm × 75 mm Si(1 0 0) wafers,
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one for structural characterization via XRD and the other for compositional characterization via electron microprobe. After deposition, the kapton pieces were carefully aligned, then stacked on top of each other to provide a thickness of deposited material adequate for M¨ossbauer effect spectroscopy. Library 1 was produced by depositing Sn through a “constant” mask and Co through a “linear out” mask yielding the compositions Snl−x Cox (0.02 < x < 0.57) [16]. Library 2 was produced using a “constant” mask on the nominally Sn0.6 Co0.4 target and a “linear out” mask on the C target. This produced the pseudo-binary library [Sn0.63 Co0.37 ]1−y Cy (0.12 < y < 0.50). Compositions of the films were determined as a function of position using a JEOL 8200 electron microprobe. A third constant-composition sample of Sn0.83 Co0.17 was sputtered using Sn and Co targets without masks. This sample was sputtered on a large area of 25 m thick polystyrene film and also onto a Si(1 0 0) wafer piece. The material sputtered on polystyrene was recovered as powder by subsequently dissolving the polystyrene in toluene and rinsing several times. The recovered powder was studied by powder X-ray diffraction using a Siemens D-5000 diffractometer equipped with a Cu-target X-ray tube and a diffracted beam monochromator. X-ray diffraction patterns for Libraries 1 and 2 were obtained using an Inel CPS-120 curved, position sensitive detector coupled to a Cu target X-ray generator equipped with a monochromator limiting the incident X-rays to Cu K␣. A programmable x–y translation stage enabled patterns to be taken every 2 mm along each library. The Si(1 0 0) wafer acts as a zero-background holder for the thin film as the ∼6◦ incident angle of the beam does not satisfy the Bragg condition for the wafer. Room temperature 119 Sn M¨ossbauer effect spectra were collected using a Wissel System II spectrometer operating in the constant acceleration mode. A Ca119m SnO3 source was used and the velocity scale was calibrated relative to CaSnO3 . Data were accumulated using an Ortec multichannel scaler with ACE-MCS software. A 4.5 mm × 25 mm lead aperture was used to select the portion of the film to be measured. Sixteen spectra were acquired by moving the sample in front of the aperture in 4.5 mm steps with a computer-controlled translation stage. A 4.5 mm aperture results in sampling a ∼3% range of compositions at each step. The average composition at each step is quoted. Acquisition times were 12 h per spectrum for Library 1 and 24 h per spectrum for Library
Fig. 1. Room temperature 119 Sn M¨ossbauer effect spectra and X-ray diffraction patterns of Sn1−x Cox (Library 1) as a function of Co content. The velocity scale is measured relative to CaSnO3 . The numbers below the M¨ossbauer spectra are the atomic percentage of Co. Solid lines through the data represent the total fits as described in the text and the dashed lines are the spectral components. The lines to the right of the X-ray diffraction patterns are the ranges for the observed phases: (dot-dashed line) Co3 Sn2 ; (short dashes) unknown phase; (long dashes) Sn. The region with no line is amorphous.
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2. Details of the combinatorial M¨ossbauer apparatus can be found in Refs. [20,21].
3. Results and discussion 3.1. SnCo thin films Room temperature 119 Sn M¨ossbauer effect spectra and Xray diffraction patterns of Sn1−x Cox (Library 1) as a function of Co content are illustrated in Fig. 1. The X-ray diffraction patterns indicate the phases present as a function of Co content as follows: up to 17 at.% Co, crystalline -Sn is present as evidenced by strong Bragg peaks at 2θ = 30.6◦ , 32.0◦ , 43.9◦ and 44.9◦ as expected for tetragonal white tin. For 17–22 at.% Co, an unidentified phase (to be discussed further below) shows a strong diffraction peak at 2θ = 29.24◦ in addition to peaks from crystalline -Sn. For 26–45 at.% Co, the diffraction patterns show broad peaks characteristic of an amorphous phase. For 48–57 at.% Co, Co3 Sn2 is the most abundant phase present. There is the possibility of a small amount of an amorphous Sncontaining phase or more likely, substitutionally placed atoms. As the composition moves away from stoichiometic Co3 Sn2 , it is likely that the excess Sn atoms are introduced substitutionally into Co sites in the Co3 Sn2 structure. The mixing of Sn and Co atoms on the lattice sites would create a greater distribution of Sn environments thereby further smearing out the quadrupole split doublet due to the stoichiometric Co3 Sn2 . There is evidence of this effect in the M¨ossbauer spectra of Fig. 2. The x = 57 spectrum (the closest composition to stoichiometric Co3 Sn2 in the library) is found to have the sharpest peaks for the spectra that contain Co3 Sn2 . Room temperature 119 Sn M¨ossbauer effect spectra as illustrated in Fig. 1 have been analyzed on the basis of models that are most compatible with the phases observed in X-ray patterns. The -Sn phase was fit to a Lorentzian singlet. The unidentified phase was fit to a Lorentzian pattern with the
Fig. 2. Measured room temperature 119 Sn M¨ossbauer center shifts, δ, of Sn1−x Cox (Library 1) as a function of Co content: (solid squares) -Sn phase; (open squares) unknown phase; (triangles) amorphous phase; (solid circles) Co3 Sn2 . The solid line is a linear interpolation from the center shift value for pure -Sn and literature value for dilute Sn in Co [21]. The velocity scale is referenced to CaSnO3 .
possibility of a quadrupole splitting. The amorphous phase was fit to a Voigt-based function [22]. Here a single Gaussian distribution of quadrupole splittings was used with a linear correlation between the center shift, δ, and the quadrupole splitting, Δ, of the form δ = δ0 + αΔ
(1)
In Eq. (1) δ0 and α were fitted parameters. The Co3 Sn2 phase was fit to a Lorentzian doublet [13]. The fitted curves are shown by the solid lines in Fig. 1. The spectra for 12, 17 and 22 at.% Co were fit to patterns for both -Sn and the unidentified phase and these sub-spectra are shown by the broken lines in the figure. For 12 at.% Co the Xray diffraction pattern did not show any clear evidence of the unidentified phase but the inclusion of a second spectral component with parameters appropriate for this phase improved the fit and suggested the presence of a small quantity of this phase. The inclusion of a non-zero quadrupole splitting for the subspectrum of the unidentified phase did not improve the fit and suggests that this component is suitably described as a singlet. The measured room temperature 119 Sn M¨ossbauer center shift, δ, of Sn1−x Cox as a function of Co content is illustrated in Fig. 2. For the amorphous phase, the mean center shift is shown. A linear interpolation between the literature values for the center shift of pure -Sn (+2.56 mm/s) and dilute Sn in Co (+1.76 mm/s) [23] is shown by the solid line in the figure. Room temperature quadrupole splittings, Δ, as a function of Co content are shown in Fig. 3 for compositions that were fit to Lorentzian or Voigt-based doublets. The values of the quadrupole splittings found for 48–57 at.% Co are consistent with value of ∼1.3 mm/s [12,13]. The center shift of the -Sn phase as seen in Fig. 2, is consistent with the value for pure -Sn. The unidentified phase shows a center shift of approximately +1.7 mm/s and an average of the Sn and unidentified phase center shifts weighted by the relative
Fig. 3. Measured room temperature 119 Sn M¨ossbauer quadrupole splittings, Δ, of Sn1−x Cox (Library 1) as a function of Co content; (triangles) amorphous phase and (solid circles) Co3 Sn2 .
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areas of the sub-spectra is consistent with the interpolated line in the figure. The center shifts for the amorphous phase are also consistent with the interpolated line. The values of the center shift for the Co3 Sn2 phase are consistent with literature value of +1.79 mm/s [12,13]. The properties of the unidentified phase may be considered from the results of the M¨ossbauer studies. The 22 at.% Co sample shows the clearest contribution from the unidentified phase. The value of the -Sn center shift for this composition indicates that the Co resides entirely in the unidentified phase. The intensities of the two components of this spectrum are 74% Sn and 26% unidentified phase. If it is assumed that the recoil-free fractions of -Sn in the two phases are equal, this gives a composition for the unidentified phase of Sn48 Co52 . The actual Sn content may be lower if the Debye temperature of the unidentified phase is higher than that for pure -Sn. The lack of detectable quadrupole splitting in this component of the spectrum suggests a high degree of symmetry of the Sn sites. Fig. 4a shows the powder XRD pattern of the unidentified phase as measured on the constant composition sample. The powder pattern shows a co-existence of an amorphous component and crystalline component having a set of Bragg peaks that were indexed based on a cubic unit cell with a lattice constant ˚ Table 1 shows that the observed and calculated Bragg of 3.06 A. peak positions agree well based on this unit cell. Fig. 4b shows the XRD pattern (measured on the thin film library) of the location in Library 1 having approximately the same composition as the constant composition sample displayed in Fig. 4a. The crystalline Bragg peaks in the two samples agree well. Thus, we believe the features in the Mossbauer spectra of the unidentified
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Fig. 4. XRD spectra of the unknown phase. Top panel: powder recovered from polystyrene showing the six peaks due to a cubic Sn–Co phase. The Miller indices are indicated next to each peak. Bottom panel: XRD pattern of a similar composition from Library 1. Peaks due to the unknown phase are indicated by vertical dashed lines and peaks due to crystalline Sn are indicated by vertical solid lines and the Miller indices of the Sn peaks are indicated. Both samples have average compositions of approximately 84 at.% Sn and 16 at.% Co.
phase correspond to those of a cubic phase with a lattice con˚ There is only one Sn–Co phase, Co3 Sn, reported stant a = 3.06 A. ˚ close to that observed having a cubic lattice constant (2.94 A) [25,26]. This Co3 Sn phase is metastable and has been prepared by splat cooling [25] and by rapid quenching [26]. It is possible that a similar metastable phase exists at the Sn-rich end of the
Fig. 5. Room temperature 119 Sn M¨ossbauer effect spectra and X-ray diffraction patterns of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content. The velocity scale is measured relative to CaSnO3 . The numbers below the M¨ossbauer spectra are the atomic percentage of C. Solid lines through the data represent the total fits as described in the text and the dashed lines are the spectral components corresponding to the amorphous phase and a small amount of Sn oxide.
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Table 1 ˚ The differences are also indicated Observed and calculated Bragg peak positions for the unidentified phase, based on a cubic lattice constant of a = 3.06 A. Index
2θ (Observed)
2θ (Corrected) [24]
2θ (Calculated)
Δ(2θ)
d (Calculated)
100 110 111 200 210 211
29.27 41.75 51.82 60.63 68.58 76.2
29.169 41.652 51.726 60.54 68.494 76.118
29.162 41.172 51.702 60.462 68.516 76.144
0.007 −0.06 0.024 0.077 −0.022 −0.026
3.062 2.165 1.768 1.531 1.369 1.25
composition range and can be accessed by the rapid quenching rate afforded by sputter deposition [27]. Such a phase would be expected to have a larger lattice constant, as observed, because Sn is larger than Co. Further studies are needed to completely characterize this new metastable phase. 3.2. SnCoC thin films Room temperature 119 Sn M¨ossbauer effect spectra and Xray diffraction patterns of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content are illustrated in Fig. 5. The X-ray patterns indicate that all compositions in this library are amorphous or nanostructured. The M¨ossbauer effect spectra are all broadened doublets as was the case for the amorphous compositions from Library 1. This suggests that the addition of carbon does not strongly affect the local environment of the Sn atoms. Thus it is expected that a detailed analysis of the spectra will be consistent with a nanostructured material consisting of small grains of amorphous Sn–Co separated by a “matrix” of carbon. This is not unexpected based on the C–Co, C–Sn and Co–Sn phase diagrams [28] that show C–Co and C–Sn are immiscible systems, while there are numerous Co–Sn intermetallic phases. The spectra in Fig. 5 were fit to a Gaussian distribution of Lorentzian doublets as described for the spectra of the amorphous Sn1−x Cox phase shown in Fig. 1. Spectral fits were improved by the inclusion of a weak singlet near zero velocity, corresponding to the presence of a small amount of Sn oxide. Room temperature M¨ossbauer center shifts, ␦, of the amorphous phase of [Sn0.63 Co0.37 ]1−y Cy as a function of C content are shown in Fig. 6 and the corresponding quadrupole splittings are shown in Fig. 7. These are mean values for the fitted distributions. These compositions are an extension of the composition Sn0.63 Co0.37 from Library 1 and the results from the analysis of this spectrum are shown in Figs. 6 and 7 for comparison. The analysis of the amorphous spectra in Library 1 (in particular, the spectrum for the composition Sn0.63 Co0.37 ) as well as those from Library 2 have been appropriately fit with a Voigtbased function. The analysis indicates a negative value for the coefficient α in Eq. (1). The data in Fig. 6 suggest a Sn center shift that becomes slightly more positive with increasing C content in [Sn0.63 Co0.37 ]1−y Cy . Fig. 7 indicates a weakly decreasing quadrupole splitting with increasing C content and is consistent with the trends suggested by the sign of α. The relatively small change in Sn center shift as a function of C content suggests that there is no direct interaction between the Sn atoms and the C
Fig. 6. Measured room temperature 119 Sn M¨ossbauer center shifts, δ, of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content (solid circles). The solid square is the value for Sn0.63 Co0.37 from Library 1. The solid line is a linear least squares fit to the data. The velocity scale is referenced to CaSnO3 .
atoms. A model that would appropriately describe the observed behavior is based on amorphous Sn–Co grains in a carbon matrix with Co–C interactions at the interface. The preference for carbon to interact with Co at the interface has been proposed in
Fig. 7. Measured room temperature 119 Sn M¨ossbauer quadrupole splittings, Δ, of [Sn0.63 Co0.37 ]1−y Cy (Library 2) as a function of C content (solid circles). The solid square is the value for Sn0.63 Co0.37 from Library 1. The solid line is a linear least squares fit to the data.
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Fig. 8. Schematic illustration of Sn–Co grains in a ‘carbon sea’. C shown in grey and the Sn–Co grains in cross-hatch.
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milling cannot reproduce the fast quenching rates that are associated with the sputtering process used here. Therefore, the amorphous phases produced here may not be attainable by alternative methods, which may lead to the formation of more a crystalline-grained structure. While these M¨ossbauer effect studies have been crucial for the understanding of the role of Sn and C in the phase formation in these films, the details of the Co–C interactions cannot be directly probed. Further studies of these libraries by other techniques, such as XPS, are planned and may help to clarify these issues. Acknowledgements
literature [7] for similar materials. With increasing C content, the SnCo grains decrease in size. The increasing surface area of the SnCo grains results in a greater degree of Co–C interaction. Thus there is an increased tendency for Co to reside near the surface of the amorphous grains and a corresponding increase in Sn near the grain center. This would result in a more positive Sn center shift as a consequence of increases in Sn–Sn contacts and decreased Sn–Co contacts within the grain. This would also increase the symmetry of the Sn environments yielding a smaller quadrupole splitting. 4. Conclusions The present studies have illustrated the details of the phase formation in sputtered Sn–Co thin films by the application of X-ray diffraction and 119 Sn M¨ossbauer spectroscopy. These results have been helpful for the interpretation of our studies of Sn–Co–C films. The present M¨ossbauer effect studies show that sputtered [Sn0.63 Co0.37 ]1−y Cy materials can be best thought of as a nanocomposite of grains of amorphous Sn0.63 Co0.37 separated by a carbon matrix. As the carbon content increases, the separation between the grains presumably increases. These results help understand the variation in the stability of differential capacity versus potential as a function of charge–discharge cycle number for sputtered [Sn0.60 C0.40 ]1−y Cy materials (e.g., see Fig. 7 in Ref. [8]). As the carbon content increases, the separation between nanoscale grains of amorphous Sn0.6 Co0.4 presumably increases, thus making it more and more difficult for tin aggregation to occur during charge–discharge cycling. Compositions with y ∼ = 0.4 were found to exhibit stable differential capacity versus potential curves for over 25 cycles, while compositions with y ∼ = 0.2 did not [8]. The latter samples showed evidence for aggregation of Sn into larger clusters, leading to poor charge–discharge cycle life, presumably because the initial Sn0.6 Co0.4 grains were not separated widely enough. It is our opinion that materials with overall compositions of Sn0.36 Co0.24 C0.40 prepared by alternative methods directed to form nanocomposites, such as high-energy ball milling, will also consist of Sn–Co regions separated by a carbon matrix as schematically illustrated in Fig. 8. This is consistent with the report from Inoue [7]. However, alternative synthesis methods, such as high-energy ball
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