X-ray Diffraction, 7Li MAS NMR Spectroscopy, and 119Sn Mössbauer

Loïc Baggetto , Hien-Yoong Hah , Jean-Claude Jumas , Charles E. Johnson , Jacqueline A. Johnson , Jong K. Keum , Craig A. Bridges , Gabriel M. Veith...
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X-ray Diffraction, 7Li MAS NMR Spectroscopy, and 119Sn Mo1 ssbauer Spectroscopy Study of SnSb-Based Electrode Materials Francisco J. Ferna´ndez-Madrigal, Pedro Lavela, Carlos Pe´rez Vicente, and Jose´ L. Tirado* Laboratorio de Quı´mica Inorga´ nica, Edificio C3, Planta 1, Campus de Rabanales, Universidad de Co´ rdoba, 14071 Co´ rdoba, Spain

Jean Claude Jumas and Josette Olivier-Fourcade Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, UPR ESA 5072, Universite´ Montpellier II, Place Euge` ne Bataillon, 34095 Montpellier, France Received November 19, 2001. Revised Manuscript Received April 29, 2002 119 Sn Mo¨ssbauer spectroscopy is used in combination with 7Li MAS NMR spectroscopy and X-ray diffraction to resolve the details of the different Li-Sn and Li-Sb intermetallics formed during the electrochemical reactions of nominal “SnSb” compounds inside a lithium battery. At a Li/SnSb ratio of 2.02, the Mo¨ssbauer spectrum indicates that the initial structure of the compounds are not deeply affected during the first steps of discharging. At 4.95 Li/ SnSb, each spectrum consists of three components. The hyperfine parameters agree well with those of β-Sn and a LiSn-related solid. At 7.27 Li/SnSb, the spectra reveal that no metallic Sn is present. During cell charging, the appearance of a poorly crystalline Li13Sn5 phase is detected at 8.9 Li/SnSb. NMR data show signals resulting from ionic Li of the passivating layer and from Li-Sn and Li-Sb intermetallics at downfield and upfield shifts, respectively, which depart from those observed in bulk solids. The partial recovery of the stistaite structure at the end of charging is confirmed in SnSb single-phase electrodes, whereas the opposite is true for mixtures of tin and antimony.

Introduction A renewed interest in the use of intermetallic compounds as anodes in lithium-ion cells has grown since the announcement of the high performance of tin composite oxides (TCOs) by employees of Fuji Film Co.1 In fact, the reactions of lithium with tin and other metals were well-known in the past to give high capacities during the discharging of lithium anode cells. However, this effect was counteracted by enhanced changes in volume, leading to a marked electrode deterioration during successive charge-discharge cycles. This, in turn, has negative effects on cell performance. To avoid the use of metals in the bulk state, new materials have been proposed in which the active element (most commonly tin or antimony) is dispersed in a matrix of electrochemically inert components generated in situ. Some examples can be cited, such as the lithium oxide matrix generated by the reduction of binary2 and composite3 tin oxides or the transition metal matrix generated from skutterudite CoSb34 or marcasite CrSb25 type compounds, where antimony is the only * Corresponding author. Tel.: +34 957 218637. E-mail: iq1ticoj@ uco.es. (1) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Science 1997, 276, 1395. (2) Chouvin, J.; Branci, C.; Sarradin, J.; Olivier-Fourcade, J.; Jumas, J. C.; Simon, B.; Biensan, Ph. J. Power Sources 1999, 81-82, 277. (3) Courtney, I. A.; Dahn, J. R. J. Electrochem. Soc. 1997, 144, 2045.

active element. The inactive elements impede the aggregation of the active atoms into larger particles, which has proven to be responsible for the above-mentioned mechanical stresses. Nevertheless, the inclusion of inert atoms implies a reduction of the theoretical mass capacity. On the other hand, Yang et al.6 showed in 1996 that intermetallic matrixes such as SnAgx and SnSbx lead to much better behavior than pure, compact, or polycrystalline tin or antimony metallic host structures, allowing for an expansion of the more active material embedded in still unreacted and ductile material. In recent years, a renewed attention has been paid by several research groups7-13 to the intermetallic SnSbx system, which provides a new family of electrochemi(4) Alca´ntara, R.; Ferna´ndez-Madrigal, F. J.; Lavela, P.; Tirado, J. L.; Jumas, J. C.; Olivier-Fourcade, J. J. Mater. Chem. 1999, 9, 2517. (5) Ferna´ndez-Madrigal, F. J.; Lavela, P.; Pe´rez-Vicente, C.; Tirado, J. L. J. Electroanal. Chem. 2001, 501, 205. (6) Yang, J.; Winter, M.; Besenhard, J. O. Solid State Ionics 1996, 90, 281. (7) Besenhard, J. O.; Wachtler, M.; Winter, M.; Andreaus, R.; Rom, I.; Sitte, W. J. Power Sources 1999, 81-82, 268. (8) Winter, M.; Besenhard, J. O. Electrochim. Acta 1999, 45, 31. (9) Yang, J.; Wachtler, M.; Winter, M.; Besenhard, J. O. Electrochem. Solid State Lett. 1999, 2, 161. (10) Yang, J.; Takeda, Y.; Imanishi, N.; Xie, J. Y.; Yamamoto, O. Solid State Ionics 2000, 133, 189. (11) Yang, J.; Takeda, Y.; Imanishi, N.; Yamamoto, O. J. Electrochem. Soc. 2000, 147, 1671. (12) Yang, J.; Takeda, Y.; Li, Q.; Imanishi, N.; Yamamoto, O. J. Power Sources 2000, 90, 64.

10.1021/cm0112800 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/13/2002

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cally active materials in which both constituent elements react with lithium, thus contributing to cell capacity. Moreover, a homogeneous distribution of tin and antimony atoms in the SnSbx electrode materials provides good mechanical behavior of the products and avoids electrode “pulverization” on prolonged cycling. Here, we study a compound with a nominal composition of SnSb, in which both elements are uniformly distributed in a slightly distorted rock-salt-type structure. By using the powerful 119Sn Mo¨ssbauer and 7Li magic-angle spinning (MAS) NMR spectroscopies in combination with electrochemical techniques and X-ray diffraction measurements, the details of the different lithium-tin compounds formed in the electrochemical process are resolved. Experimental Section The polycrystalline compound with a nominal composition of SnSb was obtained by direct synthesis. The powdered metallic elements Sn and Sb were used as supplied by Strem. The mixtures were weighed in a 1:1 atomic ratio, ground to ensure good homogeneity, introduced as pellets in vacuumsealed silica tubes, and heated at 5 °C/min to 750 °C. The tubes were annealed at this temperature for two successive 1-week periods between which the tubes were cooled and opened and their contents were ground. Two-electrode Swagelok cells of the type Li|LiPF6 (mixed organic solvents)|SnSb were used for the electrochemical measurements. The electrodes were prepared as 7-mmdiameter pellets by pressing a mixture of 60% active compound, 10% PTFE polymer, and 30% graphite (Merck) to improve the mechanical and electronic conduction properties. Lithium electrodes consisted of a clean 7-mm-diameter lithium metal disk. The commercial electrolyte solution [Merck, 1 M LiPF6 in a 1:1 w/w mixture of ethylene carbonate (EC) and diethylene carbonate (DEC)] was supported by porous glasspaper disks from Whatman. Discharges were carried out under galvanostatic conditions by using a multichannel MacPile II system. Galvanostatic cycling was performed at a rate of C/4 [C ) 1 (mol of Li) (mol of SnSb)-1 h-1]. During sample preparation for ex situ characterization, the first dischargecharge cycle was carried out at C/8. X-ray powder diffraction (XRD) patterns were recorded on a Siemens D5000 instrument, using Cu KR radiation and a graphite monochromator. The samples were always prepared inside a glovebox, by carefully opening the cells, placing the products on a glass sample holder, and finally covering them with a plastic film to avoid exposure to air. 119Sn Mo ¨ ssbauer (MB) spectra of pristine and lithiated samples were recorded using a conventional EG&G constantacceleration spectrometer. In the case of 119Sn, the γ source was 119mSn in a matrix of BaSnO3, and spectra were collected at room temperature. The velocity scale was calibrated from the magnetic sextet of a high-purity R-Fe foil absorber using a 57Co(Rh) source. The origin of the isomer shift range was determined from the center of the peak of BaSnO3. Samples were prepared as thin films, avoiding any contact with moisture in the atmosphere by using a glovebox and a specially designed sealed measurement cell. 7Li MAS NMR spectra were recorded at room temperature on a Bruker ACP-400 spectrometer working at a resonance frequency of 155.52 MHz and at a spinning rate of ca. 5.5 kHz. The lithium reference was a 1 M LiCl aqueous solution.

Results and Discussion The phase purity of the polycrystalline “SnSb” sample was checked by X-ray diffraction. Figure 1 shows the (13) Li, H.; Zhu, G. Y.; Huang, X. J.; Chen, L. Q. J. Mater. Chem. 2000, 10, 693.

Figure 1. Powder X-ray diffraction patterns of pristine SnSb and the product of cell discharging to 0 V and recharging to 1.30 V. The asterisks (*) indicate graphite additive reflections.

pattern of the pristine material. A comparison of the diffraction data and the unit cell parameters with those included in the JCPDS file14 showed good agreement with a rhombohedral SnSb phase with the mineral name stistaite. However, recent studies15,16 have indicated that only two phases, Sn4Sb3 and Sn3Sb4, exist in the binary SnSb system. Because of the close unit cell parameters the two phases are poorly resolved. Moreover, a lowintensity peak at 34° (2θ) can be ascribed to the presence of minor impurities of unreacted antimony. The amount of antimony impurity was semiquantitatively estimated to be 7%. The first electrochemical discharging and charging of the cell made of the pristine material versus a lithium metal anode was monitored by recording cell voltage versus lithium concentration, as shown in the plot in Figure 2. An extended discharge with a maximum capacity value of 10 F mol-1 was observed. In a previous work, Besenhard et al.7 proposed the existence of a ternary phase LiδSnSb (0 < δ < 0.1) in the early stages of the electrochemical reaction resulting from lithium insertion into this solid. According to the same authors, (14) JCPDS File 33-0118. (15) Albering, J. H.; Besenhard, J. O. In Vortragstagung der Fachgruppe Festko¨ rperchemie und Materialforschung; Mu¨nster, Germany, 2000; p A4. (16) Obendorff, P. J. T. L.; Kodentsov, A. A.; Vuorinen, V.; Kivilahti, J. K.; van Loo, F. J. J. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1321.

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Figure 2. Voltage vs composition and first derivative profiles for the first cycle of a Li/LiPF6 (EC:DEC)/SnSb cell recorded under galvanostatic conditions at C/4. The filled spots show compositions for which XRD and/or MB data were obtained. The theoretical compositions calculated from Li22Sn5 at 0 V are shown as vertical bars.

the occurrence of this phase is followed by tin extrusion, as revealed by the coexistence of LiδSnSb with LiδSn and Li3-xSnySb (x, y , 1). Irrespective of the formation of these ternary phases, the extended discharging of SnSb electrodes is expected to ultimately destroy the distorted rock-salt structure of SnSb, yielding the maximum allowable lithium-metal reaction extension for tin and antimony, according to the following reaction8

SnSb + 7.4Li f Li3Sb + Li4.4Sn

(1)

From these reactions, a maximum capacity of 7.4 F mol-1 could be achieved. The 2.6 F mol-1 of charge excess can thus be ascribed, in a first approach, to the occurrence of side reactions. In previous works,4,5 we found FTIR evidence that supports an electrolyte decomposition process at the surface of CoSb3 and CrSb2 electrodes at a cell potential coincident with that of the Li-Sb reaction at ca. 0.8 V. The nature of this porous film has been discussed by several authors, who indicate the presence of species such as Li2CO3 or ROCO2Li.17-19 These solid products cover the metallic particles without stopping the ionic exchange between the electrode and the electrolyte. Henceforth, the lithium consumption associated with the creation of this passivating film around the particles and its contribution to the total capacity during the first discharge cannot be discarded. Further discharging of the cell originates two main reversible plateaus above 0 V. A closer inspection of the first -dV/dx derivative curve also shown in Figure 2 makes evident the presence of less intense electrochemical effects that are visible as resolved peaks. In these plots, each peak is located at the end of each voltage step, and thus, each peak can be associated with the formation of a different single-phase product. The occurrence of a triplet at ca. 5 F mol-1 and a doublet at ca. 8 F mol-1 during cell discharging is comparable to two similar signals at ca. 6 and ca. 9 F mol-1 observed during charging. The charging ends at ca. 2.5 F mol-1, (17) Aurbach, D.; Ein-Eli, Y. J. Electrochem Soc. 1995, 142, 1746. (18) Aurbach, D.; Ein-Eli, Y.; Chusid, O.; Carmeli, Y.; Babai, M.; Yamin, H. J. Electrochem. Soc. 1995, 142, 1746. (19) Li, H.; Huang, X. J.; Chen, L. Q. J. Power Sources 1999, 82, 340.

coinciding with a sharp increase of the cell potential. These results are in good agreement with the high reversibility of the lithium reaction below 0.8 V and the presence of irreversible side reactions at the beginning of the electrochemical process. The theoretical compositions at each peak, also included in Figure 2, were calculated using the assumption that the composition reached at 0 V corresponded to the products in eq 1 and that the formation-decomposition of lithium-tin compounds occurred before the formation of Sb/Li3Sb.8 The experimental evidence to support these data and the nature of the reversible reactions with lithium are discussed below with the aid of the X-ray diffraction and Mo¨ssbauer spectroscopy results. Ex situ XRD patterns of electrochemically lithiated samples are shown in Figure 1. The whole range of the first cycle was checked, and only weak and highly broadened lines were visible in some samples. For comparative purposes, the positions where the lines of crystalline phases in the Li-Sn and Li-Sb systems should occur are also plotted in Figure 1 as vertical lines. Although the lines are broadened and show shifts from their ideal positions, the presence of poorly crystalline solids structurally related to β-Sn, Li17Sn4, and Li3Sb is suggested by Figure 1. The discrepancy between the calculated and experimental peak positions might indicate a homogeneity range, reflecting the fact that the composition is spatially inhomogeneous because of to the cycling conditions chosen. Other possible intermediate phases cannot be detected under our experimental measurement conditions, probably because of their lower crystallinity. It should be noted that the lithium intermetallics formed from a precursor phase, such as SnSb, that is completely decomposed during cell discharging are usually found in the literature as poorly crystalline materials.20-23 In fact this is the reason for using alternative techniques such as Mo¨ssbauer and 7Li NMR spectroscopies to complete the characterization. Nevertheless, from Figure 1 the first poorly crystalline solid that occurs during discharging at ca. 5.80 F mol-1 is related to Li3Sb, which is in good agreement with LiδSnSb decomposition and subsequent Li-Sb compound formation. No diffraction maxima that could be ascribed to solids related to phases in the Li-Sn system were detected in the range of composition up to 10 F mol-1. Finally, a poorly crystalline lithium-rich compound was observed at the last point of the discharging process. The compound with the highest Li/Sn ratio has been usually referred to as Li22Sn5 in the literature, although a recent study showed that it corresponds to a Li17Sn4 compositions.24 The presence of this solid confirms the complete discharging of the electrode according to reaction 1. During extraction of lithium, several samples at intermediate charging depth were also checked by the (20) Chouvin, J.; Olivier-Fourcade, J.; Jumas, J. C.; Simon, B.; Godiveau, O. Chem. Phys. Lett. 1999, 308, 413. (21) Dedryve`re, R.; Olivier-Fourcade, J.; Jumas, J. C.; Denis, S.; Lavela, P.; Tirado, J. L. Electrochim. Acta 2000, 46, 127. (22) Chouvin, J.; Olivier-Fourcade, J.; Jumas, J. C.; Simon, B.; Biensan, Ph.; Ferna´ndez-Madrigal, F. J.; Tirado, J. L.; Pe´rez-Vicente, C. J. Electroanal. Chem. 2000, 494, 136. (23) Wang, Y.; Sakamoto, J.; Huang, C. K.; Surampudi, S.; Greenbaum, S. G. Solid State Ionics 1998, 110, 167. (24) Goward, G. R.; Taylor, N. J.; Souza, D. C. S.; Nazar, L. F. J. Alloys Compd. 2001, 329, 82.

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diffraction technique (Figure 1). The most significant result is the recovery of the initial SnSb at the end of charging. This finding has been previously reported by Yang et al.25 in SnSb0.14. These authors point out the gradual disappearance of this pattern after 16 cycles. In fact, the presence of β-Sn can be detected during charging in our work, possibly as a consequence of the increase in particle size during the previous electrochemical reaction. An additional result that supports this conclusion is the more defined shape of peaks obtained in the first derivate curve for the charging process. To evaluate the reversibility of reaction 1, i.e., the possible recovery of stistaite, powdered tin and antimony metals in a 1:1 molar ratio were hand-ground in a mortar. The mixture was used as the active electrode material in lithium cells similar to those used for the compound. Aside from the differences in cell capacity, which will be discussed below, a recording of the electrochemical discharge-charge curves of this cell (Figure 3) shows a close resemblance to the profile in Figure 2. Such similarities are better resolved in a plot of -dV/dx vs cell voltage, in which the peaks occur at almost coincident voltage values. This, in turn, reveals that the formation of the binary compounds LixSb and LixSn at fixed voltages comprises the main contributions to the reversible capacity of SnSb. In addition, the XRD data of the cycled mixture electrode allows us to confirm the absence of any SnSb formation from the lithium compounds. This result can be interpreted by considering the deleterious effects of particle aggregation on the reversibility of compound formation. Consequently, the use of mixtures of bulk Sn and Sb metals inhibits the intimate contact between LixSb and LixSn compounds

after charging and, thus, the recovery of SnSb. Moreover, the reversible capacity of SnSb is significantly higher than that of the separate metals, as evidenced by comparing the voltage vs composition curves for the two cells (Figures 2 and 3). This effect might be associated with the loss of electrical contacts and mechanical properties of the bulk electrode. 119Sn Mo ¨ ssbauer spectroscopy allows us to complete the information previously supported by XRD. The experimental 119Sn Mo¨ssbauer spectra of selected samples obtained at different depths of discharging/charging are shown in Figure 4, together with the calculated profiles. The corresponding hyperfine parameters are included in Table 1. The spectrum of pristine SnSb powders shows an unresolved quadrupole split signal at 2.702(5) mm/s, which can be ascribed exclusively to tin nuclei in unresolved Sn3Sb4 and Sn4Sb3 mixtures.15,16 The low value of QS is in agreement with the slight rhombohedral distortion from the cubic NaCl-type structure. In addition, a second weak signal was detected at ca. 0.1 mm/s and assigned to a SnO2 impurity. A semiquantitative evaluation of the intensities reveals that the SnO2 content always lower than 3%. A small but nonnegligible contribution of tin(IV) reduction in the impurity to the irreversible capacity and excess capacity is then expected. In fact, Figure 4 shows, that, during the early stages of discharging, the electrochemical reaction causes first a decrease in the SnO2 signal (at 2.02 Li/ SnSb in Figure 4), which then becomes almost undetectable (at 4.95 Li/SnSb) and eventually completely disappears (at 5.80 Li/SnSb). It is worth noting that, at 2.02 Li/SnSb, the Mo¨ssbauer spectrum corresponds to that of pristine SnSb, thus indicating that SnSb is not affected during the first steps of discharging. This result also confirms the occurrence of side reactions, such as passivating film formation (as discussed above), and SnO2 reduction. At 4.95 Li/SnSb, the MB spectrum changes significantly in both shape and isomer shift (IS). This spectrum consists of three components. The hyperfine parameters of the first component, centered at IS ) 2.57 mm/s with almost negligible quadrupole splitting of QS ) 0.14 mm/s, correspond well to those of β-Sn. The other two components can be assigned to binary lithium-tin compounds. They are similar to those described for the chemically obtained LiSn-related solid.26 At 5.80 Li/ SnSb, a similar profile is observed. A decrease of the metallic Sn signal is accompanied by an increase of the signals attributed to a LiSn-related solid. Thus, the Sn contribution varies from 40.4% at 4.95 Li/SnSb to 16.7% at 5.80 Li/SnSb, as expected from the advancement of the electrochemical reaction. At 7.27 Li/SnSb, the MB spectrum can be analyzed in terms of a mixture of two components, I and II. The first component (I) is due to a solid similar to the abovedescribed LiSn-related material, with a decreased contribution as compared with the previous sample. The second contribution (II) has hyperfine parameters that partially agree with previously published data20,26 and can be tentatively ascribed to a Li7Sn3-related solid, with an increasing contribution. For clarity, the components of this spectrum corresponding to both LixSn

(25) Yang, J.; Takeda, Y.; Imanishi, N.; Yamamoto, O. J. Electrochem. Soc. 1999, 146, 4009.

(26) Dunlap, R. A.; Small, D. A.; MacNeil, D. D.; Obrovac, M. N.; Dahn, J. R. J. Alloys Compd. 1999, 289, 135.

Figure 3. (a) Voltage vs composition for a Li/LiPF6 (EC:DEC)/ (Sn + Sb) cell recorded under galvanostatic conditions at C/4. (b) Comparison of the first derivative profiles, -dV/dx vs V, for the first cycle of cells using SnSb and Sn + Sb electrodes.

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Ferna´ ndez-Madrigal et al. Table 1. Refined Hyperfine Parameters of the 119Sn Mo1 ssbauer Spectra Included in Figure 4 voltage Li/SnSb

hyperfine parameters (mm/s) ISa QSb LWc

I (%)

attribution

pristine

2.702(5) 0.13(2)

0.24(1) 0.50(2)

0.784(8) 0.65(4)

89.9 11.0

SnSb SnO2

0.78 Vd 2.02

2.71(1) 0.16(5)

0.26(2) 0.26(2)

0.77(2) 0.6(1)

92.7 7.3

SnSb SnO2

0.58 Vd 4.95

2.57(3) 2.39(3) 2.39(3) 0.32(3)

0.14(2) 0.10(2) 0.86(2) 0.77(2)

0.72(2) 0.72(2) 0.72(2) 0.72(2)

40.4 20.0 37.2 2.4

Sn LixSn

0.42 Vd 5.80

2.58(5) 2.45(5) 2.45(5)

0.14(9) 0.16(9) 0.76(9)

0.82(3) 0.82(3) 0.82(3)

16.7 47.2 36.1

Sn LixSn

0.30 Vd 7.27

2.41(2) 2.41(2) 2.17(2) 2.12(2)

0.25(1) 0.74(1) 0.46(1) 1.19(1)

0.76(3) 0.76(3) 0.77(3) 0.77(3)

26.7 23.1 20.4 29.8

LixSn (I)

0.32 Ve 8.90

2.14(1) 2.13(1) 2.01(1)

1.04(2) 0.45(2) 0.73(2)

0.73(4) 0.73(4) 0.73(4)

47.4 42.2 10.4

0.60 Ve 8.00

2.47(1) 2.31(1)

0.38(2) 0.94(2)

0.75(4) 0.75(4)

64.4 35.6

LixSn

0.76 Ve 6.28

2.57(1) 2.44(1) 2.44(1)

0.11(1) 0.43(1) 0.92(2)

0.73(4) 0.73(4) 0.73(4)

26.1 49.3 24.6

Sn LixSn

1.00 Ve 5.42

2.52(1) 2.30(2) 2.30(2)

0.31(2) 0.42(4) 0.83(4)

0.78(3) 0.78(8) 0.78(8)

42.8 25.2 32.0

Sn LixSn

1.30 Ve 2.46

2.67(2) 0.24(3)

0.22(4) 0.63(4)

0.88(3) 0.63(7)

87.2 12.8

SnSb SnO2

SnO2

LixSn (II)

LixSn

a IS, isomer shift. b QS, quadrupole splitting. c LW, line width at half-maximum. d Samples prepared by interrupting discharging of lithium anode cells. e Samples prepared by interrupting charging of lithium anode cells.

Figure 4. Changes in the 119Sn Mo¨ssbauer spectra of the SnSb-based electrodes during (a) cell discharging, (b) cell charging. Small circles: experimental data. Dotted line: β-Snrelated phase. Full and dashed lines: LixSn phases

phases have been plotted separately in the upper part of Figure 4. The QS values of the signal corresponding to these solids vary from 0.10 to 1.20 mm/s, these values being close to those of the pure compounds.20 Higher values, around 1.5-1.6 mm/s, have been detected in the electrochemical Li-Sn intermediates formed from CuInSnS4 as the anodic material.21 This result was interpreted in terms of the existence of interactions with sulfur present in the amorphous matrix. Also, during the lithium-tin reaction using SnO as the active material, values of QS (of ca. 1.4 mm/s) higher than

those of pure compounds have been obtained,22 because of the interaction with the Li2O matrix. The main conclusion from the above Mo¨ssbauer data is the deviation of the materials formed in the discharged electrodes from the crystalline LixSn phases prepared thermally. The Mo¨ssbauer data show local order and Li-Sn interactions, but the long-range ordering is not fully coincident with that of the crystalline phases. From these spectra, the reliability of the Mo¨ssbauer fitting parameters is limited. Figure 5 shows a plot of the centroid isomer shift of each sample electrode and a comparison with the values for crystalline and noncrystalline electrodes obtained in previous studies.20,26 As this parameter is unequivocally indicative of the average electron density at the 119Sn nuclei, it shows coincident behavior with previous systems described in the literature, irrespective of the quadrupolar effect, which is highly influenced by symmetry considerations. The reversible nature of the charging process was also monitored by 119Sn Mo¨ssbauer spectroscopy at several intermediate compositions (Figure 4b). From the hyperfine MB parameters, it can be deduced that electrode oxidation to 8.90 Li/SnSb induces a transformation toward lower lithium contents. The Mo¨ssbauer spectrum of the 8.00 Li/SnSb sample was similar to that of a poorly crystalline LiSn-related solid. Moreover, at 6.28 and 5.42 Li/SnSb, the MB spectrum corresponds to a mixture of metallic tin- and LiSn-related solids, as

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Figure 5. Plot of weighted averages of isomer shifts of Li-Sn compounds described in (0) ref 20, (O) ref 26, and (*) this work.

observed previously during discharging at 4.95 and 5.80 Li/SnSb. As expected, the contribution of LiSn-related materials decreases with the lithium content, whereas the contribution of metallic tin increases. Finally, at the end of charging, at 2.45 Li/SnSb, we again found the typical spectrum of SnSb, indicating that SnSb is reformed, as observed in previously published works.13 This result also confirms the observations made by XRD. To gain further insight into the products of reaction with lithium, the 7Li MAS NMR technique was used. NMR spectroscopy can be considered as a powerful tool for obtaining information about the environments that lithium adopts on insertion into the electrode.23,27 Figure 6 shows the spectra of discharged electrodes starting with (a) Sb and (b and c) SnSb. In all spectra, a component at ca. 0 ppm was observed that can be unequivocally ascribed to ionic lithium in the electrolyte and/or traces in the passivating layer that is formed by electrolyte decomposition. On discharge to 0.58 V, decomposition of the spectrum into its components (Figure 6c, Table 2) reveals the partial destruction of the SnSb structure by the appearance of an upward field signal. To gain information about the nature of the signal with negative shifts, the spectra of discharged LixSb electrodes were recorded (Figure 6a), in which the signal at ca. 0 ppm and two upward field signals are visible. The two signals at -3.3 and -4.8 ppm result from the two different environments of lithium in LixSb for x close to 3. For LixSnSb electrodes, these two signals cannot be fully resolved, giving an average negative shift of -2.8 ppm. The shift of the signal from that obtained for pure Sb (Figure 6a and c) can be tentatively explained by considering the interactions of LixSb with the surrounding metallic tin matrix. The 7Li MAS NMR spectrum varies significantly from 0.58 to 0.42 V (Figure 6c). Four new signals appear at 1.5, 2.0, 42, and 77 ppm. Because the spectra were recorded at different spinning rates, one can see that all shifts were isotropic. It is expected that these signals imply different lithium environments surrounded by the (27) Goward, G. R.; Taylor, N. J.; Souza, D. C. S.; Nazar, L. F. J. Mater. Chem. 2000, 10, 1241.

Figure 6. Experimental and calculated 7Li MAS NMR spectra of electrodes discharged to 0 V using (a) Sb and (b) SnSb as the active material. (c) Intermediate depths of discharging of SnSb electrodes.

tin atoms resulting from the reaction of lithium with the metallic tin formed by the previous decomposition of SnSb to give LixSb. The first three signals have

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Table 2. Deconvolution of Chemical Shift and Relative Intensity of the Signals in the 7Li MAS NMR Spectra of Discharged Electrodesa shift (ppm)

width (Hz)

intensity (%)

0.58 V

-2.7 -0.8

265 70

100 electrolyte

0.42 V

1.4 2.0 42.0 77.4 -0.8

119 171 62 648 62

43 40 4 13 electrolyte

0.30 V

2.0 3.4 14.8 41.8 76.4 -0.8

345 531 1154 750 330 56

49 18 22 10 1 electrolyte

0.10 V

1.89 3.5 -2.2 -0.9

427 448 1428 45

66 8 26 electrolyte

0V

1.6 3.0 30.3 -0.8 -0.9

322 696 2392 3177 38

13 34 7 46 electrolyte

electrode

a Electrolyte/passivating layer contribution excluded in the calculation of the relative intensities.

chemical shifts similar to those observed for cycled SnO electrodes,27 which can be ascribed to Li-Sn contacts. The signal observed at 77 ppm (not shown in Figure 6c) can be ascribed to a situation in which the Knight shift is significant. However, these values are far from the 114 ppm shift reported for crystalline LixSn phases.23

Because of the sensitivity of NMR spectroscopy to the structural and electronic environment of lithium in metallic hosts, the positive shift components cannot be ascribed to a particular environment. At 0.30 V (Figure 6c, Table 2), the pattern reaches maximum complexity with the appearance of a new intermediate shift at ca. 15 ppm. It should be noted that, at both 0.42 and 0.30 V, no signal with negative shift is visible. This can be tentatively explained by assuming that Li-Sn interactions in this voltage range are not completely independent from Li-Sb interactions. The joint effects of the two environments increase the complexity of the spectra in the downfield region. Finally, at 0.1 and 0.0 V, the pattern is reduced to the signals below 10 ppm, and negative shifts can be detected. This result can be interpreted by assuming that the initial dilute Li-in-Sn phases leading to higher Knight shifts are finally converted to lithium-rich LixSn phases, with low conductivity, in which the Knight shift is not present. Simultaneously, the negative shift can be ascribed to lithium in an Sb-rich environment, as in Figure 6a. It should be noted that negative chemical shift values were also found upon complete discharge in CoSb3-related electrodes.28 Acknowledgment. The authors acknowledge financial support from the PICASSO program and Project MAT99-0741. CM0112800 (28) Monconduit, L.; Jumas, J. C.; Alca´ntara, R.; Tirado, J. L.; Pe´rez Vicente, C. J. Power Sources 2002, 107, 72.