57Fe Mössbauer Spectroscopy Study of the Electrochemical Reaction

Oct 26, 2009 - ... Spain, and Institut Charles Gerhardt (UMR 5253), Equipe Agrégats, Interfaces et Matériaux pour l'Energie, Université Montpellier...
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J. Phys. Chem. C 2009, 113, 20081–20087

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Fe Mo¨ssbauer Spectroscopy Study of the Electrochemical Reaction with Lithium of MFe2O4 (M ) Co and Cu) Electrodes P. Lavela,*,† J. L. Tirado,† M. Womes,‡ and J. C. Jumas‡ Laboratorio de Quı´mica Inorga´nica, UniVersidad de Co´rdoba, Edif. C3, Campus de Rabanales, 14071 Co´rdoba, Spain, and Institut Charles Gerhardt (UMR 5253), Equipe Agre´gats, Interfaces et Mate´riaux pour l’Energie, UniVersite´ Montpellier II, CC 15, Place E. Bataillon, 34095 Montpellier Cedex 5, France ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: October 9, 2009

Cycled electrodes of CoFe2O4 and CuFe2O4 were analyzed by 57Fe Mo¨ssbauer spectroscopy. On decreasing the recording temperature, the spectra evidence the superparamagnetic-ferromagnetic transition of metallic iron formed during the reduction of iron oxides in a lithium cell. In addition, the full reoxidation of iron to the 3+ oxidation state is evidenced after cell recharge. As compared with CoFe2O4, the presence of copper led to three effects: a lower percentage of surface atoms, a higher average oxidation state of surface iron atoms, and smaller metallic particles in the reduced state. The relationships between the Mo¨ssbauer spectra and particle size and morphology of the raw spinel materials are studied. The charging process involves an important reordering of iron atoms between bulk and surface locations in CoFe2O4, which leads to minor differences in the spectra of the charged electrodes of both compounds. Introduction In recent years, a new field of research on the negative electrodes for lithium ion batteries was developed by taking into account electrochemical conversion reactions of transition metal compounds. Although the complete reduction of CoO to the metallic state was verified in the early 1980s,1 the performance of a LiAl/LiCl,KCl/CoO cell deteriorated with further cycling. The poor response was ascribed to the instability of cobalt oxide in contact with the electrolyte and to the dispersion of the discharge product.2 More recently, it was demonstrated that the nanometric size of the metal particles produced electrochemically after the discharging process favors the reversible reaction. However, highly polarized charging curves are recorded,3,4 which is considered as a major drawback for an eventual commercial application of these electrode materials. Research on the parameters influencing the nanostructure of the discharged particles may lead to improvements in the reversibility of the electrochemical reaction.5 The electrochemical performance of transition metal mixed oxides with spinel structure has been tested in lithium test cells.6-8 Although the composition and structure are related, the strong changes occurring during cell discharge led to distinct performances as active materials in lithium cells. In fact, the initial particle morphology and metal composition can also be pointed out as relevant factors in the reversibility of the Lidriven conversion reaction.9-11 To understand the nature of the nanometric particles yielded by the electrochemical conversion reaction of MFe2O4 in lithium cells, 57Fe Mo¨ssbauer spectroscopy was considered as a useful technique to reveal the oxidation-reduction reaction mechanism and the appearance of new locations for the probe atom upon cell cycling.12-14 The aim of this work is to analyze the 57Fe Mo¨ssbauer spectra of discharged and charged CoFe2O4 and CuFe2O4 electrodes. * Corresponding author. Phone: 957 218 637. Fax: 957 218 621. E-mail: [email protected]. † Universidad de Co´rdoba. ‡ Universite´ Montpellier II.

The different electrochemical behavior of these solids has been previously reported by our group.15-17 It has been shown that the discharging and charging process lead to an important reordering of iron atoms.18 Our previous work allowed us to distinguish between surface and bulk sites of iron atoms in CoFe2O4.14 However, to date, no evidence of any magnetic ordering has been revealed. Such an ordering would be expected if metallic iron was formed in discharge or if the initial ferrites would be reformed in charge. Here, an in-depth study is carried out by recording Mo¨ssbauer spectra down to 12 K to search for unequivocal proof of the existence of metallic iron in a highly dispersed state as well as the maximum iron oxidation state reached during reoxidation. Moreover, the occurrence of different iron environments will be considered as a function of composition and annealing temperature. Finally, from the hyperfine parameters measured at different temperatures, we will evaluate the transition from the superparamagnetic to the magnetically blocked state in the nanostructured material yielded in the electrochemical conversion reaction. Experimental Methods Spinel oxides with nominal stoichiometries CoFe2O4 and CuFe2O4 were prepared by a sol-gel method using a citrate ligand to complex the metal ions, as described elsewhere.15 The precursors were annealed at several temperatures ranging from 400 to 1000 °C for 24 h in air. The Service of Support to Research at the University of Co´rdoba provided a JEOL JEM 2010 microscope to obtain HRTEM images. The test cells were assembled in two-electrode Swageloktype lithium cells. For this purpose, 9 mm lithium metal discs were used as counter electrodes. Working electrodes were prepared from a mixture of 75% active material, 10% graphite, 10% carbon black, and 5% PVDF binder. A paste, formed with the mixed powders and N-methyl pirrolidone, was coated on a 9 mm copper support. Whatman glass fiber discs supported a 1 M LiPF6 (EC/DEC ) 1:1) electrolyte solution. An Arbin potentiostat/galvanostat multichannel system was used to cycle the cells at C/10 rate in both charge and discharge runs.

10.1021/jp9056362 CCC: $40.75  2009 American Chemical Society Published on Web 10/26/2009

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Fe Mo¨ssbauer spectra (MS) were recorded in transmission mode at room temperature using an EG&G constant acceleration spectrometer. The source was 57Co in a Rh matrix (10 mCi). The velocity scale was calibrated from the magnetic sextet of a high-purity iron foil absorber. Experimental data were fitted to Lorentzian lines by using a least-squares-based method.19 The quality of the fit was controlled by the classical test of χ2. All isomer shifts are given relative to the center of the R-Fe spectrum at room temperature. Mo¨ssbauer spectra at 77 K were recorded at the UM II using a liquid nitrogen flow cryostat coupled to a temperature controller. The spectra recorded at 12 and 40 K were obtained at UCO using a Cryogenics cryostat coupled to a Cryo.Con temperature controller. Samples of airsensitive cycled materials were extracted from the Swagelok cells in a glovebox under argon atmosphere and were transferred to specific airtight sample holders equipped with mylar windows transparent for γ radiation. Results and Discussion CoFe2O4 has a cubic spinel structure corresponding to the Fd3m space group, whereas in CuFe2O4, the presence of Cu2+ ions induces a tetragonal distortion as a consequence of the Jahn-Teller effect provoked by the transition metal ions in d9 configuration. The corresponding XRD pattern shows additional reflections that can be indexed in the I41/amd space group (34425 JCPDS-ICDD card). Phase purity of the samples obtained in this work was dependent on the chemical composition and annealing temperature. Pure CoFe2O4 samples were obtained, regardless of the heating conditions. In contrast, CuFe2O4 showed additional minor reflections in samples prepared at 400 and 600 °C, attributed to hematite (*) and CuO (O) (Figure 1). An annealing temperature of 800 °C is needed to obtain a pure tetragonal phase.20 In a previous work on CuFe2O4, we have demonstrated that other factors than impurities, such as particle morphology, are relevant for electrochemical performance due to the strong structural degradation of these materials during the first discharge.17 For this reason, copper-containing samples annealed at low temperatures were also evaluated. An analysis by scanning electron microscopy (SEM) of initial CoFe2O4 and CuFe2O4 samples annealed at 400 °C revealed large, curled, layered particles (Figure 2a, c). On increasing the annealing temperature to 1000 °C, a different texture is observed, depending on the composition (Figure 2). Submicrometer particles mutually interconnected were observed for CoFe2O4, and large particles larger than 5 µm appeared for CuFe2O4. Hence, the effect of the transition metal on sample morphology is evidenced. Previous reports related to the electrochemical properties of these samples as electrodes in lithium cells evidenced a better cycling behavior of CoFe2O4, which was attributed to the more favorable texture of the cobalt ferrite.15,17 The macroporous system created by the mutually interconnected submicrometer particles provides a better electrode/electrolyte interphase. 57 Fe Mo¨ssbauer spectra of MFe2O4 ferrites have been extensively studied to unfold their structural and magnetic properties.21-23 For this reason, the evaluation of the spectra of raw materials was not considered as a main objective in this work. However, the spectra (not shown) showed overlapped sextets attributed to Fe3+ located at octahedral and tetrahedral sites in a ferromagnetic state. They reflect the inverse character of these spinel oxides. The Li-driven conversion reaction leads to the complete reduction of the transition metal, whereas lithium ions link to oxygen to form the corresponding oxide. The conversion

Figure 1. X-ray diffraction patterns of CoFe2O4 and CuFe2O4 annealed at various temperatures.

Figure 2. SEM micrographs of CoFe2O4 annealed at (a) 400 and (b) 1000 °C and CuFe2O4 annealed at (c) 400 and (d) 1000 °C.

involves the complete disappearance of the structure of the starting oxide (spinel structure in our case). At the end of the cell discharge, the reaction product consists of Li2O/transition metal mixtures, in which the transition metal is in the form of nanoparticles. However, these conversion products are not randomly dispersed through the sample, but tightly bonded among themselves, almost preserving the morphology of the initial particle. Thus, large agglomerates of metal and Li2O can be observed at the end of the cell discharge. In most cases, only

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Figure 3. Room temperature 57Fe Mo¨ssbauer spectra of CoFe2O4 and CuFe2O4 prepared at 1000 °C. These spectra correspond to fully discharged and charged electrodes.

rounded edges are observed, as compared to the sharp ones of the raw material.17,24 Our Mo¨ssbauer study is focused in the totally reduced and amorphous products. The Mo¨ssbauer spectra of the discharged electrodes revealed a characteristic asymmetric profile that can be decomposed into two doublets associated with superparamagnetic iron atoms in new distorted environments induced by the structural degradation. The hyperfine field, characteristic of the initial ferromagnetic spinel oxides, is then effectively zero due to thermal excitation, giving rise to superparamagnetic behavior. Because the grains are so small, much of the iron is in the region of grain boundaries. This iron would reside in a nonisotropic environment, giving rise to the observed to the high split quadrupole doublet. Hence, both doublets can be assigned to iron atoms located at the surface and inside the particles, respectively. This assumption is based on the low size of the metallic nanoparticles (around 20 Å), which makes significant the contribution of Fe atoms located at the particle surface, as compared to those located in the bulk of the particle.12 Moreover, Larcher et al. have reported that the contribution of both surface and bulk iron is observed to increase when the reaction with lithium progresses during the first discharge. This fact is closely related to the electrochemical grinding effect.13 As an example, Figure 3 shows the spectra of the spinel oxides, annealed at 1000 °C, that were discharged in a lithium cell down to 0.02 V. Even if the overall profiles match well with the interpretation described above, differences among profiles can be discerned. Therefore, composition and morphology of the original materials

Figure 4. Plots of (a) isomer shift, (b) quadrupole splitting, and (c) relative contribution versus annealing temperature. CoFe2O4 and CuFe2O4 electrodes were fully discharged to 0.02 V. Filled symbols refer to the weakly split signal, and open symbols, to the strongly split one.

may influence the final state of the discharged electrode, deserving a deeper inspection. The hyperfine parameters isomer shift and quadrupole splitting and the relative contributions of both doublets are plotted in Figure 4 for both CoFe2O4 and CuFe2O4 discharged electrodes as a function of the annealing temperature. For CoFe2O4, the weakly split signal (filled symbols) showed isomer shift values close to 0 mm/s, indicating an effective reduction of iron atoms located inside the particles. The strongly split signal gives values ranging from 0.10 to 0.14 mm/s. Both values are clearly different

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Figure 6. Single design of iron atoms locations in a reduced particle after full discharge.

Figure 5. HRTEM images of electrodes discharged at 0.02 V corresponding to the following samples: (a) CoFe2O4 and (b) CuFe2O4. Raw materials were annealed at 1000 °C.

from the isomer shift of 0.34 mm/s obtained for pure nanosized CoFe2O4,25 indicating the effective reduction of the starting material. For CuFe2O4, the weakly split signal also has an isomer shift close to 0 mm/s, whereas the isomer shifts of the strongly split signal lie above 0.20 mm/s and have been attributed to the chemical oxidation of iron atoms on the surface in contact with the electrolyte.14 This fact would undoubtedly affect the reversibility of the main redox reaction in further charging. As for CoFe2O4, these parameters are different from those of pure nanosized material, indicating the reduction of the initial copper ferrite.26 Quadrupole splitting values (Figure 4b) again evidenced a similarity of the properties of the iron atoms in discharged cells when located inside the particles, irrespective of the sample. However, a more anisotropic environment is detected for surface irons in CoFe2O4. These differences may explain the different electrochemical behavior of copper and cobalt ferrites. On the contrary, the effect of the annealing temperature on either the isomer shift or the quadrupole splitting did not show a clear tendency. Relative contributions of both signals revealed significant differences among the studied spectra (Figure 4c). CoFe2O4 electrodes discharged at 0 V revealed a large population of iron atoms located on surface sites (∼60%), as compared to the copper oxide, where this fraction was ∼40%. From this result, it could be inferred that there were larger reduced metal particles for CuFe2O4 than for CoFe2O4 sample. However, a close inspection of HRTEM images of cobalt and copper samples annealed at 1000 °C and discharged to 0.02 V evidence even smaller metallic domain sizes for CuFe2O4 (Figure 5). Therefore, the larger contribution of inner iron atoms for CuFe2O4 cannot be ascribed to larger metal particles upon discharge. A more suitable explanation of this result may arise from considering as surface iron atoms only those located on the surface of the large agglomerates (Figure 6). It would involve that the nanometric size of the inner metal and Li2O particles preserve a highly isotropic environment of iron atoms irrespective of their location, at least, under the limit of sensitivity of the technique. An increase in the relative contribution with the

annealing temperature was more clearly detected for CuFe2O4 samples, as expected from the huge increase in the particle size up to 1000 °C. The Li-driven conversion reaction is partially reversible during the first cycle. Although metal oxidation has been reported, a complete structural recovery has not been detected in any case.14,27,28 The oxidation process takes place through highly polarized charge curves in which the unique horizontal discharge plateau is replaced by two overlapped stepped plateaus.15 The charge reaction was monitored up to 3.0 V, ensuring the complete oxidation of metal atoms. The 57Fe Mo¨ssbauer spectra of the charged electrodes were characterized by broadened and asymmetric doublets, which could be decomposed into two overlapped split signals (Figure 3). None of the spectra showed a six-line structure expected for magnetic ordering, like in the starting material. This fact precludes an easy assignation of the geometrical environment. However, the different values recorded for the quadrupole splitting would confirm the validity of the description based on distinct locations with different isotropy. The evaluation of the isomer shifts (Figure 7a) revealed values ranging from 0.3 to 0.4 mm/s, corresponding to Fe3+. Thus, the charging process was successful, irrespective of the location of the iron atoms in the particle. This fact can be beneficial for the electrode performance in a long-term cycling. On increasing the annealing temperature, the isomer shifts tend to decrease, most probably as a consequence of kinetic problems arising from the larger particle size with a small electrode/electrolyte interface. The quadrupole splitting values of both signals remained different, regardless of the effective iron oxidation (Figure 7b). Thus, values between 0.4 and 0.6 mm/s were recorded for the weakly split signal for both compounds, whereas the strongly split signal showed values between 1.0 and 1.2 mm/s. These values are significantly higher than those found after the discharge process. According to the previous interpretation of the quadrupole splitting values, this result evidences an enhancement of the anisotropy in the local environment of the probe atoms, most probably as a consequence of the back migration of oxygen from lithium to transition metal atoms to form MO bonds. It is, however, worth noting that in the case of CoFe2O4, the splittings of both signals are different from the value of 0.76 mm/s reported for the unique doublet found for nanosized material.25 Concerning CuFe2O4, the parameter isomer shift and quadrupole splitting of one of our doublets (δ = 0.3 mm/s; ∆ ) 1.0 ... 1.2 mm/s) agree quite well with those of one doublet found by Jiang et al. for nanosized particles. The splitting of their second doublet (1.1 ... 1.6 mm/s), however, differs considerably from our result. These authors leave the question open whether their doublets correspond to a Fe3+ core and surface atoms or to Fe3+ atoms on tetrahedral and octahedral sites. The differences between these results for nanosized materials and our recharged electrodes confirm statements in the literature according to which a complete structural recovery has never been detected by XRD in related oxides.14,24

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Figure 8. 57Fe Mo¨ssbauer spectra, recorded at 77 K, of fully discharged and charged (a) CoFe2O4 and (b) CuFe2O4 electrodes. Raw materials were annealed at 1000 °C.

Figure 7. Plots of (a) isomer shift, (b) quadrupole splitting, and (c) relative contribution versus annealing temperature. CoFe2O4 and CuFe2O4 electrodes were fully discharged to 0.02 V and then charged to 3 V. Filled symbols refer to the weakly split signal, and open symbols, to the strongly split one.

The relative contributions of the doublets after cell charging are shown in Figure 7c. For the weakly and strongly split signals, values around 40% and 60% were measured, respectively. A clear tendency of these values with respect to the annealing temperature or composition could not be inferred. The observed similarity in the behavior for both samples differs from the observation on fully discharged electrodes. In a previous report, on the basis of the use of conversion electron Mo¨ssbauer spectroscopy (CEMS), we revealed the validity of the different local bulk and surface iron environments after oxidation.29 Thus,

the charging process involves an important reordering of iron atoms between bulk and surface locations in both compounds. Samples annealed at 1000 °C were selected to record 57Fe Mo¨ssbauer spectra at low temperatures. Figure 8a shows the spectra of both a fully discharged and a fully charged CoFe2O4 electrode recorded at 77 K. A strongly broadened six-line spectrum is now observed instead of the doublets seen at room temperature. According to Klabunde et al., metal particles in the range of 4.5-7 nm are expected.30 These values agree well with the particle diameter observed in Figure 5. Due to their nanometric size, the metallic particles each form a single magnetic domain. The magnetization vector of such domains switches rapidly from one direction of easy magnetization to another. The time it spends in one orientation depends, for a given material, on particle size and temperature. At room temperature, this time is much shorter than the characteristic measuring time of Mo¨ssbauer spectroscopy. As a consequence, the average local magnetic field seen by the iron nucleus becomes zero, and the sample is superparamagnetic. At 77 K, the fluctuation rate of the magnetization is sufficiently slowed down that a static magnetic field is seen by Mo¨ssbauer spectroscopy. A rough fit of the spectrum of the discharged sample by two sextets gives isomer shift values of δ ) 0.07 and 0.36 mm/s and hyperfine magnetic fields of 33.3 and 34.9 T (see Table 1). The first set of values is close to the parameters of bulk R-Fe at 77 K of δ ) 0.10 mm/s and Bhf ) 33.9 T.31 The second one would match with oxidized iron atoms, though the low magnetic field does not allow assigning it to either CoFe2O4 or any binary iron oxide. This result agrees with the

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TABLE 1: Hyperfine Parameters of Fully Discharged and Charged CoFe2O4 Electrodes Recorded at 77 Ka sample

δb (mm/s)

∆c (mm/s)

2Γd (mm/s)

Bhfe (T)

Cf (%)

discharged at 0.02 V

0.36(8) 0.07(6) 0.39(3)

0.09(6) 0.10(4) 0.00(1)

1.20(5) 1.20(5) 1.01(1)

34.9(1) 33.3(2) 37.1(2)

42.4(9) 57.6(8) 37.6(8)

0.24(3) 0.24(3)

0.00(1) 0.00(1)

1.01(1) 1.01(1)

31.7(2) 41.1(2)

33.4(7) 29(1)

discharged and then charged at 3 V

a CoFe2O4 was prepared at 1000 °C. b δ: isomer shift. c ∆: quadrupole splitting. magnetic field. f C: contribution to total absorption. g χ2: goodness of the fitting.

Figure 9. 57Fe Mo¨ssbauer spectra of fully discharged CuFe2O4 electrodes recorded at (a) 40 and (b) 12 K and CuFe2O4 electrodes after full discharge and recharge to 3 V recorded at (c) 40 and (d) 12 K. Raw materials were annealed at 1000 °C.

presence of oxidized iron on discharged electrodes, as inferred from the spectra recorded at room temperature, and supports our hypothesis of iron oxidation through reaction with the electrolyte, which would certainly not lead to any binary oxide or to a ferrite-like structure. The enhanced width of these sextuplets can be attributed to the occurrence of a distribution in the hyperfine parameters, as is usual for nanostructured materials. A more sophisticated analysis would give a statistically better fit, but is not likely to add any meaningful information to the analysis. The spectrum of the recharged sample recorded at 77 K shows that iron occupies a large number of inequivalent sites differing by the number and arrangement of magnetic neighbors and characterized by different local magnetic fields. The spectral shape suggests the occurrence of at least three different sites (Figure 8a, Table 1). Previous work on crystalline CoFe2O4 has

d

2Γ: full line width at half-maximum.

χ2

g

0.503 0.912

e

Bhf: hyperfine

reported the decomposition of the spectrum in several sextets. One sextet at about 0.24 mm/s is ascribable to iron atoms located at the tetrahedral sites. Four additional sextets centered at about 0.4 mm/s are associated with iron in octahedral sites with different next-nearest neighbor configurations.32 The isomer shift values of all these subspectra indicate a +3 oxidation state. Nevertheless, the hyperfine magnetic fields of the subspectra are significantly lower than those commonly recorded for crystalline samples. This evidences the partial recovery of the initial structure after the electrochemical oxidation. For CuFe2O4, the spectrum of a fully discharged and of recharged samples recorded at 77 K are characterized by strongly broadened components, doublets, and sextets (Figure 8b). This spectral shape is characteristic of small particles with a distribution in particle size. The smallest particles appear as superparamagnetic and give rise to the doublet, whereas the largest particles appear as magnetically blocked. Ku¨ndig et al. proposed a method to calibrate the average particle size in R-Fe2O3 as a function of the ratio of the ferromagnetic to the superparamagnetic contribution in the spectrum.33 In a first approach, we can consider minor deviations in our CuFe2O4 samples electrochemically reoxidized from the behavior of the reported R-Fe2O3. If we apply the calibration curve obtained for the hematite to our copper ferrite particles, diameters as low as 8 nm are obtained. Further spectra were recorded at 40 and 12 K (Figure 9a, b) to assess this assumption. At 12 K, due to the further reduced fluctuation rate, all magnetization vectors appear as blocked, and we observe at least two magnetic contributions in the spectra, clearly reflected by the spectral shape. The components of the discharged sample give magnetic hyperfine fields of 31.3 and 31.7 T (Table 2), which are close to the magnetic field of bulk R-Fe at 12 K of 34 T.31 As in the case of CoFe2O4, the isomer shift values agree with the model previously established from the spectra recorded at room temperature of two different environments in the bulk and on the surface. The blocking temperature of the magnetic moments of the iron particles formed by the discharge process is, thus, lower in CuFe2O4 than in CoFe2O4, indicating a smaller average particle size in the copper compound. The different behavior of the two compounds CoFe2O4 and CuFe2O4 could be correlated

TABLE 2: Hyperfine Parameters of Fully Discharged and Charged CuFe2O4 Electrodes Recorded at 12 Ka sample

δb (mm/s)

∆c (mm/s)

2Γd (mm/s)

Bhfe (T)

Cf (%)

discharged at 0.02 V

0.16(2) 0.18(1) 0.58(3)

0.01(1) 0.02(1) 0.00(1)

1.04(1) 1.15(1) 0.84(1)

33.5(1) 29.1(2) 49.6(2)

49(1) 51.0(9) 59.8(6)

0.56(4)

0.00(1)

0.84(1)

44.1(2)

40.2(8)

discharged and then charged at 3 V

CuFe2O4 was prepared at 1000 °C. δ: isomer shift. ∆: quadrupole splitting. magnetic field. f C: contribution to total absorption. g χ2: goodness of the fitting. a

b

c

d

2Γ: full line width at half-maximum.

χ2

g

0.538 1.69

e

Bhf: hyperfine

Reaction with Lithium of MFe2O4 Electrodes with the lower size of both metallic and oxidized nanoparticles when copper is present in the ferrite composition. Like the discharged sample, the recharged CuFe2O4 electrode gives at 77 K a spectrum characterized by two strongly broadened components: a doublet and a sextet (Figure 9c, d). Additional spectra were recorded to identify the various components. The hyperfine parameters obtained at 12 K (Table 2) agree with a reoxidation of iron atoms, as expected from the electrochemical reaction. The magnetic fields of the components are 44.2 and 49.6 T. These values are lower than those obtained for bulk material but agree with some of the subspectra found for nanosized material obtained from a well crystallized sample by prolonged ball-milling.26 Like for the discharged samples, we observe here the same phenomenon of lower blocking temperatures in the copper ferrite, as compared to CoFe2O4. Although it appears likely that the smaller iron particles formed in CuFe2O4 after discharge would lead to smaller oxidized particles, in charge, it is difficult to draw a similar conclusion on particle sizes here for the oxidized samples. We have to keep in mind that the recharged electrodes represent different materials. The superparamagnetic relaxation time depends not only on particle size and temperature but also on the value of the magnetic anisotropy constant of the material. Blaskov et al. determined this value for 5 nm CoFe2O4 to K ) 2.5 × 106 erg/cm3, whereas 10 nm particles of CuFe2O4 have, according to Jiang et al., an anistropy constant of K ) 1.8 × 105 erg/cm3.25,26 This difference by 1 order of magnitude alone is sufficient to give, for similar particle sizes of 5-10 nm, a magnetically blocked state at 77 K for the cobalt ferrite. For CuFe2O4, the lifetime of a spin orientation at that same temperature would be on the order of the characteristic Mo¨ssbauer measuring time of 5 × 10-9 and would produce a spectral shape like in Figure 9. Conclusions Electrochemically reduced CoFe2O4 and CuFe2O4 electrodes to form Co/Li2O and Cu/Li2O have been evaluated by 57Fe Mo¨ssbauer spectroscopy. Whereas bulk iron atoms are efficiently reduced to their metallic state, the high isomer shift of surface atoms in Cu-containing samples indicates a favored chemical oxidation by contact with the electrolyte. The contribution of surface and bulk iron atoms of the metallic particles produced after cell discharge is virtually independent of the annealing temperature of the spinel starting solid. In contrast, the chemical composition plays a major role. 57 Fe Mo¨ssbauer spectra recorded at low temperatures (77, 40, and 12 K) gave the first direct evidence that superparamagnetic metallic iron is formed during the reduction of iron oxides in a lithium cell. The lower blocking temperature of the magnetization for copper-containing samples is associated with the lower particle size for discharged materials derived from CuFe2O4 cycled electrodes. Superparamagnetic behavior was also observed for recharged samples. Low temperature spectra recorded at 12, 40, and 77 K revealed magnetic ordering in electrodes of both compounds. The hyperfine magnetic fields agree with data from the literature reported previously for nanosized particles of CuFe2O4 and CoFe2O4.

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