3842 7Li
J. Phys. Chem. B 2002, 106, 3842-3847
NMR Studies of Chemically-Delithiated Li1-xCoO2 Michael C. Tucker, Jeffrey A. Reimer,* and Elton J. Cairns Ernest Orlando Lawrence Berkeley National Laboratory and Department of Chemical Engineering, UniVersity of California, Berkeley, Berkeley, California 94720
S. Choi and A. Manthiram Materials Science and Engineering Program, The UniVersity of Texas at Austin, Austin, Texas 78759 ReceiVed: August 30, 2001; In Final Form: December 11, 2001
7 Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and Rietveld analysis of X-ray diffraction data have been used to study chemically delithiated Li1-xCoO2. Samples of LiCoO2 were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li0.5CoO2 composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li0.5CoO2 displayed layered structure, whereas all LT materials and the 200 °C heated MT- Li0.5CoO2 displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li0.5CoO2 approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The 7Li MAS NMR results provide evidence for electronic phase segregation in layered and spinel Li1-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co3+ environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 °C and 800 °C gave rise to only one NMR peak, associated with a mixed-valence cobalt environment, and produced a low NMR signal intensity, arising from an environment containing localized t2g holes (Co4+ ions). A large NMR shift was observed for lithium in the mixed-valence environment, attributed to a Knight shift for the as-prepared HT-Li0.5CoO2 and to a hyperfine shift in the other samples. Variable-field studies showed homonuclear dipolar coupling to be the dominant source of residual line broadening in Li1-xCoO2, and chemical shift dispersion to be a probable secondary source.
Introduction Lithium intercalation materials have been studied intensively during the past decade for use as cathodes in lithium ion batteries, primarily for the popular application of portable electronic devices. Several types of spectroscopic techniques have been used to determine the structural and electronic properties which are crucial to the understanding of the materials’ performance as electrodes. Among these techniques, Li NMR is becoming increasingly popular in the study of electrode materials for lithium rechargeable batteries because it allows direct observation of lithium in the bulk of the material. It is thus complementary to other spectroscopic techniques which are sensitive to heavier elements or limited to surface studies. Li NMR has been successfully applied to the study of both diamagnetic materials such as LiCoO21,2 and paramagnetic materials such as LiMn2O4.3-5 Layered LiCoO2 has been the primary choice of cathode material for commercial lithium-ion batteries since LiCoO2 has the most favorable electrochemical properties for this application.6 While the layered rhombohedral form of LiCoO2, prepared at high temperature (∼900 °C), is a well-established electrode material, a novel low temperature (≈400 °C) form of LiCoO2 was found to have spinel-like cubic structure.7-10 The low temperature form, usually denoted as LT-LiCoO2, has been studied with X-ray, neutron, and electron diffraction techniques to determine exact structural information.7-10 These early studies * Corresponding author. Tel.: 1-510-642-8011. Fax: 1-510-642-4778. E-mail:
[email protected].
found that LT-LiCoO2 has a lithiated spinel structure with a minor layered-phase impurity, and that LT-Li0.5CoO2 prepared by acid delithiation has the normal spinel structure, denoted as (Li)8a[Co2]16dO4, containing lithium in the 8a tetrahedral sites and cobalt in the 16d octahedral sites. Several previous NMR studies of LiCoO2-related materials have focused on as-synthesized materials at high temperature (∼900 °C). In a study of the solid solution LiAlxCo1-xO2,11 all compositions showed single, narrow 7Li lines at 0 ppm, as expected from the diamagnetic electron configurations of Al3+ (t1u6) and low spin Co3+ (t2g6). The observed line width, however, went through a maximum near x ) 0.3, suggesting the greatest disorder in lithium nearest neighbor occupancy for that composition. A study of the solid solution LiNixCo1-xO212 found that substitution of the paramagnetic Ni3+ ion (t2g6 eg1) for a Co ion in the first coordination sphere of lithium resulted in a new NMR peak at about 111 ppm. This shift was ascribed to hyperfine coupling between the lithium nucleus and unpaired electron density in the lithium orbitals transferred from the nickel ion through the covalent Ni-O-Li bond. Substitution into the second coordination sphere of lithium produced a -15 ppm shift for each substituted Ni3+, consistent with antiparallel electron density transferred to the lithium s orbital, as predicted by Goodenough-Anderson superexchange rules.13 Hyperfine coupling is also responsible for the large shifts in the highly paramagnetic manganese spinels.3,4 The motion of Li+ defects in LiCoO2 was investigated by measuring the Li NMR spin-lattice relaxation time (T1) under static conditions.1 The temperature-dependence of T1 above
10.1021/jp0133541 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/23/2002
Chemically-Delithiated Li1-xCoO2 room temperature was well-modeled by the BloembergenPurcell-Pound theory of relaxation in which lithium motion in the layered planes gives rise to the dominant relaxation pathway. At room temperature, the spin-lattice relaxation rate was a rapid 20 s-1. At lower temperatures where hopping would not be an efficient relaxation mechanism, the authors suggested that dipolar contributions from paramagnetic impurities offer the dominant relaxation pathway. The relative contributions of quadrupolar and homonuclear dipolar interactions to the relaxation above room temperature were estimated and the quadrupolar contribution was found to dominate by a few orders of magnitude. Two notable studies on Li1-xCoO2 at various states of charge2,14 have provided further insight into the shift mechanisms in these materials, as well as the effects of the oxidation and reduction of the Co3+ ion. The NMR spectrum of Li1-xCoO2 was observed upon lithium deintercalation over the range 0.5 e (1 - x) e 1.0.14 The NMR spectra and electronic conductivity measurements showed a change from localized t2g holes (Co4+ ions) to delocalized holes upon deintercalation. Over the composition range 0.94 e (1 - x) e 1.0, the authors reported a single 7Li peak at 0 ppm, the intensity of which decreased significantly with deintercalation. Only 24% of the original NMR intensity was observed after 6% of the lithium was removed. This loss of intensity was attributed to strong hyperfine and dipolar interactions between localized t2g holes and the surrounding lithium nuclei. Upon further deintercalation of lithium, the expected Li NMR intensity was recovered, following delocalization of the t2g holes in the composition range 0.5 e (1 - x) e 0.75. The authors attributed the shift of about 100 ppm for Li0.5CoO2 to a Knight shift arising from participation of the Li orbitals in global metallic conduction. Similar studies were carried out on lithium-rich Li1+xCoO2 (x > 0).2 The peaks shifted to -8 and -40 ppm, providing indirect evidence of localized Co2+ (t2g6 eg1) arising from defects in the structure. While rhombohedral layered LiCoO2 has been well-studied with Li NMR, we are unaware of any NMR studies of cubic LT-LiCoO2. In this work, 7Li MAS NMR is used to study the local environment of lithium in Li0.5CoO2 prepared by chemical extraction of lithium from LiCoO2 that was synthesized at various temperatures (400, 600, and 800 °C). The chemical shift, line width, and quantitative intensity of the 7Li NMR peaks are discussed in terms of their implications for the electronic and atomic structures in these materials. Complementary bulk structural information is determined by Rietveld refinement of the X-ray diffraction data. Experimental Techniques Three samples of LiCoO2 were synthesized by firing required amounts of intimately mixed Li2CO3 and Co3O4 at three different temperatures, T ) 400, 600, and 800 °C in air (solidstate reactions). These are designated as LT-LiCoO2, MTLiCoO2, and HT-LiCoO2, respectively. Li0.5CoO2 samples were prepared by chemically extracting lithium from each LiCoO2 sample. The extraction was performed by stirring the respective LiCoO2 powder with an aqueous solution consisting of optimized amounts of an oxidizing agent, Na2S2O8, for 2 days. During this process, the following reaction occurred:
2LiCoO2 + xNa2S2O8 f 2Li1-xCoO2 + xNa2SO4 + xLi2SO4 The product formed was filtered, washed repeatedly first with water and finally with acetone, and air-dried. The samples were
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3843
Figure 1. X-ray diffraction patterns of LiCoO2: (a) HT-LiCoO2 (layered), (b) MT-LiCoO2 (layered), (c) LT-LiCoO2 (spinel).
then characterized by X-ray powder diffraction. Structural information and lattice parameters were obtained by a Rietveld refinement of the X-ray data with a fitting of atomic positions with the DBWS-9411 PC program.15 Lithium contents were determined by atomic adsorption spectroscopy. Oxygen contents were determined by a redox (iodometric) titration as described below.16 About 20 mg of sample was dissolved in a mixture of 15 mL of 10% KI and 10 mL of 3.5 N HCl. During this process, all Co(2+x)+ is reduced to Co2+ and iodine is liberated according to the following reaction:
2xI- + 2Co(2+x)+ f 2Co2+ + xI2 The liberated I2 was then titrated against a 0.03 N Na2S2O3 (sodium thiosulfate) solution using starch as an indicator. During this process, the following reaction occurs:
I2 + 2S2O32- f 2I- + S4O62From the titer value, the oxidation state of cobalt and hence the oxygen content of the sample were obtained using the charge neutrality principle. NMR experiments were performed on a Bruker AMX spectrometer, with a 7 mm MAS probe (Doty Scientific) tuned to 7Li frequency of 38.9 MHz and a home-built spectrometer with the same probe tuned to 7Li frequency of 70.3 MHz. All shifts were referenced to 1 M LiCl(aq). A 90°-τ-180° (90° ) 1-2 µs) pulse sequence with τ ) (spinning frequency)-1 was used to obtain the spectra. Spinning frequencies were 11 ( 0.1 kHz. A recycle delay of 2 s was used to avoid saturation of the signal. Isotropic peaks were identified by varying the spinning speed. All samples were diluted with NaCl to roughly 30 wt % sample to minimize the effect of sample magnetism on probe tuning and reduce the bulk conductivity of the sample. Results and Discussion Structural Analysis and NMR Shifts. Figure 1 shows the X-ray diffraction patterns of the three as-prepared LiCoO2 samples (HT-, MT-, and LT-). Both the HT-LiCoO2 and MTLiCoO2 samples have the rhombohedral layer structure, while the LT-LiCoO2 sample has the lithiated cubic spinel structure as has been previously reported in the literature.7-10 Figure 2
3844 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Tucker et al. TABLE 1: Oxidation State Analysis Data average oxidation state of cobalt Li0.5CoO2
synthesis temperature
LiCoO2
as-prepared
200 °C heated
800 °C 600 °C 400 °C
3.00 3.02 3.03
3.24 3.28 3.38
3.00 3.19 3.28
TABLE 2: Comparison of Total Integrated Intensities of the 7Li MAS NMR Spectra Acquired at 38.9 MHz
Figure 2. X-ray diffraction patterns of Li0.5CoO2: (a) as-prepared HTLi0.5CoO2 (layered), (b) HT-Li0.5CoO2 after heating at 200 °C (layered), (c) as-prepared MT-Li0.5CoO2 (layered), (d) MT-Li0.5CoO2 after heating at 200 °C (spinel), (e) as-prepared LT-Li0.5CoO2 (spinel), and (f) LTLi0.5CoO2 after heating at 200 °C (spinel).
R
sample
integrated 7Li NMR intensitya
HT-LiCoO2 MT-LiCoO2 LT-LiCoO2 as-prepared HT-Li0.5CoO2 200 °C heated HT-Li0.5CoO2 as-prepared MT-Li0.5CoO2 200 °C heated MT-Li0.5CoO2 as-prepared LT-Li0.5CoO2 200 °C heated LT-Li0.5CoO2
2.0 ( 0.1 1.98 1.88 0.61 0.94 0.57 1.0 0.95 0.96
Normalized to MT-Li0.5CoO2 annealed at 200 °C.
Figure 4. TGA plot of LT-Li0.5CoO2 spinel recorded in air at a heating rate of 2 °C/min.
Figure 3. Rietveld refinement results of Li0.5CoO2: (a) as-prepared LT-Li0.5CoO2, (b) sample (a) after heating at 200 °C, and (c) MT-Li0.5CoO2 after heating at 200 °C. The observed and calculated X-ray profiles, peak positions, the difference between the observed and calculated profiles, and lattice parameters are shown.
shows the X-ray diffraction patterns of the Li0.5CoO2 samples that were obtained from the HT-, MT-, and LT-LiCoO2 samples. The HT-Li0.5CoO2 sample maintains the initial rhombohedral layer structure even after heating at 200 °C as indicated by a splitting of the (018) and (110) reflections in Figure 2a and b. The as-prepared MT-Li0.5CoO2 sample also has the rhombohedral symmetry, but with a smaller c/a ratio as indicated by a smaller separation between the (018) and (110) reflections, suggesting that it has some disordering of the lithium and cobalt ions. The MT-Li0.5CoO2 sample, however, transforms to a cubic spinel-like phase on heating at 200 °C as indicated by a merging of the (018) and (110) reflections of the initial rhombohedral structure into a single (440) reflection of the cubic spinel structure (Figure 2c and d). On the other hand, both the asprepared and 200 °C heated LT-Li0.5CoO2 have the cubic-spinellike structure like the 200 °C heated MT-Li0.5CoO2 sample. The formation of spinel-like phase is also confirmed by a Rietveld refinement of the X-ray diffraction data (Figure 3). The
formation of spinel phase for the LT-Li0.5CoO2 is in agreement with that found before by Thackeray’s group.7-10 Table 1 gives the average oxidation state of cobalt determined by the iodometric titration of various samples. While all the three parent LiCoO2 samples have an average oxidation state of around 3+, the lithium-extracted samples (as-prepared) have an oxidation state of >3+ as one would expect. Although the samples obtained after lithium extraction are expressed as Li0.5CoO2 throughout the text and in Tables 1 and 2, their actual compositions obtained from atomic absorption spectroscopy and iodometric titration are as follows: Li0.53CoO1.89, Li0.48CoO1.88, and Li0.53CoO1.96 for the as-prepared HT-, MT-, and LT- Li0.5CoO2, respectively, and Li0.53CoO1.77, Li0.48CoO1.84, and Li0.53CoO1.91 for the 200 °C heated HT-, MT-, and LT- Li0.5CoO2, respectively. The Li0.5CoO2 samples lose oxygen on heating to higher temperatures as indicated by the TGA data in Figure 4 and disproportionate to LiCoO2 and Co3O4 at T g 300 °C as indicated by the X-ray data in Figure 5. The Li NMR spectra of the LiCoO2 samples, shown in Figures 6 and 7, are characterized by a single peak at 0 ppm with small spinning sidebands indicative of lithium in a diamagnetic environment as expected from the Co3+:t2g6eg0 electronic configuration. These results are in agreement with the previous NMR studies of LiCoO2.12,17 Upon removal of 50% of the lithium from the HT-LiCoO2 and concomitant oxidation of some Co3+ (t2g6) to Co4+ (t2g5), significant changes in the
Chemically-Delithiated Li1-xCoO2
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3845
Figure 8. 7Li MAS NMR isotropic peaks of HT-Li0.5CoO2: asprepared (thick line) and 200 °C heated (thin line) samples.
Figure 5. X-ray diffraction patterns of (a) as-prepared LT-Li0.53CoO1.96 and after heating it at (b) 150 °C, (c) 200 °C, (d) 300 °C, (e) 400 °C, and (f) 800 °C.
Figure 9. 7Li MAS NMR isotropic peaks of MT-Li0.5CoO2: asprepared (thick line) and 200 °C heated (thin line) samples.
Figure 6. 7Li MAS NMR spectrum of HT-LiCoO2 synthesized at 800 °C. Isotropic peak is marked; all other peaks are spinning sidebands.
Figure 7. Comparison of the 7Li MAS NMR isotropic peaks of LiCoO2 synthesized at 800 °C (solid thick line), 600 °C (dashed thick line), and 400 °C (solid thin line).
NMR spectrum are observed, as shown in Figure 8. The peak at 0 ppm disappears and new peaks appear at approximately 30 and 80 ppm. The breadth and position of the peak at 80 ppm are in qualitative agreement with that reported previously for Li0.5CoO2.14 In that study, Li0.5CoO2 was found to be a metallic conductor due to the holes introduced into the t2g band by the Co4+ ions, supporting the assignment of the large NMR shift to a Knight shift mechanism, whereby the Li orbitals participate in global conduction and the conduction electrons scatter off the Li nucleus. The additional small peak at 30 ppm observed in this and all other Li0.5CoO2 samples studied here probably arises from lithium in the vicinity of an unspecified structural defect, as each sample was determined to be singlephase by XRD. On heating the HT-Li0.5CoO2 to 200 °C, the peak at 80 ppm disappears and a new peak at -4 ppm appears. This can be explained by the decrease in the oxidation state from 3.24+ to 3+ due to the reduction of Co4+ during the 200
°C heating step (Table 1) accompanied by a metal-to-insulator transition. However, the peak at 30 ppm remains indicating that not all of the Co4+ is reduced. The small negative shift of the main peak may suggest the presence of a small amount of Co2+ in the material; the oxidation state analysis indicates an average oxidation state of 3+, which would require equal amounts of Co4+ and Co2+ defects. Similar behavior is observed upon heating MT-Li0.5CoO2 to 200 °C as seen in Figure 9 and Table 1. The as-prepared sample gives rise to overlapping broad and narrow peaks with a maximum intensity at 63 ppm. This shift is considerably lower than the 80 ppm Knight shift observed for the as-prepared HTLi0.5CoO2 even though the average oxidation state of cobalt is higher in MT-Li0.5CoO2. This is the opposite trend from that observed in ref 14, where the magnitude of the Knight shift was seen to increase with increasing Co oxidation state. This implies that a different shift mechanism may dominate in the materials prepared at lower temperatures. A likely alternative is a hyperfine shift arising from unpaired Co:t2g electrons similar to that observed in LiNiyCo1-yO211 and LiMn2O4 spinel.3-5 Variable-temperature NMR experiments would be able to determine the relative contribution from Knight and hyperfine coupling shift mechanisms as the Knight shift is independent of temperature whereas the hyperfine shift scales as the bulk magnetization, which is generally quite temperature dependent. Such a technique has been used to attribute the large shifts in LiMn2O4-based spinels to hyperfine coupling between the lithium nucleus and unpaired Mn:3d electrons, as the magnitude of the NMR shift followed the Curie-Weiss temperature dependence of the bulk magnetization of the sample.3 Upon heating to 200 °C, the broad component of the spectrum of MT-Li0.5CoO2 is removed and the peak at around 65 ppm becomes narrower (Figure 9); additionally, a new peak appears at 0 ppm. Again, this suggests reduction of some of the Co4+ to Co3+ consistent with the change in the average oxidation state of cobalt from 3.28+ to 3.19+ in Table 1. The presence
3846 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Figure 10. 7Li MAS NMR isotropic peaks of Li0.5CoO2: as-prepared LT-Li0.5CoO2 (solid thick line), LT- Li0.5CoO2 after heating at 200 °C (solid thin line), and MT- Li0.5CoO2 after heating at 200 °C (dashed thick line).
of two peaks in the 200 °C heated sample implies phase separation in this material. Rietveld refinement of the XRD peaks of the 200 °C heated MT-Li0.5CoO2 shows the material to be single-phase spinel (Figure 3c), however, so the phase separation is most likely of an electronic nature. A phase separation could, for example, arise from nonuniformities in the lithium content at the microstructural level, although it seems unlikely that the 200 °C heating would result in a less uniform lithium content. We surmise that the two distinct lithium sites are distinguished by their local electronic structure: one site in which the lithium is surrounded by only Co3+ neighbors giving rise to the peak at 0 ppm, and another site in which the lithium is surrounded by cobalt neighbors that participate in a t2g hole hopping process. The shape of the peak at 65 ppm seems to be characteristic of the spinel structure, as a similar peak shape is observed for the other spinel-phase samples, discussed below. The shoulder to lower shift probably arises from disorder in the second coordination shell of the lithium, perhaps in the form of oxygen vacancy or cobalt substitution for lithium, both of which would lead to a lower local average cobalt oxidation state and thus a lower shift for lithium in the vicinity of the defect. While the XRD results show the as-prepared MT-Li0.5CoO2 to be a layered phase with some cobalt in the lithium plane, the NMR results suggest that the local atomic and electronic structure approach that of the LT phase, as shown below. Structural refinement of the as-prepared and 200 °C heated LT- Li0.5CoO2 (Figure 3a and b) indicates they have the normal spinel structure, in agreement with previous studies.7-10 The characteristic NMR peak at 65 ppm for spinel, which is observed in the case of MT-Li0.5CoO2, as well as the peaks at 30 and 0 ppm are also observed for the LT-Li0.5CoO2 samples (Figure 10). The average oxidation state of cobalt decreases on heating the LT-Li0.5CoO2 at 200 °C (Table 1), which results in a transfer of the NMR intensity from the spinel peak at 65 ppm, characteristic of a mixed valence state, to the peak at 0 ppm, characteristic of a Co3+ environment (Figure 10). The relative intensity of the spinel peak clearly follows the bulk average oxidation state of cobalt in the sample, indicating that the presence of spinel phase is possible for a wide range of average cobalt oxidation state, and two electronic phases exist within the bulk material. The low intensity of the diamagnetic peak at 0 ppm for the as-prepared LT-Li0.5CoO2 indicates that the average oxidation state of cobalt in the mixed-valence phase is near the bulk average of 3.38+ measured for that sample (Table 1). The relative extent of these phases can, therefore, be tuned by the preparation conditions. Relative Intensities of the NMR Spectra. NMR is an inherently quantitative technique. When the spectral intensities obtained for a sample deviate from that expected on the basis
Tucker et al. of its chemical composition, it is often the result of interesting structural features. For instance, lines arising from largequadrupole-moment nuclei in highly asymmetric environments are often broadened so much as to be virtually unobservable. Interactions between nuclei and unpaired electrons can also lead to loss of observability of the affected nuclei. In a 51V NMR study of V2O5,18 a substantial reduction in the intensity of the spectra was observed after ball milling of the sample. The authors attributed this loss of intensity to extreme broadening due to localized electrons on paramagnetic V3+ sites, which were created during the milling process. They estimated that no vanadium ion within 10 Å of a V3+ would be observable. A similar argument was used to explain the loss of lithium NMR intensity upon deintercalation of lithium from LiCoO2.14 On the basis of these results, we would expect to see a loss of lithium NMR intensity in the present samples if they contain localized paramagnetic Co4+ or Co2+ ions. The integrated intensities of the present NMR spectra, including spinning sidebands, are shown in Table 2. Most samples show the expected intensities within the limits of the technique. This is true even for many Li0.5CoO2 samples, which contain Co4+ ions, suggesting that the t2g holes are delocalized to some extent. The LT-LiCoO2 sample fired at 400 °C shows slightly less intensity than expected, suggesting the presence of localized paramagnetic Co4+ or Co2+. Rietveld refinement of the XRD data showed that the LT-LiCoO2 has some cobalt ions in the lithium plane. It is plausible that the cobalt substituting for lithium may have a reduced oxidation state such as Co2+. Furthermore, the geometric configuration of cobalt in the lithium plane would allow direct overlap of the Co:eg and Li:s orbitals leading to a very strong coupling between the lithium nucleus and unpaired electrons. Both the as-prepared MT-Li0.5CoO2 and HT-Li0.5CoO2 also show significantly less intensity than expected. Due to the high average oxidation state of cobalt in these samples, the lost intensity is likely to be due to localized Co4+ ions. The large shift for the observed NMR peak for both these samples indicates that there are delocalized t2g holes as well. Thus the band structure may not be uniform throughout the samples, providing supporting evidence for the coexistence of multiple electronic phases. It should be noted that HT-Li0.5CoO2 and possibly MT-Li0.5CoO2 are expected to display metallic conduction.14 Possible alternative explanations for the loss of the NMR intensity could be the incomplete penetration of the radio frequency (RF) radiation into the particles due to skin effects or loss of RF power due to eddy currents in the bulk sample.19 However, results for nutation of the lithium nucleus in as-prepared HT-Li0.5CoO2, annealed MTLi0.5CoO2, and the aqueous LiCl reference sample were identical, making spectral artifacts arising from the conductive nature of the samples highly unlikely. Linebroadening in Li1-xCoO2. The line width of the dominant NMR peaks for the Li0.5CoO2 samples was measured (in Hz) at two fields and found to be very nearly independent of field for all of the peaks. This suggests that dipolar broadening is the dominant contributor to the residual MAS line width in these materials. Dipolar broadening arises from coupling of the 7Li magnetic moments to6Li, 59Co, other 7Li nuclei, or unpaired electrons. Coupling to 6Li and 59Co can be ruled out due to their low gyromagnetic ratio, as the magnitude of the interaction depends linearly on the gyromagnetic ratios of the coupled species. Electron-nuclear coupling is an unlikely contributor because the interaction between 7Li and delocalized electrons should be completely averaged by magic angle spinning, and significant line widths are observed for the peaks at 0 ppm where
Chemically-Delithiated Li1-xCoO2 hyperfine 7Li-electron coupling is nonexistent. Thus, homonuclear dipolar coupling is the probable source of linebroadening. Furthermore, the line width measured at 38.9 MHz for the as-prepared HT-Li0.5CoO2 decreased with spinning speed over the range of 8-12 kHz indicating that the dipolar interaction is strong and partly homogeneous and therefore not completely removed by magic angle spinning at reasonable spinning speeds.20 This is not surprising, given that each 7Li nucleus is coupled to several other 7Li nuclei. Similar results have been observed for 7Li in lithium-intercalated SnS2.21 The line widths for the LiCoO2 samples were only measured at 38.9 MHz. They display increasing line width with decreasing firing temperature as seen in Figure 7 suggesting chemical dispersion may be an important secondary contributor to linebroadening. A similar trend was observed previously,17 and the increased line width was attributed to structural disorder in the second coordination sphere of the lithium ions. Previous results for LiAlyCo1-yO211 also support chemical dispersion as an important secondary contributor to the residual MAS line width. Conclusions Lithium has been chemically extracted from LiCoO2 samples synthesized at various temperatures to obtain Li0.5CoO2; the samples have been characterized by both X-ray diffraction and 7Li MAS NMR. It is found that the initial preparation temperature of LiCoO2 has a profound effect on the macroscopic and microscopic structures. LT-LiCoO2 prepared at 400 °C has a lithiated cubic spinel structure, which upon extraction of 50% of the lithium gives the normal cubic spinel phase (Li)8a[Co2]16dO4, as indicated by Rietveld analysis of the X-ray data. On the other hand, HT-Li0.5CoO2 obtained by extracting lithium from the HT-LiCoO2 prepared at 800 °C maintains the initial rhombohedral layer structure of HT-LiCoO2 even after heating at 200 °C. MT-Li0.5CoO2 obtained by extracting lithium from the MT-LiCoO2 prepared at 600 °C similarly maintains the initial layered structure, but it transforms to the spinel phase upon heating at 200 °C. 7Li MAS NMR results also provide indirect evidence for the formation of spinel phase by showing characteristic peak positions and shapes in the case of LT- and MT-Li0.5CoO2 samples. The NMR results additionally suggest that the local atomic and electronic structures of the as-prepared MT-Li0.5CoO2 approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The 7Li MAS NMR results also provide evidence for electronic phase segregation, which could not be detected by X-ray diffraction, in layered and spinel Li1-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co3+ environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 °C and 800 °C displayed only one NMR peak, associated with a mixed-valence cobalt environment, and produced a low NMR signal intensity, arising from an environment containing localized t2g holes (Co4+ ions).
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3847 A large NMR shift was observed for lithium in the mixedvalence environment, attributed to a Knight shift for the asprepared HT-Li0.5CoO2 and to a hyperfine shift in the other samples. Variable-field studies showed homonuclear dipolar coupling to be the dominant source of residual linebroadening in Li1-xCoO2, and chemical shift dispersion to be a probable secondary source. Acknowledgment. The work at UC-Berkeley was supported by the National Science Foundation through a fellowship for M.C.T. and by the Director, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy, under Contract DE-AC03-76SF00098. The work at UT-Austin was supported by the Welch Foundation Grant F-1254, Texas Advanced Technology Program Grant 003658-0488-1999, and Center for Space Power at the Texas A&M University (a NASA Commercial Space Center). References and Notes (1) Nakamura, K.; Yamamoto, M.; Okamura, K.; Michihiro, Y.; Nakabayashi, I.; Kanashiro, T. Solid State Ionics 1999, 121, 301-306. (2) Levasseur, S.; Menetrier, M.; Suard, E.; Delmas, C. Solid State Ionics 2000, 128, 11-24. (3) Gee, B.; Horne, C. R.; Cairns, E. J.; Reimer J. A. J. Phys. Chem. B 1998, 102, 10142-10149. (4) Lee, Y. J.; Wang, F.; Grey, C. P. J. Am. Chem. Soc. 1998, 120, 12601-12613. (5) Tucker, M. C.; Reimer, J. A.; Cairns, E. J. J. Electrochem. Soc. 2001, 148, A951-A959. (6) Mizushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783. (7) Gummow, R. J.; Thackeray, M. M.; David, W. I. F.; Hull, S. Mater. Res. Bull. 1992, 27, 327. (8) Gummow, R. J.; Liles, D. C.; Thackeray, M. M. Mater. Res. Bull. 1993, 28, 235. (9) Gummow, R. J.; Liles, D. C.; Thackeray, M. M. Mater. Res. Bull. 1993, 28, 1177. (10) Shao-Horn, Y.; Hackney, S. A.; Johnson, C. S.; Kahaian, A. J.; Thackeray, M. M. J. Solid State Chem. 1998, 140, 116. (11) Alcantara, R.; Lavela, P.; Relano, P. L.; Tirado, J. L.; Zhecheva, E.; Stoyanova, R. Inorg. Chem. 1998, 37, 264-269. (12) Marichal, C.; Hirschinger, J.; Granger, P.; Menetrier, M.; Rougier, A.; Delmas, C. Inorg. Chem. 1995, 34, 1773-1778. (13) Goodenough, J. B. Magnetism and the Chemical Bond; Wiley & Sons: New York, 1963. (14) Menetrier, M.; Saadoune, I.; Levasseur, S.; Delmas, C. J. Mater. Chem. 1999, 9, 1135-1140. (15) Young, R. A.; Shakthivel, A.; Moss, T. S.; Paiva Santos, C. O. J. Appl. Crystallogr. 1995, 28, 366. (16) Manthiram, A.; Swinnea, S.; Sui, Z. T.; Steinfink, H.; Goodenough, J. B. J. Am. Chem. Soc. 1987, 109, 6667. (17) Garcia, B.; Barboux, P.; Ribot, F.; Kahn-Harari, A.; Mazerolles, L.; Baffier, N. Solid State Ionics 1995, 80, 111-118. (18) Shubin, A. A.; Lapina, O. B.; Bosch, E.; Spengler, J.; Knozinger, H. J. Phys. Chem. B 1999, 103, 3138-3144. (19) Yahnke, M. The Application of Solid-State NMR Spectroscopy to Electrochemical Systems: CO Adsorption on PT Electrocatalysts at the Aqueous-Electrode Interface; M.S. Thesis, University of California, Berkeley, Berkeley, CA, 1996. (20) Modern NMR Techniques and their Application in Chemistry; Popov, A. I., Hallenga, K., Eds.; Practical Spectroscopy, Brame, E. G., Ed.; Marcel Dekker: New York, 1991; Chapter 3. (21) Pietrass, T.; Taulelle, F.; Lavela, P.; Olivier-Fourcade, J.; Jumas, J.-C.; Steuernagel, S. J. Phys. Chem. B 1997, 101, 6715-6723.
