Article pubs.acs.org/cm
Effect of Cooling Rates on Phase Separation in 0.5Li2MnO3·0.5LiCoO2 Electrode Materials for Li-Ion Batteries Brandon R. Long,† Jason R. Croy,*,† Fulya Dogan,† Matthew R. Suchomel,‡ Baris Key,† Jianguo Wen,§ Dean J. Miller,§ Michael M. Thackeray,† and Mahalingam Balasubramanian*,‡ †
Chemical Sciences and Engineering Division, ‡X-ray Science Division, Advanced Photon Source, and §Electron Microscopy Center, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: The results of a detailed structural investigation on the influence of cooling rates in the synthesis of lithium- and manganese-rich 0.5Li2MnO3· 0.5LiCoO2 composite electrode materials, which are of interest for Li-ion battery applications, are presented. It is shown that a low-temperature, intermediate firing step, often employed in cathode synthesis, yields a minor secondary component representing a polydisperse distribution of lattice parameters, not found in the absence of low-temperature treatments. However, regardless of the heating and cooling conditions employed, all samples present two distinctly different local environments as evidenced by X-ray absorption fine structure spectroscopy (XAFS) and nuclear magnetic resonance (NMR) analysis. Transmission electron microscopy (TEM) data show discrete domain structures that are consistent with the XAFS and NMR findings. Furthermore, high resolution synchrotron X-ray diffraction (HR-XRD), as well as the XAFS and NMR data show no discernible differences between sample sets heated in similar fashion and subsequently cooled at different rates. The results contradict recent reports, using X-ray diffraction, that rapidly quenched samples of the same composition are true solid solutions. This apparent discrepancy is assigned, in part, to the inherent nature of conventional diffraction, which firmly elucidates the average long-range structure but does not capture the local domain microstructure of these nanocomposite materials. The combined use of HR-XRD, XAFS, NMR, and TEM data indicate that charge ordering, which is initiated at relatively low temperatures, is the dominant force that produces a nanoscale, inhomogeneous composite structure, irrespective of the cooling rate.
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INTRODUCTION Layered lithium- and manganese-rich (LMR) systems, Li1+xMnyTMzO2 (TM = Ni, Co, Fe, Mn, Cr, or some combination thereof, and x + y + z = 1), are of significant interest due to their potential use as next generation cathode materials in Li-ion batteries.1 Despite over 15 years of extensive research, the basic structures of these materials continue to be studied, and literature presents a debate as to whether these systems should be classified as having composite structures or better described as solid solutions. Specifically, the debate is centered on the following criteria: Are LMR systems composed of nanodomains (as in a composite structure) of Li2MnO3 and LiMO2, or can the material be described inarguably as a single phase? The compatibility of the Li2MnO3 (monoclinic C2/m) and the LiMO2 (trigonal R3m ̅ ) structures arises from the fact that the interlayer spacing of the close-packed layers is ∼4.7 Å for both structures.2 Therefore, an intimate, atomic level integration is possible. X-ray powder diffraction has long played a vital role in elucidating the structure of battery materials and in helping to develop structure−activity relationships of battery cells.3−5 However, in these systems XRD data are often difficult to interpret uniquely because all peaks in the R3̅m structure can also be indexed to the C2/m structure. However, reflections that appear in the ∼20−23° 2θ range (Cu Kα) can only be indexed to the monoclinic structure and are a signature of © XXXX American Chemical Society
lithium and transition metal (TM) ordering which produces, for example, the LiMn6 units in the TM layers of Li2MnO3.2,6 To get a complete picture, XRD can be complemented with other techniques, especially those that have specific sensitivity to local order. K-edge X-ray absorption fine structure spectroscopy (XAFS)7−9 and Li magic-angle-spinning nuclear magnetic resonance (MAS NMR),10−13 which probe the local atomic structures with element-specificity (TM and Li local environments by XAFS and Li NMR, respectively), show that when the TM is trivalent, such as Cr3+ or Co3+, the system exhibits nanoscale phase separation, with local regions enriched either with Mn4+ or TM3+.14−17 Furthermore, NMR studies show that the excess-lithium ions occupy sites in the TM layers with a strong preference for a local LiMn6 coordination.18,19 The presence of such an environment manifests itself in Mn XAFS data, primarily as a reduction of the Mn-TM coordination number (lithium is a very weak scatterer and is largely invisible as TM-Li two-body correlation in the XAFS data of these oxides); the spectra are thus consistent with a tendency for the excess lithium to cluster preferentially around Mn4+ ions.16,20 Coincidentally, the LiMn6 environment is Received: April 7, 2014 Revised: May 12, 2014
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the prepared materials: (1) the entire line joining the endmember phases is a true solid solution and therefore a homogeneous single phase, when samples are quenched from 800−900 °C, (2) when slow cooled deliberately to room temperature, the samples phase separate to two distinct layered phases, and (3) for regular or intermediate cooling, nanodomains in the 2−10 nm length scale lying on the same lattice are formed; this nanoscale phase separation is due to the lack of time during cooling required to form large scale crystallites of each phase. Similar findings were also reported for nickelcontaining composite materials.35 However, an earlier study on quenching lithium-rich, Li[NixLi1/3−2x/3Mn2/3−x/3]O2 materials found no differences in the XRD data when compared to those of samples which were slow cooled.36 Furthermore, peak broadening sometimes observed in the XRD data has been proposed by McCalla et al.34,35 to be a result, and therefore evidence, of phase separation under certain cooling conditions. Wang et al. have also reported that shoulders in the XRD data of lithium-rich compositions can be used as an indication of phase separation.37 However, the authors propose that lithium loss at high temperatures results in elemental inhomogeneity (e.g., nickel-rich R3̅m regions) and drives phase separation. The authors also note that cooling rates had no effect on the electrochemistry of the final products. The above studies suggest that differing heating/cooling conditions could account for the discrepancies reported in the literatures on the topic of nanoscale inhomogeneity in lithium-rich materials. In this article, we show that regardless of the cooling rate employed, the local structure of these materials is not greatly affected. This finding is significant because the local structure dictates the overall electrochemical properties and related degradation mechanisms (e.g., voltage fade) in this class of cathodes.20,38 Therefore, synthesis strategies which do not address these materials on a local scale will likely fail to produce significant advancements in electrode stability, as evidenced in the vast amount of literature on the synthesis of lithium and manganese-rich cathode materials.37,39−41 XAFS, NMR, HRXRD, and TEM have been used to obtain an integrated picture of the transition metal and lithium environments present in 0.5Li2MnO3·0.5LiCoO2 (Li1.2Co0.4Mn0.4O2) samples for various heating and cooling conditions. This particular composition was selected because any potential structural difference between quenched and slow-cooled samples should be plainly evident, as espoused by the studies of McCalla et al.34,35
similar to that of lithium in the mixed TM-lithium plane of Li2MnO3, wherein the Li+ and Mn4+ ions charge order to form a 31/2 × 31/2 sublattice.21 The nanoscale phase separation also reveals itself in transmission electron microscopy (TEM) as local regions with different space groups, R3m ̅ or C2/m, archetypal of layered LiMO2 and Li2MnO3, respectively.2 Scanning transmission electron microscopy (STEM) images show the coexistence of regions with and without lithium ordering; the lithium-ordered regions have a dot contrast pattern akin to those found for Li2MnO3.22−25 Although STEM sheds light on the potential ordering, metal neighbors of similar atomic numbers cannot be differentiated in the images; therefore, the associated partitioning of the sample into Mnrich and TM-rich regions cannot be directly ascertained. In addition, in other electron microscopic methods that are sensitive to the specific TM ions, such as electron energy loss (EELS) or energy dispersive (EDS) spectroscopies, no regions enriched in either Mn or other TM ions are seen within the probed sample volume (for either Co- or Ni-containing systems).24,25 Using atomistic modeling that explicitly invokes consistency with XAFS and STEM findings, the absence of this enrichment has been rationalized to occur due to volume averaging inherent to such EELS measurements.25 In a different EELS study of an Fe-containing system, coexistence of Mn4+and Fe3+-rich regions that is consistent with nanoscale inhomogeneities has been reported.26 Ni-containing systems exhibit added complexity due to the interplay of several competing interactions, such as Li+-Mn4+ and Ni2+-Mn4+ charge ordering, the presence of nickel antisite defects in the lithium plane, and configurational entropy.27 Addition of cobalt increases the complexity even further, as Co3+ is expected to partially disrupt the Ni2+-Mn4+ charge ordering.21,28 Nevertheless, these Ni-containing systems also present local LiMn6 environments and potential nanoscale inhomogeneity, as evidenced by NMR, XAFS, and electron microscopy observations.19,23,28 Conversely, other studies purportedly find no evidence for nanoscale phase separation, and it is argued that these layered lithium-rich systems are examples of true solid solutions of the end-members, Li2MnO3 and LiMO2, and as such, should be strictly regarded as homogeneous, single-phase compounds. In this regard, the original studies of Lu et al. (using XRD), Jarvis et al. (using TEM), and Koga et al. (using XRD, neutron diffraction (ND), NMR, Raman, and TEM) stand out.29−32 In fact, close inspection of the XRD data of Jarvis et al. reveals the presence of clear shoulders on the low-energy side of several main peaks, indicating the presence of an additional secondary component even in the average structure length scale that is probed by XRD.29 Surprisingly, at variance with the XRD data, analysis by various electron microscopy techniques has been interpreted to show the presence of a single phase compound.29 In sharp contrast, an earlier study by Armstrong et al. found that the same composition investigated by Jarvis et al. cannot be described as a single phase.33 Recently, McCalla et al. have performed conventional XRD experiments to map out the phase diagram of Li−Ni−Mn-O and Li−Co−Mn-O systems; most importantly, these studies appear to provide key evidence to bridge the gap in understanding that exists on the structures of these materials.34,35 A salient point they make is that phase separation and the formation of nanodomains is a direct consequence of the cooling rate employed. Specifically, they have studied the synthesis of xLi2MnO3·(1−x)LiCoO2, and they proclaim three distinct scenarios regarding the structure of
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EXPERIMENTAL SECTION
(MnCo)C2O4 precursors were prepared from CoSO4·7H2O, MnSO4· H2O, and Na2C2O4. An aqueous solution containing the required stoichiometric amounts of metal sulfates was added, while stirring, to a solution of sodium oxalate. The solution was then stirred for 3 h at 70 °C. The coprecipitated powder was filtered, washed, and dried in air at 105 °C. A portion of the dried powders was thoroughly mixed with stoichiometric amounts of lithium carbonate and preheat treated at 550 °C (∼2 °C/min ramp) for 12 h in air followed by uncontrolled cooling and subsequent grinding. This baseline material was then used in three separate firing experiments. For all three experiments, ∼2 g batches of the baseline were first fired to 850 °C (∼2 °C/min) for 12 h. Subsequently, the samples were (1) allowed to cool in the furnace uncontrolled, taking ∼6.5 h to reach 100 °C (labeled 550/ 850[Uncontrolled]); (2) cooled at a rate of ∼1 °C/min, taking ∼14.5 h to reach 100 °C (labeled 550/850[SlowCool]); or (3) quenched between two metal plates cooled beforehand in liquid nitrogen, taking less than 30 s to reach temperatures below 0 °C (labeled 550/850[Quench]). The temperature of the quenched sample was monitored with an infrared temperature sensor. A lowB
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temperature, intermediate firing is often employed in the synthesis of cathode materials.31,42−44 In order to examine the effect of this step on the final products, a parallel experiment was carried out on a separate portion of the same precursors as just described but omitting the intermediate 550 °C, 12 h firing step, followed by quenching or slow cooling (labeled as 850[Quench] and 850[SlowCool] respectively). Inductively coupled plasma mass spectrometry (ICP) measurements were carried out on the first set of samples for elemental analysis. High-resolution (ΔQ/Q ≈ 2 × 10−4) synchrotron X-ray powder diffraction data (HR-XRD) were collected using beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory. Scans were collected in transmission mode on spinning Kapton capillaries using a fixed wavelength of 0.413893 Å. For XAFS studies, the as-prepared powders were diluted with cellulose and pressed into pellets. Measurements were conducted at beamline 20-BM of the APS. The incident beam was monochromatized using a Si(111) fixed-exit, double-crystal monochromator. Manganese and cobalt foils were used for energy calibration with the zero energy (E0) defined according to Kraft et al.45 Spectra were acquired in transmission mode utilizing gas ionization chambers as detectors. XANES and EXAFS data were extracted with established methods using the ATHENA software package.46 The normalized EXAFS data were converted from energy to k-space and weighted by k3. These data were then Fourier transformed to R-space and left uncorrected for photoelectron phase shifts. As such, distances in Rspace are ∼0.4−0.5 Å shorter than actual bond distances. 6 Li MAS NMR experiments were performed at 7.02 T (300 MHz) on a Bruker Avance III HD spectrometer operating at a Larmor frequency of 44.21 MHz, using a 1.3 mm MAS probe. All spectra were acquired at 67 kHz with a rotor synchronized echo pulse sequence (90°τ180°τacq), where τ = 1/νr. A π/2 pulse width of 1.5 μs was used with pulse recycle delays of 0.2 s. The spectra were collected at a constant controlled temperature of 283 K, and chemical shifts were referenced to 1 M LiCl at 0 ppm. High-resolution imaging and selected area diffraction was carried out using TEM in a chromatic aberration-corrected FEI Titan 80-300 ST transmission electron microscope at voltages from 80 to 300 keV. TEM specimens were prepared by initial thinning using a focused ion beam approach followed by low-energy (100 eV) Ar ion thinning.
Figure 1. (a) Powder high-resolution X-ray diffraction patterns of the as-prepared 0.5Li2MnO3·0.5LiCoO2 samples (λ = 0.413893 Å). (b−d) Zoomed in regions of patterns shown in panel a. All peaks are labeled with respect to R3̅m symmetry.
symmetry, trigonal space group, R3̅m, for convenience, whereas the weaker superstructure peaks, for example, at ∼6° 2θ (Figure 1a), could be indexed to the lower symmetry, monoclinic space group, C2/m. The most obvious and striking observation related to these data is the degree of uniformity between the three samples. Specifically, despite different postheating cooling rates, all three patterns match one another almost perfectly and would certainly appear identical in lower resolution XRD data from a laboratory diffractometer. (As such, refinements are not strictly necessary as they will not provide any unique insights about such small differences in these samples. However, results from the Rietveld analysis are given in the Supporting Information for completeness.) This finding implies that the final cooling rate does not strongly influence the average crystallinity for these compositions as measured by HR-XRD. Another noteworthy observation is the existence of shoulders on the high angle side (smaller d-spacing) of most peaks, which is more pronounced for peaks at high scattering angles. Peak asymmetry in these HR-XRD patterns reflects at least a bimodal distribution of lattice parameters (see Supporting Information), likely representing some nonhomogenous mixed distribution of cations within the sample. Kim et al. observed broadening in low-resolution XRD for similar compositions which was interpreted by McCalla et al. to result from the existence of phase separation on the ∼10 nm length scale.34,47 Figure 2a shows the HR-XRD data of 0.5Li2MnO3· 0.5LiCoO2 samples 850[Quench] and 850[SlowCool]; these did not undergo the intermediate 550 °C annealing step and were subsequently quenched or slow cooled. The patterns for 850[Quench] and 850[SlowCool] are essentially indistinguishable, again demonstrating that in our experiments the cooling rate from the final firing temperature does not significantly alter the long-range crystalline structure. However, the intermediate 550 °C firing step does appear to have an effect. Samples that were not subject to the intermediate 550 °C firing step show more symmetric diffraction peaks, with much less pronounced high-angle broadening and peak shoulders. This is highlighted in Figure 2b−d, comparing samples with the same, final cooling
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RESULTS Elemental Analysis. In order to verify the elemental composition of our materials, ICP analysis was performed on the baseline set of samples. Table 1 lists the results. All samples Table 1. ICP-MS Elemental Analysis of As-Prepared 0.5Li2MnO3·0.5LiCoO2 Sample Powders Assuming O = 2.00 sample
ICP-MS composition
baseline (550 °C) 550/850[Quench] 550/850[Uncontrolled] 550/850[SlowCool] (∼1 °C/min)
Li1.19Mn0.39Co0.42O2 Li1.19Mn0.39Co0.42O2 Li1.19Mn0.39Co0.42O2 Li1.20Mn0.39Co0.41O2
show excellent agreement with the intended stoichiometry and with each other after their respective treatments. These data show that the intended composition of the metal-oxide precursor, as well as the final products, was achieved and that lithium loss was not an issue. As such, any structural differences in the final products can be directly related to synthesis conditions and not differences in composition. Synchrotron X-ray Diffraction. Figure 1a−d shows the HR-XRD data of the baseline 0.5Li2MnO3·0.5LiCoO2 material first preheated to 550 °C, then heated in the three ways described above and labeled as 550/850[Quench], 550/ 850[Uncontrolled], and 550/850[SlowCool]. All of the strong peaks in the diffraction patterns could be indexed to the higher C
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°C intermediate heating step. Like the HR-XRD data, the XAFS data of all samples overlap with one another with little distinction between them. The results are consistent with the previously published XAFS data of this system, showing that the average oxidation state of Mn is 4+ and that of Co is 3+.25,48 In addition, the EXAFS data of Figure 3c shows that the Mn−metal correlations at ∼2.5 Å are much lower in amplitude than the corresponding Co−metal correlations (Figure 3d). Bareño et al. have shown that this is due to the preferential ordering of Li and Mn (e.g., LiMn6) resulting in both Mn−Li and Mn−metal (Mn or Co) correlations in the second coordination shell.25 The local Co environment, however, is largely similar to that of LiCoO2 with an almost full complement of six, nearest-neighbor TM atoms. In fact, a detailed analysis reveals that coordination numbers, bond distances, and Debye−Waller factors for similar edges are the same, within error, for all samples (see Supporting Information). Therefore, on a local scale, XAFS clearly reveals two distinctly different environments for Mn and Co regardless of the cooling rates used. Figure 4a−d shows XAFS data for the samples without the 550 °C firing step. Again, all samples overlap and are identical
Figure 2. (a) Powder high-resolution X-ray diffraction patterns of 850[Quench] (blue line) and 850[SlowCool] (black line) 0.5Li2MnO3·0.5LiCoO2 samples (λ = 0.413893 Å). (b−d) Comparison of HR-XRD patterns from quenched samples with (black line) and without (blue line) a 550 °C intermediate firing step. All peaks are labeled with respect to R3̅m symmetry.
rate (Quench) but with and without the intermediate 550 °C firing step. Evidently, the low-temperature, intermediate firing at 550 °C has an effect on the long-range average structure of the final product. Remarkably, this effect remains even after subsequently heating the samples to higher temperatures (e.g., 850 °C), irrespective of the various cooling rates used. X-ray Absorption Spectroscopy. Figure 3a−d shows XAFS data taken at the Mn and Co K-edges for the samples shown in Figure 1, all of which had been subjected to the 550
Figure 4. (a) Mn and (b) Co K-edge XANES of the 0.5Li2MnO3· 0.5LiCoO2 samples prepared without a 550 °C intermediate firing step. (c and d) Corresponding Fourier-transformed magnitudes of the Mn and Co K-edge EXAFS, respectively. The inset in panel a shows a magnified view of the Mn K pre-edge region. The EXAFS is uncorrected for photoelectron phase shift. The k-ranges used for analysis were 2.3−12.1 Å−1 (Co) and 2.6−12.8 Å−1 (Mn).
(see Supporting Information), revealing two distinctly different local environments regardless of heat treatment. Figure 5a and b shows a comparison of the Mn K-edge XANES and EXAFS, respectively, of the quenched samples with and without the 550 °C firing step. Small differences between the two samples can be seen, particularly in the intensity of the pre-edge peaks (inset of Figure 5a), indicating slight differences in their averaged electronic and atomic local structures. Co K-edge data (not shown) showed similar results. These subtle, but detectable, differences in the XAFS of these samples are in general agreement with the HR-XRD data (Figure 2) that clearly show the effects of the intermediate
Figure 3. (a) Mn and (b) Co K-edge XANES of the as-prepared 0.5Li2MnO3·0.5LiCoO2 samples. (c and d) Corresponding Fouriertransformed magnitudes of the Mn and Co K-edge EXAFS, respectively. The inset in panel a shows a magnified view of the Mn K pre-edge region. The EXAFS is uncorrected for photoelectron phase shift. The k-ranges used for analysis were 2.3−12.1 Å−1 (Co) and 2.6− 12.8 Å−1 (Mn). D
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resonance at 400 ppm is tentatively assigned to second-shell coordinations having a combination of transition metal- and lithium cations. The second coordination shell can be summarized, to a best possible estimation, as 6Mn × 180° + 3Mn × 90° + [(3Li × 90° and 0Co × 90°), (2Li × 90° and 1Co × 90°), (1Li × 90° and 2Co × 90°), and/or (0Li × 90° and 3Co × 90°). Finally, the sharp resonance at 0 ppm is assigned to Li in LiCoO2 (assignment based on composition and the well-established diamagnetic NMR signal of LiCoO2).49 It is important to point out that variations in the cooling step (quenched vs slow cooled) resulted in no significant changes in the local Li structure of the final powders. This is clearly illustrated by the superimposed spectra of the 550/850 sample set and the 850 sample set (Figure 6, blue/green and red/ black). The NMR data therefore confirm that Li2MnO3 local ordering occurs in all investigated samples irrespective of the cooling rates used. However, some differences in the average lithium environments are seen for sample sets with and without the intermediate firing. These differences are largely related to the local environment of lithium in the lithium layers; for instance, peaks at ∼732 ppm (Li2MnO3-like, lithium layer coordinations) and 0 ppm (LiCoO2-like coordinations) show changes in relative intensity. The presence of such differences, in samples with and without the intermediate firing, is consistent with HR-XRD data. Transmission Electron Microscopy. TEM was used to study the slow-cooled and quenched samples with and without the 550 °C firing step. In each case, discrete domains with Li2MnO3-like order and LiMO2-like order were observed. Selected area electron diffraction patterns are shown for the 550/850[SlowCool] and 550/850[Quench] samples in Figure 7a and b, respectively. These electron diffraction patterns reveal structural characteristics over a length scale of about 100 nm.
Figure 5. (a) Mn K-edge XANES and (b) Fourier-transformed magnitudes of Mn K-edge EXAFS comparing quenched samples with and without the intermediate 550 °C firing step. The inset in panel a shows a magnified view of the Mn K pre-edge region. The k-ranges used for analysis were 2.3−12.1 Å−1 (Co) and 2.6−12.8 Å−1 (Mn).
firing treatment. A detailed EXAFS analysis (see Supporting Information) shows that the average, local transition metal environments are, within error, the same for similar edges, as expected from the mostly overlapping Mn K-edge FTs of Figure 5b. The subtle differences that are seen, however, are too small to quantify as the absolute uncertainties (which arise mainly from correlation between various fitting parameters) are much larger. This is not surprising as the additional component seen in the HR-XRD is only minor and consequently need not alter the ensemble-averaged local structure of the metal atoms significantly. Solid State NMR. Figure 6 shows 6Li MAS NMR for the quenched and slow cooled samples, with and without the
Figure 7. Selected area electron diffraction patterns for (a) the 550/ 850[SlowCool] and (b) the 550/850[Quench] samples. The diffraction patterns are indexed according to the Li2MnO3 structure, and reflections distinctive to that structure are marked by arrows.
Figure 6. 6Li MAS NMR of slow cooled and quenched 0.5Li2MnO3· 0.5LiCoO2 samples with and without the intermediate 550 °C firing step. * indicates spinning sidebands.
The two diffraction patterns are similar, with each exhibiting primary reflections that are common to both the Li2MnO3 and LiMO2 structures but also reflections at the 1/3 and 2/3 positions that are distinctive of the Li2MnO3 structure. Figure 8a and b shows high-resolution TEM images from the 550/ 850[SlowCool] and 550/850[Quench] samples, respectively. These images reveal the domains in real space. For this orientation, the LiMO2 regions show a continuous atomic structure along the Li and TM layers, whereas the Li2MnO3 regions show the periodic structure associated with the presence of Li in the TM layers.22 The structure of these
intermediate 550 °C firing step. Two groups of resonances were observed for the Li species present in the lattice: (1) Li in the Li layers in the region between 50 and 1100 ppm and (2) Li in the transition metal layers in the region between 1300 and 1500 ppm. For the octahedral lithium sites in lithium layers, two main, ordered Lorentzian resonances and a very broad disordered Gaussian resonance were observed. The resonance observed in all four spectra at 732 ppm is due to Li2MnO3-like domains which have been well characterized in various Li-rich transition metal oxides in previous literature.19 The Lorentzian E
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ments), the domain microstructures of these composite materials, driven by charge ordering, have already formed. Firing to 550 °C followed by cooling appears to create a unique, secondary long-range averaged component that remains partially intact even after subsequent high-temperature treatments, regardless of associated cooling rates. However, this component appears to be minor even in our synchrotron, HRXRD data and may be difficult to detect in low-resolution diffractometers. With respect to the electrochemical properties of these composite structures and intercalation materials in general, the electrochemical potential depends on the electron energy (determined by the Fermi level) and the Li+-site energy (which in turn depends on the local interactions). Thus, the electrochemical properties and reactions of these materials, for example, internal phase transitions that give rise to voltage fade,20,52 are expected to be greatly influenced by localized structural configurations in the electrode material. In this work, we have used a combination of techniques to probe different length scales ranging from atoms and atomic local structure to bulk ensembles and average structure. The broad agreement between these measurements supports the conclusion of local ordering in 0.5Li2MnO3·0.5LiCoO2 materials, regardless of the processing route used. The apparent discrepancies about the nature of this and other composite materials arise partly from the exclusive use and interpretation of conventional diffraction methods, which inherently provide only long-range and averaged structural information. Specifically, such techniques do not necessarily contain information about the details of the local structure, and the sole analysis of such data might not reveal the kind of nanoscale inhomogeneity clearly observed in the data presented above. It is only by the judicious use of complementary techniques, which provide information at local and long-range length scales, can a comprehensive, detailed understanding of these complex structures be obtained.
Figure 8. High-resolution TEM images for (a) the 550/850[SlowCool] and (b) the 550/850[Quench] samples. Local diffraction information obtained by fast Fourier transform (FFT) from the regions marked is shown below each image.
domains is confirmed by local diffraction information extracted from the HRTEM image using a Fast Fourier Transform (FFT) from the regions marked. The 850[Quench] sample prepared without the 550 °C firing step also shows regions with Li2MnO3-like order and with LiMO2-like order, as shown in Figure 9. The high-resolution images of Figure 9 reveal the presence of both types of domains, and the FFT data of highlighted sections further confirm the local structure.
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CONCLUSIONS We have demonstrated that despite different cooling rates used in the synthesis of 0.5Li2MnO3·0.5LiCoO2 electrode materials, the products possess similar structures on both local (XAFS and NMR) and long-range (HR-XRD) scales. XAFS and NMR data provide evidence for Li and Mn ordering in all samples and show that the Mn environments have significant similarity to those found in pure Li2MnO3. The local coordination of the TMs evidenced by EXAFS data clearly does not support true solid solution behavior, consistent with the analyses of related Mn- and Ni-based composite structures.20 TEM data confirm that Li2MnO3-like character is present in all samples, regardless of cooling rate or intermediate thermal history. It appears, therefore, that charge ordering, which is initiated at a relatively low onset temperature, is the dominant driving force that dictates the configuration of localized structures in these composite electrode materials. We conclude that solid solution type behavior cannot be definitively assessed by long-range averaging techniques, such as powder X-ray diffraction.53 Furthermore, even in highly crystalline samples that provide relatively sharp XRD patterns, information about nanoscale inhomogeneity and distinct local ordering is lost on techniques which do not probe the proper coherence lengths. Further studies to correlate nanoscale and macroscale structural inhomogeneities with electrochemical properties are being initiated to optimize synthesis and design strategies for this class of electrode materials. For example, studies on the time-
Figure 9. High-resolution TEM image for the 850[Quench] without the 550 °C firing step. Insets show local diffraction information obtained by FFT from the regions marked.
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DISCUSSION The XAFS data show that the second-shell, metal−metal coordination is clearly different for Mn with respect to Co in all samples. Specifically, charge ordering between Li and Mn is very strong. This agrees with the NMR results and implies that charge ordering in these materials is the dominant force that drives their structural formation. In addition, we have recently reported strong Li and Mn ordering in the Mn- and Ni-based xLi2MnO3·(1−x)LiMn0.5Ni0.5O2 composites for all values of x, 0 ≤ x ≤ 0.5.20 It is also known that disordered, nanocrystalline Li2MnO3 can be synthesized below 500 °C.50,51 Therefore, it is likely that very early on in the annealing process, especially during slow heating-ramp times (∼2 °C/min in our experiF
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scale of domain formation via rapid heating protocols are currently under way in an attempt to overcome the tendencies of cation ordering reported herein.
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ASSOCIATED CONTENT
S Supporting Information *
Results obtained from Rietveld refinement and EXAFS fitting. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(J.R.C.) E-mail:
[email protected]. *(M.B.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support from the Vehicle Technologies Program, Hybrid and Electric Systems, in particular, David Howell, Tien Duong, and Peter Faguy, at the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy is gratefully acknowledged. Sector 20 facilities at the Advanced Photon Source of Argonne National Laboratory, and research at these facilities, are supported by the U.S. DOE, Basic Energy Sciences, and National Sciences and Engineering Research Council of Canada and its founding institutions. Electron microscopy was carried out in the Electron Microscopy Center at Argonne, which is supported by the Office of Science under contract No. DEAC02-06CH11357. We thank Dr. Eungje Lee for his assistance in preparing structural models. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.
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