Comments pubs.acs.org/cm
Comment on “Positive Electrode Materials for Li-Ion and Li-Batteries”
R
ecently, Ellis et al.1 published a review article regarding the cathode materials in Li-ion rechargeable batteries, giving a nice overview of a variety of Li-intercalation compounds. However, we have found that this paper includes TEM results and assertions concerning the iron-rich secondary phase that completely contradict those in the original paper published by the same research group in 2004.2 More seriously, some claims in ref 1 reverse the conclusion drawn in ref 2. As the distribution of second phases in polycrystalline LiFePO4 has been one of the most significant issues in the olivine phosphates research, we believe that any contradiction and misinterpretation should be pointed out for clarification. In this Comment, we raise five specific concerns regarding the TEM data published in both refs 1 and 2. 1. The EDS point analysis in ref 1 and the EELS map in ref 2 for the Fe distribution contradict each other. The authors of ref 1 provided the same bright-field (BF) TEM image and EELS map as in the previous paper in 2004 (ref 2). Figure 1a shows the TEM results that have been published in ref 1. As revealed in this figure, Point B is noted to be iron-rich Fe2P with an indication of Fe:P = 2:1. However, when a direct comparison is made between the data published in refs 1 and 2, this EDS analysis is completely contrary to the results of the EELS mapping. As reproduced in Figure 1b, the region denoted by the red circle for “Point B” is in fact rather iron-deficient in the Fe EELS map, even though Fe2P should exhibit much higher intensity. Furthermore, if one compares the schematic diagram and the TEM results published in ref 2, such contradiction becomes more apparent. Figure 2a is the schematic description showing the composite along with the EDS data presented in ref 2. The authors clarified in their paper that the “gray regions” were iron-rich Fe2P. In particular, their EDS spectrum indicates that the region denoted by a red circle is of an iron-rich composition. However, none of the gray regions (for example, those indicated by a red square) in the schematic diagram shows an iron-rich composition in the corresponding Fe EELS map (Figure 2b). Moreover, some of the gray regions display no EELS signal for Fe in the map, failing to provide any evidence for the presence of “percolating metal-rich phosphides”. It is very hard to understand how the contradictory EDS and EELS results for the Fe composition have been obtained. 2. An arbitrarily altered conclusion regarding the percolating phase has been drawn upon the same TEM results. While Subramanya Herle et al. in 20042 asserted the existence of “percolating non-carbonaceous metal-rich phosphides or phosphocarbides”, Ellis et al. in 20101 reversed their initial conclusion, arbitrarily arguing that “carbon makes a percolating conductive network through the sample with the phosphides” upon precisely the same TEM results. It must be very puzzling to readers how such totally different conclusions were drawn from the same experimental results. 3. Iron-rich phases cannot show brighter image features than the LiFePO4 matrix in BF-TEM due to their strong massabsorption contrast. Incident electrons in TEM are much more scattered by Fe2P and Fe75P15C10 than by LiFePO4 because the © 2012 American Chemical Society
Figure 1. (a) BF-TEM image and EDS point-analysis data provided in the paper by Ellis et al.1 These are the identical data published by the same research group, Subramanya Herle et al.2. (b) BF-TEM image and Fe map by EELS provided in the paper by Subramanya Herle et al.2 Contrary to the EDS point analysis shown in the paper by Ellis et al. in 2010, an iron-deficient composition is inconsistently demonstrated for the region denoted by a red circle in the Fe map by Subramanya Herle et al. in 2004. Part a reprinted with permission from refs 1 and 2. Part c reprinted with permission from ref 2. Copyright 2010 American Chemical Society. Copyright 2004 Nature Publishing Group.
two iron-rich phases contain more Fe atoms, which are much heavier than Li and O, in the unit volume.3 As a result, comparatively darker contrast should appear in BF-TEM images for Fe2P. In good agreement with the results of recent reports,4,5 Figure 3a illustrates such “mass-absorption contrast”, demonstrating much darker features for Fe2P in the polycrystalline LiFePO4 matrix, as denoted by red arrows. By contrast, as shown in Figure 3b, the BF-TEM image published by Subramanya Herle et al.2 contrarily displays much brighter contrast for Fe2P. The regions denoted by a red circle Received: November 24, 2011 Revised: January 17, 2012 Published: May 15, 2012 2240
dx.doi.org/10.1021/cm203525f | Chem. Mater. 2012, 24, 2240−2243
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Figure 2. (a) Schematic diagram of polycrystalline structure and EDS point-analysis data appearing in the paper by Subramanya Herle et al. in 2004.2 (b) Comparison between the schematic diagram (left), adaptation of iron-rich secondary phase distribution only (middle), and Fe map (right) provided in the paper by Subramanya Herle et al.2 Note that the iron-rich secondary phases contradictorily display an iron-deficient composition in the Fe EELS map. Parts a and b reprinted with permission from ref 2. Copyright 2004 Nature Publishing Group.
in Figure 3b are typical examples showing unreasonable contrast reversal. This does not make any sense in terms of simple electron-scattering phenomena. Taking a closer look at the TEM image in ref 2, it is most likely that the gray regions in the schematic diagrams in Figure 3b correspond to merely relatively thin locations or perforated holes in the LiFePO4 matrix and are not related to any iron-rich secondary phases. 4. Acquisition of Li mapping by electron energy-loss spectroscopy (EELS) in general TEM is not possible in the case of LiFePO4. Figure 4a is the BF-TEM image (left) and corresponding Li mapping by EELS (right) that appeared in ref 2. It should be noted that the Li K-edge (55 eV) and the Fe M2,3-edge (54 eV) are extremely close to each other in the EELS spectra. Considering that the energy resolution in EELS spectra is usually worse than 1.2 eV in most TEM analyses, these two edges cannot be resolved in EELS spectra for LiFePO4. Achieving better than 1 eV energy resolution is still challenging in real energy-filtered mapping,3 even if a monochromator is used. Direct comparison between EELS spectra from Fe2O3 and LiFePO4 clearly supports this. As shown in Figure 4b, no peak splitting for Li is observed in this typical EELS spectrum from a LiFePO4 crystal, revealing just one single energy-loss peak between 50 and 60 eV, analogous to the case of Fe2O3. Therefore, it should be clarified how such an elemental mapping of Li has been obtained. 5. Great care should be taken in the interpretation of EELS data on carbon when a TEM sample is prepared by ultramicrotomy. Whereas an ultramicrotome is generally used for the preparation of biological or polymeric soft-matter TEM specimens, it is rarely utilized for brittle inorganic samples because the preparation of sufficiently thin specimens
Figure 3. (a) BF-TEM image showing darker image contrast for Fe2P phases in the LiFePO4 matrix. (b) Comparison between the schematic diagram (left) and BF-TEM image (right) appearing in the paper by Subramanya Herle et al. in 2004.2 Note that the Fe2P secondary phases denoted by a red circle conflictingly display rather brighter contrast in the BF-TEM image. Part b reprinted with permission from ref 2. Copyright 2004 Nature Publishing Group. 2241
dx.doi.org/10.1021/cm203525f | Chem. Mater. 2012, 24, 2240−2243
Chemistry of Materials
Comments
Figure 5. (a) Low-magnification image showing an overall specimen geometry provided by Subramanya Herle et al. in 2004.2 (b) BF-TEM image and carbon map by EELS appearing in the paper by Subramanya Herle et al. in 2004.2 Note that the locations where carbon is detected by EELS in the carbon map exactly coincide with the locations that are thin and concave in the TEM image, as indicated by the yellow curves. Parts a and b reprinted with permission from ref 2. Copyright 2004 Nature Publishing Group.
Figure 4. (a) BF-TEM image and corresponding Li map obtained in EELS appearing in the paper by Subramanya Herle et al. in 2004.2 Reprinted with permission from ref 2. Copyright 2004 Nature Publishing Group. (b) EELS spectra of LiFePO4 and Fe2O3 in the lowloss range. Note that the Li K-edge (55 eV) and the Fe M2,3-edge (54 eV) are too close to be resolved.
with constant thickness is quite challenging. In addition, as a soft resin along with a small diamond knife is used in an ultramicrotome to embed and slice a sample, detectable carbon contamination from the resin is plausible unless plasma cleaning or ion-beam polishing is applied as a final step during specimen preparation. Figure 5a is the image of an overall specimen geometry provided by Subramanya Herle et al. in ref 2. As easily recognized in this figure, the specimen prepared by ultramicrotomy is considerably hill-and-dale, showing irregular thickness from region to region. Therefore, the concave locations may be contaminated with carbon from the resin after slicing. This seems very likely when the TEM image and the carbon map in ref 2 are compared, as shown in Figure 5b. The yellow curves in the images indicate the thin and concave regions, which appear to be a prototypical slicing texture created during ultramicrotomy. It is clearly observable that the locations where carbon is detected by EELS in the carbon map (right) exactly coincide with the locations that are thin and concave in the TEM image (lef t), which are not related to real grain boundaries. Consequently, we believe that the carbon shown by Subramanya Herle et al. was introduced during specimen preparation rather than originating from the bulk crystal lattice of the sample.
Sung-Yoon Chung*
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Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea Nalphates LLC, Wilmington, Delaware 19801, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea, Grants 2011-0003894, 2011-0004918, and 20110030297.
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REFERENCES
(1) Ellis, B. L.; Lee, K. T.; Nazar, L. F. Chem. Mater. 2010, 22, 691. (2) Subramanya Herle, P.; Ellis, B.; Nazar, L. F. Nat. Mater. 2004, 3, 147. 2242
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Chemistry of Materials
Comments
(3) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science; Plenum Press: New York, 1996. (4) Chung, S.-Y.; Kim, Y.-M.; Choi, S.-Y. Adv. Funct. Mater. 2010, 20, 4219. (5) Chung, S.-Y.; Kim, J.-G.; Kim, Y.-M.; Lee, Y.-B. Adv. Mater. 2011, 23, 1398.
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