Revealing Electrochemically Induced Antisite Defects in LiCoPO4

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Revealing Electrochemical induced Antisite Defects in LiCoPO4: Evolution Upon Cycling Adrien Boulineau, and Thibaut Gutel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503716p • Publication Date (Web): 05 Jan 2015 Downloaded from http://pubs.acs.org on January 13, 2015

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Chemistry of Materials

Revealing Electrochemical induced Antisite Defects in LiCoPO4: Evolution Upon Cycling Adrien Boulineau* and Thibaut Gutel Univ. Grenoble Alpes, F-38000, Grenoble, France CEA, LITEN, 17 rue des Martyrs, F-38054, Grenoble, France. KEYWORDS : LiCoPO4, olivine structure, lithium-ion battery, structural defects ,electron microscopy, HAADF STEM imaging. ABSTRACT: This article is aimed to reveal the formation of antisite defects that are induced in LiCoPO4 crystals upon electrochemical charge/discharge cycles. This is achieved using Cs-corrected High Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM) that allows their direct visualization. By comparison with simulated images, their evolution is discussed and their quantification is performed. Within free of defects sample, a disordering due to the exchange between lithium and cobalt atoms is progressively created. It is the first time that the evidence of antisite defect creation in an olivine type compound upon electrochemical cycling is reported. It is showed their formation occurs during the charging process. While they are heterogeneously distributed after the first charge/discharge because their concentration is low, such exchange defects appear to be more homogeneously dispersed in the lattice when their amount is much higher after the 30th charge/discharge cycle. This article gives a new insight on the behavior of this compound and contributes to explain the reason why such a high capacity fading is observed when this material is used in a battery.

1. Introduction The need of environmentally friendly, safe, stable and low cost materials for application in lithium-ion batteries has led to a strong interest in developing and in optimizing olivine type LiMPO4 (M= Mn, Fe, Co, Ni) cathode materials. Among them, LiFePO4 has been extensively studied. Due to its high discharge voltage plateau close to 4.8V vs Li+/Li° and its large theoretical capacity1 of 167mAh/g, LiCoPO4 is an interesting alternative cathode material offering a remarkable high energy density² around 800Wh/kg that outclasses either LiFePO4 or LiCoO2 which remains the main material used as positive electrode in commercial lithium-ion batteries. However, the practical use of LiCoPO4 is at the moment precluded due to its poor cycling performances related either to intrinsic low electronic conductivity and limited lithium diffusion3 but also to the detrimental electrolyte degradation above 4.5V vs Li+/Li°4. The structure of LiCoPO4 is represented in Figure 1a. It adopts the olivine structure whose lattice is orthorhombic, described in the space group Pnma, with the following cell parameters: a = 10.338 (1) Å , b = 6.011 (1) Å and c = 4.695 (1 ) Å1, 5, 6. The olivine type structure can be described as a distorted compact hexagonal stacking of oxygen atoms into which octahedral sites are occupied by Li+ and Co2+ cations. These sites are distinct and allow the ordering of lithium and cobalt due to their different ionic radii, 0.74 Å and 0.65Å respectively7. This ordering is responsible for the distortion of the oxygen sublattice. Meanwhile, phosphorus cations, much smaller, occupy tetrahedral sites. The material structure thus consists of CoO6 octahedra and PO4 tetrahedra linked by edges

and corners. LiO6 octahedra that are sharing edges form tunnels along the direction [010], along which lithium (de)intercalation occurs. Each lithium atomic column is surrounded by 6 cobalt and phosphorous ones. It is important to note that the view along the [100] direction, Figure 1b, shows that lithium colums contain as twice as many atoms as the cobalt ones. CoO6 octahedra are bonded to four other CoO6 octahedra by corner sharing. This makes difficult the electron delocalization and thus reduces the electronic conductivity. PO4 tetrahedra play the role of “pillars” in the structure, see Figure 1c. One of their triangular bases shares its 3 edges with 2 LiO6 and 1 CoO6 and its 3 corners with 3 CoO6. The opposite corner of this base is connected with 2LiO6 and one CoO6. The highly covalent nature of the P-O bonds8 ensures the stability of the material, in particular by limiting the oxygen loss that occurs in traditional layered and spinel oxides.

2. Experimental 2.1 Synthesis The material was prepared using solvothermal synthesis. The stoichiometric amounts of CoSO4.7H2O, (NH4)3PO4 3H2O, glucose and LiOH.H2O (mole ratio is 1:1:0.5:3) were introduced in 40mL of mixed solvent containing water and benzylalcohol (1:1 by volume). The concentration of Co2+ was 0.1mol.L−1. After vigorous stirring at room temperature for 20 min, the suspension was poured into a 100mL Teflon-lined stainless steel autoclave (Parr® Bomb). The sealed autoclave was heated in an oven at 200°C for 24h and then slowly cooled. The product was centrifuged then washed with deion-

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ized water and absolute alcohol and finally dried in air at 60°C.

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at an accelerating voltage of 200 kV. A 20 mrad beam convergence semi-angle was used. STEM Images were collected using High Angle Annular Dark Field detector which inner and outer semi-angles were 60 and 180 mrad respectively ensuring so called “Z-contrast” conditions. STEM Image simulations were achieved considering these experimental conditions using QSTEM package. The size of the electron probe was close to 1Å. The Thermal diffuse scattering has been taken into account using 10 frozen phonon configurations. The sample thickness was estimated using Electron Energy Loss Spectroscopy (EELS) by calculating the ratio of the integrated intensity of zero loss peak over that of the whole low loss spectrum.

3. Results and discussion The aim of this article is not to present a LiCoPO4 compound with the best performances but a compound which particles morphology and size would allow to carry out High Resolution Scanning Transmission Electron Microscopy experiments to study the structural evolutions and to perform quantitative analyses. In this way, the material was prepared using solvothermal synthesis according to the procedure reported by Wang et al9 in order to be composed of plateletsshaped particles with a small thickness (i.e. less than 100nm). This is confirmed by the SEM micrograph presented in Figure 2 where agglomerated platelets can be observed. Moreover, this study has been performed on an uncoated material in order to be able to discriminate possible amorphous parts of the material formed during the electrochemical process from carbon coating contribution.

Figure 1. Crystal structure of LiCoPO4 viewed along the b axis (a) and the a axis (b). Grey, green, blue and red refer to Li, Co, P and O respectively. c) Detail of the environment of a PO4 tetrahedron that is sharing edges with two LiO6 and one CoO6 surrounding octahedron. Four others CoO6 and two others LiO6 octahedra are sharing corners

2.2 Electrochemical testings Electrochemical cyclings were carried out using coin cells prepared as follows. First, a slurry was obtained by mixing 80wt% active material, 10wt% polyvinylidene fluoride (PVDF) and 10wt% carbon black in N-methyl pyrolidinone (NMP) solvent. This slurry was coated on an Al current collector using a 100 µm doctor blade and dried at 55°C overnight. 14 mm diameter electrodes were punched and dried under vacuum at 80°C for 48 h. Finally, coin cells were assembled in an Ar filled glove box using Li-metal as counter electrode and LiPF6 1M in EC:PC:DMC in a 1:1:3 vol. ratio as electrolyte. 2.3 Transmission Electron Microscopy studies STEM images were recorded using a FEI Titan Ultimate microscope equipped with a monochromator and double spherical aberration (Cs) correctors for both the probe-forming and the image-forming lenses. The microscope was operated

Figure 2. a) First cycle charge-discharge curve from our LiCoPO4/Li cell at C/10 rate between 2.5 and 5.2V vs Li+/Li0. A SEM micrograph in insert depicts the platelet shape of the particles. b) Plot of the electrochemical specific capacity of the material as a function of the number of charge/discharge electrochemical cycles.


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The initial galvanostatic charge/discharge curve for our uncoated LiCoPO4 compound is presented in Figure 2a. During the charge/discharge reactions, the material exhibits a two-step (de)lithiation process. Indeed, as has already been reported6, two potential plateaus which are very close to each other can be observed around 4.8V. These two plateaus are related to two biphasic reactions within a three phase system (LiCoPO4, the intermediate phase Li0.7CoPO4 and CoPO4). A small polarization revealing the ionic diffusion-limited process and electronic resistance is observed and has the effect of increasing the gap between them on discharge. According to the literature, the fully delithiated phase, CoPO4, is found to be unstable at room temperature and to rapidly change to an amorphous state10. As shown in Figure 2b, the first discharge capacity is low, 60mAh/g, and a high capacity fading is observed; the compound delivers only 15mAh/g after 30 cycles. These results we obtained are similar to those obtained by others also with uncoated materials1, 11, 12, the low capacity and the fading being mainly attributed to the structural and electrolyte instabilities at high voltage4, 6, 13, 14.

The relationship between the crystal structure orientation and the particle morphology was established using electron diffraction experiments. First, the platelet shown in Figure 3a was oriented perpendicular to the electron beam and the corresponding diffraction pattern was recorded, see Figure 3b. Then, a series of diffraction patterns was recorded tilting the particle by around 15°, 7° and 17° successively, Figure 3c, 3d and 3e. All these patterns have been recorded so that they have in common the a* direction (plans (h00) are kept in diffraction condition). In this condition, all the patterns are linked together and can be indexed unambiguously. They are all consistent with the olivine structure of the LiCoPO4 compound. The zone axis of the pattern presented in Figure 3a that is [010], ensures that Li channels (that are parallel to the b parameter) are oriented perpendicular to the extended face of the platelets and parallel to the smallest dimension of them (i.e. their thickness). From Electron Energy Loss Spectroscopy measurements, the thickness of the platelets was found to be around 70nm. Such an orientation is the ideal one to ensure the best electrochemical performances and to validate the synthesis.

Figure 3. Image of a particle and corresponding series of electron diffraction patterns collected from the pristine LiCoPO4. a) TEM image. b) Pattern associated to the orientation of the as presented particle. c), d) and e) Patterns obtained from the first one successively tilting the particle by around 15°, 7° and 17° respectively.

Figure 4 presents high resolution HAADF-STEM image recorded along the [010] direction for the pristine LiCoPO4 (Figure 4a) and for the compounds recovered after the 1st charge (Figure 4b), after the first discharge (Figure 4c) and after the 30th one (Figure 4d). The HAADF-STEM imaging technique has the advantage that atomic columns are directly identified15, 16, whereas complicated simulations of images are needed for the analysis of conventional TEM images. In such an image the contrast is approximately proportional to Z2 (Z being the atomic number). Thus, both structural and chemical information can be obtained at an atomic resolution. In

LiCoPO4, Li and O are not heavy enough to produce any contrast and thus are not visible. To make the link with the structure of the compound, the left hand corner of the images in Figure 4 are enlarged and the locations of the Li, Co and P atomic columns in the cell are superimposed in the inset. Due to the distance between each other and their relative arrangement, Co and P atomic columns are not independently resolved and appear as coma-shaped columns. One must note the perfect crystallinity of the materials and that no amorphous part could be detected; the difference between these images comes from the intensity of the lithium atomic column. While no intensity can be seen over the lithium columns on the image rela-



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tive to the pristine material (Figure 4a), a visible white contrast can be observed over some of the lithium columns after the first charge (Figure 4b) and after the first discharge (Figure 4c) and over all of them after 30 elec-

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trochemical cycles (Figure 4d). For the latter two cases, this implies the partial occupancy of cobalt on the lithium site.

Figure 4. High Resolution STEM images recorded along the b axis. The atomic array is superimposed in the insets. Lithium, cobalt and phosphorous ions stand in grey, green and blue respectively. The orange rectangle represents the areas from which are extracted the line profiles depicted in Figure 5. a) pristine material. b) and c) Materials recovered after the first charge and first discharge respectively. The red arrows highlight some of the Li columns that present a bright contrast. d) Material recovered after the 30th discharge. Image simulations as a function of the degree of Li/Co exchange were performed to quantitatively interpret these modifications. The series of simulations is presented in Figure 5 for a sample thickness of 70nm as experimentally determined by EELS (and in good accordance with the SEM observation as depicted in Figure 2a), and considering various Li/Co exchanges up to 30%. A thickness of 70nm corresponds to the average thickness of the platelets deduced from EELS measurements by calculating the ratio between the intensity under the zero-loss peak and the total intensity of the spectrum. The series of simulated images along the [010] zone axis shows that intensity from the Li columns starts to exhibit a white contrast when around 5% of Li/Co exchange is considered at each octahedral site of the atomic column. Then, increasing the degree of exchange enhances the contrast of the Li columns as much as cobalt is substituting lithium. The line profiles extracted from the orange areas (insets of figure 4) are presented on Figure 6a. This gives an objective evolution of how much is increasing the contrast on the

lithium atomic column, that is to say how much cobalt and lithium are exchanged. The plot of the line profiles are in agreement with the projected structure depicted in the insets. Indeed they reflect the Co and P atomic columns that are not independently resolved and generate shouldered peaks between which another one appears when cobalt is partially occupying the lithium site. Indeed the ratio of the intensities of the lithium column over the cobalt column are 0, 0.24, 0.25 and 0.66 for the pristine material, these recovered after the first charge, after the first discharge and after the 30th respectively. Thus, it shows that the intensities on the lithium site are equal after the first charge and after the first discharge revealing that the amounts of cobalt on the lithium site are similar for both materials and that these exchange defects are formed during the charging process while the discharge does not produce any modification into the material. Also one can note that no intensity is observed for the pristine material ensuring that it is free of these defects. Interestingly, they are much more numerous after 30 charge/discharge cycles. The profiles deduced from the


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simulated images are plotted in figure 6b. As for the experimental ones, they exhibit shouldered peaks linked to the Co and P unresolved atomic columns separated by the lithium atomic column. While the lithium itself does not produce any intensity, its substitution by cobalt makes the intensity increase. The intensity ratio of the lithium atomic column over the cobalt one is plotted in Figure 6c in function of the amount of Li/Co exchange. One can note it is very sensitive to their exchange and the increase is not linear but follows a power law. This dependence is linked to the contrast formation of the as used imaging mode (i.e. Z-contrast STEM HAADF) that is approximately proportional to Z2, Z being the average atomic number of the observed atomic column.

Figure 5. Simulated High Resolution STEM images calculated in projection along the b axis for a sample thickness of 70nm and for various Li/Co amount of exchange 0% (a) , 5% (b), 10% (c), 15% (d) , 20% (e), 25% (f) and 30% (g). The atomic array is superimposed in the inset. Lithium, cobalt and phosphorous ions stand in grey, green and blue respectively. The orange rectangles represent the areas from which are extracted the line profiles depicted in Figure 6.

Thus, with x being the Co occupancy on the Li sites, the average Z number of the Li column (partially occupied by Co) is 1 3 27and the average Z number of the Co column (partially occupied by Li) is 1 27 3. The Li(Co) columns contain twice as many atoms as the Co(Li) ones, see Figure 1b. As a result the intensity ratio (R) of the lithium atomic column (partially occupied by Co) over the cobalt one (partially occupied by Li) should obey the following relationship:

so

² ²

.

This function is plotted in figure 6c and fits well with the locations of the experimental points related to the contrast of the atomic columns of the simulated images.

Figure 6. (a) Line profiles extracted from the areas depicted in the experimental images presented in Figure 4. (b) Line profiles extracted from the areas depicted in the simulated images presented in Figure 5. The profiles are normalized so that their maximum corresponds to the highest grey scale value. (c). Plot of the intensity ratio of the lithium atomic column (partially occupied by Co) over the cobalt one (partially occupied by Li) deduced from the simulated images in function of the lithium-cobalt exchange (blue points). The theoretical curve making the link between the intensity ratio and the exchange based on a Z² dependence is overimposed (red curve). The correspondence linking the intensity ratio measured experimentally and the corresponding amount of Li/Co exchange is depicted by the dotted lines.

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As showed by the plot, the difference between the theoretical ratio and the one deduced from the experimental images is not exceeding the percentage; this ensures a reliable accuracy to the used methodology. Thanks to these simulations, the quantification of cobalt on the lithium atomic column could be performed for all the experimental images presented in figures 4b, 4c and 4d related to the materials recovered after the 1st charge, the first discharge and the 30th electrochemical cycle respectively. The intensity ratios deduced from the experimental images are 0.24, 0.25 and 0.66 respectively and these values are placed on the figure 6c by the dotted lines. Thanks to the correspondence previously established, around 12% of exchange is found into some of the lithium atomic columns for both materials recovered after the 1st charge and after the first discharge, and close to 24% of exchange is deduced into all the lithium atomic columns after 30 cycles. The fact that these defects are formed during the charge rather than the discharge can be understood thanks to the calculus of the cobalt Ligand Field Stabilizing Energy. To migrate from their octahedral site to the lithium one, cobalt cations have to travel via a tetrahedral one. Based on Ligand Field Stabilizing Energy (LFSE) calculation, this diffusion is the most favourable when cobalt cations are still in their 3+ oxidation state prior to the migration, the energy cost being only 0.13*∆0 (with ∆0 the ligand filed splitting energy in the octahedral symmetry). Indeed the migration in the 2+ oxidation state that is equal to 0.27*∆0 + 2*P (with P the electron pairing energy) is at least two times less energetically favourable. The last possibility that consist in the migration followed by the oxidation that is equal to 0.53*∆0 + P is at least four times less energetically favourable than the first possibility. This confirms our observations that defect formation occurs during the charging process. To our knowledge, this is the first time such formation of antisite defects due to electrochemical cycling is reported in olivine-based materials. Nevertheless, such kind of defect has been reported yet for LiFePO4 in a freshly synthesised material. What we observe after the first charge/discharge cycle in our LiCoPO4 is very similar to what Chung et al17 have shown in a pristine LiFePO4 sample annealed at relatively low temperature (600°C). Indeed, by using quantitative STEM, they have shown the presence of around 15% of antisite defects in some of the Li columns. Moreover, it is noticeable that we also observed the aggregation of these defects (but formed electrochemically in our case) when they are present in a reduced amount. Indeed, as showed in Figures 4b and 4c, defects are not homogeneously distributed within the crystal but rather seem to be clustered. This phenomenon appear more clearly on the plot of intensity profiles recorded along several lines and depicted in supported information, see Figure S1. According to the study of A. J. Fisher et al18 based on ab-inito calculations, such clustering of defects is expected into the LiMPO4 (M = Fe, Co, Mn, Ni) compounds and act as precursor to larger clusters.

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tain any antisite defects in a significant proportion (i.e. less than 5%), after the first charge/discharge cycle, such defects are already visible. To our knowledge, it is the first time that the evidence of antisite defect creation in a olivine-based compound upon electrochemical cycling is reported. After the 1rst charge/discharge electrochemical cycle, Li atomic columns can contain around 12% of defects, but all of them do not contain such amount. Indeed these point defects are not homogeneously distributed but seem to be clustered. In contrast, after the 30th charge/discharge electrochemical cycle, all the Li atomic columns contain defects with a similar amount that is around 24%. As a result, the mobility and the diffusion of lithium must be largely reduced and the electrochemical capacity of the material is consequently much lowered. Stabilizing cobalt into its crystallographic site, for example, by making substitutions for strengthening bounding, would greatly improve the electrochemical properties of such compound that delivers an interesting voltage for a practical use in a lithium ion battery, in particular for the transportation industry.

SUPPORTING INFORMATION High Resolution STEM image recorded from the material recovered after the first discharge and four extracted line profiles are presented highlighting the heterogeneous distribution of the defects when present in a reduced amount.

This information is available free of charge via the Internet at http://pubs.acs.org/

5. Conclusion In summary, we have studied the structural evolutions occurring into the LiCoPO4 compound upon electrochemical cycling. While no amorphization can be observed, antisite defect creation due to the exchange between lithium and cobalt is pointed out. Whereas the pristine material does not con-

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Chemistry of Materials [6] Bramnik, N.N.; Nikolowski, K.; Baehtz, C.; Bramnik, K.G.; Ehrenberg, H. Chem. Mater. 2007, 19, 908-915. [7] Shannon, R.D. and Prewitt, C.T. Acta Cryst. 1969, B25, 925946. [8] Geller, S. and Durand, J.L. Acta. Cryst. 1960, 13, 325-331. [9] Wang, F.; Yang, J.; NuLi, Y.; Wang, J. J. Power Sources. 2011, 196, 4806-4810. [10] Ehrenberg, H.; Bramnik, NN.; Senyshyn, A.; Fuess, H. Solid State Sci. 2009, 11, 18–23. [11] Jin, B.; Gu, H.-B.; Kim, K.-W. J. Solid State Electr. 2008, 12, 105-111. [12] Li, H.H.; Jin, J.; Wei, J.P.; Zhou, Z.; Yan, J. Electrochem. Commun. 2009, 11, 95-98. [13] Bramnik, N.N.; Bramnik, K.G.; Buhrmester, T.; Baehtz, C.; Ehrenberg, H.; Fuess, H. J. Solid State Electr. 2004, 8, 558. [14] Andersson, A.S. and Thomas, J.O. J. J. Power Sources. 2001, 97-98, 498-502. [15] Spence, J.C.H. Mater. Sci. Eng. 1999, R26, 1-49. [16] Browning, N.D. and Pennycook, S.J. J. Phys. D.: Appl. Phys. 1996, 29, 1779-1798. [17] Chung, S.-Y.; Choi, S.-Y.; Yamamoto, T.; Ikuhara, Y. Phys. Rev. Lett. 2008, 100, 1-4. [18] Fisher, C.A.J.; Hart Prieto, V.M.; Islam, M.S. Chem. Mater. 2008, 20, 5907-5915.

AUTHOR INFORMATION Corresponding Author *Phone: +33 (0)4 38 78 27 88 Email address: [email protected]

ACKNOWLEDGMENT The authors are grateful to Claire Fongy and Eric Prestat for fruitful discussions about the material and concerning the simulations of the STEM images respectively. This work is part of the AMELIE project and founded within the 7th Framework Program of European Union.

REFERENCES [1] Amine, K.; Yasuda, H.; Yamachi, M. Electrochem. Solid St. 2000, 3, 178-179. [2] Okada, S.; Sawa, S.; Uebeou, Y.; Egashira, M.; Yamaki, J.; Tabuchi, M.; Kobayashi, H.; Fukumi, K.; Kageyama, H. Electrochemistry. 2003, 71, 1136-1138. [3] Wolfenstine, J.; Read, J.; Allen, J.L. J. Power Sources. 2007, 163, 1070-1073. [4] Markevich, E.; Sharabi, R.; Gottlieb, H.; Borgel, V.; Fridman, K.; Salitra, G.; Aurbach, D.; Semrau, G.; Schmidt, M.A.; Schall, N.; Bruenig, C. Electrochem. Commun. 2012, 15, 22-25. [5] Okada, S.; Sawa, S.; Egashira, M.; Yamaki, J.; Tabuchi, M.; Kageyama, H.; Konishi, T.; Yoshino, A. J. Power Sources. 2001, 9798, 430-432.

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Crystal structure of LiCoPO4 viewed along the b axis (a) and the a axis (b). Grey, green, blue and red refer to Li, Co, P and O respectively. c) Detail of the environment of a PO4 tetrahedron that is sharing edges with two LiO6 and one CoO6 surrounding octahedron. Four others CoO6 and two others LiO6 octahedra are sharing corners 171x288mm (96 x 96 DPI)


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Figure 2. a) First cycle charge-discharge curve from our LiCoPO4/Li cell at C/10 rate between 2.5 and 5.2V vs Li+/Li0. A SEM micrograph in insert depicts the platelet shape of the particles. b) Plot of the electrochemical specific capacity of the material as a function of the number of charge/discharge electrochemical cycles. 119x196mm (150 x 150 DPI)



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Image of a particle and corresponding series of electron diffraction patterns collected from the pristine LiCoPO4. a) TEM image. b) Pattern associated to the orientation of the as presented particle. c), d) and e) Patterns obtained from the first one successively tilting the particle by around 15°, 7° and 17° respectively. 257x179mm (96 x 96 DPI)


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High Resolution STEM images recorded along the b axis. The atomic array is superimposed in the insets. Lithium, cobalt and phosphorous ions stand in grey, green and blue respectively. The orange rectangle represents the areas from which are extracted the line profiles depicted in Figure 5. a) pristine material. b) and c) Materials recovered after the first charge and first discharge respectively. The red arrows highlight some of the Li columns that present a bright contrast. d) Material recovered after the 30th discharge. 170x153mm (150 x 150 DPI)



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Simulated High Resolution STEM images calculated in projection along the b axis for a sample thickness of 70nm and for various Li/Co amount of exchange 0% (a) , 5% (b), 10% (c), 15% (d) , 20% (e), 25% (f) and 30% (g). The atomic array is superimposed in the inset. Lithium, cobalt and phosphorous ions stand in grey, green and blue respectively. The orange rectangles represent the areas from which are extracted the line profiles depicted in Figure 6. 129x225mm (150 x 150 DPI)


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Figure 6. (a) Line profiles extracted from the areas depicted in the experimental images presented in Figure 4. (b) Line profiles extracted from the areas depicted in the simulated images presented in Figure 5. The profiles are normalized so that their maximum corresponds to the highest grey scale value. (c). Plot of the intensity ratio of the lithium atomic column (partially occupied by Co) over the cobalt one (partially occupied by Li) deduced from the simulated images in function of the lithium-cobalt exchange (blue points). The theoretical curve making the link between the intensity ratio and the exchange based on a Z² dependence is overimposed (red curve). The correspondence linking the intensity ratio measured experimentally and the corresponding amount of Li/Co exchange is depicted by the dotted lines. 111x255mm (150 x 150 DPI)



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Figure S1. a) High Resolution STEM image recorded from the material recovered after the first discharge. b) Profiles extracted from the four red lines. The red stars depict the location of the lithium sites along the lines that can be partially occupied by cobalt. Note that profiles are smoothed and background subtracted for highlighting Li/Co exchanges. 301x433mm (150 x 150 DPI)


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Revealing Electrochemically Induced Antisite Defects in LiCoPO4

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