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Subsurface Distribution of Antisite Defects in LiMnPO: Direct Comparison with LiFePO 4

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Sung-Yoon Chung, Si-Young Choi, Tae-Hwan Kim, and Seongsu Lee J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09730 • Publication Date (Web): 26 Oct 2016 Downloaded from http://pubs.acs.org on October 31, 2016

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The Journal of Physical Chemistry 1

Subsurface Distribution of Antisite Defects in LiMnPO4: Direct Comparison with LiFePO4 Sung-Yoon Chung,*,†, 4 Si-Young Choi,‡, 4 Tae-Hwan Kim,§ and Seongsu Lee§

†

Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea ‡ Korea Institute of Materials Science, Changwon 51508, Korea § Korea Atomic Energy Research Institute, Daejeon 34057, Korea

*Corresponding Author: [email protected] and [email protected] Theses authors contributed equally to this work.

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The Journal of Physical Chemistry

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Abstract We demonstrate a thermodynamically stable distribution of antisite exchange defects within a single LiMnPO4 crystal through a combination of atomic-resolution scanning transmission electron microcopy and ab initio calculations. In contrast to the strong segregation of exchange defects in subsurface layers with selective orientations in LiFePO4, the highly ordered arrangement between Li and Mn in the bulk is preserved near the surface of LiMnPO4 crystals, showing no preferential aggregation of defects irrespective of the location in a particle. Based on the identical crystal structure and the analogous compositional basis of the two olivine phosphates, the distinct distribution characteristics of the antisite exchange defects are notably unexpected findings, highlighting the value of direct visualization at an atomic scale.


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Introduction An ordered configuration of Li and transition metals (TM) among the oxygen interstitials is a significant structural feature in many Li-intercalation oxides, and results in rapid Li-ion mobility within a close-packed oxygen framework during the electrochemical Li (de)insertion reactions. In addition to the macroscopic information provided by the conventional powder diffraction and refinement analyses,1,2 direct identification of local variation in the cation ordering and segregation has been a central issue in structural chemistry dealing with Li-intercalation oxides over the past decade.3−9 In particular, unequivocal experimental evidence based on atomic columnby-column imaging in aberration-corrected scanning transmission electron microscopy (STEM) has provided an important contribution to a better understanding of the correlation between cation disordering and resulting electrochemical performance. Many notable findings observed via STEM at an atomic scale have been reported since 2008, visualizing the Li−TM antisite exchange defects,10−16 the local compositional segregation,8,17,18 and even structural transitions19−25 in various layered rocksalt-, olivine-, and spinel-type Li-intercalation oxides. The Li (de)intercalation reaction is initiated at the surface of particles during electrochemical cycling. As a result, the apparent Li diffusion and subsequent reaction kinetics can be greatly affected by the local Li−TM configuration perturbed from the complete ordering in the surface regions. A substantial number of studies have thus recently focused on elucidating the atomic configurational variations near the particle surface.5,8,9,15,16,21,22,25 Among many intriguing aspects, a strong orientation-dependent distribution of the Li−TM intermixing at the surface is one of the most noteworthy features.15,16,25 Furthermore, such anisotropy is not confined to a specific oxide system but rather commonly occurs in many layered oxides encompassing Li1+x(Mn,Ni)O2 and Li(Mn,Ni,Co)O2. Both the composition and the atomic structure at each surface in



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ordered Li-intercalation oxides vary remarkably with the crystallographic orientation of the surface. Consequently, the facet-dependent anisotropic formation of Li−TM intermixing near the surface is understood in terms of an energetically favorable configuration for the thermodynamic stability of entire particles, as exemplified in Li1+x(Mn,Ni,Co)O2.16 Very recently, analogous behavior in the subsurface distribution of antisite exchange defects was demonstrated in olivine-type LiFePO4.25 For example, while LiFe−FeLi exchanges were hardly observed in the (100) surface region during the STEM analysis, a substantial amount of exchange defects were clearly imaged in the (001) surface regions, showing the highly anisotropic formation of defects. In this article, we report that antisite defects are not detected in significant amounts in the surface regions of LiMnPO4 particles, in stark contrast to LiFePO4. More importantly, such LiMn−MnLi exchanges are remarkably insensitive to the surface facet orientation. Recalling the compositional analogy and identical crystal structure of LiMnPO4 and LiFePO4, these two distinct subsurface defect formation characteristics appear to be unusual.

Experimental Methods Particle synthesis. LiMnPO4 and LiFePO4 crystalline particles were prepared via a solid-state reaction by using high-purity lithium carbonate (Li2CO3, Aldrich), manganese oxalate dihydrate (Mn(II)C2O4⋅2H2O, Alfa Aesar), iron oxalate dihydrate (Fe(II)C2O4⋅2H2O, Aldrich), and ammonium dihydrogenphosphate (NH4H2PO4, Aldrich). Stoichiometric powder mixtures of the starting materials were ball-milled in acetone for 24 h with zirconia milling media. Dried slurries were calcined at 350°C for 5 h under a flow of high-purity Ar (99.999%, 400 sccm). The calcined amorphous powder samples were further annealed at 500°C for LiMnPO4 and at 600C for LiFePO4 in the same Ar atmosphere for a sufficiently long time, 72 h, to exclude kinetic formation of exchange defects during synthesis26 and thus obtain crystalline particles


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having a thermodynamic equilibrium state of defect configurations. HAADF-STEM. Atomic-column-resolved HAADF-STEM images were acquired using a transmission electron microscope with a spherical-aberration corrector (CEOS GmbH) for an electron probe. The size of the probe in STEM mode was 0.96 Å. The collection semiangles of the HAADF detector were adjusted from 71 to 190 mrad in order to utilize incoherently scattered electrons at large angles. The obtained raw images were band-pass filtered to reduce background noise. DFT calculations. Ab initio calculations were carried out within the spin-polarized generalized-gradient approximation (GGA) along with the Perdew-Burke-Ernzerhof (PBE) functional for exchange correlation and the ultrasoft pseudopotentials for ionic cores, as implemented in the CASTEP code (Biovia). A sufficiently long slab along with a 10-Å vacuum layer was constructed as an optimum supercell for each calculation to make the relaxation layer of each slab more than 10 Å in thickness. As previously suggested, stoichiometric [LiMnPO4]x-type slabs were adopted as proper supercells for reasonable determination of the surface termination.25 To exclude any energy variation induced by change of the surface termination, a pair of LiMn−MnLi exchange defects was introduced into the second Li and Mn rows beneath the top-most surface plane, preserving the initial surface termination. The formation energy of a defect pair in the subsurfaece layer (ES) was obtained by ES = E(defects) − E(w/o defects) where E(defects) and E(w/o defects), respectively, are the total energies of a supercell with and without a defect pair after geometry optimization. The plane-wave basis set for the kinetic energy cutoff was 450 eV. Optimization of the internal coordinates for each case was performed using the BFGS algorithm with convergence tolerances of 0.1 eV/Å for the maximum ionic force, 5 × 10−5 eV/atom for the total energy, and 0.005 Å for the maximum ionic displacement.

Results and Discussion Figure 1 shows the atomic-scale HAADF-STEM images obtained near the (001) vicinal

surface

planes

of

LiFePO4

and

LiMnPO4


particles,

respectively.


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Comprehensively different formation behavior of subsurface exchange defects between the two phosphates is readily recognized in this set of images. As consistently shown in the previous study,25 substantial column intensity in most of the Li sites (green arrows) is detected in the surface region of a LiFePO4 particle (Figure 1a), indicating the presence of a significant amount of antisite exchange defects. In contrast, Figure 1b reveals that the ordered image feature with invisible Li columns and bright Mn columns is maintained over the surface region of a LiMnPO4 particle. Very few Li columns showing sufficiently detectable intensity near this surface were found during the STEM observation. As demonstrated in Figure 2, as an example, merely several Li columns out of more than a hundred columns in a magnified image exhibited intensity that exceeds the background noise, verifying the distinct distribution of antisite defects in the subsurface region between LiMnPO4 and LiFePO4. For further comparison, intensity profiles of Li columns in LiMnPO4 and LiFePO4 are demonstrated in Figure S1 in Supporting Information. To examine whether any surface-orientation-dependent characteristics of the defect distribution are present in LiMnPO4, atomic-resolution HAADF-STEM images were collected near various surface facets of a single particle. The low-magnification STEM image in Figure 3 shows a LiMnPO4 particle in the [010] projection. A series of HAADF-STEM images were subsequently obtained from the three different surface regions denoted by orange rectangles with the numeric labels of 1, 2, and 3. The most noticeable aspect in this set of images is that the highly ordered configuration between Li and Mn observed in the bulk is preserved well up to the top-most surfaces, irrespective of the surface-facet orientation. Few Li columns with detectable intensity stemming from Li−Mn exchanges are identified in all three regions. The maintenance of a high degree of Li−Mn ordering demonstrated in Figure 3 implies

that

the

formation

of

exchange

defects


has

similar

likelihood

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thermodynamically regardless of the location in a particle. We thus carried out ab initio DFT calculations to theoretically compare the defect formation energies near surfaces of a different crystallographic orientation. As adopted in previous studies of olivine phosphates,25,27 (LiMnPO4)x-type stoichiometric slabs with a vacuum layer were utilized as proper supercells. In addition, an exchange-defect pair, LiMn−MnLi, was introduced into the subsurface layers, not at the top-most surface layers, in order to maintain the initial surface termination of each slab. Figure 4 illustrates the supercell models with seven different surface-facet orientations for DFT calculations. As denoted by a red ellipse in each of the cells, a pair of LiMn−MnLi was introduced beneath the top surface rather than the very first Li and Mn rows. Based on these two approaches for supercell construction, we can avoid unnecessary concern regarding the configurational change of surface termination. We can therefore exclude possible unknown influence induced by the termination change on the total energy variation, thereby allowing a reasonable comparison of the formation energies between subsurface regions. Three crystallographically distinct neighboring Mn atoms are placed around each Li in this ordered olivine-type lattice (see Figure S2 in Supporting Information). We thus calculated the LiMn−MnLi formation energies for 21 combinations in total, considering the three possible exchange geometries for each of the seven supercells. The lowest formation energy (Es) among the three values is listed in Table 1 in each case (see Table S1 in Supporting Information for the entire set of formation energies). For a direct comparison, the Es values of LiFePO4 obtained in our recent study in the same manner are also included in the right column of the table. One of the notable findings from the DFT calculations for LiMnPO4 is that the defect formation energy does not considerably vary with the surface orientation. While highly negative values of Es are obtained for the (001) and (111) surface planes of LiFePO4, the calculation results of LiMnPO4 in Table 1 show no negative formation



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energies in any surface plane. Although the defect formation energies for the (010) and (001) surfaces in LiMnPO4 are comparatively lower than that in the bulk (0.75 eV), the overall Es does not deviate from a range of 0.39−0.76 eV, in striking contrast to the strong surface-orientation-dependent defect formation in LiFePO4. This relatively small variation in the defect formation energy in LiMnPO4 also agrees well with the HAADFSTEM observations shown in Figures 1 and 2, consistently supporting a distribution of antisite exchange defects insensitive to the location within a particle. Precise and direct probing of the initial atomic arrangements of Li and TM in pristine Li-intercalation oxides prior to any electrochemical cycling is of great significance as it is the first step to understand any plausible correlations between the thermodynamically stable configurations and the electrochemically induced structural changes. If two Li-intercalation oxides have an identical crystal structure and compositional analogy, it is usually accepted that their point defect structure and distribution will not completely differ in general. Contrary to this conventional wisdom, our STEM observations along with the DFT calculations in this study demonstrate an important discrimination in the distribution of antisite exchange defects between two identical olivine frameworks, LiMnPO4 and LiFePO4, highlighting the value of an atomic column-by-column analysis.

Conclusion Using HAADF-STEM, we have demonstrated the distinct distribution characteristics of antisite exchange defects between LiMnPO4 and LiFePO4 in the surface regions. While strong subsurface segregation of the defects was observable in some selective surface orientations in LiFePO4, no substantial differences in the highly ordered Li−Mn arrangement were identified near surface facets during the STEM analysis. Ab initio DFT calculations also showed that the formation energies of a pair of


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exchange defects do not vary significantly with the surface crystallographic orientation in LiMnPO4, consistently providing theoretical support to the experimental observations. This rapid communication revealing the unusual discrimination of defect distribution in two olivine phosphates emphasizes the necessity of direct atomic-scale probing for understanding the unexpected defect chemistry in Li-intercalation compounds.

Associated Content

Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Unit cell structure of LiMnPO4; intensity profiles of Li columns; and additional table for defect formation energies (PDF)

Author Information

Corresponding Author *E-mail: [email protected] or [email protected] Author Contributions 4 S.-Y. Chung and S.-Y. Choi contributed equally to this work. Notes The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF), grant numbers 2014R1A4A1003712 (BRL Program) and 2012M2A2A6002461 (Nuclear R&D Program) and also by the Ministry of Tarde, Industry, and Energy, grant number 10065691.

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Figure 1. HAADF-STEM images showing the (001) vicinal surface regions in LiFePO4 and LiMnPO4 along with their unit-cell structures. This set of two atomic-column images directly compares the distinct distribution of subsurface exchange defects. (a) Most Li columns (green arrows) in LiFePO4 reveal detectable bright intensity, indicating a large amount of Li−Fe exchange defects near the surface. (b) In contrast, few Li columns containing antisite Mn are identified in LiMnPO4. Schematic illustrations for atomic arrangement are superimposed on each of the images.


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Figure 2. HAADF-STEM image showing a Li column with detectable intensity (denoted by a yellow arrow) due to antisite Mn in the subsurface region of LiMnPO4. The intensity profile in the right panel consistently verifies the higher contrast in Column 4 (yellow arrow) among Columns 1−4.



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Figure 3. Series of HAADF-STEM images with different surface orientations in a LiMnPO4 particle. Three atomic-column images in the vicinal (001), (100) and (10 1 ) surfaces were obtained from Regions 1−3 denoted by orange rectangles in the lowmagnification image. The highly ordered feature between the bright Mn columns and the invisible Li columns is maintained well over all three images, irrespective of the surface orientation.


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Figure 4. Supercell models for DFT calculations of subsurface defect formation energies in LiMnPO4. A pair of antisite exchange defects, LiMn−MnLi, is denoted by a red ellipse in each of the illustrations. Note that this defect pair has been introduced in the subsurface layer rather than on the top-most surface so as not to disturb the initial atomic termination in each slab.



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Table 1. Formation energies of an antisite defect pair in each subsurface layer and differences with its formation energy in the bulk. Surface Plane

Defect Formation Energy (ES )*, eV LiMnPO4

LiFePO4

(100)

0.75

0.59

(010)

0.39

0.14

(001)

0.41

−0.56

(110)

0.76

0.15

(101)

0.64

0.38

(011)

0.43

0.25

(111)

0.50

−0.70

(*Defect Formation Energy of LiMnPO4 in Bulk, EB : 0.75 eV. The formation energy in red in LiFePO4 denotes a larger value than the formation energy in bulk (0.43 eV). The negative formation energies are also represented in blue.)


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