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Atomic-Scale Observations of (010) LiFePO4 Surfaces Before and After Chemical Delithiation Shunsuke Kobayashi, Craig A.J. Fisher, Takeharu Kato, Yoshio Ukyo, Tsukasa Hirayama, and Yuichi Ikuhara Nano Lett., Just Accepted Manuscript • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on July 29, 2016

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Atomic-Scale Observations of (010) LiFePO4 Surfaces Before and After Chemical Delithiation

Shunsuke Kobayashi*,†, Craig A. J. Fisher†, Takeharu Kato†, Yoshio Ukyo‡, Tsukasa Hirayama† and Yuichi Ikuhara†, § †

Nanostructures Research Laboratory, Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587, Japan

‡

Office of Society-Academia Collaboration for Innovation, Kyoto University, Uji, Kyoto 611-0011,

Japan §

Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan

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ABSTRACT The ability to view directly the surface structures of battery materials with atomic resolution promises to dramatically improve our understanding of lithium (de)intercalation and related processes. Here we report the use of state-of-the-art scanning transmission electron microscopy techniques to probe the (010) surface of commercially important material LiFePO4, and compare the results with theoretical models. The surface structure is noticeably different depending on whether Li ions are present in the topmost surface layer or not. Li ions are also found to migrate back to surface regions from within the crystal relatively quickly after partial delithiation, demonstrating the facile nature of Li transport in the [010] direction. The results are consistent with phase transformation models involving metastable phase formation and relaxation, providing atomic-level insights into these fundamental processes.

KEYWORDS Lithium iron phosphate, scanning transmission electron microscopy, surface structure, delithiation, single crystal

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MAIN TEXT Impressive increases in the electrochemical performance of lithium iron phosphate (LiFePO4), first proposed as an alternative Li-ion battery positive electrode material in 1997,1 have been achieved primarily by nanosizing2 and addition of conducting phases.3-5 Control of particle morphology has also been a major focus of recent research to optimize charging rates, chemical stability and cyclability in response to the underlying anisotropy of its olivine-type crystal structure.6-10 Because of the large surface-to-volume ratios of LiFePO4 nanoparticles, understanding surface structures and their properties is also essential for formulating effective strategies to improve electrode performance further.11 Olivine-type LiFePO4 belongs to orthorhombic space group Pmna, and Li migration is known to occur predominantly one-dimensionally parallel to the b axis.12,13 This means that the (010) surface is the most critical in terms of Li-ion exchange between positive electrode and electrolyte. Removal of Li from LiFePO4 results in the formation of FePO4, which has only a small Li solubility, so that phase separation with narrow LiFePO4/FePO4 interphase boundaries is typically observed in micron-sized particles under equilibrium conditions after delithiation.14 Under non-equilibrium conditions, however, especially those encountered at high charging rates, metastable phases of intermediate Li contents have been observed.1521

Analysis of the growth kinetics of FePO4 during chemical delithiation has also shown how

microcracking and pore formation in large single crystals open up larger areas of (010) faces, resulting in uneven growth of the FePO4 phase.22

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While theoretical studies have provided valuable information about the underlying conduction mechanisms and defect chemistry of the crystal bulk and surfaces at the atomic level,13,23-27 experimental methods have generally been limited to resolutions of a few nanometers at best. Annular bright field (ABF) imaging in aberration-corrected scanning transmission electron microscopy (STEM) now makes it possible to characterize crystal structures of battery materials directly with sub-angstrom resolution.2830

Given the dependence of lithiation/delithiation kinetics on particle surfaces, direct visualization of

their structures at the atomic level should contribute considerably to our understanding of electrode materials. Visualizing surface structures remains challenging, however, as it requires high-quality surface samples and more stringent viewing conditions than when imaging bulk crystals.

In this paper, we report atomic-scale observations of the reconstructed (010) surface of LiFePO4 before and after delithiation. Although other low-index surfaces such as (100) and (001) also play a role in the overall (de)lithiation behavior, the (010) surface is of particular interest because it is both the most stable planar surface (according to theoretical modeling studies26,27) and perpendicular to the main Li-ion migration channels; its detailed characterization is thus important for understanding the fundamental (de)lithiation mechanism in this system and any structural changes that this involves.

Numerous studies have shown how LiFePO4 particles can be formed with different morphologies by altering the synthesis method and conditions, making it difficult to determine “intrinsic” surface structures and properties. To avoid such extrinsic effects, we prepared pristine (010) surfaces of LiFePO4

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by cleaving large single crystals grown by a flux method, cutting them perpendicular to each principal axis and polishing them into a cubic shape (see Figure S1). Lattice parameters calculated from out-ofplane x-ray diffraction (XRD) analysis were a = 1.0336 nm, b = 0.6012 nm, c = 0.4696 nm, in good agreement with previously reported values.31 Out-of-plane XRD also confirmed that no secondary phases were present in the crystal (Figure S2), making it an excellent candidate for examining surface structures of LiFePO4 at the atomic scale.

Surface structures were examined using a scanning transmission electron microscope with aberration correction. ABF STEM imaging of the pristine (010) surface revealed that it was atomically flat, which implies that the surface has a low energy and high stability, consistent with theoretical calculations predicting the (010) surface to have lower excess energy than (100) and (001) surfaces.26,27 Figure 1a shows an ABF STEM image of the (010) surface taken down the [001] zone axis. The imaging conditions were chosen to observe light atoms (O and Li). Figure 1b shows a magnified image of the same surface, in which the contrast is inverted for the ABF STEM image so as to see each atom position clearly. The terminating layer corresponds to the y = 0.5 plane (related to the y = 0.0 plane by a rotation of 180o) of the orthorhombic unit cell, as shown by the atomic model overlaid on the micrograph in Figure 1b. No other terminations were observed for this surface orientation, consistent with theoretical calculations showing that the (Li+PO4)-terminated cut is energetically more stable than the Feterminated cut.26,27

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Li columns are clearly visible in the first layer below the terminating layer, as highlighted by the dotted circles in Figure 1b. However, Li and O columns above the surface are not visible in the image. Even if the lattice sites in the terminating layer were fully occupied, light elements such as Li and O are expected to undergo large displacements from their ideal positions/scattering centers (as reflected in their large Debye-Waller factors). Consequently the signals for light elements on the surface are invariably very weak and the corresponding atom positions difficult to pinpoint. Although P and Fe columns also undergo comparatively large displacements near the surface, they still provide sufficiently strong signals to be detected. In this case, the P atoms either shift outwards from the surface or laterally towards neighboring Fe atoms, as indicated by white arrows in Figure 1c.

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Figure 1. (a) ABF STEM image of the (010) surface of LiFePO4 taken down the [001] zone axis. (b to c) Magnified ABF STEM color images. The contrast has been inverted to make each atom position clearer. The dotted circles and white arrows in (b) indicate Li atom columns below the surface and sites of large cation displacements, respectively. The white arrows in (c) indicate the directions of shift of P and Fe atom columns. (d) Cross-section of the reconstructed (010) surface model of LiFePO4 obtained from first-principles calculations. (e) Simulated ABF STEM image generated using the model in (f), with spacings between large cations near the surface as indicated. The presence of lighter atoms on the surface can be inferred by comparing the observed displacements of subsurface columns with those predicted from first-principles calculations. To remove the intrinsic dipole of the stoichiometric as-cut (010) surface in the simulation box, half the Li ions on one side of the slab were moved to the corresponding sites on the opposite side. This in effect creates 50% Li vacancies 7


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in the outermost surface layer, which turns out to be the most stable arrangement for the ideal stoichiometric surface after performing energy minimization calculations (Figure S3).

The observed P and Fe displacements are consistent with structural relaxation behavior predicted from first-principles calculations indicated in Figure 1d. Figure 1e shows a simulated ABF image generated from the theoretical structure model in Figure 1d using xHREM (HREM Research Inc.). When Li atoms align as [001] columns in the outermost surface layer, the P atoms below them relax vertically outwards by about 25.5 pm. When the Li sites in the outermost layer are vacant, however, this vertical shift is only 15.0 pm, while the distance between P and Fe atoms decreases from 194.0 pm to 179.9 pm. Differences in P-Fe interatomic distances in the observed (010) surface thus provide evidence for the presence or absence of Li atoms in the outermost layer. The smaller fraction of widely spaced Fe-P columns and larger vertical shifts in the observed subsurface layer compared to flatter and more-closely-spaced pairs over a wide area (several nanometers) suggests that the Li content in the terminating layer of the ascleaved surface is significantly less than the theoretical concentration of 50% occupancy expected if local stoichiometry had been maintained. This may reflect the ease of removing Li ions from the (010) surface, or simply the high volatility of Li atoms, with a high proportion of the outermost Li atoms lost during synthesis.

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Figure 2. (a) Low-magnification BF STEM image taken within 100 h after delithiation. (b) Electron diffraction pattern obtained from surface region in (a). (c) Low-magnification BF STEM image taken more than 3000 h after delithiation. (d) Electron diffraction pattern obtained from surface region in (c).

To observe changes in the crystal and surface structures after removing Li, a cleaved crystal was chemically delithiated in an acetonitrile solvent using NO2BF4 as the oxidant. Figure 2a shows a low magnification BF STEM image of the (010) surface around 100 h after delithiation by chemical oxidation. Several microcracks can be seen to have formed at more-or-less regular intervals along the surface, producing strong strain contrasts in the STEM image. As reported previously, microcrack formation can be understood as a result of the large change in molar volume and large strain differentials induced by formation of the FePO4 phase.14,22 These cracks extend to a depth of around 100 nm into the crystal, suggesting that this is the extent of FePO4 phase formation under the conditions used in this

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study. The variations in contrast in the surface region also indicate that FePO4 phase formation and relithiation from the crystal bulk did not occur uniformly.

The electron diffraction pattern of the surface region in Figure 2b indicates a mixed two-phase region, although the intensities of the FePO4 phase are considerably weaker than those of LiFePO4. Comparison of the peak positions of the two-phase system with those in electron diffraction patterns for delithiated and lithiated regions (evidenced by EEL spectroscopy measurements, Figure S4), confirmed that the two phases are LiFePO4 and FePO4. The smaller fraction of FePO4 phase detected after only 100 h indicates that, even within this short time, some Li has relaxed back into the chemically delithiated regions.

Observation of the delithiated (010) surface of the main crystal again after 3000 h revealed that after this long period the microcracks had largely healed, with only a residual strain contrast appearing in STEM images (e.g., Figure 2c). Electron diffraction measurements (Figure 2d) of the surface also showed the crystal had returned to that of a single Li1-αFePO4 phase. These changes can be contrasted with what happens when FePO4 regions of crystal are physically separated from the bulk LiFePO4 crystal by lateral cracking. In this case, Li ions are prevented from migrating in the [010] direction back in to the FePO4 regions, so that delithiated (FePO4) regions are retained near the surface even after long relaxation times (Figures S4 and S5).

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Figure 3. (a) Low-magnification BF STEM image of the (010) surface taken over 3000 h after delithiation viewed down the [001] zone axis. (b) ABF STEM image (in color) of the surface in (a) in which the contrast has been inverted to make the atom positions clearer. (c and d) magnified images of regions A and B, respectively, in (b), spanning a step in the surface. (e) A magnified ABF STEM image of a region of the as-cleaved surface in Figure. 1a showing large P column displacements.

Figure 3a shows an ABF STEM image of the (010) surface 3000 h after delithiation. The surface is rougher than in the pristine state, indicating that delithiation degraded the crystallinity in this region (Figure S5). In the magnified view in Figure 3b, Li columns (indicated by dotted circles) can be distinguished in the subsurface region, confirming the relithiation inferred from the electron diffraction pattern.

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According to the analysis of Abdellahi et al.,32 an intermediate solid-solution interphase region between FePO4 and LiFePO4 is thermodynamically more stable than a sharp interface parallel to the ac plane because of the high coherent strain energy of the latter; this intermediate phase is predicted to occupy a significant fraction of a two-phase particle’s volume. The energy differential between high and low strain energies at coherent interfaces thus provides sufficient driving force for lithium to migrate facilely back to the (010) surface region, as proposed by Malik et al.33 for non-equilibrium crystals. In our sample the volume of delithiated crystal at the (010) surface is much smaller than that of the remaining lithiated crystal, so after a sufficient amount of time the ~100 nm thick FePO4 region, being narrower than the equilibrium intermediate phase region, was almost entirely relithiated. The reduction in strain results in closing of cracks perpendicular to the surface. Similar phenomena have been observed in nanoparticles of LiFePO4 using time-resolved XRD.15,16

A monolayer-high surface step (indicated by the dotted line between regions A and B) is also visible in Figure 3b. Figures 3c and 3d show highly magnified views of the regions A and B, respectively, labelled in Figure 3b. P atoms in both Figures 3c and 3d are displaced towards the surface even though crystal structures in regions A and B are mirror symmetric around the b axis. Cation displacement in this case, however, is smaller than that of the as-cleaved surface when Li ions are present in the terminating layer (Figure 3e). A survey of the cation displacements over a wide area found no instances of larger displacements, suggesting that there are no Li ions in the topmost surface layer after delithiation even after 3000 h. 12


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Figure 4. Li-K edge EEL spectra of (a) the as-cleaved (010) surface and (b) the same surface some 3000 h after delithiation. The Fe-M2,3 edge obtained from an FePO4 crystallite is included for comparison. Dotted lines indicate the peak-top positions of Fe-M2,3 edges, and red arrows indicate positions of Li-K edges.

Electron energy loss (EEL) spectra taken at various locations near the surface before and after delithiation using an EEL spectrometer attached to a Wien filter monochromated aberration corrected STEM are shown in Figure 4. The Li-K edge originates mainly from excitations from 1s to 2p orbitals. And, in LiFePO4, Fe-M2, 3 and Li-K edges are observed around from 53 to 57 eV and from 60 to 65 eV.34 Scans were taken in STEM mode within rectangular areas (about 30 nm wide × 0.6 nm deep) to avoid damaging the surface with the electron beam. Figures 4a and 4b show Li-K edge spectra as a function of distance from the surface into the bulk crystal before delithiation and 3000 h after delithiation, respectively. An Fe-M2,3 edge spectrum taken from a pure FePO4 particle is included for comparison. 13


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The first peaks of the Li-K edges in Figure 4 are indicated by red arrows. Prior to delithiation, a distinct Li-K edge is detected in the region closest to the surface (Figure 4a). After delithiation, however, the LiK edge peak in this region (up to 1 nm from the terminating plane) has disappeared almost entirely (Figure 4b). In the subsurface regions, the Li-K edge peak begins to reappear, consistent with the return of Li when the crystal is left to equilibrate. The absence of Li ions in the topmost layer after relaxation may be a reflection of the larger energy barrier for Li transport between subsurface and surface layers predicted from first-principles calculations.23

Although in this study we examined the effect of chemical delithiation on the (010) surface of LiFePO4, similar structural changes are expected to occur during electrochemical delithiation under potentiostatic conditions, as the transport of Li ions is the rate-determining step in both cases.32 The chemical delithiation rate under the conditions used in this study is comparable to a high rate of charging of a battery using LiFePO4 as the positive electrode.22,32 A similar amount of surface roughening and loss of crystallinity to a depth of ~100 nm as seen in Figure 3a can thus be expected to occur at (010) surfaces of LiFePO4 particles during the first (conditioning) cycle of such an electrode. Delithiation from (010) surfaces and the formation of the intermediate phase also has implications for nanostructural design of well-aligned LiFePO4 nanoparticles, or epitaxial growth of LiFePO4 thin films for use as positive electrodes.

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In conclusion, we have successfully observed the structure of the electrochemically important (010) surface of positive electrode material LiFePO4 before and after delithiation by applying STEM techniques to a high-quality cleaved single crystal subjected to chemical delithiation. Structural changes at the surface were found to be in good agreement with reconstructed (010) surfaces calculated from density functional theory. In particular, displacements of P and Fe atoms were found to depend on whether they were shielded by Li atoms on the surface or not. Relithiation of the surface region from the crystal bulk was also found to begin soon after chemical delithiation was halted. After being allowed to relax for long periods, the crystal returned almost entirely back to the Li1-αFePO4 phase, although the outermost surface layer remained Li-free and was much rougher than the pristine surface. These results provide further evidence for the size-dependence of the miscibility gap, as well as phase transformation and relaxation mechanisms, in LixFePO4. Extension of these methods to other surfaces and materials promises to rapidly enhance our understanding of these phenomena and contribute to the rational design of battery materials.

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Experimental Section. Commercially available LiFePO4 singe crystals (Oxide Corp.) were used for all experiments. The crystals were cut perpendicular to each principal axis and polished (Figure S1a). Lattice parameters were determined using an out-of-plane XRD (Superlab, Rigaku Co.). Clean surfaces were prepared by manually applying a bending force to a crystal with a pair of tweezers. Various cleaved surfaces were topotactically delithiated by chemical oxidation in an acetonitrile solvent, for which the oxidant NO2BF4 was selected with a proposed chemical reaction of LiFePO4 + xNO2BF4 → Li1-xFePO4 + xLiBF4 + xNO2. The molar ratio, x, was about 0.2. After a reaction time of 5 min, the reactant was washed in acetonitrile several times. As-cleaved and delithiated surfaces were embedded into epoxy resin (G2, Gatan, Inc.) and thermally cured at 120 ° C for 1 h for STEM observation. Cross-sectional STEM samples were prepared by dual-beam focused ion beam scanning microscopy (NB5000, Hitachi High-Technologies Co.) using Ga ions. Next, Ar ion milling (PIPS, Gatan Inc.) with a cold stage was performed to prepare TEM specimens. The structure of the (010) surface of LiFePO4 was investigated using an aberration-corrected (CEOS GmbH) scanning transmission electron microscope (JEM-2100F, JEOL Ltd.). STEM observations were performed at accelerating voltages of 200 kV. The probe-forming aperture semiangle used was 17 mrad, and ABF STEM images were recorded with 10–23 mrad detectors. A radial difference ﬁlter (HREM Filters Lite v1.5.1, HREM Research Inc.) was applied to the image to reduce noise. ABF STEM image simulations were generated using the program xHREM (HREM Research Inc.). 16


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Chemical compositions and charge states were analyzed using an EEL spectrometer (Tridiem ERS, Gatan, Inc.) attached to a Wien filter monochromated aberration corrected STEM (JEM-2400FCS, JEOL Ltd.) operated at 200 kV. EEL spectra were recorded within a rectangular area (about 30 nm × 0.6 nm) in STEM mode, using 0.1 eV per channel and an energy resolution of 300 meV (full-width at halfmaximum of zero-loss peak). The background signal of the EEL spectra was subtracted by power law fitting. The Li-K edge in these spectra originates mainly from excitations from 1s to 2p orbitals of Li atoms. In bulk LiFePO4, Fe-M2, 3 and Li-K edges are observed at around 53 to 57 eV and 60 to 65 eV, respectively.34,35 The structure and stability of reconstructed (010) surfaces were investigated using first-principles calculations within the framework of density functional theory (DFT). The projected augmented wave (PAW) method was used, as implemented in the Vienna ab initio Simulation Package (VASP).36 An energy cutoff of 400 eV and appropriate k-point meshes were chosen for bulk crystal and surface calculations to ensure that total energies were converged to within 3 meV per formula unit of LiFePO4. The GGA+U approach37 was used to accurately calculate the surface redox potentials and to guarantee that the excess holes or electrons are properly localized.

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ASSOCIATED CONTENT Supporting Information Detailed methods and additional data are included. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author E-mail: [email protected] (S. K.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS This work was supported by the Research & Development Initiative for Scientific Innovation of New Generation Batteries (RISING) and the Research & Development Initiative for Scientific Innovation of New Generation Batteries II (RISING II) project from the New Energy and Industrial Technology Development Organization (NEDO), Japan. X-ray diffraction measurements were conducted at the Research Hub for Advanced Nano Characterization, The University of Tokyo, with support from the "Nanotechnology Platform" (project No.12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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