Article pubs.acs.org/cm
Scanning X-ray Fluorescence Imaging Study of Lithium Insertion into Copper Based Oxysulfides for Li-Ion Batteries Rosa Robert,†,‡ Dongli Zeng,‡ Antonio Lanzirotti,§ Paul Adamson,∥ Simon J. Clarke,∥ and Clare P. Grey*,†,‡ †
Department of Chemistry, Cambridge University, Lensfield Rd, Cambridge, CB2 1EW, U.K. Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794-3400, United States § Center for Advanced Radiation Sources, The University of Chicago, Upton, New York 11973, United States ∥ Department of Chemistry, Oxford University, South Parks Rd, OX1 3QR, U.K. ‡
S Supporting Information *
ABSTRACT: Ex situ and in situ Synchrotron X-ray fluorescence imaging coupled with selective micro-X-ray absorption near-edge spectroscopy (μXANES) and micro-X-ray diffraction (μXRD) were used to investigate the electrochemical lithiation of the layered oxysulfide Sr2MnO2Cu3.5S3. Microfocused X-ray fluorescence (XRF) imaging was used to image the elemental components within the battery electrode while μXANES and μXRD provided information about the Cu oxidation state and phase distribution, respectively. Sr2MnO2Cu3.5S3 operates by a combined insertion/displacement mechanism. After 1 mol of Li intercalation, Cu metal extrusion is observed by μXRD, which also reveals the formation of the Sr2MnO2Cu3.5−xLixS3 phase. Ex situ μXRF images of the electrode after 3.75 mol of Li intercalation show segregated Cu metal and Sr2MnO2Cu3.5−xLixS3 particles, while in situ μXRF imaging experiments reveal that the Cu and Mn elemental distribution maps are highly correlated to the particle orientation giving different results when the particle is oriented either perpendicular or parallel to the incident beam. In situ electrochemical synchrotron XRF imaging has the advantage over the ex situ mode in that it allows the reaction mechanism of a single particle to be followed vs time. In situ μXRF imaging data suggest that the microstructure of the electrode, on a microscale level, is not affected by the Cu extrusion process. KEYWORDS: scanning X-ray fluorescence imaging, Li-ion batteries, cathode materials, insertion/displacement reactions, oxysulfides
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INTRODUCTION Li-ion batteries (LIBs) are of considerable importance since they are key components in portable electronic devices, such as laptop computers and cellular phones. They also represent the most viable energy storage technology for electric vehicles/ hybrid electric vehicles. Although intercalation compounds such as LiCoO2 are the most widely used electrode materials in LIBs, the constant demand for higher energy density batteries has motivated a study of a wider range of materials and electrode materials that react with lithium via alternative mechanisms.1,2 In this regard, the series of layered oxysulfides, Sr2MnO2Cu2m−δSm+1 (m = 1, 2, and 3, δ ∼ 0.5) are interesting model systems that operate via a so-called intercalation/ displacement mechanism.3−5 These materials consist of alternating perovskite-type [Sr2MnO2] sheets, which remain essentially unchanged during the electrochemical reactions, and antifluorite-type [Cu2S] sheets with varying thicknesses. The Li ions can be reversibly inserted into the framework structure replacing Cu+ in the MS4 tetrahedral sites, leading to the extrusion of elemental Cu.3,5 Sr2MnO2Cu4−δS3, the m = 2 member of the family contains double thickness antifluoritetype Cu2−δS2 layers, that is, Cu4−δS3 (Figure 1a), and exhibits a theoretical capacity of 187 mAh/g and good capacity retention upon cycling. The crystal structure of this member is tetragonal © 2012 American Chemical Society
with P4/mmm space group (a = b = 4.0217(1) Å and c = 11.4180(3) Å).3 It has a copper deficiency of δ ≈ 0.5 at the tetrahedral site resulting in the molecular formula Sr2MnO2Cu3.5S3, diffraction data revealing highly delocalized positions for the copper ions within the layers.3 The Mn oxidation state was determined to be +2.5 based on XANES and neutron diffraction refinement results.3 Upon lithiation, Mn2.5+ is reduced to Mn2+ and Cu+ to Cu metal,4 so that a total of 4 Li+ ions can be inserted into the structure, a process that requires 4 electrons. This class of materials should be contrasted with intercalation materials, structures that preserve their host crystal structure upon Li insertion and extraction. Intercalation materials have a capacity that is limited by the number of vacant sites present in their crystal structure that can reversibly accommodate lithium. In contrast, materials that operate via a combination of intercalation and displacement mechanisms, such as Cu2.33V4O11,1 CuTi2S4,2 and here, Sr2MnO 2Cu2m−δSm+1, provide a potential route to higher energy densities since more possible sites are available to Li+, fresh sites being made Received: February 17, 2012 Revised: May 31, 2012 Published: June 13, 2012 2684
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Figure 1. (a) Sr2MnO2Cu4−δS3 (Sr2MnO2Cu3.5S3) crystallizes in tetragonal symmetry with P4/mmm space group (a = b = 4.0217(1) Å and c = 11.4180(3) Å)3 and consists of alternating perovskite-type Sr2MnO2 sheets and double thickness antifluorite-type CuS2 layers. Figure adapted from ref 4 (Copyright 2006, American Chemical Society) and (b) the voltage profile for the in situ and ex situ Sr2MnO2Cu3.5S3/Li cells during the first discharge/charge cycle, obtained with a discharge rate of C/15 and C/20, respectively. Both the Li content x, referring to the amount of Li insertion per formula unit, and the specific capacity in mAh/g are denoted in the figure. The red circles indicate the compositions at which the ex situ maps were acquired (Figure 4). The Li content varies continuously throughout the in situ data collection, each map taking 48 min to record. The 6 highlighted intervals correspond to the 6 maps shown in the Figure 7.
available by the metal ions (in this case Cu) that are displaced (extruded). Additionally, and similar to conversion materials, these displacement materials can accommodate multiple electrons per formula unit, leading to high overall capacities, since the transition metals of the host (e.g., V, Ti, Mn) can be reduced in addition to the metal ions that are displaced (e.g., Cu), the latter being reduced to their metallic state.1,6 Different types of advanced characterization techniques such as synchrotron spectroscopies,7,8 diffraction techniques,7,9 solid state NMR,10,11 optical microscopy, atomic force microscopy,12−14 and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (EDS),1 high resolution transmission electron microscopy (TEM),15−18 among others, have been used to study battery materials in situ, that is, while they are operating in a working cell, to obtain mechanistic and dynamic information about the electrochemical reactions. For example, Morcrette et al. used in situ SEM and EDS to observe copper extrusion and dendrite formation as Cu2.33V4O11 reacts electrochemically with Li.1 Of recent note, a novel X-ray imaging setup based on the combination of full-field Transmission X-ray Microscopy (TXM) with X-ray Absorption Nearedge Structure (XANES) spectroscopy has enabled the imaging, in two/three-dimensions, of large sample areas (up to millimeters) with a resolution of tens of nanometers offering chemical speciation at the nanoscale.19 Use of these techniques is aimed at answering fundamental questions about a material’s elemental composition, its crystal and local structures, and microstructure variations during the reaction processes. Longstanding goals in the study of the electrochemical reactions are to identify the microstructural properties of the electrodes with high spatial resolution and to understand how these properties affect the battery performance. Therefore, information on a micrometer-scale is of substantial importance, for example, in evaluating how individual atomic/ionic species are distributed as function of location in the electrode film and how the oxidation state of the metal cations vary spatially during the
electrochemical reactions. This information serves to complement other in situ imaging methods such as TEM,15−18 which probe electrode reactions at the particle or subparticle level. Synchrotron X-ray fluorescence imaging allows for convenient visualization of the reactions that occur on the electrode at a micrometer scale or smaller and, when combined with other synchrotron-based microbeam techniques such as X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), this method, and applied in situ, provides insight into the dynamics of electrode reactions and shape changes of particles. Synchrotron-based X-ray fluorescence microprobe imaging utilizes a focused beam of X-rays as the excitation source to make measurements of the characteristic X-ray emission lines from the de-excitation of electronic core levels for each atom in a material. Thus, this technique is element specific with detection limits that are typically on the order of 1 ppm or better for transition metals. Information about the bulk and surface morphology can be obtained over large areas of an electrode surface by raster scanning the sample through the microfocused beam, producing two-dimensional elemental maps. These elemental distribution maps of the electrode can be recorded with micrometer-scale spatial resolution by extracting the fluorescence counts from element-specific X-ray emission lines in the energy dispersive spectra. Since synchrotron X-ray fluorescence imaging allows for direct analysis of the electrode heterogeneity present within the observation area with micrometer resolution, we have utilized this advanced technique, to help us understand the electrochemical processes that occur in the layered oxysulfide Sr2MnO2Cu3.5S3, at the electrode level at different stages of electrochemical lithiation. The oxidation state and spatial distribution of Cu and Mn in the electrode film are evaluated for both ex situ samples and in situ cells with the X-ray microprobe. Selective micro-X-ray absorption near-edge spectroscopy (μXANES) and micro-X-ray diffraction (μXRD) can then be performed at several selected locations. In this study, X2685
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normalized to an incident beam flux measured by an ion chamber just upstream of the focusing optics and then normalized to the edge-step. The XANES spectra at the Cu K-edge were collected at room temperature. The absorption spectrum of a standard Cu foil was measured for calibration. The energy scale was calibrated with respect to the first inflection point in the K-edge spectrum of Cu metal (8979 eV). Spectra were acquired from 8930 to 9060 eV (energy resolution of the beamline, ΔE/E = 2 × 10−4) in 10 eV increments within the pre-edge region of 8930−8965 eV, 0.2 eV near the edge of 8965−8990 eV, and 1 eV within the region of 8991−9060 eV. For all compounds, the first root in the first derivative of the near-edge region was taken as the edge position (i.e., threshold energy). XAS data evaluation was carried out using the IFEFFIT interactive software package (with the ATHENA graphical interface).21,22 Microfocused XRD experiments were conducted in transmission mode using a Rayonix SX-165 CCD area detector at an incident energy of 17479 eV (MoKα = 0.7093 Å). XRD detector calibration was done using the SRM 674a diffraction standard α-Al2O3 and AgBehenate (AgC22H43O2). Calibrations and corrections for detector distortions (camera-sample distance, the camera tilt and rotation, and the beam center on the camera plane) were done using Fit2DTM software.23 For easy comparison, all the patterns were converted to those corresponding to a Cu Kα1 wavelength (λ = 1.5406 Å).
ray fluorescence maps are used to image the elemental components within the battery electrode at different stages of electrochemical lithiation. The oxidation state of Cu and phase distribution as function of the location in the cell is investigated by means of μXANES and μXRD, respectively.
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EXPERIMENTAL SECTION
Sr2MnO2Cu3.5S3 was prepared using a solid-state synthesis method which had been adapted for air-sensitive materials and is described elsewhere.3 The positive electrode film used for ex situ experiments consisted of 80 wt % of the active material, 10 wt % of carbon black, and 10 wt % of poly vinylidene fluoride (PVDF) binder, named as Film-I. Alternatively, for the in situ experiments, cathode films consisting of 33 wt % of the active material, 40 wt % of carbon black (the additional carbon being used to dilute the particles somewhat), and 27 wt % of polytetrafluoroethylene (PTFE) binder were prepared, and named as Film-II. Coin cells (CR2032, Hohsen corp.) were assembled in an argon-filled glovebox. Each cell typically contains a film of several tens of micrometers with about 15 mg of active material, separated from Li foil as negative electrode by two pieces of Celgard separator (Celgard Inc., U.S.A.). A 1 M solution of LiPF6 in EC:DMC (1:1) has been used as the electrolyte. The in situ coin cell was prepared as described above but modified so that a small window (sealed using Kapton film and epoxy) was made on the cathode side. The electrochemical experiments were carried out on a battery cycler (Arbin Instrument, College Station, TX) in galvanostatic mode at a C/ 20 rate. Batteries stopped at desired stages of discharge (e.g., for 1.0, 2.0, and 3.75 Li insertion, as shown in Figure 1b) were opened in an Ar-filled glovebox and the cathode films were washed with DMC and dried. The resulting samples were sealed as films between layers of Kapton tape for the ex situ experiments. The in situ coin cell was cycled at a rate of C/15, and the electrochemistry is shown in Figure 1b. The morphology of the prepared cathode material, at different stages of Li insertion, was characterized using a LEO-1550 scanning electron microscope (SEM). Ex situ and in situ X-ray microprobe experiments were performed at the beamline X26A at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY. The beamline was utilized for microfocused X-ray fluorescence (μXRF) imaging of elemental components within the batteries, microfocused X-ray diffraction (μXRD) for phase identification, and microfocused X-ray absorption near-edge spectroscopy (μXANES) to evaluate the oxidation state of metallic components as the experiment progressed. A monochromatic X-ray beam was utilized for all experiments, and tuned using a Si(111) monochromator. This monochromatic beam was then collimated to 350 × 350 μm and Rh-coated reflective focusing mirrors arranged in a Kirkpatrick−Baez geometry were used to achieve a focused beam size of ∼9 μm in the horizontal by ∼5 μm in the vertical. Compositional μXRF mapping was conducted at an incident beam energy of 8981.2 eV and the higher energy of 9400.0 eV (which efficiently excites the K-edge spectra of all the Cu and Mn species), by raster scanning the battery through the incident X-ray beam. The incident beam at the tuned energy can probe into the electrode by about 15 μm and ∼4 μm for Sr2MnO2Cu3.5S3 and Cu metal, respectively, and >1000 μm assuming a bulk C composition. Maps were taken in an area/step size of either (170 μm × 170 μm)/5 μm, (200 μm × 200 μm)/5 μm, or (400 μm × 400 μm)/10 μm with a 1- to 2-s dwell time per pixel. Energy dispersive fluorescence data were collected using a combination of a single element silicon drift diode detector from SII Nanotechnologies and a 9-element HPGe solid-state detector array from Canberra Industries. All detectors were processed digitally using the XMap series of digital spectrometers from XIA. Each map represents 48 min of measurement. Micro-X-ray diffraction (μXRD) and X-ray absorption near-edge spectroscopy (XANES) were performed on selected locations on each 2D map. All μXANES measurements were performed in fluorescence detection mode, that is, the characteristic X-ray emission lines from deexcitation of electronic core levels for the atom under study are taken as an approximation for X-ray absorption.20 Each spectrum was
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RESULTS AND DISCUSSION Figure 2a shows a representative SEM photomicrograph of the cathode films (Film-I) used for these electrochemical studies
Figure 2. SEM images of the cathode films: (a) Film-I (consisting of Sr2MnO2Cu3.5S3: carbon black: PVDF (80: 10: 10) wt %) and (b) an isolated Sr2MnO2Cu3.5S3 particle in a diluted cathode Film-II (consisting of Sr2MnO2Cu3.5S3: carbon black: PVDF (33: 40: 27) wt %).
and the ex situ X-ray fluorescence experiments. The fresh electrode consists of 10−40 μm Sr2MnO2Cu3.5S3 particles in contact with carbon black that was added to the electrode for enhancement of the electronic conductivity. Figure 2b displays a particle in the diluted cathode film (Film-II), which contain a lower Sr2MnO2Cu3.5S3 concentration and higher carbon content. This film is characterized by dispersed, (see Supporting Information, Figure S1) larger particles that are approximately ∼40 μm in size (as shown in Figure 2b) and smaller flakes that have broken off. Considerable cracking/ delamination of these particles has occurred, possibly during electrode production, the delamination largely appearing to occur parallel to the layers. Ex situ X-ray fluorescence two-dimensional elemental maps (200 μm × 200 μm in size; 5 μm steps) of the pristine Sr2MnO2Cu3.5S3-based electrode film (Film-I), at the Cu Kα (Figure 3a) and Mn Kα edges (Figure 3b) show a homogeneous distribution of the active material in the matrix within a measured area of 400 μm2. The high intensity areas in both the Cu Kα and Mn Kα maps correspond to locations in which the active material particles are located, whereas the lower intensity regions correspond to the carbon matrix. Regions with what appear to be isolated, primary Sr2MnO2Cu3.5S3 particles of length 0.03−0.06 mm (30−60 2686
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Figure 3. Two-dimensional (a) Cu Kα and (b) Mn Kα elemental maps for the Sr2MnO2Cu3.5S3-based electrode film (Film-I). The measurement was taken on a film electrode area of 400 μm2 with a step size of 5 μm (200 × 200 μm/5 μm) an incident energy of 9400 eV. The color bars indicate the counting scales with unit count per second (cps). The x and y axes define the area of the electrode film that was irradiated by the photon flux.
μm), are observed, consistent with the SEM images, along with regions that clearly contain overlapping particles. Relative differences in the detected count rates for Cu Kα and Mn Kα X-ray fluorescence emissions primarily reflect differences in the distributed elemental masses of Cu and Mn in the sample but are also affected by the relative differences in the degree of Xray self-absorption of the Cu Kα and Mn Kα emission lines respectively by the sample and the air path between the sample and detector because of the differences in the energy of the two photon emissions. Mn Kα emissions will provide a calculated fluorescence yield in this material of approximately 0.625 × 108 Photons/scan ratio/gram, compared to 0.169 × 109 Photons/ scan ratio/gram for Cu Kα. This should be borne in mind when examining the compositional images since it means that for the same number of atoms, the detected Mn Kα fluorescence intensity would be roughly 37% lower than that of Cu Kα Taking this into account, stoichiometric Sr2MnO2Cu3.5S3 with an atomic fraction of Mn/Cu ratio of approximately 0.25 will yield a detected fluorescence intensity ratio of Mn Kα to Cu Kα of ∼0.09. The microstructural changes that occur during lithiation of Film-I were then investigated by ex situ X-ray fluorescence imaging measurements. The electrochemistry profile during the first discharge is similar to that obtained for the in situ experiment (discussed below) and shown in Figure 1b. Figure 4 shows the Cu Kα 2D maps (400 × 400 μm2) collected with step sizes of 10 μm and the Cu K-edge XANES spectra of three lithiated samples Lix (x = 1, 2, and 3.75), where x denotes the electrochemically inserted amount of Li per formula unit (i.e., Sr2MnO2LixS3) (Figure 4a to 4c, respectively). Two or three different locations in each cathode film, denoted as I, II, (and III), have been selected for the μXANES measurements. The ex situ 2D maps for Li2 and Li3.75 (Figure 4b and c), show a more dispersed distribution of the active material within the matrix, while the region of the Li1 electrode film (see Figure 4a) correspond to an area where the active material is highly agglomerated and the carbon matrix is barely visible. As clear differences are seen among these ex situ samples in terms of the distributions of the particles, which can in part be ascribed to the fact that we are examining different parts of the electrode films, in situ experiments where a selected electrode area was constantly observed during cycling have been performed to determine how the microstructure of the
Figure 4. Two-dimensional elemental maps for Cu at an energy level of 9400 eV and the corresponding XANES spectra scanned through the Cu K-edge (8970-8995 eV) at selected locations in the map (assigned as I, II, and III) for three lithiated samples Lix, (a to c) x = 1, 2, and 3.75, respectively, where x denotes the amount of Li inserted electrochemically per formula unit. The color bar indicates cps. The x and y axes define the area of the electrode film that was irradiated by the photon flux.
electrode film evolves during discharging, and these are described later on in the paper. In the Li1 sample, no significant difference (apart from a small shift for region III) is observed in the Cu edge at all the three selected locations in the 2D map (Figure 4a), the average copper oxidation state being very close to that of the pristine sample. However, the (111) reflection from Cu metal is clearly seen from an XRD pattern taken from a selected location (I) on electrode film sample Li1 (see Figure 5), which indicates that copper metal is extruded after 1 mol of Li insertion in an amount that is sufficiently large to be detected by diffraction methods. Furthermore, the XRD pattern reveals the appearance of the (001) Bragg reflection at a low 2-θ angle (∼ 7° 2θ) characteristic of the lithiated Sr2MnO2Cu3.5S3 phase.5 This reflection is extremely weak in the Sr2MnO2Cu3.5S2 parent phase and increases in intensity on lithiation because of the different scattering factors of Li and Cu. Similar XRD patterns (see Figure 5) are recorded for spots II and III in the same map shown in Figure 4a. It is worth pointing out that because of the small diameter of the incident beam (∼9 μm in diameter), the crystallites may not be small enough to produce well-defined Debye−Scherrer rings (i.e., there may be a poor powder average). Furthermore, the anisotropic shape characteristic of 2687
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and the diffraction pattern for this sample shows narrow and intense peaks associated with copper metal, clearly indicating the existence of crystalline copper metal particles (see Figure 5). At this stage of discharge, the inserted amount of lithium is larger than the initial amount of copper (around 3.5 mol per formula unit) in the layered oxysulfide, indicating the complete occupation of Li in the tetrahedral sites in the structure. A comparison between the Cu Kα and Mn Kα elemental maps of the 3.75Li sample (see Figure 6a and b) reveals different spatial distributions of these elements within the same electrode film area (highlighted by dashed area), that is, Mn and Cu are segregated at the nearly fully discharged stage. Since Mn remains in the structure of the lithiated phase Sr2MnO2Li3.75S3, the different spatial distribution of Cu and Mn suggests that the copper metal is extruded from the main particle. Nonetheless, the electrode materials are still homogenously dispersed in the carbon matrix. SEM images along with EDS analysis of the Li inserted materials (Figure 6c and Supporting Information, Figure S1) confirm that the Li-driven displacement process leads to the growth of Cu dendrites. Furthermore, the SEM images suggest that the Cu filaments are extruded from between the layers, which is consistent with similar observations made for the layered Cu2.33V4O11 system.1 As discussed in previous studies of electrode reactions involving Cu dendrite formation,24 since these materials show high electronic conductivities (the resistivity for Sr2MnO2Cu3.5S3 is only 20 Ω cm 3), reduction and the associated lithium insertion copper extrusion processes do not have to commence at the point where electrical contact is made between the carbon conducting matrix of the electrode and the active material. Rather, the process will be controlled by structural factors, the layered structure serving to confine the migration path of copper (and lithium) in two dimensions. Thus the copper particles will likely nucleate and grow on the edges of the particles that bisect the perovskite and sulfide layers. This proposal is consistent with the spatial separation of Cu and Mn seen in many places in the 2D images. In situ Scanning X-ray fluorescence-yield imaging data of an area of the positive electrode in the Sr2MnO2Cu3.5S3|Li cell were also collected to obtain a better insight into the chemical composition and microstructural changes of this layered
Figure 5. XRD patterns of Sr2MnO2Cu3.5S3 and samples with 1 mol, 2 mol, and 3.75 mol of Li electrochemically inserted per formula unit at the location I, II, II in the electrode films (see Figure 4). The green ticks denote the Bragg peaks corresponding to the Sr2MnO2Cu3.5S3 phase with tetragonal symmetry and the P4/mmm space group,3 and the orange ticks correspond to elemental copper. The (001) Bragg reflection characteristic of Li insertion into the tetragonal structure is labeled along with the characteristic Cu metal reflections at 43.32° 2θ (111) and 50.45° 2θ (200).
the Sr2MnO2Cu3.5S3 particles results in the introduction of nonrandom crystallite orientations (i.e., preferred orientation) which cause considerable distortions of the scattered intensity leading to some differences on the collected XRD patterns at different sample locations. A significant Cu edge shift in the μXANES spectra of the sample after 2 mol of Li insertion is detected (sample Li2, Figure 4b). In addition, the XRD patterns taken from location I, II, and III in sample Li2 (see Figure 5) show that the Bragg reflections corresponding to metallic Cu gain in intensity, confirming the progressive copper metal accumulation due to its extrusion from the structure during discharge. After 3.75 mol of Li insertion (sample Li3.75, see Figure 4c), the copper absorption edges in both μXANES spectra (taken at location I and II as illustrated) are close to that of the Cu metal,
Figure 6. (a, b) Two-dimensional elemental maps of the almost fully lithiated sample, Li3.75 (Sr2MnO2Li3.75S3)-based electrode film at an energy level of 9400 eV for (a) Cu Kα and (b) Mn Kα. The color bar indicates cps. The x and y axes define the area of the electrode film that was irradiated by the photon flux, and (c) SEM ex situ image of the reduced electrode (Li3.75) consisting of layered particles, mainly composed by Sr, Mn, S, and O, and Cu dendrites as deduced from EDS analysis. 2688
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Figure 7. X-ray fluorescence compositional maps for Cu Kα and Mn Kα of an in situ Sr2MnO2Cu3.5S3|Li cell at different stages of dis/charge, (a) discharge maps (from top to bottom): Li0, Li1.4−1.6, and Li4.0 (fully discharged), and (b) charge maps (from bottom to top): Li2.25−2, Li1.5− 1.26, and Li0.1 (fully charged to 3.75 V). The Li contents, denoted by x in Lix, indicate the amount of Li inserted/remaining in the Sr2MnO2Cu3.5−xLixS3 structure. The Cu Kα map was collected at 8981 eV (corresponding to the first maxima of the K-edge for Cu+ in these samples) and normalized to the Cu fluorescence at 9400 eV (the edge step) for each pixel. This allowed the changes in distribution of Cu+ to be mapped with time. The Mn Kα map was collected with a 8981 eV incident beam energy. The maps were collected at a same region on the electrode film of an area (120/130 μm × 140 μm) at 5 μm/step with dwell time of 2s/pixel. The elemental maps correspond to the highlighted intervals in the electrochemistry profile recorded for the in situ cell and are shown in Figure 1b. The range of Li contents listed for each map reflects the time taken to acquire the maps.
Figure 8. Schematic illustration of the differently oriented Sr2MnO2Cu3.5S3 particles in the electrode film investigated in the X-ray fluorescence imaging maps (Figure 7): (a) particles A and (b) B, showing the direction in which the Cu filaments are extruded.
Supporting Information, Figure S2). Representative Cu and Mn Kα elemental maps, corresponding to the crosses labeled in the electrochemistry profile recorded for the in situ cell (Figure 1b), at Li0, Li1.4−1.6, fully discharged to Li4.0) and charge: Li2.25−2, Li1.5−1.25 and fully charged to 3.75 V (Li0.1), are
oxysulfide during the electrochemical processes in real time. The voltage profile for the Sr2MnO2Cu3.5S3/Li cell during the first cycle was shown in Figure 1b. These experiments made use of the cathode film with a more dispersed active material to enable the observation of more isolated particles, (see 2689
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situ data, segregation between Cu and Mn elemental distribution can be associated to the different particle orientations. In situ experiments reveal that the Cu and Mn elemental distribution maps are highly correlated with the particle orientation giving different results depending on whether the particle is oriented perpendicular or parallel to the incident beam. This effect was clearly seen at the end of the first discharge, where the copper metal and the fully lithiated phase Sr2MnO2Li4S3 appear to be either in the same region of the map or spatially separated. In situ imaging during charge suggests that a reversible Li insertion/Cu extrusion process is occurring in Sr2MnO2Cu3.5S3. Observing and recording electrode reactions using in situ electrochemical X-ray fluorescence imaging has the advantage over the ex situ mode in that the mechanisms involving single particles can be resolved. The results shown provide a clear illustration of the behavior of micrometer-scale Sr2MnO2Cu3.5S3 particles as the active components of a carbon black/binder/Sr2MnO2Cu3.5S3 composite electrode during lithiation. Further in situ studies are planned to evaluate in more detail how the microstructures affect and evolve during the following further cycles as a function of location in the electrode.
shown in Figure 7. Each map took 48 min to record, the Li content changing continuously throughout the mapping. Thus the Li content given here represents the range of Li contents during the map collection. Two well-defined particles, highlighted with dashed rectangles in the top maps shown in Figure 7 (labeled as particle A and B), were analyzed in more detail, revealing different variations of the Cu distribution for the two studied particles. In particle A, copper metal agglomerates in its center upon full discharging, while in particle B the copper agglomerates at the side of the particle. Assuming that the Cu dendrites are extruded between the layers of the particle as seen by SEM, particle A and B are most likely oriented differently in the electrode film as depicted schematically in Figure 8. The results strongly suggest that the ab layers of particle A are oriented perpendicular to the plane of the electrode while in particle B they lie in the electrode plane. This assumption is supported by the Mn Kα maps in Figure 7: In particle A, the Mn and Cu elemental distributions remain uniform within the particle throughout the whole discharge process, as Cu is extruded. In contrast in particle B, a segregation of the Cu and Mn elemental distributions is observed, as Cu is extruded from the side of the particle, as was observed for the ex situ Li3.75 maps (Figure 6a). These observations show the importance of in situ experiments versus ex situ experiments, the in situ studies allowing us to establish a correlation between particle orientation and elemental map distribution which is lacking in the ex situ experiments where we are unable to track the reaction of an individual particle as a function of state of charge. The μ-XANES spectra performed at this stage of discharge show that all the Cu is in its copper metal form. Copper elemental mapping indicates that Cu0 is formed by all the particles, and therefore, all the particles have reacted during the electrochemistry reaction. The processes operating on charge differ noticeably from those occurring during discharge, which we ascribe to the different diffusivities of Li and Cu and the difficulty of reinserting Cu into the host lattice. As discussed in more detail elsewhere,25 removal of much of the Li is accompanied with little reinsertion of the Cu up to a charge voltage of 2.75 V and consequently almost no intensity differences are observed in the in situ maps corresponding to the Li2.0−Li1.25 Li content range (Figure 7b). The complete reinjection of Cu into the structure is sluggish, and a large overpotential is required to complete the process, that is, the cell voltage rises to over 3 V. A more uniform Cu distribution across the 2 particles is seen in the Cu map at the end of charge (Figure 7b) consistent with Cu insertion into the particles during charge. μ -XANES spectra performed at full charge, (3.75 V, Li0.1), reveals that Cu+ is present in the reconverted Sr2MnO2Cu3.5S3. Concerning the microstructure of the electrode, in situ imaging data suggest that, on a microscale level (i.e., at a 5 μm resolution), the microstructure is not affected during the reaction process at least in the first cycle.
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ASSOCIATED CONTENT
S Supporting Information *
Scanning electron microscopy images of the electrode-base films. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Generalitat de Catalunya is gratefully acknowledged for the Postdoctoral research grant awarded to R. Robert (BP-DGR 2008). The authors thank Dr. Jordi Cabana, Lawrence Berkeley National Laboratory, for helpful discussions. C.P.G. and D.Z. thank the Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC0376SF00098, via subcontract No. 6517749 with the Lawrence Berkeley National Laboratory, for support. Portions of this work were performed at Beamline X26A, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. X26A is supported by the Department of Energy (DOE) Geosciences (DE-FG02-92ER14244 to The University of Chicago - CARS) and DOE - Office of Biological and Environmental Research, Environmental Remediation Sciences Div. (DE-FC09-96-SR18546 to the University of Kentucky). Use of the NSLS was supported by DOE under Contract No. DE-AC02-98CH10886.
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CONCLUSIONS In summary, this study has shown directly that Sr2MnO2Cu3.5S3 takes up Li by a process that involves extrusion of copper as dendrites. At the initial stage of Li insertion (sample Li1), the average copper oxidation state is close to that of the pristine compound and is homogeneously spatially distributed. Upon further Li intercalation (sample Li2), Cu metal extrusion is observed, and a segregation between the Cu metal particles and the Sr2MnO2Cu3.5−xLixS3 phase starts to be present. From in
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