Article pubs.acs.org/cm
Correlation between Chemical and Morphological Heterogeneities in LiNi0.5Mn1.5O4 Spinel Composite Electrodes for Lithium-Ion Batteries Determined by Micro-X-ray Fluorescence Analysis Ulrike Boesenberg,*,† Mareike Falk,‡ Christopher G. Ryan,§ Robin Kirkham,§ Magnus Menzel,∥ Jürgen Janek,‡ Michael Fröba,∥ Gerald Falkenberg,† and Ursula E. A. Fittschen∥,⊥ †
Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany Justus-Liebig University Gießen, Institute of Physical Chemistry, Heinrich-Buff-Ring 58, 35392 Gießen, Germany § Commonwealth Scientific and Industrial Research Organisation CSIRO, Clayton, Victoria 3169, Australia ∥ University of Hamburg, Institute of Inorganic and Applied Chemistry, Martin-Luther-King-Platz 6, 20146 Hamburg, Germany ⊥ Washington State University, Department of Chemistry, Post Office Box 644630, Pullman, Washington 99164-4630, United States ‡
S Supporting Information *
ABSTRACT: The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel is a promising material for high-energy battery applications, despite problems of capacity fade. This is due in part to transition metal leaching that produces chemical and morphological inhomogeneities. Using fast microX-ray fluorescence spectroscopy to scan the sample at medium spatial resolution (500 nm) over millimeter ranges, effects of cycling rate and state-of-charge on the elemental distribution (Ni and Mn) for LiNi0.5Mn1.5O4/carbon composite electrodes in LNMO/Li cells are visualized. Charge distribution is imaged by mapping the Ni oxidation state by acquisition of a stack of elemental maps in the vicinity of the Ni K edge. Our results show significant effects on morphology and elemental distribution, such as formation of elemental hot-spots and material erosion, becoming more pronounced at higher cycling rates. In nickel hot-spots, we observed hampered oxidation of nickel during charging.
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INTRODUCTION The high-voltage LiNi0.5 Mn 1.5 O4 (LNMO) spinel is a promising material for high-energy battery applications because of its high operation potential of ∼4.7 V versus Li+/Li0. However, this high potential lies outside the thermodynamic window of stability for conventional electrolyte solutions used in Li-ion batteries, and therefore presents a critical hurdle toward application.1 As one result, oxidation of the electrolyte by the active material produces passivating products, causing reduced cycle life and capacity fade of the material.2−4 Furthermore, Mn-containing phases such as the LiMn2O4 spinel and also the LiNi0.5Mn1.5O4 spinel are known to be susceptible to leaching of Mn2+ and Ni2+ to the electrolyte,5 possibly as a consequence of the parasitic reactions with the electrolyte,6,7 thereby causing elemental inhomogeneities in the electrodes. Formation of hydrofluoric acid, caused by trace amounts of water in the electrolyte, is regarded as one origin for this cathode corrosion.8,9 The effect on capacity fade is especially pronounced in LNMO/graphite full cells, because of the detrimental contribution of the deposited 3d metals on the anode impacting the solid-electrolyte-interphase (SEI) growth on its surface.2,10−13 The interplay between Ni and Mn cations leads to complex ordering effects, formation of secondary rock-salt structures, variable Mn3+ content, and oxygen vacancies that all © XXXX American Chemical Society
significantly influence electrochemical performance and are determined by the synthesis conditions.6,14−16 Detailed investigations of the composition−structure relationship in LNMO spinel showed systematic deviations from the theoretical stoichiometry involving an excess of Mn. To compensate for the formation of excess Mn3+, formation of a rock-salt-type structure with a lower Mn/Ni ratio was observed.17 A systematic study by Choi and Manthiram5 on manganese dissolution into the electrolyte revealed a correlation with the Mn oxidation state, with samples containing initially more Mn3+ being prone to the disproportionation reaction and thus to dissolution of the Mn2+ species in the electrolyte. Furthermore, they found a correlation with increasing difference in lattice parameter between the two cubic phases formed during the charge−discharge process and Mn dissolution. However, the consequences of the ordering effect for 3d metal dissolution remains unclear. Recent studies employing soft X-ray absorption spectroscopy have shown oxygen deficiencies at the surface of the LNMO spinel (in the top 10 nm surface layer) and a corresponding higher concentration of Ni in a lower oxidation state (reduced) Received: January 12, 2015 Revised: March 5, 2015
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DOI: 10.1021/acs.chemmater.5b00119 Chem. Mater. XXXX, XXX, XXX−XXX
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cycled using either a VMP3 measurement bridge (Bio-Logic USA, LLC, Knoxville, TN, USA) at room temperature (approximately 23 °C) or a Maccor Series 4000 Automated Test System (Maccor Inc., Tulsa, OK, USA) under a controlled temperature regime of 25 °C with LP30 electrolyte (Merck, Darmstadt, Germany) containing 1 M LiPF6 in EC/DMC, 1:1, in the potential range of 2.9−5.0 V versus Li+/Li0. Either cyclic voltammograms or constant-current chronopotentiometric measurements at C/n rate, defined as the current density required to achieve full theoretical capacity (Ctheor. = 147 mAh/g) in n hours, unless noted otherwise, were performed. The number of cycles, cycling rate, and final voltage were varied to investigate the influence of these parameters and are noted with each experiment. The cycled cells were disassembled in a glovebox under protective atmosphere. To avoid oxidation during subsequent measurements, the electrodes were sealed in PET pouches. XRF and XAS in fluorescence mode were conducted at the Hard Xray Microprobe end-station of the undulator beamline P06 at the storage ring PETRA III at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany.29 A primary energy of 17 keV (unless noted otherwise) was selected by means of a cryogenically cooled Si(111) double crystal monochromator. The beam was focused to 0.5 × 0.5 μm2 (horizontal × vertical) using a Kirkpatrick-Baez mirror optic, yielding a flux in the focused beam of ∼1010 photons/s. Incident and transmitted flux were recorded using an ionization chamber and photodiode, respectively. For analysis, all data were normalized to the incident photon intensity. A Maia detector with an annular array of 384 elements was used to collect the fluorescent radiation in backscattering geometry.30,31 The Maia detector uses an integrated field programmable gate array (FPGA) processor to perform real-time analysis of the X-ray events and to correlate sample position through direct coupling to the encoder readout of the continuously moving sample stages. Typical pixel sizes were 0.5 μm to match the beam size, with 1 ms dwell time per pixel unless noted otherwise. For data analysis, the software package GeoPIXE was used.32 To visualize relative elemental concentrations and the Ni/Mn ratio, red−green− blue (RGB) images were constructed. Each color presents one element (Ni, red; Mn, green; and transmission, blue), and the intensity of the color corresponds to a relative concentration of the respective element or transmission signal. X-ray absorption near edge spectroscopy (XANES) mapping was performed by acquiring and fitting a full fluorescence spectrum for each pixel in a 2D data set at each of 52 selected energies in the vicinity of the Ni K edge (8333 eV). Energy stacking and alignment of the Ni-XRF maps to construct spectroscopic images as well as linear combination fitting of the single pixel XANES spectra were performed using TXMwizard software.33,34 Each single-pixel XANES spectrum is fitted as a linear combination of reference materials and used to visualize the distribution of components as a phase map in a heterogeneous sample. Bulk XANES spectra were collected using a Vortex EM Si-drift detector (SII Nano Technology) with a 50 mm2 active area, illuminating approximately an area of 1 mm2 of the composite electrodes with the unfocused beam. These XANES spectra were processed using the ATHENA program in the IFFEFIT package and used as reference materials for linear combination fitting in TXMwizard.35−37
compared to that of the bulk after cycling of the material, adding to the complexity of the problem.18 While these investigations often average over many particles (bulk measurements) and large areas, our studies utilize spatially resolved Xray fluorescence spectrometry (micro-XRF) to visualize changes in the stoichiometry and morphology throughout the electrode. Chemical and structural heterogeneities in electrodes are critical factors for aging and degradation of active battery materials causing, e.g., localized heat generation and overcharge or overdischarge.19 Although bulk characterization using X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) is well-established for battery materials, only recently were the opportunities for highly spatially resolved characterization of the distribution of chemical phases using X-rays properly recognized.20−24 Elemental migration on the nanoscale in lithium- and manganese-rich Li1.2Mn0.525Ni0.175Co0.1O2 secondary particles upon cycling has been visualized using full-field transmission Xray microscopy.24 Although overall leaching and elemental migration was established for these materials, their effects on morphology and spatial distribution of the elements on the electrode scale have yet to be visualized. X-rays are particularly valuable when it comes to chemical speciation in correlation to morphology because of the high spectral resolution and their long penetration depth.25 X-ray fluorescence spectroscopy (XRF) is a widely known tool used to probe distribution and correlation of various elements26 or different chemical species of the same element, through scanning in the vicinity of an absorption edge using XAS. Recently, a confocal setup was employed to investigate the oxidation states at multiple depth levels of the electrode in 3D.27 Robert et al. have illustrated the beneficial effect of combining spatially resolved XRF with XRD for active battery electrodes under operando conditions, by detecting and correlating the distribution of Cu and Mn in layered oxysulfide Sr2MnO2Cu3.5S3 particles.28 Identifying the origin and correlation of structural and chemical heterogeneities on the electrode level is essential for an improved understanding of the aging mechanism, and thus improved electrode design and handling. To take a first step toward characterization of the effects on the mesoscale, fast micro-XRF scanning techniques with medium spatial resolution (500 nm) were applied to visualize changes in elemental and charge distribution in LNMO composite electrodes cycled at different rates and at different states of charge. Charge distribution is imaged by mapping the Ni oxidation state by measuring a stack of elemental maps in the vicinity of the Ni K edge. The results of our exemplary study show significant effects on morphology and elemental distribution, such as formation of elemental “hot-spots” and material erosion hundreds of micrometers in size, becoming more pronounced at higher sweep rates. In nickel “hot-spots”, we observed hampered oxidation of nickel during charging.
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RESULTS A selected part of a typical cyclic voltammetry (CV) measurement of LiNi0.5Mn1.5O4 composite electrodes versus metallic Li between 2.9 and 5 V for cycles 1, 2, 3, 5, 10, and 15 is shown in Figure 1a. The CV over the full voltage range is supplied in Supporting Information Figure S4. During charge, the main electrochemical response (Ni2+/4+ oxidation) starts above 4.65 V, whereas the main discharge reaction (Ni4+/2+ reduction) occurs at voltages below 4.75 V. The small dip/ splitting of the electrochemical response at 4.78 and 4.66 V during charge and discharge, respectively, is indicative of the degree of cation ordering of Mn and Ni.15,38,39 Figure 1b
EXPERIMENTAL SECTION
The composite electrodes were prepared with 70% LiNi0.5Mn1.5O4 bulk material (synthesized according to a solid-state route described elsewhere),16 10% conductive carbon, and 20% polyvinylidene fluoride as a binder in N-methylpyrrolidone and stirred overnight. Further details on the characterization of the active materials can be found in the Supporting Information in Figures S1−S3. From XRD, the material was characterized as phase-pure Fd3̅m (disordered) LNMO with secondary particle sizes of 2−10 μm. The samples were then cast on aluminum foil at a thickness of 250 μm and dried under vacuum for 12 h at 120 °C. Punches of this sheet (12 mm in diameter) were then B
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Figure 2. Electrodes cycled with (a) C/4, (b) C/2, and (c) 5 C for approximately 25 cycles. Green, Mn; red, Ni; and blue, transmission. The angled green area in b was caused by a piece of copper tape used to fix the electrode in its pouch.
Figure 1. (a) Selected region of cyclic voltammetry (CV) measurements of LiNi0.5Mn1.5O4 composite electrodes vs metallic Li between 2.90 and 5.00 V for cycles 1, 2, 3, 5, 10, and 15 recorded at a scan rate of 0.1 mV/s at room temperature. (b) Capacity fade in LiNi0.5Mn1.5O4 half cells as a function of sweep rate during CV.
At higher sweep rates, the tendency toward inhomogeneity increases, both morphologically and chemically. The effect is strongest for the cell cycled at the highest rate. Here, formation of craters or holes, where most of the active material is depleted, was observed. Interestingly, in the direct vicinity of the holes, Ni is enriched, whereas in the wider area around the holes Ni is depleted and Mn is enriched. We therefore find not only dissolution and depletion of Mn and/or Ni but also redistribution of the transition metals. Furthermore, formation of cracks in the electrode was detected for all cycling rates, indicating mechanical stresses. Their origin is likely repeated lithium incorporation and release, but no direct correlation between the cracks and chemical inhomogeneities was observed. Bulk XANES spectra of pristine and fully charged electrodes using the unfocused beam were acquired as standard references later used for micro-XANES evaluation (Figure 3). The large beam averages information over about 1 mm2 of the electrode. Relative to the discharged sample, which was measured after cycling to 2.90 V, the absorption edge of the oxidized sample is shifted by approximately 3 eV after charging to 5.00 V. Exemplary XANES imaging of electrodes was performed ex situ at selected charge states at 4.78 and 5.00 V during charge and at 4.68 V during discharge. The intermediate voltages were selected as the local minima/maxima between the two peaks upon charge/discharge during the Ni2+/Ni4+ charge-transfer reaction (Figure 1a). To guarantee completeness of the
depicts capacity values recorded at different cycling rates from 0.05 to 0.5 mV/s (approximately 12 and 1.2 h per charge/ discharge), showing the distinct degradation especially at the highest sweep rate (0.5 mV/s, approximately 110 mAh/g), which is not fully recovered to the previous capacity at subsequent cycling at 0.1 mV/s. The capacity degradation and efficiency over 300 cycles with constant-current measurements at 0.5 C is shown in Supporting Information Figure S3. After 300 cycles, the capacity has decreased to less than 50 mAh/g. The influence of the (galvanostatic) cycling rates on solvation and migration of transition metal elements as a contribution to irreversible capacity losses can be evaluated from the following figures. Corresponding maps of Ni (red), Mn (green), and transmission (blue) are shown in Figure 2. The transmission signal is indicative of the morphology because the absorption of the primary beam varies with the thickness of the material. An area of 2 × 2 mm2 was scanned at 0.5 μm per pixel. Each half cell underwent approximately 25 cycles with C rates of C/4, C/2, and 5 C in Figure 2a,b,c, respectively. During processing, all data were normalized to the incident photon flux to accommodate for fluctuations in beam intensity. Individual intensity maps (counts/pixel) for Ni and Mn as well as an intensity profile over one of the defects are provided in Supporting Information Figure S5 for the sample with the highest cycling rate shown in Figure 2c. C
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significant change in Mn concentration, which we will call Ni hot-spots. By collecting a number of 2D XRF maps at selected energies in the vicinity of the Ni K edge, fitting each XRF spectrum at each spatial pixel and each energy, and aligning the stack of extracted Ni-intensity images, a spectroscopic image was created. Further processing of this image allows us to extract single-pixel XANES spectra or to perform a principal component analysis.33 The previously obtained bulk XANES spectra of pristine and charged samples were used as standards for linear combination (LC) fitting of each single-pixel spectrum to obtain the chemical phase maps. The corresponding Ni intensity distribution and chemical phase maps are shown in Figure 5. For the sample at 4.78 V during charge, which is above the 4.7 V equilibrium voltage for the reaction, the distribution of Ni shows distinct hot-spots (Figure 5a). The distribution of the oxidation state mirrors this distribution; the Ni hot-spots show less oxidized nickel in comparison to the more homogeneous matrix (Figure 5b). It is not possible to determine if the reduced oxidation is caused by either sluggish kinetics resulting from the high Ni density and would occur at a higher overpotential or a side reaction forming a stable species with Ni2+ oxidation state, e.g., NiF2. Figure S8 in the Supporting Information shows exemplary single-pixel XANES spectra and corresponding LC fits of selected marked pixels of this sample. In the fully charged state at 5.00 V (Figure 5c,d), a quite homogeneous distribution of highly oxidized Ni is observed. Though it seems that there is a tendency for lower Ni oxidation states in regions with higher Ni concentration, this is not a decisive characteristic. Figure 5d shows a large particle (arrow) with relatively high Ni concentration but stoichiometric Mn
Figure 3. Bulk XANES spectra with the normalized absorption coefficient μ(E) obtained for reference charged and discharged electrodes.
reaction, the cells were kept at the respective voltages for at least 43 h. The region of interest for XANES imaging of electrodes was selected from a large area map, illustrating inhomogeneities in elemental distribution and morphology. Figure 4 shows an example of an electrode charged to 4.78 V. The large area maps for the samples charged to 5.00 V and discharged to 4.68 V can be found in Supporting Information Figures S6 and S7, respectively. The individual maps for the Ni and Mn distribution are included in Figure 4a,b, respectively, whereas these signals are combined in Figure 4c for a RGB image (red Ni, green Mn and blue transmission). We assume that the coarse structure of the images corresponds to the particle structure in the composite electrodes. The marked region clearly shows a locally enhanced Ni concentration without
Figure 4. Large overview map of a sample charged to 4.78 V with (a) Mn element map, (b) Ni element map, and (c) combined RGB image of Ni (red), Mn (green) and transmission (blue). The marked region (square) indicates the selected region for XANES mapping. D
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Figure 5. Ni elemental distribution maps for electrodes (a and c) charged to 4.78 and 5.00 V, respectively, and (e) discharged to 4.68 V. (b, d, and f) Corresponding phase maps for a, c, and e, respectively, obtained by linear combination fitting of each XANES pixel.
cycle. In contrast, these particles also show a Mn-enriched zone at the surface of the secondary particle after 200 cycles. In their study using XANES at the Ni and Mn L edge and O K edge, Zhou et al.18 could determine less oxidized states of the transition metals (especially Ni) at the surface (5−10 nm), compared to the bulk of the material by comparing total electron yield (TEY) with fluorescence yield (FY). This detected oxygen deficiency in the surface layer may cause formation of Mn3+ at the surface. As pointed out in the introduction, there is a close relationship between the oxidation state of Ni and Mn and ordering effects in the crystal structure, therefore suggesting atomistic and nanoscale effects impacting the mesoscale of the electrode. However, it should be pointed out that the mentioned methods utilize a high spatial resolution in the depth of the electrode and probe the top surface layer, but either evaluate single secondary particles or average over a much larger surface region, 50 × 50 μm2 in case of SIMS2 and likely even larger for XANES. In this thin top layer (10 nm), contributions from the SEI formation are not negligible.7 In contrast, our measurements probe the elemental distribution on the mesoscale over the width of the electrode. Working in the hard X-ray regime (Mn Kα and Kβ emission lines at 5.899 and 6.490 keV, respectively, and Ni Kα and Kβ emission lines at 7.478 and 8.265 keV, respectively),41 the present experiments probe approximately 20−30 μm, which is a significant portion of the full depth of the electrode. This large probing depth coupled with the inhomogeneous morphology of the electrode, does bear the disadvantage of heterogeneous self-absorption and shadowing effects, which limit a more quantitative analysis. The origin of the observed severe structural damage remains unknown, but is possibly related to inhomogeneities in the current density, such as a rough and porous electrode structure or imperfect alignment of both electrodes and/or the electrolyte soaked separators. This is supported by the fact that the physical damage is increased with higher sample rates, as shown in Figure 2c. It will therefore be valuable to investigate electrodes from cells which were prepared to higher industrial standards and under operando conditions. Additionally, Talyosef et al. observe localized oxidation reactions in LNMO from the electrolyte under simple storage conditions. Interestingly, such pitted surfaces with pits of 1−2
concentration, which is fully oxidized at 5 V. This indicates that particle size itself is not the limiting factor of the reaction. For the sample at 4.68 V during discharge, both the distribution of Ni (Figure 5e) and the Ni oxidation state (Figure 5f) are quite homogeneous. The sample appears to be reduced overall to Ni2+ in the scanned region.
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DISCUSSION
In contrast to studies focusing on the dissolution of 3d metals in the electrolyte,2,5 the present work targets the chemical and structural effects of 3d metal leaching and redistribution of LNMO as cathode material using fast micro-XRF techniques. The results reveal significant structural and chemical heterogeneities on the electrode level. The effects are especially pronounced for composite electrodes cycled at high rates, where the electrode became severely physically damaged. The eroded spots were surrounded by Ni-enriched regions. In turn, in the surrounding zones the Ni/Mn ratio decreased. We therefore assume dissolution of Ni atoms and redeposition around the eroded spots. This indicates a more complex aging mechanism than simple dissolution of Mn (and Ni) in the electrolyte and reduction at the lithium counter electrode.9 A study by Kim et al. also showed redeposition of Mn on LMO electrodes, and this was found to be even more pronounced in full cells using graphite anodes in comparison to half cells using lithium metal as the counter electrode.40 Material from these eroded spots and the distorted zones in the surroundings are either not available or available only to a limited extent for Li/ charge storage, inducing capacity fade of the electrode. The observations draw a somehow different image compared to that obtained by the measurements of other groups, who only propose a surface effect.2,9,18,24 Talyosef et al. identified localized formation of γ-MnO2 on the surface of LNMO particles after extended storage in electrolyte at elevated temperatures by using Raman spectroscopy at the surface.9 Pieczonka et al.2 have found a Ni- and Mn-depleted zone in the top 10 nm region using XPS and TOF-SIMS, but constant bulk concentration below it. Results obtained on secondary particles by Yang et al.24 indicate a transition-metal-depleted surface region in lithium- and manganese-rich Li1.2Mn0.525Ni0.175Co0.1O2 secondary particles after one full E
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Chemistry of Materials μm in size were observed for pure LMO by Matsuo et al. and Blyr et al. after storing electrodes in electrolyte at elevated temperatures where it was ascribed to Mn dissolution.42,43 Using micro-XRF in this study bears the advantage that we can visualize not only the surface morphology but also the elemental distribution of Ni and Mn or the oxidation state using XANES mapping. Another contribution could come from the oxidation and thus degradation of the electrical conduction agent, the carbon black particles,44 which could directly affect the local current densities and structural integrity of the electrode. In the sample showing multiple eroded spots and Ni and Mn inhomogeneities, the characterization of the oxidation state in the Nienriched region is pending. However, Ni hotspots in the partially charged state were probed using spatially resolved XANES maps (Figure 5a,b). Here, these Ni-enriched regions clearly showed a lower oxidation state. The collected spectra could not reveal, however, whether this Ni2+ was completely electrochemically inactive, e.g., through side reactions with the electrolyte forming compounds such as NiF2,2,3 or kinetically hampered and would occur at a higher overpotential. To investigate this in more detail and without artifacts from using multiple electrodes, in operando measurements will be necessary.
(2) Pieczonka, N. P. W.; Liu, Z.; Lu, P.; Olson, K. L.; Moote, J.; Powell, B. R.; Kim, J. J. Phys. Chem. C 2013, 117, 15947−15957. (3) Alva, G.; Kim, C.; Yi, T.; Cook, J. B.; Xu, L.; Nolis, G. M.; Cabana, J. J. Phys. Chem. C 2014, 118, 10596−10605. (4) Etacheri, V.; Marom, R.; Elazari, R.; Slitra, G.; Aurbach, D. Energy Environ. Sci. 2011, 4, 3243−3262. (5) Choi, W.; Manthiram, A. J. Electrochem. Soc. 2006, 153, A1760− A1764. (6) Kim, J.-H.; Pieczonka, N. P. W.; Yang, L. ChemPhysChem 2014, 15, 1940−1954. (7) Choi, N.-S.; Han, J.-G.; Ha, S.-Y.; Park, I.; Back, C.-K. RSC Adv. 2015, 5, 2732−2748. (8) Choi, N.-S.; Chen, Z.; Freunberger, S. A.; Ji, X.; Sun, Y.-K.; Amine, K.; Yushin, G.; Nazar, L. F.; Cho, J.; Bruce. Angew. Chem. 2012, 51, 9994−10024. (9) Talyosef, Y.; Markovsky, B.; Salitra, G.; Aurbach, D.; Kim, H. J.; Choi, S. J. Power Sources 2005, 146, 664−669. (10) Amine, K.; Liu, J.; Kang, S.; Belharouak, I.; Hyung, Y.; Vissers, D.; Henriksen, G. J. Power Sources 2004, 129, 14−19. (11) Kim, J.-H.; Pieczonka, N. P. W.; Li, Z.; Wu, Y.; Harris, S.; Powell, B. R. Electrochim. Acta 2013, 90, 556−562. (12) Gowda, S. R.; Gallagher, K. G.; Croy, J. R.; Bettge, M.; Thackeray, M. M.; Balasubramanian, M. Phys. Chem. Chem. Phys. 2014, 16, 6898−902. (13) Shkrob, I. A.; Kropf, A. J.; Marin, T. W.; Li, Y.; Poluektov, O. G.; Niklas, J.; Abraham, D. P. J. Phys. Chem. C 2014, 118, 24335−24348. (14) Manthiram, A.; Chemelewski, K.; Lee, E.-S. Energy Environ. Sci. 2014, 7, 1339. (15) Liu, D.; Zhu, W.; Trottier, J.; Gagnon, C.; Barray, F.; Guerfi, A.; Mauger, A.; Groult, H.; Julien, C. M.; Goodenough, J. B.; Zaghib, K. RSC Adv. 2014, 4, 154. (16) Kraas, S.; Vijn, A.; Falk, M.; Ufer, B.; Luerßen, B.; Janek, J.; Fröba, M. Prog. Solid State Chem. 2014, 42, 218−241. (17) Cabana, J.; Omenya, F. O.; Chernova, N. A.; Zeng, D.; Whittingham, M. S.; Grey, C. P. Chem. Mater. 2012, 24, 2952−2964. (18) Zhou, J.; Hong, D.; Wang, J.; Hu, Y.; Xie, X.; Fang, H. Phys. Chem. Chem. Phys. 2014, 16, 13838−42. (19) Liu, J.; Kunz, M.; Chen, K.; Tamura, N.; Richardson, T. J. J. Phys. Chem. Lett. 2010, 1, 2120−2123. (20) Meirer, F.; Cabana, J.; Liu, Y.; Mehta, A.; Andrews, J. C.; Pianetta, P. J. Synchrotron Radiat. 2011, 18, 773−81. (21) Boesenberg, U.; Meirer, F.; Liu, Y.; Shukla, A. K.; Dell’Anna, R.; Tyliszczak, T.; Chen, G.; Andrews, J. C.; Richardson, T. J.; Kostecki, R. M.; Cabana, J. Chem. Mater. 2013, 25, 1664−1672. (22) Ebner, M.; Marone, F.; Stampanoni, M.; Wood, V. Science 2013, 342, 716−20. (23) Wang, J.; Chen-Wiegart, Y. K.; Wang, J. Chem. Commun. (Cambridge, U.K.) 2013, 49, 6480−2. (24) Yang, F.; Liu, Y.; Martha, S. K.; Wu, Z.; Andrews, J. C.; Ice, G. E.; Pianetta, P.; Nanda, J. Nano Lett. 2014, 14, 4334−4341. (25) Andrews, J. C.; Weckhuysen, B. M. ChemPhysChem 2013, 14, 3655−3666. (26) Fittschen, U. E. A.; Falkenberg, G. Anal. Bioanal. Chem. 2011, 400, 1743−50. (27) Menzel, M.; Schlifke, A.; Falk, M.; Janek, J.; Fröba, M.; Fittschen, U. E. A. Spectrochim. Acta, Part B 2013, 85, 62−70. (28) Robert, R.; Zeng, D.; Lanzirotti, A.; Adamson, P.; Clarke, S. J.; Grey, C. P. Chem. Mater. 2012, 24, 2684−2691. (29) Schroer, C. G.; Boye, P.; Feldkamp, J. M.; Patommel, J.; Samberg, D.; Schropp, A.; Schwab, A.; Stephan, S.; Falkenberg, G.; Wellenreuther, G.; Reimers, N. Nucl. Instrum. Methods Phys. Res., Sect. A 2010, 616, 93−97. (30) Ryan, C. G.; Siddons, D. P.; Kirkham, R.; Li, Z. Y.; De Jonge, M. D.; Paterson, D.; Cleverley, J. S.; Kuczewski, A.; Dunn, P. A.; Jensen, M.; De Geronimo, G.; Howard, D. L.; Godel, B.; Dyl, K. A.; Fisher, L. A.; Hough, R. H.; Barnes, S. J.; Bland, P. A.; Moorhead, G.; James, S. A.; Spiers, K. M.; Falkenberg, G.; Boesenberg, U.; Wellenreuther, G. In Proc. SPIE, Vol. 8851; Lai, B., Ed.; 2013; DOI: 10.1117/12.2027195.
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CONCLUSIONS Fast micro-XRF is a powerful method to characterize correlations between chemical and structural inhomogeneities at the electrode level. Severe structural and chemical defects were identified with higher sweep rates in an exemplary study of LNMO composite electrodes. In the zones surrounding these eroded spots, the Ni/Mn ratio was altered. Mapping of the oxidation state of Ni at multiple degrees of charge/ discharge revealed less oxidized regions in these Ni hot-spots.
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ASSOCIATED CONTENT
* Supporting Information S
Additional electrochemical and spectroscopic data and XRF elemental maps. 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 authors acknowledge the help of Teresa Nunez Pardo de Vera, Matthias Alfeld (both Deutsches Elektronen-Synchrotron, DESY, Hamburg, Germany), Florian Meirer (Utrecht University, The Netherlands), Annalena Vijn (University of Hamburg, Germany), Kouichi Tsuji (Osaka City University, Japan) and Jan Garrevoet (Ghent University, Belgium) during the experiments and analysis of the data. Parts of this research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF). Part of this project was funded by the project STORE-E within the LOEWE program (State of Hessen).
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REFERENCES
(1) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587−603. F
DOI: 10.1021/acs.chemmater.5b00119 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.5b00119 Chem. Mater. XXXX, XXX, XXX−XXX
