Molecular Surface Modification of NCM622 Cathode Material Using

May 29, 2018 - Phone: +49 721 60828907 (S.N.)., *E-mail: [email protected]. ... modification route for NCM622 (60% Ni) using organophosphates, ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 20487−20498

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Molecular Surface Modification of NCM622 Cathode Material Using Organophosphates for Improved Li-Ion Battery Full-Cells Sven Neudeck,*,† Felix Walther,§ Thomas Bergfeldt,‡ Christian Suchomski,§ Marcus Rohnke,§ Pascal Hartmann,†,∥ Jürgen Janek,*,†,§ and Torsten Brezesinski*,†

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†

Battery and Electrochemistry Laboratory, Institute of Nanotechnology and ‡Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § Institute of Physical Chemistry & Center for Materials Research, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany ∥ BASF SE, 67056 Ludwigshafen, Germany S Supporting Information *

ABSTRACT: Surface coating is a viable strategy for improving the cyclability of Li1+x(Ni1−y−zCoyMnz)1−xO2 (NCM) cathode active materials for lithium-ion battery cells. However, both gaining synthetic control over thickness and accurate characterization of the surface shell, which is typically only a few nm thick, are considerably challenging. Here, we report on a new molecular surface modification route for NCM622 (60% Ni) using organophosphates, specifically tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate. The functionalized NCM622 was thoroughly characterized by state-of-the-art surface and bulk techniques, such as attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (ToFSIMS), to name a few. The comprehensive ToF-SIMS-based study comprised surface imaging, depth profiling, and threedimensional visualization. In particular, tomography is a powerful tool to analyze the nature and morphology of thin coatings and is applied, to our knowledge, for the first time, to a practical cathode active material. It provides valuable information about relatively large areas (over several secondary particles) at high lateral and mass resolution. The electrochemical performance of the different NCM622 materials was evaluated in long-term cycling experiments of full-cells with a graphite anode. The effect of surface modification on the transition-metal leaching was studied ex situ via inductively coupled plasma optical emission spectroscopy. TNPP@NCM622 showed reduced transition-metal dissolution and much improved cycling performance. Taken together, with this study, we contribute to optimization of an industrially relevant cathode active material for application in highenergy-density lithium-ion batteries. KEYWORDS: lithium-ion battery, lithium nickel cobalt manganese oxide, phosphate ester, molecular coating, surface shell, ToF-SIMS imaging

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INTRODUCTION Apart from LiNi0.80Co0.15Al0.05O2 (NCA), layered lithium nickel cobalt manganese oxides (NCMs) of the general formula Li1+x(Ni1−y−zCoyMnz)1−xO2 are the most relevant cathode active materials in rechargeable lithium-ion batteries (LIBs) for electric vehicles at present. In particular, Ni-rich NCMs, such as NCM622 (60% Ni) and NCM811 (80%), are either already being used or have the potential to be used in automotive applications.1−6 To further improve these materials, the deactivation and capacity fading pathways in full-cells need to be better understood and then strategies developed to suppress them. Structural changes, mechanical degradation, and surface reactions are considered the main issues in NCM-based cells.5,7 Structural transformations lead to impedance rise during cycling operation and therefore eventually to capacity decay. Mechanical degradation, such as secondary particle fracture and pulverization, results in electronic contact losses © 2018 American Chemical Society

and continuous exposure of fresh surface material to the electrolyte. Surface reactions at the positive electrode/liquid electrolyte interface comprise those that involve residual lithium compounds, such as Li2CO3, electrolyte decomposition (oxidation), and transition-metal dissolution from the active material. Increases in pressure and cell impedance as well as irreversible lithium losses are inevitable results of these adverse side reactions. Note that the different processes are not separated phenomena but rather are interrelated.5,7−13 Bulk-doping and coating of the material’s surface have been shown to be effective strategies for mitigating most of the deactivation pathways.5,7,10,14,15 While doping mainly addresses the structural stability, coating tackles the problem of surface Received: March 17, 2018 Accepted: May 29, 2018 Published: May 29, 2018 20487

DOI: 10.1021/acsami.8b04405 ACS Appl. Mater. Interfaces 2018, 10, 20487−20498

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powerful tool for characterizing battery materials, even though its use in this particular field of research is still rather limited.

reactions. A robust surface shell acts as a physical barrier between the highly reactive cathode active material, especially when in charged state, and the electrolyte solution. Ideally, it should be electronically insulating but Li-ion conducting (and “soft” enough to allow direct contact of secondary particles after electrode fabrication and calendaring). However, the synthesis of coating layers of uniform thickness and composition that fulfill these requirements is complicated. An additional problem is the characterization of the coating material since the thickness is usually well below 10 nm. Thus, new synthesis protocols and a variety of special characterization techniques are required. Highly stable oxides such as aluminum oxide are common coating materials in the area of LIBs.16−20 Besides their protective function, they may also act as an hydrogen fluoride (HF) scavenger.21−23 HF is believed to be responsible, at least partly, for the leaching of transition metals at the cathode side.16 Furthermore, in recent years, Li-ion conducting materials like Li3PO4 have been employed.24−26 Other efforts with phosphate-type coatings concentrated on using (lithium) transition-metal phosphates, including Co3(PO4)2 and LiCoPO4.27−29 Because of the covalent P−O bonds, these polyanionic compounds exhibit high stability, as known, for example, for olivine materials.4 Only few studies (patents) reported on the use of organic molecules containing POx groups.30,31 In contrast, phosphonates (OPR1(OR2)2) are widely used for anchoring molecules on solid supports.32 The anchoring occurs through reaction between surface hydroxy groups (−OH) and the phosphonate. In fact, phosphonates have already been used for modification of LiFePO4 (LFP), however, mainly aiming at improving the electronic charge transfer.33,34 Despite being few, these examples clearly demonstrate that organic phosphonates are well suited for surface functionalization of battery materials. Major advantages of molecular coating strategies are the high degree of control over both structure and thickness of the surface shell. In this context, it is also important to note that molecular coatings are considered robust because of covalent attachment to the surface. In general, precursor routes are scarce and they mostly make use of the reduction of diazonium salts.35 Here, we describe the surface modification of NCM622 using organophosphates (OP(OR)3), with the objective of improving its cycling performance in LIB full-cells. NCMs naturally contain reactive OH groups and thus are prone to surface functionalization (targeted deactivation of these groups is thought to increase the cell performance).36−38 In particular, two different phosphate esters were tested as potential coating agents, namely, tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate (TMSP). TMSP is a common electrolyte additive for LIB applications, and the available studies suggest formation of a TMSP-derived protective film on the cathode surface.39−42 Consequently, precoating of the cathode active material using TMSP might help to achieve a uniform surface layer, whereas TNPP, with its chromophoric nitrophenolate (NP) group, can be used to optimize the reaction conditions. Furthermore, we demonstrate the application of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to obtain detailed molecular information with spatial resolution from the samples. Different modes of operation allowed imaging the top surface, depth profiling, and three-dimensional (3D) reconstruction of secondary particles. Overall, we show that ToF-SIMS is a unique and

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EXPERIMENTAL SECTION

Materials. NCM622 (60% Ni) was obtained from BASF SE (Ludwigshafen, Germany). Tris(4-nitrophenyl) phosphate (TNPP, ≥98.0%) was purchased from TCI (Tokyo Chemical Industry Co., Ltd.; Tokyo, Japan). Tris(trimethylsilyl) phosphate (TMSP, ≥98%), anhydrous dimethyl carbonate (DMC, ≥99%), and anhydrous diethyl carbonate (DEC, ≥99%) were obtained from Sigma-Aldrich. Deuterochloroform (99.80% D, 0.03% tetramethylsilane, H2O 99.5%) from Merck. All reactions were routinely performed using standard Schlenk techniques. Synthesis of TNPP@NCM622 and TMSP@NCM622. A mixture of NCM622 (20.8 g) and either TNPP (810 mg, 1.76 mmol) or TMSP (1.42 g, 4.51 mmol) in DMC (50 mL) was heated to reflux for 10.5 h under argon (5.0, ≥99.999%, Air Liquid). The solid product was filtered off and, subsequently, washed using DMC (4 × 10 mL). After drying in vacuum at 120 °C overnight, the product was obtained as a black powder (19.8 g for TNPP@NCM622 and 20.0 g for TMSP@NCM622). In the case of TNPP@NCM622, the filtrate was collected and the solvent was removed under reduced pressure. The remaining white solid was dried prior to further analysis. Preparation of Electrodes. Slurries for electrode fabrication were prepared by dispersing binder (Solef5130 poly(vinylidene difluoride), 3 wt %) in NMP (7.5 wt % solution), graphite (SFG6L, 2 wt %), and carbon black (Super C65, 1 wt %) in NMP. The quantity of NMP was chosen such to achieve a total solid content of 65% in the final slurry (including NCM622). The suspension was mixed for 3 min at 2000 rpm and for additional 3 min at 400 rpm using a planetary centrifugal mixer (ARE 250, Thinky Corp.; Tokyo, Japan). After addition of cathode active material (94 wt %), the slurry was mixed twice at the conditions described above, followed by coating onto aluminum foil using a doctor blade (UA3000, MTV Messtechnik oHG; Erftstadt, Germany) on a film applicator (Coatmaster 510, Erichsen GmbH & Co. KG; Hemer, Germany). The electrodes were dried at 120 °C in a vacuum oven (VDL 53, Binder GmbH; Tuttlingen, Germany), and then they were calendared at 10 N mm−1 using a laboratory calendar (CA 5/200, Sumet Systems GmbH; Denklingen, Germany). The NCM622 loading was in the range between 10.5 and 11.0 mg cm−2. Coin Cell Assembly and Electrochemical Testing. Coin cells (CR2032, Hohsen Corp.; Osaka, Japan) were assembled in an argonfilled glovebox (MBraun) and sealed using a crimping tool (MSK160D, MTI Corp.; Richmond, CA). Prior to use, all materials were dried at 100 °C in vacuum overnight. Half-cells were assembled using NCM622 cathode (Ø14 mm), lithium-metal anode (99.9%, Ø15 mm, Gelon LIB Co.; Linyi, China), and glass fiber separator (GF/D, Whatman, GE Healthcare Life Sciences) soaked with electrolyte solution (200 μL of LP47, 1 mol L−1 LiPF6 in 3:7 by weight ethylene carbonate/DEC, BASF SE). For full-cells, NCM622 cathode (Ø14 mm), graphite anode (Ø15 mm, 6.8 mg cm−2 active material loading, BASF SE), and glass fiber separator (GF/A, Whatman) soaked with electrolyte solution (100 μL of LP472, LP47 containing 2 wt % vinylene carbonate, BASF SE) were used. Electrochemical testing was performed at 25 °C using a multichannel battery cycler (Series 4000, MACCOR Inc.; Tulsa, OK). After 2 h of equilibration, half-cells were cycled between 3.0 and 4.3 V with a potentiostatic step at the upper cutoff voltage. This constant voltage (CV) step was limited either by time (1 h) or by residual current (0.01C). After two cycles at 0.1C were completed, the cycling was performed at 0.25C charge and discharge at different rates from 0.1 to 3C. Half-cell cycling data are averaged from three to four cells. The standard deviation of the average initial specific discharge capacities was ≤0.3 mAh gNCM−1. Full-cells were equilibrated for 6 h, followed by cycling between 2.8 and 4.2 V with a CV step at 4.2 V. The CV step was also limited either by time (1 h) or by residual current (0.02C). After three formation cycles at 0.1C were completed, the cells were cycled at 1C, with a rate capability test every 100 cycles. Long-term 20488

DOI: 10.1021/acsami.8b04405 ACS Appl. Mater. Interfaces 2018, 10, 20487−20498

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ACS Applied Materials & Interfaces cycling data are averaged from two cells. The standard deviation of the average initial specific discharge capacities was ≤0.4 mAh gNCM−1. Instrumentation. NMR spectra were recorded on a Bruker Avance III 500 in CDCl3 (δ = 7.26 ppm). Ultraviolet-visible (UV−vis) spectra were collected on a Cary 500 Scan (Varian) instrument using crown glass cuvettes (d = 1 cm). Attenuated total reflection infrared (ATR-IR) spectra were recorded on an ALPHA FT-IR spectrometer (Bruker) equipped with an EcoATR sampling module with a germanium crystal in an argon-filled glovebox. The spectra were background corrected using OPUS software (Bruker). Thermogravimetric analysis mass spectrometry (TGA-MS) data were acquired on a Netzsch STA 409 PC coupled to a Balzers QMG 421 quadrupole mass spectrometer. The ionization energy was 70 eV. Both the phosphorus content of the different materials and the amount of transition-metal species deposited on the anode were determined via inductively coupled plasma optical emission spectroscopy (ICP-OES) on both a PerkinElmer Optima 4300 DV and a Thermo Scientific iCAP 7600. As for the latter, the cells were discharged to 2.8 V and then transferred to an argon-filled glovebox, followed by opening using a decrimping tool for the crimping machine (MSK-160D, MTI Corp.). The cathode was separated and washed using DEC (4 × 250 μL). Both separator and anode were soaked in DEC (500 μL). After removal of the separator, the anode was washed using DEC (2 × 250 μL). All electrodes were dried in vacuum. The data presented are averaged from measurements conducted on two different anodes. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) in positive and negative-ion modes was performed on a TOF.SIMS 5 instrument (IONTOF GmbH; Muenster, Germany) equipped with a Bi cluster primary ion gun and a dual source column for depth profiling. The spectrometry mode was applied to investigate the fragmentation of both coatings and to draw conclusions about the composition. Bi3+ ions (25 keV) were used for analysis. The analysis area was (150 × 150) μm2, and the primary ion beam was rastered with (128 × 128) pixels at 1 shot per pixel. The data shown are based on a single measurement per sample. To demonstrate the reproducibility of results, ToF-SIMS data with error bars from two different measurements conducted on TNPP@NCM622 are shown in the Supporting Information. The mass resolution (m/Δm) achieved was >5000 for the masses 62.97 (PO2−) and 78.96 (PO3−). Depth profiling in fast imaging mode was combined with delayed extraction to allow 3D tomography studies. The latter measurements were performed in negative-ion mode and in non-interlaced mode with Bi3+ (25 keV) as primary ion species and O2+ (1 keV) for sputtering with 3 s low-energy electron flooding per cycle for charge compensation. The analysis area was (20 × 20) μm2 and centered in the (100 × 100) μm2 sputter crater. The analysis area was rastered with (128 × 128) pixels at 5 shots per pixel and 50 frames. Using delayed extraction enabled a mass resolution of >6000 for the masses 62.97 (PO2−) and 78.96 (PO3−). X-ray photoelectron spectroscopy (XPS) was performed using a PHI5000 Versa Probe II XPS (Physical Electronics GmbH; Ismaning, Germany) equipped with a monochromatic Al Kα radiation source (1486.6 eV). The power of the X-ray source was 50 W at a voltage of 15 keV. The analytical beam had a diameter of 200 μm. Sputtering using Ar+ ions (0.5 keV) was performed by rastering over a (2 × 2) mm2 area. All spectra were calibrated to the signal of adventitious carbon at 284.8 eV, and Shirley background was applied.

Scheme 1. Reaction Scheme of the Surface Modification of NCM622 Using Phosphate Triestera

“M” represents any of the three transition metals present in NCM622. Only monodentate and tridentate binding modes are shown for clarity.

a

react once, twice, or three times with the surface species, leading to mono, bi, or tridentate binding modes and corresponding release of one, two, or three equivalents of R− OH. In practice, both NCM622 and phosphate triester were heated in DMC. DMC is a common electrolyte solvent and therefore believed to not undergo reaction with either NCM622 or phosphate ester. The reaction conditions were optimized by analyzing the hydrolysis of TNPP@NCM622, as described below. Different analytical techniques were applied to characterize the modified NCM622 materials and estimate the amount of surface coating. NMR spectroscopy of the reaction solution after filtration provided a first indication as to whether the synthesis was successful. In the case of TNPP, the released alcohol, 4nitrophenol (R−OH in Scheme 1), is found by 1H NMR (Figure S1), thereby clearly demonstrating that a reaction with the triester took place. More importantly, partially or fully hydrolyzed phosphate species are not detected in the filtrate via 31 P NMR (Figure S2). Consequently, the phosphate is indeed incorporated into the surface structure. The phosphorus content of pristine NCM622, TNPP@ NCM622, and TMSP@NCM622 was determined by means of ICP-OES. As indicated in Table 1, TNPP@NCM622 and TMSP@NCM622 reveal similar quantities, whereas the phosphorus content of pristine NCM622 is negligible. Table 1. Phosphorus Content of Pristine NCM622, TNPP@ NCM622, and TMSP@NCM622 from ICP-OES

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sample

P (μmol gNCM−1)

NCM622 TNPP@NCM622 TMSP@NCM622

100 cycles later for cells using TNPP@NCM622. Overall, this is a 20494

DOI: 10.1021/acsami.8b04405 ACS Appl. Mater. Interfaces 2018, 10, 20487−20498

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Ni) full-cells after cycling.66,67 Collectively, the ICP-OES results demonstrate that although phosphate ester coatings, in general, affect the transition-metal leaching from the cathode, only TNPP is capable of preventing the dissolution to a greater extent. The amount of transition metals found on the graphite anode accounts for around 0.6% of all transition metals in the as-made cathode active material in the case of pristine NCM622. Consequently, the capacity fading can be attributed not only to loss of active material but also to loss of active lithium at the anode side. The reduced leaching (and subsequent deposition on graphite) is certainly one of the reasons for the superior cycling performance of TNPP@NCM622. However, it should be noted that the anode contribution to the rise in cell impedance with cycling is usually small compared to that of the cathode.68,69 This is likely also why there are no significant differences in the average charge/discharge voltages between pristine NCM622 and TNPP@NCM622, even though the capacity retention is much improved for the latter material. Taken together, we conclude from the data shown above that the TNPP-derived coating mainly affects the cathode transitionmetal dissolution. However, we cannot rule out that other adverse effects such as gassing play a major role, too.70

in capacity with increasing C-rate is much more pronounced for pristine NCM622 (2C: 14.0 mAh gNCM−1, 3C: 25.2 mAh gNCM−1) and TMSP@NCM622 (2C: 15.2 mAh gNCM−1, 3C: 28.6 mAh gNCM−1). However, after 900 cycles, both pristine NCM622 and TNPP@NCM622 show similar discharge kinetics, which is still superior to that of TMSP@NCM622 (Figure S11). Overall, this result suggests that either the TNPPderived coating is not long-term stable or other effects determined the rate performance in the later cycles. The different cycling performance of TNPP@NCM622 and TMSP@NCM622 demonstrates that the improvement seen for the former material is in fact caused by the surface shell, rather than a kind of activation of the actual NCM622 during the synthesis process. Also, it is evident that a larger coating thickness or surface coverage does not necessarily result in a more protective film and TMSP apparently functions differently when used as an electrolyte additive.39−42 As for the latter, the beneficial effect is probably more related to its reactivity in solution. One of the deactivation pathways in full-cells is the dissolution of transition-metal species from the cathode and subsequent deposition at the anode side. This process is believed to result in continuous poisoning/destruction (and rebuilding) of the solid electrolyte interphase on graphite and therefore loss of active lithium.7 The amounts of Co, Mn, and Ni on the graphite anode were determined via ICP-OES, aiming at studying the effect of surface coating on the transition-metal leaching. To this end, discharged cells were disassembled inside an argon-filled glovebox, followed by rinsing the anode using DEC. Regardless of the type of transition metal, the lowest quantity is found when TNPP@ NCM622 is used as the cathode material (Figure 8); the total

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CONCLUSIONS Two different phosphate esters, namely, tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate (TMSP), have been successfully used for formation of molecular coatings on NCM622. The presence of phosphate ester species on the top surface was confirmed by detection of characteristic P−O, N−O, and Si−O vibrational bands via ATR-IR spectroscopy. Besides, for TNPP@NCM622, the presence of nitrophenyl groups was demonstrated in hydrolysis and TGA-MS experiments. The surface shells were also thoroughly characterized by means of ToF-SIMS. Among others, PO2− and PO3− fragments were clearly detected for both materials. Depth profiling in fast imaging mode combined with delayed extraction allowed characterization at high lateral and mass resolution. 3D reconstructions made from NiO2− and PO2−/PO3− signals revealed the spherical nature of the secondary cathode particles and, more importantly, provided evidence that the NCM622 material is uniformly coated by a thin shell, with minor inhomogeneities in the case of TNPP. The characterization using XPS supported the successful incorporation of phosphate ester onto the NCM622. Electrochemical testing was performed in graphite-based fullcells. TNPP@NCM622 showed superior cycling performance, followed by pristine NCM622 and TMSP@NCM622. Although there were no significant differences in measures for cell impedance between TNPP@NCM622 and pristine NCM622, ICP-OES indicated much reduced transition-metal dissolution (and deposition at the anode side) in the case of TNPP@NCM622, which partly explains the improved longevity. Ultimately, our results demonstrate that molecular coating is a useful option for surface modification of cathode active materials, in addition to the more common coating with thin ceramic films.

Figure 8. Amounts of Co, Mn, and Ni species deposited after 963 cycles at 1C rate on the graphite anode of full-cells using pristine NCM622, TNPP@NCM622, and TMSP@NCM622. Error bars are highlighted in orange.

amount of transition metals is lower by about 25% compared with pristine NCM622 and TMSP@NCM622. The proportion of Mn is much more than expected from the NCM622 stoichiometry. In the case of pristine NCM622, Co, Mn, and Ni account for roughly 10, 30, and 60% of the total amount of transition metals on the anode. For both coated samples, even larger proportions of Mn are observed. It is known that the dissolution of Mn, especially from charged NCMs, is much higher than that for Co and Ni species. However, there are contradictory reports as to whether Co or Ni exhibits higher dissolution rates.64,65 Larger quantities of Mn species were also found on the anode of NCM111 (33% Ni) and NCM523 (50%

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b04405. 20495

DOI: 10.1021/acsami.8b04405 ACS Appl. Mater. Interfaces 2018, 10, 20487−20498

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Details of the calculation of coating amount together with experimental results, 1H and 31P NMR spectra, UV−vis data and analysis, TGA-MS curves, positive and negative ToF-SIMS data and depth profiling results, and additional cycling data of half-cells and full-cells (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +49 721 60828907 (S.N.). *E-mail: [email protected]. Phone: +49 721 60828827 (J.J.). *E-mail: [email protected]. Phone: +49 721 60828827 (T.B.). ORCID

Marcus Rohnke: 0000-0002-8867-950X Jürgen Janek: 0000-0002-9221-4756 Torsten Brezesinski: 0000-0002-4336-263X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is a part of the projects being funded within the BASF International Network for Batteries and Electrochemistry. The work was partially carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.knmf.kit. edu), a Helmholtz research infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).

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REFERENCES

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