Understanding the Air-Exposure Degradation Chemistry at a

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Understanding the Air-Exposure Degradation Chemistry at Nanoscale of Layered Oxide Cathodes for Sodium-Ion Batteries Ya You, Andrei Dolocan, Wangda Li, and Arumugam Manthiram Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03637 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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Understanding the Air-Exposure Degradation Chemistry at Nanoscale of Layered Oxide Cathodes for SodiumIon Batteries

Ya You, Andrei Dolocan, Wangda Li, and Arumugam Manthiram*

Materials Science and Engineering Program and Texas Materials Institute; The University of Texas at Austin; Austin, Texas 78712, United States. *Corresponding Authors: [email protected]

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ABSTRACT

Undesired reactions between layered sodium transition-metal oxide cathodes and air impede their utilization in practical sodium-ion batteries. Consequently, a fundamental understanding of how layered oxide cathodes degrade in air is of paramount importance, but it has not been fully understood yet. Here a comprehensive study on a model material NaNi0.7Mn0.15Co0.15O2 reveals its reaction chemistry with air and the dynamic evolution of the degradation species upon air exposure. We find that besides the extraction of Na+ ions from the crystal lattice to form NaOH, Na2CO3, and Na2CO3•H2O in contact with air, nickel ions gradually dissolve from the bulk to form NiO and accumulate on the particle surface as revealed by sub-nanometer surface sensitive time-of-flight secondary ion mass spectroscopy. The degradation species on the surface are insulating, leading to an increase in interfacial resistance and declined electrochemical performance. We also demonstrate a feasible surface coating strategy for suppressing the unfavorable degradation process. Understanding the degradation mechanism at nanoscale can facilitate future development of high-energy cathodes for sodium-ion batteries.

KEYWORDS. sodium-ion batteries • high-Ni layered oxides • air sensitivity • secondary-ion mass spectroscopy • transition-metal dissolution • surface nanocoating


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The urgency to develop clean energy technologies has prompted extensive investigation of high-energy, low-cost electrical energy storage devices for large-scale grid storage.1-4 From a cost and sustainability perspective, it is appealing to use sodium-ion rather than lithium-ion based batteries due to the abundance and cost advantage of sodium over lithium.5-13 The basic principles and characteristics of sodium-ion batteries and lithium-ion batteries are similar in many aspects. Therefore, we could start from structures and chemistries that work well for Li + ion intercalation in the search for viable cathode materials for sodium-ion batteries.7, 14-19 HighNi layered oxide cathodes are being implemented in high-energy lithium-ion batteries; undoubtedly their sodium analogs NaNixM1-xO2 (M = Co, Mn and their combinations) have also received considerable interest due to their high theoretical capacity, low cost, and facile synthesis.20-21 Unfortunately, the prospect of high-Ni cathodes for sodium-ion batteries is challenging due to their poor chemical stability against air.22 High-Ni cathodes normally show notable performance losses during the handling and storage processes in ambient environment, which are believed to originate from the formation of electrochemically inactive NaOH or Na2CO3.23 In addition, these alkaline species initiate the defluorination of polyvinylidene fluoride (PVDF) binder, which leads to particle agglomeration and slurry gelation during electrode preparation process,24 drastically impeding their practical viability. Undoubtedly, the development of practical Na-ion batteries requires a fundamental understanding of the reaction mechanism and kinetics of the reaction of high-Ni oxides with air. In our previous work, we found that LiNi0.94Co0.06O2 is prone to react with moisture and carbon dioxide in air, primarily generating LiOH, LiHCO3, Li2CO3, NiO, and O2.25 Their sodium analogs are expected to have a higher dynamic affinity towards moisture and carbon dioxide because of the higher electropositivity of sodium relative to lithium. Various reaction


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mechanisms have been proposed describing the degradation process of sodium transition-metal (TM) layered oxides in air, including intercalation of water molecules between the TMO2 slabs,26-27 insertion of carbonate ions into the TM layer,28 and oxidation of NaxTMO2 by water and oxygen.24 It appears that both the degradation mechanism and reactivity towards air depend largely on the specific structure and compositions of the cathodes. Unfortunately, the understanding of the reaction chemistry between sodium high-Ni oxides and air is rather limited. Despite some early success,29-33 the effective suppression of the degradation process upon air exposure still remains a major challenge. Therefore, investigation of the dynamic formation and evolution of the cathode-air interphases is highly required to develop viable strategies for the practical implementation of high-Ni cathodes for sodium-ion batteries. Herein, for the first time, we systematically investigate the degradation mechanism of high-Ni oxides in air and the dynamic evolution of chemical species at the cathode-air interphase using NaNi0.70Mn0.15Co0.15O2 (NMC701515) as a model system. A facile chemical titration method has been developed to quantitively determine the concentration of the residual sodium species (NaOH and Na2CO3). With sub-nanometer surface sensitive time-of-flight secondary ion mass spectroscopy (TOF-SIMS), we analyze the spatial distribution of the degradation species and their surface composition and structure at nanoscale. We also provide a feasible surface coating strategy to improve the chemical stability of high-Ni oxides against ambient air. Results and Discussion The NMC701515 cathode was synthesized by a two-step process: a transition-metal hydroxide co-precipitation to synthesize the Ni0.70Mn0.15Co0.15(OH)2 precursor followed by a hightemperature sodiation process to obtain the NMC701515 sample. The X-ray diffraction (XRD) pattern of NMC701515 and its refinement results are shown in Figure 1a. All the XRD peaks are


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well indexed to the hexagonal α-NaFeO2 structure (space group: R3m) without any impurities, indicative of a typical O3-type layered structure. The refined crystallographic data summarized in Table S1 show that the lattice parameters of NMC701515 are a = b = 2.94309 Å and c = 15.79021 Å. The stoichiometry of NMC701515 determined by inductively coupled plasmaatomic emission spectroscopic (ICP-AES) analysis indicated a Na : Ni : Mn : Co atomic ratio of 0.97 : 0.70 : 0.15 : 0.15, which is quite close to the designed composition. The as-prepared NMC701515 powder was exposed inside an air-filled glovebox with a controlled relative humidity (RH) value of 60% to investigate the chemical reaction between NMC701515 and moisture/carbon dioxide in air. The air-filled glovebox was connected to an air tank to keep a constant concentration of CO2 during the exposure process. The concentration of the reaction products, such as NaOH and Na2CO3, termed hereafter “residual sodium”, was determined by a simple chemical titration method. The fresh and air-exposed powder was immersed into ethanol and ethylene glycol, respectively, to dissolve residual NaOH and Na2CO3 species.34 Ethanol and ethylene glycol were used as solvents to distinguish NaOH and Na2CO3 because NaOH selectively dissolves in ethanol while Na2CO3 selectively dissolves in ethylene glycol. It is worth noting that organic solvents rather than water were employed to dissolve the residual sodium species given the facile reaction between NMC701515 and water. Then, the obtained solutions were diluted with deionized (DI) water and titrated against 0.1 M HCl solution using methyl orange as an indicator; the color change from bright yellow to orange was used as the end point (Figure S1). The contents of NaOH and Na2CO3 were calculated based on the volume of HCl consumed. Figure 1b shows the residual sodium concentration (based on the mass of each product) in both freshly prepared and 7-days exposed NMC701515 samples. Even for the fresh sample with an exposure time of less than 2 h, the concentrations of NaOH and


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Na2CO3 are, respectively, 18,838 and 46,334 ppm, corresponding to a reaction of 15.2 % Na+ ions in the crystal structure with air. After 7-days exposure, about 32.4 % of Na+ ions diffuse out of the crystal lattice to form residual sodium species, and the concentration of surface Na2CO3 increases by more than 3 times (144,775 ppm). The content of NaOH, however, decreases to 5,238 ppm after air exposure because of the facile reaction between NaOH and CO2. Note that another possible residual sodium species, NaHCO3, is also soluble in ethylene glycol, the chemical titration method cannot separate NaHCO3 from Na2CO3. To validate (or exclude) the possible existence of NaHCO3, we employed Fourier transform infrared spectroscopy (FTIR) to characterize the fresh and exposed NMC701515 samples (Figure 1c). The characteristic peaks of Na2CO3 can be observed at 1434 and 879 cm-1 for both the fresh and 7-days air exposed NMC701515 samples while no peaks representing NaHCO3 were observed, suggesting that Na2CO3 is the dominant degradation product. The formation of Na2CO3 was further confirmed by XRD (Figure 1d and e). During the air exposure process, the diffraction peaks indicative of Na2CO3, Na2CO3•H2O, and NiO gradually emerge and their intensities increase with the exposure time, while the characteristic (104) peak of the O3-phase NMC701515 gradually weakens and vanishes. In addition, a gradual shift of the (003) peak toward lower angles is observed upon exposure to air, which results from a migration of Na+ ions out from the lattice to form impurities, increasing the electrostatic repulsion between the oxygen layers and thereby leading to an expansion of the c lattice parameter. Thermogravimetric analysis (TGA) results of the fresh and 7-days exposed NMC701515 are shown in Figure 1f. The weight loss occurring before 100 oC is attributed to the desorption of dissociated water and the loss of lattice water from sodium carbonate monohydrate. The weight loss between 200 and 500 oC may be assigned to some adsorbed species. The weight percentage


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of water and adsorbed species in the exposed sample are, respectively, 5.7% and 2.4%, notably higher than in the fresh sample (2.5% and 1.4%). Based on the results above, one can conclude that the NMC701515 sample reacts intensively with moisture and carbon dioxide during air exposure. Na+ ions diffuse out of the crystal lattice to form residual sodium species, such as sodium hydroxide, sodium carbonate, and their hydrates. After the Na+ ions are leached out, nickel oxide is also formed accompanied by a reduction of Ni3+ to Ni2+. To balance this redox reaction, O2- is assumed to be oxidized to form O2 as suggested by Liu et al.35 The proposed reaction are given below: NaNi0.7Mn0.15Co0.15O2 + xCO2 → xNa2CO3 + yNiO + Na1-2xNi0.7-y Mn0.15Co0.15O2-m + 0.5(m-xy) O2↑

Eq. (1)

NaNi0.7Mn0.15Co0.15O2 + xH2O → 2xNaOH + yNiO + Na1-2xNi0.7-y Mn0.15Co0.15O2-m + 0.5(m-xy) O2 ↑

Eq. (2)

2NaOH + CO2 → Na2CO3 + H2O

Eq. (3)

Na2CO3 +H2O → Na2CO3•H2O

Eq. (4)

The concentration and spatial distribution of the degradation species on the fresh and exposed NMC701515 powders were analyzed by TOF-SIMS depth profiling and high-resolution imaging. To start, depth profiles using Bi1+ as analysis species and Cs+ as sputtering species were acquired on both the fresh and exposed samples. TOF-SIMS spectra of several fragments of interest integrated over the first 4,000 s of Cs+ sputtering along the depth profiles are shown in Figure 2a-f. The severe reaction of NMC701515 with moisture and carbon dioxide in air is demonstrated by a significant increase in the C- signal (Figure 2a), together with a notable buildup of adsorbed species (represented by CH2-, C2HO-, and O2H-) and residual sodium species (represented by NaCO3-) after air exposure (Figure 2b-e). Moreover, the NaO2H- concentration in


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the exposed sample is also significantly higher than that in the fresh sample (Figure 2f) due to the increased water content after air exposure (Figure 1f). Note that the NaO2H- fragment can be produced by NaOH, Na2CO3•H2O, and even bulk materials with adsorbed superficial water. The depth profiling in Figure 2g shows that with Cs+ sputtering, the signals of the degradation products, including CH2-, C2HO-, O2H-, NaO2H-, and NaCO3-, reach the maximum concentration at the surface of the NMC701515 particles. The degradation products on particle surfaces actually exhibit a “multilayered” structure, with a centralized distribution of carbon-containing adsorption species (CH2- and C2HO-) at the outermost part of the surface, a middle layer contributed by Na residual species (NaCO3-), and a penetrated distribution of inner layer into the bulk contributed mainly by adsorbed water (O2H-) and NaOH (NaO2H-). The spatial distribution of the outer adsorption layer (CH2- and C2HO-) is almost the same for both the fresh and exposed samples. However, the normalized intensities of NaCO3-, O2H-, and NaO2H- profiles collected on the 7-days air exposed sample exhibit peaks with maxima appearing at larger depths, which decline at a much slower rate with sputtering relative to those of the fresh sample, indicating a significant increase in the thickness of the surface degradation products. The spatial distribution of NaO- (Figure 2g) and Na3O+ (Figure S2) signals also clearly show that the formed Na residues accumulate on the surface and continue to increase in thickness upon air exposure. We also notice a distinctive accumulation of nickel-containing species (represented by

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Ni+) at the

surface as evidenced by the depth profiling in positive mode (Figure S2). Different from the profiles of the bulk signal (represented by NaNiO+) of which the content gradually increases with the sputtering time, the

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Ni+ signal reaches the maximum concentration after 800 s sputtering

for the fresh sample and shows a very high concentration at the outmost surface (~ 90% of its maximum concentration) for the exposed sample. This indicates that the nickel ions gradually


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dissolve from the bulk structure and accumulate on the particle surface. Since NiO is one of the major degradation products as evidenced by XRD, it is expected that a NiO layer is also formed on the surface, which will be discussed later. Scanning electronic microscopy (SEM) images in Figures 3a-c illustrate the evolution of the surface morphology of the NMC701515 particles during air exposure. The freshly-prepared NMC701515 particles exhibit a spherical morphology with an average diameter of 12 μm (Figure 3a and Figure S3), and each particle is composed of nanosized primary particles. The cross-section SEM image (Figure S4) demonstrates that the secondary particle is assembled by radially aligned columnar structure. Upon air exposure, the degradation products gradually grow on the NMC701515 composite particle surface and between adjacent particles (Figure 3b, c and S5). Eventually, the surface of the NMC701515 particles are entirely covered by a thick layer of impurities and the primary particles are barely seen. The energy dispersive X-ray spectroscopy (EDS) elemental mappings (Figure 3d) collected on the impurities formed between adjacent particles (Figure 3b) clearly show that Na, C, and Ni elements are well distributed among the surface degradation products. Illustrative TOF-SIMS chemical mappings were collected on the 24 h air-exposed NMC701515 electrode (Figure 3e) in a cross-sectional view. These maps indicate that the surface of active cathode particles (represented by the NaNi+ fragment) is covered by degradation-induced chemical species, which includes adsorbed species (represented by C3H2-), sodium residues (represented by NaC2O2-) and nickel oxide (represented by

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NiO-).

This observation is in good agreement with the TOF-SIMS depth profile results in Figure 2 and S2. Besides the above-mentioned chemical species, we also note a significant accumulation of Fat the surface of the active cathode particles and a large penetration into their bulk as evidenced in Figure 3e, indicating that there is an intense chemical reaction between the particles and the


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PVDF binder. The surface alkaline residual sodium species (e.g., NaOH and Na2CO3) initiate the defluorination of PVDF, which not only deteriorate the mechanical strength of the electrode (Figure S6a), but also generate NaF (Figure 3e) on the surface of the active compound particles during the electrode preparation and air exposure process. The poor chemical stability of NMC701515 also leads to a corrosion of the Al current collector (Figure S6b) during air exposure. The intensive reaction between NMC701515 cathode and air, which is accompanied by a depletion of Na+ ions in the lattice, reduction of Ni3+ to Ni2+, and loss of O, is responsible for the capacity decay during air-exposure. Figure 4a shows the initial galvanostatic charge-discharge (GCD) profiles of the freshly prepared and 24 h and 7-days air exposed NMC701515 electrodes cycled at a 0.1C rate between 1.5 and 3.9 V. The initial charge and discharge capacities are, respectively, 118 and 128 mA h g-1 for the fresh NMC701515 electrode, and they dramatically decline to 67 and 100 mA h g-1 after 24 h air exposure. The initial charge capacities are notably lower than the discharge capacities for both the fresh and exposed cathodes and the discrepancy remarkably increases with exposure time, indicating that the Na+ ions are depleted from the bulk structure during the degradation process. After 7 days of exposure, the NMC701515 electrode shows an abnormally high charge capacity (or “infinite charging”) with a long charge plateau at 3.4 V (Figure 4a and S7), which might correspond to the decomposition process of sodium carbonates and the reaction between electrolytes and Na residual species. Given that the reaction between NMC701515 and moisture/carbon dioxide in air mostly occurs at the surface as revealed by TOF-SIMS, we modified the surface of NMC701515 particles with an inert ZrO2 layer to suppress this degradation process. The ZrO2 coated NMC701515 cathode (ZrO2@NMC701515) was obtained through a wet-chemical process followed by heat treatment.


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SEM images (Figure S8) show that ZrO2@NMC701515 still preserves the spherical morphology and EDS data confirm the existence of ZrO2 (Figure S9). Surface coating of ZrO2 on the NMC701515 cathode notably improves its stability against air. After 24 h and 7-days exposure, the ZrO2@NMC701515 cathode still delivers a capacity of 113 and 96 mA h g-1, respectively, which corresponds to 90% and 76% of the capacity delivered by the fresh sample. The ZrO2@NMC701515 cathode shows improved rate capabilities as well compared to the unmodified NMC701515 cathode (Figure 4c) after being exposed to air for 24 h. At a high rate of 5C, the ZrO2@NMC701515 cathode still maintains a reversible capacity of 72 mA h g -1, while the NMC701515 cathode delivers a specific capacity of only 47 mA h g-1. Apart from the improved kinetics, the exposed ZrO2@NMC701515 cathode also shows a high cycling stability (Figure S10). After 200 cycles at a 0.5C rate, the discharge capacities of the exposed ZrO2@NMC701515 and NMC701515 are, respectively, 75 and 57 mA h g-1. Electrochemical impedance spectroscopy (EIS) was employed to characterize the electrochemical interphases of the 24 h air exposed NMC701515 and ZrO2@NMC701515 samples (Figure 4d). Table S2 shows the values of impedance parameters Re, Rf, and Rct obtained by fitting the Nyquist plots with an equivalent circuit (inset in Figure 4d), where Re denotes the ohmic resistance of the cell, Rf indicates the migration resistance of Na+ through the solid electrolyte interface (SEI) layer, and Rct is assigned to the charge-transfer resistance. Na-ion transport appears slower for the ZrO2@NMC701515 cathode relative to the unmodified NMC701515 due to the poor ionic conductivity of ZrO2. The charge-transfer kinetics is, however, significantly improved by ZrO2 coating, which might be attributed to the stable surface. The overall resistance of the ZrO2@NMC701515 cathode is still lower than the unmodified cathode. As a result, the ZrO2@NMC701515 cathode shows high capacity, high-rate capability,


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and stable cycling performance after being exposed to air. The improved air stability of ZrO2@NMC701515 cathode demonstrates the feasibility of surface coating to suppress the unfavorable degradation process in air. Conclusion In summary, our findings reveal the degradation chemistry of a model high-Ni cathode in air and the dynamic changes of the chemical species at the cathode-air interphase. The selected NMC701515 compound displays a much higher dynamic affinity towards moisture and carbon dioxide during air exposure compared to its lithium analogues, primarily forming NaOH, Na2CO3, Na2CO3•H2O, and NiO at the particle surface. The reaction between NMC701515 cathode and air is so intense that the cathode degrades in air within hours. A “multilayered” structure of the degradation products on particle surface was observed and nickel ions gradually dissolve from the bulk and accumulate on the particle surface. In addition, a strong reaction between the alkaline residual sodium species and PVDF binder occurs during electrode preparation of the air exposed samples,24 generating a layer of F-containing species, such as NaF, at the particles surface. The surface degradation layers formed are electronic/ionic insulators, leading to significant electrochemical performance losses for the air-exposed cathodes. We also demonstrate that the chemical stability against air could be improved by modifying the particle surface with a ZrO2 layer, and this strategy could be also applied to other types of air-sensitive materials to enable their industrial deployment. Our work demonstrates the importance of developing air-stable cathodes and provides insights on the practical viability of high-Ni oxides for high-energy-density sodium-ion batteries.


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FIGURES

Figure 1. Characterization of the degradation products of the high-Ni layered NMC701515 cathode after being exposed to air with an RH value of 60%. (a) The experimental and Rietveld refined XRD profiles of the fresh NMC701515 sample; black circles denote the experimental data and red lines indicate the calculation results; green lines denote the difference between the experimental and calculation results. (b) Concentration of the residual sodium species on the freshly prepared (< 2 h air exposure) and 7-days air exposed NMC701515 cathodes, wherein blank bars indicate NaOH concentration, orange bars denote Na2CO3 concentration, and green bars indicate Na+ ion atomic percentage in residual sodium species relative to the total amount. (c) FTIR spectra collected with Na2CO3, NaHCO3, and fresh and air-exposed (7 days) NMC701515 cathodes. (d) XRD patterns of the NMC701515 samples after being exposed to air for various times. (e) Enlarged XRD peaks in figure 1(d). (f) TGA curves of the freshly prepared and 7-days exposed NMC701515 cathodes.


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Figure 2. TOF-SIMS characterization of the freshly prepared and 7-days exposed NMC701515 powders. TOF-SIMS spectra of several secondary ion fragments of interests integrated over the first 4,000 s of Cs+ sputtering along the depth profiles on both samples, including (a) C-, (b) CH2-, (c) C2HO-, (d) O2H-, (e) NaCO3-, and (f) NaO2H-. (g) Depth profiling of several fragments of interests collected on the fresh sample (top) and 7-days exposed sample (bottom).


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Figure 3. Morphology characterization of the NMC701515 cathode during air exposure, including SEM images of the (a) fresh sample as well as samples exposed to air for (b) 24 h and (c) 7-days. (d) SEM image of the dendrite-like impurity formed between NMC701515 particles in (b) and elemental mappings of Na, C, and Ni. (e) TOF-SIMS chemical mapping (burst alignment mode) of the 24 h exposed NMC701515 electrodes in a cross-sectional view showing the distribution of various secondary ions of interests, including NaNi+, C3H2-, NaC2O2-, 62NiO-, F-, and Na2F+ fragments.


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Figure 4. Electrochemical performance of the NMC701515 and ZrO2@NMC701515 cathodes cycled between 1.5 and 3.9 V. The initial GCD profiles of two cathodes (0.1C) after being exposed to air for various times: (a) NMC701515 and (b) ZrO2@NMC701515. (c) Rate capability of two cathodes after being exposed to air for 24 h. (d) Nyquist plots showing the impedance spectra of the 24 h air exposed NMC701515 and ZrO2@NMC701515 electrodes after 20 cycles in half cells with Na anodes. The inset shows the equivalent circuit for fitting and the solid line represents the fitting results.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. SEM images, TOF-SIMS depth profiles; crystal structure parameters; GCD profiles; cycling performance; impedance data (PDF) AUTHOR INFORMATION Corresponding Author *Corresponding authors: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award number DE-SC0005397.


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SYNOPSIS


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Understanding the Air-Exposure Degradation Chemistry at a

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