New Chemical Insights into the Beneficial Role of Al2O3 Cathode

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New Chemical Insights into the Beneficial Role of AlO Cathode Coatings in Lithium-ion Cells 2

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David Scott Hall, Roby B Gauthier, Ahmed Eldesoky, Vivian S Murray, and Jeff R. Dahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22743 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Materials & Interfaces

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New Chemical Insights into the Beneficial Role of Al2O3 Cathode Coatings in Lithium-ion Cells David S. Hall,a Roby Gauthier,a Ahmed Eldesoky,b Vivian S. Murray,c J.R. Dahnab* a

Department of Physics and Atmospheric Science, Halifax, Nova Scotia, B3H 4R2, Canada

b

Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada

c

Department of Process Engineering and Applied Science, Dalhousie University, Halifax, Nova Scotia, B3H 4R2, Canada

*

[email protected]

Abstract Inorganic surface coatings, such as Al2O3, are commonly applied on positive electrode materials to improve the cycling stability and lifetime of lithium-ion cells. The beneficial effects are typically attributed to the chemical scavenging of corrosive HF and the physical blockage of electrolyte components from reaching the electrode surface. The present work combines published thermochemistry data with new density functional theory calculations to propose a new mechanism of action: the spontaneous reaction of the LiPF6 electrolyte salt with Al2O3-based surface coatings. Using

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F and

31

P solution nuclear magnetic resonance spectroscopy, it is

demonstrated that the storage of LiPF6-containing electrolyte solution with Al2O3 produces LiPO2F2, a well-known electrolyte additive. The production of LiPO2F2 is also observed for electrolyte solutions that were stored for 14 days at 40°C with Al2O3-coated LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Co0.15Al0.05O2 (NCA) materials. Given the beneficial nature of this species for the lifetime and stability of lithium-ion cells, this reaction is here proposed to similarly benefit the performance of cells that use Al2O3-coated cathode materials. Keywords Cathode coatings, Positive electrode materials, Electrode coatings, Aluminum oxide, Lithium difluorophosphate, Electrolytes, Additives, Atomic layer deposition 1 ACS Paragon Plus Environment

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2 Introduction Lithium-ion cells currently find diverse use in portable electronics, power tools, medical implants, electric vehicles, and grid energy storage.1 To further progress their adoption, especially for renewable energy applications, it is desirable to develop lithium-ion cell chemistries that provide longer lifetimes at high cell voltages and at high temperatures, without significantly increased cost. Over prolonged cycling and use, positive electrode materials can develop high charge transfer impedance leading to impedance growth in cells and loss of capacity at high discharge rates. 2–4 The gradual decomposition of the electrolyte solution components also occurs at the positive electrode, with rates that are linked to cell temperature and operational voltage limits.5 The application of an inorganic surface coating, such as Al2O3, AlPO4, or TiO2, on the positive electrode material is a common approach to impeding these degradation pathways. 6–12 The protective effects of these surface coatings are typically attributed to the scavenging of HF, limiting transition metal dissolution, altering the composition of the solid electrolyte interface on the positive electrode, and the physical blockage of electrolyte components from reaching the electroactive material surface.2,13–20 This work suggests a new mechanism of action for Al2O3 and other oxide coatings on positive electrode materials; the reaction of the coating with the LiPF6 salt is herein demonstrated to produce lithium difluorophosphate (LiPO2F2), and the general reaction (1) is proposed: − − 2 𝐴𝑙2 𝑂3(𝑠) + 𝑥 𝑃𝐹6(𝑠𝑜𝑙𝑣.) → 2 𝐴𝑙2 𝑂3−𝑥 𝐹2𝑥(𝑠) + 𝑥 𝑃𝑂2 𝐹2(𝑠𝑜𝑙𝑣.)

(1)

Reaction (1) presents the case where a mixed aluminum oxyfluoride is produced, similar to those observed following heat treatment of AlF3 coatings.21 In the case where x = 3, this reaction simplifies to produce aluminum fluoride (2): − − 2 𝐴𝑙2 𝑂3(𝑠) + 3 𝑃𝐹6(𝑠𝑜𝑙𝑣.) → 4 𝐴𝑙𝐹3(𝑠) + 3 𝑃𝑂2 𝐹2(𝑠𝑜𝑙𝑣.)

(2)

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3 LiPO2F2 is a well-known electrolyte additive that improves the cycling stability and lifetime of a variety of lithium-ion cell chemistries,22–26 such as the LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite cells shown in Figure 1. The data demonstrates what a significant effect this compound has on the rate of cell capacity loss (Figure 1a). The difference in the average cell voltage during the charge and the discharge steps, here denoted ΔV (Figure 1b), as well as the difference in capacity of fast vs. slow cycles (Figure 1a) are both representative of the cell impedance. It is therefore observed that small amounts of LiPO2F2 also inhibit cell impedance growth. Although there is evidence that LiPO2F2 forms from the reaction of LiPF6 with LiCoPO4 positive electrode materials,27 this work is the first report, to the best of the authors’ knowledge, that proposes it forms when electrolytes are exposed to Al2O3, and presumably other oxide, coatings.

Figure 1

a) Normalized discharge capacity and b) ΔV for NMC532/graphite pouch cells cycled between 3.0 – 4.3 V at C/3 and at 40°C. A slow C/20 cycle was performed every 50 cycles. Cells contained 1.2 M LiPF6 in 3EC:7DMC and 0% LiPO2F2 (blue squares) or 1% LiPO2F2 (red circles).



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4 Experimental Methods

Materials and Electrolytes Aluminum oxide powder (Aldrich, < 50 nm particle size) was dried at 500°C for 2 days to remove adsorbed water. The material was cooled to 100°C and immediately transferred into an argonatmosphere glove box while still hot. The powder was then dried again under vacuum at 100°C for 14 h and transferred directly back into the glove box. Laminate pouch bags made of the same material as pouch cells and as described previously,28,29 were dried under vacuum at 100°C for 14 h and transferred directly into the glove box. Electrolyte solutions were prepared with LiPF6 (ShenZhen Capchem, China ≥ 99.9%) in a 3:7 solvent blend, by mass, of ethylene carbonate and dimethyl carbonate (DMC, Capchem, < 20 ppm H2O). Reference solutions were prepared by adding 1%, by mass, LiPO2F2 (Capchem). Pouch bags were sealed at -10 kPa gauge pressure using an MSK-115A compact vacuum sealer (MTI Corp.) Solution preparation, materials handling, and pouch bag sealing was performed in an argon glove box. LiNi0.6Mn0.2Co0.2O2 (NMC622) and LiNi0.8Co0.15Al0.05O2 (NCA) powders were received from a reputable manufacturer either uncoated or coated with Al2O3. Coated materials were prepared using proprietary atomic layer deposition (ALD) methods. The surface coatings were continuous over the materials and on the order of 5 – 10 nm thick. Materials were dried under vacuum at 100°C for 14 h to remove adsorbed water and transferred directly into an argon glove box.

Lithium-ion Cells Dry (no electrolyte), vacuum-sealed LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite pouch cells, with capacity of ~220 mAh, were received from LiFun Technology (Tianyuan District, Zhuzhou, Hunan, China). The NMC532 active material particles were uncoated (NMC532u) and were ‘single crystal’, as described previously.30,31 However, the NMC532 material used in references 30 and 31 did have Ti-based coating applied. Therefore, special single crystal NMC532u material, without a coating, was used in the pouch cell experiments reported in this paper. The negative 4 ACS Paragon Plus Environment

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5 electrodes were made of an artificial graphite. The cells were cut open in an argon glove box and dried under vacuum at 80°C for 14 h. Cells were returned to the glove box, filled with 1.0 ± 0.1 g of solution, and sealed at -90 kPa gauge pressure using an MSK-115A compact vacuum sealer (MTI Corp.), and then maintained at 1.5 V for 24 h at room temperature (21 – 25°C). Cells were connected to a Maccor 4000 test system (Maccor Inc) for cell formation. Cells were maintained at 40.0 ± 0.1°C for all electrochemical testing in this work. Because gas formation is frequently observed during formation, storage, and cycling, the pouch cells were clamped during all electrochemical testing using soft rubber (at ~25 kPa gauge pressure), which has been observed to improve the experimental precision. The SEI was formed by charging at C/20 to 4.3 V (at 40°C), holding at 4.3 V for 1 h, discharging at C/20 to 3.8 V, and then holding at 3.8 V for 1 h. Cells were degassed in an argon glove box by cutting the pouch cells open and then resealing them with the compact vacuum sealer. Cells were cycled at C/3 between 3.0 – 4.3 V using a Neware testing system. A slow cycle was performed every 50 cycles at C/20 to evaluate impedance growth.

Scanning Electron Microscopy The morphology of the NMC622 and NCA cathode materials used in this work was imaged using a Hitachi S-4700 scanning electron microscope (SEM). Residual lithium carbonate was removed by washing samples three times in a 4:1 ratio, by volume, with high purity water (≥ 18.2 MΩ cm, Barnstead NANOpure) and centrifuging to collect the powder. After rinsing and drying, powder samples were mounted on double-sided adhesive carbon tape for imaging. Accelerating voltages, working distances, and magnification factors are indicated in-figure for each image. Images were collected using secondary electron detection mode.

Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance (NMR) spectra were collected with a Bruker Avance 500 spectrometer using Topspin software. Samples were prepared by cutting open the pouch bags in 5 ACS Paragon Plus Environment


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6 an argon-atmosphere glove box and filling fluorinated ethylene propylene (FEP) NMR tube liners. Solutions that were stored with NMC622 or NCA were filtered through a 0.2 μm PTFE membrane. A few drops of benzene-d6 was placed into standard NMR tubes, between the outer glass tube and the filled inner FEP liners. This was done to facilitate the Topspin auto-lock and shimming procedures. Tubes were sealed using Parafilm M to prevent moisture contamination.

Density Functional Theory Calculations Density functional theory (DFT) geometry optimizations, energy calculations, and normal mode analyses were performed with Gaussian (G09.a02),32 using the B3LYP hybrid functional,33 the IEFPCM-UFF implicit solvation model (ε = 20),34–36 and the 6-311++G(2df,2pd) basis set. The polarizable continuum model (PCM) was selected for its balance of speed and accuracy.34,37 Implicit solvation models lead to some loss of accuracy because there are factors they do not account for, such as solvation sheath structure or preferential solvation in binary solvents.38–41 However, more detailed representations of solvation, such as hybrid DFT-MM-PCM models (MM = molecular mechanics) were considered outside the scope of this work. Results & Discussion

Predicted Spontaneity of the Reaction Having demonstrated that LiPO2F2 is beneficial for cell lifetime and performance (Figure 1), it was considered whether reaction (1) is thermodynamically spontaneous. Whereas there is previous evidence of stable aluminum oxyfluorides,21 the present work only considers the case where the reaction product is AlF3, i.e., reaction (2). This was chosen because of the abundance of thermochemical data for AlF3. The Gibbs free energy of reaction (2) was evaluated by combining density functional theory (DFT) calculations and published experimental thermodynamic values. From the standard free energy of formation for AlF3(s) (ΔG0f = -1431 kJ mol-1) and Al2O3(s) (ΔG0f = -1582 kJ mol-1),42 the reaction energy for (3) is readily calculated (4):


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7 3 2

𝐴𝑙2 𝑂3(𝑠) + 3 𝐹2(𝑔) → 2 𝐴𝑙𝐹3(𝑠) + 𝑂2(𝑔)

(3)

0 0 0 Δ𝐺(𝟑) = 2 Δ𝐺𝑓,𝐴𝑙𝐹 − Δ𝐺𝑓,𝐴𝑙 3 2 𝑂3

(4)

It is found that ΔG0(3) = -1280 kJ mol-1. Next, the total free energy at room temperature was evaluated (following a geometry optimization) for each component of reaction (5): − − 𝑂2(𝑔) + 𝑃𝐹6(𝑠𝑜𝑙𝑣.) → 2 𝐹2(𝑔) + 𝑃𝑂2 𝐹2(𝑠𝑜𝑙𝑣.)

(5)

0 Δ𝐺(𝟓) = 2 GF2(g) + GPO2 F−2 − GO2(g) − 𝐺𝑃𝐹6−

(6)

It is found that under standard conditions ΔG0(5) = +703 kJ mol-1. Finally, these two reactions may be combined to determine the free energy change (7) for the overall reaction (2), i.e., reaction (1) when x = 3: 0 0 0 Δ𝐺(𝟐) = 2 Δ𝐺(𝟑) + 3 Δ𝐺(𝟓)

(7)

The result gives ΔG0(2) = -451 kJ mol-1, indicating the reaction is spontaneous at standard state. It is noted that neither fluorine nor oxygen gas are actually proposed to be involved in this process, but rather they are included as part of a thermodynamic cycle to allow the combination of published free energies of formation for the solid materials with calculated stabilities of the dissolved anions.

Reactivity of Electrolyte and Pure Aluminum Oxide Having predicted that reaction (1) will be spontaneous (at least for the case where x = 3), experiments were performed to detect whether appreciable quantities of LiPO 2F2 are produced when electrolyte solution is exposed to pure Al2O3 powder. Electrolyte solutions were sealed in pouch bags either by themselves or in a 10:1 ratio, by mass, with high surface area Al2O3 material (10 g electrolyte solution and 1 g of Al2O3 powder). In general, interfacial chemical reaction rates are directly proportional to the surface area of the reacting material. It is for this reason that high



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8 surface area material was used in this work. The sealed pouch bags were then stored at either 40°C or 60°C for 1 week. Given that the introduction of water to electrolyte solutions is known to produce LiPO2F2,43 all of the materials in this work were meticulously dried and handled to avoid contamination. It is also known from previous work that solution NMR is a suitable method for the detection of small quantities of LiPO2F2, based on the characteristic 19F and 31P chemical shift positions and multiplet splitting patterns (Figure 2).

Figure 2

The (a) hexafluorophosphate (PF6–) and (b) difluorophosphate (PO2F2–) anions. Typical 19F and 31P NMR peak positions (centre of the multiplet) and 19F-31J coupling constants in electrolyte solution are shown.

Following the storage period, pouch bags were returned into the argon-atmosphere glove box, cut open, and NMR samples were prepared. The

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F and

31

P spectra of the as-prepared

electrolyte, a standard 1% LiPO2F2 solution, and the post-storage samples are shown in Figure 3 and Figure 4, respectively. The results clearly show that peaks corresponding to LiPO 2F2 are present in samples stored with Al2O3 at high temperature (Figure 3(d,f) and Figure 4(d,f)). Moreover, these peaks are not present in the control samples that did not contain any Al2O3 (Figure 3(c,e) and Figure 4(c,e)). It is presumed that the peak broadening and shifting, observed at around -84.5 ppm and -86.7 ppm in Figure 3(d,f) and at -8 ppm, -12 ppm, and -16 ppm in Figure 4(d,f) is attributable to the presence of Al2O3 nanoparticles that remain in solution, lowering the homogeneity of the magnetic field.


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Figure 3

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F solution NMR were collected from 1.2 M LiPF6 in 3EC:7DMC electrolyte solution (a) as-prepared, (b) with the addition of 1% LiPO2F2, and (c-f) following storage in pouch bags for 1 week. The pouch bags contained electrolyte solution either by itself or in a 10:1 (w/w) ratio with Al2O3 powder and were stored at either 40°C or 60°C, as indicated on the figure. Insets show expanded views of the PO2F2– region.



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Figure 4

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P solution NMR were collected from 1.2 M LiPF6 in 3EC:7DMC electrolyte solution (a) as-prepared, (b) with the addition of 1% LiPO2F2, and (c-f) following storage in pouch bags for 1 week. The pouch bags contained electrolyte solution either by itself or in a 10:1 (w/w) ratio with Al2O3 powder and were stored at either 40°C or 60°C, as indicated on the figure. Insets show expanded views of the PO2F2– region.

These results support the hypothesis that Al2O3 cathode coatings chemically react with the electrolyte solution to form LiPO2F2 in lithium-ion cells, which may contribute to the lifetime improvements that such coatings afford. In order to further test this theory, it was decided that the experiment must be repeated using positive electrode materials, both pristine (i.e., uncoated) and with an Al2O3 coating. The presence of additional features in Figures 2 and 3 indicate the PO2F2– anion exists in multiple chemical environments. This is perhaps attributable to an adsorbed state on the aluminum oxide nanoparticles.


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Reactivity of Electrolyte and Al2O3-Coated Positive Electrode Materials The experiments discussed above were repeated using Al2O3-coated electrode materials. Two positive electrode materials with relevance to electric vehicle and grid storage were chosen, NMC622 and NCA. The pristine materials (Figure 5a-d) are composed of secondary particles, ~10 – 15 μm in diameter, made up of smaller primary particles, ~1 μm diameter (NMC622) or ~0.5 μm diameter (NCA). This polycrystalline structure is typical for these positive electrode materials. The darker and lighter regions in these secondary electron images correspond to regions of different morphology, indicating these samples have some small amount of nonuniformity. The NMC622 material has lower particle size (Figure 5b) than the NCA material (Figure 5d). The materials were tested as-prepared (i.e., uncoated) and coated with Al2O3 by ALD. Figure 5 shows the morphology of the materials, measured by SEM, and that the ALD coatings are continuous (Figure 5e-h). These samples were rinsed prior to SEM imaging, whereas the samples used for the storage experiments below were unrinsed. It is possible that the unrinsed materials had small amounts of Li2CO3 on the surface, which is often present following the lithiation stage of NMC preparation. This is significant, because it has been shown that Li2CO3 and LiPF6 can react to form LiPO2F2.44

Figure 5

SEM images showing the morphology of the (a-b) uncoated NMC622, (c-d) uncoated NCA, (e-f) Al2O3-coated NMC622, and (g-h) Al2O3-coated NCA materials.



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12 Electrolyte solutions were stored in pouch bags with each of the four materials shown in Figure 5 in a 2:1 ratio, by mass (2 g electrolyte solution and 1 g positive electrode material), and then stored at 40°C for 14 days. Following storage, the pouch bags were opened in a glove box, the solutions were filtered with a 0.2 μm PTFE membrane, and prepared for NMR analysis. The NMR tubes were carefully sealed with Parafilm to prevent moisture from entering the samples. The 19F spectra show that small amounts of LiPO2F2 are present for both coated and uncoated materials (Figure 6). Some amount of LiPO2F2 is expected from the uncoated materials due to reaction of the LiPF6 with a) residual moisture at the surface and/or b) residual Li2CO3 from the lithiation process. There were no peaks present in the

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F spectra at -190 ppm, indicating there was

negligible HF, a known by-product of the reaction of LiPF6 and H2O.43 This is significant because HF can lead to cell degradation. Further analysis was performed by comparing the integral of the PF6– and PO2F2– peaks at -75 ppm and -85.8 ppm, respectively (Figure 7). The results clearly show that more LiPO2F2 is present in the electrolytes that were exposed to coated materials, relative to the electrolytes exposed to the corresponding uncoated materials. It is suggested that the relative amounts of LiPO2F2 between the two materials tested in this work (NMC622 and NCA) should not be compared, since the surface areas of the two materials were not the same.


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Figure 6

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Figure 7

The ratio of the PF6– and PO2F2– peak areas at -75 ppm and -85.8 ppm, respectively, for the samples in Figure 6.

F NMR spectra of electrolyte solutions that were sealed in pouch bags with a) NMC622 or b) NCA material in a 2:1 solution-to-material ratio and stored for 14 d at 40°C. Electrode materials were uncoated or ALD-coated with Al2O3, as indicated on the figure. Insets show expanded views of the PO2F2– region.

The results demonstrate that LiPF6 salts in electrolyte solutions react spontaneously with Al2O3 materials and with Al2O3-coated positive electrode materials to form LiPO2F2. Unlike the reaction of LiPF6 with water, HF is not produced during this reaction. Given the beneficial nature of this species for the lifetime and stability of lithium-ion cells, this reaction is here proposed to similarly benefit the performance of cells that use Al2O3-coated or oxide-coated cathode materials. It may be the case that for sufficiently thick Al2O3 coatings this reaction continues to proceed over the 13 ACS Paragon Plus Environment


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14 years and years of Li-ion cell operation, thus continually increasing or replenishing the LiPO2F2 content in the electrolyte. Conclusions From published thermochemical values and new DFT calculations, this work predicts that Al2O3 cathode coating materials and LiPF6 electrolyte salt solutions will spontaneously react to form LiPO2F2, a well-known beneficial electrolyte additive. Using 19F and 31P solution nuclear magnetic resonance spectroscopy, it is demonstrated that storing LiPF6-containing electrolyte solution with Al2O3 indeed produces LiPO2F2. The production of LiPO2F2 is also shown to occur when electrolyte solution is stored with Al2O3-coated NMC and NCA materials. This is a new mechanism of action for inorganic oxide surface coatings, the benefits of which are conventionally attributed to the chemical scavenging of corrosive HF and the physical blockage of electrolyte components from reaching the electrode surface. Acknowledgments This work was jointly funded by Tesla Canada and the Natural Sciences and Engineering Research Council of Canada. Computational facilities were provided by ACENET (Atlantic Computational Excellence Network) and funded by the Canada Foundation for Innovation (CFI), the Atlantic Canada Opportunities Agency (ACOA), and the provinces of Newfoundland and Labrador, Nova Scotia, and New Brunswick.


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21

Figure for Table of Contents only.



a) Normalized discharge capacity and b) ΔV for NMC532/graphite pouch cells cycled between 3.0 – 4.3 V at C/3 and at 40°C. A slow C/20 cycle was performed every 50 cycles. Cells contained 1.2 M LiPF6 in 3EC:7DMC and 0% LiPO2F2 (blue squares) or 1% LiPO2F2 (red circles). 82x99mm (300 x 300 DPI)

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The (a) hexafluorophosphate (PF6–) and (b) difluorophosphate (PO2F2–) anions. Typical 19F and 31P NMR peak positions (centre of the multiplet) and 19F-31J coupling constants in electrolyte solution are shown. 62x32mm (300 x 300 DPI)



19F

solution NMR were collected from 1.2 M LiPF6 in 3EC:7DMC electrolyte solution (a) as-prepared, (b) with

the addition of 1% LiPO2F2, and (c-f) following storage in pouch bags for 1 week. The pouch bags contained electrolyte solution either by itself or in a 10:1 (w/w) ratio with Al2O3 powder and were stored at either 40°C or 60°C, as indicated on the figure. Insets show expanded views of the PO2F2– region. 81x130mm (300 x 300 DPI)


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31P

solution NMR were collected from 1.2 M LiPF6 in 3EC:7DMC electrolyte solution (a) as-prepared, (b) with

the addition of 1% LiPO2F2, and (c-f) following storage in pouch bags for 1 week. The pouch bags contained electrolyte solution either by itself or in a 10:1 (w/w) ratio with Al2O3 powder and were stored at either 40°C or 60°C, as indicated on the figure. Insets show expanded views of the PO2F2– region. 83x130mm (300 x 300 DPI)



SEM images showing the morphology of the (a-b) uncoated NMC622, (c-d) uncoated NCA, (e-f) Al2O3coated NMC622, and (g-h) Al2O3-coated NCA materials. 152x57mm (300 x 300 DPI)


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19F

NMR spectra of electrolyte solutions that were sealed in pouch bags with a) NMC622 or b) NCA material in a 2:1 solution-to-material ratio and stored for 14 d at 40°C. Electrode materials were uncoated or ALDcoated with Al2O3, as indicated on the figure. Insets show expanded views of the PO2F2– region. 73x84mm (300 x 300 DPI)



The ratio of the PF6– and PO2F2– peak areas at -75 ppm and -85.8 ppm, respectively, for the samples in Figure 6. 77x53mm (300 x 300 DPI)


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Abstract Figure 81x34mm (300 x 300 DPI)


New Chemical Insights into the Beneficial Role of Al2O3 Cathode

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