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SupplementalInformation

DetectionofElectrochemicalReactionProductsfromtheSodium-OxygenCellwithSolid-State23NaNMRSpectroscopy

Zoë E. M. Reeve,(1) Christopher J. Franko,(1) Kristopher J. Harris,(1) Hossein Yadegari,(2) Xueliang Sun,(2) and Gillian R. Goward*(1)

*goward@mcmaster.ca

(1) Department of Chemistry, McMaster University 1280 Main Street West, Hamilton, Ontario, L8S 4M1 (Canada)

(2) Department of Mechanical and Materials Engineering, University of Western, London, Ontario N6A 5B9 (Canada)

TableofContents Solid-State NMR Spectroscopy Experimental Details S3 NaO2 Synthesis Experimental Details S5 Figure S1 Variable Temperature 23Na NMR of Synthetic NaO2 S6 Powder X-Ray Diffraction Experimental Details for NaO2 S7 Figure S2 PXRD of Synthetic NaO2 S7 Sample Preparation: Na2O2 and Na2CO3 S8 Table S1 Comparison of Experimental and Literature 23Na NMR Parameters for the Expected Na-O2 Species

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Figure S3 2D 23Na-3QMAS spectrum of Na2CO3 S10 Figure S4 Simulated and Experimental 1D NMR Spectrum of Na2O2 S11 Figure S5 2D 23Na-3QMAS spectrum of Na2O2 S12 Figure S6 2D 23Na-3QMAS spectrum of the Na2O2 / Na2CO3 mixture S13 Table S2 23Na Spin-Lattice Relaxation Times for the Expected Na-O2 Species S14 Figure S7 23Na NMR T1 Filtering experiment of Na2O2 / NaO2 Mixture S14 Electrochemistry Experimental Details S15 Figure S8 Representative Na-O2 Electrochemical Discharge Profile S16 Figure S9 23Na NMR spectra of Comparing Sodium Triflate and Na2CO3 S16

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Investigations of Potential Na-O2 Degradation Products S17 Sample Preparation: Potential Na-O2 Degradation Products S18 Table S3 Experimental 23Na NMR Parameters for Potential Degradation Products

S18

Figure S10 23Na NMR Spectral Library of Potential Na-O2 Degradation Products

S19

Sample Preparation: Na2O2 / NaO2 Composite Cathode S20 Figure S11 PXRD of Na2O2 / NaO2 Composite Cathode S20 Sample Preparation: NaO2 Reactivity Tests S21 Figure S12 23Na NMR spectrum of NaO2 and PVDF Reaction Products S21 Figure S13 Direct 19F NMR spectrum of NaO2 and PVDF Reaction Products S22 Figure S14 19F NMR spectrum of NaO2 and PVDF Reaction Products where the PVDF signal is selectively saturated revealing NaF

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Figure S15 23Na NMR spectrum of NaO2 and Carbon Black Reaction Products S24 Figure S16 2D 23Na-3QMAS spectrum of NaO2 and Carbon Black Reaction Products

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Figure S17 23Na NMR spectrum of NaO2 and electrode Reaction Products S26 References S27

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Solid-State NMR Spectroscopy Experimental Details

The 23Na NMR spectra of the expected Na-O2 reaction products were acquired on a

Bruker Avance I spectrometer (B0 = 11.7 T, ν0(23Na) = 132.294 MHz) using a custom-

built double-resonance probe supporting 1.8 mm rotors capable of magic angle spinning

(MAS) frequencies from 20 kHz up to 45 kHz. The spectra were referenced to a 1 M

NaCl solution at 0 ppm. All 1D spectra were collected via single pulse experiments using

a hard 90 pulse at an RF field of 125 kHz, and signal averaging over eight scans. The

delay times used for the synthesized NaO2, pristine Na2O2, pristine Na2CO3 and sodium

triflate were 1 s, 30 s, 5 s, and 5 s respectively. A 1 s delay time was used for the cycled

cathodes. Variable temperature 23Na MAS experiments were performed for NaO2

between 300 – 350 K. T1 measurements used a standard inversion-recovery sequence

where twelve relaxation times were used for each T1 determination.

The multiple quantum magic angle spinning (MQMAS)[1-2] pulse sequence used in

all experiments was the three pulse, z-filtered sequence. All 2D 23Na triple quantum

magic angle spinning (3QMAS) spectra were acquired with 64 t1 increments at 100 µs in

the F1 dimension with 64 scans per t1 increment at a MAS rate of 20 kHz. The RF field

used in the excitation and conversion pulses was 125 kHz and a 20 kHz RF field was

used for the selective pulse. The respective pulse lengths for the excitation, conversion

and selective pulses were 4.7 µs, 1.7 µs and 11.5 µs. The 23Na-3QMAS spectrum of the

electrochemically-cycled electrode was collected under the same conditions; with 192

scans used per t1 increment, due to the small sample size (~2mg per electrode).

The 23Na NMR spectra of the NaO2/Na2O2 mixture, the composite cathode, sodium

triflate and the potential Na-O2 degradation products (sodium acetate, sodium formate,

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sodium bicarbonate, sodium oxalate, sodium tartrate, sodium tartrate dihydrate, sodium

hydroxide and sodium fluoride) were acquired on a Bruker Avance III 850HD

spectrometer (B0 = 19.9 T, ν0(23Na) = 224.876 MHz) using a Bruker 1.9 mm probe with a

MAS frequency of 30 kHz. All spectra were referenced to a 1 M NaCl solution at 0 ppm

and collected with a soft 90 degree pulse at a RF field of 20 kHz. The spectra of the

NaO2/Na2O2 mixture and the composite cathode were collected with a 45 s delay time in

16 scans. The spectra of sodium tartrate dihydrate, sodium fluoride and sodium

bicarbonate were collected with a 60 s delay time in 8 scans. The spectra of sodium

oxalate and sodium formate were collected with a delay time of 30 s in 8 scans. The

spectra of sodium acetate and sodium hydroxide were collected with a delay time of 10 s

in 8 scans. The spectra of the dried sodium tartrate sample and sodium triflate were

collected with a 5 s delay time in 8 scans.

All 23Na spectra for experiments relating to NaO2 reactivity with PVDF binder,

carbon black, and carbon cathodes were collected on the Bruker Avance I spectrometer

(B0 = 11.7 T, ν0(23Na) = 132.294 MHz) using the custom-built double-resonance probe

supporting 1.8 mm rotors described above. The spectra of NaO2 with the carbon cathode

and NaO2 with carbon black were collected using a D1 of 30 s over 512 scans. The

spectrum of NaO2 with PVDF was also collected using a D1 of 30 s but with 16 scans.

19F experiments were collected on the same instrument with the same probe. All

19F spectra were referenced to CFCl3 at 0 ppm. Spectra of PVDF, NaO2 with PVDF, and

NaF were collected using a D1 of 30 s over 32 scans.

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NaO2 Synthesis Experimental Details

Sodium superoxide was chemically synthesized under an argon atmosphere

following a previously published procedure.[3] Oxygen gas was rapidly bubbled through

liquid NH3 and then individual pieces of sodium metal were added to the solution. As

each sodium metal piece was solvated, a blue colour appeared and subsequently

dissipated as the reduced O2 reacted with the solvated Na+ ions. Once the desired amount

of sodium metal fully reacted, the liquid ammonia was boiled off. The final product was

dried under vacuum for several hours and stored in an argon glovebox. The synthetic

NaO2 was confirmed to be the disordered pyrite structure (Fm3m, ICSC 87176) with

powder X-Ray diffraction (PXRD). Variable temperature 23Na NMR was applied to

confirm that the material is paramagnetic.

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Figure S1 (Left) Plot of the NaO2 (Na(sup-1)) 23Na chemical shift vs. 1000/T and (Right) the experimental 23Na MAS NMR spectra of the synthesized sodium superoxide, where the NaO2 (Na(sup-1)) resonance moves to lower frequencies as the sample temperature is increased compared to the diamagnetic impurity which remains at a constant chemical shift with temperature. All measurements were made at a MAS speed of 20 kHz at a magnetic field of 11.7 T.

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Powder X-Ray Diffraction Experimental Details for NaO2

The synthetic NaO2 PXRD sample was prepared under inert conditions inside an

argon glovebox. NaO2 was mounted into a 0.5 mm diameter capillary which was sealed

with grease inside the glovebox. The X-ray diffraction characterization was performed on

a Bruker 3-circle D8 goniometer system equipped with a Bruker SMART 6000 CCD area

detector, Rigaku RU-200 rotating anode Cu K̅α generator, and cross-coupled parallel

focusing optics. An exposure time of 300s/frame was used. Integrated diffractograms

were obtained with a 2θ range of 5-95°.

Figure S2 Comparison of the chemically synthesized NaO2 and reference NaO2 powder X-ray diffraction patterns, confirming that the synthesized NaO2 has a disordered pyrite structure.

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Sample Preparation: Na2O2 and Na2CO3

Sodium peroxide (Alfa Aesar) was used without further purification where as

sodium carbonate (Sigma Aldrich) was dried at 120 0C for several hours under active

vacuum prior to use. Both materials were packed in NMR rotors inside an argon

glovebox.

The experimental 23Na NMR parameters for the expected Na-O2 reaction products

are reported in Table S1 and compared to existing literature values where available. The

NMR spectra were simulated using the TopSpin 3.2 software.

Table S1– Comparison of the Experimental and Literature 23Na NMR Parameters at Room Temperature for the Relevant Na-O2 Species

Compound / Assigned NMR Resonance δiso (ppm) CQ (MHz) ±

0.05 ηQ ± 0.1

Na2O2 Lit.[4] 6.9 0.47 - Na2O2 / Na(per-1) Expt. 7.0 0.47 0.1 Na2O2 / Na(per-2) Expt. 11.8 0.47 0.1

Na2O Lit [5] 48.0 - - Expt. 55.0 0.00 0.00

NaO2 / Na(sup-1) Lit.[5] -28 - - Expt. -23 0.00 0.00

Na2CO3 / Na(carb-1) Expt. 7 1.20 0.60 Na2CO3 / Na(carb-2) Expt. 7.5 1.28 0.50 Na2CO3 / Na(carb-3) Expt. -4 2.48 0.65

With 2D 23Na-3QMAS, Na2CO3 is confirmed have three Na sites (Figure S3) in

accordance with the reported crystal structure.[6] The NMR parameters reported in Table

S1 for Na2CO3, were determined by simulated the extracted 1D slice for each of the three

sodium carbonate sites shown in Figure S3.

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The 23Na NMR parameters of Na2O2 were determined by evaluating the spinning

side band manifolds, which is reflective of the first order quadrupole powder pattern

(Figure S4). With 2D 23Na-3QMAS, Na2O2 is confirmed have two Na sites (Figure S5)

in accordance with the reported crystal structure.[7] The two Na sites of sodium peroxide,

were found to have the same quadrupole coupling constant, in accordance with 23Na Cq

value previously reported by Bastow et al.[4]. However Bastow et al. modeled the

spectrum as a single site[4], where the data sets were collected at a magnetic field of 9.4 T

and at a MAS frequency ranging from 12 and 16 kHz. Our 1D 23Na and 2D 23Na-3QMAS

spectra, (Figure 1 and S5 respectively) were collected at a higher magnetic field of

11.7 T and at a faster MAS frequency of 20 kHz, demonstrating that Na2O2 has two

distinct Na sites.

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Figure S3 2D 23Na-3QMAS spectrum of Na2CO3 collected at 11.7 T and a MAS rate of 20 kHz, illustrating good site resolution for the three Na sites, where the 1D extracted slices for each site is shown,simulated using the parameters reported in Table S1.

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Figure S4 (Pink) simulated and (black) experimental 23Na NMR spectra of Na2O2collected at 11.7 T with a MAS rate of 20 kHz, where an evaluation of the central transition illustrates that the two Na sites; Na(per 1) (dark blue) and Na(per 2) (light blue) are resolved.

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Figure S5 2D 23Na-3QMAS spectrum of Na2O2 collected at 11.7 T and a MAS rate of 20 kHz, illustrating good site resolution for the two Na sites

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Figure S6 23Na-3QMAS spectrum of the Na2O2 and Na2CO3 physical mixture collected at 11.7 T and with a MAS rate of 20 kHz, where the 1D extracted slices for each site is shown, simulated using the reported parameters from Table S1.

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Table S2 – 23Na Spin-Lattice (T1) Relaxation Times for the Expected Na-O2

Reaction Species collected at 11.7 T

NMR Resonance T1 value (s) Na(per-1) & Na(per-2) 14.0 ± (0.1)

Na(sup-1) 7.0 x10-3 ± (5.0 x10-3) Na2CO3 / Na(carb-1,2) 9.0 x 10-1 ± (3.0 x10-2)

Na(carb-3) 1.5 ± (0.1)

Figure S7 Results from a T1 filtering experiment where the delay varied from 0.1 s to 60 s, for a sodium superoxide and sodium peroxide mixture, highlighting the rapid relaxation of the paramagnetic NaO2 (Na(sup1)) compared to the diamagnetic Na2O2 (Na(per1) and Na(per2)). The 1D 23Na MAS NMR spectra were collected at a MAS frequency of 20 kHz and at a magnetic field of 11.7 T.

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Electrochemistry Experimental Details

The Na-O2 cells were constructed following previously published methods.[8] Gas

diffusion electrodes were prepared by casting a mixture of commercial carbon black

(N330) and Polyvinylidene fluoride (PVDF, Alfa Aesar) with a weight ratio of 9:1 on a

separator (Celgard 3500). The electrodes were 3/8 inch in diameter with a loading of

~0.25 mg. Swagelok type cells comprised of sodium foil anode, Celgard 3500 separator,

gas diffusion electrode and a stainless steel mesh as current collector were used to

prepare the discharge product. A fresh sodium foil was prepared with the aid of a

homemade press machine using sodium metal blocks (Sigma Aldrich), inside the argon-

filled glove box. The electrolyte was a 0.5 M solution of sodium triflate (NaSO3CF3 98%,

Aldrich) dissolved in diethylene glycol dimethyl ether (DEGDME reagent grade ≥ 98%,

Aldrich). The sodium triflate electrolyte salt was dried at 80 ˚C under vacuum for 48

hours and the water content of diethylene glycol dimethyl ether solvent was removed

using molecular sieves for at least 10 days. The assembled Na-O2 cells were sealed into a

specially designed testing chamber and then taken out of the glove box. A moderate

vacuum was applied to remove the argon, and subsequently the chamber was back-filled

with pure oxygen (purity 99.993%). The oxygen pressure was brought to 1.0 atm and

maintained under static conditions throughout the discharge cycle. The Na-O2 cells were

discharged at a current density of 75 mA g-1 and stopped at the specific capacities of

interest along the discharge curve. Following electrochemical cycling the Na-O2 cells

were disassembled in the glovebox and packed in an NMR rotor without further

modification.

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Figure S8 Representative Electrochemical discharge profile of the first discharge for a Na-O2 cell with a 0.5 M electrolyte solution of sodium triflate in diethylene glycol dimethyl ether, specifying the specific capacities D100 and D750 that correspond to the two extracted cells.

Figure S9 Comparison of the sodium carbonate and sodium triflate 23Na MAS NMR spectra collected at 11.7 T with a MAS rate of 20 kHz, highlighting the overlap between the sodium triflate and sodium carbonate (Na(carb 3)) NMR signals, where sodium triflate has five unique crystallographic sites.[9]

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Investigation of Potential Na-O2 Degradation Products

In order to test whether 23Na NMR can detect and resolve all of the likely products

(either electrochemical or from secondary reactions), a wide range of oxygen- and

carbon-containing sodium salts were investigated. The experimental 23Na NMR spectra

of the potential degradation products are provided in Figure S10, while the 23Na NMR

parameters determined from them (via lineshape simulation with TopSpin 3.2 software)

are reported in Table S3. Each of the wide selection of materials is found to be easily

distinguishable, which further confirms the assignment of NaF as the breakdown product

where noted in the manuscript.

Sample Preparation: Potential Na-O2 Degradation Products

All materials were purchased from Sigma Aldrich. Sodium bicarbonate, sodium

tartrate dehydrate and anhydrous sodium acetate were used without further purification.

Additionally sodium bicarbonate and anhydrous sodium acetate were stored under inert

conditions. Dehydrated sodium tartrate, sodium hydroxide, sodium formate, sodium

oxalate and sodium triflate were dried at 120 0C for several hours under active vacuum

prior to use.

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Table S3 – Experimental 23Na NMR Parameters of Potential Na-O2 Degradation Products

Compound δiso (ppm) Cq (MHz) ηq Sodium acetate -2.5 0.70 1 Sodium formate 1.6 0.85 0.75

Sodium bicarbonate -4.0 0.60 1.00 Sodium oxalate 3.0 2.49 0.76

dehydrated Sodium tartrate (Na1) 7.9 2.34 0.85 dehydrated Sodium tartrate (Na2) 1.5 2.46 0.15 Sodium tartrate dihydrate (Na1) 3.4 0.71 0.5 Sodium tartrate dihydrate (Na2) 2.3 0.22 1

Sodium hydroxide 20.5 3.60 0.15 Sodium fluoride 7.4 0 0

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Figure S10 - 23Na MAS NMR spectral library of potential Na-O2 degeneration products, comparing the experimental spectra (black) and simulated best fits (red) collected at MAS rate of 30 kHz at a 19.9 T magnetic field of (a) sodium acetate, (b) sodium formate, (c) sodium bicarbonate, (d) sodium oxalate, (e) dehydrated sodium tartrate, (f) sodium tartrate dihydrate, (g) sodium hydroxide and (h) sodium fluoride

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Sample Preparation: Na2O2 / NaO2 Composite Cathode

The Na2O2 / NaO2 mixed sample and the composite cathode (Na2O2 / NaO2 mixture

+ pristine carbon + PVDF electrode) were fabricated by grinding the required

components together under an argon atmosphere. The composite cathode PXRD sample

was prepared and collected following the conditions outlined in the NaO2 PRXD

experimental details.

Figure S11 Experimental PXRD diffraction pattern of the composite cathode (black) overlaid with the reference PXRD patterns for Na2CO3 (red) and NaF (purple).

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Sample Preparation: NaO2 Reactivity Tests

The reactivity of NaO2 was tested by fabricating three mixtures; NaO2 + PVDF,

NaO2 + carbon black and NaO2 + carbon PVDF electrode. The mixtures were made by

grinding NaO2 with carbon black, PVDF powder and a carbon PVDF electrode

respectively, under an inert argon atmosphere. Each sample remained in an argon

glovebox for two weeks prior to analysis with solid state MAS NMR spectroscopy.

Figure S12 23Na MAS NMR spectrum of the reaction products from grinding NaO2 and PVDF binder, in addition to the reference spectra of each component, collected at 11.7 T with a MAS rate of 20 kHz.

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Figure S13 19F MAS NMR spectrum of the reaction products of grinding NaO2 and PVDF binder, as well as the reference spectra of PVDF and NaF, collected at 11.7 T with a MAS rate of 27 kHz. The spinning side bands of the PVDF signal overlap significantly with the NaF signal at this MAS speed, thus making the small NaF signal observed in the reaction products difficult to resolve. Multiple spinning speeds ranging from 20 kHz to 35 kHz were tested in an attempt to resolve the NaF site, but all speeds within the capacity of the probe used were insufficient. To address the issue, a selective saturation experiment was used and is presented in S14.

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Figure S14 19F MAS NMR spectrum of the reaction products from grinding NaO2 and PVDF binder using a train of pulses to selectively saturate the PVDF signal, at a magnetic field strength of 11.7 T and a MAS rate of 27 kHz. Identical conditions were applied to collect the PVDF and NaF reference spectra. The NaF peak is sufficiently resolved using this method, with only trace PVDF signal present in the NaO2 + PVDF spectrum.

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Figure S15 23Na MAS NMR spectrum of the reaction products from grinding NaO2 and carbon black as well as the reference spectra of each component collected at 11.7 T with a MAS rate of 20 kHz.

25

Figure S16 2D 23Na-3QMAS spectrum of the reaction products from grinding NaO2 and carbon black, collected at 11.7 T with a MAS rate of 20 kHz. Overlaid is the reference spectrum of sodium carbonate.

20 10 0 -10 -20

10

8 6

4 2

0

NaO2+PVDF

Na2CO3

δiso+ansio/(ppm)

δiso/(ppm

)

26

Figure S17 23Na MAS NMR spectrum of the reaction products from grinding NaO2 and carbon black, collected at 11.7 T with a MAS rate of 20 kHz. Additionally the 23Na NMR spectra of the pristine reference materials are provided.

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