| ICP-OES solubility in G1 (n = 3) | NaF: 5.50 × 10⁻⁶ M, SD 1.75 × 10⁻⁶ M; Na₂CO₃: 1.62 × 10⁻⁶ M, SD 3.91 × 10⁻⁷ M; Na₂O: 1.67 × 10⁻⁴ M, SD 3.15 × 10⁻⁵ M; NaOCH₃: 5.41 × 10⁻³ M, SD 2.58 × 10⁻⁴ M; background Na: 2.55 × 10⁻⁶ M, SD 3.86 × 10⁻⁷ M |
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| ICP-OES solubility in G2 (n = 3) | NaF: 2.66 × 10⁻⁵ M, SD 1.54 × 10⁻⁶ M; Na₂CO₃: 1.34 × 10⁻⁵ M, SD 1.48 × 10⁻⁶ M; Na₂O: 2.34 × 10⁻⁴ M, SD 1.05 × 10⁻⁵ M; NaOCH₃: 3.96 × 10⁻⁴ M, SD 1.95 × 10⁻⁵ M; background Na: 2.63 × 10⁻⁵ M, SD 3.67 × 10⁻⁶ M |
| ICP-OES solubility in G2 (n = 3) | NaOH: 2.14 × 10⁻⁴ M, SD 1.42 × 10⁻⁵ M; background Na: 2.26 × 10⁻⁵ M, SD 8.86 × 10⁻⁶ M |
| ICP-OES solubility in G2 (n = 3) | Na₂O₂: 1.34 × 10⁻³ M, SD 2.36 × 10⁻⁴ M; background Na: 3.05 × 10⁻⁵ M, SD 2.13 × 10⁻⁶ M |
| ICP-OES solubility in G4 (n = 3) | NaOCH₃: 3.22 × 10⁻³ M, SD 9.55 × 10⁻⁵ M; background Na: 3.48 × 10⁻⁵ M, SD 1.51 × 10⁻⁷ M |
| ICP-OES solubility in DMC (n = 3) | NaF: 8.27 × 10⁻⁷ M, SD 4.82 × 10⁻⁷ M, below quantification limit; Na₂CO₃: 7.90 × 10⁻⁷ M, SD 5.37 × 10⁻⁷ M, below quantification limit; Na₂O: 4.13 × 10⁻⁶ M, SD 1.45 × 10⁻⁶ M; NaOCH₃: 1.04 × 10⁻⁴ M, SD 1.48 × 10⁻⁵ M; background Na: 1.44 × 10⁻⁶ M, SD 1.56 × 10⁻⁶ M, below quantification limit |
| ICP-OES solubility in DEC (n = 3) | NaOCH₃: 4.88 × 10⁻⁴ M, SD 2.10 × 10⁻⁴ M; background Na: 6.56 × 10⁻⁶ M, SD 2.76 × 10⁻⁶ M |
| ICP-OES solubility in PC (n = 3) | NaF: 1.78 × 10⁻⁵ M, SD 2.47 × 10⁻⁶ M; Na₂CO₃: 9.87 × 10⁻⁶ M, SD 2.28 × 10⁻⁶ M; Na₂O: 9.94 × 10⁻⁵ M, SD 7.85 × 10⁻⁵ M; background Na: 1.57 × 10⁻⁵ M, SD 3.27 × 10⁻⁶ M |
| ICP-OES solubility in PC (n = 3) | NaOCH₃: 4.33 × 10⁻⁴ M, SD 4.67 × 10⁻⁵ M; background Na: 1.45 × 10⁻⁴ M, SD 8.66 × 10⁻⁶ M |
| Reporting qualifier for Na₂O solubility | The measured solubility can be impacted by NaOH and Na₂O₂, two common impurities in Na₂O |
Chien-Rung Shih, Sean A. Fernandez, Sabine P. Bohn, Jihyeon Kim, Lauren E. Marbella, Betar M. Gallant. “Solid-electrolyte interphase reactivity with the bis(fluorosulfonyl)imide anion governs cycling instabilities of sodium metal anodes.” ChemRxiv (). https://doi.org/10.26434/chemrxiv.15005534/v2
| ICP-OES solubility in G1 (n = 3) | LiF: 9.12 × 10⁻⁷ M, SD 4.00 × 10⁻⁷ M, below quantification limit; Li₂CO₃: 4.35 × 10⁻⁷ M, SD 1.77 × 10⁻⁷ M, below quantification limit; Li₂O: 1.71 × 10⁻⁵ M, SD 2.32 × 10⁻⁵ M; LiOCH₃: 1.88 × 10⁻⁴ M, SD 1.50 × 10⁻⁵ M; background Li: 2.41 × 10⁻⁷ M, SD 8.08 × 10⁻⁸ M, below quantification limit |
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| ICP-OES solubility in G2 (n = 3) | LiF: 5.65 × 10⁻⁶ M, SD 5.02 × 10⁻⁸ M, below quantification limit; Li₂CO₃: 2.80 × 10⁻⁶ M, SD 8.11 × 10⁻⁸ M, below quantification limit; Li₂O: 2.31 × 10⁻⁵ M, SD 2.50 × 10⁻⁵ M; LiOCH₃: 1.04 × 10⁻⁴ M, SD 4.30 × 10⁻⁵ M; background Li: 5.61 × 10⁻⁶ M, SD 1.88 × 10⁻⁷ M, below quantification limit |
| ICP-OES solubility in G2 (n = 3) | LiOH: 2.82 × 10⁻⁵ M, SD 4.32 × 10⁻⁶ M; background Li: 9.37 × 10⁻⁶ M, SD 3.25 × 10⁻⁶ M |
| ICP-OES solubility in G2 (n = 3) | Li₂O₂: 4.67 × 10⁻⁴ M, SD 2.56 × 10⁻⁴ M; background Li: 3.02 × 10⁻⁶ M, SD 3.23 × 10⁻⁷ M |
| ICP-OES solubility in DMC (n = 3) | LiF: 2.52 × 10⁻⁷ M, SD 1.46 × 10⁻⁷ M, below quantification limit; Li₂CO₃: 6.18 × 10⁻⁸ M, SD 1.62 × 10⁻⁸ M, below quantification limit; Li₂O: 7.83 × 10⁻⁷ M, SD 5.84 × 10⁻⁷ M, below quantification limit; LiOCH₃: 6.65 × 10⁻⁶ M, SD 9.59 × 10⁻⁷ M, below quantification limit; background Li: 1.76 × 10⁻⁷ M, SD 7.97 × 10⁻⁸ M, below quantification limit |
| ICP-OES solubility in PC (n = 3) | LiF: 1.19 × 10⁻⁶ M, SD 6.03 × 10⁻⁷ M, below quantification limit; Li₂CO₃: 5.05 × 10⁻⁷ M, SD 4.30 × 10⁻⁸ M, below quantification limit; Li₂O: 2.01 × 10⁻⁶ M, SD 2.19 × 10⁻⁶ M, below quantification limit; LiOCH₃: 1.90 × 10⁻⁴ M, SD 3.20 × 10⁻⁵ M; background Li: 8.35 × 10⁻⁷ M, SD 8.90 × 10⁻⁸ M, below quantification limit |
| Reporting qualifier for Li₂O solubility | The measured solubility can be impacted by LiOH and Li₂O₂, two common impurities in Li₂O |
| AccuTOF-DART MS, negative-ion mode (after overnight at 28 °C) | m/z 101, [PC−H]⁻; deprotonated PC and series of signals separated by Δm/z 58, equal to the molecular mass of PC − CO₂, were observed in all spectra |
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| ¹H NMR (Bruker Avance NEO; 500.18 MHz) | Methanol δ 3.36–3.37 ppm, corresponding to methoxide after protonation by water or acetic acid, was observed in every sample |
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| ¹H NMR quantity of NaOCH₃ (× 10⁻⁷ mol) | NaPF₆/G2 after 50 cycles: 2.10; NaOTf/G2 after 30 cycles: 0.318; NaFSI/G1 after 8 cycles: 1.61; NaFSI/G2 after 8 cycles: 2.67; NaClO₄/G2 after 2 cycles: 55.6 |
| ¹H NMR normalization and referencing | Spectra were normalized by the intensity of TSP-d₄; except for NaPF₆/G2, peaks were aligned to the G2 reference at 3.39 ppm or the G1 reference at 3.38 ppm; NaPF₆/G2 was referenced to TSP-d₄ in D₂O |
| ¹H NMR of digested Cu electrode and Cu-side separator | Only the G2 solvent peak was observed between 3.24 and 3.5 ppm; methanol at approximately 3.36 ppm was not observed |
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| ¹⁹F NMR broad scan (Bruker Avance Neo 600.14 MHz spectrometer; 64 scans; relaxation delay 1 s; spectral width 300 ppm) | No additional fluorine species were observed |
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| ¹⁹F NMR integrated area (32 scans; relaxation delay 8 s; a.u.) | NaOTf/G2: 211195439 without NaOCH₃ and 198794637 with NaOCH₃, −5.9 % relative to without NaOCH₃; NaPF₆/G2: 194550640 without NaOCH₃ and 206905878 with NaOCH₃, 6.4 % relative to without NaOCH₃ |
| ¹⁹F NMR reporting qualifier | The percentage differences can arise from experimental errors; systematic and random errors for pipetting 100 µL were ±8.0 % and ±3.0 %, respectively |
| ¹⁹F NMR narrow scan | Trace impurities in both electrolytes did not persist when exposed to NaOCH₃; the impurities in NaPF₆/G2 were likely hydrolyzed PF₆⁻ products such as [POₓFᵧ]⁵⁻²ˣ⁻ʸ anions |
| ¹⁹F NMR of NaFSI/G2 with NaOCH₃ (Bruker Ascend 500 MHz spectrometer; external DMSO-d₆ lock) | Additional product signal approximately 0.1 ppm downfield from FSI, assigned to (OCH₃)FSI |
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| ¹H NMR of NaFSI/G2 with NaOCH₃ (Bruker Ascend 500 MHz spectrometer; external DMSO-d₆ lock) | Two additional signals near 3.65 ppm, assigned to (OCH₃)FSI and (OCH₃)₂SI |
| ¹H–¹³C HSQC of NaFSI/G2 with NaOCH₃, with separately acquired ¹³C{¹H} projection | Protons near 3.65 ppm correlated with carbons near 55 ppm |
| AccuTOF-DART MS, negative-ion mode (heater 350 °C; m/z 100–1000; ion-guide RF 900 V; PEG m/z 446.24 reference) | NaFSI/G2: m/z 180.13, FSI; 383.05 and 385.04, derivatives of anions and solvent; NaFSI/G2 with NaOCH₃: m/z 180.13, FSI; 192.12, (OCH₃)FSI; 204.11, (OCH₃)₂SI; 382.78 and 384.77, derivatives of anions and solvent; m/z 180.13, 192.12, and 204.11 were also observed in methanol and G1 mixtures |
| ¹⁹F NMR | FSI, an FSI decomposition product, and NaF were observed |
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| ¹⁹F and ¹H NMR (200 µL recovered electrolyte with 150 µL G2) | Signals assigned to (OCH₃)FSI and (OCH₃)₂SI; additional unassigned ¹⁹F signals near 53.1 and 38 ppm |
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| ¹H–¹³C HSQC with separately acquired ¹³C{¹H} projection | Two cross peaks assigned to (OCH₃)FSI and (OCH₃)₂SI |
| ¹H–¹³C HSQC reporting qualifier | The displayed spectrum was manually masked to remove residual t₁ noise and sidebands from imperfect ¹³C decoupling; an unmasked spectrum was also provided |
| ¹⁹F NMR (500.18 MHz; equal volume DMSO-d₆; n = 3 per cycle, except n = 2 at cycle 1 after an outlier) | FSI signal near 52.5 ppm; (OCH₃)FSI signal near 52.8 ppm; FSI concentration decreased over cycling, with large measurement errors in the initial cycles |
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| Karl Fischer titration (n = 2) | NaOH in commercial NaOCH₃: 8.4 wt%, SD 1.9 wt%; LiOH in commercial LiOCH₃: 1.5 wt%, SD 0.1 wt% |
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| XRD (Panalytical X’Pert PRO; Cu Kα, λ = 1.5406 Å; step 0.0167°; scan speed 0.0213° s⁻¹) | Trace crystalline Na₂O₂ was detected in Na₂O, and trace crystalline Li₂O₂ was detected in Li₂O; crystalline NaOH and LiOH were not detected; amorphous hydroxides could not be ruled out |
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| Titration gas chromatography with thermal-conductivity detection of inactive Na⁰ (n = 3 or 4 per point; 0.5 mL water for NaOTf/G2 or ethanol for NaFSI/G2 and NaFSI/G1; 5 mL headspace; LiAlH₄ calibration) | Inactive Na⁰ increased with cycle number in all electrolytes; absolute quantities were smaller for NaOTf/G2; NaFSI/G2 and NaFSI/G1 accumulated comparable quantities through four cycles, after which accumulation was faster for NaFSI/G2 |
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| ¹⁹F NMR quantification of NaF (n = 3 or 4 per point) | NaF concentration was determined from the signal near −120 ppm; NaF accumulated linearly with cycling in all electrolytes; total quantities were smaller for NaOTf/G2 than for NaFSI/G2 and NaFSI/G1; accumulation in NaFSI/G2 exceeded NaFSI/G1 after four cycles |
| 2-Butoxyethanol conversion and Karl Fischer titration of Na₂O (n = 3 or 4 per point; 1 mL 2-butoxyethanol overnight; empty-vial and solvent background subtracted) | With the assumption that little native NaOH was present and detected NaOH came primarily from conversion of Na₂O, Na₂O accumulated linearly with cycling in all electrolytes; total quantities were smaller for NaOTf/G2 than for NaFSI/G2 and NaFSI/G1; accumulation in NaFSI/G2 exceeded NaFSI/G1 after four cycles |
| NaF and Na₂O quantities converted to mAh, assuming every Na⁺ accounted for one electron loss | At least twice as much NaF as Na₂O accumulated in the NaFSI electrolytes; less NaF than Na₂O accumulated in NaOTf/G2 |
| Sulfuric-acid titration gas chromatography of Na₂CO₃ (0.5 mL 3.5 M H₂SO₄; empty-vial and glovebox-Ar CO₂ background) | Na₂CO₃ was insignificant across all samples |