Supplementary Information

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1 Supplementary Information High Capacity Na-O 2 Batteries Key Parameters for Solution- Mediated Discharge L. Lutz a,b,c,d, W.Yin a, A. Grimaud a,f*, D. Alves Dalla Corte a,f, M. Tang e, f, L. Johnson b, E. Azaceta h, V. Sarou-Kanian e,f, A. J. Naylor b, S. Hamad g, J. A. Anta g, E. Salager e,f, R. Tena-Zaera h, P. G. Bruce b,d and J-M.Tarascon a,d,f. a) Chimie du Solide et de l Energie, FRE 3677, Collège de France, Paris Cedex 05, France b) Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK c) Department of Chemistry, Université Pierre et Marie Curie, 4 Place Jussieu, Paris, France d) ALISTORE-European Research Institute, FR CNRS 3104, Amiens, France e) CNRS, CEMHTI UPR3079, Université d Orléans, 1D avenue de la recherche scientifique, Orléans Cedex 2, France f) Réseau sur le Stockage Electrochimique de l Energie (RS2E), CNRS FR3459,33 rue Saint Leu, Amiens Cedex, France g) Physical Chemistry Group, Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, ES-41013, Spain h) IK4-CIDETEC, Parque Technologico de San Sebastian, P Miramon, 196, Donostia-San Sebastian, Spain *Corresponding author: alexis.grimaud@college-de-france.fr S1

2 Figure S 1: Correlation between 23 Na-NMR shifts and DN in 0.5 mol.l -1 solutions of NaTFSI in various solvents. A linear fit between chemical shift and DN was created based on the values reported for DMSO, DMA, DMF and ACN. 1-3 The DN of the ethers are deduced from the linear fit. Vertical error is calculated by linear regression and horizontal error is obtained by the FWHM of the 23 Na-NMR shifts. The resulting DN values for DME, DGME and TGME are in good agreement with literature. 3-4 Figure S 2: (solid line) Multiple Na-O 2 cell discharges using DME showing good reproducibility of the capacities, which is approximately 7,5 mah/cm 2 (geom). (dashed line) Corresponding oxygen pressure decay indicating a 1e - /O 2 ratio displayed in matching to color. S2

3 Figure S 3: (solid line) Multiple Na-O 2 cell discharges using DGME showing good reproducibility of the capacities, which is approximately mah/cm 2 (geom). (dashed line) Corresponding oxygen pressure decay indicating a 1e - /O 2 ratio displayed in matching to color. Figure S 4: (solid lines) Multiple Na-O 2 cell discharges using TGME, showing fast cell death at capacities in the range mah/cm 2 (geom). (dashed lines) Corresponding oxygen pressure decay displayed in matching to color. S3

4 . Figure S 5: SEM images of three GDL-cathodes from different NaO 2 cells, discharged using TGME (0.5 M NaOTf) as solvent. The images show the formation of small agglomerates as well as a thin-film across the electrode surface. S4

5 Figure S 6: Raman spectrum of the discharge product found in Na-O 2 cells using TGME. The peak at 1137 cm 1, indicates the thin film consists of NaO 2. Cell and GDL background indicated by *. Figure S 7: Influence of Na + ion concentration (NaTFSI) on the solubility of electrochemically formed NaO 2 in O 2 -saturated DME as detected by RRDE. Showing increasing O 2 - detection with decreasing Na + concentration. RRDE measurements were performed with an Au/Au electrode at a rate of 20 mv s -1 and a rotation speed of 1000 rpm. S5

6 Figure S 8: UV-Vis extinction of various KO 2 saturated TiOSO 4 solutions at room temp. In presence of NaO 2 the TiOSO4 complex gets reduced to [Ti(O 2 2- ] 2+ (solutions turn yellow). The specific absorption maximum at 405 nm was used to calculate the amount of solubilized KO 2 in each solvent. Figure S 9: Radial Distribution Functions (RDF) and coordination numbers of oxygen atoms around the Na + cation, as calculated from 50 ps long Molecular Dynamics simulations for DME, DGME and TGME. See text for details. S6

7 Solvent D Na+ [10-6 cm 2 /s] D H+ [10-6 cm 2 /s] TGME DGME DME Table S1: Self-diffusion coefficient of the Na + cation obtained from 23 Na-NMRpulse field gradient (D Na+ ) and self-diffusion coefficient of the solvent obtained from 1 H-NMRpulse field gradient (D H+ ) for 0.5 mol.l -1 NaTFSI solutions in the three glyme-ethers at 25 C, uncertainty c.a. 5%. Figure S10: Cyclic voltammograms O 2 reduction in O 2 -saturated 0.5M NaTFSi (black line) and TBATFSi (red line) in DME. The difference in onset potential (approx. 300 mv) indicates the effect of Na + during the reduction of O 2 to O 2 -, which stabilizes the superoxide and thereby shifts the reduction potential to positive values. S7

8 References: 1. Katayama, M.; Shinoda, M.; Ozutsumi, K.; Funahashi, S.; Inada, Y., Reevaluation of Donor Number Using Titration Calorimetry. Analytical Sciences 2012, 28, Gutmann, V., Empirical Parameters for Donor and Acceptor Properties of Solvents. Electrochim. Acta 1976, 21, Mamantov, G., Chemistry of Nonaqueous Solutions: Current Progress Wiley: New York, Plewa, A.; Kalita, M.; Siekierski, M., Estimation of Ion Pair Formation Constants of Lithium Salts in Mixtures of Glymes and 1,4-Dioxane. Eleoctrochemica Acta 2007, 53, S8