Supplementary Information

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1 Supplementary Information The role of electrolyte anions in the Na-O 2 battery: implications for NaO 2 solvation and the stability of the sodium SEI in glyme-ethers. Lukas Lutz a,b,c,d, Daniel Alves Dalla Corte a,f, Mingxue Tang e, f, Elodie Salager e, f, Michael Deschamps, e,f Alexis Grimaud a,f*, Lee Johnson b, Peter, G. Bruce b,d, and Jean-Marie 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 S1

2 23 Na-NMR measurements: DME (ppm) ACN (ppm) DMSO (ppm) - ClO PF TFSi Otf Variation Table S 1: DN calculation of solvent-salt complexes by means of the 23 Na-NMR peak position. Influence of anion DN on Solubility of NaO 2 : Figure S 1: Rotating ring disc electro response, obtained in O 2 saturated DGME solutions using 0.5 M NaOTf and NaClO 4 at 1000 rpm. The Au-disc was scanned at 20 mv/s between 3-1V (vs.na + /Na) and the Pt-ring potential fixed to 2.5 V. S2

3 Cycling Na-O 2 batteries using NVP anodes: Figure S 2: Activation and prechargeing of NVP material. Typical discharge/charge profile of a Na-ion half-cell using NVP as cathode, Na metal as anode and DME M NaPF 6 as electrolyte. S3

4 Figure S 3: Electrochemical discharge profiles of Na-air cells using GDL as a cathode, NVP as anode and DME with 0.5 M NaClO 4 (blue line) and NaTFSi (organe line) as the electrolyte, as described by Zhang et al. 1, showing similar capacities for both electrolyte systems. Stripping of Na and plating on Cu-foil in Na-coin cells using various sodium salts in DME: Ability of sodium anodes in various DME based electrolytes to sustain a Na-O 2 cell discharge. When NaPF 6, NaOTf and NaClO 4 are used, the anode is not limiting the discharge in Na-O 2 cells. Only in case NaTFSi is used as conductive salt, the anode is limiting. Figure S 4: Electrochemical voltage profiles of coin cells containing a Na anode (stripping) and Cu-foil (plating) as working electrode, using DME with 0.5 M NaPF 6 (a), NaOTf (b), NaClO 4 (c) and NaTFSi (d) as the electrolyte. Cells were discharged until a cutoff limit of -0.5 V (vs. Na + /Na) at a rate of 25 µa/cm 2. S4

5 Early EIS analysis of SEI formation when using NaTFSi salt: Figure S 5: First 24 h of SEI formation analyzed by Electrochemical Impedance Spectroscopy (EIS) on symmetrical Na-Na coin cells employing electrolytes based on DME and 0.5 M NaTFSi. Spectra recorded every 6 h. Ageing of metallic sodium in various ethereal electrolytes Figure S 6: Sodium metal aged 48 h in electrolyte solutions using 05 M NaTFSi, NaClO 4, NaPF 6 and NaOTf as conductive salts in DME (a),dgme (b) and TGME (c). S5

6 Detailed XPS analysis of various SEI s. Spectrum Compound NaClO 4 NaPF 6 NaOTf NaTFSi C1s O-C-O 268,5 286,4 286,4 286,5 C1s C-O=O 289,1 289,1 288,9 288,7 C1s CO C1s C-F 2 292, O1s C-O 231,4 531,2 531,2 531,3 O1s C-O-Na 533,4 533, ,2 O1s Na-auger 536,3 536,1 536,1 536,1 Na1s Na-F/Cl 1071,6 1071,2 1071,5 1071,0 Na1s Na 1072,5 1071,9 1072, Cl2p 198,6 / 200,2 F1s C-F 3 291,7 F1s Na-F 683,8 684,1 683,7 F1s P-F 687,2 F1s C-F 3 685,2 F1s C-F 2 688,7 688,7 Table S 2: XPS signals of chemical compounds present in the SEI formed on metallic sodium when aged in DME based electrolytes using various sodium salts. S6

7 Reactivity of decomposition products towards cell components: The analysis of the cell components of a standard Swagelok setup using a Mylar insulator demonstrates the high reactivity of the herein reported decomposition products towards fluorinated compounds like e.g.: Mylar. Therefore fluorinated compounds should be avoided when using ethereal solvents in combination with metallic sodium. Figure S7: Disassembled Swagelok cell setup after stripping/plating tests using a DME based electrolyte with a sodium working and counter electrode as well as a Mylar insulator sheet. S7

8 Pressure analysis of sodium-sei formation in DME based electrolytes: After imbedding metallic sodium into various DME based electrolytes, the pressure evolution during the first 24 h of SEI formation was observed. The initial sharp pressure rise was assigned to temperature equilibration from room temperature to incubation temperature of 25 C. However after stabilization, a continuous pressure rise was observed for the first 24 h. By further Differential electron mass spectroscopy (DEMS), the gas was confirmed to be H 2 and no significant amounts of other gases like e.g.: CO or CO 2 were found. The H 2 formation most like results from residual H 2 O reduction at the sodium anode. 2 This measurement further demonstrates that residual water amounts are consumed on the electrode. Figure S 8: Pressure analysis of a 1 cm 2 Na-anode imbedded in DME-based electrolyte using various sodium salts at 25 C. REFERENCES 1. Zhang, Y.; Ma, L.; Zhang, L.; Peng, Z., Identifying a Stable Counter/Reference Electrode for the Study of Aprotic Na O2 Batteries. J.Electrochem. Soc. 2016, 163, A A1274. S8