SUPPLEMENTARY INFORMATION

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1 In the format provided by the authors and unedited. SUPPLEMENTARY INFORMATION DOI: /NCHEM.2923 Oxygen redox chemistry without excess alkali-metal ions in Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 Urmimala Maitra a, Robert A. House a, James W. Somerville a, Nuria Tapia-Ruiz a, Juan Lozano a, Niccoló Guerrini a, Rong Hao a, Kun Luo a, Liyu Jin a, Miguel A. Pérez-Osorio a, Felix Massel c, David M. Pickup d, Silvia Ramos d, Xingye Lu e, Daniel E. McNally e, Alan V. Chadwick d, Feliciano Giustino a, Thorsten Schmitt e, Laurent C. Duda c, Matthew R. Roberts a, Peter G. Bruce ab * joint first author a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. b Department of Chemistry, University of Oxford, Parks Road, Oxford OX1 3PH, UK. c Department of Physics and Astronomy, Division of Molecular and Condensed Matter Physics, Uppsala University, Box 516, S Uppsala, Sweden. d School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK. e Swiss Light Source, PSI, 5232 Villigen, Switzerland. Table of contents Methods Section Supplementary Figure Supplementary Table Supplementary Table Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Figure Supplementary Table References NATURE CHEMISTRY Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

2 Methods Section Powder X-ray diffraction (PXRD) patterns were recorded on a 9 KW Rigaku Smartlab diffractometer using Cu Kα1 radiation (λ = Å). The PXRD data were analysed using the Rietveld refinement method as implemented in the GSAS software suite with the EXPGUI software interface. Samples were mounted in customized holders with a Kapton window to avoid any exposure to the atmosphere and measurements were carried out in reflection mode. In-situ XRD was carried out in an in-situ cell with an X-ray transparent beryllium window. A very thin Al film was placed between the Be window and the cathode to prevent Be oxidation at high potentials (around 4.5 V vs Na + /Na). (Further information about the cell design can be obtained at The cell was controlled by a Biologic MPG potentiostat. Electrochemistry Electrodes were prepared by mixing 80 wt% active material, 10 wt% Super P carbon and 10 wt% polytetrafluoroethylene (PTFE) binder (in a mortar pestle and rolling out thin free standing films of the mixture. For testing the effect of adding Na 2 CO 3 into the electrodes additional electrodes were prepared containing 70 wt% active material, 10 wt% Super P carbon and 10 wt% polytetrafluoroethylene (PTFE) binder and 10 wt% Na 2 CO 3. Electrochemical testing was carried out in coin cells with a Na metal-disk as the anode and a 1M NaPF6 (Alfa Aesar, 99.0%) in battery grade propylene carbonate (PC) electrolyte (BASF Selectilyte). The PC was distilled using a packed bed column and dried for several days over freshly activated molecular sieves (type 4 Å) prior to making up the electrolyte. NaPF 6 (Alfa) was dried at 60 C under vacuum before preparing the electrolyte solution. Galvanostatic charge-discharge was carried out in CR2032 coin cells using a Maccor Series 4000 at a rate of 10 mag -1. Operando differential electrochemical mass spectrometry (DEMS) analysis was carried out to study the different gases generated during cell cycling. The set up consisted of a quadrupole mass spectrometer (Thermo Fischer) equipped with turbomolecular pump (Pfeiffer Vacuum) and mass-flow controllers (Bronkhorst). Two electrode type cells (ECC-Std from EL-CELL) with gas inlet and outlet ports were used for the operando measurements. The cell consisted of Na anode, 1M NaPF6 in PC electrolyte and the same cathode as described above. More details of the DEMS set-up are given in a previous publication. 1 Mn K-edge X-ray absorption near edge structure (XANES) measurements were undertaken at beamline B18 at the Diamond Light Source, Harwell, UK. The beamline is equipped with a doublecrystal monochromator (with two crystals Si(111) and Si(311)) and works in the range kev. The XANES spectra were collected in transmission mode and the intensities of both the incident and transmitted X-ray beams were measured using gas-filled ionisation chambers. To correct for any drift in monochromator, Mn metal foil was placed in front of a third ionisation chamber. For each sample three scans were taken, summed, calibrated, background subtracted, and normalised using the program Athena. Mn 2 O 3 and MnO 2 were used as references for Mn 3+ and Mn 4+, respectively. Soft XAS and RIXS The O K-edge soft x-ray absorption spectra (SXAS) and resonant inelastic x-ray scattering (RIXS) spectra were recorded at the ADRESS beamline of the Swiss Light Source, PSI, using the SAXES spectrometer. 2,3 To obtain SXAS spectra at the O K-edge, we simultaneously recorded the total fluorescence yield (TFY) signal using an x-ray sensitive photodiode and the total electron yield (TEY) signal by measuring the sample drain current. TFY data was used for analysis due to the lower signal to noise ratio of the TEY data, possibly because of conductivity issues. For recording the O-K SXAS as well as the RIXS spectra the monochromator band width was set to about 40 mev. The total resolution for the RIXS spectra was about 55 mev. 2

3 Raman Spectroscopy Raman spectra of the materials at different states of charge were collected using a Raman Renishaw InVia spectrometer equipped with a diode laser (λ = 785 nm) and a laser power of 1.5 mw. For these measurements all samples were sealed between two glass slides under argon atmosphere. Scanning Transmission Electron Microscopy: ADF-STEM and ABF-STEM data were collected on an aberration corrected JEOL ARM 200F operated at 200 kv. The convergence semi-angle used was 22mrad, and the collection semi-angles were mrad (ABF) and mrad (ADF). In all cases, sets of fast-acquisition multi-frame images were recorded and subsequently corrected for drift and scan distortions using SmartAlign O NMR method: Solution-state 17 O NMR experiments were performed in a Bruker Diff50 probe coupled with a 17O insert on a 400 MHz Bruker Avance III spectrometer at the 17 O Larmor frequency of 54.3 MHz. The spectra were recorded with zg30 pulse sequence; the applied π/2 pulse length was 23 μs. All samples were loaded in 5mm NMR tubes sealed with air-tight caps in an Ar-filled glovebox and the volumes of the liquid samples were kept the same (700 μl). The spectra were externally referenced with water at 0.0 ppm. The electrolyte of a charged battery was collected by soaking its separator in the pristine electrolyte for 24 hrs inside an Ar-filled glovebox. TGA-MS: Thermogravimetric Analysis was carried out on powder samples of ~30-40 mg quantities under inert Ar atmosphere using a NETZSCH Jupiter STA 449 F3 TGA. This was coupled with a NETZSCH Aëolos QMS 403 D mass spectrometer to provide in operando mass spec. SQUID: FC magnetization measurements of the Ar-treated powder was carried out in Quantum Design Inc. SQUID-VSM. 3

4 Figure 1. PXRD pattern of the pristine Na 0.67 Mg 0.28 Mn 0.72 O 2 material refined in space group P63/mcm. (Refined parameters are shown in table S1). The pink tick marks indicate the allowed reflections. The 2 theta ranges (highlighted in light grey) have been excluded from the refinement as peaks in these regions arose from the air sensitive sample holder. The black curve is the experimental diffraction pattern, the red curve is the calculated diffraction pattern as obtained from Rietveld refinements, the green curve is the background subtracted and the blue curve is the difference plot. 4

5 Table 1. Refined parameters of the Na 0.67 Mg 0.28 Mn 0.72 O 2 pristine material. Atoms Wycoff positions X Y Z Occupancy U iso Mg1/Mn1 2b (5)/ 0.186(2) 0.015(2)/0.018(4) Mg2/Mn2 4d 1/3 2/ (10)/0.989(5) 0.016(5)/0.016(2) O 12k (4) Na1 6g ¼ 0.397(4) 0.028(8) Na2 4c 1/3 2/3 ¼ 0.403(4) 0.032(8) Space groups P63/mcm a = b = (3), c = (4), χ 2 = 3.3. Stoichiometry from refinement Na 0.67 Mg 0.28 Mn 0.72 O 2 5

6 Table 2. Stoichiometry at various points of charge and discharge as determined from ICP analysis Composition from ICP Na (±0.027) Mg (±0.01) Mn (±0.026) Pristine V Dis 2V

7 Figure 2. Temperature dependence of magnetization under FC condition. The effective magnetic moment was calculated from equation χ " = $ * * %& ' $ ()) $ + = C= 3.479,- +. BM, where C is the Curie constant obtained from fitting 1/χ vs T. For fitting, χ is converted into the cgs units (emu/mol.oe). The effective spin only magnetic moment for composition Na Mg 0.28 Mn(III) 0.11 Mn(IV) 0.61 O 2 is calculated to be μ 566 = ; ; = 3.434BM. However, considering the 0.14 moles of Na extracted in the region 1 being contributed by Mn(III)/Mn(IV) redox (consistent with electrochemistry), μ 566 = ; ; = BM. 7

8 Temperature ( o C) Ion Current (A) E E E E Oxygen m/z=32 Carbon Dioxide m/z= Time (mins) Mass Change (%) Figure 3. TGA-MS of Na 0.67 Mg 0.28 Mn 0.72 O 2 from which Na 2 CO 3 has been remove using a heating step in Argon. After heat treatment, no mass loss is observed in this cleaned sample below 750 C at which point a small amount of oxygen is lost from the material. The TGA experiment was performed on a ~40 mg sample contained in an alumina crucible which was ramped at 10 C min -1 from room temperature to 800 C in Argon. This result confirms that these materials are almost free from Na 2 CO 3 impurities. 8

9 Figure 4. SEM images of the Na 0.67 Mg 0.28 Mn 0.72 O 2 pristine material. The images were taken at an operating voltage of 5keV (Zeiss Gemini SEM-500) 9

10 Capacity (mah/g) Voltage (V) Charge Discharge Avg Discharge voltage Cycle no. Figure 5. Capacity (top) and average discharge voltage (bottom) plotted as a function of cycle number for Na 0.67 Mg 0.28 Mn 0.72 O 2 over 50 cycles in the voltage range 2-4.5V at a rate of 10 mag -1. Not that the capacity fading here, ~1 mah g -1 per cycle, is significantly lower than reported in ref 19 main manuscript (Yabucchi et.al.) where the voltage range for cycling was larger, from 1.5 V to 4.4 V 10

11 Figure 6. PXRD patterns recorded in situ using a cell constructed with a Na 0.67 Mg 0.28 Mn 0.72 O 2 cathode during the first cycle. The pattern shown after charging to 100 mahg -1 shows a peak for the O2 structure and this increases in intensity by the end of charge. On discharge the peaks of the original P2 structure are regained fully. The sample discharged to 2 V (orange) shows a general broadening of all the peaks along with a reduction of the c parameter and expansion of the a/b parameter, compared with the sample discharged to 2.3 V (yellow). The lattice parameter changes are consistent with insertion of excess Na (beyond Na 0.67 ). Peaks labelled with arrows represent reflections from the in situ cell components. 11

12 Figure 7: ADF-STEM micrograph after 1 cycle, where a high density of stacking faults along the [1-10] -direction can be observed. The stacking faults result in streaking of reflections in the fast Fourier transform of the image in the inset. 12

13 Flux CO 2 (x10-9 mol min -1 ) Voltage (V) Time (mins) m/z = 35 ( 16 O 18 O) m/z = 36 ( 18 O 18 O) m/z = 44 (C 16 O 16 O) m/z = 46 (C 16 O 18 O) m/z = 48 (C 18 O 18 O) Figure 8. Operando mass spectrometry data collected during the first cycle of Na 2/3 [Mg 0.28 Mn 0.72 ]O 16 2-xO 18 x where x is approximately 1 as determined by TGA-MS. An identical trace of CO 2 is observed as reported in Fig 3. No evidence of any O 2 or CO 2 containing O 18 was observed as reported 13

14 Figure O NMR spectra of pristine electrolyte containing 1M NaPF 6 in propylene carbonate (blue spectrum) and an electrolyte which had been extracted from a battery (with ~50 at. % 17 O enriched cathode) red spectrum after charging to 4.5 V vs. Na/Na +. The two spectra are identical Therefore, oxygen has not been released from the material and contained within the electrolyte in any form. 14

15 Figure 10. TGA-MS data on as (a) prepared and (b) charged electrodes. The O 18 labelled O 2 and CO 2 traces show no evidence of new decomposition products containing 18 O. 15

16 Figure 11. Operando mass spectrometry data collected during the first charge for as-prepared Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 i.e. with no procedure for removal of Na 2 CO 3 applied (red curve), material with 10 wt. % of Na 2 CO 3 intentionally added (blue curve) and Na 2/3 [Mg 0.28 Mn 0.72 ]O 2 after heat treatment under Ar (black curve). Quantities given are in units of moles of CO 2 per mole of active material (AM) 16

17 Figure 12. Evolution of the XANES Mn K-edge (1s-4p). Inset shows the pre-edge (1s-3d). Figure in bottom right panel shows the variation of Mn oxidation state, calculated from the position of the centroid of the pre-edge, with Na content on charge then discharge. There is no significant change in the peak positions of the pre-edge on charging across the plateau and until around 2.3 V on discharge implying almost no change in Mn oxidation state through these regions. Lines shown on the bottom right hand graph indicate the oxidation states expected for the material at various states of charge when considering Mn oxidation and reduction as the only mechanism of charge compensation in this material. 17

18 Figure 13. Raman spectra of samples with various sodium contents prepared through charging the pristine Na 0.67 Mg 0.28 Mn 0.72 O 2 material. Samples a-g represent various states of charge as represented in Fig 4(a). Raman spectra of Li 2 O 2, ZnO 2, Na 2 O 2 and the as prepared material are also shown for comparison. Absence of O-O vibrations indicate no true (1.4 Å O-O bond length) peroxide species are formed. Note that the standards were chosen (alkali and transition metal) to span the environments expected around any true peroxo species in Na 0.67 Mg 0.28 Mn 0.72 O 2. The feature at around 650 cm -1 appears to evolve with charging. Peaks at similar frequencies have also been observed in other materials which exhibit the P2 to O2 phase transition on charging e.g. P2-type Na 2/3 Mn 1/2 Fe 1/2 O 2. 5 As a result of changes in the Raman spectrum associated with the P2/O2 transition and with the distortions of the O coordination environment around Mn, it is unfortunately not possible to extract unambiguous information on the formation of O 2 n- peroxo-type species by comparing the charged and pristine Raman spectra. 18

19 Figure 14. One electron energy level diagram representing M-O more covalent vs less covalent bonding interactions. More ionic interactions like Mn/Mg-O places the O2p states at relatively high energies, which are then accessible within the stability window of the electrolyte. More covalent M- O interaction pushes the O 2p states down in energy well above the voltage window of electrolyte oxidation. 19

20 Figure 15. Spin resolved total and partial density of states for Na 2/3 [Mg 1/3 Mn 2/3 ]O 2. The total density of states is represented by the black area while red and blue areas correspond to O 2p and Mn 3d states, respectively. The Fermi energy is set to 0 ev, and is arbitrarily placed in the middle of the calculated band gap, as indicated by the dashed black line. Within the valence band, the O 2p states are dominant near the top of the valence band, while Mn 3d states are located primarily a few ev's below the top of the valence band. This is consistent with O oxidation occurring prior to Mn 4+/5+ oxidation. Table 3. Calculated parameters of the supercell Na 16 Mg 8 Mn 16 O 48 (2x2x1 supercell) Calculated from DFT Deviation from experiment a b c α β γ Å Å Å % 2.19% 0.47% 0.00% 0.00% 0.01% 20

21 References 1. Luo, K. et al. Charge-compensation in 3d-transition-metal-oxide intercalation cathodes through the generation of localized electron holes on oxygen. Nat. Chem. 8, (2016). 2. Strocov, V. N. et al. High-resolution soft X-ray beamline ADRESS at the Swiss Light Source for resonant inelastic X-ray scattering and angle-resolved photoelectron spectroscopies. J. Synchrotron Radiat. 17, (2010). 3. Schmitt, T. et al. High-resolution resonant inelastic X-ray scattering with soft X-rays at the ADRESS beamline of the Swiss light source: Instrumental developments and scientific highlights. J. Electron Spectros. Relat. Phenomena 188, (2013). 4. Jones, L. et al. Smart Align a new tool for robust non-rigid registration of scanning microscope data. Advanced Structural and Chemical Imaging, 8, 1-16 (2015). 5. Mortemard De Boisse, B., Carlier, D., Guignard, M., Bourgeois, L. & Delmas, C. P2- NaxMn1/2Fe1/2O2 phase used as positive electrode in Na batteries: Structural changes induced by the electrochemical (De)intercalation process. Inorg. Chem. 53, (2014). 21