Readme file for data relating to Na2Fe2OS2, a new earth abundant oxysulphide cathode material for Na-ion batteries

1.	Materials
Na (ACS reagent, 99.99 %, dry), Na2S (97.3 %, anhydrous) and S (99.998 %, trace metal basis) were purchased from Sigma Aldrich. FeS (99.98%, metals basis), Fe2O3 (99.9 %, metal basis) were obtained from Alfa Aesar. Na2O (98%) from abcr GmbH and methanol (99.9%, Extra Dry, AcroSeal™, ACROS Organics™) from Thermo Fisher. 

2.	Synthesis
Solid State synthesis. The synthesis procedure is similar to that of Na2Fe2OSe2 reported by He et al.[14]  Na (97 mg; 4.219 mmol), Fe2O3 (112mg; 0.701 mmol), FeS (247 mg; 2.809 mmol) and S (45 mg; 1.403 mmol) were weighed according to the stoichiometry 2:1/3:4/3:2/3 ratio, transferred into an alumina crucible and placed in a quartz tube before sealing under vacuum (10-4 mbar). The tube containing the sample was heated to 800 °C at a ramp rate of 5 °C·min-1, held at 500 °C for 20 hours, and cooled down at a ramp rate of 5 °C·min-1. The resulting powder was ground in an agate mortar for 15 min before transferring in an alumina crucible and re-annealing under vacuum at 600 °C for 20 hours using the same heating and cooling ramp rates. The resulting powder was then manually ground in order to obtain a fine powder.
Mechanosynthesis. In order to increase the purity of the sample as well as speed up the screening of synthetic conditions, ball-milling of the precursor powders was used to synthesize samples in the Na-Fe-O-S phase field. In an Argon filled glovebox, Na2O, FeS and Na2S were weighed according to the desired nominal composition in order to yield a total mass of powder of 3 g, and transferred into an 80 mL zirconia grinding bowl of the Planetary Micro Mill Pulverisette 7 (Fritsch). Ten 15 mm zirconia balls were added in the bowl which was sealed with an air-tight gassing lid before placing into the planetary mill. A grinding time of 20 min alternating with 10 min pause with the inverse mode off and a repeat of 18 cycles (6 hours total grinding time) was then used for the mechanosynthesis. 
Removal of excess Na2S. The powders containing excess Na2S were washed using methanol (99.9%, Extra Dry, AcroSeal™, ACROS Organics). The whole procedure was carried out in an Argon filled glovebox. Typically, 500 mg of the NFOS-M powder was dispersed into 10 mL of methanol using a 15 mL centrifuge tube. After 15 min  , the mixture was centrifuged at 6000 rpm for 10 min using an EBA 200 Hettish centrifuge. The supernatant was collected and 10 mL of fresh methanol were added and dispersed with the residual powder. The washing procedure was repeated 6 times after which the resulting powder was dried under vacuum overnight. 398(2) mg of the powder is recovered, corresponding to 78(1) % of the initial mass of powder used. 

3.	Stability test
To test the stability of Na2Fe2OS2 under different atmospheres, ~200 mg of the sample prepared by mecanosynthesis before methanol washing (NFOS-M) was maintained under ambient atmosphere for 20 hours on the bench, before performing an XRD scan of the powder. In parallel, ~200 mg of the same sample was loaded in a glass tube which was then connected to a Swagelok® tube fitting and closed with valves in an Argon filled glovebox. Outside the glovebox, a compressed air supply was linked to a molecular sieve desiccant the output of which was connected to the tube containing the sample. A flow of dry air was thus maintained inside the glass tube for 20 hours. Another experiment consisted of synthesizing the NFOS-M sample under a dry air atmosphere, following the procedure described above. Prior loading in the planetary mill, the bowl was filled with dry air by flowing synthetic air through the gassing valves of the gassing lid for ~5 min. 

4.	Elemental analysis
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Compositions were determined using a PerkinElmer Optima 2000 instrument. Solutions were prepared by digesting approximately 10 mg of sample in 10 mL of an aqua regia solution (HNO3 20%, HCl 30%) which was diluted further in deionized water to a volume of 100 mL. The methanol filtrate was diluted 10 times in deionized water. The measurements were repeated three times for each sample. 
CHNS analysis. Sulphur composition was determined using an Elementar Vario MicroCube instrument. Samples were prepared by loading, in an Argon filled glovebox, approximately 1 mg of sample into a tin vial that was then sealed with a press in order to avoid contamination from the air. The measurements were repeated three times for each sample and averaged.
ICP-OES: data are in a 7 columns format (ElementLabel, Wavelength(nm), Concentration (mg/L), %RSD, UnadjustedConc, Intensity, %RSD)
CHNS: data are in a 8 columns format (%C (NFOS-M), %H(NFOS-M), %N(NFOS-M), %S(NFOS-M), %C(NFOS-MW), %H(NFOS-MW), %N(NFOS-MW), %S(NFOS-MW)


5.	Electron microscopy
Wavelength Dispersive X-ray spectroscopy (WDX) was performed using a Tescan S8000 scanning electron microscope (SEM) equipped with a WDX detector from Oxford Instruments. The detector was calibrated with appropriate standards for each chemical element. Data acquisition and analysis was performed using INCA software.  Powder sample was sprinkled on a carbon tape and coated with a thin layer of carbon with a Quorum sputter coater in order to avoid charging.
Transmission Electron Microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) mapping was performed with a Jeol 2100+ equipped with an EDX detector from Oxford instruments. Powder sample was ball milled in methanol under inert atmosphere to reduce particles size. Few drops of the methanol suspension were deposited on a holey carbon film Ni TEM grid. The grid was loaded into a beryllium TEM holder and inserted in the JEOL 2100+. In order to avoid exposing the material to air the sample preparation and loading was perform under inert atmosphere.
Data for SEM-WDX are in a 5 columns format (Fe%mol, S%mol, Na%mol, O%mol, Zr%mol)

6.	Infrared spectroscopy
Infrared spectroscopy measurements were done in ATR mode using a Nicollet iS50 FTIR spectrometer from ThermoFisher Scientific inside a glovebox.
Data are in a 12 columns format (Wavelength_NFOS-MW,	Intensity_NFOS-MW, Wavelength_NFOS-SS,	Intensity_NFOS-SS,	Wavelength_Na3Fe2S4,	Intensity_Na3Fe2S4,	Wavelength_NFOS made in air,	Intensity_NFOS made in air	Wavelength_NFOS left in dry air,	Intensity_NFOS left in dry air,	Wavelength_NFOS left in humid air,	Intensity_NFOS left in humid air)

7.	Diffraction
Routine analysis of phase purity and lattice parameters were performed on a Bruker D8 Advance diffractometer with a monochromated Cu source (Ka1, ? = 1.54418 Å) in powder transmission Debye Scherrer geometry (capillary) with sample rotation. Synchrotron X-ray diffraction (SXRD) was performed on pristine and ex situ samples at the I11 beamline at Diamond Light Source (Oxfordshire, UK), with an incident wavelength of 0.826540(1) Å using a wide-angle position sensitive detector, and samples sealed in Ø = 0.3 mm glass capillaries to prevent air exposure. Time-of-flight (ToF) neutron powder diffraction (ND) data was collected for the pristine material at room temperature using the Polaris instrument at ISIS neutron source (Oxfordshire, UK). Samples were loaded in Ø = 6 mm vanadium cylindrical cans and sealed in an Argon-filled glovebox. The structural models were refined by the Rietveld method[74,75] as implemented in the Fullprof suite.[76] In situ XRD was performed using an electrochemical cell equipped with a Be window (250 µm thick) and an Al current collector (8 µm thick) on a Rigaku SmartLab diffractometer with a 9 kW rotating anode providing a parallel beam of Mo Ka1 radiation (?Ka1 = 0.709032 Å).
The program FullProf was used to obtain information about the microstructure, following the method described by Rodriguez-Carjaval et al. This model uses the Scherrer formula, which considers that the size broadening can be written as a linear combination of spherical harmonics. Peak shapes were modelled using the spherical harmonics expansions 15  in a tetragonal material with Laue class 4/m. The microstructural information given in the output files were then treated using GFOURIER program16  to visualize the particle shapes.
The amount of amorphous content was determined by using the quantitative phase analysis method described in the literature . This method consists of mixing a known amount of the Na2Fe2OS2 material with an internal standard of similar absorption coefficient, Na3Fe2S4, and to compare the relative weight percent calculated by the program with the relative weight percent determined experimentally, which includes the contribution of amorphous phases. This method is described in the ESI, part 2.3.4. 
lab XRD for stability test: Data are in a 5 columns format (2theta, Iobs_NFOS-M, Iobs_humid air, Iobs_dry air, Iobs_synthesized dry air)
ex situ SXDR and Neutron: Data are in a 4 columns format (Q-Angstrom-1, Iobs, Q-Angstrom-1, Icalc)
in situ XRD: Data are in a 93 columns formats (2theta, #run)

8.	Electrochemical characterization
Electrochemical characterization was performed in 3-electrode Swagelok cells and 2025-type coin cells. The positive electrode consisted of a laminated mixture of active material (Na2Fe2OS2), conductive carbon (C65 from Timcal) and binder (polytetrafluoroethylene, PTFE dried from a 60% aqueous suspension from Sigma-Aldrich) in proportions 85:10:5 by weight for measurement in Na cells. This ratio was changed to 55:30:15 for electrodes used in full cells in order to balance cathode/anode capacities. For ex situ characterization by XPS/XRD, the active material was simply mixed with 10 wt% C65 conductive carbon and used as a powder. Active material loadings were typically between 5 and 10 mg. Metallic Na was used as an anode for half cells and hard carbon (HC) electrodes for full cells. Hard carbon electrodes were prepared by ball-milling hard carbon with C65 conductive carbon for 20 min, then dispersing the powder in a 1 % aqueous solution of carboxymethylcellulose (CMC) to obtain a 81:9:10 ratio of HC:C65:CMC, and casted over an Al foil. Electrodes were punched after solvent evaporation and further dried under vacuum at 100 °C for 12 h before use. 1 M NaPF6 in EC:DMC (1:1) with or without 2 wt% fluoroethylenecarbonate (FEC) additive was used for the electrolyte. The positive and negative electrodes were separated by Whatman GF/D borosilicate glass fiber membranes soaked with the electrolyte. All parts were assembled in an Ar-filled glovebox. Galvanostatic cycling was performed at a C/10 rate (defined as 1 Na+ exchanged in 10 h, considering the chemical formula Na2Fe2OS2) between 1.5 and 3 V versus Na+/Na. After cycling, samples for ex situ characterization were recovered inside the glovebox, washed three times in anhydrous DMC and dried under vacuum. 
Voltage curve: data are in a 3 columns format (x	Ewe	Ece)
differential capacity curve: data are in a 2 columns format (Ewe	dQ/dE
Capacity retention and Coulombic efficiency: data are in a 7 columns format (Cycle	Discharge capacity_FEC	Charge capacity_FEC	Coulombic efficiency_FEC	Discharge capacity_noFEC	Charge capacity_noFEC	Coulombic efficiency_noFEC)
In  situ XRD: data are in a 3 columns format (Time(h)	x	Ewe)	

9.	X-ray Absorption Spectroscopy
Self-standing electrodes, with an optimized density for X-ray absorption measurements at the Fe K-edge, were prepared and cycled at different charge/discharge states, washed in DMC, and sealed in a pouch bag to prevent air exposure of the samples. X-ray absorption spectra were measured at the B18 beamline at Diamond Light Source (Oxfordshire, UK) in transmission mode. The data were calibrated by fixing at 7112 eV the maximum of the derivative of an Fe metal foil reference placed after the samples, and normalized with the Athena software.[79] Fitting of the EXAFS data was done using the Artemis software of the same suite, starting from the crystallographic model obtained from diffraction experiment to build the initial paths used for fitting. The data were fitted in R space using k, k2 and k3-weighed data together, with 2.5 - 12.5 Å-1 k-range/1 – 6 Å R-range for the pristine material, and 3 – 12.6 Å-1 k-range/1 – 3 Å R-range for the cycled samples. E0 and S02 were first fixed at values obtained from fitting reference materials (FeO, Fe2O3, FeS, FeS2, NaFeO2, Na3Fe2S4) and let to refine only at the end of the procedure when a satisfactory fit was obtained.
raw data are in a 8 columns formats (#qexafs_energy	time	I0	It	Iref	lnI0It	lnItIref	QexafsFFI0)
Figure 6a: data are in a 5 columns format (r	Pristine	2p05V	3V	1p5V)
Figure 6b: data are in a 9 columns format (r	Pristine_data	Pristine_fit	O1	S1	S2	Fe1	Na1)

10.	X-ray Photoemission Spectroscopy
Measurements were performed on a Thermo Fisher Scientific NEXSA spectrometer using a micro-focused monochromatic Al Ka source, with an X-ray source power of 150 W. Low energy electrons and Ar ions were used for charge neutralization. Survey spectra were run at a pass energy of 200 eV and high-resolution spectra at a pass energy of 40 eV. All samples for XPS analysis were prepared in a glovebox and loaded in the spectrometer using an air-tight sample transfer chamber to avoid any exposure to air. 
Data are in a 3 columns format (Kinetic Energy	Binding energy	Intensity)