| XRD after 240 h moisture exposure | Hydration was identified by the appearance of FeSO₄·H₂O diffraction peaks; hydration onset occurred at ~35 % RH (0.82 kPa) at 20 °C, ~36 % RH (1.14 kPa) at 25 °C, and ~38 % RH (2.13 kPa) at 35 °C |
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| Gravimetry at the deliquescence boundary | At ~95 % RH, rapid mass uptake and formation of a liquid phase were observed; the boundary remained effectively constant between 20 and 35 °C |
| Gravimetry and time-resolved XRD, 30 % RH, 25 °C | Minimal mass change (~0.44 wt.%) and no detectable phase evolution over 240 h; no new diffraction peaks and unchanged phase composition within experimental uncertainty |
| Gravimetry and time-resolved XRD, 45 % RH, 25 °C | 0–6 h: mass gain ~0.79 wt.% with no detectable phase change; from 12 h: diffraction peaks related to Na₂Fe(SO₄)₂·2H₂O and FeSO₄·H₂O emerged, with a minor increase in Na₆Fe(SO₄)₄; at 144 h: mass gain plateaued at ~7.6 wt.% and the alluaudite phase fraction reached ~37.94 % through 240 h |
| Time-resolved XRD, 45 % RH, 25 °C | Hydrated phases emerged as sharp crystallographic transitions at 16.5–19° and 25.6–27.8° 2θ |
| Gravimetry and time-resolved XRD, 60 % RH, 25 °C | ~3.7 % of the parent phase remained after 96–240 h; Na₆Fe(SO₄)₄ initially increased and disappeared by 48 h; Na₂Fe(SO₄)₂·2H₂O, Na₂Fe(SO₄)₂·4H₂O, FeSO₄·H₂O, and FeSO₄·7H₂O evolved sequentially, with higher hydrates increasingly prominent at later stages |
| Gravimetry after 240 h at 25 °C | Mass gain reached ~0.44, 8.4, and 27.3 wt.% at 30, 45, and 60 % RH, respectively |
| Raman microscopy after 12 h at 25 °C | 30 % RH: spectra remained unchanged; 45 and 60 % RH: SO₄²⁻ vibrational modes split at 990 cm⁻¹ and 1034 cm⁻¹; ≥80 % RH: sulfate Raman bands broadened; ~95 % RH: features were consistent with aqueous Na–Fe–SO₄ species, including Na/Fe–SO₄ contact ion-pair formation at 1080–1200 cm⁻¹, [Fe(H₂O)₆]²⁺ coordination at 370–410 cm⁻¹, and the H₂O bending mode at 1600 cm⁻¹ |
| Macroscopic morphology after 12 h at 25 °C | 30 % RH: unchanged; 45 and 60 % RH: progressive colour change from grey to light brown and particle aggregation; ≥80 % RH: significant particle agglomeration and loss of powder flowability; ~95 % RH: rapid moisture uptake (~49 wt.%) produced a viscous slurry |
Tristram Jenkins, Koyo Tokunaga, Hengyi Teah, Masakazu Sugiyama, Yasunori Kikuchi, Atsuo Yamada. “Thermodynamic hydration boundaries guide storage and closed-loop recycling of sodium-ion battery cathodes.” ChemRxiv (). https://doi.org/10.26434/chemrxiv.15004936/v2
Values presented only in diffraction, microscopy and spectroscopic plots or images were not transcribed. Textual ICP-OES and UV–Vis–NIR spectroscopy results were not found in the main paper. The main paper refers to supporting information, but no separate supporting information file was found among the files available from the ChemRxiv record
| Quantitative XRD phase purity | 95 % ± 0.6 % |
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