Aqueous Na-ion batteries with highly concentrated NaClO4 aq. electrolytes are drawing attention as candidates for large-scale rechargeable batteries with a high safety level. However, the detailed mechanism by which the potential window in 17 m NaClO4 aq. electrolyte was expanded remains unclear. Therefore, we investigated the local structure around a Na+ ion or a ClO4- ion using X-ray diffraction combined with empirical potential structure refinement (EPSR) modelling and Raman spectroscopy. The results showed that in 17 m NaClO4 aq. electrolyte, most of the water molecules were coordinated to Na+ ions and few free water molecules were present. The 17 m NaClO4 aq. electrolyte could be interpreted as widening the potential window because almost all water molecules participated in hydration of the Na+ ions.
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