NaClO₄ Ethylene Glycol–water Binary Solution as an Electrolyte for Aqueous Sodium Ion Batteries (Supporting Information)

<p>Aqueous Na-ion batteries are attracting attention as candidates for large-scale rechargeable batteries with a high safety level. We proposed a novel electrolyte composed of NaClO₄ dissolved in water, along with ethylene glycol, to serve as an innovative solution for aqueous Na-ion batteries. The potential window of NaClO₄ aqueous-based electrolyte expanded as the ethylene glycol concentration increased. Specifically, when incorporating ethylene glycol into a 7 m (= mol kg⁻¹) NaClO₄ aqueous electrolyte, we observed a favorable cyclability pattern in the context of the NaTi₂(PO₄)₃ anode, akin to that exhibited by a 17 m NaClO₄ aqueous electrolyte. Moreover, the irreversible capacity of NaTi₂(PO₄)₃ decreased as the ethylene glycol concentration increased. This effect was evident even at low rates such as 0.2 mA cm⁻². Notably, the NaTi₂(PO₄)₃ anode’s capacity, when utilizing a 7 m NaClO₄ solution with a higher fraction of ethylene glycol (<i>X</i>_E = 0.5), remained stable at 120 mAh g⁻¹ even after the completion of 15 cycles. On the other hand, the Na₂MnP₂O₇ cathode properties 7 m NaClO₄ aqueous electrolyte could not be improved by adding a small amount of ethylene glycol.</p>