Aqueous sodium-ion batteries have been proposed as an attractive alternative to large-scale energy storage in terms of safety and economic efficiency. We experimentally confirmed that the potential width of the practical electrochemical window of a highly concentrated aqueous NaClO4 electrolyte had 2.8 V by using the method of cyclic voltammetry. This positive effect allowed the Na2MnFe(CN)6 cathode to operate reversibly at unusually higher potentials in highly concentrated aqueous electrolyte without any side reactions such as undesirable oxidation of water.
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