Polarity Switching Effect on an Aqueous Symmetric Sodium-Ion Battery (Supporting Information)

The polarity reversal operation of a symmetric battery —a strategy uniquely enabled by identical electrodes— using NASICON-type Na2.5V1.5Ti0.5(PO4)3 electrodes in a highly concentrated aqueous sodium trifluoroacetate electrolyte was examined to gain insight into the inherent asymmetric properties that develop during unidirectional charge-discharge cycling. The asymmetrically grown electrode-electrolyte interfaces (EEIs) at both electrodes exerted persistent effects on subsequent reversely directed charge-discharge cycles following electrode switching, whereas the electrolyte pH split responded rapidly. A certain number of repeated charge-discharge cycles in the same direction was required to form a robust EEI tolerant to drastic pH changes. Polarity reversal operation combined with occasional electrode switching may promote quasi-symmetric EEI formation on both electrodes, which could be beneficial for improving cyclability and coulombic efficiency.