A mixed aqueous/aprotic sodium/air cell using a NASICON ceramic separator

An aqueous (mixed aqueous/aprotic) Na-air cell is fabricated and its discharge properties are examined experimentally. This design is attractive because Na is not only more abundant and cheaper than Li, but also potentially advantageous for enhancing cell output. The theoretical energy density of the aqueous Na-air cell exceeds that of a Li one under conditions where precipitation of hydroxides in the aqueous electrolyte is not allowed. Na ₃Zr₂Si₂PO₁₂ (NASICON) ceramics, which has a Na⁺ ion conductivity of 2 × 10^-3 S · cm^-1 at 50°C, is used as a solid electrolyte separator in the Na-air cell. Involvement of O₂ gas in the cell reaction is verified by changing cell voltage in the presence or absence of O₂ flow through the aqueous electrolyte. The Na-air cell exhibits an electrochemical discharge of ∼600 mAh· g^-1 and energy density of ∼1500 Wh· kg^-1 based on the weights of reacted Na and H₂O. Its maximum areal output power is 11 mW· cm ^-2 and depends on the thickness of the NASICON ceramic separator, the concentration of NaOH in the aqueous electrolyte and the type of cathode, suggesting that cell performance can be improved by optimization of these components.