Electrochemical Sodium Ion Intercalation Properties of Na2.7Ru4O9in Nonaqueous and Aqueous Electrolytes

This paper represents the first description of the electrochemical sodium ion intercalation properties of Na 2.7 Ru 4 O 9 in both nonaqueous and aqueous electrolytes. Na 2.7 Ru 4 O 9 was synthesized at 700–850°C by solid-state reaction under Ar flow. It has single, double and triple chains of edge-sharing RuO 6 octahedra and one-dimensional tunnels between them parallel to the b -axis, accommodating three different sites of Na1, Na2 and Na3. In an organic electrolyte (1 M NaClO 4 in PC), Na 2.7 Ru 4 O 9 shows three redox peaks in its cyclic voltammogram between 2.8 and 3.9 V vs. Na. In an aqueous (1 M Na 2 SO 4 ) electrolyte, Na 2.7 Ru 4 O 9 exhibits three anodic peaks during the first charge, similar to the behavior observed in the organic electrolyte. However, the three peaks become a single broad peak over the following cycles. Electrochemical data and ex-situ XRD results indicate that once Na ions are removed from the Na 2.7 Ru 4 O 9 structure containing the maximum amount of sodium, only a portion (<60%) can be re-intercalated and that the aqueous electrolyte allows a lower reversible capacity than the nonaqueous electrolyte.