Electrochemical sodium ion intercalation property of Na2.7Ru4O9 in nonaqueous and aqueous electrolytes

Abstract

Although lithium ion batteries (LIB) have become wide-spread in the past decades, there are still some limitations in terms of lithium source deficiency in the world and safety problems using flammable organic electrolytes. In order to overcome these drawbacks of LIB, researches on aqueous sodium ion batteries have received increasing attention because they may solve shortage of cation sources as well as safety issues. However, there are few reports on sodium ion insertion materials in aqueous media1-3 compared to investigations of intercalation materials using Na metal in nonaqueous electrolytes. In our previous work4, we verified the structural and electrochemical properties of Na0.44MnO2, one of sodium intercalation host materials. In this work, we propose a new sodium ion intercalation material, Na2.7Ru4O9, with a similar structure to, but different polyhedral networks from, Na0.44MnO2. We have examined the electrochemical sodium ion intercalation property of Na2.7Ru4O9 in both aqueous and nonaqueous solutions. Na2.7Ru4O9 powders were synthesized by solid state reaction at 750~850 oC under Ar flow. The Na2.7Ru4O9 structure has 1-dimensional tunnels along b-axis with single, double and triple chains of edge-sharing RuO6 octahedra, which can accommodate three different sites of Na1, Na2 and Na3 (Fig.1). The cyclic voltammogram in organic solvents with Na metal as a counter electrode represented a reversible sodium de/insertion reaction from the three oxidative peaks (sodium desertion) at 3.25, 3.7 and 3.8 V vs. Na/Na+, and the corresponding reduction peaks (sodium insertion) at 3.1, 3.5 and 3.6 V, respectively (Fig.2). The X-ray diffraction studies showed structural changes during electrochemical tests. The comparison study of electrochemical performance in aqueous versus nonaqueous system, and structural relationship between the tunnel structure and electrochemical sodium de/insertion property will be discussed.