Effect of Fe3+ for Ru4+ substitution in disordered Na1.33Ru0.67O2 cathode for sodium-ion batteries: Structural and electrochemical characterizations

Abstract

Sodium ruthenium oxides (Na2RuO3 and Na1.33Ru0.67O2) belong to a special Na-rich type of cathode material which show high specific capacities and structural stability. Even though these Na-rich materials possess attractive electrochemical properties, owing to the low abundance of ruthenium (Ru) within the Earth’s crust, commercialization of sodium ruthenium oxide (NRO) cathodes is extremely difficult. To mitigate this disadvantage, we have substituted Ru in NRO using Fe, which is electrochemically active in many cathode materials for sodium-ion batteries (SIBs), Earth-abundant and cheap. Herein, a series of Fe-substituted (Fe% = 5 mol. %, 10 mol. %, 20 mol. %, and 30 mol. %) sodium ruthenium iron oxide (NRFOx) phases were synthesized using a conventional solid-state synthesis method. When used as a cathode material, among these novel Fe substituted phases, the composition with the lowest Fe mol. % displays the highest specific discharge capacities of 115 mAh g−1 and 61 mAh g−1, at 0.2C and 2C rate respectively, within a 2.0–3.7 V potential range. It was seen that the Fe substitution level significantly influences structural stability and discharge capacities. Cyclic voltammetry displays that Fe3+ is surprisingly electrochemically inert inside the NRO matrix and only plays a structural role.