Flexible free-standing Na₄Mn₉O₁₈/reduced graphene oxide composite film as a cathode for sodium rechargeable hybrid aqueous battery

A novel free-standing Na₄Mn₉O₁₈/reduced graphene oxide (NMO-RGO) composite was successfully prepared by a facile vacuum filtration technique. Scanning and transmission electron microscopy studies revealed formation of a film with needle-like Na₄Mn₉O₁₈ homogeneously distributed and anchored on the RGO matrix. A sodium rechargeable hybrid aqueous battery with this novel cathode and zinc anode was first investigated and reported. The NMO-RGO film delivered a reversible discharge capacity of about 83 mAh g⁻¹ when cycled galvanostatically at a current density of 0.1 A g⁻¹, which was for about 78% higher than that of Na₄Mn₉O₁₈ prepared by a slurry casting technique. This significant enhancement of electrochemical properties of NMO-RGO was due to the presence of graphene sheets in this free-standing composite cathode. These sheets form an agile substrate with a high conductivity to guarantee a successive electronic conduction channels in the electrode. In the same time, such composition and structure enables accommodation of the volume changes of Na₄Mn₉O₁₈ during sodium insertion-extraction.