Relating Synthesis Conditions and Electrochemical Performance for the Sodium Intercalation Compound Na4Mn9O18 in Aqueous Electrolyte

The sodium intercalation compound Na 4 Mn 9 O 18 , more commonly Na 0.44 MnO 2 , was studied as a potential positive electrode in an aqueous electrolyte hybrid energy storage device. Varying ratios of precursors were used in a solid-state synthetic route in an effort to compensate for volatile loss of sodium during processing. The powders were characterized using X-ray powder diffraction and thermogravimetric analysis, while particle morphology and formation were studied by scanning electron microscopy/electron dispersive spectroscopy and transmission electron microscopy. Electrochemical behavior was characterized by galvanostatic cycling and cyclic voltammetry. With a positive electrode voltage window of -0.3 to 0.3 V vs a Hg/HgSO 4 reference electrode, a specific capacity of 35 mAh/g was observed after 20 cycles at a C/1.4 rate (25 mA/g) with little capacity loss. The most stable of the materials were made with a Na:Mn precursor ratio equal to 0.55 and showed excellent performance through many charge/ discharge cycles. These samples also contained varying amounts of β-Na 0.70 MnO 2 and α-Mn 2 O 3 impurity phases. The results indicated a relationship between the precursor Na/Mn ratio and the resultant redox potentials associated with the multiple hybrid Mn oxidation states encountered during cycling although no significant variance in the crystallography of the Na 0.44 MnO 2 phase was observed.