The sodium intercalation compound Na4Mn9O18, more commonly Na0.44MnO2, 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/HgSO4 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 beta-Na0.70MnO2 and alpha-Mn2O3 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 Na0.44MnO2 phase was observed. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428667] All rights reserved.