Na4Mn9O18 as a positive electrode material for an aqueous electrolyte sodium-ion energy storage device
Introduction
Several sodium-ion based energy storage devices that work at room temperature have been reported. For example, a class of organic solvent based Na-ion batteries have been suggested, though these systems appear to have lower specific energies and rate capabilities than Li-ion batteries while still needing costly electrolytes, thin electrode structures, and ultra-dry fabrication conditions [1], [2], [3], [4]. An aqueous electrolyte sodium-ion analog could in principle be made for a lower cost. This approach has been described by several groups, who have investigated the functionality of sodium birnessite or Na0.7MnO2 positive electrode material in an aqueous electrolyte using activated carbon as a negative electrode material [5], [6], [7].
In this work, we use this same style of hybrid device to examine a completely different manganese-based positive electrode: Na4Mn9O18 (also known informally as Na0.44MnO2). The ability of this material to be reversibly cycled in an aqueous electrolyte was first documented by our group in 2008 [8]. To our knowledge we are the first to study its potential as a positive electrode in an aqueous electrolyte based energy storage device, although the promise for using it as the basis of a sensor immersed in an aqueous media has been reported elsewhere [9].
Section snippets
Materials synthesis
To make Na4Mn9O18, Na2CO3 was ball milled with Mn2O3 (both from Alfa Aesar) in a 0.55:1 M ratio in a SiC crucible using a Spex 8000 mixer mill for 60–120 min. The significant molar excess of Na2CO3 was found to result in an acceptable degree of phase purity. The precursor mix was fired at 750–800 °C for 8–12 h with heating and cooling ramp rates of 5 °C/min. Fig. 1 shows X-ray diffraction data from the solid-state synthesized material with the predicted diffraction peaks included for comparison,
Results/discussion
Fig. 2(a) shows a cyclic voltammogram collected from a Na4Mn9O18 positive electrode at a scan rate of 5 mV/s between −0.4 and 0.25 V vs. Hg/Hg2SO4 in a three-electrode cell with activated carbon as a counter electrode. The data show two main redox couples at approximately −0.07 and 0.16 V vs. Hg/Hg2SO4, with at least 3 other couples of lesser magnitude. These data are essentially identical (in the potential range studied) to those reported by Sauvage et al. for Na4Mn9O18 in an organic solvent
Summary/conclusions
The orthorhombic Na4Mn9O18 sodium intercalation compound was found to be electrochemically stable in an aqueous 1 M Na2SO4 electrolyte, and has demonstrated electrochemical characteristics nominally similar to those found for the same material in non-aqueous electrolytes. The rate capability of the material system has been shown to be appealing, and energy storage devices were made based on this material as the positive electrode and activated carbon as the negative electrode. It was found that
Acknowledgements
The authors thank Carnegie Mellon University and Aquion Energy (formerly 44 Tech) for financial support.
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