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Low-Temperature Performance of Symmetric MnOx-Carbon Nanofoam-Based Ecs with Mild-pH Aqueous Electrolytes

Tuesday, 3 October 2017
Prince George's Exhibit Hall D/E (Gaylord National Resort and Convention Center)
M. B. Sassin (U.S. Naval Research Laboratory), J. Wallace (Formerly of Nova Research Inc.), J. Ko (NRC Postdoc at the U.S. Naval Research Laboratory), and J. W. Long (U.S. Naval Research Laboratory)
Electrochemical capacitors, ECs, should provide high performance over the broadest range of operating temperatures which is primarily determined by the properties of the electrolyte (freezing and boiling points, ionic conductivity and salt solubility as a function of temperature, flash point for flammable solvents). In the case of mild-pH aqueous electrolytes, where flammability is a non-issue, the sub-ambient temperature performance is the most critical factor. En route to demonstrating the performance of aqueous-electrolyte ECs at low temperature, we have characterized both the bulk thermal properties of single and multi-component electrolytes and their behavior when used in EC pouch cells. The ionic conductivities of bulk electrolytes with a fixed cation concentration of 5 M for single-component (e.g. Li2SO4, NaNO3) and multi-component electrolytes (e.g. Li2SO4 + NaNO3) ranged from 63 to 136 mS cm-1 at 20oC. The freezing points of these electrolytes were measured with differential scanning calorimetry. It was found that the anion dominates the freezing-point behavior, with sulfate-based electrolytes supporting freezing points down to –45oC and nitrate-based electrolytes down to –35oC. To assess the impact of temperature on performance as a function of electrolyte composition, we fabricated symmetric pouch cells comprising MnOx‑carbon nanofoam electrodes and tested them over the temperature range of 25oC to –45oC in an environmental chamber. All ECs exhibited a gradual decrease in capacitance as the temperature decreased; however, upon re-warming to 25°C, the capacitance was recovered, indicating that freezing does not cause significant degradation to electrode or separator components. Aqueous electrolytes comprising mixtures of Li2SO4 and NaNO3 yielded the best combination of depressed freezing point and ionic conductivity among the compositions evaluated in EC pouch cells.