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Combining Battery-like and Pseudocapacitive Charge Storage in 3D MnOx–Carbon Electrode Architectures for Zinc-Ion Cells

Tuesday, 3 October 2017: 09:20
Chesapeake 6 (Gaylord National Resort and Convention Center)
J. S. Ko (Naval Research Laboratory, National Research Council Postdoctoral Associate), M. B. Sassin, D. R. Rolison, and J. W. Long (U.S. Naval Research Laboratory)
The ability of manganese oxides to exhibit charge-storage behavior characteristic of either batteries or electrochemical capacitors (ECs) offers an intriguing opportunity to design electrode materials that innately deliver both high power and high capacity. We previously demonstrated that lamellar birnessite-type MnOx, when expressed in carbon nanofoam–based electrode architectures (MnOx@CNF), amplifies charge storage by pseudocapacitance in aqueous electrolytes (e.g., Na2SO4; Li2SO4). Herein, we extend the functionality of these established electrode materials by varying the ionic composition of the electrolyte in which they are cycled. For example, cycling MnOx/C nanofoam electrodes in aqueous 1 M ZnSO4 yields battery-like voltammetric features reminiscent of those previously reported for “rechargeable Zn-ion batteries.” When cycling in mixed Na2SO4:ZnSO4 electrolytes, we retain Zn2+ redox peaks, but now superimposed on a pseudocapacitive background arising from the presence of Na2SO4. The Zn2+ redox provides high capacity (up to 230 mAh g–1 normalized to MnOx) at low rates (<5 mV s–1), while pseudocapacitance mechanisms take over at high rates to mitigate the effects of slow kinetics for Zn2+ insertion. Fundamental electrochemical properties are established using a three-electrode half-cell configuration. We apply peak-current kinetic analyses (for CV data) and AC impedance to deconvolve the complex mechanisms that give rise to battery-like and pseudocapacitive charge-storage features. On the basis of three-electrode studies, we down-select electrolyte compositions for testing MnOx@CNF, paired against Zn metal, in two-electrode Swagelok cells that are subjected to galvanostatic cycling at challenging C-rates (1C–20C).