Deciphering charge-storage mechanisms in 3D MnOx@carbon electrode nanoarchitectures for rechargeable zinc-ion cells
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Research Letter
Deciphering charge-storage mechanisms in 3D MnOx@carbon electrode nanoarchitectures for rechargeable zinc-ion cells Jesse S. Ko , Naval Research Laboratory–National Research Council Postdoctoral Associate, Washington, DC, 20375, USA Martin D. Donakowski, Exponent, Inc., 9 Strathmore Road, Natick, Massachusetts, 01760, USA Megan B. Sassin, Joseph F. Parker, Debra R. Rolison, and Jeffrey W. Long, U.S. Naval Research Laboratory, Surface Chemistry Branch (Code 6170), Washington, DC, 20375, USA Address all correspondence to Jeffrey W. Long at [email protected] (Received 26 October 2018; accepted 7 January 2019)
Abstract We previously demonstrated that electrode architectures comprising nanoscale birnessite-like MnOx affixed to three-dimensional carbon nanofoam (CNF) scaffolds offer performance advantages when used as cathodes in rechargeable zinc-ion cells. To discern chemical and physical changes at the MnOx@CNF electrode upon deep charge/discharge in aqueous Zn2+-containing electrolytes, we deploy electroanalytical methods and ex situ characterization by microscopy, elemental analysis, x-ray photoelectron spectroscopy, x-ray diffraction, and x-ray pair distribution function analyses. Our findings verify that redox processes at the MnOx are accompanied by reversible precipitation/dissolution of crystalline zinc hydroxide sulfate (Zn4(OH)6(SO4)·xH2O), mediated by the more uniformly reactive electrode structure inherent to the CNF scaffold.
Introduction Rechargeable “zinc-ion” batteries (ZIBs) are a promising energy-storage technology that borrows the benefits—and some components—from well-established alkaline Zn–MnO2 batteries, but uses Zn2+-containing mild aqueous electrolytes that enable previously unattainable cycle life.[1–3] Such cells are constructed from a zinc anode (commonly a metal foil) paired with a metal oxide-based cathode that undergoes Zn2+supported redox reactions. Most ZIB-based research focuses on developing active materials for the cathode,[3–6] with manganese oxides being the most promising due to their combination of high capacity (up to the theoretical one-electron value, 308 mAh/g), high operating voltage (∼1.4 V versus Zn), nominally low cost, and environmentally benign nature.[1,3,7–16] We recently reported on the electrochemical properties in aqueous zinc-ion electrolytes of electrode architectures comprising birnessite-type MnOx affixed to carbon nanofoams (MnOx@CNF).[16] These three-dimensional (3D)-wired electrodes express oxide-normalized specific capacity approaching the theoretical MnOx value, delivering that capacity at rates that meet or exceed those reported for MnOx-containing powder-composite electrodes.[3] When cycled in mixed-salt electrolytes that also contain Na+, MnOx@CNF exhibits an additional functionality—pseudocapacitance—that can be tapped to deliver charge at time scales where Zn2+-supported redox reactions are limited. We explored the dynamics of charge storage using a suite of electroanalytical protocols (voltammetry and impedance) that revealed cle
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