Achieving High Capacity at High Rates in Aqueous-Based Zn-Ion Batteries Via Advanced 3D Electrode Design and Mixed-Salt Electrolytes

Abstract

Manganese oxides (MnOx) are a well- established class of active materials for electrochemical energy- storage technologies ranging from primary alkaline to rechargeable Li- ion batteries and more recently, have shown great promise in electrochemical capacitors. The widespan applicability of MnOx in these different devices stems from the use of different electrode structures and electrolyte compositions. However, we have demonstrated that with the use of an advanced 3D electrode architecture in which the MnOx component exists as a conformal nanoscale coating on a carbon nanofoam paper with a through-connected pore structure, it is possible to deliver full theoretical capacity of LiMn2O4, a nominal battery material, at EC-like rates in an aqueous electrolyte. We are now extending the performance enhancements gained via the 3D electrode architecture demonstrated for Li-ion battery chemistries to aqueous-based Zn-ion batteries comprised of a MnOx@carbon nanofoam cathode and a Zn anode. We show that the MnOx@carbon nanofoam delivers full theoretical capacity (308 mA h g-1) at 1 C when cycled in an aqueous mixed salt electrolyte (e.g., 0.75 M Na2SO4 + 0.25 M ZnSO4) – delivering high capacity at high rate in a single electrode. To gain a better understanding of the charge-storage reactions occurring in this complex system, we perform a suite of characterization methods as a function of potential. Electrochemical impedance spectroscopy confirms that MnOx stores charge via pseudocapacitance, supported by the presence of Na ions in the electrolyte, at potentials not associated with Zn2+ insertion/extraction. Ex-situ surface characterization of discharged MnOx@carbon nanofoam electrodes reveal the formation of Zn4(OH)6SO4·5H2O crystallites on the exterior surface, but no evidence of those precipitates after recharge. The ability to deposit/dissolve the Zn4(OH)6SO4·5H2O precipitates in conjunction with pseudocapacitive charge-storage supports long-term cycling ability, with minimal decrease in capacity over 1000 cycles.