The primary alkaline Zn/MnO2 battery remains a ubiquitous energy-storage solution for many consumer uses, though its application space is narrowing due to limited rechargeability. The inherent cost and safety benefits of the Zn/MnO2 pairing electrodes can be translated to a new class of rechargeable “zinc-ion” batteries that are enabled by specific nanostructured manganese oxides that undergo reversible redox reactions in mild-pH Zn2+-based electrolytes. We are exploring two such MnOx polymorphs—layered, birnessite-type and cubic-spinel ZnMn2O4—for zinc-ion storage, where the oxide is expressed as a nanoscale coating on carbon nanofoam (CNF) paper substrates. The optimized electron/ion transport characteristics of MnOx–CNF electrode architectures ensures high oxide-normalized capacity delivered at moderate-to-high rates and over hundreds of charge–discharge cycles. Pseudocapacitive reactions at the MnOx are also activated when Na+ or Li+ salts are added to the Zn2+-based aqueous electrolyte, enabling pulse-power functionality at time scales approaching those for electrochemical capacitors [1]. Redox reactions at MnOx zinc-ion electrolytes may follow multiple pathways that including direct Zn2+ insertion/association and proton-insertion processes that shift local pH to induce the precipitation of Znx(OH)y(SO4)z at the MnOx surface. Ex situ characterization by diffraction, spectroscopy, and microscopy confirms that the latter process is dominant with birnessite-MnOx–CNF electrodes, with the reversibility of this complex precipitation/dissolution process promoted by the pore–solid architecture of the CNF [2]. To further unravel the mechanisms of Zn-ion storage, we examine 2D-planar MnOx–carbon electrodes that mimic the surfaces of their 3D counterparts, applying such in situ techniques as scanning-probe microscopy and quartz-crystal microbalance measurements. Insights from these investigations inform the design of advanced electrode architectures for high-performance, rechargeable zinc-ion batteries. [1] “Combining Battery-Like and Pseudocapacitive Charge Storage in 3D MnOx@Carbon Electrode Architectures in Zinc-Ion Cells.” J.S. Ko, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, Sustain. Energy Fuels, 2 (2018) 626–636. [2] J.S. Ko, M.D. Donakowski, M.B. Sassin, J.F. Parker, D.R. Rolison, and J.W. Long, MRS Comm., 9 (2019) 99–106.