The remarkable chemical diversity of Mn oxides comes from the ability of these structures to adopt multiple Mn oxidation states and host a variety of intercalated ions. When H + is the charge compensating ion to Mn redox, the reduction potential of MnO 2 is near the oxidative voltage stability window of water (1.23 V vs. RHE). This makes MnO 2 a suitable positive electrode for aqueous batteries like the ubiquitous alkaline Zn-MnO 2 primary battery. In addition to protons, MnO x crystal structures can accommodate various monovalent (e.g. Na + , K + ) and multivalent (e.g. Mg 2+ , Zn 2+ ) cations inside their tunnels or between their layers. As a result, Mn oxides are also targeted for low-cost "beyond-Li" battery electrodes, both aqueous and non-aqueous, with large theoretical capacities (617 mAh/g MnO 2 ) if the full 2-electron redox (Mn 2+ ⇌ Mn 4+ + 2e -) can be cycled reversibly. Higher energy densities, lower costs and greater safety could potentially all be achieved relative to current-generation Li-ion batteries. The sheer number of possible combinations of structures and ions in the Mn oxide system frustrates understanding its electrochemical ion selectivity. Exhaustive sampling of this large phase space would be prohibitively time-consuming, so experimental works typically focus on a single ion-polymorph couple. In contrast, computational approaches can efficiently evaluate the thermodynamics of materials, enabling holistic comparison of a wide range of chemistries. Comparing different ions and Mn oxidation states with a single approach requires a special computational scheme. Previous approaches, however, have been tailored to computing material properties at constant Mn oxidation state, like structure selection during polymorph synthesis. To compare between crystal structures with identical oxidation state, these works used the meta-GGA SCAN functional, which was shown to reproduce the experimentally-observed energetic ordering of b and R-MnO 2 . The behavior of the polymorphs with changing voltage was not considered. Coarser methodologies have been used to screen MnO 2 polymorphs as potential Caion battery cathodes, and study the proton-coupled redox behavior of select polymorphs in alkaline Zn-MnO 2 batteries. Yet, the absence of a tailored benchmarking scheme with Hubbard-U corrections limits the predictive power of these approaches across the broad scope of ion insertion electrochemistry and manganese redox. Conversely, computational investigations with Hubbard-U corrections have typically been limited to a single polymorph/electrolyte pair. In this work, we compare the accuracy of the SCAN and PBE functionals including the Hubbard-U parameter and develop a rigorous correction scheme to compute accurate redox energetics. We find that the PBE+U functional is a less computationally expensive and slightly higher accuracy approach to calculate ion insertion voltages in the Mn oxide system. Using this efficient approach, we investigate a broad range of Mn oxide electrochemical behavior, directly comparing the voltages and ion capacities of 𝛼, β, R, 𝛿 and λ-MnO 2 in H + , Li + , Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ and Al 3+ electrolytes. We place particular emphasis on the implications of our calculations for energy storage and conversion in aqueous electrolytes, whereby we compare the affinity of MnO 2 polymorphs to insert protons versus non-protonic cations. While the selectivity of non-protonic cation insertion hinges on oxygen coordination environments, protons are usually favored when present in the electrolyte. We also evaluate the stability of these polymorphs with respect to dissolution and structural reorganization to elucidate promising design strategies for nextgeneration batteries. Finally, we examine the effect of ions inserting with partial solvation shells in 𝛼 and 𝛿-MnO 2 , which are large enough to accommodate water molecules.