No ion displays preferential insertion relative to H + over the entire 2-electron capacity of MnO 2 in any of the polymorphs. This finding has important implications for MnO 2 electrodes in batteries, pseudocapacitors, and other ion insertion applications. Notably, in the presence of protic solvents, either proton-only or mixed ion/H + insertion reactions are likely to occur depending on pH and polymorph. Great interest exists in developing secondary aqueous batteries with MnO 2 cathodes, particularly those paired with Zn/Zn 2+ anodes. While promising performance improvements have been made, the precise operating mechanism of these batteries remains a subject of debate. Experimental reports on different polymorphs have hypothesized proton insertion, 17,70 Zn 2+ insertion, and Zn 2+ /H + co-insertion mechanisms. Our results introduce polymorph-specific explanations for these findings and demonstrate that co-insertion with H + is a general feature of the MnO 2 system, not just in Zn 2+ electrolytes. In fact, in protic electrolytes like water, dual ion insertion is likely to be the only possible charge storage mechanism other than a proton-only mechanism, and an effective ion-shuttling battery should pair a proton-discharging anode for maximum energy density.