Unraveling the Charge Storage Mechanism of β-MnO2 in Aqueous Zinc Electrolytes

MnO2-based zinc-ion batteries have emerged as a promising candidate for next-generation energy storage systems. Despite extensive research on MnO2 electrodes, the charging mechanism in mildly acidic electrolytes remains debated. Most studies have focused on α-MnO2, and this study aims to shed light on the identity of the charge carrier in β-MnO2 and the role of the Mn2+ cations. By employing in situ EQCM-D measurements, along with ssNMR, XRD, TEM, and in situ pH monitoring, we demonstrated that the charging mechanism is primarily governed by proton de/intercalation. Compared to α-MnO2, with its larger 2 × 2 tunnels that accommodate hydronium ions, the β-phase has smaller 1 × 1 tunnels, permitting only the insertion of bare protons. During cycling, we observed the formation of new phases on β-MnO2 originating from the repetitive electrodeposition/dissolution of Mn2+. In addition, these phases can reversibly host hydronium ions, resulting in a mixed charging mechanism that involves the insertion of both H3O+ and H+.