Aqueous Zn-MnO₂ batteries with mildly acidic electrolytes are a promising alternative to Li-ion batteries for large-scale energy storage due to their low cost and high safety and still remarkable energy density. These advantages arise from the high capacity of the Zn metal anode and its potential allowing plating in aqueous electrolytes. Zn is in addition widespread, cost-effective and easily recyclable. Among possible cathode materials, MnO₂ stands out due to its abundance, environmental advantages, and long-standing use, in Zn/MnO₂ alkaline batteries. The demonstration of rechargeability in mildly acidic electrolytes has expanded its potential for energy storage.

Despite their promise, the mechanism of mildly acidic Zn/ MnO₂ batteries is complex and remains debated. Early studies ruled out Zn²⁺ intercalation, instead suggesting proton intercalation or a dissolution-precipitation process involving MnO₂ dissolving into Mn²⁺ and the simultaneous formation of Zinc Hydroxide Sulfate (ZHS, ZnSO₄[Zn(OH₂)]₃·xH₂O) during discharge. This dissolution-precipitation mechanism, capable of a two-electron transfer, offers a higher theoretical capacity compared to intercalation. However, practical capacities are often closer to those of a one-electron process. Furthermore, during charging, the process is not simply reversed; the electrochemical profile reveals distinct stages, including at least two plateaus and a pseudocapacitive region, indicating a more intricate mechanism. Similar multistage profiles are observed in subsequent discharges.

Clarifying the overall reaction mechanism and the factors limiting practical capacity is challenging due to the poorly crystalline nature of many involved Mn compounds, often hygroscopic, which limits structural information from diffraction and ex-situ studies.

X-ray absorption (XAS), which is capable of detecting bulk speciation of most elements regardless of crystalline state even when embedded in heterogeneous matrix is therefore a very powerful technique for such investigation, and we applied it in several different modalities. We studied the mechanism by operando XAS at the Mn and Zn K-edges to follow speciation simultaneously and quantitatively in the cathode and in the electrolyte, essentially confirming the nowadays mostly accepted dissolution/deposition mechanism, however showing some deviations in the ZHS formation rate, and quantifying parasitic H₂ evolution reaction.  In addition, the significant evolution of the absorption fine structure region (EXAFS) of the Mn K-edge suggested existence of Mn(III) intermediate. Further evidence of Mn(III) was provided by soft X-ray microscopy, providing access to the Mn-L edge with higher sensitivity to the oxidation state. Operando conditions were in this case consistent with more detailed ex-situ data. Finally, magnetic moment derived from Mn K_β X-ray emission also confirm presence of Mn(III). Principal component analysis suggests that actually several kinds of reduced Mn species do exist, which can be associated with MnO₂ initial defects, reaction intermediates and newly formed species.

The complementarity of operando and ex-situ techniques and the beam effects present in this system will be also discussed. Overall, such pieces of information provide a more complete overview of the reaction complexity. Nonetheless, understanding these processes is key to optimizing Zn-MnO2 batteries for large-scale, sustainable energy storage applications.