Unravelling the Mechanism of Rechargeable Aqueous Zn–MnO₂ Batteries: Implementation of Charging Process by Electrodeposition of MnO₂

Abstract

Poor cycling stability and mechanistic controversies have hindered the wider application of rechargeable aqueous Zn–MnO2 batteries. Herein, direct evidence was provided of the importance of Mn2+ in this type of battery by using a bespoke cell. Without pre‐addition of Mn2+, the cell exhibited an abnormal discharge–charge profile, meaning it functioned as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO2. Additionally, a dynamic pH variation was observed during the discharge–charge process, with a precipitation of Zn4(OH)6(SO4)⋅5H2O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 was first converted into MnOOH, which reverted to MnO2 through disproportionation, resulting in the dissolution of Mn2+. The charging process occurred by the electrodeposition of MnO2, thus improving the reversibility through the availability of Mn2+ ions in the solution.

Hierarchical K‐Birnessite‐MnO2 Carbon Framework for High‐Energy‐Density and Durable Aqueous Zinc‐Ion Battery

“…[21] Along with the consumption of H + , the locally generated OH − can induce a pH-dependent conversion reaction with ZnSO 4 and H 2 O to form micrometer-scaled flake-like ZHS with 2D layered structure on its surface. [18,25,33,58,69,70] A further discharging degree to No. 6-1.00 V (fully discharged state) gives significant rises to the intensity of the peak ≈12°.…”

The Role of Al3+‐Based Aqueous Electrolytes in the Charge Storage Mechanism of MnOx Cathodes

“…Small 2021, 17, 2101515 reservoir for MnO 2 electrodeposition during charging. [12,29,33] This is the principle of a conversion battery, where finally the capacity of the electrolyte has to be taken into account for the energy density calculation. Some publications report on lower oxidation state manganese oxide cathode materials (i.e., spinel Mn 3 O 4 and MnO) capable of cycling in highly concentrated Al(OTf ) 3 aqueous electrolytes, free of Mn 2+ ions.…”

“…Given the limited pH increase occurring at the metal oxide/ electrolyte interface during the discharge (i.e., a local pH estimated to be 3 as discussed above), no precipitation of Al 3+based hydroxides/oxides is observed in our spectroelectrochemical experiments, in contrast to what is reported in Zn 2+ -based aqueous electrolytes. [28][29][30]…”

Towards a high MnO2 loading and gravimetric capacity from proton-coupled Mn4+/Mn2+ reactions using a 3D free-standing conducting scaffold

“…Because relying on protoncoupled electron transfers, this reaction is associated with significant local pH gradients, triggering precipitation of zinc hydroxides and restricting the gravimetric capacity, as well as to potential drifts. [10][11][12][13] To overcome these limitations, Mateos et al demonstrated the benefit of using a buffered aqueous electrolyte (pH = 5) to fully convert MnO 2 to water-soluble Mn 2+ (aq) at stable potential values and with low hysteresis through the highly reversible proton-coupled reaction given below: where AH and A are the weak Brønsted acid and base of the buffer, respectively. When performed on transparent 2D ITO electrodes, accumulation of an electrochemically inactive fraction of MnO 2 (ca.…”