Tracking Mn and Zn in Rechargeable Aqueous Zn-MnO2 Batteries By Operando X-Ray Absorption

Abstract

Zn-MnO2 batteries with mildly acidic electrolytes are a promising chemistry for large scale storage thanks to their remarkable energy density, low cost, and high safety. This is mainly obtained thanks to the high capacity of the Zn metal anode, and the nonflammable character of the aqueous electrolyte. MnO2 is one of the most common cathode of choice, not only for being Earth-abundant, but also because it can undergo a two-electron mechanism, which is however complex and still not fully understood. There is currently agreement in considering for discharge a MnO2 dissolution, leading to soluble Mn2+ and simultaneous precipitation of Zinc Hydroxide Sulfate (ZHS, ZnSO4[Zn(OH2)]3·xH2O). When charging the process is not simply reverted. In fact, a distinct electrochemical profile is observed, with at least two distinct plateaus and a third, apparently pseudocapacitive stage (Figure 1a). A similar multistage profile is observed during the second discharge. Although such profile is characteristic and observed with different MnO2 phases and architectures, the underlying mechanism remains elusive, as it seems to involve mainly poorly crystallized phases.We studied the mechanism by operando X-ray absorption (XAS) at the Mn and Zn K-edges to follow speciation simultaneously and quantitatively in the cathode and in the electrolyte via principal component analysis. Beam intensity needed appropriate regulation to avoid interference with the experiment.Simultaneous X-ray diffraction allowed precise correlation with the MnO2 dissolution and ZHS formation. We found evidence of Mn(III) intermediate occurring during local bond reorganization, which is inferred by the significant evolution of the absorption fine structure region (EXAFS) of the Mn K-edge (Figure 1b). In contrast, minor Zn spectral changes reflect primarily processes of precipitation and dissolution, suggesting that no Zn-Mn mixed phases form during cycling. Figure 1