Electrochemical Reaction Mechanism in a High Capacity Zinc-Ion Battery System
A combination analyses of in-situ synchrotron XANES and XRD reveal that the tunnel-type parent γ-MnO2 experiences a structural transformation to spinel-type Mn(III) phase (ZnMn2O4) and two new intermediary Mn(II) phases namely, tunnel-type γ-ZnxMnO2 and layered-type L-ZnyMnO2 and that these phases with multioxidation states co-exist after complete electrochemical Zn-intercalation. On sequential Zn-de-intercalation, a majority of these phases with multi-oxidation states was observed to revert back to the parent γ-MnO2 phase. The mesoporous γ-MnO2 cathode exhibits an initial discharge capacity of 285 mAh g-1 at 0.05 mA cm-2 with a defined plateau at around 1.25 V vs. Zn/Zn2+. Further, ex-situ HR-TEM studies of the discharged electrode aided to identify the lattice fringe widths corresponding to the Mn(III) and Mn(II) phases and the stoichiometric composition estimated by ICP appear to be in agreement with the in-situ findings.
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