Electrochemical Reaction Mechanism in a High Capacity Zinc-Ion Battery System

Earlier efforts to develop an aqueous zinc-ion battery (ZIB), which facilitates energy storage/conversion via Zn-ion intercalation/de-intercalation, has paved the way not only to realizing safe and environmentally benign energy devices, but also to reducing the processing costs of next generation batteries. [1,2] From the viewpoint of developing low-cost and environmentally safe rechargeable batteries that can be easily scaled up for large-scale production, the present study reports on a high surface area mesoporous tunnel-type γ-MnO₂ cathode obtained by a simple template-free ambient temperature strategy and subsequent annealing at low temperatures for ZIB applications. An in-depth study on the structural transformation in a mesoporous γ-MnO₂ cathode during electrochemical reaction in a zinc-ion battery (ZIB) has been conducted.

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-Zn_xMnO₂ and layered-type L-Zn_yMnO₂ 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.

References

1. C. Xu, B. Li, H. Du, F. Kang, Angew. Chem. Int. Ed., 51, 933, (2012).
2. C. Yuan, Y. Zhang, Y. Pan, X. Liu, G. Wang, D. Cao, Electrochim. Acta, 116, 404 (2014).