A relatively new type of aqueous battery that is based on MnO₂/Zn chemistry in near-neutral pH electrolytes is receiving significant attention in the research community for its potential commercial applications. This chemistry offers many advantages such as relatively high cycle life [1], low cost, high natural abundance and availability of the active materials, and environmental friendliness. On the other hand, issues with the formation of parasitic Zn-layered double hydroxide (ZnLDH) creates challenges for the capacity utilization in high mass loading electrodes. Revealing the charge storage mechanism and the products of the discharge reaction would significantly help with the scaling up of this technology.

At present, there is a debate on the nature of the reaction mechanism considering proton or zinc intercalation (or co-insertion of both) into the MnO₂ structure, and what structural transformations occur during charge/discharge. We present new insights on the reaction mechanism and show that novel, not reported yet, Mn-Zn based layered double hydroxide (MnZnLDH, Mn_xZn_y(OH)_zSO₄·5H₂O) forms during the first discharge reaction (Figure 1) [2]. MnZnLDH forms through a conversion reaction that occurs between MnO₂ and Zn²⁺ during the first discharge plateau and it is a structural analogue of ZnLDH. Electrochemical experiments in different aqueous/organic electrolytes with and without the presence of Zn ions have shown that protons and water play significant roles in the charge-storage mechanism, while Zn ions form species at the surface rather than intercalating in the bulk of MnO₂ structure. Excellent rechargeability was achieved for the ramsdellite (MnO₂) and hausmannite (Mn₃O₄) phases, i.e., over 1000 [1] and 800 [3] cycles, respectively, without considering the formation of MnZnLDH. However, improving the reversibility of MnZnLDH transformation during the cycling will enhance the battery cycle life even further.

References

[1] I. Stoševski, A. Bonakdarpour, F. Cuadra, & D. P. Wilkinson, Chem. Comm., 2019, 55, 2082-2085.
[2] I. Stoševski, A. Bonakdarpour, B. Fang, P. Lo, & D. P. Wilkinson, Electrochim. Acta, 2021, 390, 138852.
[3] I. Stoševski, A. Bonakdarpour, B. Fang, S. T. Voon, & D. P. Wilkinson, Int. J. Energy Res., 2021, 45, 220-230.

Figure 1. Mechanism of the first discharge reaction