Rechargeable alkaline Zn/MnO₂ batteries are currently attractive candidates for grid-scale energy storage due to its low cost, safety characteristics, and high gravimetric capacity of MnO₂ (308 mAh/g per electron). However, commercialization of this battery has not achieved big success due to its limited cycle life and restricted depth of discharge (DOD). Accessing higher DOD results in higher energy densities, but leads to detrimental characteristics like phase transformation of MnO₂ and the zinc redistribution. Recently, a class of Bi-birnessite (a layered MnO₂ structure with bismuth oxide) material intercalated with Cu²⁺ has been reported as being able to stabilize the MnO₂ structure and deliver near-full two-electron capacity for more than 6,000 cycles in the absence of zinc [1]. However, when a zinc anode is used, the most detrimental effect arises from zincate ions ([Zn(OH)₄]^2-) that traverse to the cathode side and react to form a spinel phase ZnMn₂O₄, a highly resistive and electrochemically inactive compound, leading to rapid energy density loss and battery failure [2]. Therefore, inhibiting ZnMn₂O₄ formation remains a major challenge for the long-term cycling of an energy dense secondary Zn/MnO₂ battery.

Previously, we reported the use of a zincate-absorbing interlayer fabricated of calcium hydroxide, which successfully prevents ZnMn₂O₄ formation [3]. In this presentation, we report the strategy of developing a separator featuring selective transport of OH^- and [Zn(OH)₄]^2- ions. We fabricate a graphene oxide-poly(vinyl alcohol) (GO-PVA) composite membrane with a layered structure. The GO membrane, which is composed of stacked and overlapped GO monolayers with a narrow interlayer spacing (~13.5Å for GO laminates swelled in water), has been demonstrated to be excellent filters for gases and liquids. The nanocapillaries formed within the membranes as well as the oxygen functional groups which repel the bulky [Zn(OH)₄]^2- anions are responsible for its ion sieving properties. PVA is impregnated to provide a conductive pathway for OH^- ions and to improve the mechanical stability. We successfully demonstrate the superiority of the GO/PVA composite membrane in suppressing zincate ion crossover (Fig.1(a)), while minimally impairing OH^- conduction. Its application in primary batteries significantly increases the energy density by obtaining capacity of more than 1.5 e^- per Mn atom above 0.9V (Fig.1(b)). In rechargeable batteries, it slows down the capacity fade of a conventional zinc/electrolytic γ-MnO₂ (Zn/EMD) battery during deep cycling within the 1^st electron capacity (Fig.1(c)), while it helps to achieve more than 200 cycles with full 2-electron capacity retained in a zinc/birnessite battery (Fig.1(d)).

References:

[1] Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S., Nat. Commun., 2017, 8, 14424

[2] Yadav, G. G.; Wei, X.; Huang, J.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Secor, J.; Banerjee, S., J. Mater. Chem. A, 2017, 5 (30), 15845-15854

[3] Huang, J.; Yadav, G. G.; Gallaway, J. W.; Wei, X.; Nyce, M.; Banerjee, S., Electrochemistry Communications 2017, 81, 136-140

Figure caption:

Fig.1 (a) crossover of zincate ions through different membranes; (b) discharge voltage curves of primary batteries with different separators; (c) discharge capacities of rechargeable Zn/EMD batteries cycling at full 1-electron capacity of MnO₂; (d) discharge capacities of rechargeable Zn/birnessite batteries cycling at full 2-electron capacity of MnO₂.