Immobilizing Zincate Ions for Long-Cycle High-Energy-Density Aqueous Batteries
Authors: Huang Jinchao , Yadav Gautam Ganapati , Turney Damon , Gallaway Joshua W , Nyce Michael , Kolhekar Snehal , Banerjee Sanjoy 
Published: February 2020

The rechargeable alkaline Zn/MnO2 battery is attractive for grid-scale energy storage due to its cheap and safe base materials, and high theoretical energy density. However, commercialization of this battery has not met with success due to its limited cycle life. Apart from the detrimental characteristics associated with MnO2 cathode, i.e. phase transformations during cycling which lead to poor rechargeability of the material, the free-moving zincate ions ([Zn(OH)4]2-) in the electrolyte also place severe limitations on the cycle life of a rechargeable Zn/MnO2 battery. The redistribution of zinc material results in problems such as shape change and dendrite formation, which eventually cause failure of the zinc anode and short circuit of the battery. Poisoning of the MnO2 cathode by zincate ions migrate to the cathode side is another leading failure mechanism for this battery, as it results in the formation of an electrochemically inactive zinc manganese spinel phase. Consequently, the current work seeks to investigate the effect of free-moving zincate ions on the battery performance, as well as to immobilize zincate ions through development of innovative separators and electrolytes, so as to improve the cycle life of a rechargeable Zn/MnO2 battery with increased energy density.

The effect of free-moving zincate ions on the MnO2 cathode is studied by both potential dynamic and galvanostatic techniques. Strategies to immobilize zincate ions are provided. We demonstrate that the use of a free standing zincate-absorbing interlayer fabricated of Ca(OH)2 is effective in localizing and trapping the zincate ions, while not affecting the transport of OH- ions. It successfully prevents short-circuit of the battery by suppressing zinc dendrites, and inhibits the formation of zinc manganese spinel phase at high depth of discharge (DOD). Separator featuring selective transport of OH- and [Zn(OH)4]2- ions are investigated as well. We fabricate a graphene oxide/ poly(vinyl alcohol) (GO/PVA) composite membrane with a layered structure. The nanocapillaries formed within the stacked and overlapped GO monolayers 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. Here we successfully demonstrate the superiority of the GO/PVA composite membrane in suppressing zincate ion crossover, while minimally impairing OH- conduction. Modification of electrolytes also provides a solution for reducing zincate ion mobility. Additives such as zinc complexing agents are investigated, and gel electrolytes are designed to restrain the movement of zincate ions.

Scaling up of a Zn/MnO2 battery is made possible based on the above technologies. Batteries each has 118 Ah full capacity are tested. We report more than 200 cycles at 20% DOD with no capacity fade, and demonstrate that with the combination of electrolyte modification and interlayer application, the lifetime and energy density of a scaled-up rechargeable Zn/MnO2 battery is significantly improved.