Effect of ZnO-Saturated Electrolyte on Rechargeable Alkaline Zn–Ni and Zn–MnO2 Batteries at High Zinc Depth-of-Discharge

Abstract

The transition from fossil fuels toward a clean, renewable energy-based economy has generated significant interest in rechargeable batteries for grid-level storage, electric vehicles, and portable electronics. Although lithium-ion batteries are currently the predominant technology for such applications, they have relatively expensive raw materials and serious safety hazards due to their use of flammable electrolyte. Rechargeable alkaline MnO2–Zn and Ni–Zn batteries are promising alternatives that have a theoretical specific energy rivaling Li-ion systems, along with inexpensive, environmentally compatible raw materials and non-flammable aqueous electrolyte. Unfortunately, alkaline Zn anodes suffer from irreversible shape change, redistribution of active material, and passivation over repeated charge­–discharge, limiting their achievable capacity and lifetime. One simple strategy to overcome these issues is to pre-saturate the electrolyte with ZnO, which minimizes dissolution and long-range migration of zinc from the anode, and also reduces the rate of hydrogen evolution.[1] Although the use of ZnO-saturated electrolyte in Ni–Zn cells has been previously reported, most of the relevant literature uses flooded cells and does not account for the large excess of Zn species in the electrolyte relative to the anode, which artificially inflates the specific capacity. In addition, there has been no systematic study to date on the effect of ZnO-saturated electrolyte at different Zn depth-of-discharge (DODZn­). Here, we present a comprehensive investigation of ZnO-saturated electrolyte in more commercially relevant, non-flooded Ni–Zn cells at various high DODZn­ (10+%), analyzing its impact on cycle life and Zn electrochemistry. With additive-free KOH electrolyte, such high levels of Zn utilization limit the cells to only ~100 cycles, but cells with saturated electrolyte can achieve 180+ cycles at 10+% DODZn, even when this DOD accounts for ZnO in the electrolyte. Besides standard anodes consisting only of a mixture of Zn and ZnO particles with a polymeric binder, we examine ZnO-saturated electrolyte with Zn anodes containing Ca- and/or Bi-based additives, which have previously been shown to improve cycle life at 15% DODZn in additive-free electrolyte.[2] Changes in the morphology, distribution, and size of the active material particles in the anodes during cycling are characterized by ex situ scanning electron microscopy. Finally, we evaluate ZnO-saturated electrolyte in a MnO2–Zn system, which offers significantly higher theoretical energy density and lower cost per kWh than Ni–Zn but has heretofore not been studied with ZnO-saturated electrolyte because zincate can react with MnO2 to form electrochemically inactive compounds. As a path forward, we are developing zincate-blocking polymeric membranes that can essentially isolate the MnO2 cathode from zincate while maintaining good hydroxide conductivity, thereby allowing for reversible cycling of a MnO2–Zn cell at high DODZn. Various aspects of this work will also be presented. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. Dr. Imre Gyuk, Energy Storage Program Manager, Office of Electricity Delivery and Energy Reliability is also thanked for his financial support of this project. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government. [1] A.R. Mainar, E. Iruin, L.C. Colmenares, J.A. Blázquez, H.-J. Grande, Energy Science & Engineering 6 (2018), 174–186. [2] D.E. Turney, J.W. Gallaway, G.G. Yadav, et al., Chem. Mater. 29 (2017), 4819–4832.