Zn/MnO2 alkaline batteries are currently attracting a lot of attention due to their potential as safe, low cost, high energy density rechargeable batteries for use in grid storage applications. These batteries are traditionally primary batteries at ~$18/kWh with long shelf life and have the lowest bill of materials cost, lowest manufacturing capital expenses, and an established supply chain for high volume manufacturing. However, this battery chemistry has yet to reliably realize > 5000 cycles, which equates to roughly 10-15 years of battery life which is needed for grid storage applications. One of the main failure mechanisms for these cells is the poisoning of the MnO2 cathode material during cycling due to zinc crossover from the anode. Advanced separators that successfully stop or limit zinc crossover are crucial to increase the cycle lifetimes of these batteries. Here, a commercial ceramic sodium ion conductor which nearly eliminates zinc crossover is evaluated as a separator in these batteries. The ionic conductivity of this separator is measured to be 3.5 mS/cm while its thickness is 1.0 mm resulting in large total membrane resistance of 26 Ω. An attempt was made to reduce this resistance by decreasing the thickness of this membrane to 0.5 mm. The effect of this reduction in thickness is demonstrated in reduced polarization of the discharge curves resulting in higher discharge potentials. To evaluate the zinc blocking performance of the membrane on battery cycle life, cells cycled under limited depth of discharge (DOD) utilizing a 30 wt% NaOH electrolyte were used and compared to traditional Celgard and cellophane separators. For a 5% DOD at a C/5 rate, the cycle lifetime was increased by over 20% using the thinner ceramic separator when compared to traditional separators. SEM/EDS and XRD characterization showed limited amounts of zinc species on the cathode utilizing the ceramic separator. Further improvement in cycle lifetimes should occur with even thinner ceramic sodium ion conducting materials.
This work was supported by Sandia National Laboratories and by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability. We thank Dr. Imre Gyuk, Manager of the Energy Storage Program for continued support. 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.