Corrosion stability of NASICON-based membranes against aqueous solutions: case study for sodium iodine batteries

Sodium batteries are attractive candidates for medium to large scale stationary energy storage applications. While high-temperature Na-NiCl batteries (Zebra batteries) have been in commercial use for decades, intensive research is being conducted into new cell concepts to mitigate some of the drawbacks, particularly the high operating temperature and the expensive Na-beta-alumina separator. Medium-temperature sodium batteries are promising low-cost alternatives with an operating temperature of only 100 degrees C (slightly above the melting point of metallic Na), a NaSICON-based separator and a high-capacity aqueous cathode based on NaI/NaI3. Compared to polycrystalline Na-beta-alumina, NaSICONs offer higher Na-ion conductivity and lower manufacturing costs. However, little is known about the stability of this large class of materials under the specific operating conditions in this new cell type. In this study, we systematically investigate the chemical stability of different NaSICONs composition as a function of the state of charge (SOC) of the catholyte. Subsequently, solid separators were prepared and tested in symmetrical NaI/NASICON/NaI3 and full Na/NaSICON/NaI + NaI3 cells to evaluate the degradation under electrochemical cycling. These experiments revealed that the main cause of degradation is Na-proton exchange in the NaSICON and reaction of iodine with secondary phases and that this depends on the SOC. From this fundamental understanding, optimization strategies were derived that led to the development of sub-stoichiometric NaSICON with optimized doping to extend the lifetime of aqueous medium temperature Na batteries. The results obtained here not only enable the use of this exciting new battery technology for medium- to large-scale energy storage, but can also help to increase the lifespan and efficiency of other aqueous-based battery systems, such as seawater-based batteries or membranes for Na extraction from brine solutions.