Stress Corrosion Cracking of Nasicon Pellets in Aqueous Electrolytes
Authors: Zhang Mengyao , Tseng Flora , Ismail Zeinab , Kwabi David G. , Sakamoto Jeff , Dasgupta Neil P. , Thouless M.D. 
Published: December 2024

Recent efforts have explored the use of sodium super ionic conductors (NaSICON) in aqueous redox flow batteries (RFBs) because of their ability to selectively conduct sodium ions while effectively preventing the crossover of active species, thereby enhancing the performance and longevity of RFBs. However, despite the potential of NaSICON membranes in enhancing the electrochemical performance of RFBs, the use of dense and thick ceramic membranes results in increased cell resistances. Modak et al. recently suggested that NaSICON membranes with thicknesses below 100 mm may achieve equivalent power densities to RFBs with polymer membrane and meet the resistance criterion.1 Therefore, to realize the commercial application of these thin ceramic films, careful attention must be paid to the membrane’s mechanical properties, especially fracture strength and toughness.
Stress-corrosion cracking (SCC) is particularly important for ceramics when they are exposed to water,2 which is defined as the growth of cracks due to the simultaneous action of a stress and a reactive environment. During cell operation, the NaSICON membrane is immersed in corrosive liquid electrolytes and is simultaneously subject to a pressure gradient due to the uneven electrolyte flow. Consequently, understanding the interaction at solid-liquid interface and its impact on NaSICON is crucial as it may affect the fracture toughness of NaSICON and cause unexpected failure.
In this study, the toughness of the samples was determined using Vickers indentation. The indentations were imaged using an optical microscope, allowing for measurement of the crack lengths. To assess the evolution of fracture toughness of NaSICON in the presence of typical RFB reactants, the pellets were soaked in selected aqueous solutions. The influence of solution composition, pH value, and molarity are investigated as well as soaking time. Significant crack propagations were observed in all pellets when immersed in solutions, suggesting a decrease in the fracture toughness of the NaSICON pellet after interacting with these liquids. Measurable crack growth was observed to occur mainly within the first 24 hours of immersion, suggesting a equilibrium had been reached between the driving forces for crack propagation (such as stress intensity and corrosive environment) and the resisting material properties (material toughness). To model the influence of SCC during operation of an aqueous RFB, theoretical analysis was conducted to quantify the relationships between the pressure gradient that a NaSICON membrane can withstand under a given geometry, and the evolution of its fracture toughness. These fundamental mechanical studies will provide valuable insights to understand and manage stress corrosion cracking of NaSICON membranes in contact with liquid electrolytes.

Modak, S., Valle, J., Tseng, K. T., Sakamoto, J. & Kwabi, D. G. Correlating Stability and Performance of NaSICON Membranes for Aqueous Redox Flow Batteries. ACS Appl Mater Interfaces 14, 19332–19341 (2022).

Spearing, S. M., Zok, F. W. & Evans, A. G. Stress Corrosion Cracking in a Unidirectional Ceramic‐Matrix Composite. Journal of the American Ceramic Society 77, 562–570 (1994).