Finite-Element Analysis Study to Predict the Thermo-Mechanical Stability of Planar-Type NaS Batteries

The sodium sulfur (NaS) battery is one of the most promising candidates for grid scale energy storage systems (ESS) applications. Within the structure of the NaS cell, there are joint parts comprised of different types of materials such as metallic alloys or glasses to bond between insulating ring/metals and insulating ring/electrolytes, respectively. During the freeze and thaw processes, the ambient temperature changes may result in interfacial decohesion or bulk fractures of these joint parts, which may lead to a catastrophic failure of the cell. It is considered that such thermomechanical stability at the joints is one of the vital issues in developing commercialized NaS cells. In this presentation, therefore, we will first review the current thermal stability problems of NaS cells, and then the computational approach will be introduced to address the quantitative prediction of such thermal stress concentration in these metallic and glass joints area of NaS batteries. Specifically, the impacts of the structures, geometries, and sizes of cell components on the thermomechanical behavior of the planar-type NaS cell have been explored based on the finite-element analysis (FEA) computational technique. In contrast to the conventional cylindrical type cells, we will introduce a planar-type NaS design recently developed at RIST as an effort to increase the efficiency and performance. In developing the computational model, we have incorporated relevant stress-strain curves of each material assuming that they follow the J2 flow theory with isotropic hardening. Realistic geometries of the battery structure were digitally transferred to the FEA solver using sophisticated mesh-generation software. The computation includes all of the detailed cell assembly procedures with corresponding temperature changes. After construction of the 3 dimensional cell geometries, a comprehensive analysis including the tangential and normal stress concentration at the interfaces and the bulk section of the joint area upon heating and cooling has been performed. The results of the FEA computation show that the thermal stress concentration in the joint areas is strongly dependent on the geometry and the size of the cells. Especially, substantial increase of the stress accumulation was predicted in the cells with angular geometries. In this presentation, we will also show quantitative analyses on the probable fracture scenarios in the cell joints. It is expected that the outcomes from the developed prediction tool employing the FEA can provide a guidance to enhance the stability of the NaS cell with optimized materials and geometrical designs.