Development of a Planar Sodium Sulfur Cell

A noble prototype planar sodium sulfur (NaS) cell with a self-standing type cell design has been developed for high power energy storage systems (ESS) applications. Upon designing the cell, a thermal stress analysis combined with a finite element method (FEM) has been conducted in order to (1) maximize the volumetric energy density of a multiple cell stack or module and to (2) predict the maximum stress accumulation and mitigate the maximum local stresses during freeze/thaw cycles. The analysis is of great importance in designing the high temperature sodium batteries since the thermal stresses at the joints become larger as the cell size increases, and to avoid the cell failure the thermal stress should be sufficiently smaller than the bonding strength at the heterogeneous joints. Based on the study, a number of cell designs with different shapes and sizes have been created, and in succession, a couple of cells were successfully materialized out of selected designs using various state-of-art joining technologies, such as electron beam welding (EBW), thermal compression bonding (TCB), and glass sealing for metal-metal, metal-ceramic, and ceramic-ceramic joints, respectively. Finally, the cell characteristics of assorted cells will be discussed.