Sodium sulfur (NaS) cell is recognized as a promising candidate for advanced grid-scale large energy storage systems (ESS). In this work, we study the impacts of planar NaS cell container materials on the accumulation of residual stresses in the cell joints and solid electrolyte during the cell assembly and operation processes. Concentration of such thermo-mechanical stress in these vulnerable areas in the modern NaS cells can lead to catastrophic cell failures, which can present a huge challenge for developing large planar NaS cells towards commercial deployment. Here, we employ the finite-element analysis (FEA) computational technique to quantitatively assess the thermo-mechanical stress accumulation using prototype planar NaS cells. Relevant experimental procedures with corresponding thermal cycling conditions for the cell assembly, operation, and maintenance processes are incorporated into the FEA model. The influences of Al alloy (Al3003), stainless steels (STS304 and STS340), and iron-nickel-cobalt superalloy (KOVAR) on the residual stress accumulation are tested and thoroughly discussed. The computation results show that high stress concentration can be developed in the cell joint area. Through the comprehensive computational analysis, it is suggested that applying smaller CTE (less than ∼12×10−6 K−1) alloys is necessary to secure the thermo-mechanical stability of NaS cells that can be implemented in a large scale ESS.