Solid oxide fuel cells (SOFCs) operate on a variety of environments and are exposed to a variety of environmental impurities. One ubiquitous impurity in the cathode gas is carbon dioxide whose concentrations are usually upwards of 400 ppm depending on the source of the air and leaks through pinholes across the electrolyte. In this work we present a comprehensive thermodynamic assessment of the phase equilibria La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) – CO₂ – O₂ system using a molten salt method and using CALPHAD (Calculation of Phase Diagrams). Detailed thermodynamic maps of LSCF in carbon dioxide environments are provided under operando conditions and compared with available experimental data. We also examine contradictions between experimental data and computation of phase equilibria, and attempt to reconcile them by paying close attention to differences in cell operating conditions. Finally we discuss future directions where the combination of experimental and CALPHAD assessment of the thermodynamics of fuel cell component materials can be used to provide design guidance.