Phase equilibria of La₂NiO_4+δ: La_xCe_1-xO_2-δ (LNO: LDC) composites (sintered at 1300°C) was studied using X-ray diffraction (XRD). The variables considered in these experiments were the dopant concentration in the ceria phase (0.2<x<0.48) and volume fractions of LNO in the composite cathode. This system has been previously evaluated and the sub-solidus phase equilibria between NiO, La₂O₃, and CeO₂ at 1200°C was revisited, and studied at 1300°C and 800°C. A thermodynamic rationale was used to re-engineer the LNO: LDC composites which stabilized the LNO phase in the composite electrode. Elemental analysis via wavelength dispersive X-ray spectroscopy (WDS) confirmed the chemical composition of the stable phases in the composites. Measurements of the coefficient of thermal expansion (CTE) of LNO: LDC composites as a function of LNO volume fraction provided further evidence of the stability of LNO and LDC (for x=0.4). Finally, we have also performed electrical conductivities of La₂NiO_4+δ: La_0.4Ce_0.6O_2-δ composites as a function of LNO volume fraction between 600 and 800°C in air. The electrical conductivity of the composites was analyzed via the resistive network approach. Based on these experiments, a window of composition is provided for stable, high performance cathode compositions in this system.