In this study, layered perovskites were synthesized using a non-stoichiometric synthesis strategy that allowed the variation of the amount of Co substitution in the basic composition of SmBa0.5Sr0.5Co2O5+d (SBSCO), used as a cathode material for Intermediate Temperature-operating Solid Oxide Fuel Cells (IT-SOFCs). SmBa0.5Sr0.5CoxO5+d (x = 1.9–2.1, SBSCO-1.9–2.1), B-site non-stoichiometric layered perovskites, were analyzed to characterize the electrical and electrochemical properties with respect to the Co substitution. Among them, SmBa0.5Sr0.5Co2.05O5+d (SBSCO-2.05) exhibited the best electrical and electrochemical properties with an ASR of 0.082 Ωcm2 and an electrical conductivity of 268.42 S/cm at 700 °C (measured in air within a series of measurements of increasing measuring temperature). Therefore, B-site non-stoichiometric perovskite compositions with increased Co substitution on their B sites represent a key strategy for achieving high performance cathodes compared to cathodes with stoichiometric compositions.