Decrease in sintering temperature of lanthanum-doped ceria (Ce0.6La0.4O2−δ, LDC), which was used as a buffer layer, was investigated using sintering additives such as Ag2O, ZnO, BaO, CaO, Bi2O3, RuO2, and TiO2 for preventing NiO diffusion into Sr- and Mg-doped LaGaO3 (LSGM) electrolyte film prepared by screen printing and co-firing method. It was found that TiO2 was the most effective in decreasing the sintering temperature to 1473 K, which was 300 K lower than that of LDC, and increasing the electrical conductivity. Ti addition in LDC was attributed to increased mass transportation caused by compensation for lattice mismatch and defect dissociation resulted in improved sintering density and electrical conductivity. However, Ti–LDC buffer layer could not completely prevent NiO diffusion into the LSGM electrolyte, although it was effective in reducing the diffusion content—probably because of the low sintering temperature of Ti–LDC. The use of fast sintering (a heating/cooling rate of 600 K/h and a holding time of 3 h) further decreased NiO diffusion into the electrolyte. Accordingly, theoretical open-circuit voltage and improved power density were attributed to reduced electron hole conduction and ohmic resistance that were assigned to suppressed NiO diffusion into the LSGM electrolyte.