Abstract: The commercialization of solid oxide fuel cells depends on the cathode, which possesses both high catalytic activity and a thermal-expansion coefficient (TEC) that aligns with the electrolyte. Although the cobalt-based cathode La_0.6Sr_0.4CoO₃ (LSC) offers excellent catalytic performance, its TEC is significantly larger than that of the electrolyte. In this study, we mechanically mix Sm_0.2Ce_0.8O_2−δ (SDC) with LSC to create a composite cathode. By incorporating 50wt% SDC, the TEC decreases significantly from 18.29 × 10⁻⁶ to 13.90 × 10⁻⁶ K⁻¹. Under thermal-shock conditions ranging from room temperature to 800°C, the growth rate of polarization resistance is only 0.658% per cycle, i.e., merely 49% that of pure LSC. The button cell comprising the LSC-SDC composite cathode operates stably for over 900 h without Sr segregation, with a voltage growth rate of 1.11%/kh. A commercial flat-tube cell (active area: 70 cm²) comprising the LSC-SDC composite cathode delivers 54.8 W at 750°C. The distribution of relaxation-time shows that the non-electrode portion is the main rate-limiting step. This study demonstrates that the LSC-SDC mixture strategy effectively improves the compatibility with the electrolyte while maintaining a high output, thus rendering it a promising commercial cathode material.