Abstract: Minimizing the thermal expansion coefficient (TEC) mismatch between the cathode and electrolyte in solid oxide fuel cells is crucial for achieving stable, durable operation and high performance. Recently, materials with negative thermal expansion (NTE) have attracted significant attention as effective additives for tailoring the thermomechanical properties of electrodes and enhancing cell durability. In this work, for the first time, single-phase NTE perovskite Sm0.85Zn0.15MnO3-δ (SZM15) was successfully synthesized via the sol-gel method, eliminating the unwanted ZnO phase typically observed in materials obtained through the conventional solid-state reaction route. The sol-gel approach proved highly advantageous, offering low cost, robustness, excellent chemical homogeneity, precise compositional control, and high phase purity. After optimization of synthesis parameters, a negative TEC of approximately −6.5×10−6 K−1 was achieved in the 400-850°C range. SZM15 was then incorporated as an additive (10–50 wt.%) into a SmBa0.5Sr0.5CoCuO5+δ (SBSCCO) cathode to tune the thermomechanical properties with a La0.8Sr0.2Ga0.8Mg0.2O3−δ (LSGM) electrolyte, achieving a minimal TEC mismatch of only 1%. Notably, the SBSCCO + 10 wt.% SZM15 composite cathode exhibited the lowest polarization resistance of 0.019 Ω·cm2 at 900°C, showing approximately 70% lower than that of the pristine cathode. Excellent long-term stability after 100 hours of operation was achieved. In addition, a high peak power density of 680 mW·cm−2 was achieved in a Ni-YSZ|YSZ|GDC10|SBSCCO+10 wt.% SZM15 anode-supported fuel cell at 850°C, highlighting the effectiveness of incorporating NTE materials as a promising strategy for regulating the thermomechanical properties and improving the long-term stability of IT-SOFCs.