<p>This study demonstrates that the compressive residual stress generated during sputter deposition of thin films can be rapidly relieved by a flash-light irradiation process, leading to enhanced electrochemical performance. In general, thin films deposited by physical vapor deposition inherently contain residual stresses, and compressive stress in the electrolyte layer has been considered one of the origins of increased ohmic resistance because it degrades ionic conductivity. Conventionally, long-term high-temperature annealing has been required to remove this stress. In this work, stress relaxation was achieved on the millisecond timescale using a light-irradiation technique instead of such prolonged thermal treatment, thereby eliminating the effects of surface morphology changes associated with long-term annealing and allowing the performance enhancement to be attributed solely to changes in the internal stress state of the thin film. When an electrolyte subjected to stress relaxation by light irradiation was used, a high peak power density of 1.81 W/cm² at 750&#xa0;°C was achieved.</p> Graphical Abstract <p>This paper introduces a novel process that uses flash-light irradiation to rapidly relieve internal stress in thin-film electrolytes, thereby enhancing their electrochemical performance.</p> <p></p>