In this study, composite pouch films incorporating ionite were fabricated, and their structural properties as well as temperature variations during charge–discharge cycles were evaluated to examine their applicability as heat-suppression pouch films for secondary batteries. The films were prepared using a film coater (Coretech, CT-AF300), with variations in ionite content and particle size. In addition, the effects of plasma treatment on the surface state of PET films were investigated to enhance coating adhesion, with the aim of determining the optimal fabrication conditions. Furthermore, an infrared thermal imaging camera and a custom-built test device were employed to measure the temperature differences with and without the pouch films during charge– discharge processes, thereby assessing the potential of developing next-generation high-performance pouch films.