Structural Color Polymer‐Based Colorimetric Sensor for Detecting Mechanical Stress in Lithium Batteries Separator

Abstract Deformation of lithium batteries is known as a hazardous problem possibly leading to a fire or fuming, while deformation sensors are not attached to the batteries. The deformation comes from complex phenomena inside the batteries, resulting in difficult deformation estimation. To display the battery deterioration status more easily, a CSDS (colorimetric stress‐detectable separator) is presented that is a separator sensor made of a structural color polymer, enabling to display of the mechanical stress with colors. The structural color polymer consists of aligned silica nanoparticles and PEGPEA (Poly (ethylene glycol) phenyl ether acrylate) acrylic polymer, displaying a specific wavelength depending on the particle gaps. To observe the CSDS color, an observation window is opened to an electrode of the battery. The CSDS showed that there are two stages for gas expansion by displaying a color change from green (≈540 nm) to red (≈580 nm). The separator also displayed blue near the edge of the observation hole, resulting from growing crystals. Those results showed that the separator sensor will contribute to an improvement in battery safety by an easy check from outside the battery packs.