Ionic Switches with Positive Temperature Coefficient Enabled by Phase Separation within Hydrogel Electrolytes

Ionic switches with a positive temperature coefficient (PTC) effect are highly desirable in the fabrication of smart electrolytes for the safety protection of electrochemical energy devices. However, most of them encounter liquid leaking or volume shrinking problems, limiting their long-term and stable operations. Herein, a PTC-type ionic switch is introduced based on a poly(acrylic acid) (PAA) hydrogel soaked by calcium acetate (CaAc), with a resistance change of six times in maximum between the homogeneous and phase separated state. The PTC effect is owing to the strong phase separation upon heating where the ion transport is restricted. Such a hydrogel-based PTC-type ionic switch is in the solid state and isochoric during phase separation without leaking or shrinking issues. The influence of different CaAc soaking concentrations is investigated. A simplified model consisting of interconnected ion channels is proposed based on microstructure analysis. A smart supercapacitor is successfully demonstrated by this PTC ionic switch with a safety protection ability. The research here would provide a new pathway for the design and development of PTC-type ionic switches in the safety protection of electrochemical energy storage devices.