On-going surge in demand for high-energy density rechargeable lithium-ion batteries has simultaneously raised serious concerns on safety failures. Such safety issues become more significant for flexible/wearable power sources because mechanical deformation exerted to cells tends to trigger internal short-circuit failures between electrodes, resulting in cell fire or explosion. From the battery materials point of view, conventional carbonate-based liquid electrolytes are considered a major cause to provoke the cell safety problems. To address this issue, solid-state electrolytes have been extensively investigated as an alternative to liquid electrolytes. Here, we demonstrate a new class of nonflammable plastic crystal polymer electrolytes (PCPEs) with well-balanced electrochemical properties and mechanical deformability as a new material strategy. The plastic crystal electrolytes (PCEs), which are composed of lithium salts and plastic crystals bearing good solvation capability, are characterized with unusual thermal stability and ionic transport behaviors. Succinonitrile (SN, NC-CH₂-CH₂-CN) is a representative organic matrix showing plastic crystal behavior with a plastic crystalline phase in temperature range between around - 40 (a transition temperature from crystalline to plastic crystalline phase) to 60 ^oC (from plastic crystalline phase to melted state). A distinct feature of the PCEs is superior thermal stability (negligibly volatile up to 150 ^oC) and nonflammability. Intrigued by this exceptional thermal characteristic, we develop PCPEs comprising PCEs and polymer matrices. Depending on final applications, a variety of polymers are combined with PCEs, including ultraviolet (UV)-cured polyacrylates, semi-interpenetrating networks (IPNs), and porous nonwoven skeleton. In-depth structural/physicochemical characterization of the PCPEs is conducted and discussed in terms of molecular interaction (and phase separation) between PCEs and polymers. Based on this material understanding, potential application of the PCPEs as a new solid-state electrolyte for safer/flexible lithium-ion batteries is explored by investigating their cell performance, safety, and flexibility. Moreover, in order to ensure wider application versatility of the PCPEs, the electrochemical performance of the cells assembled with the PCPEs are quantitatively examined under harsh operating conditions such as physically deformed state (e.g., wrinkled form), high temperature, and high current densities. The PCPEs presented herein hold great promise as a promising solid-state electrolyte with outstanding thermal tolerance, mechanical deformability, and electrochemical performance which lie far beyond those accessible with conventional liquid electrolytes. (Insert Figure 1.png)

Figure 1. A schematic illustration of UV curing-assisted fabrication process and chemical structure of the PCPE, along with a photograph showing mechanical flexibility of a lithium-ion cell containing the PCPE.