Near-Room-Temperature Transformations in Redox-Active and Superionic Conducting Ion-Plastic Crystals

Ion-plastic crystals (IPCs) belong to a category of solid-state electrolytes. The plastic crystal transition temperature (T_C) and melting point (T_m) are their two crucial parameters as they greatly influence practical usability. Ideally, an IPC should have a low T_C value and a high T_m. However, a paradox arises where decreasing T_C leads to a decrease in T_m and vice versa. This study achieved IPC [DMPip][Ni(mnt)₂] (1; DMPip⁺ = N, N-dimethylpiperidinium, mnt^2- = maleonitriledithiolate). In the crystal structure, the planar electron-delocalized anions form columns via π-orbital interactions and antiferromagnetic (AFM) couplings. The globular shape cations occupy the interstitial spaces of anion columns. This unique arrangement enhances the lattice’s thermal stability, facilitates cation migration, reduces T_C, and increases T_m. Thus, 1 exhibits T_C near room temperature, ΔT = T_m - T_C of over 120 K, and reversible redox activity in the plastic crystal state. 1 represents a novel type of plastic crystal electrolyte with inherent redox activity. These findings hold great potential for driving innovation in the development of all-solid-state redox-enhanced electrochemical devices.