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 (TC) and melting point (Tm) are their two crucial parameters as they greatly influence practical usability. Ideally, an IPC should have a low TC value and a high Tm. However, a paradox arises where decreasing TC leads to a decrease in Tm and vice versa. This study achieved IPC [DMPip][Ni(mnt)2] (1; DMPip+ = N, N-dimethylpiperidinium, mnt2– = 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 TC, and increases Tm. Thus, 1 exhibits TC near room temperature, ΔT = Tm – TC 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.