«Structure–property» correlations and rational design of organic ionic plastic crystals

Organic ionic plastic crystals (OIPCs) are promising solid electrolyte materials due to their high ionic conductivity, intrinsic plasticity, and thermal stability. However, the absence of universal structure-property relationships has hindered their targeted design. This work analyzes the thermal, crystallochemical, and transport properties of 49 OIPCs from diverse structural families, including tetraalkylammonium, tetraalkylphosphonium, pyrrolidinium, oxazolidinium, and guanidinium salts. Strong correlations (rPearson up to –0.79) were identified between ionic conductivity and key parameters: the enthalpy ( ) and entropy ( ) of melting, and the anion-to-cation volume ratio. The highest conductivities are achieved in compounds with low and an optimal volume ratio of ~0.2–0.3. Empirical equations for predicting conductivity were developed based on these correlations. The predictive power of this model was experimentally validated by synthesizing and characterizing a novel OIPC, N-methyl-N-ethylmorpholinium tetrafluoroborate ([C₂mmor][BF₄]). Its experimental conductivity (log(σ, S∙cm−1) = −4.15 at 100 °C) aligns closely with predictions. These findings provide a robust framework for the accelerated discovery of high-performance OIPCs for advanced electrochemical devices.