Molecular Insights into the Protic Organic Ionic Plastic Crystal (POIPC): Effect of Dopants and Vacancies

In recent years, significant research effort has been dedicated to the development of robust, high-performance solid electrolyte batteries. A wide range of materials, including polymers, ceramics, and glasses, etc., have been tested and characterized as potential solid electrolyte candidates. In this respect, organic ionic plastic crystals (OIPCs) have attracted significant research attention owing to their promising properties. However, the OIPCs from different crystal classes, composed of distinct cations and anions, may display widely different phase behaviors and properties. Herein, following our previous work on pure POIPC, we conduct extensive molecular dynamics simulations, in conjunction with experimental methods, to investigate the effect of dopants and vacancies on the structural, thermodynamic, and dynamical properties of POIPC [DBUH]-[FSI], consisting of the protic [DBUH]⁺ cation with labile proton (H⁺) on the nitrogen atom. Simulations have been performed for the LiFSI doping fractions of 1.31%, 2.52%, 5.39%, and 10.55%. Additionally, the effect of Schottky vacancies was examined at concentrations of 0.23% and 0.46% for the 1.31% and 2.52% doped systems. The simulated solid-solid and the solid-liquid phase transition temperatures are in very good agreement with the DSC data. Our findings indicate that the effect of the vacancy is more pronounced at the lower doping levels. Hydrogen bonding analysis reveals that the protic N2-H1 site forms the strongest hydrogen bond in the system, with N2-H1--O being the dominant hydrogen bond. Using rotational autocorrelation functions (RACFs), we identify distinct solid phases associated with different rotational modes in the systems, confirming the presence of at least three distinct solid phases across the doping range. Translational dynamics analysis affirms that the [FSI] anion is the most mobile component of the system. The activation energy (E _a) computed using the Arrhenius plot, is in considerable agreement with the experimental data. The lithium cation transference number (t ^Li+ ) agrees very well with the experimental analysis.