Molecular Insights into the Structure and Dynamics of Protic Organic Ionic Plastic Crystal (OIPC) 1,7-Diazabicyclo[5.4.0]undec-7-ene ([DBUH])–bis(fluorosulfonyl)imide ([FSI])

In recent years, organic ionic plastic crystals (OIPCs) have emerged as promising materials for various applications due to their unique properties, such as high ionic conductivity and high melting temperatures. The macroscopic properties of OIPCs are closely related to their molecular structure, and a deep understanding of the molecular level behavior and the associated thermo-physical properties is necessary to determine their potential application as solid electrolytes for all-solid-state batteries (ASSBs). It was shown that both [DBUH] and [FSI]-based OIPCs behave as good ionic conductors. Recently, protic OIPC (POIPC), [DBUH][FSI] ((10aR)-decahydropyrimido[1, 2-a]azepine <i>N</i>-fluorosulfonylsulfamoyl fluoride), has been synthesized and successfully tested as a solid electrolyte for ASSBs. In this work, the various structural features and dynamical properties of [DBUH][FSI] have been explored in detail by carrying out molecular dynamics simulations for the first time. The initial configuration is generated using the experimental crystalline structure from Hydro-Quebec. The temperature evolution of the various structural and dynamic properties has been studied. The simulated density, melting point, hydrogen bond distances, and angles are in excellent agreement with the experimental data. The effect of hydrogen bond strength, interactions, and structural correlation on the ion's mobility has been investigated. The mean square displacement and rotational autocorrelation functions reveal the greater dynamics of the anion compared to that of the cation, specifically at low temperatures. We find that the higher mobility of the anion can impact the dynamical properties of [DBUH]-based POIPCs at low temperatures.