Effect of Cation Size and Symmetry in Substituted Ammonium Tetrafluoroborates on Their Thermal, Structural, and Transport Properties

In recent years, organic ionic plastic crystals (OIPCs) have been considered as a promising class of solid electrolytes due to their exceptional thermal and electrochemical stability, as well as their plasticity. However, the relationship between their chemical structures and physicochemical properties remains poorly understood. This study investigates how thermal properties and ionic conductivity correlate with cation size and symmetry in two series of substituted ammonium tetrafluoroborates: (C_nH_2n+1)₄NBF₄ (n = 1–6) and (C₄H₉)_4–y(CH₃)_yNBF₄ (y = 0–2). Combined thermal and X-ray diffraction analyses reveal that low-symmetry cations with extended alkyl chains (e.g., (C₄H₉)₂(CH₃)₂NBF₄) exhibit the widest plastic phase range (19–128 °C) and highest ionic conductivity (3.5 × 10^–7 S/cm at 100 °C). In contrast, symmetrical cations show size-dependent conductivity, peaking at 4.0 × 10^–8 S/cm at 100 °C for (C₄H₉)₄NBF₄. Both candidates demonstrate electrochemical stability windows of 4.7–5.2 V. These findings establish clear design principles for optimizing OIPCs, enabling the targeted synthesis of materials with enhanced physicochemical properties.