Ammonium-Based Plastic Crystals as Solid-State Electrolytes for Lithium and Sodium Batteries

Organic ionic plastic crystals (OIPCs) are a promising class of solid materials composed of organic cations and inorganic anions, increasingly explored for use as solid-state electrolytes (SSEs). These materials offer a safer alternative to conventional carbonate-based electrolytes in lithium and sodium ion batteries. In this study, lithium and sodium salts were incorporated into tetramethylammonium bis(fluorosulfonyl)imide ([N₁₁₁₁][FSI]), yielding solid state electrolytes with notable properties, including high ionic conductivities (1.79 mS·cm^-1 for LiFSI doped and 3.2 mS·cm^-1 NaFSI doped, both at 80 °C), elevated diffusion coefficients (up to 3.83 × 10^-11 m²·s^-1 for Li⁺ at 80 °C), and high transference numbers (0.8 and 0.4, for Li and Na, respectively). To date, except for ceramic and glassy ion conductors, there has been no significant research demonstrating true solid-state behavior with Li⁺ or Na⁺ ion transport fully decoupled from the motion of the host structure. Furthermore, these electrolytes have exhibited impressive current densities up to 3.5 mA·cm^-2 during Li|Li and 2.9 mA·cm^-2 for Na|Na symmetric cell cycling at room temperature. As a result, these materials hold considerable potential for enhancing both Li and Na electrochemical energy storage technologies, combining both improved efficiency and safety features.