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 ([N][FSI]), yielding solid state electrolytes with notable properties, including high ionic conductivities (1.79 mS·cm for LiFSI doped and 3.2 mS·cm NaFSI doped, both at 80 °C), elevated diffusion coefficients (up to 3.83 × 10 m·s 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 during Li|Li and 2.9 mA·cm 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.