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(1111)][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) m(2)·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.