Interphase-driven ion conduction in organic ionic plastic crystal-based solid electrolytes: A review of symmetric cell studies

Solid-state batteries (SSBs) have great promise to address ever-growing demands for energy-storage technologies in our society, but their widespread adoption faces various challenges stemming from solid electrolytes (e.g., low bulk ionic conductivities) and their interfaces (e.g., poor electrode/electrolyte contacts). To tackle these issues, soft solid electrolytes have been developed. Among them, organic ionic plastic crystals (OIPCs) have been comparatively less studied, but OIPCs and their composites would be breakthrough materials in SSBs as they usually show complex ion-conduction behaviors that involve interphases often considered as liquid or disordered phases. This chapter outlines ion-conduction behaviors for the 10 categories of OIPC electrolytes (8 for Li+ systems and 2 for Na+ systems) to aid in understanding the effect of OIPC interphases on ion-transport properties. The pioneering demonstrations of OIPC electrolytes as solid-electrolyte membranes in solid-state symmetric cells are also reviewed, and their electrochemical performance is deciphered based on the ion-conduction behaviors of the OIPC electrolytes.