Hard–Soft Acid–Base Interactions Control Ionic Conductivity in Molecular-Crystal-Based Electrolytes

The influence of hard and soft base coordination on ionic conductivity was systematically investigated using diamine (hard base)- and dinitrile (soft base)-based ligands in molecular crystal-based solid electrolytes. For molecular crystals (Gln)2LiPF6, (Gln)2CuPF6, (DAB)2LiPF6, and (DAB)2CuPF6 (Gln = glutaronitrile, DAB = 1,4-diaminobutane) with isomorphic crystal structures (P4̅21c space group) and comparable metal–metal (Li+–Li+ or Cu+–Cu+) distances, less favorable soft–hard interactions (Cu+–amine or Li+–nitrile) resulted in nearly 2 orders of magnitude improvement in ionic conductivity compared to the more favorable soft–soft and hard–hard pairings (Cu+–nitrile or Li+–amine). This significant enhancement is attributed to the weaker, more labile coordination between soft nitrile donors and hard Li+ ions or hard -NH2 donors and soft Cu+, facilitating faster ion migration, reinforcing the critical role of Hard–Soft Acid–Base theory (HSAB)-guided coordination chemistry in modulating ion mobility in soft-solid molecular electrolytes, and providing valuable insights to rationally design high-performance solid electrolytes.