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) 2 LiPF 6, (Gln) 2 CuPF 6, (DAB) 2 LiPF 6, and (DAB) 2 CuPF 6 (Gln = glutaronitrile, DAB = 1,4-diaminobutane) with isomorphic crystal structures ( P 4̅2 1 c 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 -NH 2 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.