Grain boundary tuning determines iodide and lithium-ion migration in a solid adiponitrile-LiI molecular crystal electrolyte.

This work presents the synthesis of a molecular crystal of adiponitrile (Adpn) and LiI via a simple melting method. The molecular crystal has both Li+ and I- channels and can be either a Li+ or an I- conductor. In the stoichiometric crystal (Adpn)2LiI, the Li+ ions interact only with four CN groups of Adpn, while the I- ions are uncoordinated. Ab initio calculations indicate that the activation energy for ion hopping is less for the I- ions (Ea = 60 kJ mol-1) than that for the Li+ ions (Ea = 93 kJ mol-1), and this crystal is predominantly an I- conductor, with a lithium-ion transference number (tLi+) of tLi+ = 0.15; no lithium plating/stripping is observed in the cyclic voltammograms (CVs), with a conductivity of σ = 10-4 S cm-1 at 30 °C. With the addition of excess adiponitrile, which resides in the grain boundaries between the crystal grains, the contribution of Li+ ions to the conductivity increases, so that for the nonstoichiometric molecular crystal (Adpn)3LiI, Li ↔ Li+ redox reactions are observed in the CVs, tLi+ = 0.63, conductivity increases to σ = 10-3 S cm-1 at 30 °C, and the voltage stability window is 4 V, and it is thermally stable up to 130 °C, showcasing the potential of this electrolyte for advanced solid-state Li-I battery applications. The solid (Adpn)3LiI electrolyte minimizes the migration of polyiodides, inhibiting the "shuttle" effect.