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 I- conductor. In the stoichiomnetric 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- (Ea = 60 kJ/mol) than for the Li+ (Ea = 93 kJ/mol) ions, and is predominantly an I- conductor, with a lithium-ion transference number (t_Li^+) of t_Li^+ = 0.15, no lithium plating/stripping observed in the cyclic voltammograms (CVs), and a conductivity of σ = 10-4 S/cm at 30 oC. 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, t_Li^+ = 0.63, conductivity increases to σ = 10-3 S/cm 30 0C, the voltage stability window is 4V, and it is thermally stable to 130 o.C, showcasing the potential of this electrolyte for advanced solid-state Li-I battery applications. The solid (Adpn)3LiI minimizes migration of polyiodides, inhibiting the “shuttle” effect.