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 ᵒC, the voltage stability window is 4V, and it is thermally stable to 130 ᵒC, 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.