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

Abstract

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 CN 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 (E_a = 60 kJ mol⁻¹) than that for the Li⁺ ions (E_a = 93 kJ mol⁻¹), and this crystal is predominantly an I⁻ conductor, with a lithium-ion transference number (t_Li⁺) of t_Li⁺ = 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, t_Li⁺ = 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.