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Abstract Insights into structure‐conductivity mechanisms are investigated for a series of six (dinitrile)₂LiPF₆ molecular crystals with varied alkyl chain lengths, N≡C─(CH₂)_n─C≡N, n  = 2, 3, 4, 5, 6, and 2Me‐glutaronitrile. The molecular crystals have separate Li⁺ and channels, with the Li⁺ions weakly coordinated by four ─C≡N groups. The following correlations are observed: i) shorter Li⁺⋯ Li⁺ hopping distances (5.72–8.08 Å) increase ionic conductivity (3.1 × 10⁻⁴–0.15 × 10⁻⁴ S cm⁻¹ at 25 °C) for all (dinitrile)₂LiPF₆; ii) when there are unrestricted anion channels, the lithium ion transference number increases ( = 0.39–0.62) as the void volume (565–250 ų) and Li⁺⋯ Li⁺ hopping distance (7.15–5.72 Å) decrease, since a greater fraction of the charge is contributed by the Li⁺ions; this correlates with n= 2, 4, 5, 6; iii) the exceptions are Gln (n  = 3) and 2Me‐Gln, where there are restricted channels for anion migration, and in this case: iv) conductivity decreases (0.57–0.15 × 10⁻⁴ S cm⁻¹ at 25 °C), since contributions to the conductivity from anion migration decrease, but v) increases (0.64–0.7) since a greater fraction of the charge is carried by the Li⁺ ions. 

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.