Critical Role of Li + ··· Li + Distance and Anion Restriction in Conductivity and Lithium‐Ion Transference Number of Molecular Crystals for Solid State Electrolytes

Abstract Insights into structure‐conductivity mechanisms are investigated for a series of six (dinitrile) 2 LiPF 6 molecular crystals with varied alkyl chain lengths, N≡C─(CH 2 ) 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) 2 LiPF 6 ; ii) when there are unrestricted anion channels, the lithium ion transference number increases ( = 0.39–0.62) as the void volume (565–250 Å 3 ) 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.