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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. 

The preparation of saturated anionic organoborate-based polymers as single-ion-conducting solid electrolytes is described. The weakly basic or absent lone pairs prevent strong binding to lithium ions, limiting affinity for the matrix. 

The single-crystal-to-single-crystal phase transition is determined using X-ray crystallography on LiBF4, resolving a longstanding ambiguity in the existence of a high-temperature polymorph of LiBF4. LiBF4 possesses an endothermic phase change at 28.2 °C with ΔH = 1180 J mol-1 and ΔS = 3.92 J mol-1K-1 based on DSC. Single-crystal X-ray diffraction shows that the low temperature phase collected at 200K is a twinned trigonal P system with a twin law indicating reflection through the 110 plane. The same crystal collected above the phase transition temperature at 313 K is a C-centered orthorhombic system describable as the superposition of the two low-temperature twin geometries undergoing interconversion. The geome-tries of the high- and low-temperature phases are consistent with the calorimetry experiments, and with previous NMR find-ings indicating BF4 geometric reorientations above 300 K. 

Alternative solid-electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft-solid electrolyte (Adpn)2LiPF6 (Adpn = adiponitrile) is synthesized and characterized, which exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A significantly high ion conductivity (~ 10-4 S/cm) and lithium-ion transference number (0.54) are obtained due to weak interactions between “hard” (charge-dense) Li+ ions and “soft” (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li+ ions migrate at the co-crystal grain boundaries with a (preferentially) lower Ea and within the interstitial regions between the co-crystals with higher Ea, where the bulk conductivity comprises a smaller but extant contribution. These cocrystals establish a special concept of crystal design to increase the thermal stability of LiPF6 by separating ions in Adpn solvent matrix, and also exhibit a unique mechanism of ion-conduction via low-resistance grain-boundaries, which is contrasting to ceramics or gel-electrolytes.