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Not AvailableSoft-solid molecular crystals consist of crystalline grains and fluid grain boundaries (GB) that enhance the grain binding and transport of Li+ ions between the grains. The total ionic conductivity consists of ion migration in both the grains and GBs. To unravel these contributions in adiponitrile (Adpn):LiPF6 molecular crystals, the GB volume fraction was varied by changing the size of the crystals and the Adpn:LiPF6 molar ratio. Molecular dynamic (MD) simulations indicate that ion motion was sub-diffusive in the grains and “well-diffusive” in the GBs, with GBs characterized as disordered nano-confined regions of higher charge carrier concentration (~1M) than in saturated Adpn:LiPF6 solutions (0.04M), and ions predominantly solvated by -C≡N groups with few contact ion pairs. The diffusivity in the GBs is at least an order of magnitude higher than in the crystalline grains. The emergent picture is the grains as a reservoir of ions that migrate to the faster-conducting GBs. 

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.