Diadiponitrilelithium hexafluorophosphate: a soft-Solid Co-Crystalline Electrolyte Combining Advantages of Organic and Ceramic Electrolytes

Soft solid electrolyte materials are promising alternative choices for conventional battery electrolytes. Here, we have synthesized, characterized and calculated structural, thermal and electrochemical properties of an adiponitrile-based lithium-ion electrolyte which combines the advantages of organic and ceramic materials. This solid material is (Adpn)2LiPF6, (Adpn = adiponitrile) wherein (Adpn)-based channels solvate Li+ ions through weak C≡N---Li+ contacts. The surface of the crystal is a liquid nanolayer that binds the grains so that ionically conductive pellets are easily formed without high pressure/temperature treatments, which self-heals if fractured and which provide liquid-like conduction paths through the grain boundaries. High conductivity (σ ~ 10-4 S/cm) and high lithium-ion transference number (tLi+ = 0.54) result from weak interactions between “hard” (charge-dense) Li+ ions and “soft” (electronically polarizable) - C≡N, compared with the stronger interactions of previously reported “hard” ether oxygen contacts of polyethylene oxide (PEO) or glymes. The proposed mechanism of conduction is one in which Li+ ion migration occurs preferentially along the low activation energy path at the co-crystal grain boundaries and within the interstitial regions between the co-crystals, with bulk conductivity comprising a smaller but extant contribution to the observed conductivity. (Adpn)2LiPF6(s) has a wide electrochemical stability window of 0 to 5 V. Li0/(Adpn)2LiPF6/LiFePO4 cells exhibit cycling for > 50 cycles at C/20, C/10, C/5 rates with capacities of 140 mAh-g-1 to 100 mAh-g-1 and Coulombic efficiencies ~ 99%, and mitigation of the deleterious reactions with Li metal due to the high ionic strength. LTO/(Adpn)2LiPF6/NMC622 full cells were cycled at C-rates of C/20 to 1C with Coulombic efficiencies > 96%, with no dendritic failure after 100 cycles. Novel MD approaches addressing multiple conduction pathways and PWDFT calculations offer insights into the molecular basis of the physical and conductivity properties.