A Soft-Solid Co-Crystalline Electrolyte Combining Advantages of Organics and Ceramics: Thermally, Electrochemically Stable, Highly Conductive (Adiponitrile)2LiPF6

The solid electrolyte (ADN)2LiPF6 is described. The structure exhibits adiponitrile (ADN)-based channels based on XRD analysis, through which the -C≡N-solvated Li+ ions migrate. High conductivity (σ ~ 10-4 S/cm) and high lithium ion transference number (tLi+ = 0.54) results 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 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. (ADN)2LiPF6(s) has a wide electrochemical stability window of 0 to 5 V. Li0/(ADN)2LiPF6/LiFePO4 half 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 efficiencies > 95%. MD simulations and PWDFT calculations offer insights into the molecular basis of the physical and conductivity properties.