| Popis: |
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. |
