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Abstract
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.
Supplementary materials
Title
Supporting Information
Description
Powder X-ray diffraction, data, TGA, SEM, DC polarization and cycling data, additional details of simulation and DFT, and crystallographic tables. Crystallographic data is also available from the CCDC under deposition number 1986269.
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Title
MD simulation of model V8g (eight grains of the cocrystals simulated in vacuum
Description
To determine the structure of the intergranular interface, model V8g (surface model) with eight nano-sized grains (1 grain = 5x5x5 unit cells) was simulated in a vacuum box of 30x30x30 nm3 (Figure 3a). The simulation conditions of model V8g were very similar to model V (e.g., NVT ensemble, presence of large evacuated space). At t = 0, it was ensured that every grain was completely isolated from the others (initial contact distance between the grains > cut-off distances for potential energy). The simulations show that within a span of a few nanoseconds, the grains interact at the surface and form a more mobile interfacial layer.
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Title
Mechanism of ion conduction in bulk phase from NEB-DFT calculations
Description
Minimum energy path for Li+ ion migration in the bulk, in b- crystallographic direction, as observed from a 1x2x1 supercell of (Adpn)2LiPF6: The geometries of initial, final and intermediate structures are shown above. The location for migrating Li+ ion can be seen in the highlighted spot. A dynamic visualization of the Li+ ion migration is presented.
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