Molecular Dynamics Simulations and Experimental Study of Lithium Ion Transport in Dilithium Ethylene Dicarbonate

Abstract

Understanding the properties of the solid electrolyte interphase (SEI) of lithium batteries is important for minimizing interfacial resistance and improving battery safety and cycling. Ion transport has been investigated in the dilithium ethylene dicarbonate (Li2EDC) component of the SEI by impedance spectroscopy and molecular dynamics (MD) simulations employing a revised many-body polarizable APPLE&P force field. The developed force field accurately described the binding energies in LiCH3CO3, its dimer, and Li2EDC calculated at the G4MP2 and MP2 levels. M05-2X and LC-ωPBE functionals predicted too high binding energy in lithium alkyl carbonates compared to the G4MP2 results, while the MP2 and M06-L predictions agreed well with the G4MP2 data. The conductivity of Li2EDC at room temperature was found to be 10–9 S/cm from impedance measurements and extrapolation of MD simulation results. A near Arrhenius temperature dependence of Li2EDC’s conductivity was found in the MD simulations with an activation energy ranging from 64 to 84 kJ/mol. At room temperature, the lithium transport was subdiffusive on time scales shorter than ∼10–2 s in MD simulations corresponding to the onset of the plateau of resistivity vs frequency occurring at frequencies lower than 102 Hz. The influence of Li2EDC ordering on the ion transport was investigated by contrasting supercooled amorphous melts and ordered material. At 393 K Li+ transport was heterogeneous, showing chainlike and looplike Li+ correlated displacements. The non-Gaussianity of Li+ transport was examined. The influence of polarization on the structure of the lithium coordination shell and ion transport has been investigated in the molten phase of Li2EDC and contrasted with the previous results obtained for room-temperature ionic liquids (RTILs). Nonpolarizable Li2EDC exhibited orders of magnitude slower dynamics below 600 K and a higher activation energy for the Li+ diffusion coefficient. Initial simulations of Li2EDC dissolved in an EC:DMC(3:7)/LiPF6 liquid electrolyte were performed at 450 K and showed a strong aggregation of Li2EDC consistent with its phase separation from the electrolyte. The plasticizing effects of carbonate electrolyte on Li2EDC dynamics were examined.