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

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.