(420e) Computing Electron Transfer Rates for Ethylene Carbonate Reduction with Marcus Theory Using Molecular Simulations

The formation of the solid-electrolyte interphase (SEI) in lithium-ion batteries (LIBs) is a phenomenon that is still not well understood mechanistically, despite the large volume of research investigating its properties and growth mechanisms. It is known, however, that this layer is formed via the reductive decomposition of the liquid electrolyte solvent, which typically consists of ethylene carbonate (EC), dimethyl carbonate, ethyl methyl carbonate, etc., as well as small quantities of additives. During charging, these species become reduced at the anode interface and later decompose, ultimately forming the SEI layer.^1,2 Many studies have investigated the decomposition pathways of these reduced species; however, there is limited work focusing on the first step of the growth process: electron transfer (ET) to the electrolyte solvent.

We combine molecular simulations and Marcus theory formalism to compute rate of ET from a graphitic anode to interfacial EC molecules in the condensed phase.³ This talk demonstrates how to construct the necessary Marcus parabolas from which the reorganization energy, free energy barrier, and free energy of reaction are defined. Further, we also show the configuration dependence on electronic coupling strength between our model graphitic anode and interfacial EC by computing couplings from an ensemble of interfacial structures.

From our constructed Marcus parabolas, we see a large reorganization energy associated with EC reduction, both inner-sphere (internal configurational change) and outer-sphere (solvent response to charge transfer), which implies that both types of fluctuations are important for computing the rate of ET. Lastly, we show how computing the free energy of reaction via changes in intrinsic solvation energy⁴ allows for the easy comparison of multiple systems.

References:

(1) Peled, E. The Electrochemical Behavior of Alkali and Alkaline Earth Metals in Nonaqueous Battery Systems—The Solid Electrolyte Interphase Model. J. Electrochem. Soc. 1979, 126, 2047.

(2) Aurbach, D.; Markovsky, B.; Shechter, A.; Ein-Eli, Y. A Comparative Study of Synthetic Graphite and Li Electrodes in Electrolyte Solutions Based on Ethylene Carbonate-Dimethyl Carbonate Mixtures. J. Electrochem. Soc. 1996, 143, 3809–3820.

(3) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Biochim. Biophys. Acta 1985, 811, 265–322.

(4) Duignan, T. T.; Baer, M. D.; Schenter, G. K.; Mundy, C. J. Electrostatic Solvation Free Energies of Charged Hard Spheres Using Molecular Dynamics with Density Functional Theory Interactions. J. Chem. Phys. 2017, 147, 161716.