(291c) Elucidating Additive Effects on Oligomerization Reaction Mechanisms during Solid-Electrolyte Interphase Formation Using Molecular Simulations

Authors: 
Gibson, L. D., California State Polytechnic University Pomona
Pfaendtner, J., University of Washington
Solid-electrolyte interphase (SEI) formation is one of the major contributors to the loss of capacity over time in lithium-ion batteries (LIBs). SEI forms as the product of reductive decomposition of a battery’s electrolyte, which typically consists of a mixture of ethylene carbonate (EC) and dimethyl carbonate in LIBs. Chemical additives are often included in small amounts to improve the cycling lifetime of LIBs, which are variants of EC such as fluoroethylene carbonate (FEC) and vinylene carbonate (VC). There are many proposed mechanisms for this marked improvement in performance and longevity; however, due to the extremely complex nature of the electrochemical environment at the battery’s electrode-electrolyte interface, the role of chemical additives is still not well understood.

We have used molecular dynamics (MD) and density functional theory (DFT) to study SEI formation with molecular-scale resolution to understand the role of chemical additives, such as FEC and VC, in oligomerization reaction mechanisms. However, chemical reactions are rare events that often occur on time scales much longer than what is typically afforded to MD simulations. To remedy this, we have used parallel-bias metadynamics (PBMetaD) to bias generic descriptors of relative connectivity, also known as SPRINT coordinates, of all atoms to enhance the rate at which chemical reactions occur.[1,2] This method of using PBMetaD+SPRINT has been shown to be an effective tool for reaction network exploration with no need for chemical intuition.[3] We explored the reaction networks for 3 model electrolyte systems. Each system contained 3 EC molecules, a Li+ ion, an extra electron, and either a FEC, VC, or fourth EC molecule. From these reaction networks, we further investigated key pathways that lead to oligomerization reactions with DFT calculations.

These new pathways involve only a singly-reduced, ring-opened EC and a neighboring EC, VC, or FEC. In addition to this pathway, we also analyzed an analogous reaction mechanism proposed by Burkhardt [4] in which ethoxide is the attacking species. In the case of an attacking ring-opened EC, the VC-containing system showed a net change in free energy of -5.5 kcal/mol relative to the starting configuration. In contrast, the pure EC exhibited a net change in free energy of approximately +11.3 kcal/mol. In the case of ethoxide as the attacking species, both FEC and VC displayed relative changes in free energy of -2.2 and -16.5 kcal/mol, respectively. However, the EC mechanism only displayed a -1.3 kcal/mol relative change in free energy. These results highlight the ability of electrolyte additives to facilitate oligomerization pathways during electrolyte degradation, possibly leading to improved SEI stability.

References:

[1] J. Pfaendtner, M. Bonomi, Efficient Sampling of High-Dimensional Free-Energy Landscapes with Parallel Bias Metadynamics, J. Chem. Theory Comput. 11 (2015) 5062–5067. doi:10.1021/acs.jctc.5b00846.

[2] F. Pietrucci, W. Andreoni, Graph theory meets ab initio molecular dynamics: Atomic structures and transformations at the nanoscale, Phys. Rev. Lett. 107 (2011) 1–4. doi:10.1103/PhysRevLett.107.085504.

[3] C.D. Fu, J. Pfaendtner, Lifting the Curse of Dimensionality on Enhanced Sampling of Reaction Networks with Parallel Bias Metadynamics, J. Chem. Theory Comput. 14 (2018) 2516–2525. doi:10.1021/acs.jctc.7b01289.

[4] S.E. Burkhardt, Impact of Chemical Follow-up Reactions for Lithium Ion Electrolytes: Generation of Nucleophilic Species, Solid Electrolyte Interphase, and Gas Formation, J. Electrochem. Soc. 164 (2017) A684–A690. doi:10.1149/2.0621704jes.