A typical liquid electrolyte for lithium (Li)-ion and Li-metal batteries is a liquid mixture with salt, solvent, and additives. The reduction products of the electrolytes form the solid-electrolyte interphase (SEI) layer, whose composition, heterogeneity and thickness largely determine the Li plating and stripping processes and thus the cycling performance. It is highly desirable to obtain atomistic insights into the reduction reactions of the electrolytes to help predict and control the formation of the SEI layer. The reduction reactions of the liquid electrolyte occur near the charged electrode surface within the electric double layer (EDL). The electrolyte structures within EDL must change under an electric field, which impacts the electrolyte reduction and SEI formation. This has been rarely considered in previous theoretical investigations of the reduction potentials and reduction products.

In this work, we have developed an interactive model combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations to predict the multi-component electrolyte reduction within the EDL.[1] First, MD simulations, which can deal with thousands of atoms and even more, are used to capture the dynamics and statistics of the EDL structures. Then DFT calculations are used to compute the reduction products and corresponding reduction potentials of the representative clusters in the EDL. Finally, a formulation was proposed to calculate averaged reduction potential based on DFT-calculated reduction potentials of all the possible Li⁺-coordinated structures and their distribution probabilities obtained through MD analyses.

We have applied this new interactive MD-DFT model to two types of essential multi-component electrolytes, the carbonate-based electrolyte (LiPF₆@EC:EMC) and the ether-based electrolyte (LITFSI@DOL:DME). We find in both electrolytes the Li⁺ ions within the EDL tend to have less coordination with other electrolyte species (salt anion, solvent and additive) compared to those in the bulk electrolyte. For LiPF₆@EC:EMC, we reveal that FEC can enter into the EDL region and is favorable to be reduced, which helps form the F-containing SEI component (e.g., LiF). In addition, we have studied the effects of the FEC additive on LiTFSI@DOL:DME at various temperatures. We find that, at -40 °C, the number of F atoms that come near the electrode is largely increased after adding FEC, while no big change is observed at 20 °C. This explains that adding FEC is more effective in forming stable F-containing SEI and in promoting cycling performance at low temperatures.[2] Our simulations have also explained why ether based electrolyte works better than carbonate base electrolyte for Li metal anode.

Reference:

[1] Qisheng Wu, Yue Qi. Manuscript under preparation, 2022.

[2] Akila C. Thenuwara, et al. ACS Energy Lett. 2020, 5, 2411−2420