(Invited) Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling
Authors: Borodin Oleg , Pollard Travis P , Vatamanu Jenel , Wang Chunsheng , Schroeder Marshall , Xu Kang , Ma Lin , Chen Ji 
Published: December 2020

Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling

A molecular scale insight into ion transport and decomposition is important for understanding deficiencies of the currently used aqueous and non-aqueous electrolytes. In this presentation I will summarize progress made towards improving molecular scale understanding of the structure and electrochemistry for a wide range of aqueous and non-aqueous electrolytes. Modeling of non-aqueous electrolytes will focus on the competitive solvent and salt reduction at the passivated electrochemical interfaces using Born Oppenheimer Molecular Dynamics (BOMD) simulations using DFT functionals. These BOMD simulations included critical factors needed to realistically represent electrolyte reactivity at electrodes such as explicit description of the substrate – electrolyte interactions; accurate representation of electrolyte structure, ion pairing and aggregation near an electrode; and collection of sufficient statistics from multiple unique simulations that were initiated with differing initial configurations. For example, 20 BOMD simulations starting from different initial conditions using 2.0M LiPF6 in THF tetrahydrofuran/2-methyl tetrahydrofuran showed no solvent decomposition nor HF formation while only LiF formation was observed as a result of LiPF6 salt decomposition.1 The most frequently observed reduction events included a PF6
− coordinated to Li+ cations from the electrolyte and LiF surface that lead to anion defluorination and formation of 3LiF and PF3 gas. The solvent separated LiPF6 and did not actively participate in reduction. When surface defects in LiF were present near a high population of PF6
− the anions there was a preference for the LiPF6 reduction and repair the SEI without ether solvent decomposition. Interestingly, a number of fast diffusion events for F- from the electrolyte | LiF interface to the LiF-lithium metal interface was observed that would be expected to occur during Li stripping indicating that F- re-arrangement in the thin LiF passivation films should be also considered. 1 Electrolyte reduction at the passivated interfaces from these simulations will be contrasted with other solvents ranging from ethers with mixed salts or carbonates and results from the representative quantum chemistry (QC) calculations performed on the small model electrolyte clusters to estimate oxidation and reduction.
In the second part of the presentation the non-reactive molecular dynamics (MD) using APPLE&P polarizable force field to examine bulk and interfacial properties of the lithium and zinc aqueous electrolytes and electrochemical interfaces with the focus on the transport mechanism and relative contribution to charge flux from the anions and cations.2 Combined together information from these modeling scales suggests numerous strategies for stabilizing the electrolyte – electrode interfaces for numerous aggressive high energy density cathodes combined coupled with graphite and metal anodes.

References

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Borodin, O.; Self, J.; Persson, K. A.; Wang, C.; Xu, K., Uncharted Waters: Super-Concentrated Electrolytes. Joule
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