(Invited) Insights into Solvation, Dynamics and Stability of Electrolytes for the Design of Novel Multivalent Systems from Coupled Molecular Dynamics and First-Principles Modeling

Within the Joint Center for Energy Storage (JCESR) we have developed a high-throughput infrastructure for the automated calculation of molecular properties with a focus on battery electrolytes. Computational screening techniques have been found to be an effective alternative to the trial and error of experimentation for discovery of new materials. Using this infrastructure, we have computed the ionization potential (IP) and electron affinities (EA) of 16,000 molecules relevant to electrolytes targeting both multivalent intercalation as well as non-aqueous redox flow systems. For some potential multi-valent electrolytes we use classical molecular dynamics simulations coupled with ab initio calculations for Mg salts in various solvents. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode¹. We elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics.

Specifically, we find that multivalent electrolytes are highly prone to ion pair formation, even at modest concentrations, for a wide range of solvents with different dielectric constants, which have implications for dynamics as well as charge transfer. For example, at Mg metal potentials, the ion pair undergoes partial reduction at the Mg cation center (Mg²⁺→Mg⁺), which competes with the charge transfer mechanism and can activate the anion to render it susceptible to decomposition. Specifically, TFSI⁻ exhibits a significant bond weakening while paired with the transient, partially reduced Mg⁺. This information provides critical design metrics for future electrolytes as it elucidates a close connection between bulk solvation and cathodic stability as well as the dynamics of the salt. We use our previous insights to design for new Mg electrolyte anions with the aim to stabilize the electrolyte during possible ion pair formation and charge transfer reactions. Through comprehensive calculations using both first-principles as well as well-benchmarked classical molecular dynamics, we proposed 16 new electrolyte salts. Based on successive property evaluation such as electrochemical stability window, chemical decomposition and hydrolysis vulnerability we down-select 4 potential new anions for Mg electrolytes.

References

[1] N. N. Rajput, X. Qu, N. Sa, A. K. Burrell, K.A. Persson, “The Coupling between Stability and Ion Pair Formation in Magnesium Electrolytes from First-Principles Quantum Mechanics and Classical Molecular Dynamics”, Journal of the American Chemical Society, 2015.

[2] X. Qu, A. Jain, N. N. Rajput,Y.Zhang, S.P.Png, M. Brafman, L.Cheng, E. Maginn, L.A. Curtiss, K.A. Persson, “Electrolyte Genome Project: A Big Data Approach in Battery Materials Discovery”, Computational Materials Science, 2015.