High Throughput Approach to Accelerate Electrolyte Discovery for Batteries

Increasing demand of high energy density and high capacity batteries require development beyond Li-ion technology and innovations in electrodes and electrolytes, alike. An approach to achieve energy density higher than the existing Li-ion batteries is to replace monovalent Li ion by multivalent ions such as Mg, Ca. The ability of Mg batteries to provide much higher volumetric capacity, particularly on the anode side where the Mg metal can theoretically provide 3833 mA h/cc as compared to the Li counterpart graphite (∼ 800 mA h/cc) at a lower cost makes the technology an attractive candidate for future batteries. However, development and commercialization of Mg batteries require not only improved electrode discovery and development but also novel electrolytes, which are compatible with the Mg metal as conventional electrolytes fail to penetrate the Mg metal passivation layer. Thus a fundamental understanding of molecular level properties of these electrolytes is required to improve the electrochemical stability and the charge transfer properties.  An automatic High-throughput infrastructure has been constructed for the electrolyte genome project supported by the US Joint Center for Energy Storage Research (JCESR)². In this work, we present 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. We find that Mg 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. Specifically, we observe that, 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⁺. Furthermore, we observe that higher order glymes as well as DMSO improve the solubility of Mg salts, but only the longer glyme chains reduce the dynamics of the ions in solution. 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.

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.