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Kristin Persson1 Trevor Seguin1

1, University of California, Berkeley, Berkeley, California, United States


Transforming transportation and the electricity grid with high performance, low cost energy storage requires development of beyond Li-ion technology and innovations in electrodes and electrolytes, alike. Proposed technologies such as multivalent systems (e.g., Mg2+, Ca2+ and Zn2+) have attracted increased interest. While these systems are inherently quite different, many of them suffer from lack of suitable electrolytes. To address the need for novel and optimized electrolytes, an automatic high-throughput computational infrastructure has been constructed and coupled with experimental analysis. Here we present our multi-scale modelling approach for understanding the the solvation structures, the stability, the conductivity and other relevant properties relevant for multivalent energy storage. We uncover a novel effect between concentration dependent ion pair formation and anion stability at reducing potential, e.g., at the metal anode. We elucidate systematic correlations between molecular level interactions and composite electrolyte properties, such as electrochemical stability, solvation structure, and dynamics. 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. In particular, 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 instability is contrasted to the behavior in Zn electrolytes, where the origin of anodic stability for a range of nonaqueous zinc electrolytes is traced to the solvent. Finally, we apply our insights to the design of novel salts and demonstrate realized synthesis and electrochemical validation.

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