Design Principles for Improved Electrochemical Performance and Stability of Nonaqueous Mg-Electrolytes
Authors: Ilic Stefan , Leon Noel Joseph , Jain Akash , Liao Chen , Assary Rajeev S , Connell Justin G. 
Published: December 2024

Discovery of stable and efficient electrolytes that are compatible with magnesium metal anodes and high-voltage cathodes is crucial to enabling energy storage technologies that can move beyond existing Li-ion systems. Many promising electrolytes for magnesium anodes have been proposed with chloride as a co-anion that helps with reversible plating and stripping, however, Cl-containing electrolytes lack the oxidative stability required by high-voltage cathodes. We have previously demonstrated as a proof-of-principle that an oxygen-containing co-anion with higher oxidative stability (triflate) can be used as a co-anion to replace Cl–, albeit, with higher plating/stripping overpotentials (1). Given that Cl– has a higher tendency to bind to Mg2+ than triflate, in this work we focus on anions beyond halides with higher anion association strength in order to minimize these overpotentials. For this purpose, we employed a theory-guided screening approach to sample a wide chemical space and enable targeted synthesis of new salts. Based on the DFT-predicted anion association strengths, we identified alkoxide-based salts as a new family of co-anions with very favorable binding properties and oxidative stability. In order to probe the applicability of these co-anions in magnesium batteries, we have performed electrochemical evaluation using cyclic voltammetry and galvanostatic stripping and plating. We demonstrate that the addition of computationally-predicted alkoxide co-anions to a weakly coordinating electrolyte containing Mg(TPFA)2 (TPFA = tetrakis(perfluoro-tert-butoxy)aluminate) results in significantly lower stripping overpotentials (up to ~400 mV). We further show that there is a correlation between the stripping overpotential and the anion association strength, where alkoxides that have higher tendencies for Mg binding exhibit lower overpotential. Finally, the applicability of newly developed electrolytes was probed using half-cell battery testing, where we observe improved Coulombic efficiency and lower plating/stripping overpotentials. Collectively, this work outlines design principles for new electrolyte discovery and opens a new class of stable salts for next-generation magnesium batteries.

References:

Ilic; S. N. Lavan; N. J. Leon; H. Liu; A. Jain; B. Key; R. S. Assary; C. Liao; J. G. Connell; Mixed-Anion Contact Ion-Pair Formation Enabling Improved Performance of Halide-Free Mg-Ion Electrolytes, ACS Appl. Mater. Interfaces, 2024 DOI: 10.1021/acsami.3c11293

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