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Mg0 is commonly used as a sacrificial anode in reductive electrosynthesis. While numerous methodologies using a Mg sacrificial anode have been successfully developed, the optimization of the electrochemistry at the anode, i.e., Mg stripping, remains empirical. In practice, electrolytes and organic substrates often passivate the Mg electrode surface, which leads to high overall cell potential causing poor energy efficiency and limiting reaction scale-up. In this study, we seek to understand and manipulate the Mg metal interfaces for a more effective counter electrode in tetrahydrofuran. Our results suggest that the ionic interactions between the cation and the anion of a supporting electrolyte can influence the electrical double layer, which impacts the Mg stripping efficiency. We find halide salt additives can prevent passivation on the Mg electrode by influencing the composition of the solid electrolyte interphase. This study demonstrates that, by tailoring the electrolyte composition, we can modify the Mg stripping process and enable a streamlined optimization process for the development of new electrosynthetic methodologies.

Lithium–sulfur (Li–S) batteries offer high theoretical gravimetric capacities at low cost relative to commercial lithium-ion batteries. However, the solubility of intermediate polysulfides in conventional electrolytes leads to irreversible capacity fade via the polysulfide shuttle effect. Highly concentrated solvate electrolytes reduce polysulfide solubility and improve the reductive stability of the electrolyte against Li metal anodes, but reactivity at the Li/solvate electrolyte interface has not been studied in detail. Here, reactivity between the Li metal anode and a solvate electrolyte (4.2 M LiTFSI in acetonitrile) is investigated as a function of temperature. Though reactivity at the Li/electrolyte interface is minimal at room temperature, we show that reactions between Li and the solvate electrolyte significantly impact the solid electrolyte interphase (SEI) impedance, cyclability, and capacity retention in Li–S cells at elevated temperatures. Addition of a fluoroether cosolvent to the solvate electrolyte results in more fluoride in the SEI which minimizes electrolyte decomposition, reduces SEI impedance, and improves cyclability. A 6 nm AlF3 surface coating is employed at the Li anode to further improve interfacial stability at elevated temperatures. The coating enables moderate cyclability in Li–S cells at elevated temperatures but does not protect against capacity fade over time.

Mg−S batteries are a promising next-generation system for beyond conventional Li-ion chemistry. The Mg−S architecture pairs a Mg metal anode with an inexpensive, highcapacity S 8 cathode. However, S 8 -based cathodes exhibit the "polysulfide shuttle" effect, wherein soluble partially reduced S x 2− species generated at the cathode diffuse to and react with the anode. While dissolved polysulfides may undergo reactions to form Li + -permeable layers in Li−S systems, the interfaces on Mg anodes are passivating. In this work, we probe the reactivity of various Mg polysulfide solutions at the Mg anode interface. Mg polysulfide solutions are prepared without any chelating agents to closely mimic conditions in a Mg−S cell. The polysulfides are synthesized by reacting Mg metal and S 8 in electrolyte, and the speciation is controlled by varying the Mg:S precursor ratio. S-poor precursor ratios produce magnesium polysulfide solutions with a higher proportion of short-chain polysulfides that react at the Mg anode faster than the longer-chain analogues. Anode passivation can be slowed by shifting the polysulfide equilibria toward longer-chain polysulfides through addition of S 8 .

Li-S batteries are a promising alternative to conventional Li-ion batteries as Li-S batteries enable low-cost, lightweight, and high capacity cells. However, the polysulfide shuttle effect and Li reactivity with common organic solvents have limited commercialization of Li-S batteries. Highly concentrated electrolytes known as solvate electrolytes have been shown to limit polysulfide solubility in Li-S cells by reducing the ability of solvent molecules to solvate polysulfides through coordination to Li+.1 The challenges with using solvate electrolytes in electrochemical cells are associated with the high viscosity and low ionic conductivity of solvates. Addition of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) significantly decreases the solvate viscosity with slight improvement in ionic conductivity.1 TTE has been shown to suppress the polysulfide shuttle effect and is suggested to act as a nonsolvent for polysulfides. Although bulk electrolyte speciation is unchanged when the solvate is exposed to Li metal,2 the reactivity of the solvate electrolyte with and without TTE with Li metal has not been studied in detail. To study the reactivity, we evaluate the behavior of Li-S cells and Li metal at various temperatures in the solvate electrolyte with and without TTE. Increasing the temperature of the solvate electrolyte affects the equilibrium between free and coordinated MeCN and results in increased electrolyte decomposition and anode passivation. We demonstrate here that reactivity between the solvate electrolyte and the Li anode significantly impacts the cyclability and capacity retention in Li-S cells. The results indicate that TTE is necessary to stabilize the electrode/electrolyte interface and mitigate Li reactivity. Introducing a protecting layer onto the Li anode enables moderate cyclability but does not protect against decomposition over time. 1. Energy Environ. Sci. 2014, 7, 2697. 2. J. Am. Chem. Soc. 2014, 136, 13, 5039.