Role of Chloride for a Simple, Non-Grignard Mg Electrolyte in Ether-Based Solvents

Abstract

Mg battery operates with Chevrel phase (Mo6S8, ∼1.1 V vs Mg) cathodes that apply Grignard-based or derived electrolytes, which allow etching of the passivating oxide coating forms at the magnesium metal anode. Majority of Mg electrolytes studied to date are focused on developing new synthetic strategies to achieve a better reversible Mg deposition. While most of these electrolytes contain chloride as a component, and there is a lack of literature which investigates the fundamental role of chloride in Mg electrolytes. Further, ease of preparation and potential safety benefits have made simple design of magnesium electrolytes an attractive alternative to traditional air sensitive Grignard reagents-based electrolytes. Work presented here describes simple, non-Grignard magnesium electrolytes composed of magnesium bis(trifluoromethane sulfonyl)imide mixed with magnesium chloride (Mg(TFSI)2-MgCl2) in tetrahydrofuran (THF) and diglyme (G2) that can reversibly plate and strip magnesium. Based on this discovery, the effect of chloride in the electrolyte complex was investigated. Electrochemical properties at different initial mixing ratios of Mg(TFSI)2 and MgCl2 showed an increase of both current density and columbic efficiency for reversible Mg deposition as the fraction content of MgCl2 increased. A decrease in overpotential was observed for rechargeable Mg batteries with electrolytes with increasing MgCl2 concentration, evidenced by the coin cell performance. In this work, the fundamental understanding of the operation mechanisms of rechargeable Mg batteries with the role of chloride content from electrolyte could potentially bring rational design of simple Mg electrolytes for practical Mg battery.

Halogenid‐basierte Materialien und Chemie für wiederaufladbare Batterien

“…Eine detaillierte Untersuchung bezüglich des Effekts der Chlorid‐Konzentration wurde von Sa et al. durchgeführt, worin gezeigt wurde, dass das feste Mg(TFSI) 2 /MgCl 2 /THF ein Ionenpaar bestehend aus [Mg 2 Cl 3 (THF) 6 ] + [TFSI] − besitzt (Abbildung ) . Als der Mg(TFSI) 2 /MgCl 2 /DME‐Elektrolyt in die Mg‐S‐Batterie eingesetzt wurde, produzierte dieser die beste Zyklenstabilität von allen zuvor berichteten Mg‐S‐Batterien (Abbildung ) .…”

“…Aus Lit. , Copyright 2016, American Chemical Society. Elektrochemische Leistung der Mg‐S‐Batterie mit MgTFSI 2 /MgCl 2 /DME‐Elektrolyt: d) Spannungsprofile im 0.25 m Elektrolyt.…”

Magnesiumbatterien – ein Aufruf an Synthesechemiker: Elektrolyte und Kathoden dringend gesucht

“…[56] Magnesiumorganoborate mit einem Magnesium-Dimer-Kation (Mg 2 (m-Cl) 3 ·6 THF) + zeigten hingegen auf Platin sowie auf Edelstahl und Aluminium ähnliche oxidative Stabilitäten. [59][60][61] Interessanterweise unterliegt in THF/CH 3 CN aufgelçstes Magnesiumhexafluorphosphat einer reversiblen Abscheidung/Ablçsung von Magnesium, und wie bei Mg(TFSI) 2 ist der Coulomb-Wirkungsgrad extrem niedrig. Hervorzuheben ist, dass die Dimer-Kationen der Magnesiumorganohalogenaluminate und Magnesiumorganoborate korrosive Chloride enthalten.…”

A fluorinated dialkoxide-based magnesium-ion electrolyte

“…This rapid decrease in cycling performance, as well as the steady state discharge capacity near half of the theoretical capacity of Mo 6 S 8 has been reported by others. 37,38 Indeed, density functional theory calculations and experimental data reported previously show that molybdenum atoms on the surface of the Mo 6 S 8 clusters can become saturated with Cl À species in chloridebased electrolytes, thus inhibiting Mg 2+ intercalation with increasing cycle count. 39 However, compared to non-AlCl 3 containing compositions, both charge and discharge occur with lower overpotentials.…”

“…20 This is likely the result of halide ions enabling more facile charge transfer kinetics at the anode -similar behavior has also been observed in Mg(TFSI) 2 :MgCl 2 electrolytes with increasing concentration of halide. 38 In this work, we sought to demonstrate the straightforward, completely Grignard-free, and atom-efficient synthesis of a Mgion electrolyte, while accepting the limitations of formulations that include halide ions. For these electrolytes that incorporate a uorinated magnesium dialkoxide combined with AlCl 3 , we show that this structure change serves to improve many of the electrolyte gures of merit compared to Mg(OR) 2 :AlCl 3 , most notably the coulombic efficiency, conductivity, and overpotentials.…”