Comparative Study of Mg(CB₁₁H₁₂)₂ and Mg(TFSI)₂ at the Magnesium/Electrolyte Interface

Abstract

An essential requirement for electrolytes in rechargeable magnesium-ion (Mg-ion) batteries is to enable Mg plating−stripping with low overpotential and high Coulombic efficiency. To date, the influence of the Mg/ electrolyte interphase on plating and stripping behaviors is still not well understood. In this study, we investigate the Mg/ electrolyte interphase from electrolytes based on two Mg salts with weakly coordinating anions: magnesium monocarborane ( M g ( C B 1 1 H 1 2 ) 2 ) a n d m a g n e s i u m b i s -(trifluoromethanesulfonyl)imide (Mg(TFSI) 2 ). Cyclic voltammetry and chronopotentiometry of Mg plating−stripping demonstrate significantly lower overpotential in the Mg-(CB 11 H 12 ) 2 electrolyte than in Mg(TFSI) 2 under the same condition. Surface characterizations including X-ray photoelectron spectroscopy and scanning electron microscopy clearly demonstrate the superior chemical and electrochemical stability of the Mg(CB 11 H 12 ) 2 electrolyte at the Mg surface without noticeable interphase formation. On the other hand, characterizations of the Mg/electrolyte interface in the Mg(TFSI) 2 electrolyte indicate the formation of magnesium oxide, magnesium sulfide, and magnesium fluoride as the interfacial compounds resulting from the decomposition of TFSI − anions because of both chemical reduction by Mg and cathodic reduction during Mg deposition.

Structure of Magnesium Chloride Complexes in Ethereal Systems: Computational Comparison of THF and Glymes as Solvents for Magnesium Battery Electrolytes

“…Therefore, the structure of magnesium electrolytes is highly influenced by both the solvent and the salt used. Due to the environmental issues and problems with current collector corrosion, much effort has been placed on the development of Cl À -free electrolytes; [11][12][13][14] however, the best performance is still achieved when chlorine anions are included in the system. [7,[15][16][17] Their presence induces MgÀ Cl bonding structures that are considered to be responsible for the success of the electrode processes.…”

“…Also, longer glymes, up to tetraglyme (G4), have been used successfully, although some increase in overpotential is observed. [12,22,23] In general, an increase in glyme length gives a (desired) lower volatility of the electrolyte, but at the cost of higher viscosity and slower transport kinetics. [22,[24][25][26] Undoubtedly, that also affects the induced structure of the complexes and the reached equilibrium, which is crucial for magnesium electrolytes, determining many parameters, including the battery lifetime.…”

“…The shortest, monoglyme (G1), was commonly used many times instead of THF, and did not show problems with desolvation of Mg 2+ at the electrode. Also, longer glymes, up to tetraglyme (G4), have been used successfully, although some increase in overpotential is observed [12,22,23] . In general, an increase in glyme length gives a (desired) lower volatility of the electrolyte, but at the cost of higher viscosity and slower transport kinetics [22,24–26] .…”

Al2O3 Thin Films on Magnesium: Assessing the Impact of an Artificial Solid Electrolyte Interphase

“…Further studies of Mg (TFSI) 2 /DME electrolyte demonstrated that there was a high overpotential for both Mg deposition (0.6 V) and stripping (1.5 V) using this electrolyte (Shterenberg et al, 2015). The high overpotential in Mg (TFSI) 2 electrolyte was attributed in some studies to the TFSI − anion's instability at the Mg anode which causes it to degrade and form MgS as well as MgF 2 (Yoo et al, 2017;Ding et al, 2018;Jay et al, 2019). Electrolytes containing the Mg (TFSI) 2 salt also demonstrate sensitivity to water, which can aid solvent decomposition (Yu et al, 2017) and passivate the Mg anode interface (Bachhav et al, 2016).…”

Synthesis and Electrochemical Properties of an Aluminum Hexafluorophosphate Electrolyte

“…Among the common weakly coordinating anions, bis(trifluoromethanesulfonyl)imide (TFSI -) may possess the highest dissociation constant, however, it is known that TFSIcan be reduced electrochemically and chemically by metals including Al. [16][17][18][19] On the other hand, PF6anion strikes a good balance between dissociation constant and mobility as well as stability. 20 In this study, we report the synthesis of electrochemical properties of Al(PF6)3 for the first time.…”