Determining Factor on the Polarization Behavior of Magnesium Deposition for Magnesium Battery Anode

Abstract

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis­(trifluoromethanesulfonyl)­amide (Mg­(TFSA)2)/triglyme, magnesium borohydride (Mg­(BH4)2)/tetrahydrofuran (THF), and Mg­(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg­(TFSA)2/triglyme and Mg­(BH4)2/THF, while the reactions do not occur in Mg­(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]− in the Mg­(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]− in Mg­(TFSA)2/2-MeTHF and [BH4]− in Mg­(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]−, strongly bound to the magnesium ion in the Mg­(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg­(BH4)2/THF electrolyte, because the [BH4]− coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]−. Conversely, because [TFSA]− is weakly bound to the magnesium ion in Mg­(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

Influence of Complexing Additives on the Reversible Deposition/Dissolution of Magnesium in an Ionic Liquid

“…[18,28,68] Similar findings were recently described by Tuerxun et al for MgTFSI 2 in different organic solvents (THF, diglyme), based on the comparison of the calculated HOMO/ LUMO energy levels of the free TFSI À and of the MgTFSI 2 cluster, respectively. [20] In this case, implicit solvation effects were included, while explicit solvent effects by coordination of solvent molecules were not considered. Furthermore, we found that the addition of one BMP-TFSI ion pair slightly reduces the electron affinity E A .…”

“…[12] In general, most investigations so far have focused on salt/solvent combinations that allow reversible Mg deposition/dissolution without the formation of a protective surface layer. Among those, a limited number of electrolytes consisting of commercially available magnesium salts such as Mg(BH 4 ) 2 [13][14][15][16][17][18][19][20] or Mg((CF 3 SO 2 ) 2 N) 2 (Mg bis (trifluoromethylsulfonyl)imide, MgTFSI 2 ) [20][21][22][23][24][25][26][27] have been found to support largely reversible Mg plating and stripping without negative impact on the cell components (such as corrosion, which poses a problem in Cl-containing electrolytes). [4] However, even for these electrolytes, the oxidative instability of the borohydride, [6] as well as the Mg 2 + - [18,28] and moistureinduced [16,25,29,30] decomposition of TFSI À , severely limit their application.…”

Progress and Perspective on Rechargeable Magnesium–Sulfur Batteries

“…In contrast, Mg 2+ ions weakly bound by [TFSA] − in Mg(TFSA) 2 /triglyme electrolyte undergo slow reduction decomposition, allowing Mg to deposit on the anode. [ 32 ]…”

“…This high oxidation stability was also observed in other studies, but was not explained satisfactorily. [ 17c,32,51,57 ] Combined with S/CMK‐3 cathodes, the resulting Mg–S batteries displayed a reversible discharge capacity of ≈200 mA h g −1 after 100 cycles. [ 51 ]…”

Remarkable electrochemical and ion-transport characteristics of magnesium-fluorinated alkoxyaluminate–diglyme electrolytes for magnesium batteries

“…[21][22][23][24] However, the interfacial instability of [TFSA] À moderates cycling efficiency and causes high interfacial resistance with magnesium anodes. 25,26 In contrast, weakly coordinated anion-based electrolytes incorporating boron-cluster, fluorinated alkoxyborate, and fluorinated alkoxyaluminate anions are a promising option for fulfilling the MB operation requirements. [27][28][29][30][31][32][33][34][35] Owing to their favorable electrochemical properties and structures those are easily modified by simple synthesis methods, fluorinated alkoxyborate and alkoxyaluminate electrolytes have attracted the attention and fascination of many researchers, and various related materials including calcium and zinc salts have also been developed.…”