Degradation Mechanisms of Magnesium Metal Anodes in Electrolytes Based on (CF₃SO₂)₂N^– at High Current Densities

Abstract

The energy density of rechargeable batteries utilizing metals as anodes surpasses that of Li ion batteries, which employ carbon instead. Among possible metals, magnesium represents a potential alternative to the conventional choice, lithium, in terms of storage density, safety, stability, and cost. However, a major obstacle for metal-based batteries is the identification of electrolytes that show reversible deposition/dissolution of the metal anode and support reversible intercalation of ions into a cathode. Traditional Grignard-based Mg electrolytes are excellent with respect to the reversible deposition of Mg, but their limited anodic stability and compatibility with oxide cathodes hinder their applicability in Mg batteries with higher voltage. Non-Grignard electrolytes, which consist of ethereal solutions of magnesium(II) bis(trifluoromethanesulfonyl)imide (Mg(TFSI)), remain fairly stable near the potential of Mg deposition. The slight reactivity of these electrolytes toward Mg metal can be remedied by the addition of surface-protecting agents, such as MgCl. Hence, ethereal solutions of Mg(TFSI) salt with MgCl as an additive have been suggested as a representative non-Grignard Mg electrolyte. In this work, the degradation mechanisms of a Mg metal anode in the TFSI-based electrolyte were studied using a current density of 1 mA cm and an areal capacity of ∼0.4 mAh cm, which is close to those used in practical applications. The degradation mechanisms identified include the corrosion of Mg metal, which causes the loss of electronic pathways and mechanical integrity, the nonuniform deposition of Mg, and the decomposition of TFSI anions. This study not only represents an assessment of the behavior of Mg metal anodes at practical current density and areal capacity but also details the outcomes of interfacial passivation, which was detected by simple cyclic voltammetry experiments. This study also points out the absolute absence of any passivation at the electrode-electrolyte interface for the premise of developing electrolytes compatible with a metal anode.

Halide‐Based Materials and Chemistry for Rechargeable Batteries

“…The existence of Mg(TFSI) 2 can promote the high solubility of MgCl 2 in DME and, importantly, the chloride ions in the electrolyte could stabilize the Mg cations and also inhibit the passivation of the Mg anode, thereby enabling reversible Mg deposition. This was confirmed by Yoo et al., who demonstrated that the degradation of Mg metal in the TFSI‐based electrolyte could be remedied by the addition of the surface‐protecting agent MgCl 2 . A surface‐science‐based approach for rationalizing and understanding this complex electrochemical processes was introduced by Connell et al .…”

“…This was confirmed by Yooe tal.,w ho demonstrated that the degra- dation of Mg metal in the TFSI-based electrolyte could be remedied by the addition of the surface-protecting agent MgCl 2 . [199] Asurface-science-based approach for rationalizing and understanding this complex electrochemical processes was introduced by Connell et al [200] Thef ormation of ap assivation layer composed of MgO and Mg(OH) 2 after Mg deposition ceases can be inhibited by Cl À ions,w hich protect the Mg surface from passivation by H 2 Ot hrough the formation of Mg À Cl(ad) and/or MgCl 2 .G ao et al [201] first reported in ac omprehensive study of this Mg/electrolyte interface that the addition of MgCl 2 contributed to the formation of ac onductive SEI layer, which allowed Mg 2+ transport to occur under am ild overpotential driving force and enabled reversible reactions on the Mg anode.This study implies that the development of electrolytes for RMBs can be achieved through interfacial design.…”

“…[318] This attempt obtained similar discharge and charge profiles but lower capacity and inferior capacity retention compared to aC IB using lithium as the anode.T he poor electrochemical performance may be due to the dissolution of the discharged product of MgCl 2 in the ionic liquid electrolyte and the possible side reactions between Mg and the TFSI À in the electrolyte upon cycling. [199] Them ultielectron electrode system of VOCl/Mg-MgCl 2 ,a su sed in ar echargeable CIB undergoing am ultielectron reaction, has also been investigated. [41] In this case,h owever, the electrochemical performance is limited by the dissolution of VOCl 2 and MgCl 2 in the ionic-liquid electrolyte.…”

Mg Cathode Materials and Electrolytes for Rechargeable Mg Batteries: A Review

“…Also, the charge‐delocalized framework makes this conjugated [NTf 2 ] − anion highly resistant to positive potential polarization, and thus high anodic stability can be expected in such electrolytes. There have been a number of studies using Mg[NTf 2 ] 2 in different solvents for reversible Mg electrochemistry, particularly in polyether(s) ,. Electrolytes using 0.5 M Mg[NTf 2 ] 2 in a glyme/diglyme mixture showed a high conductivity of 5.2 mS cm −1 , and the anodic stability on stainless steel (SS) substrate was up to 4.2 V vs Mg …”

“…[51] Reductive decomposition via scission of the SÀC bond has been discussed also in Li[NTf 2 ] based electrolytes and ionic liquids. [63] In the presence of water,…”