Instability at the Electrode/Electrolyte Interface Induced by Hard Cation Chelation and Nucleophilic Attack

Abstract

Electrochemistry is necessarily a science of interfacial processes, and understanding electrode/electrolyte interfaces is essential to controlling electrochemical performance and stability. Undesirable interfacial interactions hinder discovery and development of rational materials combinations. By example, we examine an electrolyte, magnesium­(II) bis­(trifluoromethanesulfonyl)­imide (Mg­(TFSI)2) dissolved in diglyme, next to the Mg metal anode, which is purported to have a wide window of electrochemical stability. However, even in the absence of any bias, using in situ tender X-ray photoelectron spectroscopy, we discovered an intrinsic interfacial chemical instability of both the solvent and salt, further explained using first-principles calculations as driven by Mg2+ dication chelation and nucleophilic attack by hydroxide ions. The proposed mechanism appears general to the chemistry near or on metal surfaces in hygroscopic environments with chelation of hard cations and indicates possible synthetic strategies to overcome chemical instability within this class of electrolytes.

Rechargeable Magnesium–Sulfur Battery Technology: State of the Art and Key Challenges

“…The incompatibility between glyme‐based solvent and Mg(TFSI) 2 was reported to result in dendrite growth on the Mg anode and, finally, in short circuits . Interestingly, mixtures of diglyme and Mg(TFSI) 2 were found to be chemically unstable even before applying any voltage due to water impurities, which are unavoidable in commercially available salts . Diglyme can easily chelate Mg 2+ ions, which introduces strain into the structure of the diglyme and expose a proton for the nucleophilic attack.…”

“…One possible way to chemically purify Mg(TFSI) 2 is the addition of dibutylmagnesium, which can react with traces of water, oxygen and free trifluoromethanesulfonic acid (HTFSI) to form butane, magnesium hydroxide, magnesium oxide and Mg(TFSI) 2 . Another explanation for the poor electrochemical performance was related to the reduced stability of the TFSI‐anion upon reduction of Mg 2+ to Mg + , which resulted in the deposition of the TFSI − anion on the Mg surface and a passivation of the Mg anode …”

“…Consequently, current density is not homogeneous over the entire surface, leading to the observed dendrite formation . Typical components on the surface of a cycled anode include Mg metal, MgF 2 , Mg(TFSI) 2 and MgO, where magnesium fluoride and oxide originate from the decomposition of Mg(TFSI) 2 dissolved in diglyme . Further on, using in situ AFM and optical microscopy, Hu et al observed that the use of Mg(HMDS) 2 –AlCl 3 dissolved in different ethers (diglyme and tetraglyme) influences the Mg anode surface due to different dynamic processes.…”

Alkoxy-functionalized ionic liquid electrolytes: understanding ionic coordination of calcium ion speciation for the rational design of calcium electrolytes

“…In the case of the TFSI À coordinated Ca 2+ cluster, both the single-and the two-electron reduction processes result in the decomposition of the TFSI À anions, which is consistent with previously reported reduction processes of the TFSI À coordinated Mg 2+ cluster. 31,38,50 It is interesting to notice that the TFSI À anion decomposes through C-S bond cleavage in the case of Mg while it decomposes through N-S bond in the presence of Ca 2+ . This process leads to the formation of an insulating SEI layer, which is responsible for the poor performance of electrolytes containing IL with the TFSI À anion.…”

Rechargeable Magnesium Batteries using Conversion‐Type Cathodes: A Perspective and Minireview

“…As discussed above, some soluble intermediates might contaminate the electrolytes and change their equilibrium species, which might negatively affect the Mg cycling efficiency and reversibility. More importantly, electrolyte decomposition or their corrosion to current collectors might occur during charging to high voltage, causing the extra charging capacity (severe overcharging behavior) and the irreversible capacity . In addition, the insufficient Mg cycling CEs together with large Mg deposition overpotentials can result in large voltage hysteresis, low capacity, and poor electrode reversibility as discussed in Section .…”