Widening Electrochemical Window of Mg Salt by Weakly Coordinating Perfluoroalkoxyaluminate Anion for Mg Battery Electrolyte

Abstract

Energy storage concepts based on multivalent ion chemistries, such as Mg 2+ , promise very high volumetric energy density, however require improvements in electrolyte (Mg salt and solvent) electrochemical window to reach their full potential. Hypothetically the window of Mg salt could be widened by disfavoring the cathodic decomposition pathway of a thermodynamically and anodically stable anion, rendering it kinetically inert toward Mg +-mediated reduction. Computational and electrochemical analyses on Mg[TPFA] 2 ([TPFA] − = [Al{OC(CF 3) 3 } 4 ] −) support this hypothesis, and showcase a widened electrochemical window as a result of mitigated cathodic decomposition as well as enhanced anodic stability from electron-withdrawing CF 3 groups. Detailed NMR and IR spectroscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy further support that the weak coordination to Mg 2+ in solution is important for maintaining the wide electrochemical window of Mg salt.

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

“…The electrolyte solutions incorporating structurally analogous magnesium salts, Mg[Al(PFTB) 4 ] 2 (PFTB = perfluoro-tert-butoxy), also showed a local maximum ionic conductivity in a comparable concentration range despite the salt inherently being coordinated by seven THF molecules. 34 This conductivity-concentration relationship is a common feature of such weakly coordinated anion-based magnesium electrolytes. Notably, the local maxima appear at ca.…”

“…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. [36][37][38] The first application of a fluorinated alkoxyaluminate electrolyte to MBs was magnesium tetrakis(hexafluoro-iso-propoxy)aluminate (hereafter denoted as Mg[Al(HFIP) 4 ] 2 ) in 2016.…”

“…The boratebased analogue, which spontaneously generated upon cycling the electrolytes containing MgF 2 and B(HFIP) 3 , was also found as an effective electrolyte component for MBs. 31 Inspired by these pioneering works, some groups have also developed fluorinatedalkoxyborate-and -alkoxyaluminate-based MB electrolytes [32][33][34][35] in which even minor structural modifications such as changes in the anion coordination centers or chemical structures of the alkoxides strongly impacted the physicochemical and electrochemical properties of the resulting electrolytes. However, because all these studies were conducted by different research groups, comparative studies have not yet been conducted.…”

Promises and Challenges of Next-Generation “Beyond Li-ion” Batteries for Electric Vehicles and Grid Decarbonization

“…449 Similarly, another Mg-[TPFA] 2 electrolyte (where TPFA is Al(OC(CF 3 ) 3 ) 4 ) with the weakly coordinating anion [TPFA] − was proven to have reversible Mg electrodeposition with a high oxidative stability of >5 V against Pt as well. 448 In these electrolytes, it is believed the anion's electron-withdrawing CF 3 groups are beneficial because they weakly coordinate with Mg which improves Mg stripping/ plating from a kinetics standpoint, protects the anion from possible parasitic Mg + -related decomposition reactions, and offering good oxidative stability. The favorable solvation structure is one aspect proposed to contribute to the success of these Mg electrolytes.…”

Rationalizing Calcium Electrodeposition Behavior by Quantifying Ethereal Solvation Effects on Ca2+ Coordination in Well-Dissociated Electrolytes

“…While the EXAFS measurements cannot definitively quantify a combination of two nearly identical coordination environments, the overall trend in coordination number as a function of coordinating solvent suggests the larger effective ionic radius of Ca 2+ species dictates a more flexible first shell environment compared to smaller, dicationic species such as Zn 2+ or Mg 2+ for which octahedral environments are typically preferred in solution. 11,[38][39][40][41][42] Additionally, the most notable difference in the calculated EXAFS fits between solvent systems is the disorder term associated with the Ca-C path (and Ca-O-C multiple scattering path) found roughly 3-4 Å away from the Ca 2+ ion. As depicted in Figure 7c, the second oscillatory feature in r-space increases with coordination to larger glyme molecules.…”