Mg rechargeable batteries: an on-going challenge

Exploration of the Detailed Conditions for Reductive Stability of Mg(TFSI)2 in Diglyme: Implications for Multivalent Electrolytes

“…Interestingly, the optimization of Mg(TFSI) 2 results in a structure where Mg has a tetrahedral coordination and one TFSI unit provides two oxygens whereas another TFSI unit provides nitrogen and oxygen. AIMD simulations (>10 ps) of Mg(TFSI) 2 show that (2O-NO) coordination dominates in the bulk G2 as well. As expected, 6,54 the electronic density is delocalized over the TFSI unit whereas the vacant orbital is largely localized on Mg, in agreement with previously reported results 31 such that an incoming electron would reside on Mg and the SOMO orbital of (MgTFSI) 0 would be similar to the LUMO of (MgTFSI) + .…”

“…The fundamental premise for our study of Mg(TFSI) 2 in diglyme is that the stability of the electrolyte is dictated by the electrochemical stability of (MgTFSI) + and its reduced form (MgTFSI) 0 . Here, we limit our consideration solely to an elementary act (electron transfer), without any further implications for the thermally random coexistence of the fully charged and partially discharged forms of the ion pair in vicinity of each other.…”

“…28,50,51 In pursuit of understanding the Mg(TFSI) 2 electrolyte behavior and its potential improvement, extensive computational studies were carried out. 5,6,27,31 Using a combination of classical MD simulations and quantum chemistry calculations hybridized with implicit solvent models, 52 it was shown that the solvent donor strength and its chelating ability largely determine the solubility of Mg(TFSI) 2 and the ion-pair solvation structure. The concentration-dependent instability of the Mg(TFSI) 2 salt at Mg metal potentials was assigned to a drastic decrease of dissociation energy of the C−S bond in the cation-reduced (MgTFSI) 0 complex leading to its decomposition into neutral [Mg + −CF 3 SO 2 NSO 2 − ] 0 and CF 3 fragments.…”

Understanding the Initial Stages of Reversible Mg Deposition and Stripping in Inorganic Nonaqueous Electrolytes

“…5 The path to fully functioning and cost-competitive Mgion batteries begins with addressing a few but complex scientific questions, and perhaps the most pressing is to develop a robust understanding of the atomistic mechanism of reversible plating (or deposition) and stripping (or dissolution) of Mg at the anode/electrolyte interface during battery operation. To date, reversible Mg plating with low over-potential and reasonable anodic stability has been achieved in practice with only a specific class of electrolytes, namely organic or inorganic magnesium aluminum chloride salts (magnesium-chloro complexes) dissolved in ethereal solvents [6][7][8][9][10][11] . Aurbach and collaborators developed the first operational Mg ion full cells using electrolytes based on Grignard reagents (e.g.…”

“…Aurbach and collaborators developed the first operational Mg ion full cells using electrolytes based on Grignard reagents (e.g. butylmagnesium chloride), 6 and improved upon their performance by switching to an in-situ prepared magnesium organo-haloaluminate, which can achieve high coulombic efficiency during stripping and deposition and anodic stability up to 3.0 V. 7,8,11 Recently, Muldoon and co-workers further improved upon air-sensitivity and anodic stability by designing a non-nucleophilic electrolyte containing a Hauser base, such as hexamethyldisilazide magnesium chloride (HMDSMgCl) and AlCl 3 . 12 Another functioning electrolyte, though displaying limiting anodic stability, can be achieved using a combination of Mg(BH) 4 and LiBH 4 in diglyme as demonstrated by Shao et al 13 Despite this substantial progress, some aspects of the electrolytes are in need of improvement, particularly the anodic stability, safety and compatibility with other cell components.…”