Correlating Structure and Properties of Super‐Concentrated Electrolyte Solutions: ¹⁷O NMR and Electrochemical Characterization

Abstract

Super‐concentrated electrolyte solutions are of increasing interest for safer and more stable lithium and post‐lithium batteries. The combination of 7Li and 17O (at natural abundance) nuclear magnetic resonance (NMR) and electrochemical characterization is proposed here as an effective approach to investigate the Li+ solvation structures and properties of electrolytes featuring tetraethylene glycol dimethyl ether (TEGDME) and lithium‐bis(trifluoromethane sulfonyl) imide (LiTFSI). Five different formulations from salt‐in‐solvent to solvent‐in‐salt with LiTFSI at different concentrations (0.1 m, 0.5 m, 2 m, 4 m, 5 m) are investigated. The NMR results, also supported by physico‐chemical characterizations such as thermal gravimetric analyses, differential scanning calorimetry, specific conductivity and viscosity, give information about the association of Li+ ions with anion and solvent molecules, allowing a deeper knowledge on the relationships among structure and functional properties of super‐concentrated solutions.

Lithium–Oxygen Battery Exploiting Highly Concentrated Glyme-Based Electrolytes

“…Furthermore, the inclusion in the solution of a sacrificial additive such as vinylene carbonate, LiNO 3 , and Mg­(NO 3 ) 2 , that form a stable SEI at the lithium metal surface, was considered to be a very promising strategy to hinder the formation of lithium dendrites and optimize the electrolytes. , This favorable phenomenon has been widely exploited for avoiding the chemical process of the dissolved polysulfide intermediates with the Li metal in the Li–S battery. − In fact, the concomitant reduction of the additive along with the partial electrolyte decomposition consolidate the SEI and limit the direct contact and reactivity of the metal anode with the solution components. − Moreover, recent studies suggested the addition of LiNO 3 to the electrolyte as a suitable strategy to decrease the polarization of the ORR/OER electrochemical process in lithium–oxygen batteries, , while relevant performances have been achieved through the employment of molten LiNO 3 without a solvating agent . In this regard, an interesting field of research for lithium metal batteries was represented by the use of electrolytes based on solutions with high concentrations of lithium salts, that is, solvent-in-salt configurations, which can lead to notable cycling efficiency and high specific capacity because of the formation of an improved and stable SEI layer, providing at the same time a suitable safety content. − We have investigated in this work the performances of lithium–oxygen batteries using electrolyte solutions consisting of diglyme [diethylene-glycol dimethyl-ether (DEGDME)] and triglyme [triethylene-glycol dimethyl-ether (TREGDME)] with a relevant amount of lithium salts [lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) and LiNO 3 ]. The exploited salt amount approached the saturation limit of the solvents in order to achieve highly concentrated solutions with increased safety content and limited evaporation, thus favoring practical applications of the Li/O 2 cell in an open environment.…”

Lithium–Metal Batteries Using Sustainable Electrolyte Media and Various Cathode Chemistries

“…Indeed, previous literature has demonstrated that high concentrations of lithium salts in glyme-based electrolytes can lead to an uneven composition and low thickness of the SEI layer, possibly leading to a modest stability. 41 We have demonstrated in a recent paper that TREGDME dissolving lithium trifluoromethanesulfonate (LiCF 3 SO 3 ) and LiNO 3 in conventional concentrations undergoes an electrochemical optimization process in a Li/LFP cell by adopting a reduction step at the first discharge occurring around 1.5 V, i.e., a voltage value far lower than the ones exploited in the galvanostatic measurements reported in Figure 4 . 36 The above-mentioned reduction deals with LiNO 3 and actually leads to the formation of stable interfaces at the electrodes surface with a remarkable improvement of the cell performance.…”

Influence of electrolyte structural evolution on battery applications: Cationic aggregation from dilute to high concentration

“…[ 86 ] (B) The 17 O NMR spectra of the electrolytes at different concentrations (in the left panel) and the region of tetraethylene glycol dimethyl ether (TEGDME) peaks (in the right panel) for the 0.1 and 0.5 m electrolytes. [ 92 ] (C) The changes of Li + solvation structures with increasing LiFSI salt concentration in DME‐based electrolyte by explaining the 6 Li and 17 O NMR spectra of electrolyte with different concentrations [ 87 ]…”

“…[86] (B) The 17 O NMR spectra of the electrolytes at different concentrations (in the left panel) and the region of tetraethylene glycol dimethyl ether (TEGDME) peaks (in the right panel) for the 0.1 and 0.5 m electrolytes. [92] (C) The changes of Li + solvation structures with increasing LiFSI salt concentration in DME-based electrolyte by explaining the 6 Li and 17 O NMR spectra of electrolyte with different concentrations [87] effect. [91] Additional NMR measurements were carried out to record the chemical shifts of 1 M LiPF 6 in EC and DMC mixtures with different ratios, proving that Li + strongly contacts with EC over DMC in traditional non-aqueous electrolytes.…”

“…Moreover, after quantifying chemical shifts, the Li +solvation sheath was also revealed, in which the maximum of six EC molecules can coexist. Piercarlo et al [92] carried out the NMR measurements with the combination of 17 O and 6 Li detection at their natural abundance, performing a deep investigation of the solvation structure of Li + in the applied solution. As exhibited in Figure 3B, the NMR results proved that Li-ions are chiefly associated with the adopted anion and coordinated with one or two solvents in concentrated solutions, which helps better understanding of the main species in the formed SEI layer for good operation of Li metal batteries.…”