Oxygen Reduction Reaction in Highly Concentrated Electrolyte Solutions of Lithium Bis(trifluoromethanesulfonyl)amide/Dimethyl Sulfoxide

Abstract

The performance of current Li–air batteries is greatly limited by critical obstacles such as electrolyte decomposition, high charging overpotentials, and limited cycle life. Thus, much effort is devoted to fundamental studies to understand the mechanisms of discharge/charge processes and overcome the above-mentioned obstacles. In particular, the search for new stable electrolytes is vital for long-lasting and highly cyclable batteries. The highly reactive lithium superoxide intermediate (LiO2) produced during discharge process can react with the electrolyte and produce a variety of byproducts that will shorten battery life span. To study this degradation mechanism, we investigated oxygen reduction reaction (ORR) in highly concentrated electrolyte solutions of lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA])/dimethyl sulfoxide (DMSO). On the basis of rotating ring disk electrode measurements, we showed that LiO2 dissolution can be limited by increasing lithium salt concentration over 2.3 mol dm–3. Our Raman results suggested that this phenomenon can be related to lack of free DMSO molecules and increasing DMSO–Li+ interactions with higher Li+ concentration. X-ray diffraction measurements for the products of ORR suggested that the side reaction of DMSO with Li2O2 and/or LiO2 could be suppressed by decreasing the solubility of LiO2 in highly concentrated electrolytes.

Unraveling the Mechanism of the Solution‐Mediated Oxygen Reduction in Metal‐O2 Batteries: The Importance of Ion Association

“…The decrease of the LiO 2 concentration would lead to a positive Nernstian shift of the potential of the ORR . Therefore this argumentation would also explain the observations of Tatara et al . There is a third explanation for this potential shift.…”

“…Previously Tatara et al . investigated the effect of concentrated LiTFSI solution in DMSO on the ORR . With RRDE and Raman studies, the authors found that the amount of LiO 2 , as an ORR intermediate, is decreasing in higher concentrated solution.…”

Improved Li–Metal Cycling Performance in High Concentrated Electrolytes for Li‐O2 Batteries

“…Figure 1 shows the recorded Raman spectra of fresh electrolytes in comparison to pure solvent. [25] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 The concentrations of coordinated DMSO ([DMSO] coor ) molecules and uncoordinated or free DMSO ([DMSO] free ) molecules along with the coordination number (N DMSO ) in the electrolyte were estimated using Raman spectra (Table 1). A shift of CÀS symmetric and asymmetric stretching band towards high wavenumbers indicates much stronger CÀS bond of DMSO, suggesting an unusual coordination structure totally different from those in dilute solutions.…”

“…[22] TFSI À is moreover reported to form a stable SEI layer upon reduction on Li metal preventing further decomposition of solvent molecules. [25] Their work includes the finding of solvation structure of different concentrated electrolytes ranging from pure solvent up to 3.4 M using Raman spectroscopy. [25] Their work includes the finding of solvation structure of different concentrated electrolytes ranging from pure solvent up to 3.4 M using Raman spectroscopy.…”

Mechanistic Insights on Concentrated Lithium Salt/Nitroalkane Electrolyte Based on Analogy with Fluorinated Alcohols

“…The in solution structure of inorganic Li salts has been studied extensively in the field of lithium ion batteries. Raman spectroscopy is specifically useful for this purpose, and many researchers have proposed that lithium salts change their form from a solvent‐separated ion pair (SSIP) to a contacted ion pair (CIP) and an aggregate (AGG) species at higher concentrations (Figure ) . SSIP is also referred to as free species, whereas CIP and AGG are referred to as bound species (Figure ).…”