Effect of Anion in Glyme-based Electrolyte for Li-O₂ Batteries: Stability/Solubility of Discharge Intermediate

Abstract

This study demonstrates that the amount of discharge product (Li 2 O 2 ) precipitated on the separator in a lithium oxygen cell using glyme-based electrolytes depends on the anion. The stability of the discharge intermediate (LiO 2 ) in the electrolyte has been shown to depend on the anionic species, which is related to Li 2 O 2 precipitation on the separator. The implications for producing an efficient and long-life Li-O 2 cell are elaborated. Keywords: Li-O 2 battery | Lithium superoxide | AnionThe Li-O 2 battery has been attracting a great deal of attention due to its high theoretical energy density.1 The solubility of the) is a critical parameter, because its solubility is believed to directly affect the morphology of the final discharge product, Li 2 O 2 .2 This morphology determines the rate of cathode clogging, thus also the maximum discharge capacity.2,3 The morphology has also been shown to affect the charging potential, 4 and thus directly influences the energy efficiency. However, a little-considered aspect is the extent to which the solubility of the O 2•¹ may allow the transport of reduced O 2 species away from the electrode. Nazar et al. reported SEM observation of discharge product deposition on glass fiber (separator) for a cell using 1 M lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA])/tetraethylene glycol dimethyl ether (G4) electrolyte. 5 The implications, in terms of cell cyclability, of precipitation of discharge product on an electronically nonconductive component of the cell are self-evident. Moreover, Liu et al. have demonstrated the presence of large amounts of predominantly amorphous precipitates in the separator of a Li-O 2 battery using a 1 M lithium trifluoromethanesulfonate (Li[OTf])/ G4 electrolyte, and commented on their detrimental effects on cell overpotential and cyclability. 6 In their later work, they also reported X-ray diffraction-based evidence for crystalline Li 2 O 2 in the separator.7 A means to tune the degree of discharge product loss to the separator (and elsewhere in the cell) by variation of the electrolyte properties is thus desirable. The effect of variation of electrolyte, and in particular the anion, on this phenomenon has not yet been reported, to the best of our knowledge.In this study, we present an analysis of the variation of O 2•¹ solubility, and its subsequent effect on Li 2 O 2 distribution in the cell, for a series of glyme-based electrolyte solutions. In our previous study, 8 we applied a 1:1 mixture of Li salt and glyme for oxygen reduction/evolution. We have also recently shown that variation of the anion in such 1:1 solutions results in large differences in the amount of free glyme depending on the choice of Li salt. 9 Thus, to fully characterize the direct effect of the anion on the intermediate solubility and stability, we have chosen to explore more dilute solutions in this study, in which the effects may be more easily observed. The molar ratio of lithium salt to triethylene glycol dimethyl ether (G3) was kept constant (1:4), and the vario...

Glyme-based electrolytes: suitable solutions for next-generation lithium batteries

“…8e shows how the fraction of Li 2 O 2 in the separator, with reference to the theoretical total amount of Li 2 O 2 , changes with the anion composition. 119…”

“…Notably, this system could store even more energy than the Li–S cell and has been suggested as a possible battery for future applications. 105–130 In this review, we discuss with chronological details various developments in the research on glyme-based electrolytes for lithium batteries, which are summarized in the scheme in Fig. 1 (panel a).…”

Solvent-Dependent Oxidizing Power of LiI Redox Couples for Li-O2 Batteries

“…1,3 Therefore, considerable efforts have been placed on attempting to catalyze the charging process in Li-O 2 batteries. While solid-state catalysts have been employed to reduce the overpotential during charge, including metal oxides, 31,32 modified carbon, [33][34][35] and metals or metal alloys, 35,38 these catalysts rely on good electrical contact between Li 2 O 2 and the catalyst throughout the entire charging process, 37 cannot oxidize Li 2 O 2 that forms electronically isolated from the positive electrode, 39 and do not suppress side reactions during charging. 36 An alternative approach is the use of soluble redox mediators to promote electron transfer to the surface of the electronically insulating Li 2 O 2 , 40 where the redox mediator is first electrochemically oxidized at the electrode surface and then the oxidized form of the redox mediator chemically oxidizes Li 2 O 2 to…”

Solvate Ionic Liquids for Li, Na, K, and Mg Batteries

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