Singlet Oxygen in Electrochemical Cells: A Critical Review of Literature and Theory

Abstract

Rechargeable metal/O2 batteries have long been considered a promising future battery technology in automobile and stationary applications. However, they suffer from poor cyclability and rapid degradation. A recent hypothesis is the formation of singlet oxygen (1O2) as the root cause of these issues. Validation, evaluation, and understanding of the formation of 1O2 are therefore essential for improving metal/O2 batteries. We review literature and use Marcus theory to discuss the possibility of singlet oxygen formation in metal/O2 batteries as a product from (electro)­chemical reactions. We conclude that experimental evidence is yet not fully conclusive, and side reactions can play a major role in verifying the existence of singlet oxygen. Following an in-depth analysis based on Marcus theory, we conclude that 1O2 can only originate from a chemical step. A direct electrochemical generation, as proposed by others, can be excluded on the basis of theoretical arguments.

Overlooked Factors Required for Electrolyte Solvents in Li–O2 Batteries: Capabilities of Quenching 1O2 and Forming Highly‐Decomposable Li2O2

“…The proposed pathways for the formation of 1 O 2 include 1) disproportionation reaction of LiO 2 , and 2) oxidation reaction of LiO 2 [10, 12, 37–39] . Notably, it was recently reported that more than 40 % of the 1 O 2 is generated via the pathway‐(1) in ether‐based electrolytes [31] .…”

“…The proposed pathways for the formation of 1 O 2 include 1) disproportionation reaction of LiO 2 , and 2) oxidation reaction of LiO 2 . [10,12,[37][38][39] Notably, it was recently reported that more than 40 % of the 1 O 2 is generated via the pathway-(1) in ether-based electrolytes. [31] On the other hand, the details of the 1 O 2 generation mechanism in other pathways and the differences depending on the kind of electrolyte have not been clarified at present.…”

“…[10,11] (Note that the mechanism of its generation has not been fully elucidated. [12] ) As a countermeasure to electrolyte decomposition by 1 O 2 , the addition of an 1 O 2 chemical or physical quencher has been proposed. [13,14] However, chemical quenchers are gradually depleted; therefore, in principle, their effect is not sustainable, and the addition of an effective amount of physical quenchers leads to a decrease in the gravimetric energy density of an LOB.…”

“…However, recent studies have pointed out that singlet oxygen ( 1 O 2 ) generated during discharge–charge cycles could be among the species responsible for decomposing the electrolyte and electrodes [10, 11] . (Note that the mechanism of its generation has not been fully elucidated [12] [13, 14] .…”

Evolving aprotic Li–air batteries

“…140,141 However, theoretical and mechanistic considerations suggest that a direct 2e À Li 2 O 2 oxidation and 1e À oxidation of LiO 2 would be impossible. 144 Instead, a chemical disproportionation of superoxide species to peroxide and oxygen (2LiO 2 -(LiO 2 ) 2 -Li 2 O 2 + 1 O 2 ) has been proposed as a more convincible pathway toward 1 O 2 generation on either discharge or charge. 145 Freunberger et al suggested that the governing factor of 1 O 2 generation might lie in the Lewis acidity of cations in the electrolyte.…”