Mechanism for Singlet Oxygen Production in Li-Ion and Metal–Air Batteries

Abstract

Singlet oxygen has emerged as a real mystery puzzling battery science, having been observed in Li−O 2 and Na−O 2 batteries, in conventional Li-ion batteries with NMC cathodes, and during the oxidation of Li 2 CO 3 . The formation of singlet oxygen has been directly linked to the degradation and catastrophic fade seen in these battery chemistries. While there are several proposed hypothesis for its origin, the exact mechanism for the formation of singlet oxygen remains unclear. In this Letter, we attempt to unify these findings by proposing a mechanism of singlet oxygen production in metal−air and Li-ion batteries. We show that a potential dependence of surface termination explains the onset potentials of singlet oxygen release, and in all considered cases the mechanism of singlet oxygen generation is through the chemical disproportionation of the uncoordinated superoxide anion in solution; therefore, the singlet oxygen yield is determined by the concentration of free superoxide versus alkali superoxide ion pairs in solution.

Li-O2 Battery Discharge Redox Mediation by Triarylmethyl Cations

“…Many compounds and functional groups have been investigated and shown success in reducing the charging overpotential 11,20,24,[30][31][32][33][34][35][36][37][38][39][40] , but the majority of them have shown poor long-term stability and performance loss after many discharge-charge cycles 41,42 . The πsystem containing cores of many redox mediators have a propensity to react with in situ generated 1 O2, leading to deactivation [42][43][44][45] . Owing to their lack of a π-system, the nitroxylbased, e.g.…”

“…For example, most of the discharge mediators are quinone-based and the large number of compounds from this family, with electron withdrawing and donating groups, have already been explored. Following our group's interest in redox-active organic compounds as catalysts and co-catalysts in electrochemistry [43][44][45][46][47][48][49][50][51][52][53] , we investigate here a new type of discharge mediator for Li-O2 batteries, based on triarylmethyl cation motifs (Scheme 1a). Computational screening at the density functional theory (DFT) level was used to evaluate the thermodynamic parameters that control the inner-sphere and outersphere catalysis mechanisms for the O2/Li2O2 reduction by the triarylmethyl cations under study.…”

PTFE Enhances Discharge Performance of Carbon Cathodes in Potassium‐Oxygen Batteries**

“…Recently, formation of highly reactive singlet oxygen ( 1 O 2 ) during all stages of cycling was recognized as a major source of cell degradation [4] . While not yet fully understood, [5] there is strong evidence that 1 O 2 generation is tied to the formation mechanism of the main discharge product lithium peroxide (Li 2 O 2 ) [6] . By contrast, 1 O 2 formation is not observed in KOB, where potassium superoxide (KO 2 ) is the main discharge product [5] .…”

“…While not yet fully understood, [5] there is strong evidence that 1 O 2 generation is tied to the formation mechanism of the main discharge product lithium peroxide (Li 2 O 2 ) [6] . By contrast, 1 O 2 formation is not observed in KOB, where potassium superoxide (KO 2 ) is the main discharge product [5] . The discharge reaction of KOB is a single electron transfer (SET) process [Eq.…”

Ambilaterality of Redox Mediators towards 1O2 in Li‐O2 Batteries: Trap and Quencher

“…[ 5 ] Specifically, a pair of superoxides (O 2 − ) generated during charging may disproportionate into 1 O 2 and peroxide (O 2 2− ). [ 11 ] Possible routes that can generate singlet oxygen are laid on both charging and discharging. In discharging, negatively single charged oxygen (O 2 − ) is generated and reacts with lithium ion so that forms Li 2 O 2 or LiO 2 .…”