Oxidation Stability of Organic Redox Mediators as Mobile Catalysts in Lithium–Oxygen Batteries

Abstract

Employing organic redox mediators (ORMs) for lithium−oxygen (Li−O 2 ) batteries has emerged as an important strategy to suppress charging overpotentials. Judicious molecular designs of ORMs can also tailor their redox potential and electrontransfer rate to optimize the catalytic efficiency. However, the stability of ORMs in Li−O 2 cells was scarcely studied. Here, the catalytic efficiency and stability of several important ORMs are assessed through in situ gas analysis and reactivity tests with singlet oxygen. Some well-known ORMs are detrimentally decomposed during the first cycle in Li−O 2 cells, whereas nitroxyl-radical-based ORMs bear the most stable and efficient response. Analogous nitroxyl-radical derivatives further increase round-trip energy efficiency and electron-transfer kinetics. This study underlines chemical stability aspects of ORMs, which are mandatory for the long-term cyclability in Li−O 2 cells. We emphasize that besides the importance of ORMs in these systems and their proper selection, an effective operation of Li−O 2 cells depends also strongly on the stability of the carbonaceous cathodes and the electrolyte solutions. The stability of all the components in these systems is inter-related.

Singlet Oxygen in Lithium−Oxygen Batteries

“…It indicates that DABCO also stabilized DMPZ by elimination of 1 O 2 and resulted in the improved stability of the Li−O 2 cell [5b] . This synergetic effect also caused decreased side products [4e,9] …”

“…Decomposing non‐aqueous electrolytes by 1 O 2 have been highlighted through experimental and mechanistic studies [5b,6a,7] . In addition, the 1 O 2 reacts with carbon electrode [8] and redox mediators that are used in Li−O 2 batteries to decrease charging overpotential or increased capacity [4e,9] . The 1 O 2 as the excited form of O 2 is indicated as 1 ▵ g .…”

Tunable Cationic Vacancies of Cobalt Oxides for Efficient Electrocatalysis in Li–O2 Batteries

“…[14] Redox mediators are employed as the soluble cocatalysts to increase roundtrip energy efficiency and electron-transfer kinetics for better rechargeability. [15,16] Alternatively, it is still highly desirable to develop efficient bifunctional electrocatalysts to lower energy barriers of both ORR and, in order to improve the battery reversibility on the basis of the rational modulation of active sites and thorough understanding on the catalytic mechanism.…”

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

“…This implies that RMs can react with the reactive O 2 species, in addition to inducing a desirable catalytic pathway. [ 14 ] Figure a,b shows the results of the round‐trip efficiency and O 2 evolved/consumed ratio of Li–O 2 cells containing 10‐isopropylphenothiazine (PPT), (2,2,6,6‐teramethylpiperidin‐1‐yl)‐oxyl (TEMPO), TTF, DMPZ, and N,N,N,N′ ‐tetramethyl‐ p ‐phenylenediamine (TMPD), respectively. The round‐trip efficiency of ORMs was much higher than that of the electrolyte without ORMs; in particular, DMPZ and TMPD have higher round‐trip efficiency, which is over 80% higher than that of any other ORMs.…”