Redox mediators for high-performance lithium–oxygen batteries

Abstract

The aprotic lithium-oxygen (Li-O2) batteries are receiving intense research interest in virtue of their ultra-high theoretical specific energy. However, current Li-O2 batteries are sufferring from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catlysts, i.e., redox mediators (RMs), have been explored to catalyze the oxygen electrochemistry in Li-O2 batteries and are regarded as an advanced solution. To fully unlock the capability of Li-O2 batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs, and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li-O2 batteries.

Recent progresses and challenges in aqueous lithium–air batteries relating to the solid electrolyte separator: A mini-review

“…In the past decade, a number of excellent books and review articles have been published regarding non-aqueous or aqueous Li–air batteries ( Feng et al, 2016 ; Kwak et al, 2020 ; Dou et al, 2022 ; Liu et al, 2022 ), but the latest article for the aqueous Li–air battery has been published for more than 3 years, best to our knowledge ( Imanishi and Yamamoto, 2019 ). As significant approaches have been made day by day, it is necessary to summarize and review them in order to accelerate the research pace for the application of aqueous Li–air batteries.…”

Completely Eradicating Singlet Oxygen in Li–O2 Battery via Cobalt(II)-Porphyrin Complex-Catalyzed LiOH Chemistry

“…Redox mediators (RMs), soluble catalysts in an electrolyte, have been confirmed to be high-efficiency diffusive catalysts for the cathode reactions. , They act as electron–hole “carriers” between the electrode surface and Li x O y , facilitating the decomposition of Li x O y , irrespective of the size and the structure of Li x O y . Since Bruce et al reported tetrathiafulvalene (TTF) as an effective RM for LOBs in 2013, various kinds of RMs (organic, organometallic, and halide) have been developed. − Among them, organometallic RMs are particularly attractive because their redox properties can be flexibly tuned by replacing the center active transition metal ions and/or modifying the organic ligands. , In addition, recent studies have revealed that RMs can induce an increased intersystem crossing rate and suppress 1 O 2 formation during the ORR and OER processes, thereby reducing parasitic reactions. , Notably, the organic RMs themselves can also be predominantly decomposed and deactivated by 1 O 2 , leading to limited efficiencies of 1 O 2 suppression. , Thus, it is essential to find a way to operate LOBs via a chemical step that does not form 1 O 2 .…”

“…12,13 They act as electron−hole "carriers" between the electrode surface and Li x O y , facilitating the decomposition of Li x O y , irrespective of the size and the structure of Li x O y . 14 Since Bruce et al reported tetrathiafulvalene (TTF) as an effective RM for LOBs in 2013, 13 various kinds of RMs (organic, organometallic, and halide) have been developed. 15−18 Among them, organometallic RMs are particularly attractive because their redox properties can be flexibly tuned by replacing the center active transition metal ions and/or modifying the organic ligands.…”

Critical Factors Affecting the Catalytic Activity of Redox Mediators on Li–O2 Battery Discharge

“…Toward unlocking the potential of Li–O 2 batteries, considerable efforts were devoted to governing the formation of Li 2 O 2 in electrolytes (i.e., Li 2 O 2 (sol)), termed as the solution-route ORR . Although strong-solvation electrolytes or functional additives can achieve this goal by regulating the solubility and/or reactivity of superoxide species, such systems are vulnerable due to the decreased electrolyte stability and increased parasitic reactions with Li anodes. − Recently, soluble redox mediators (RMs) have demonstrated their ability to modulate the formation pathway and deposition behavior of Li 2 O 2 . − Upon operation, the electrochemically reduced RMs chemically reduce O 2 to Li 2 O 2 while recovering to the pristine neutral state. Mediators allow, in principle, the reduction of O 2 in electrolytes away from the electrode surface, resulting in significant remission of cathode passivation. , …”