Redox Mediators for Li–O₂ Batteries: Status and Perspectives

Abstract

Li–O2 batteries have received much attention due to their extremely large theoretical energy density. However, the high overpotentials required for charging Li–O2 batteries lower their energy efficiency and degrade the electrolytes and carbon electrodes. This problem is one of the main obstacles in developing practical Li–O2 batteries. To solve this problem, it is important to facilitate the oxidation of Li2O2 upon charging by using effective electrocatalysis. Using solid catalysts is not too effective for oxidizing the electronically isolating Li‐peroxide layers. In turn, for soluble catalysts, red‐ox mediators (RMs) are homogeneously dissolved in the electrolyte solutions and can effectively oxidize all of the Li2O2 precipitated during discharge. RMs can decompose solid Li2O2 species no matter their size, morphology, or thickness and thus dramatically increase energy efficiency. However, some negative side effects, such as the shuttle reactions of RMs and deterioration of the Li‐metal occur. Therefore, it is necessary to study the activity and stability of RMs in Li–O2 batteries in detail. Herein, recent studies related to redox mediators are reviewed and the mechanisms of redox reactions are illustrated. The development opportunities of RMs for this important battery technology are discussed and future directions are suggested.

Electrolytes for Rechargeable Lithium–Air Batteries

“…A variety of organic, organometallic, and inorganic compounds have been identified as OERMs . Organic OERMs such as TTF and tetramethylpiperidinyloxyl (TEMPO) are molecules with double bonds and/or an aromatic nature which undergo redox reactions through electron exchange of noncovalent/resonance structures.…”

Promoting Li–O2 Batteries With Redox Mediators

“…On charge, a RM char is required to transfer electrons between Li 2 O 2 and electrode. A qualified RM char should satisfy the following criteria: (1) A redox potential higher than 2.96 V vs. Li + /Li, the standard potential of the O 2 /Li 2 O 2 couple; (2) a fast rate constant of Li 2 O 2 oxidation reaction to allow a high charge current; and (3) a high stability during discharge and charge. The stability mentioned here includes not only the ability to resist attack by reactive oxygen species but also its inherent stability.…”

Defect Chemistry in Discharge Products of Li–O2 Batteries

“…Redox mediators are soluble catalysts dissolved in electrolytes used to effectively oxidize Li 2 O 2 , thereby reducing charge overpotential . LiI is a well‐studied redox mediator due to its appropriate redox potential and stability in conventional organic electrolytes .…”