Understanding the fundamentals of redox mediators in Li–O₂ batteries: a case study on nitroxides

Abstract

The development of aprotic lithium-oxygen (Li-O2) batteries suffers from high charging overvoltages. Dissolved redox mediators, like nitroxides, providing increased energy efficiency and longer lifetime are promising tools to overcome this challenge. Since this auspicious concept is still in its infancy, the underlying chemical reactions as well as the impact of the different (electro)chemical parameters are poorly understood. Herein, we derive an electrochemical model for the charging reactions, which is validated by potentiostatic measurements. The model elucidates the impact of the major factors including basic cell parameters and the chemical properties of the redox mediator. The model is applied to the promising class of nitroxides, which is systematically investigated by using derivatives of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), AZADO (2-azaadamantane-N-oxyl), and an azaphenalene based nitroxide. The nitroxides are electrochemically characterized by cyclic voltammetry and their performance as redox mediators is studied in Li-O2 batteries with an ether-based electrolyte. Based on the presented model, the charging profiles of the different nitroxide redox mediators are correlated with their molecular structures.

Electrolytes for Rechargeable Lithium–Air Batteries

“…Av ariety of organic, organometallic,a nd inorganic compounds have been identified as OERMs. [77] Organic OERMs such as TTF [72,78] and tetramethylpiperidinyloxyl (TEMPO) [79][80][81] are molecules with double bonds and/or an aromatic nature which undergo redox reactions through electron exchange of noncovalent/resonance structures. Their electronic structures strongly determine the redox potentials;therefore,alarge number of organic materials can be designed to fulfill the requirements of OERMs.Ionization energy is an important parameter, according to which possible rules for the design of organic OERMs have been proposed.…”

Promoting Li–O2 Batteries With Redox Mediators

“…Bergner et al. systematically investigated the typical representatives of different nitroxide subgroups and found that all nitroxide radicals can greatly reduce the charge overpotential, some of which can directly affect the discharge process …”

Improving Electrochemical Performances of Rechargeable Li−CO2 Batteries with an Electrolyte Redox Mediator

“…When used in Li−O 2 batteries, redox mediators electrochemically oxidize firstly in the charging process, and then chemically oxidize Li 2 O 2 to release O 2 . With the aid of redox mediators, such as tetrathiafulvalene (TTF), 2,2,6,6‐tetramethylpiperidinyloxyl (TEMPO), tri[4‐(diethylamino)phenyl]amine (TDPA), 10‐methylphenodiazine (MPT), and lithium bromide (LiBr), the discharge products would be decomposed easily and completely, as the overpotential could be reduced to a great extent. In rechargeable Li−CO 2 batteries, the discharge products are much more difficult to decompose even with the aid of electrocatalysts, and the accumulated insulating Li 2 CO 3 could increase the charging voltage and even the sudden death of batteries.…”