Singlet Oxygen in Electrochemical Cells: A Critical Review of Literature and Theory.

Rechargeable metal/O2 batteries have long been considered a promising future battery technology in automobile and stationary applications. However, they suffer from poor cyclability and rapid degradation. A recent hypothesis is the formation of singlet oxygen (1O2) as the root cause of these issues. Validation, evaluation, and understanding of the formation of 1O2 are therefore essential for improving metal/O2 batteries. We review literature and use Marcus theory to discuss the possibility of singlet oxygen formation in metal/O2 batteries as a product from (electro)chemical reactions. We conclude that experimental evidence is yet not fully conclusive, and side reactions can play a major role in verifying the existence of singlet oxygen. Following an in-depth analysis based on Marcus theory, we conclude that 1O2 can only originate from a chemical step. A direct electrochemical generation, as proposed by others, can be excluded on the basis of theoretical arguments.

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

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.

TEMPO: a mobile catalyst for rechargeable Li-O2 batteries.

Nonaqueous Li-O2 batteries are an intensively studied future energy storage technology because of their high theoretical energy density. However, a number of barriers prevent a practical application, and one of the major challenges is the reduction of the high charge overpotential: Whereas lithium peroxide (Li2O2) is formed during discharge at around 2.7 V (vs Li(+)/Li), its electrochemical decomposition during the charge process requires potentials up to 4.5 V. This high potential gap leads to a low round-trip efficiency of the cell, and more importantly, the high charge potential causes electrochemical decomposition of other cell constituents. Dissolved oxidation catalysts can act as mobile redox mediators (RM), which enable the oxidation of Li2O2 particles even without a direct electric contact to the positive electrode. Herein we show that the addition of 10 mM TEMPO (2,2,6,6-tetramethylpiperidinyloxyl), homogeneously dissolved in the electrolyte, provides a distinct reduction of the charging potentials by 500 mV. Moreover, TEMPO enables a significant enhancement of the cycling stability leading to a doubling of the cycle life. The efficiency of the TEMPO mediated catalysis was further investigated by a parallel monitoring of the cell pressure, which excludes a considerable contribution of a parasitic shuttle (i.e., internal ionic short circuit) to the anode during cycling. We prove the suitability of TEMPO by a systematic study of the relevant physical and chemical properties, i.e., its (electro)chemical stability, redox potential, diffusion coefficient and the influence on the oxygen solubility. Furthermore, the charging mechanisms of Li-O2 cells with and without TEMPO were compared by combining different electrochemical and analytical techniques.