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...

TEMPO: a mobile catalyst for rechargeable Li-O₂ batteries

Nonaqueous lithium–air batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities and potentially low cost. Significant adv...

Current challenges and routes forward for nonaqueous lithium–air batteries

The seemingly simple reaction of Li-O2 batteries involving lithium and oxygen makes this chemistry attractive for high-energy-density storage systems; however, achieving this reaction in practical rechargeable Li-O2 batteries has proven difficult. The reaction paths leading to the final Li2O2 discharge products can be greatly affected by the operating conditions or environment, which often results in major side reactions. Recent research findings have begun to reveal how the reaction paths may be affected by the surrounding conditions and to uncover the factors contributing to the difficulty in achieving the reactions of lithium and oxygen. This progress report describes the current state of understanding of the electrode reaction mechanisms in Li-O2 batteries; the factors that affect reaction pathways; and the effect of cell components such as solvents, salts, additives, and catalysts on the discharge product and its decomposition during charging. This comprehensive review of the recent progress in understanding the reaction chemistry of the Li-O2 system will serve as guidelines for future research and aid in the development of reliable high-energy-density rechargeable Li-O2 batteries.

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Reaction chemistry in rechargeable Li-O2 batteries.

Aprotic Li–O2 batteries represent promising alternative devices for electrical energy storage owing to their extremely high energy densities. Upon discharge, insulating solid Li2O2 forms on cathode surfaces, which is usually governed by two growth models, namely the solution model and the surface model. These Li2O2 growth models can largely determine the battery performances such as the discharge capacity, round-trip efficiency and cycling stability. Understanding the Li2O2 formation mechanism and controlling its growth are essential to fully realize the technological potential of Li–O2 batteries. In this review, we overview the recent advances in understanding the electrochemical and chemical processes that occur during the Li2O2 formation. In the beginning, the oxygen reduction mechanisms, the identification of O2−/LiO2 intermediates, and their influence on the Li2O2 morphology have been discussed. The effects of the discharge current density and potential on the Li2O2 growth model have been subsequently reviewed. Special focus is then given to the prominent strategies, including the electrolyte-mediated strategy and the cathode-catalyst-tailoring strategy, for controlling the Li2O2 growth pathways. Finally, we conclude by discussing the profound implications of controlling Li2O2 formation for further development in Li–O2 batteries.

https://pubs.rsc.org/en/Content/ArticlePDF/2017/CS/C7CS90095C

Recent advances in understanding of the mechanism and control of Li2O2 formation in aprotic Li–O2 batteries

The aprotic lithium-oxygen (Li-O2 ) battery has excited huge interest due to it having the highest theoretical energy density among the different types of rechargeable battery. The facile achievement of a practical Li-O2 battery has been proven unrealistic, however. The most significant barrier to progress is the limited understanding of the reaction processes occurring in the battery, especially during the charging process on the positive electrode. Thus, understanding the charging mechanism is of crucial importance to enhance the Li-O2 battery performance and lifetime. Here, recent progress in understanding the electrochemistry and chemistry related to charging in Li-O2 batteries is reviewed along with the strategies to address the issues that exist in the charging process at the present stage. The properties of Li2 O2 and the mechanisms of Li2 O2 oxidation to O2 on charge are discussed comprehensively, as are the accompanied parasitic chemistries, which are considered as the underlying issues hindering the reversibility of Li-O2 batteries. Based on the detailed discussion of the charging mechanism, innovative strategies for addressing the issues for the charging process are discussed in detail. This review has profound implications for both a better understanding of charging chemistry and the development of reliable rechargeable Li-O2 batteries in the future.

Understanding the Reaction Chemistry during Charging in Aprotic Lithium–Oxygen Batteries: Existing Problems and Solutions

Lithium peroxide (Li2O2), the solid and intrinsically electronic insulating discharge product of Li–O2 batteries strongly influences the discharge and charge kinetics. In a series of experiments, we investigated the growth of Li2O2 upon discharge and the corresponding reduction and oxidation processes by varying the depth of discharge. The results indicate that insulating Li2O2 particles with a disc-like shape were formed during the initial discharge stage. Afterward, the nucleation and growth of Li2O2 resulted in the formation of conducting Li2O2 shells. When the discharge voltage dropped below 2.65 V, the Li2O2 discs evolved to toroid-shaped particles and defective superoxide-like phase presumably with high conductivity was formed on the rims of Li2O2 toroids. Both Li2O2 and the superoxide-like phase are unstable in ether-based electrolytes resulting in the degradation of the corresponding cells. Nevertheless, by controlling the growth of Li2O2, the chemical reactivity of the discharge product can be su...

Evolution of Li2O2 growth and its effect on kinetics of Li-O2 batteries.

In lithium/oxygen batteries with liquid nonaqueous electrolytes, metal or metal oxide electrocatalysts are generally used in the carbon cathode to decrease the cell overvoltage and increase the cap...

Silica Nanoparticles as Structural Promoters for Oxygen Cathodes of Lithium–Oxygen Batteries

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Evolution of Li2O2 growth and its effect on kinetics of Li-O2 batteries.

Silica Nanoparticles as Structural Promoters for Oxygen Cathodes of Lithium–Oxygen Batteries