The discovery of effective catalysts is an important step towards achieving Li–O2 batteries with long cycle life and high round-trip efficiency. Soluble-type catalysts or redox mediators (RMs) possess great advantages over conventional solid catalysts, generally exhibiting much higher efficiency. Here, we select a series of organic RM candidates as a model system to identify the key descriptor in determining the catalytic activities and stabilities in Li–O2 cells. It is revealed that the level of ionization energies, readily available parameters from a database of the molecules, can serve such a role when comparing with the formation energy of Li2O2 and the highest occupied molecular orbital energy of the electrolyte. It is demonstrated that they are critical in reducing the overpotential and improving the stability of Li–O2 cells, respectively. Accordingly, we propose a general principle for designing feasible catalysts and report a RM, dimethylphenazine, with a remarkably low overpotential and high stability. Soluble catalysts such as redox mediators are promising in enhancing energy efficiency of Li–O2 batteries. Here, the authors propose a design principle for finding efficient redox mediators and demonstrate the application of such a new catalyst.

Rational design of redox mediators for advanced Li–O2 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.

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

Multi-electron redox phenazine for ready-to-charge organic batteries

We describe the morphological, electrochemical, spectroelectrochemical characterization, and electrochromic device properties of films of poly(2,2‘-[10-methyl-3,7-phenothiazylene]-6,6‘-bis[4-phenylquinoline]) (PPTZPQ). Cyclic voltammograms of PPTZPQ polymer films showed reversible electrochemical oxidation accompanied with a color change that was switchable as a function of the potential. However, the reduction behavior of PPTZPQ was unstable and caused a loss of the electrochromic effect. The spectroelectrochemical behavior of PPTZPQ was interpreted as the combination of the properties of the constituent donor and acceptor moieties. Two PPTZPQ solid-state electrochromic devices were constructed using indium tin oxide (ITO) deposited on polyester and poly(ethylene oxide) as a polymeric electrolyte. The electrochromic active materials were PPTZPQ as the primary electrode and ladder poly(benzobisimidazobenzophenanthroline) (BBL) or vanadium pentoxide (V2O5) as the secondary electrode. The PPTZPQ−BBL and PPT...

Barbara Paduszek

Papers

Redox behaviour of phenothiazine and phenazine in organic solvents