No abstract

As the demand for rechargeable lithium-ion batteries (LIBs) with higher energy density increases, the interest in lithium-rich oxide (LRO) with extraordinarily high capacities is surging. The capacity of LRO cathodes exceeds that of conventional layered oxides. This has been attributed to the redox contribution from both cations and anions, either sequentially or simultaneously. However, LROs with notable anion redox suffer from capacity loss and voltage decay during cycling. Therefore, a fundamental understanding of their electrochemical behaviors and related structural evolution is a prerequisite for the successful development of high-capacity LRO cathodes with anion redox activity. However, there is still controversy over their electrochemical behavior and principles of operation. In addition, complicated redox mechanisms and the lack of sufficient analytical tools render the basic study difficult. In this review, we aim to introduce theoretical insights into the anion redox mechanism and in situ analytical instruments that can be used to prove the mechanism and behavior of cathodes with anion redox activity. We summarized the anion redox phenomenon, suggested mechanisms, and discussed the history of development for anion redox in cathode materials of LIBs. Finally, we review the recent progress in identification of reaction mechanisms in LROs and validation of engineering strategies to improve cathode performance based on anion redox through various analytical tools, particularly, in situ characterization techniques. Because unexpected phenomena may occur during cycling, it is crucial to study the kinetic properties of materials in situ under operating conditions, especially for this newly investigated anion redox phenomenon. This review provides a comprehensive perspective on the future direction of studies on materials with anion redox activity.

The synergistic combination of oxygen vacancies, dual OER/ORR catalysts, and the complementary catalyst with oxygen-mediating capability in the Co3O4/Ru–CeO2/Li2O2−x successfully activated reversible anion redox reaction in lithia-based cathodes.

To realize the potential high capacity of lithium–oxygen (Li–O2) batteries, a double oxygen supply system for cells with high-loading cathodes is devised in this study. High-loading thick electrodes can achieve exceptionally high capacities, but this promise has been plagued by partial utilization of thick electrodes in Li–O2 cells due to the kinetic limitation imposed by oxygen transport. The proposed double oxygen supply system provides oxygen gas to the cathode not only from the cathode opening but also from the separator side to ensure sufficient oxygen supply to the whole high-loading electrode. Subsequently, the entire region of the high-loading cathode is rendered active, resulting in a uniform vertical distribution of discharge products. The maximum utilization of the high-loading electrodes is, thus, achieved, along with a remarkably increased capacity, low overpotential, and cycle life. By this strategy, CNT cathodes can be cycled with a capacity of 5 mAh cm–2, without using any additional catalyst.

Chemical compatibility of various polymeric binders and solvents with lithia is investigated for improved cycling stability of lithia-based cathodes.