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

The Li–O2 battery (LOB) represents a promising candidate for future electric vehicles owing to its outstanding energy density. However, the practical application of LOB cells is largely blocked by the poor cycling performance of cathode materials. Herein, an ultralong 440‐cycle life of an LOB cell is achieved using CeO2 nanocubes super‐assembled on an inverse opal carbon matrix as the cathode material without any additives. CeO2 is proved to be effective for the complete and sensitive decomposition of loosely stacked Li2O2 films during the oxygen evolution reaction process and full accommodation of volume changes caused by the fast growth of Li2O2 films during the oxygen reduction reaction process. The super‐assembled porous CeO2/C frameworks satisfy critical requirements including controlled size, morphology, high Ce3+/Ce4+ ratio, and efficient volume change accommodation, which dramatically increase the cycle life of LOB cell to 440 cycles. This study reveals the design strategy for high performance CeO2 catalyst cathodes for LOB cells and the generation mechanisms of Li2O2 films during the discharge process by using density functional theory calculations, showing new avenues for improving the future smart design of CeO2‐based cathode catalysts for Li–O2 batteries.

Interfacial Super-Assembled Porous CeO2/C Frameworks Featuring Efficient and Sensitive Decomposing Li2O2 for Smart Li–O2 Batteries

Advances in modeling and simulation of Li–air batteries

Photoassisted electrochemical reaction is regarded as an effective approach to reduce the overpotential of lithium–oxygen (Li–O2) batteries. However, the achievement of both broadband absorption and long term battery cycling stability are still a formidable challenge. Herein, an oxygen vacancy‐mediated fast kinetics for a photoassisted Li–O2 system is developed with a silver/bismuth molybdate (Ag/Bi2MoO6) hybrid cathode. The cathode can offer both double advantages for light absorption covering UV to visible region and excellent electrochemical activity for O2. Upon discharging, the photoexcited electrons from Ag nanoplate based on the localized surface plasmon resonance (LSPR) are injected into the oxygen vacancy in Bi2MoO6. The fast oxygen reaction kinetics generate the amorphous Li2O2, and the discharge plateau is improved to 3.05 V. Upon charging, the photoexcited holes are capable to decompose amorphous Li2O2 promptly, yielding a very low charge plateau of 3.25 V. A first cycle round‐trip efficiency is 93.8% and retention of 70% over 500 h, which is the longest cycle life ever reported in photoassisted Li–O2 batteries. This work offers a general and reliable strategy for boosting the electrochemical kinetics by tailoring the crystalline of Li2O2 with wide‐band light.

Oxygen Vacancy‐Mediated Growth of Amorphous Discharge Products toward an Ultrawide Band Light‐Assisted Li–O2 Batteries

We simulated the discharge process of Li–O2 batteries and the growth of Li2O2 thin films at the mesoscale with a novel kinetic Monte Carlo model, which combined a stochastic description of mass transport and detailed elementary reaction kinetics. The simulation results show that the ordering of the Li2O2 thin film is determined by the interplay between diffusion and reaction kinetics. Due to the fast reaction kinetics on the catalyst, the Li2O2 formed in the presence of catalyst (cat-CNF) shows a low degree of ordering and is more likely to be amorphous. Moreover, the mobility of the LiO2 ion pair, which depends largely on the nature of the electrolyte, also impacts the homogeneity of the compactness of the Li2O2 thin film. These results are of high importance for understanding the role of the catalyst and reaction kinetics in Li–O2 batteries.

Compactness of the Lithium Peroxide Thin Film Formed in Li-O2 Batteries and Its Link to the Charge Transport Mechanism: Insights from Stochastic Simulations.

Yue Deng

Papers

Compactness of the Lithium Peroxide Thin Film Formed in Li-O2 Batteries and Its Link to the Charge Transport Mechanism: Insights from Stochastic Simulations.