Progress on Liquid Organic Electrolytes of Li-O2 Batteries

Abstract

Lithium-oxygen (Li-O₂) batteries have garnered significant attention due to their ultrahigh theoretical energy density, comparable to that of gasoline. However, despite this promise, several challenges have hindered the commercial application of Li-O₂ batteries. These challenges include poor reversibility, unsatisfactory cycling duration, and high overpotential during battery operation. The key factor behind the poor reversibility of current Li-O₂ batteries is the occurrence of side reactions between various battery components and discharge products or intermediates. The electrolyte, an essential component in Li-O₂ batteries, plays a crucial role in ion transport and mass transfer within the battery. Among the available electrolytes used in Li-O₂ batteries, liquid organic electrolytes have been predominantly investigated as the potential option. However, they suffer from insufficient chemical and electrochemical stability, which induce the overall poor reversibility. Substantial progress has been made to understand the factors that lead to the degradation of liquid organic electrolytes and to enhance their stability. However, there are still requirements for more significant improvements to achieve practical performance. This review comprehensively introduces the development of liquid organic electrolytes for Li-O₂ batteries, focusing on solvents, Li salts, and additives. It outlines the specific requirements of electrolytes for Li-O₂ batteries and highlights the importance of reducing charge overpotentials as a critical strategy to mitigate both electrochemical and chemical degradations. The review proceeds to detail the composition of liquid organic electrolytes, beginning with solvents. Carbonates, ethers, amides, and ionic liquids (ILs) are discussed, along with their respective advantages, disadvantages, and strategies to overcome limitations. The role of Li salts is then discussed, with an emphasis on the relationship between the properties of Li salts and electrolyte performance. Some Li salts exhibit additional functions, such as the ability to form a stable solid electrolyte interface (SEI) on the anode side and reduce overpotentials during charging. Additives in liquid organic electrolytes are also discussed. Redox mediators (RMs) and singlet oxygen (¹O₂) quenchers are discussed as representative additives, highlighting their importance in Li-O₂ batteries. RMs can influence the reaction mechanism, leading to lower overpotentials in both discharge and charge processes and increased capacity. Notably, classical RMs like LiI are introduced in detail, and criteria for selecting appropriate RMs are outlined. On the other hand, ¹O₂ quenchers convert aggressive ¹O₂ into harmless triplet oxygen (³O₂), thereby suppressing unwanted side reactions in Li-O₂ batteries. The mechanism behind ¹O₂ generation is also discussed. In summary, this review aims to provide a comprehensive overview of the progress in liquid organic electrolytes for Li-O₂ batteries. This comprehensive understanding will guide future research efforts towards developing more stable and efficient electrolytes for Li-O₂ batteries, thereby advancing their practical applications.

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