The origin of potential rise during charging of Li-O2 batteries

Morphology-Dictated Mechanism of Efficient Reaction Sites for Li2O2 Decomposition

In the pursuit of a highly reversible lithium-oxygen (Li-O₂) battery, control of reaction sites to maintain stable conversion between O₂ and Li₂O₂ at the cathode side is imperatively desirable. However, the mechanism involving the reaction site during charging remains elusive, which, in turn, imposes challenges in recognition of the origin of overpotential. Herein, via combined investigations by in situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), we propose a universal morphology-dictated mechanism of efficient reaction sites for Li₂O₂ decomposition. It is found that Li₂O₂ deposits with different morphologies share similar localized conductivities, much higher than that reported for bulk Li₂O₂, enabling the reaction site not only at the electrode/Li₂O₂/electrolyte interface but also at the Li₂O₂/electrolyte interface. However, while the mass transport process is more enhanced at the former, the charge-transfer resistance at the latter is sensitively related to the surface structure and thus the reactivity of the Li₂O₂ deposit. Consequently, for compact disk-like deposits, the electrode/Li₂O₂/electrolyte interface serves as the dominant decomposition site, which causes premature departure of Li₂O₂ and loss of reversibility; on the contrary, for porous flower-like and film-like Li₂O₂ deposits bearing a larger surface area and richer surface-active structures, both the interfaces are efficient for decomposition without premature departure of the deposit so that the overpotential arises primarily from the sluggish oxidation kinetics and the decomposition is more reversible. The present work provides instructive insights into the understanding of the mechanism of reaction sites during the charge process, which offers guidance for the design of reversible Li-O₂ batteries.

Revealing the Intrinsic Atomic Structure and Chemistry of Amorphous LiO2-Containing Products in Li-O2 Batteries Using Cryogenic Electron Microscopy

Aprotic lithium-oxygen batteries (LOBs) are promising energy storage systems characterized by ultrahigh theoretical energy density. Extensive research has been devoted to this battery technology, yet the detailed operational mechanisms involved, particularly unambiguous identification of various discharge products and their specific distributions, are still unknown or are subjects of controversy. This is partly because of the intrinsic complexity of the battery chemistry but also because of the lack of atomic-level insight into the oxygen electrodes acquired via reliable techniques. In the current study, it is demonstrated that electron beam irradiation could induce crystallization of amorphous discharge products. Cryogenic conditions and a low beam dosage have to be used for reliable transmission electron microscopy (TEM) characterization. High-resolution cryo-TEM and electron energy loss spectroscopy (EELS) analysis of toroidal discharge particles unambiguously identified the discharge products as a dominating amorphous LiO₂ phase with only a small amount of nanocrystalline Li₂O₂ islands dispersed in it. In addition, uniform mixing of carbon-containing byproducts is identified in the discharge particles with cryo-EELS, which leads to a slightly higher charging potential. The discharge products can be reversibly cycled, with no visible residue after full recharge. We believe that the amorphous superoxide dominating discharge particles can lead researchers to reconsider the chemistry of LOBs and pay special attention to exclude beam-induced artifacts in traditional TEM characterizations.

Revealing the Correlations between Morphological Evolution and Surface Reactivity of Catalytic Cathodes in Lithium-Oxygen Batteries

Lithium-oxygen batteries suffer from the degradation of the catalytic cathode during long-term operation, which limits their practical use. Understanding the direct correlations between the surface morphological evolution of catalytic cathodes at nanoscale and their catalytic activity during cycling has proved challenging. Here, using in situ electrochemical atomic force microscopy, the dynamic evolution of the Pt nanoparticles electrode in a working Li-O₂ battery and its effects on the Li-O₂ interfacial reactions are visualized. In situ views show that repeated oxidation-reduction cycles (ORCs) trigger the increase in the size of Pt nanoparticles, eventually causing the Pt nanoparticles to fall off the electrode. In 0-80 ORCs, the grown Pt nanoparticles promote the conversion of the Li-O₂ reaction route from the surface-mediated pathway to the solution-mediated pathway during discharging and significantly increase the discharge capacity. After 250 ORCs, accompanied by the part of the Pt nanoparticles detaching from the electrode, the nucleation potential of reaction product decreases, and the reaction dynamic slows down, which cause the performance to degrade. Modification of a proper amount of Au nanoparticle on the Pt nanoparticles electrode could improve its stability and maintain the high catalytic activity. These results provide a direct evidence for clarifying the correlations between morphological evolution and surface reactivity of catalytic cathodes during cycling, which is critical for developing high-performance catalysts.

Surface Mechanism of Catalytic Electrodes in Lithium-Oxygen Batteries: How Nanostructures Mediate the Interfacial Reactions

The use of catalysts is the key to boost electrode reactions in lithium-oxygen (Li-O₂) batteries. In-depth understanding of the nanoscale catalytic effect at electrode/electrolyte interfaces is of great significance for guiding a design of functionally optimized catalyst. Here, using electrochemical atomic force microscopy, we present the real-time imaging of interfacial evolution on nanostructured Au electrodes in a working battery, revealing that the nanostructure of Au is directly related to the catalytic activity toward oxygen reduction reaction (ORR)/oxygen evolution reaction (OER). In situ views show that nanoporous Au with a size of ∼14 nm for ligaments and ∼5 nm for nanopores promote the nucleation and growth of discharge product Li₂O₂ with large size at a high discharge voltage, yet densely packed Au nanoparticles with a diameter of ∼15 nm could catalyze Li₂O₂ to fully decompose via the top-bottom approach at a low charge potential. In addition, the difference in the nucleation potential of Li₂O₂ on the electrode with hybrid nanostructures could result in an uneven distribution of discharge products, which is alleviated at a large discharge rate and the capacity of the battery is improved significantly. These observations provide deep insights into the mechanisms of Li-O₂ interfacial reaction catalyzed by nanostructured catalysts and strategies for improving Li-O₂ batteries.

Current Challenges and Routes Forward for Nonaqueous Lithium-Air 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 advances have been achieved both in the mechanistic understanding of the cell reactions and in the development of effective strategies to help realize a practical energy storage device. By drawing attention to reports published mainly within the past 8 years, this review provides an updated mechanistic picture of the lithium peroxide based cell reactions and highlights key remaining challenges, including those due to the parasitic processes occurring at the reaction product-electrolyte, product-cathode, electrolyte-cathode, and electrolyte-anode interfaces. We introduce the fundamental principles and critically evaluate the effectiveness of the different strategies that have been proposed to mitigate the various issues of this chemistry, which include the use of solid catalysts, redox mediators, solvating additives for oxygen reaction intermediates, gas separation membranes, etc. Recently established cell chemistries based on the superoxide, hydroxide, and oxide phases are also summarized and discussed.