Redox Mediators: A Solution for Advanced Lithium–Oxygen Batteries

Advances in Functional Organosulfur-Based Mediators for Regulating Performance of Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are promising next-generation energy storage systems due to their high theoretical energy density. However, their practical applications are hindered by lithium dendrite growth and various intricate issues associated with the cathodes. These challenges can be mitigated by using organosulfur-based mediators (OSMs), which offer the advantages of abundance, tailorable structures, and unique functional adaptability. These features enable the rational design of targeted functionalities, enhance the interfacial stability of the lithium anode and cathode, and accelerate the redox kinetics of electrodes via alternative reaction pathways, thereby effectively improving the performance of LMBs. Unlike the extensively explored field of organosulfur cathode materials, OSMs have garnered little attention. This review systematically summarizes recent advancements in OSMs for various LMB systems, including lithium-sulfur, lithium-selenium, lithium-oxygen, lithium-intercalation cathode batteries, and other LMB systems. It briefly elucidates the operating principles of these LMB systems, the regulatory mechanisms of the corresponding OSMs, and the fundamentals of OSMs activity. Ultimately, strategic optimizations are proposed for designing novel OSMs, advanced mechanism investigation, expanded applications, and the development of safe battery systems, thereby providing directions to narrow the gap between rational modulation of organosulfur compounds and their practical implementation in batteries.

A Computational Study on Halogen/Halide Redox Mediators and Their Role in 1O2 Release in Aprotic Li-O2 Batteries

We present a computational study on the redox reactions of small clusters of Li superoxide and peroxide in the presence of halogen/halide redox mediators. The study is based on DFT calculations with a double hybrid functional and an implicit solvent model. It shows that iodine is less effective than bromine in the oxidation of Li2O2 to oxygen. On the basis of our thermodynamic data, in solvents with a low dielectric constant, iodine does not spontaneously promote either the oxidation of Li2O2 or the release of singlet oxygen, while bromine could spontaneously trigger both events. When a solvent with a large dielectric constant is used, both halogens appear to be able, at least on the basis of thermodynamics, to react spontaneously with the oxides, and the ensuing reaction sequence turned out to be strongly exoergic, thereby providing a route for the release of significant amounts of singlet oxygen. The role of spin-orbit coupling in providing a mechanism for singlet-triplet intersystem crossing has also been assessed.

Edge-Site-Free and Topological-Defect-Rich Carbon Cathode for High-Performance Lithium-Oxygen Batteries

The rational design of a stable and catalytic carbon cathode is crucial for the development of rechargeable lithium-oxygen (LiO2 ) batteries. An edge-site-free and topological-defect-rich graphene-based material is proposed as a pure carbon cathode that drastically improves LiO2 battery performance, even in the absence of extra catalysts and mediators. The proposed graphene-based material is synthesized using the advanced template technique coupled with high-temperature annealing at 1800 °C. The material possesses an edge-site-free framework and mesoporosity, which is crucial to achieve excellent electrochemical stability and an ultra-large capacity (>6700 mAh g-1 ). Moreover, both experimental and theoretical structural characterization demonstrates the presence of a significant number of topological defects, which are non-hexagonal carbon rings in the graphene framework. In situ isotopic electrochemical mass spectrometry and theoretical calculations reveal the unique catalysis of topological defects in the formation of amorphous Li2 O2 , which may be decomposed at low potential (∼ 3.6 V versus Li/Li+ ) and leads to improved cycle performance. Furthermore, a flexible electrode sheet that excludes organic binders exhibits an extremely long lifetime of up to 307 cycles (>1535 h), in the absence of solid or soluble catalysts. These findings may be used to design robust carbon cathodes for LiO2 batteries.

Single-Atomic Zn/Co-Nx Sites Boost Solid-Soluble Synergistic Catalysis for Lithium-Oxygen Batteries

Lithium-oxygen batteries have attracted widespread attention owing to their superior theoretical energy density. However, they are obstructed by sluggish oxygen reduction (ORR) and evolution reaction (OER) kinetics at air cathodes. Herein, different from using single solid or soluble catalysts, solid-soluble synergistic catalysis is proposed to conjointly enhance ORR/OER performances. During discharge, single-atomic zinc/cobalt embedded in nitrogen-doped carbon (Zn, Co-N/C) is judiciously engineered as a solid catalyst to regulate the growth pathway of Li₂O₂ and promote ORR kinetics. During charge, a typical redox mediator (RM, LiI) is added as a soluble catalyst to permit efficient oxidation of Li₂O₂. Of note is that the atomic Zn/Co-N_x sites can chemically adsorb oxidized iodine (I₂) and accelerate OER kinetics, which plays a decisive role in eliminating the shuttle effect of I₃^-/I₂ to the Li anode. Coupling a single-atomic catalyst with restricted oxidized iodine offers an exceptional discharge capacity, remarkably low polarization, and superior long-term cycling stability.

The key to improving the performance of Li-air batteries: Recent progress and challenges of the catalysts

Li-air batteries are considered to be one of the most promising energy storage devices due to their high energy density and large specific capacity. But the high overpotential, the sluggish...

OUP accepted manuscript

Aprotic lithium–oxygen (Li–O₂) batteries are receiving intense research interest by virtue of their ultra-high theoretical specific energy. However, current Li–O₂ batteries are suffering from severe barriers, such as sluggish reaction kinetics and undesired parasitic reactions. Recently, molecular catalysts, i.e. redox mediators (RMs), have been explored to catalyse the oxygen electrochemistry in Li–O₂ batteries and are regarded as an advanced solution. To fully unlock the capability of Li–O₂ batteries, an in-depth understanding of the catalytic mechanisms of RMs is necessary. In this review, we summarize the working principles of RMs and their selection criteria, highlight the recent significant progress of RMs and discuss the critical scientific and technical challenges on the design of efficient RMs for next-generation Li–O₂ batteries.

Versatile Redox-Active Organic Materials for Rechargeable Energy Storage

With the ever-increasing demand on energy storage systems and subsequent mass production, there is an urgent need for the development of batteries with not only improved electrochemical performance but also better sustainability-related features such as environmental friendliness and low production cost. To date, transition metals that are sparse have been centrally employed in energy storage devices ranging from portable lithium ion batteries (e.g., cobalt and nickel) to large-scale redox flow batteries (e.g., vanadium). Toward the sustainable battery chemistry, there are ongoing efforts to replace the transition metal-based electrode materials in these systems to redox-active organic materials (ROMs). Most ROMs are composed of the earth abundant elements (e.g., carbon, nitrogen, oxygen, sulfur), thus are less restrained by the resource, and their production does not require high-energy consuming processes. Furthermore, the structural diversity and chemical tunability of organic compounds make them more attractive for the versatile design of future energy storage systems. Accordingly, the timely development of high-performance ROM-based electrodes would expedite the shift from the current resource-limited battery chemistry to more sustainable energy solutions.In this Account, we provide an overview of the endeavors to employ and develop ROMs as high-performance active materials for various battery systems. Diverse approaches will be introduced starting from the new ROM design mimicking the energy carrying molecules in biological metabolism to the chemical modifications to tailor the properties for specific battery systems. The molecular redesign of ROM, for example, can be carried out by substituting heteroatoms in the redox center, which leads to the enhancement of the redox potential by the inductive effect. Or, tailoring the ROM molecule by removing redox-inactive functionals results in a reduced molecular weight, thereby an increased specific capacity. The intrinsic limitations of ROMs, such as the low electrical conductivity and the dissolving nature, have been under extensive scrutiny; however, they can be partly addressed through efforts including intermolecular fusion and/or nanoscale hybridization with a conducting scaffold. On the other hand, this problematic dissolving nature of ROMs makes them appealing for some new battery configurations such as redox flow batteries that employ the liquid-state active materials. The high solubility and the stability of the ROM were found to be beneficial in attaining the enhanced energy density and the cycle stability of flow batteries, which could be further optimized by the chemical modifications of ROMs. Besides the role of active materials, the redox activity of ROMs has also enabled their use as catalysts to promote the electrode reaction in metal-air batteries. The redox capability of the ROM was often proven to be effective in the solution-based redox mediation that facilitates both the charging and discharging reaction in metal-air batteries. Finally, we conclude this account by proposing the future research directions regarding the fundamental electrochemistry and the further practical development of ROMs for the sustainable rechargeable energy storage.

Suppressing Singlet Oxygen Formation during the Charge Process of Li-O2 Batteries with a Co3O4 Solid Catalyst Revealed by Operando Electron Paramagnetic Resonance

Aprotic lithium-oxygen (Li-O₂) batteries promise high energy, but the cycle life has been plagued by two major obstacles, the insulating products and highly reactive singlet oxygen (¹O₂), which cause higher overpotential and parasitic reactions, respectively. A solid-state catalyst is known to reduce overpotential; however, it is unclear whether it affects ¹O₂ generation. Herein, Co₃O₄ was employed as the representative catalyst in Li-O₂ batteries, and ¹O₂ generation was investigated by ex-situ and operando electron paramagnetic resonance (EPR) spectroscopy. By comparing a carbon nanotube (CNT) cathode with a Co₃O₄/CNT cathode, we find that ¹O₂ generation in the charge process can be suppressed by the Co₃O₄ catalyst. After carefully studying the discharge products on the two electrodes and the corresponding decomposition processes, we conclude that a LiO₂-like species is responsible for the ¹O₂ generation during the early charge stage. The Co₃O₄ catalyst reduces the amount of LiO₂-like species in discharge products, and thus the ¹O₂ formation is suppressed.

Mechanism of mediated alkali peroxide oxidation and triplet versus singlet oxygen formation

Aprotic alkali metal-O₂ batteries face two major obstacles to their chemistry occurring efficiently, the insulating nature of the formed alkali superoxides/peroxides and parasitic reactions that are caused by the highly reactive singlet oxygen (¹O₂). Redox mediators are recognized to be key for improving rechargeability. However, it is unclear how they affect ¹O₂ formation, which hinders strategies for their improvement. Here we clarify the mechanism of mediated peroxide and superoxide oxidation and thus explain how redox mediators either enhance or suppress ¹O₂ formation. We show that charging commences with peroxide oxidation to a superoxide intermediate and that redox potentials above ~3.5 V versus Li/Li⁺ drive ¹O₂ evolution from superoxide oxidation, while disproportionation always generates some ¹O₂. We find that ¹O₂ suppression requires oxidation to be faster than the generation of ¹O₂ from disproportionation. Oxidation rates decrease with growing driving force following Marcus inverted-region behaviour, establishing a region of maximum rate.

Bifunctional 1-Boc-3-Iodoazetidine Enhancing Lithium Anode Stability and Rechargeability of Lithium-Oxygen Batteries

Lithium anode protection is an effective strategy to prohibit the continuous loss of redox mediators (RMs) resulting from the unfavorable "shuttle effect" in lithium-oxygen batteries. In this work, an in situ Li anode protection method is designed by utilizing an organic compound, 1-Boc-3-iodoazetidine (BIA), as both a RM and an additive, to form a lithium anode protective layer. The reaction between Li metal and BIA can form lithium iodide (LiI) and lithium-based organometallic. LiI can effectively reduce the charging overpotential. Meanwhile, the in situ-formed anode protection layer (lithium-based organometallic) can not only effectively prevent RMs from being reduced by the lithium metal, but also inhibit the growth of lithium dendrites. As a result, the lithium-oxygen battery with BIA shows a long cycle life of 260 cycles with a notably reduced charging potential. In particular, the battery with BIA achieves an excellent lifespan of 160 cycles at a large current density of 2000 mA g^-1.

Operando Synchrotron XRD of Bromide Mediated Li-O2 Battery

Li-O₂ battery technology offers large theoretical energy density, considered a promising alternative energy storage technology for a variety of applications. One of the main advances made in recent years is the use of soluble catalysts, known as redox mediators (RM), decreasing the charge overpotential and improving cyclability. Despite its potential, much is still unknown regarding its dynamic, especially over higher loading electrodes, where mass transport may be an issue and the interplay with common impurities in the electrolyte, like residual water. Here we perform for the first time an operando XRD characterization of a DMSO-based LiBr mediated Li-O₂ battery with a high loading electrode based on CNTs aiming to reveal these dynamics and track chemical changes in the electrode. Our results show that, depending on the electrode architecture, the system's issue can move from catalytic to a mass transfer. We also assess the effect of residual water in the system to better understand the reaction routes. As a result, we observed that with DMSO, the system is even more sensitive to water contamination compared to glyme-based studies reported in the literature. Despite the activity of LiBr on the Li-peroxide oxidation and its contribution to cyclability, with the system and electrode configuration used in this study, we verified that a mass transfer limitation caused a cell "sudden death" caused by clogging after cycling.

Mechanism for Preserving Volatile Nitrogen Dioxide and Sustainable Redox Mediation in the Nonaqueous Lithium-Oxygen Battery

Excessive overpotential during charging is a major hurdle in lithium-oxygen (Li-O₂) battery technology. NO₂^-/NO₂ redox mediation is an efficient way to substantially reduce the overpotential and to enhance oxygen efficiency and cycle life by suppressing parasitic reactions. Considering that nitrogen dioxide (NO₂) is a gas, it is quite surprising that NO₂^-/NO₂ redox reactions can be sustained for a long cycle life in Li-O₂ batteries with such an open structure. A detailed study with in situ differential electrochemical mass spectrometry (DEMS) elucidated that NO₂ could follow three reaction pathways during charging: (1) oxidation of Li₂O₂ to evolve oxygen, (2) vaporization, and (3) conversion into NO₃^-. Among the pathways, Li₂O₂ oxidation occurs exclusively in the presence of Li₂O₂, which suggests that NO₂ has high reactivity to Li₂O₂. At the end of the charging process, most of the volatile oxidized couple (NO₂) is stored by conversion to a stable third species (NO₃^-), which is then reused for producing the reduced couple (NO₂^-) in the next cycle. The dominant reaction of Li₂O₂ oxidation involves the temporary storage of NO₂ as a stable third species during charging, which is an innovative way for preserving the volatile redox couple, resulting in a sustainable redox mediation for a high-performance Li-O₂ battery.

Predicting the chemical reactivity of organic materials using a machine-learning approach

Stability and compatibility between chemical components are essential parameters that need to be considered in the selection of functional materials in configuring a system. In configuring devices such as batteries or solar cells, not only the functionality of individual constituting materials such as electrodes or electrolyte but also an appropriate combination of materials which do not undergo unwanted side reactions is critical in ensuring their reliable performance in long-term operation. While the universal theory that can predict the general chemical reactivity between materials is long awaited and has been the subject of studies with a rich history, traditional ways proposed to date have been mostly based on simple electronic properties of materials such as electronegativity, ionization energy, electron affinity and hardness/softness, and could be applied to only a small group of materials. Moreover, prediction has often been far from accurate and has failed to offer general implications; thus it was practically inadequate as a selection criterion from a large material database, i.e. data-driven material discovery. Herein, we propose a new model for predicting the general reactivity and chemical compatibility among a large number of organic materials, realized by a machine-learning approach. As a showcase, we demonstrate that our new implemented model successfully reproduces previous experimental results reported on side-reactions occurring in lithium–oxygen electrochemical cells. Furthermore, the mapping of chemical stability among more than 90 available electrolyte solvents and the representative redox mediators is realized by this approach, presenting an important guideline in the development of stable electrolyte/redox mediator couples for lithium–oxygen batteries.

Stability and compatibility between chemical components are essential parameters that need to be considered in the selection of functional materials in configuring a system.

Lithium-Oxygen Batteries and Related Systems: Potential, Status, and Future

The goal of limiting global warming to 1.5 °C requires a drastic reduction in CO₂ emissions across many sectors of the world economy. Batteries are vital to this endeavor, whether used in electric vehicles, to store renewable electricity, or in aviation. Present lithium-ion technologies are preparing the public for this inevitable change, but their maximum theoretical specific capacity presents a limitation. Their high cost is another concern for commercial viability. Metal-air batteries have the highest theoretical energy density of all possible secondary battery technologies and could yield step changes in energy storage, if their practical difficulties could be overcome. The scope of this review is to provide an objective, comprehensive, and authoritative assessment of the intensive work invested in nonaqueous rechargeable metal-air batteries over the past few years, which identified the key problems and guides directions to solve them. We focus primarily on the challenges and outlook for Li-O₂ cells but include Na-O₂, K-O₂, and Mg-O₂ cells for comparison. Our review highlights the interdisciplinary nature of this field that involves a combination of materials chemistry, electrochemistry, computation, microscopy, spectroscopy, and surface science. The mechanisms of O₂ reduction and evolution are considered in the light of recent findings, along with developments in positive and negative electrodes, electrolytes, electrocatalysis on surfaces and in solution, and the degradative effect of singlet oxygen, which is typically formed in Li-O₂ cells.

Guidelines and trends for next-generation rechargeable lithium and lithium-ion batteries

This review article summarizes the current trends and provides guidelines towards next-generation rechargeable lithium and lithium-ion battery chemistries.

Electrolytes for Rechargeable Lithium-Air Batteries

Lithium-air batteries are promising devices for electrochemical energy storage because of their ultrahigh energy density. However, it is still challenging to achieve practical Li-air batteries because of their severe capacity fading and poor rate capability. Electrolytes are the prime suspects for cell failure. In this Review, we focus on the opportunities and challenges of electrolytes for rechargeable Li-air batteries. A detailed summary of the reaction mechanisms, internal compositions, instability factors, selection criteria, and design ideas of the considered electrolytes is provided to obtain appropriate strategies to meet the battery requirements. In particular, ionic liquid (IL) electrolytes and solid-state electrolytes show exciting opportunities to control both the high energy density and safety.