Promoting Surface-Mediated Oxygen Reduction Reaction of Solid Catalysts in Metal–O2 Batteries by Capturing Superoxide Species

Controllable Regulation of the Oxygen Redox Process in Lithium-Oxygen Batteries by High-Configuration-Entropy Spinel with an Asymmetric Octahedral Structure

Designing bifunctional electrocatalysts to boost oxygen redox reactions is critical for high-performance lithium-oxygen batteries (LOBs). In this work, high-entropy spinel (Co0.2Mn0.2Ni0.2Fe0.2Cr0.2)3O4 (HEOS) is fabricated by modulating the internal configuration entropy of spinel and studied as the oxygen electrode catalyst in LOBs. Under the high-entropy atomic environment, the Co-O octahedron in spinel undergoes asymmetric deformation, and the reconfiguration of the electron structure around the Co sites leads to the upward shift of the d-orbital centers of the Co sites toward the Fermi level, which is conducive to the strong adsorption of redox intermediate LiO2 on the surface of the HEOS, ultimately forming a layer of a highly dispersed Li2O2 thin film. Thin-film Li2O2 is beneficial for ion diffusion and electron transfer at the electrode-electrolyte interface, which makes the product easy to decompose during the charge process, ultimately accelerating the kinetics of oxygen redox reactions in LOBs. Based on the above advantages, HEOS-based LOBs deliver high discharge/charge capacity (12.61/11.72 mAh cm-2) and excellent cyclability (424 cycles). This work broadens the way for the design of cathode catalysts to improve oxygen redox kinetics in LOBs.

Accelerating S↔Li2 S Reactions in Li-S Batteries through Activation of S/Li2 S with a Bifunctional Semiquinone Catalyst

The reaction rate bottleneck during interconversion between insulating S8 (S) and Li2 S fundamentally leads to incomplete conversion and restricted lifespan of Li-S battery, especially under high S loading and lean electrolyte conditions. Herein, we demonstrate a new catalytic chemistry: soluble semiquinone, 2-tertbutyl-semianthraquinone lithium (Li+ TBAQ⋅- ), as both e- /Li+ donor and acceptor for simultaneous S reduction and Li2 S oxidation. The efficient activation of S and Li2 S by Li+ TBAQ⋅- in the initial discharging/charging state maximizes the amount of soluble lithium polysulfide, thereby substantially improve the rate of solid-liquid-solid reaction by promoting long-range electron transfer. With in situ Raman spectra and theoretical calculations, we reveal that the activation of S/Li2 S is the rate-limiting step for effective S utilization under high S loading and low E/S ratio. Beyond that, the S activation ratio is firstly proposed as an accurate indicator to quantitatively evaluate the reaction rate. As a result, the Li-S batteries with Li+ TBAQ⋅- deliver superior cycling performance and over 5 times higher S utilization ratio at high S loading of 7.0 mg cm-2 and a current rate of 1 C compared to those without Li+ TBAQ⋅- . We hope this study contributes to the fundamental understanding of S redox chemical and inspires the design of efficient catalysis for advanced Li-S batteries.

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal-O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Amphi-Active Superoxide-Solvating Charge Redox Mediator for Highly Stable Lithium-Oxygen Batteries

A multifunctional electrolyte additive for lithium oxygen batteries (LOBs) was designed to have (1) a redox-active moiety to mediate decomposition of lithium peroxide (Li₂O₂ as the final discharge product) during charging and (2) a solvent moiety to solvate and stabilize lithium superoxide (LiO₂ as the intermediate discharge product) in electrolyte during discharging. 4-Acetamido-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidin-1-yl)oxyl) or AAT was employed as the additive working for both charge and discharge processes (amphi-active). The redox-active moiety was rooted in TEMPO, while the acetamido (AA) functional group inherited the high donor number (DN) of N,N-dimethylacetamide (DMAc). Integrating two functional moieties (TEMPO and AA) into a single molecule resulted in the bifunctionality of AAT (1) facilitating Li₂O₂ decomposition by the TEMPO moiety and (2) encouraging the solvent mechanism of Li₂O₂ formation by the high-DN AA moiety. Significantly improved LOB performances were achieved by the superoxide-solvating charge redox mediator, which were not obtained by a simple cocktail of TEMPO and DMAc.

Critical Factors Affecting the Catalytic Activity of Redox Mediators on Li-O2 Battery Discharge

Redox mediators (RMs) have a substantial ability to govern oxygen reduction reaction (ORR) in Li-O₂ batteries, which can realize large capacity and high-rate capability. However, studies on understanding RM-assisted ORR mechanisms are still in their infancy. Herein, a quinone-based molecule, vitamin K1 (VK1), is first used as the ORR RM for Li-O₂ batteries, together with 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), to elucidate key factors on the catalytic activity of RMs. By combining experiments and first-principle computations, we demonstrate that the reduced VK1 has strong oxygen affinity and can effectively retard the deposition of Li₂O₂ films on the electrode surface, thereby guaranteeing enough active sites for electron transfer. Besides, the low reaction free energy of disproportionation of the Li(VK1)O₂ intermediate into Li₂O₂ also significantly accelerates the ORR process. Consequently, the catalytic activity of VK1 is significantly boosted, and the discharge capacity of VK1-assisted batteries is 3.2-4.5 times that of DBBQ-assisted batteries. This study provides new insight for better understanding the working roles of RMs in Li-O₂ batteries.

Hydrogen-Bond-Assisted Solution Discharge in Aprotic Li-O2 Batteries

Surface discharge mechanism induced cathode passivation is a critical challenge that blocks the full liberation of the ultrahigh theoretical energy density in aprotic Li-O₂ batteries. Herein, a facile and universal concept of hydrogen-bond-assisted solvation is proposed to trigger the solution discharge process for averting the shortcomings associated with surface discharge. 2,5-Di-tert-butylhydroquinone (DBHQ), an antioxidant with hydroxyl groups, is introduced as an exemplary soluble catalyst to promote solution discharge by hydrogen-bond-assisted solvation of O₂ ^- and Li₂ O₂ (OH···O). Thus, a Li-O₂ battery with 50 × 10^-3 m DBHQ delivers an extraordinary discharge capacity of 18 945 mAh g^-1 (i.e., 9.47 mAh cm^-2 ), even surpassing the capacity endowed by the state-of-the-art reduction mediator of 2,5-di-tert-butyl-1,4-benzoquinone. Besides, an ultrahigh Li₂ O₂ yield of 97.1% is also achieved due to the depressed reactivity of the reduced oxygen-containing species (O₂ ^- , LiO₂ , and Li₂ O₂ ) by the solvating and antioxidative abilities of DBHQ. Consequently, the Li-O₂ battery with DBHQ exhibits excellent cycling lifetime and rate capability. Furthermore, the generalizability of this approach of hydrogen-bond-assisted solution discharge is verified by other soluble catalysts that contain OH or NH groups, with implications that could bring Li-O₂ batteries one step closer to being a viable technology.

Soluble and Perfluorinated Polyelectrolyte for Safe and High-Performance Li-O2 Batteries

The severe performance degradation of high-capacity Li-O₂ batteries induced by Li dendrite growth and concentration polarization from the low Li⁺ transfer number of conventional electrolytes hinder their practical applications. Herein, lithiated Nafion (LN) with the sulfonic group immobilized on the perfluorinated backbone has been designed as a soluble lithium salt for preparing a less flammable polyelectrolyte solution, which not only simultaneously achieves a high Li⁺ transfer number (0.84) and conductivity (2.5 mS cm^-1 ), but also the perfluorinated anion of LN produces a LiF-rich SEI for protecting the Li anode from dendrite growth. Thus, the Li-O₂ battery with a LN-based electrolyte achieves an all-round performance improvement, like low charge overpotential (0.18 V), large discharge capacity (9508 mAh g^-1 ), and excellent cycling performance (225 cycles). Besides, the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.

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.

Direct In Situ Spectroscopic Evidence for Solution-Mediated Oxygen Reduction Reaction Intermediates in Aprotic Lithium-Oxygen Batteries

A fundamental understanding of the reaction process is essential to predict and enhance the performance of electrochemical devices. As a central reaction in aprotic lithium-oxygen (Li-O₂) batteries, the oxygen reduction reaction (ORR) has been confronted with the "sudden-death" phenomenon caused by the cathode passivation from discharge product Li₂O₂. The soluble catalyst (e.g., reduction mediator) promoted solution-mediated ORR represents an elegant solution. However, no direct molecular evidence is available so far, and its link to Li-O₂ batteries performance remains hypothetical. Here, we present in situ surface-enhanced Raman spectroscopy and obtain direct spectroscopic evidence (i.e., LiAQ and LiAQO₂) of the solution-mediated ORR on a model anthraquinone (AQ, a typical reduction mediator)-immobilized Au electrode. With the assistance of density functional theory calculations and differential electrochemical mass spectrometry, the related elementary reaction steps of the solution-mediated ORR are proposed. This work provides intuitive insights into the AQ-catalyzed solution-mediated ORR mechanism that is helpful in the optimization and tailor-design of soluble catalysts for excellent next-generation Li-O₂ batteries.

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...

Cation-Tuning Induced d-Band Center Modulation on Co-based Spinel Oxide for Rechargeable Zn-Air Batteries

Atomic substitutions at the tetrahedral site (A Td ) could theoretically achieve an efficient optimization of the charge at the octahedral site (B Oh ) through the A Td -O-B Oh interactions in the spinel oxides (AB2O4). However, the precise control and adjustment of the spinel oxides are still challenging owing to the complexity of their crystal structure. In this work, we demonstrate a simple solvent method to tailor the structures of spinel oxides and further use the spinel oxide composites (ACo2O4/NCNTs, A = Mn, Co, Ni, Cu, Zn) for oxygen electrocatalysis. And the optimized MnCo2O4/NCNTs exhibit high activity and excellent durability for oxygen reduction/evolution reactions. Remarkably, the rechargeable liquid Zn-air battery equipped the MnCo2O4/NCNTs cathode affords a specific capacity of 827 mAh gZn-1 with high power density of 74.63 mW cm-2 and no voltage degradation after 300 cycles at a high charging-discharging rate (5 mA cm-2). The density functional theory (DFT) calculations reveal that the substitution could regulate the ratio of Co3+/Co2+ and thereby lead to the electronic structure modulated accompanied with the movement of d-band center. The tetrahedral and octahedral sites interact through the Mn-O-Co, the Co3+ Oh of MnCo2O4 with the optimal charge structure allows more suitable binding interaction between the active center and the oxygenated species, resulting in superior oxygen electrocatalytic performance. This work not only proves the influence of the charge modulation mechanism on the oxygen catalysis process but also provides novel strategies for the subsequent design of other oxygen catalysis materials.

Revealing the catalytic pathway of a quinone-mediated oxygen reduction reaction in aprotic Li-O2 batteries

Soluble redox species that facilitate the oxygen reduction reaction by mediating the LiO₂ intermediate and consequently the formation of the Li₂O₂ have attracted considerable interest for Li-O₂ batteries. Based on extensive radical studies, this work discloses a distinct solution reaction route when a quinone derivative was employed as a redox mediator.

Lithium superoxide encapsulated in a benzoquinone anion matrix

Lithium peroxide is the crucial storage material in lithium-air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to p-benzoquinone (p-C6H4O2) vapor, develops a deep blue color. This blue powder can be formally described as [Li2O2][Formula: see text] [LiO2][Formula: see text] {Li[p-C6H4O2]}0.7, though spectroscopic characterization indicates a more nuanced structural speciation. Infrared, Raman, electron paramagnetic resonance, diffuse-reflectance ultraviolet-visible and X-ray absorption spectroscopy reveal that the lithium salt of the benzoquinone radical anion forms on the surface of the lithium peroxide, indicating the occurrence of electron and lithium ion transfer in the solid state. As a result, obligate lithium superoxide is formed and encapsulated in a shell of Li[p-C6H4O2] with a core of Li2O2 Lithium superoxide has been proposed as a critical intermediate in the charge/discharge cycle of Li-air batteries, but has yet to be isolated, owing to instability. The results reported herein provide a snapshot of lithium peroxide/superoxide chemistry in the solid state with redox mediation.

Synergetic Advantages of Atomically Coupled 2D Inorganic and Graphene Nanosheets as Versatile Building Blocks for Diverse Functional Nanohybrids

2D nanostructured materials, including inorganic and graphene nanosheets, have evoked plenty of scientific research activity due to their intriguing properties and excellent functionalities. The complementary advantages and common 2D crystal shapes of inorganic and graphene nanosheets render their homogenous mixtures powerful building blocks for novel high-performance functional hybrid materials. The nanometer-level thickness of 2D inorganic/graphene nanosheets allows the achievement of unusually strong electronic couplings between sheets, leading to a remarkable improvement in preexisting functionalities and the creation of unexpected properties. The synergetic merits of atomically coupled 2D inorganic-graphene nanosheets are presented here in the exploration of novel heterogeneous functional materials, with an emphasis on their critical roles as hybridization building blocks, interstratified sheets, additives, substrates, and deposited monolayers. The great flexibility and controllability of the elemental compositions, defect structures, and surface natures of inorganic-graphene nanosheets provide valuable opportunities for exploring high-performance nanohybrids applicable as electrodes for supercapacitors and rechargeable batteries, electrocatalysts, photocatalysts, and water purification agents, to give some examples. An outlook on future research perspectives for the exploitation of emerging 2D nanosheet-based hybrid materials is also presented along with novel synthetic strategies to maximize the synergetic advantage of atomically mixed 2D inorganic-graphene nanosheets.

In situ Construction of Robust Biphasic Surface Layers on Lithium Metal for Lithium-Sulfide Batteries with Long Cycle Life

Lithium-sulfur (Li-S) batteries have potential in high energy density battery systems. However, intermediates of lithium polysulfides (LiPSs) can easily shuttle to the Li anode and react with Li metal to deplete the active materials and cause rapid failure of the battery. A facile solution pretreatment method for Li anodes involving a solution of metal fluorides/dimethylsulfoxide was developed to construct robust biphasic surface layers (BSLs) in situ. The BSLs consist of lithiophilic alloy (Li_x M) and LiF phases on Li metal, which inhibit the shuttle effect and increase the cycle life of Li-S batteries. The BSLs allow Li⁺ transport and they inhibit dendrite growth and shield the Li anodes from corrosive reaction with LiPSs. Li-S batteries containing BSLs-Li anodes demonstrate excellent cycling over 1000 cycles at 1 C and simultaneously maintain a high coulombic efficiency of 98.2 %. Based on our experimental and theoretical results, we propose a strategy for inhibition of the shuttle effect that produces high stability Li-S batteries.

Boosting the Performance of Graphene Cathodes in Na-O2 Batteries by Exploiting the Multifunctional Character of Small Biomolecules

Graphene aerogels derived from a biomolecule-assisted aqueous electrochemical exfoliation route are explored as cathode materials in sodium-oxygen (Na-O₂ ) batteries. To this end, the natural nucleotide adenosine monophosphate (AMP) is used in the multiple roles of exfoliating electrolyte, aqueous dispersant, and functionalizing agent to access high quality, electrocatalytically active graphene nanosheets in colloidal suspension (bioinks). The surface phenomena occurring on the electrochemically derived graphene cathode is thoroughly studied to understand and optimize its electrochemical performance, where a cooperative effect between the nitrogen atoms and phosphates from the AMP molecules is demonstrated. Moreover, the role of the nitrogen atoms in the adenine nucleobase of AMP and short-chain phosphate is unraveled. Significantly, the use of such cathodes with a proper amount of AMP molecules adsorbed on the graphene nanosheets delivers a discharge capacity as high as 9.6 mAh cm^-2 and performs almost 100 cycles with a considerably reduced cell overpotential and a coulombic efficiency of ≈97% at high current density (0.2 mA cm^-2 ). This study opens a path toward the development of environmentally friendly air cathodes by the use of natural nucleotides which offers a great opportunity to explore and manufacture bioinspired cathodes for metal-oxygen batteries.

Separation of Metal-N4 Units in Metal-Organic Framework for Preparation of M-Nx/C Catalyst with Dense Metal Sites

Metal and nitrogen codoped carbon (M-N_x/C) materials with good metal dispersion in a carbon matrix have attracted great attention because they can efficiently catalyze various kinds of chemical/electrochemical reactions. However, the existing strategies for preparing M-N_x/C materials still face the challenge of metal site aggregation when the carbon skeleton in the precursor is substantially lost during pyrolysis. Herein, we propose a general strategy that the increase of metal site density and inhibition of metal aggregation could be realized by separating M-N₄ units in metal-organic frameworks (MOFs). The metal sites can be well separated and distributed in carbon materials during the pyrolysis of a metal-coordinated block copolymer, one of the specific MOFs. The strategy can be widely applicable for the synthesis of M-N_x/C materials with dense metal sites, such as Fe-, Mn-, Ni-, and Co-N_x/C materials, which are potentially used as the efficient catalysts for various kinds of reactions. Taking Fe-N_x/C as a model oxygen reduction reaction (ORR) electrocatalyst, it shows the ORR half-wave potentials of 0.90 and 0.81 V vs RHE in alkaline and acidic electrolytes, respectively.

Organogermanium Nanowire Cathodes for Efficient Lithium-Oxygen Batteries

We report a technique for effectively neutralizing the generation of harmful superoxide species, the source of parasitic reactions, in lithium-oxygen batteries to generate stable substances. In organic electrolytes, organogermanium (Propa-germanium, Ge-132) nanowires can suppress solvated superoxide and induce strong surface-adsorption reaction due to their high anti-superoxide disproportionation activity. Resultantly, the effect of organogermanium nanowires mitigate toxic oxidative stress to stabilize organic electrolytes and promote good Li₂O₂ growth. These factors led to long duration of the electrolytes and impressive rechargeability of lithium-oxygen batteries.

Unraveling the Promotion Effects of a Soluble Cobaltocene Catalyst with Respect to Li-O2 Battery Discharge

The discharge process of a nonaqueous Li-O₂ battery at the cathode is the direct oxygen reduction reaction (ORR) with the formation of discharge product, e.g., Li₂O₂, deposits on the cathode surface. The aggressive superoxide intermediate generated during the ORR severely degrades the organic electrolyte and carbon-based cathodes. To avoid the formation of superoxide species and promote the growth of Li₂O₂ in the electrolyte solution, we employ a soluble cobaltocene [Co(C₅H₅)₂, Cp₂Co] as a homogeneous molecule catalyst to boost the discharge performance of Li-O₂ batteries. Owing to the unique chemical reactivity of Cp₂Co with molecular oxygen, the electrochemistry of the discharge process at the cathode is the (Cp₂Co)₂^II-O₂^2- adduct-mediated process rather than direct electrochemical oxygen reduction, thereby avoiding the formation of aggressive superoxide intermediate. In addition, the strong intermolecular attraction between Cp₂Co and the newly formed Li₂O₂ promotes the solution phase growth of Li₂O₂, which effectively suppresses electrode passivation.

High-Capacity and Stable Li-O2 Batteries Enabled by a Trifunctional Soluble Redox Mediator

Li-O₂ batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products. Here, a trifunctional ether-based redox mediator, 2,5-di-tert-butyl-1,4-dimethoxybenzene (DBDMB), is introduced into the electrolyte to capture reactive O₂ ^- and alleviate the rigorous oxidative environment of Li-O₂ batteries. Thanks to the strong solvation effect of DBDMB towards Li⁺ and O₂ ^- , it not only reduces the formation of by-products (a high Li₂ O₂ yield of 96.6 %), but also promotes the solution growth of large-sized Li₂ O₂ particles, avoiding the passivation of cathode as well as enabling a large discharge capacity. Moreover, DBDMB makes the oxidization of Li₂ O₂ and the decomposition of main by-products (Li₂ CO₃ and LiOH) proceed in a highly effective manner, prolonging the stability of Li-O₂ batteries (243 cycles at 1000 mAh g^-1 and 1000 mA g^-1 ).

Inhibition of Discharge Side Reactions by Promoting Solution-Mediated Oxygen Reduction Reaction with Stable Quinone in Li-O2 Batteries

Aprotic lithium-oxygen (Li-O₂) batteries with an ultrahigh theoretical energy density have great potential in rechargeable power supply, while their application still faces several challenges, especially poor cycle stability. To solve the problems, one of the effective strategies is to inhibit the generation of the LiO₂ intermediate produced via a surface-mediated oxygen reduction reaction (ORR) pathway, which is an important species inducing byproduct generation and low cell cyclic stability. Herein, a series of quinones and solid materials serve as the solution-mediated and surface-mediated ORR catalysts, and it was found that the generation of LiO₂ and byproducts from solid catalysts was inhibited by quinones. Among the studied quinones, benzo[1,2-b:4,5-b']dithiophene-4,8-dione, a quinone molecule with the advantage of a highly symmetrical planar and conjugated structure and without α-H, exhibits high redox potential, diffusion coefficient, and electrochemical stability, and consequently the best ORR activities and the capability to inhibit byproduct generation. It indicated that the increase of the solution-mediated ORR pathway plays an important role in restraining the discharging side reaction, substantially improving cell cycle stability and capacity. This study provides the theoretical and experimental basis for better understanding the ORR process of 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.

Electrochemical Oxidation of Li2O2 Surface-Doped with Li2CO3

Electrochemical oxidation of Li₂O₂, i.e., the charging reaction of the aprotic lithium-oxygen batteries (Li-O₂ batteries), is significantly influenced by its surface chemistry. Here, the surface species of Li₂CO₃, widely identified together with Li₂O₂ at the end of discharge, is investigated to understand its implication for the oxidation of Li₂O₂. In situ doping Li₂O₂ with various amounts of Li₂CO₃ has been obtained by reacting with CO₂ gas in a controlled way, and the electrochemical oxidation of the doped Li₂O₂ is studied with a quantitative differential electrochemical mass spectrometer (DEMS). Instead of a single charging potential plateau and one O₂ gas evolution stage for the pristine Li₂O₂, Li₂CO₃-doped Li₂O₂ exhibits two O₂/CO₂ gas evolution stages and three charging plateaus characterized with the larger overpotential for the initial and final stages. The conductivity of Li₂CO₃ dopant is invoked to explain the different oxidation behaviors of Li₂CO₃-doped Li₂O₂. The DEMS study of the electrochemical oxidation of isotope-labeled Li₂¹³CO₃ is also conducted to identify the origins of O₂ and CO₂ evolution during the oxidation of Li₂CO₃-doped Li₂O₂. The results reported here provide an improved understanding of the Li₂O₂ oxidation in the presence of parasitic Li₂CO₃ species and will contribute to the future development of Li-O₂ batteries.

Voltammetric monitoring of a solid-liquid phase transition in N,N,N′,N′-tetraoctyl-2,6-diamino-9,10-anthraquinone (TODAQ)

Exploratory experiments on effects from a phase transition are reported for a low-melting microcrystalline anthraquinone (N,N,N′,N′-tetraoctyl-2,6-diamino-9,10-anthraquinone or TODAQ). Data for the solid-liquid phase transition are obtained by differential scanning calorimetry and then compared to data obtained by voltammetry. In preliminary electrochemical measurements, microcrystal deposits on a basal plane pyrolytic graphite electrode are shown to undergo a solid-state 2-electron 2-proton reduction in contact to aqueous 0.1 M HClO4 with a midpoint potential Emid,solid = − 0.24 V vs. SCE. The reduction mechanism is proposed to be limited mainly by the triple phase boundary line and some transport of TODAQ molecules towards the electrode surface for both solid and melt. A change in the apparent activation energy for this reduction is observed at 69 °C, leading to an enhanced increase in reduction current with midpoint potential Emid,liquid = − 0.36 V vs. SCE. A change of TODAQ transport along the crystal surface for solid microcrystalline material (for the solid) to diffusion within molten microdroplets (for the liquid) is proposed. Upon cooling, a transition at 60 °C back to a higher apparent activation energy is seen consistent with re-solidification of the molten phase at the electrode surface. Differential scanning calorimetry data for solid TODAQ dry and for TODAQ in contact to aqueous 0.1 M HClO4 confirm these transitions.