High-Energy-Density Metal-Oxygen Batteries: Lithium-Oxygen Batteries vs Sodium-Oxygen Batteries

Probing Interfacial Nanostructures of Electrochemical Energy Storage Systems by In-Situ Transmission Electron Microscopy

The ability to control the electrode interfaces in an electrochemical energy storage system is essential for achieving the desired electrochemical performance. However, achieving this ability requires an in-depth understanding of the detailed interfacial nanostructures of the electrode under electrochemical operating conditions. In-situ transmission electron microscopy (TEM) is one of the most powerful techniques for revealing electrochemical energy storage mechanisms with high spatiotemporal resolution and high sensitivity in complex electrochemical environments. These attributes play a unique role in understanding how ion transport inside electrode nanomaterials and across interfaces under the dynamic conditions within working batteries. This review aims to gain an in-depth insight into the latest developments of in-situ TEM imaging techniques for probing the interfacial nanostructures of electrochemical energy storage systems, including atomic-scale structural imaging, strain field imaging, electron holography, and integrated differential phase contrast imaging. Significant examples will be described to highlight the fundamental understanding of atomic-scale and nanoscale mechanisms from employing state-of-the-art imaging techniques to visualize structural evolution, ionic valence state changes, and strain mapping, ion transport dynamics. The review concludes by providing a perspective discussion of future directions of the development and application of in-situ TEM techniques in the field of electrochemical energy storage systems.

Ultralow Overpotential in Rechargeable Li-CO2 Batteries Enabled by Caesium Phosphomolybdate as an Effective Redox Catalyst

Rechargeable lithium-CO2 batteries are emerging as attractive energy storage devices due to their potential for high capacity and efficient CO2 reduction, making them promising candidates for post-lithium-ion batteries with high energy densities. However, their practical applications have been restricted by low reversibility, poor cycle life, and sluggish redox kinetics induced by the high potential required for decomposing the discharge product Li2CO3. Despite the various cathode catalysts explored, their application is often limited by availability, high cost, and complexity of synthesis. Herein, caesium phosphomolybdate (CPM) is synthesized through a facile and low-cost method. The Li‒CO2 battery based on the CPM cathode demonstrates a high discharge capacity of 15 440 mAh g-1 at 50 mA g-1 with 97.3% coulombic efficiency. It further exhibits robust stability, operating effectively over 100 cycles at 50 mA g-1 with a capacity limitation of 500 mAh g-1. Remarkably, the CPM catalyst yields a low overpotential of 0.67 V, surpassing most catalysts reported in prior research. This study reports, for the first time, the application of a Keggin-type polyoxometalate as a bifunctional redox catalyst, significantly improving the reversible cycling of rechargeable Li-CO2 batteries.

Tuning Dual Catalytic Active Sites of Pt Single Atoms Paired with High-Entropy Alloy Nanoparticles for Advanced Li-O2 Batteries

To achieve a long cycle life and high-capacity performance for Li-O2 batteries, it is critical to rationally modulate the formation and decomposition pathway of the discharge product Li2O2. Herein, we designed a highly efficient catalyst containing dual catalytic active sites of Pt single atoms (PtSAs) paired with high-entropy alloy (HEA) nanoparticles for oxygen reduction reaction (ORR) in Li-O2 batteries. HEA is designed with a moderate d-band center to enhance the surface adsorbed LiO2 intermediate (LiO2(ads)), while PtSAs active sites exhibit weak adsorption energy and promote the soluble LiO2 pathway (LiO2(sol)). An optimal ratio between LiO2(ads) and LiO2(sol) pathway was realized to modulate PtSAs and HEA active sites via regulating the etching conditions in the dealloying synthesis process for obtaining high-performance Li-O2 batteries. The ORR kinetics are accelerated, and the parasitic reactions are restrained in the Li-O2 batteries. As a result, Li-O2 batteries based on the HEA@Pt-PtSAs catalyst demonstrate an ultralow overpotential (0.3 V) and ultralong cycling performance of 470 cycles at 1000 mA g-1. The insights into the synthetic strategies and the importance of balancing the ORR pathways will offer guidance for devising multisite synergistic catalysts to accelerate redox-reaction kinetics for Li-O2 batteries.

Enhanced bifunctional electrocatalysis of Co5.47N nanocrystals in porous carbon nanofibers for high-efficiency zinc-air batteries

As a promising energy conversion and storage device, recently, rechargeable zinc-air batteries (ZABs) have developed rapidly, and the exploitation of excellent electrode catalysts to improve the energy efficiency and long-term performance of ZABs has become a focus of current research. Herein, the Co5.47N nanocrystals embedded in porous carbon nanofibers (Co5.47N PCNFs) were designed to act as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and iodide oxidation reaction (IOR), which occur on the electrode in the charging-discharging process of ZABs. The electrochemistry results showed that the ORR activity of Co5.47N PCNFs is comparable to the commercial Pt/C electrocatalyst, and the IOR activity and stability are higher than those of the Pt/C electrocatalyst. Importantly, Co5.47N PCNFs electrocatalyst endows ZABs with a low charge-discharge voltage difference (0.49 V), a high round-trip energy efficiency (72.1 %), as well as a large specific capacity (791.5 mAh gZn-1), surpassing the performance of Pt/C electrocatalyst. Density functional theory calculation demonstrates that Co5.47N PCNFs have lower Gibbs free energy for the formation of IOR intermediate species, thereby displaying outstanding IOR catalytic performance compared to that of Pt/C electrocatalyst. These findings offer crucial insights into the rational design of cobalt nitride-based electrocatalysts for application in ZABs with high energy efficiency.

Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries

Sodium-oxygen batteries have been regarded as promising energy storage devices due to their low overpotential and high energy density. Its applications, however, still face formidable challenges due to the lack of understanding about the influence of electrocatalysts on the discharge products. Here, a phosphorous and nitrogen dual-doped carbon (PNDC) based cathode is synthesized to increase the electrocatalytic activity and to stabilize the NaO2 superoxide nanoparticle discharge products, leading to enhanced cycling stability when compared to the nitrogen-doped carbon (NDC). The PNDC air cathode exhibits a low overpotential (0.36 V) and long cycling stability (120 cycles). The reversible formation/decomposition and stabilization of the NaO2 discharge products are clearly proven by in-situ synchrotron X-ray diffraction and ex-situ X-ray diffraction. Based on the density functional theory calculation, the PNDC has much stronger adsorption (-2.85 eV) for NaO2 than that of NDC (-1.80 eV), which could efficiently stabilize the NaO2 discharge products.

Achieving High-Capacity Cathode Presodiation Agent Via Triggering Anionic Oxidation Activity in Sodium Oxide

Compensating for the irreversible loss of limited active sodium (Na) is crucial for enhancing the energy density of practical sodium-ion batteries (SIBs) full-cell, especially when employing hard carbon anode with initially lower coulombic efficiency. Introducing sacrificial cathode presodiation agents, particularly those that own potential anionic oxidation activity with a high theoretical capacity, can provide additional sodium sources for compensating Na loss. Herein, Ni atoms are precisely implanted at the Na sites within Na2O framework, obtaining a (Na0.89Ni0.05□0.06)2O (Ni-Na2O) presodiation agent. The synergistic interaction between Na vacancies and Ni catalyst effectively tunes the band structure, forming moderate Ni-O covalent bonds, activating the oxidation activity of oxygen anion, reducing the decomposition overpotential to 2.8 V (vs Na/Na+), and achieving a high presodiation capacity of 710 mAh/g≈Na2O (Na2O decomposition rate >80%). Incorporating currently-modified presodiation agent with Na3V2(PO4)3 and Na2/3Ni2/3Mn1/3O2 cathodes, the energy density of corresponding Na-ion full-cells presents an essential improvement of 23.9% and 19.3%, respectively. Further, not limited to Ni-Na2O, the structure-function relationship between the anionic oxidation mechanism and electrode-electrolyte interface fabrication is revealed as a paradigm for the development of sacrificial cathode presodiation agent.

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Recent Advances in Understanding of the Singlet Oxygen in Energy Storage and Conversion

Singlet oxygen (term symbol 1Δg, hereafter 1O2), a reactive oxygen species, has recently attracted increasing interest in the field of rechargeable batteries and electrocatalysis and photocatalysis. These sustainable energy conversion and storage technologies are of vital significance to replace fossil fuels and promote carbon neutrality and finally tackle the energy crisis and climate change. Herein, the recent progresses of 1O2 for energy storage and conversion is summarized, including physical and chemical properties, formation mechanisms, detection technologies, side reactions in rechargeable batteries and corresponding inhibition strategies, and applications in electrocatalysis and photocatalysis. The formation mechanisms and inhibition strategies of 1O2 in particular aprotic lithium-oxygen (Li-O2) batteries are highlighted, and the applications of 1O2 in photocatalysis and electrocatalysis is also emphasized. Moreover, the confronting challenges and promising directions of 1O2 in energy conversion and storage systems are discussed.

Fine nanostructure design of metal chalcogenide conversion-based cathode materials for rechargeable magnesium batteries

Magnesium-ion batteries (MIBs) a strong candidate to set off the second-generation energy storage boom due to their double charge transfer and dendrite-free advantages. However, the strong coulombic force and the huge diffusion energy barrier between Mg2+ and the electrode material have led to need for a cathode material that can enable the rapid and reversible de-insertion of Mg2+. So far, researchers have found that the sulfur-converted cathode materials have a greater application prospect due to the advantages of low price and high specific capacity, etc. Based on these advantages, it is possible to achieve the goal of increasing the magnesium storage capacity and cycling stability by reasonable modification of crystal or morphology. In this review, we focus on the application of a variety of sulfur-converted cathode materials in MIBs in recent years from the perspective of microstructural design, and provide an outlook on current challenges and future development.

Evaluation of Polymetallic Phosphide Cathodes for Sodium-Air Batteries by Distribution of Relaxation Time

Sodium-oxygen batteries are emerging as a new energy storage system because of their high energy density and low cost. However, the cycling performance of the battery is not satisfying due to its insulating discharge product. Here, we synthesized metallic phosphides with gradient concentration (g-CoNiFe-P) and their uniform counterpart (CoNiFe-P) as cathode catalysts in a Na-O2 battery. Notably, the distribution of relaxation time (DRT) was utilized to identify the rate-determining step in a Na-O2 battery, evaluate the catalytic performance of the catalysts, and monitor the change of every single electrochemical process along the whole cycling process to study the degradation mechanism. The g-CoNiFe-P catalyst presented better initial capacity and cycling performances. The evolution of the kinetic processes resulting in battery degradation has been investigated by DRT analysis, which assists with characterizations. Our work demonstrates the application of DRT in battery diagnosis to evaluate the catalytic performance of catalysts and monitor the changes in different kinetic processes of new energy systems.

Recent advances in electrolyte molecular design for alkali metal batteries

In response to societal developments and the growing demand for high-energy-density battery systems, alkali metal batteries (AMBs) have emerged as promising candidates for next-generation energy storage. Despite their high theoretical specific capacity and output voltage, AMBs face critical challenges related to high reactivity with electrolytes and unstable interphases. This review, from the perspective of electrolytes, analyzes AMB failure mechanisms, including interfacial side reactions, active materials loss, and metal dendrite growth. It then reviews recent advances in innovative electrolyte molecular designs, such as ether, ester, sulfone, sulfonamide, phosphate, and salt, aimed at overcoming the above-mentioned challenges. Finally, we propose the current molecular design principles and future promising directions that can help future precise electrolyte molecular design.

3D mixed ion/electron-conducting scaffolds for stable sodium metal anodes

Sodium (Na) metal batteries represent an optimal choice for the forthcoming generation of large-scale, cost-effective energy storage systems. However, Na metal anodes encounter several formidable challenges during the Na plating and stripping processes, which encompass the formation of an unstable solid electrolyte interface, uncontrollable dendrite growth, and infinite volume expansion. These issues result in a reduced Coulombic efficiency, shortened battery lifespan, and potential safety hazards, thereby constraining their commercial development. Therefore, addressing these challenges to ensure the cycling stability of Na metal anodes stands as a paramount requirement for practical applications. Among the reported strategies, three-dimensional conductive scaffolds possessing a high surface area and porous structure are acknowledged for their significant potential in stabilizing Na metal anodes. Compared with conventional electron-conducting scaffolds, emerging mixed ion/electron-conductive (MIEC) scaffolds provide rapid ion/electron transport pathways, which enable uniform Na+ flux and promote dendrite-free Na deposition, thus improving the cycle life of Na metal anodes, even at high current densities and large areal capacities. Therefore, this review primarily emphasizes the recent progress in applying MIEC scaffolds to Na metal anodes. It introduces diverse design methods, examines the electrochemical performance of MIEC scaffolds, and delves into their regulation mechanisms over Na deposition behaviour. Finally, the development prospects and research strategies for MIEC scaffolds from both fundamental research and practical application perspectives are discussed, suggesting directions for further designing high-performance Na metal batteries.

Highly Stable Organic Molecular Porous Solid Electrolyte with One-Dimensional Ion Migration Channel for Solid-State Lithium-Oxygen Battery

Solid-state lithium-oxygen (Li-O2 ) batteries have been widely recognized as one of the candidates for the next-generation of energy storage batteries. However, the development of solid-state Li-O2 batteries has been hindered by the lack of solid-state electrolyte (SSE) with high ionic conductivity at room temperature, high Li+ transference number, and the high stability to air. Herein, the organic molecular porous solid cucurbit[7]uril (CB[7]) with one-dimensional (1D) ion migration channels is developed as the SSE for solid-state Li-O2 batteries. Taking advantage of the 1D ion migration channel for Li+ conduction, CB[7] SSE achieves high ionic conductivity (2.45 × 10-4 S cm-1 at 25 °C). Moreover, the noncovalent interactions facilitated the immobilization of anions, realizing a high Li+ transference number (tLi + = 0.81) and Li+ uniform distribution. The CB[7] SSE also shows a wide electrochemical stability window of 0-4.65 V and high thermal stability and chemical stability, as well as realizes stable Li+ plating/stripping (more than 1000 h at 0.3 mA cm-2 ). As a result, the CB[7] SSE endows solid-state Li-O2 batteries with superior rate capability and long-term discharge/charge stability (up to 500 h). This design strategy of CB[7] SSE paves the way for stable and efficient solid-state Li-O2 batteries toward practical applications.

K-O2 electrochemistry at the Au/DMSO interface probed by in situ spectroscopy and theoretical calculations

The reaction mechanism underpinning the operation of K-O2 batteries, particularly the O2 reactions at the positive electrode, is still not completely understood. In this work, by combining in situ Raman spectroelectrochemistry and density functional theory calculations, we report on a fundamental study of K-O2 electrochemistry at a model interface of Au electrode/DMSO electrolyte. The key products and intermediates (O2-, KO2 and K2O2) are identified and their dependency on the electrode potential is revealed. At high potentials, the first reduction intermediate of O2-* radical anions (* denotes the adsorbed state) can desorb from the Au electrode surface and combine with K+ cations in the electrolyte producing KO2via a solution-mediated pathway. At low potentials, O2 can be directly reduced to on the Au electrode surface, which can be further reduced to at extremely low potentials. The fact that K2O2 has only been detected in the very high overpotential regime indicates a lack of KO2 disproportionation reaction both on the Au electrode surface and in the electrolyte solution. This work addresses the fundamental mechanism and origin of the high reversibility of the aprotic K-O2 batteries.

Polycyclic Aromatic Hydrocarbons as Anode Materials in Lithium-Ion Batteries: A DFT Study

The structure and energetics of the interactive behavior of Li⁺ and Li with polycyclic aromatic hydrocarbons (PAHs) have been studied at the wB97XD/6-311G(d,p) level of DFT. The electron distribution in the PAHs, analyzed using the topology of the molecular electrostatic potential (MESP), led to the categorization of their aromatic rings into five types, viz R_s, R_n, R_d, R_b, and R_e. Among the different rings, sextet-type R_s and naphthalene-type R_n rings showed the highest interaction with Li⁺. The change in MESP at the nucleus of Li⁺ (ΔV_Li⁺) due to the formation of the complex Li⁺...PAH is found to be proportional to the adsorption energy (E₁). In Li...PAH, the spin density on Li is close to zero, suggesting the formation of Li⁺...PAH^•- due to the electron transfer from Li to PAH. The adsorption energy (E₂) for Li...PAH does not correlate with the change in MESP at the nucleus of Li, whereas the dissociation energy (E₃) of Li⁺...PAH^•- to yield Li⁺ and PAH^•- correlates well with the MESP data, ΔV_Li. The study confirms that the change in MESP at the nucleus of Li⁺ due to complex formation gives a quantitative measure of the electronic effect of the cation-π binding. The cell potential (V_cell) is predicted for the lithium ion battery (LIB) using the Li⁺...PAH and Li...PAH adsorption energies. On the basis of the V_cell data, "carbon nanoflake"-type systems, viz coronene, circumbiphenyl, C₄₂H₁₆, and C₅₀H₁₈ are suggested as good anode materials for LIBs.

Ionic Liquid Electrolyte with Weak Solvating Molecule Regulation for Stable Li Deposition in High-Performance Li-O2 Batteries

Li-O₂ batteries with bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte are promising because TFSI-IL can stabilize O₂ ^- to lower charge overpotential. However, slow Li⁺ transport in TFSI-IL electrolyte causes inferior Li deposition. Here we optimize weak solvating molecule (anisole) to generate anisole-doped ionic aggregate in TFSI-IL electrolyte. Such unique solvation environment can realize not only high Li⁺ transport parameters but also anion-derived solid electrolyte interface (SEI). Thus, fast Li⁺ transport is achieved in electrolyte bulk and SEI simultaneously, leading to robust Li deposition with high rate capability (3 mA cm^-2 ) and long cycle life (2000 h at 0.2 mA cm^-2 ). Moreover, Li-O₂ batteries show good cycling stability (a small overpotential increase of 0.16 V after 120 cycles) and high rate capability (1 A g^-1 ). This work provides an effective electrolyte design principle to realize stable Li deposition and high-performance Li-O₂ batteries.

NiS2 nanoparticles by the NaCl-assisted less-liquid reaction system for the magnesium-ion battery cathode

Rechargeable magnesium batteries are expected to be the next generation of energy storage devices. Therefore, it is of great significance to develop low-cost and long-life magnesium (Mg) electrode materials. However, the traditional method of synthesizing electrode materials is complicated, and it is difficult to remove potentially dangerous impurities. In this study, without adding any additional solvent, the crystal water in the reactant provides a liquid environment directly for the reaction, such that the whole reaction could be carried out safely and efficiently in the less liquid reaction system. Furthermore, NiS₂ in the cotton-like form was synthesized under the spatial effect of NaCl solution in a confined space. The fabricated material was tightly connected and has abundant active sites, which promote the rapid transport of charge. This work provides a general strategy of preparation methods for metal sulfides and also points in a new direction for the improvement of electrochemical performance with less-liquid reaction systems without additional solvents.

Metal-air batteries: progress and perspective

The metal-air batteries with the largest theoretical energy densities have been paid much more attention. However, metal-air batteries including Li-air/O₂, Li-CO₂, Na-air/O_2, and Zn-air/O₂ batteries, are complex systems that have their respective scientific problems, such as metal dendrite forming/deforming, the kinetics of redox mediators for oxygen reduction/evolution reactions, high overpotentials, desolution of CO₂, H₂O, etc. from the air and related side reactions on both anode and cathode. It should be the main direction to address these shortages to improve performance. Here, we summarized recently research progress in these metal-air/O₂ batteries. Some perspectives are also provided for these research fields.

Ion-in-Conjugation: A Promising Concept for Multifunctional Organic Semiconductors

Most organic semiconductors (OSCs) consist of conjugated skeletons with flexible peripheral chains. Their weak intermolecular interactions from dispersion and induction forces result in environmental susceptibilities and are unsuitable for many multifunctional applications where direct exposure to external environments is unavoidable, such as gas absorption, chemical sensing, and catalysis. To exploit the advantages of inorganic semiconductors in OSCs, ion-in-conjugation (IIC) materials are proposed. An IIC material refers to any conjugated material (molecules, polymers, and crystals) in Kekule's structural formula containing stoichiometric ionic states in its conjugated backbone in the electronic ground state. In this review, the definitions, structures, synthesis, properties, and applications of IIC materials are described briefly. Four types of IIC material, including zwitterionic conjugated molecules/polymers, conjugated ionic dyes, π-d conjugated molecules and polymers, and coordinatively doped polymers, are reported. Their applications in gas sensing, humidity sensing, resistive memory devices, and thermal/photo-/electro-catalysis are demonstrated. The challenges and opportunities for future research are also discussed. It is expected that this work will inspire the design of new organic electronic information materials.

Plasma Surface Engineering of NiCo2S4@rGO Electrocatalysts Enables High-Performance Li-O2 Batteries

The sluggish redox reaction kinetics for aprotic Li-O₂ batteries (LOBs) caused by the insulating discharge product of Li₂O₂ could result in the poor round-trip efficiency, low rate capability, and cyclic stability. To address these challenges, we herein fabricated NiCo₂S₄ supported on reduced graphene oxide (NiCo₂S₄@rGO), the surface of which is further modified via a unique low-pressure capacitive-coupled nitrogen plasma (CCPN-NiCo₂S₄@rGO). The high ionization environment of the plasma could etch the surface of NiCo₂S₄@rGO, introducing effective nitrogen doping. The as-prepared CCPN-NiCo₂S₄@rGO has been employed as an efficient catalyst for advanced LOBs. The electrochemical analysis, combined with theoretical calculations, reveals that the N-doping can effectively improve the thermodynamics and kinetics for LiO₂ adsorption, giving rise to a well-knit Li₂O₂ formation on CCPN-NiCo₂S₄@rGO. The LOBs based on the CCPN-NiCo₂S₄@rGO oxygen electrode deliver a low overpotential of 0.75 V, a high discharge capacity of 10,490 mA h g^-1, and an improved cyclic stability (more than 110 cycles). This contribution may pave a promising avenue for facile surface engineering of the electrocatalyst in LOBs and other energy storage systems.

Sodium Pre-Intercalated Carbon/V2 O5 Constructed by Sustainable Sodium Lignosulfonate for Stable Cathodes in Zinc-Ion Batteries: A Comprehensive Study

The aqueous zinc-ion battery (AZIB) has been widely investigated in recent years because it has the advantages of being green, safe, and made from abundant raw materials. It is necessary to continue to study how to prepare cathode materials with excellent performance and high cycling stability for future commercialization. In this work, a strategy was proposed that uses sustainable sodium lignosulfonate as both carbon and sodium sources to obtain a sodium pre-intercalated vanadium oxide/carbon (VO/LSC) composite as the cathode of AZIB. The carbon matrix could improve the electronic conductivity of vanadium oxide, while the sodium lignosulfonate could provide sodium ions pre-intercalated into the layered vanadium oxide simultaneously. Through this strategy, vanadium-based cathode materials with high stability and excellent rate capability were obtained. The VO/LSC cathode delivered high capacities of 350 and 112.8 mAh g^-1 at 0.1 and 4.0 A g^-1 , respectively. Zinc sulfate and zinc trifluoromethyl sulfonate were selected as electrolytes, and the influence of electrolytes on the performance of VO/LSC was analyzed. The oxygen in the environment was used to oxidize the low-priced vanadium oxide to achieve a self-charging AZIB. This paper provides a valuable strategy for the design of vanadium-based cathode material for AZIB, which can broaden the research and application of AZIB.

Light-Assisted Li-O2 Batteries with Lowered Bias Voltages by Redox Mediators

The enormous overpotential caused by sluggish kinetics of the oxygen reduction reaction and the oxygen evolution reaction prevents the practical application of Li-O₂ batteries. The recently proposed light-assisted strategy is an effective way to improve round-trip efficiency; however, the high-potential photogenerated holes during the charge would degrade the electrolyte with side reactions and poor cycling performance. Herein, a synergistic interaction between a polyterthiophene photocatalyst and a redox mediator is employed in Li-O₂ batteries. During the discharge, the voltage can be compensated by the photovoltage generated on the photoelectrode. Upon the charge with illumination, the photogenerated holes can be consumed by the oxidization of iodide ions, and thus the external circuit voltage is compensated by photogenerated electrons. Accordingly, a smaller bias voltage is needed for the semiconductor to decompose Li₂ O₂ , and the potential of photogenerated holes decreases. Finally, the round-trip efficiency of the battery reaches 97% with a discharge voltage of 3.10 V and a charge voltage of 3.19 V. The batteries show stable operation up to 150 cycles without increased polarization. This work provides new routes for light-assisted Li-O₂ batteries with reduced overpotential and boosted efficiency.

Recent advances in heterostructured cathodic electrocatalysts for non-aqueous Li-O2 batteries

Developing efficient energy storage and conversion applications is vital to address fossil energy depletion and global warming. Li-O2 batteries are one of the most promising devices because of their ultra-high energy density. To overcome their practical difficulties including low specific capacities, high overpotentials, limited rate capability and poor cycle stability, an intensive search for highly efficient electrocatalysts has been performed. Recently, it has been reported that heterostructured catalysts exhibit significantly enhanced activities toward the oxygen reduction reaction and oxygen evolution reaction, and their excellent performance is not only related to the catalyst materials themselves but also the special hetero-interfaces. Herein, an overview focused on the electrocatalytic functions of heterostructured catalysts for non-aqueous Li-O2 batteries is presented by summarizing recent research progress. Reduction mechanisms of Li-O2 batteries are first introduced, followed by a detailed discussion on the typical performance enhancement mechanisms of the heterostructured catalysts with different phases and heterointerfaces, and the various heterostructured catalysts applied in Li-O2 batteries are also intensively discussed. Finally, the existing problems and development perspectives on the heterostructure applications are presented.

Recent Advances in Metal-Gas Batteries with Carbon-Based Nonprecious Metal Catalysts

Metal-gas batteries draw a lot of attention due to their superiorities in high energy density and stable performance. However, the sluggish electrochemical reactions and associated side reactions in metal-gas batteries require suitable catalysts, which possess high catalytic activity and selectivity. Although precious metal catalysts show a higher catalytic activity, high cost of the precious metal catalysts hinders their commercial applications. In contrast, nonprecious metal catalysts complement the weakness of cost, and the gap in activity can be made up by increasing the amount of the nonprecious metal active centers. Herein, recent work on carbon-based nonprecious metal catalysts for metal-gas batteries is summarized. This review starts with introducing the advantages of carbon-based nonprecious metal catalysts, followed by a discussion of the synthetic strategy of carbon-based nonprecious metal catalysts and classification of active sites, and finally a summary of present metal-gas batteries with the carbon-based nonprecious metal catalysts is presented. The challenges and opportunities for carbon-based nonprecious metal catalysts in metal-gas batteries are also explored.

In Situ Defect Engineering Route to Optimize the Cationic Redox Activity of Layered Double Hydroxide Nanosheet via Strong Electronic Coupling with Holey Substrate

A defect engineering of inorganic solids garners great deal of research activities because of its high efficacy to optimize diverse energy-related functionalities of nanostructured materials. In this study, a novel in situ defect engineering route to maximize electrocatalytic redox activity of inorganic nanosheet is developed by using holey nanostructured substrate with strong interfacial electronic coupling. Density functional theory calculations and in situ spectroscopic analyses confirm that efficient interfacial charge transfer takes place between holey TiN and Ni-Fe-layered double hydroxide (LDH), leading to the feedback formation of nitrogen vacancies and a maximization of cation redox activity. The holey TiN-LDH nanohybrid is found to exhibit a superior functionality as an oxygen electrocatalyst and electrode for Li-O2 batteries compared to its non-holey homologues. The great impact of hybridization-driven vacancy introduction on the electrochemical performance originates from an efficient electrochemical activation of both Fe and Ni ions during electrocatalytic process, a reinforcement of interfacial electronic coupling, an increase in electrochemical active sites, and an improvement in electrocatalysis/charge-transfer kinetics.

Lewis-Acidic PtIr Multipods Enable High-Performance Li-O2 Batteries

The sluggish oxygen reaction kinetics concomitant with the high overpotentials and parasitic reactions from cathodes and solvents is the major challenge in aprotic lithium-oxygen (Li-O₂ ) batteries. Herein, PtIr multipods with a low Lewis acidity of the Pt atoms are reported as an advanced cathode for improving overpotentials and stabilities. DFT calculations disclose that electrons have a strong disposition to transfer from Ir to Pt, since Pt has a higher electronegativity than Ir, resulting in a lower Lewis acidity of the Pt atoms than that on the pure Pt surface. The low Lewis acidity of Pt atoms on the PtIr surface entails a high electron density and a down-shifting of the d-band center, thereby weakening the binding energy towards intermediates (LiO₂ ), which is the key in achieving low oxygen-reduction-reaction (ORR) and oxygen-evolution-reaction (OER) overpotentials. The Li-O₂ cell based on PtIr electrodes exhibits a very low overall discharge/charge overpotential (0.44 V) and an excellent cycle life (180 cycles), outperforming the bulk of reported noble-metal-based cathodes.

Unconventional Stoichiometries of Na-O Compounds at High Pressures

It has been realized that the stoichiometries of compounds may change under high pressure, which is crucial in the discovery of novel materials. This work uses systematic structure exploration and first-principles calculations to consider the stability of different stoichiometries of Na-O compounds with respect to pressure and, thus, construct a high-pressure stability field and convex hull diagram. Four previously unknown stoichiometries (NaO5, NaO4, Na4O, and Na3O) are predicted to be thermodynamically stable. Four new phases (P2/m and Cmc21 NaO2 and Immm and C2/m NaO3) of known stoichiometries are also found. The O-rich stoichiometries show the remarkable features of all the O atoms existing as quasimolecular O2 units and being metallic. Calculations of the O-O bond lengths and Bader charges are used to explore the electronic properties and chemical bonding of the O-rich compounds. The Na-rich compounds stabilized at extreme pressures (P > 200 GPa) are electrides with strong interstitial electron localization. The C2/c phase of Na3O is found to be a zero-dimensional electride with an insulating character. The Cmca phase of Na4O is a one-dimensional metallic electride. These findings of new compounds with unusual chemistry might stimulate future experimental and theoretical investigations.

An Ionic Liquid Electrolyte with Enhanced Li+ Transport Ability Enables Stable Li Deposition for High-Performance Li-O2 Batteries

Bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte could endow Li-O₂ batteries with low charging overpotential. However, their weak Li⁺ transport ability (LTA) leads to non-uniform Li deposition. Herein, guided by Sand formula, the LTA of TFSI-IL electrolyte is greatly enhanced to realize robust Li deposition through introducing hydrofluoroether (HFE) and optimizing electrolyte component ratios to regulate solvation environment. The solvation environment changes from Li(TFSI)₂ ^- ion pair into ionic aggregate clusters in the optimal electrolyte thanks to the slicing function of HFE toward ionic aggregate network. The transport parameters of Sand formula are synchronously enhanced, resulting in highly robust Li deposition behavior with greatly improved Coulombic efficiency (ca. 97.5 %) and cycling rate (1 mA cm^-2 ). Cycling stability of Li-O₂ batteries was greatly improved (a tiny overpotential rise of 64 mV after 75 cycles).

Reactions in non-aqueous alkali and alkaline-earth metal-oxygen batteries: a thermodynamic study

Multivalent aprotic metal-oxygen batteries are a novel concept in the applied electrochemistry field. These systems are variants of the so-called Li-air batteries and up to present are in their research infancy. The superoxide disproportionation reaction is a crucial step for the operation of any metal-oxygen redox system using aprotic solvents: in the best scenario, disproportionation leads to peroxide formation while in the worse one it releases singlet molecular oxygen. In this work we address the fundamental thermodynamics of such reaction for alkali (Li, Na and K) and alkaline earth (Be, Mg and Ca) metal-O₂ systems using multiconfigurational ab initio methods. Our aim is to draw a comprehensive description of the disproportionation reaction from superoxides to peroxides and to provide the thermodynamic likelihood of the pathways to singlet oxygen release.

Nano Polymorphism-Enabled Redox Electrodes for Rechargeable Batteries

Nano polymorphism (NPM), as an emerging research area in the field of energy storage, and rechargeable batteries, have attracted much attention recently. In this review, the recent progress on the composition and formation of polymorphs, and the evolution processes of different redox electrodes in rechargeable metal-ion, metal-air, and metal-sulfur batteries are highlighted. First, NPM and its significance for rechargeable batteries are discussed. Subsequently, the current NPM modulation strategies of different types of representative electrodes for their corresponding rechargeable battery applications are summarized. The goal is to demonstrate how NPM could tune the intrinsic material properties, and hence, improve their electrochemical activities for each battery type. It is expected that the analysis of polymorphism and electrochemical properties of materials could help identify some "processing-structure-properties" relationships for material design and performance enhancement. Lastly, the current research challenges and potential research directions are discussed to offer guidance and perspectives for future research on NPM engineering.

Single Atom Catalysts for Fuel Cells and Rechargeable Batteries: Principles, Advances, and Opportunities

Owing to the energy crisis and environmental pollution, developing efficient and robust electrochemical energy storage (or conversion) systems is urgently needed but still very challenging. Next-generation electrochemical energy storage and conversion devices, mainly including fuel cells, metal-air batteries, metal-sulfur batteries, and metal-ion batteries, have been viewed as promising candidates for future large-scale energy applications. All these systems are operated through one type of chemical conversion mechanism, which is currently limited by poor reaction kinetics. Single atom catalysts (SACs) perform maximum atom efficiency and well-defined active sites. They have been employed as electrode components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance. In this Review, we briefly summarize the related background knowledge, motivation and working principle toward next-generation electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air batteries, Li-CO₂ batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation electrochemical energy storage and conversion devices are discussed.

The Oxygen Reduction Reaction in Ca2+ -Containing DMSO: Reaction Mechanism, Electrode Surface Characterization, and Redox Mediation*

In this study the fundamental understanding of the underlying reactions of a possible Ca-O2 battery using a DMSO-based electrolyte was strengthened. Employing the rotating ring disc electrode, a transition from a mixed process of O2 - and O2 2- formation to an exclusive O2 - formation at gold electrodes is observed. It is shown that in this system Ca-superoxide and Ca-peroxide are formed as soluble species. However, there is a strongly adsorbed layer of products of the oxygen reduction reaction (ORR) s on the electrode surface, which is blocking the electrode. Surprisingly the blockade is only a partial blockade for the formation of peroxide while the formation of superoxide is maintained. During an anodic sweep, the ORR product layer is stripped from the electrode surface. With X-ray photoelectron spectroscopy (XPS) the deposited ORR products were shown to be Ca(O2 )2 , CaO2 , and CaO as well as side-reaction products such as CO3 2- and other oxygen-containing carbon species. It is shown that the strongly attached layer on the electrocatalyst, that was partially blocking the electrode, could be adsorbed CaO. The disproportionation reaction of O2 - in presence of Ca2+ was demonstrated via mass spectrometry. Finally, the ORR mediated by 2,5-di-tert-1,4-benzoquinone (DBBQ) was investigated by differential electrochemical mass spectrometry (DEMS) and XPS. Similar products as without DBBQ are deposited on the electrode surface. The analysis of the DEMS experiments shows that DBBQ- reduces O2 to O2 - and O2 2- , whereas in the presence of DBBQ2- O2 2- is formed. The mechanism of the ORR with and without DBBQ is discussed.

Dissociation of (Li2O2)0,+ on graphene and boron-doped graphene: insights from first-principles calculations

Reducing charge overpotential is of great significance to enhance the efficiency and cyclability of Li-O₂ batteries. Here, a dramatically reduced charge overpotential via boron-doped graphene as a catalytic substrate is successfully predicted. By first-principles calculations, from the perspective of reaction thermodynamics and kinetics, the results show that the electrochemical oxidation of the Li₂O₂⁺ cation is easier than the chemical oxidation of the neutral Li₂O₂ molecule, and the oxidation of (Li₂O₂)^0,+ is facilitated by boron-doping in pristine graphene. More importantly, the results reveal the oxidation mechanism of (Li₂O₂)^0,+: two-step dissociation with the LiO₂ molecule as a reactive intermediate has advantages over one-step dissociation; the rate-determining step for the dissociation of (Li₂O₂⁺)_G is the oxygen evolution process, while the lithium removal process is the rate-determining step for the dissociation of (Li₂O₂⁰)_G, (Li₂O₂⁰)_BG, and (Li₂O₂⁺)_BG.

Hybrid solid electrolyte enabled dendrite-free Li anodes for high-performance quasi-solid-state lithium-oxygen batteries

The dendrite growth of Li anodes severely degrades the performance of lithium-oxygen (Li-O₂) batteries. Recently, hybrid solid electrolyte (HSE) has been regarded as one of the most promising routes to tackle this problem. However, before this is realized, the HSE needs to simultaneously satisfy contradictory requirements of high modulus and even, flexible contact with Li anode, while ensuring uniform Li⁺ distribution. To tackle this complex dilemma, here, an HSE with rigid Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) core@ultrathin flexible poly (vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) shell interface has been developed. The introduced large amount of nanometer-sized LAGP cores can not only act as structural enhancer to achieve high Young's modulus but can also construct Li⁺ diffusion network to homogenize Li⁺ distribution. The ultrathin flexible PVDF-HFP shell provides soft and stable contact between the rigid core and Li metal without affecting the Li⁺ distribution, meanwhile suppressing the reduction of LAGP induced by direct contact with Li metal. Thanks to these advantages, this ingenious HSE with ultra-high Young's modulus of 25 GPa endows dendrite-free Li deposition even at a deposition capacity of 23.6 mAh. Moreover, with the successful inhibition of Li dendrites, the HSE-based quasi-solid-state Li-O₂ battery delivers a long cycling stability of 146 cycles, which is more than three times that of gel polymer electrolyte-based Li-O₂ battery. This new insight may serve as a starting point for further designing of HSE in Li-O₂ batteries, and can also be extended to various battery systems such as sodium-oxygen batteries.

Li-CO2 and Na-CO2 Batteries: Toward Greener and Sustainable Electrical Energy Storage

Metal-CO₂ batteries, especially Li-CO₂ and Na-CO₂ batteries, offer a novel and attractive strategy for CO₂ capture as well as energy conversion and storage with high specific energy densities. However, some scientific issues and challenges existing restrict their practical applications. Here, recent progress of crucial reaction mechanisms on cathodes in Li-CO₂ and Na-CO₂ batteries are summarized. The detailed reaction pathways can be modified by operation conditions, electrolyte compositions, and catalysts. Besides, specific discussions from aspects of catalyst design, stability of electrolytes, and anode protection are presented. Perspectives of several innovative directions are also put forward. This review provides an intensive understanding of Li-CO₂ and Na-CO₂ batteries and gives a useful guideline for the practical development of metal-CO₂ batteries and even metal-air batteries.

Interface-Controlled Rhombohedral Li3V2(PO4)3 Embedded in Carbon Nanofibers with Ultrafast Kinetics for Li-Ion Batteries

We present a unique composite assembly of rhombohedral Li₃V₂(PO₄)₃ and carbon nanofiber, which simultaneously facilitates Li-ion transport as well as electron transfer. For the synthesis of this composite, the inorganic precursors were confined in electron-spun nanofibers, and then, through controlled annealing, Na₃V₂(PO₄)₃ particulates were grown with controllable crystallite size and partially embedded into carbon nanofibers with precisely controlled diameter. The rhombohedral Li₃V₂(PO₄)₃ could be successfully obtained by ion exchange from Na to Li in the prepared Na₃V₂(PO₄)₃. The final rhombohedral Li₃V₂(PO₄)₃ particles anchored onto the carbon nanofibers exhibited excellent electrochemical performance with fast kinetics for Li-ion batteries. Suprisingly it maintains 69 and 41 mAh/g even at 100C as cathode and anode. Several advanced characterizations revealed that its ultrafast kinetics could be attributed to synergistic effect resulting from the distinctive microstructure of the composite and the structural superiority of highly symmetric rhombohedral Li₃V₂(PO₄)₃ over its monoclinic homologue for Li-ion transport.

An investigation of commercial carbon air cathode structure in ionic liquid based sodium oxygen batteries

In order to bridge the gap between theoretical and practical energy density in sodium oxygen batteries challenges need to be overcome. In this work, four commercial air cathodes were selected, and the impacts of their morphologies, structure and chemistry on their performance with a pyrrolidinium-based ionic liquid electrolyte are evaluated. The highest discharge capacity was found for a cathode with a pore size ca. 6 nm; this was over 100 times greater than that delivered by a cathode with a pore size less than 2 nm. The air cathode with the highest specific surface area and the presence of a microporous layer (BC39) exhibited the highest specific capacity (0.53 mAh cm⁻²).

From Black Liquor to Green Energy Resource: Positive Electrode Materials for Li-O2 Battery with High Capacity and Long Cycle Life

Black liquor has caused a tremendous degree of pollution and waste. Exploring the utilization of lignin, which is the major component of black liquor, has become a key factor in dealing with the problem. In this study, lignin derived from black liquor was used as a raw material to prepare carbon materials through different activation methods including KOH, H₃PO₄, and steam activation. The structure and properties of obtained samples were characterized as well as electrochemical performance when applied on a lithium-oxygen battery. Results of N₂ adsorption/desorption showed that all obtained samples possessed high surface area of over 1000 m²/g. XRD, Raman, and XPS also indicated that obtained samples possessed a large defect area and many functional groups. Electrochemical measurements illustrated that all obtained samples exhibited a high discharge capacity over 2.8 mAh/cm² at 0.02 mA/cm², while LKAC exhibited the highest discharge capacity of 7.2 mAh/cm². Cycling tests of all obtained samples indicated a long cycle life of at least 300 cycles. LSAC maintained a 100% retention rate of capacity and stable terminal voltage even after 800th cycle, and its cycling performance was investigated further by XRD and EIS. This study demonstrated excellent performance for lignin-based carbon materials, and provided alternative materials for positive electrode of lithium-oxygen battery.

An Overview of Fiber-Shaped Batteries with a Focus on Multifunctionality, Scalability, and Technical Difficulties

Flexible and wearable energy storage devices are receiving increasing attention with the ever-growing market of wearable electronics. Fiber-shaped batteries display a unique 1D architecture with the merits of superior flexibility, miniaturization potential, adaptability to deformation, and compatibility with the traditional textile industry, which are especially advantageous for wearable applications. In the recent research frontier in the field of fiber-shaped batteries, in addition to higher performance, advances in multifunctional, scalable, and integrable systems are also the main themes. However, many difficulties exist, including difficult encapsulation and installation of separators, high internal resistance, and poor durability. Herein, the design principles (e.g., electrode preparation and battery assembly) and device performance (e.g., electrochemical and mechanical properties) of fiber-shaped batteries, including lithium-based batteries, zinc-based batteries, and some other representative systems, are summarized, with a focus on multifunctional devices with environmental adaptability, stimuli-responsive properties, and scalability up to energy textiles, with the hope of enlightening future research directions. Finally, technical challenges in the realistic wearable application of these batteries are also discussed with the aim of providing possible solutions and new insights for further improvement.