Semiconducting Metal–Organic Polymer Nanosheets for a Photoinvolved Li–O2 Battery under Visible Light

A Highly Reversible Sn-Air Battery Possessing the Ultra-Low Charging Potential with the Assistance of Light

Aqueous Sn-air batteries are attracting a great deal of interest in recent years due to the ultra-high safety, low cost, dendrite-free and highly reversible Sn anode. However, the slurry oxygen reduction/evolution reaction (ORR/OER) kinetics on the air cathodes seriously affect the Sn-air battery performances. Although various advanced catalysts have been developed, the charge overpotentials (~1000 mV) of these Sn-air batteries are still not satisfactory. Herein, iron oxide (Fe2O3) modified titanium dioxide (TiO2) nanorods with heterogeneous structure are firstly synthesized on Ti mesh (Fe2O3@TiO2/Ti), and the obtained Fe2O3@TiO2/Ti films are further applied as catalytic electrodes for Sn-air batteries. The core-shell heterogeneous structure of Fe2O3@TiO2/Ti can effectively facilitate the conversion of electrochemical intermediates and separation of photo-excited electrons and holes to activate oxygen-related reaction processes. Density functional theory (DFT) and experimental results also confirm that Fe2O3@TiO2/Ti can not only act as the electrocatalysts to improve ORR/OER properties, but also exhibit the superior photo-catalytic activity to promote charging kinetics. Hence, the Fe2O3@TiO2/Ti-based Sn-air batteries show ultra-low overpotential of ~40 mV, excellent rate capability and good cycling stability under light irradiation. This work will shed light on rational photo-assisted catalytic cathode design for new-type metal-air batteries.

Exciton Dissociation into Charge Carriers in Porphyrinic Metal-Organic Frameworks for Light-Assisted Li-O2 Batteries

Light-assisted Li-O2 batteries exhibit a high round-trip efficiency attributable to the assistance of light-generated electrons and holes in oxygen reduction and evolution reactions. Nonetheless, the excitonic effect arising from Coulomb interaction between electrons and holes impedes carrier separation, thus hindering efficient utilization of photo-energy. Herein, porphyrinic metal-organic frameworks with (Fe2Ni)O(COO)6 clusters are used as photocathodes to accelerate exciton dissociation into charge carriers for light-assisted Li-O2 batteries. The coupling of Ni 3d and Fe 3d orbitals boosts ligand-to-metal cluster charge transfer, and hence drives exciton dissociation and activates O2 for superoxide (•O2 -) radicals, rather than singlet oxygen (1O2) under photoexcitation. These enable the light-assisted Li-O2 batteries with a low total overvoltage of 0.28 V and round-trip efficiency of 92% under light irradiation of 100 mW cm-2. This work highlights the excitonic effect in photoelectrochemical processes and provides insights into photocathode design for light-assisted Li-O2 batteries.

Photo-Assisted Rechargeable Metal Batteries: Principles, Progress, and Perspectives

The utilization of diverse energy storage devices is imperative in the contemporary society. Taking advantage of solar power, a significant environmentally friendly and sustainable energy resource, holds great appeal for future storage of energy because it can solve the dilemma of fossil energy depletion and the resulting environmental problems once and for all. Recently, photo-assisted energy storage devices, especially photo-assisted rechargeable metal batteries, are rapidly developed owing to the ability to efficiently convert and store solar energy and the simple configuration, as well as the fact that conventional Li/Zn-ion batteries are widely commercialized. Considering many puzzles arising from the rapid development of photo-assisted rechargeable metal batteries, this review commences by introducing the fundamental concepts of batteries and photo-electrochemistry, followed by an exploration of the current advancements in photo-assisted rechargeable metal batteries. Specifically, it delves into the elucidation of device components, operating principles, types, and practical applications. Furthermore, this paper categorizes, specifies, and summarizes several detailed examples of photo-assisted energy storage devices. Lastly, it addresses the challenges and bottlenecks faced by these energy storage systems while providing future perspectives to facilitate their transition from laboratory research to industrial implementation.

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.

In-situ dynamic catalysis electrode electrolyte interphase enabling Mg2+ insertion in 2D metal-organic polymer for high-capacity and long-lifespan magnesium batteries

Rechargeable magnesium battery is regarded as the promising candidate for the next generation of high-specific-energy storage systems. Nevertheless, issues related to severe Mg-Cl dissociation at the electrolyte-electrode interface impede the insertion of Mg2+ into most materials, leading to severe polarization and low utilization of Mg-storage electrodes. In this study, a metal-organic polymer (MOP) Ni-TABQ (Ni-coordinated tetramino-benzoquinone) with superior surface catalytic activity is proposed to achieve the high-capacity Mg-MOP battery. The layered Ni-TABQ cathode, featuring a unique 2D π-d linear conjugated structure, effectively reduces the dissociation energy of MgxCly clusters at the Janus interface, thereby facilitating Mg2+ insertion. Due to the high utilization of active sites, Ni-TABQ achieves high capacities of 410 mAh/g at 200 mA g-1, attributable to a four-electron redox process involving two redox centers, benzoid carbonyls, and imines. This research highlights the importance of surface electrochemical processes in rechargeable magnesium batteries and paves the way for future development in multivalent metal-ion batteries.

Ruddlesden-Popper 2D Perovskite-MoS2 Hybrid Heterojunction Photocathodes for Efficient and Scalable Photo-Rechargeable Li-Ion Batteries

Photo-rechargeable batteries (PRBs) can provide a compact solution to power autonomous smart devices located at remote sites that cannot be connected with the grid. The study reports the Ruddlesden-Popper (RP) metal halide perovskite (MHP) and molybdenum disulfide (MoS2) hybrid heterojunction-based photocathodes for Li-ion photo-rechargeable battery (Li-PRB) applications. Hybrid Lithium-ion batteries (LIBs) have demonstrated an average discharge specific capacity of 144.46 and 129.17 mAhg-1 for 50 cycles when operating at 176 and 294 mAg-1, respectively compared to the pristine LIBs which have shown specific capacity of 37.48 and 25.60 mAhg-1 under similar conditions. Hybrid Li-PRB has achieved an average dark discharge specific capacities of 128.66 mAhg-1 (capacity retention: 96.56%) which enhanced to 180.67 mAhg-1 under illumination (capacity retention: 97.39%; photo-enhancement: 40.42%) at 64 mAg-1. Excellent performance of hybrid Li-PRB is attributed to the formation of type-II heterojunction that leads to improved crystallinity and film morphology. The PRB has demonstrated a high photo conversion and storage efficiency (PC-SE) of 0.52% under standard 1 Sun illumination, which outperforms other previously reported MHP based LIBs and PRBs. This work provides a novel approach of harnessing the potential of MHPs for PRBs and offers new avenues for MHP photocathodes for various applications beyond PRBs.

Reversible Carbon Dioxide/Lithium Oxalate Regulation toward Advanced Aprotic Lithium Carbon Dioxide Battery

Li-CO2 batteries have received significant attention owing to their advantages of combining greenhouse gas utilization and energy storage. However, the high kinetic barrier between gaseous CO2 and the Li2CO3 product leads to a low operating voltage (<2.5 V) and poor energy efficiency. In addition, the reversibility of Li2CO3 has always been questioned owing to the introduction of more decomposition paths caused by its higher charging plateau. Here, a novel "trinity" Li-CO2 battery system was developed by synergizing CO2, soluble redox mediator (2,2,6,6-tetramethylpiperidoxyl, as TEM RM), and reduced graphene oxide electrode to enable selective conversion of CO2 to Li2C2O4. The designed Li-CO2 battery exhibited an output plateau reaching up to 2.97 V, higher than the equilibrium potential of 2.80 V for Li2CO3, and an ultrahigh round-trip efficiency of 97.1 %. The superior performance of Li-CO2 batteries is attributed to the TEM RM-mediated preferential growth mechanism of Li2C2O4, which enhances the reaction kinetics and rechargeability. Such a unique design enables batteries to cope with sudden CO2-deficient environments, which provides an avenue for the rationally design of CO2 conversion reactions and a feasible guide for next-generation Li-CO2 batteries.

Dynamic Modulation of Li2O2 Growth in Li-O2 Batteries through Regulating Oxygen Reduction Kinetics with Photo-Assisted Cathodes

Widely acknowledged that the capacity of Li-O2 batteries (LOBs) should be strongly determined by growth behaviors of the discharge product of lithium peroxide (Li2O2) that follows both coexisting surface and solution pathways. However until now, it remains still challenging to achieve dynamic modulation on Li2O2 morphologies. Herein, the photo-responsive Au nanoparticles (NPs) supported on reduced oxide graphene (Au/rGO) have been utilized as cathode to manipulate oxygen reduction reaction (ORR) kinetics by aid of surface plasmon resonance (SPR) effects. Thus, we can experimentally reveal the importance of matching ORR kinetics with Li+ migration towards battery performance. Moreover, it is found that Li+ concentration polarization caused "sudden death" of LOBs is supposed to be just a form of suspended animation that could timely recover under irradiation. This work provides us an in-depth explanation on the working mechanism of LOBs from a kinetic perspective, offering valuable insights for the future battery design.

Accelerated Confined Mass Transfer of MoS2 1D Nanotube in Photo-Assisted Metal-Air Batteries

Applying solar energy into energy storage battery systems is challenging in achieving green and sustainable development, however, the efficient progress of photo-assisted metal-air batteries is restricted by the rapid recombination of photogenerated electrons and holes upon the photocathode. Herein, a 1D-ordered MoS2 nanotube (MoS2-ONT) with confined mass transfer can be used to extend the lifetime of photogenerated carriers, which is capable of overcoming the challenge of rapid recombination of electron and holes. The tubular confined space cannot only promote the orderly separation and migration of charge carriers but also realize the accumulation of charge and the rapid activation of oxygen molecules. The concave surface of MoS2-ONT can improve the carrier separation ability and prolong the carrier lifetime. Meanwhile, the ordered tubular confined space can effectively realize the rapid transfer of charge, ion, and oxygen. Under light irradiation, a fast oxygen reduction reaction kinetics of 70 mW cm-2 for photo-assisted Zn-air battery is achieved, which is the highest value reported for photo-assisted Zn-air batteries. Significantly, the photo-assisted Li-O2 battery based on MoS2-ONT also shows superior rate capability and other exciting battery performance. This work shows the universality of the confined carrier separation strategy in photo-assisted metal-air batteries.

Decoding Niobium Carbide MXene Dual-Functional Photoactive Cathode in Photoenhanced Hybrid Zinc-Ion Capacitor

The coupling of energy harvesting and energy storage discrete modules in a single architecture as a "two-in-one" concept is significant in off-grid energy storage devices. This approach can decrease the device size and the loss of energy transmission in common integrated energy harvesting and storage systems. This work systematically investigates the photoactive characteristics of niobium carbide MXene, Nb2CTx, in a photoenhanced hybrid zinc-ion capacitor (P-ZIC). The unique configuration of the Nb2CTx photoactive cathode absorbs light to charge the capacitor and enables it to operate continuously in the light-powered mode. The Nb2CTx-based P-ZIC shows a photodriven capacitance enhancement of over 60% at the scan rate of 10 mV s-1 under 50 mW cm-2 illumination with 435 nm wavelength. Furthermore, a photoenhanced specific capacitance of ∼27 F g-1, an impressive photocharging voltage response of 1.0 V, and capacitance retention of ∼85% (over 3000 cycles) are obtained.

Copper and conjugated carbonyls of metal-organic polymers as dual redox centers for Na storage

Metal-organic polymers (MOPs) are fascinating electrode materials for high-performance sodium-ion batteries due to their multiple redox centers and low cost. Herein, a flower-like π-d conjugated MOP (Cu-TABQ) was synthesized using tetramino-benzoquinone (TABQ) as an organic ligand and Cu2+ as a transition metal node under the slow release of Cu2+ from [Cu(NH3)4]2+ and subsequent dehydrogenation. It possesses dual redox centers of Cu2+/Cu+ and C[double bond, length as m-dash]O/C-O to render a three-electron transfer reaction for each coordination unit with a high reversible capacity of 322.9 mA h g-1 at 50 mA g-1 in the voltage range of 1.0 to 3.0 V. The flower-like structure enhances fast Na+ diffusion and highly reversible organic/inorganic redox centers. This results in excellent cycling performance with almost no degradation within 700 cycles and great rate performance with 198.8 mA h g-1 at 4000 mA g-1. The investigation of the Na-storage mechanism and attractive performance will shed light on the insightful design of MOP cathode materials for further batteries.

Self-sufficient metal-air battery systems enabled by solid-ion conductive interphases

Metal-air batteries including Li-air, Na-air, Al-air, and Zn-air, have received significant scientific and technological interest for at least the last three decades. The interest stems primarily from the fact that the electrochemically active material (O2) in the cathode can in principle be harvested from the surroundings. In practice, however, parasitic reactions with reactive components other than oxygen in dry air passivate the anode, limit cycling stability of air-sensitive (e.g., Li, Na, Al) and electrolyte-sensitive (e.g., Zn) anodes, in most cases obviating the energy-density benefits of harvesting O2 from ambient air. As a compromise, so-called metal-oxygen batteries in which pure O2 is used as the active cathode material have been extensively studied but are understood to be of little practical relevance because of the large infrastructure required to produce the pure O2 stream. Here, we report on the design of solid-ion conductive chemically inert metal interphases that simultaneously protect a metal anode from parasitic reactions with electrolyte components and which facilitate rapid interfacial ion transport. Interphases composed of indium (In) are reported to be of particular interest for protecting Li and Na anodes from passivation in air whereas interphases composed of Sn are shown to prevent chemical and electrochemical corrosion of Zn anodes in alkaline electrolytes. We report further that these protections enable so-called self-sufficient metal-air batteries capable of extended cycling stability in ambient air environments.

High-Yield Synthesis of Colloidal Carbon Rings and Their Applications in Self-Standing Electrodes of Li-O2 Batteries

The use of carbon materials in porous electrodes has impressive advantages. However, precisely tailoring the multilevel pore structure of carbon electrodes remains an unsolved challenge. Here, we report a highly efficient site-selective growth strategy to synthesize colloidal carbon rings by templating patchy droplets. Carbon rings are used for the direct fabrication of self-standing porous electrodes with hierarchical pores for lithium-oxygen batteries (LOBs). In situ atomic force microscopy reveals that during discharge the discharge products densely nucleate and grow on carbon rings, demonstrating that such rings are a very potential electrode material in LOBs. The hollow carbon ring electrode (HCRE) possesses micrometer-scale channels formed by random packing of rings and nanochannels consisting of ring-shaped hollow cavities connected by nanosized pores in the wall. Both channels contribute to ion transportation and gas diffusion, but the storage of the discharge products mainly lies in micrometer-scale channels, leading to a high discharge capacity of LOBs (20 658 mAh/g). Our work paves a new way to construct hierarchically porous electrodes for application in electrocatalysis and electrochemical energy storage.

Molecular Engineering of a Metal-Organic Polymer for Enhanced Electrochemical Nitrate-to-Ammonia Conversion and Zinc Nitrate Batteries

Metal-organic framework-based materials are promising single-site catalysts for electrocatalytic nitrate (NO3 - ) reduction to value-added ammonia (NH3 ) on account of well-defined structures and functional tunability but still lack a molecular-level understanding for designing the high-efficient catalysts. Here, we proposed a molecular engineering strategy to enhance electrochemical NO3 - -to-NH3 conversion by introducing the carbonyl groups into 1,2,4,5-tetraaminobenzene (BTA) based metal-organic polymer to precisely modulate the electronic state of metal centers. Due to the electron-withdrawing properties of the carbonyl group, metal centers can be converted to an electron-deficient state, fascinating the NO3 - adsorption and promoting continuous hydrogenation reactions to produce NH3 . Compared to CuBTA with a low NO3 - -to-NH3 conversion efficiency of 85.1 %, quinone group functionalization endows the resulting copper tetraminobenzoquinone (CuTABQ) distinguished performance with a much higher NH3 FE of 97.7 %. This molecular engineering strategy is also universal, as verified by the improved NO3 - -to-NH3 conversion performance on different metal centers, including Co and Ni. Furthermore, the assembled rechargeable Zn-NO3 - battery based on CuTABQ cathode can deliver a high power density of 12.3 mW cm-2 . This work provides advanced insights into the rational design of metal complex catalysts through the molecular-level regulation for NO3 - electroreduction to value-added NH3 .

A 3D-printed freestanding graphene aerogel composite photocathode for high-capacity and long-life photo-assisted Li-O2 batteries

The construction of a high-performance photocathode is essential for improving Li-O2 battery performance and solar energy utilization. However, the single pore and few active reaction sites of the photocathode result in insufficient discharge capacity and unsatisfactory cycling durability. Herein, we designed and fabricated a self-standing 3D-printed multi-pore graphene-based photocathode via direct ink writing (DIW) featuring non-competitive three-phase transmission channels to promote the transport of Li+, e-, and O2. The macropore provides adequate space for storage of the Li2O2 discharge product; the mesopore facilitates the reactant transport, while the micropore stores the active ions. Furthermore, the photogenerated carriers of the photocathode promote overpotential reduction. Under illumination, the charging voltage of the Li-O2 battery with a reduced graphene oxide/titanium dioxide (rGO/TiO2) photocathode is decreased from 4.55 V to 3.77 V, and the battery exhibits stable cycling for 1000 hours. Notably, the photocathode's pore structure and specific surface area are further optimized after adding carbon nanotubes (CNTs). Compared with rGO/TiO2, the specific surface area of reduced graphene oxide/titanium dioxide/carbon nanotubes (rGO/TiO2/CNTs) is increased by 12 times to 194.13 m2 g-1, and the discharge capacity can reach up to 33.37 mA h cm-2. This self-standing 3D-printed photocathode structure paves a new way for developing high-performance energy storage systems.

Metal-Organic Framework-Based Mixed Conductors Achieve Highly Stable Photo-assisted Solid-State Lithium-Oxygen Batteries

The demand for high-energy sustainable rechargeable batteries has motivated the development of lithium-oxygen (Li-O₂) batteries. However, the inherent safety issues of liquid electrolytes and the sluggish reaction kinetics of existing cathodes remain fundamental challenges. Herein, we demonstrate a promising photo-assisted solid-state Li-O₂ battery based on metal-organic framework-derived mixed ionic/electronic conductors, which simultaneously serve as the solid-state electrolytes (SSEs) and the cathode. The mixed conductors could effectively harvest ultraviolet-visible light to generate numerous photoelectrons and holes, which is favorable to participate in the electrochemical reaction, contributing to greatly improved reaction kinetics. According to the study on conduction behavior, we discover that the mixed conductors as SSEs possess outstanding Li⁺ conductivity (1.52 × 10^-4 S cm^-1 at 25 °C) and superior chemical/electrochemical stability (especially toward H₂O, O₂^-, etc.). Application of mixed ionic electronic conductors in photo-assisted solid-state Li-O₂ batteries further reveals that a high energy efficiency (94.2%) and a long life (320 cycles) can be achieved with a simultaneous design of SSEs and cathodes. The achievements present the widespread universality in accelerating the development of safe and high-performance solid-state batteries.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Decoupled Artificial Photosynthesis

Natural photosynthesis (NP) generates oxygen and carbohydrates from water and CO₂ utilizing solar energy to nourish lives and balance CO₂ levels. Following nature, artificial photosynthesis (AP), typically, overall water or CO₂ splitting, produces fuels and chemicals from renewable energy. However, hydrogen evolution or CO₂ reduction is inherently coupled with kinetically sluggish water oxidation, lowering efficiencies and raising safety concerns. Decoupled systems have thus emerged. In this review, we elaborate how decoupled artificial photosynthesis (DAP) evolves from NP and AP and unveil their distinct photoelectrochemical mechanisms in energy capture, transduction and conversion. Advances of AP and DAP are summarized in terms of photochemical (PC), photoelectrochemical (PEC), and photovoltaic-electrochemical (PV-EC) catalysis based on material and device design. The energy transduction process of DAP is emphasized. Challenges and perspectives on future researches are also presented.

Directly Growing Graphdiyne Nanoarray Cathode to Integrate an Intelligent Solid Mg-Moisture Battery

A continuous humidity and solar-light dual responsive intelligent solid Mg-moisture battery (SMB) with a graphdiyne nanosheets array was fabricated. The integrated battery works based on a new concept of chemical bond conversion on the surface of the graphdiyne nanosheets array that is grown in situ on a 3D melamine sponge (GDY/MS). The unique structure, excellent catalytic, and semiconductor performance of GDY endows the GDY/MS with some outstanding characteristics on trapping and transferring water molecules, catalyzing HER, and utilizing solar energy, making the GDY/MS a new generation cathode for a high-performance intelligent SMB. The performance of the GDY/MS-based smart SMB (GSMB) can be continuously tuned by humidity and solar-light. The GSMB shows a significant positive correlation between open circuit potential (OCP) and humidity, while the natural band gap of GDY makes it further act as a photoelectrode to capture light and generate photoelectrons. The GSMB can be applied as a self-power humidity monitor with an ultrafast response time of <0.24 s, a recovery time of <0.16 s, and a sensitive (36,600%) respiratory sensing performance. This simple and efficient battery-made strategy represents the future development direction of self-power supply equipment, intelligent electronic devices, and intelligent battery integration.

Reversible Metal and Ligand Redox Chemistry in Two-Dimensional Iron-Organic Framework for Sustainable Lithium-Ion Batteries

Metal-organic frameworks (MOFs) are emerging as attractive electrode materials for lithium-ion batteries, owing to their fascinating features of sustainable resources, tunable chemical components, flexible molecular skeletons, and renewability. However, they are faced with a limited number of redox-active sites and unstable molecular frameworks during electrochemical processes. Herein, we design a novel two-dimensional (2D) iron(III)-tetraamino-benzoquinone (Fe-TABQ) with dual redox centers of Fe cations and TABQ ligands for high-capacity and stable lithium storage. It is constructed of square-planar Fe-N₂O₂ linkages and phenylenediamine building blocks, between which the Fe-TABQ chains are connected by multiple hydrogen bonds, and then featured as an extended π-d-conjugated 2D structure. The redox chemistry of both Fe³⁺ cations and TABQ anions is revealed to render its remarkable specific capacity of 251.1 mAh g^-1. Benefiting from the intrinsic robust Fe-N(O) bonds and reinforced Li-N(O) bonds during cycling, Fe-TABQ delivers high capacity retentions over 95% after 200 cycles at various current densities. This work will enlighten more investigations for the molecular designs of advanced MOF-based electrode materials.

Bifunctional Photoassisted Li-O2 Battery with Ultrahigh Rate-Cycling Performance Based on Siloxene Size Regulation

Directly integrating the bifunctional photoelectrode into Li-O2 batteries has been considered an effective way to reduce the overpotential and promote electric energy saving. However, more regular investigations on various bifunctional photocatalysts have still been desired for high-performance photoassisted Li-O2 batteries. Herein, a systematic exploration of various-sized siloxene photocatalysts affected by Li-O2 batteries has been introduced. Compared with the utilization of larger-sized siloxene nanosheets (SNSs), the photoassisted Li-O2 battery with a siloxene quantum dot (SQD) photoelectrode delivers a superior round-trip efficiency of 230% based on the highest discharge potential up to 3.72 V and lowest charge potential of 1.60 V and enables the maintenance of a long-term cycling life with only 13% efficiency attenuation after 200 cycles at 0.075 mA/cm2. Furthermore, this system exhibits a record-high rate-cycling performance (162% round-trip efficiency, even at 3 mA/cm2) and a high discharge capacity of 2212 mAh/g at 1 mA/cm2. These ground-breaking performances could be attributed to the synergistic effect of the photocatalytic and electrocatalytic activities of SQD photocatalysts with the ideal conduction band/valence band values, the abundant defective sites, and the stronger O2 and lower LiO2 adsorption strengths of SQD photocatalysts. These systematic research studies highlight the significance of SQD bifunctional photocatalysts and could be extended to other photocatalysts for further high-efficiency photoelectric conversion and storage.

Titration Mass Spectroscopy (TMS): A Quantitative Analytical Technology for Rechargeable Batteries

Development of high-energy-density rechargeable battery systems not only needs advanced qualitative characterizations for mechanism exploration but also requires accurate quantification technology to quantitatively elucidate products and fairly assess numerous modification strategies. Herein, as a reliable quantification technology, titration mass spectroscopy (TMS) is developed to accurately quantify O-related anionic redox reactions (Li-O₂ battery and nickel-cobalt-manganese (NCM)/Li-rich cathodes), parasitic carbonate deposition and decomposition (derived from air-exposure degradation and electrolyte oxidation), and dead Li⁰ formation (Li-metal battery and over-discharged graphite anode). TMS technology can harvest key information on products (e.g., quantification of oxidized lattice oxygen and solid electrolyte interphase (SEI)/cathode electrolyte interphase (CEI) components) and guide corresponding design strategy by enhancing understanding of the mechanism (e.g., clearly distinguish the catalytic target of highly oxidative Ni⁴⁺ on the NCM cathode). Not limited as a rigid quantification tool for widely known products/mechanisms, TMS technology has been demonstrated as a powerful and versatile tool for the investigations of advanced batteries.

Solar utilization beyond photosynthesis

Natural photosynthesis is an efficient biochemical process which converts solar energy into energy-rich carbohydrates. By understanding the key photoelectrochemical processes and mechanisms that underpin natural photosynthesis, advanced solar utilization technologies have been developed that may be used to provide sustainable energy to help address climate change. The processes of light harvesting, catalysis and energy storage in natural photosynthesis have inspired photovoltaics, photoelectrocatalysis and photo-rechargeable battery technologies. In this Review, we describe how advanced solar utilization technologies have drawn inspiration from natural photosynthesis, to find sustainable solutions to the challenges faced by modern society. We summarize the uses of advanced solar utilization technologies, such as converting solar energy to electrical and chemical energy, electrochemical storage and conversion, and associated thermal tandem technologies. Both the foundational mechanisms and typical materials and devices are reported. Finally, potential future solar utilization technologies are presented that may mimic, and even outperform, natural photosynthesis.

Light-Assisted Metal-Air Batteries: Progress, Challenges, and Perspectives

Metal-air batteries are considered one of the most promising next-generation energy storage devices owing to their ultrahigh theoretical specific energy. However, sluggish cathode kinetics (O₂ and CO₂ reduction/evolution) result in large overpotentials and low round-trip efficiencies which seriously hinder their practical applications. Utilizing light to drive slow cathode processes has increasingly becoming a promising solution to this issue. Considering the rapid development and emerging issues of this field, this Review summarizes the current understanding of light-assisted metal-air batteries in terms of configurations and mechanisms, provides general design strategies and specific examples of photocathodes, systematically discusses the influence of light on batteries, and finally identifies existing gaps and future priorities for the development of practical light-assisted metal-air batteries.

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.

Integrated Design for Regulating the Interface of a Solid-State Lithium-Oxygen Battery with an Improved Electrochemical Performance

A composite solid-state electrolyte (SSE) with acceptable safety and durability is considered as a potential candidate for high-performance lithium-oxygen (Li-O₂) batteries. Herein, to address the safety issues and improve the electrochemical performance of Li-O₂ batteries, a solvent-free composite SSE is prepared based on the thermal initiation of poly(ethylene glycol) diacrylate radical polymerization, and an integrated battery is achieved by injecting an electrolyte precursor between electrodes during the assembly process through a simple heat treatment. The Li-metal symmetric cells based on this composite SSE achieve a critical current density of 0.8 mA cm^-2 and a stable cycle life of over 900 h at a current density of 0.2 mA cm^-2. This composite SSE effectively inhibits the erosion of O₂ on the Li metal anode, optimizes the interface between the electrolyte and cathode, and provides abundant reaction sites for the electrochemical reactions during cycling. The integrated solid-state Li-O₂ battery prepared in this work achieves stable long cycling (118 cycles) at a current density of 500 mA g^-1 at room temperature, showing the promising future application prospects.

Photo-Assisted Metal-Air Batteries: Recent Progress, Challenges and Opportunities

To meet the need of high energy density, long durability, safe and cost-efficient energy conversion and storage devices, metal-air batteries like Li-O₂ and Zn-O₂ batteries have received enormous attention and were subject to exciting development in the past decade. Photo-assisted strategies that enable the effective combination of photo/electric energy conversion/storage render a new dimension for the conventional metal-air batteries techniques with mere electric energy utilization. Therefore, tremendous research is ongoing in search of more efficient and durable devices with photo-assisted strategies. This review provides an overview of photo-assisted Li-O₂ batteries, Zn-O₂ batteries, and batteries with various metal/air components. The working mechanism, the basic device architecture and practical performances of various photo-assisted systems are summarized and discussed. Furthermore, certain technical challenges and future opportunities for the photo-assisted metal air batteries are emphasized and discussed in the hope of stimulating further research.

Photo-Enhanced Magnesium-Ion Capacitors Using Photoactive Electrodes

Off-grid power sources are becoming increasingly important for applications ranging from autonomous sensor networks to fighting energy poverty. Interactions of light with certain classes of battery and capacitor materials have recently gained attention to enhance the rate performance or to even charge energy storage devices directly with light. Interestingly, these devices have the potential to reduce the volume and cost of autonomous power sources. Here, a light-enhanced magnesium (Mg)-ion capacitor is shown. The latter is interesting because of the large natural abundance of Mg and its ability to operate in low cost and non-flammable aqueous electrolytes. Photoelectrodes using a combination of vanadium dioxide and reduced graphene oxide can achieve capacitance enhancements of up to 56% under light exposure alongside a 21% higher energy density of 20.5 mAh kg^-1 .

Quenching singlet oxygen via intersystem crossing for a stable Li-O2 battery

Aprotic Li-O₂ batteries are a promising energy storage technology, however severe side reactions during cycles lead to their poor rechargeability. Herein, highly reactive singlet oxygen (¹O₂) is revealed to generate in both the discharging and charging processes and is deterimental to battery stability. Electron-rich triphenylamine (TPA) is demonstrated as an effective quencher in the electrolyte to mitigate ¹O₂ and its associated parasitic reactions, which has the tertiary amine and phenyl groups to manifest excellent electrochemical stability and chemical reversibility. It reacts with electrophilic ¹O₂ to form a singlet complex during cycles, and it then quickly transforms to a triplet complex through nonradiative intersystem crossing (ISC). This efficiently accelerates the conversion of ¹O₂ to the ground-state triplet oxygen to eliminate its derived side reactions, and the regeneration of TPA. These enable the Li-O₂ battery with obviously reduced overvoltages and prolonged lifetime for over 310 cycles when coupled with a RuO₂ catalyst. This work highlights the ISC mechanism to quench ¹O₂ in Li-O₂ battery.

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.

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.

Boosting Cycling Stability and Rate Capability of Li-CO2 Batteries via Synergistic Photoelectric Effect and Plasmonic Interaction

Sluggish CO₂ reduction/evolution kinetics at cathodes seriously impede the realistic applications of Li-CO₂ batteries. Herein, synergistic photoelectric effect and plasmonic interaction are introduced to accelerate CO₂ reduction/evolution reactions by designing a silver nanoparticle-decorated titanium dioxide nanotube array cathode. The incident light excites energetic photoelectrons/holes in titanium dioxide to overcome reaction barriers, and induces the intensified electric field around silver nanoparticles to enable effective separation/transfer of photogenerated carriers and a thermodynamically favorable reaction pathway. The resulting Li-CO₂ battery demonstrates ultra-low charge voltage of 2.86 V at 0.10 mA cm^-2 , good cycling stability with 86.9 % round-trip efficiency after 100 cycles, and high rate capability at 2.0 mA cm^-2 . This work offers guidance on rational cathode design for advanced Li-CO₂ batteries and beyond.

Internal Electric Field and Interfacial Bonding Engineered Step-Scheme Junction for a Visible-Light-Involved Lithium-Oxygen Battery

Li-O₂ batteries have aroused considerable interest in recent years, however they are hindered by high kinetic barriers and large overvoltages at cathodes. Herein, a step-scheme (S-scheme) junction with hematite on carbon nitride (Fe₂ O₃ /C₃ N₄ ) is designed as a bifunctional catalyst to facilitate oxygen redox for a visible-light-involved Li-O₂ battery. The internal electric field and interfacial Fe-N bonding in the heterojunction boost the separation and directional migration of photo-carriers to establish spatially isolated redox centers, at which the photoelectrons on C₃ N₄ and holes on Fe₂ O₃ remarkably accelerate the discharge and charge kinetics. These enable the Li-O₂ battery with Fe₂ O₃ /C₃ N₄ to present an elevated discharge voltage of 3.13 V under illumination, higher than the equilibrium potential 2.96 V in the dark, and a charge voltage of 3.19 V, as well as superior rate capability and cycling stability. This work will shed light on rational cathode design for metal-O₂ batteries.

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.

Oxygen Vacancy-Mediated Growth of Amorphous Discharge Products toward an Ultrawide Band Light-Assisted Li-O2 Batteries

Photoassisted electrochemical reaction is regarded as an effective approach to reduce the overpotential of lithium-oxygen (Li-O₂ ) batteries. However, the achievement of both broadband absorption and long term battery cycling stability are still a formidable challenge. Herein, an oxygen vacancy-mediated fast kinetics for a photoassisted Li-O₂ system is developed with a silver/bismuth molybdate (Ag/Bi₂ MoO₆ ) hybrid cathode. The cathode can offer both double advantages for light absorption covering UV to visible region and excellent electrochemical activity for O₂ . Upon discharging, the photoexcited electrons from Ag nanoplate based on the localized surface plasmon resonance (LSPR) are injected into the oxygen vacancy in Bi₂ MoO₆ . The fast oxygen reaction kinetics generate the amorphous Li₂ O₂ , and the discharge plateau is improved to 3.05 V. Upon charging, the photoexcited holes are capable to decompose amorphous Li₂ O₂ promptly, yielding a very low charge plateau of 3.25 V. A first cycle round-trip efficiency is 93.8% and retention of 70% over 500 h, which is the longest cycle life ever reported in photoassisted Li-O₂ batteries. This work offers a general and reliable strategy for boosting the electrochemical kinetics by tailoring the crystalline of Li₂ O₂ with wide-band light.

Photoelectrochemistry of oxygen in rechargeable Li-O2 batteries

Rechargeable lithium-oxygen (Li-O₂) batteries are promising energy storage devices due to their high theoretical energy density. However, the sluggish kinetics of the oxygen reduction and evolution reactions (ORR/OER) at the cathodes results in large polarization and low energy efficiency. Although advances have been achieved in electrode material designs and battery configurations, large discharge/charge voltage gaps remain. The introduction of light into Li-O₂ batteries has been demonstrated to boost the reaction kinetics of the ORR/OER, leading to enhanced electrochemical performances, but the understanding of the photoelectrochemical process at oxygen cathodes is limited. This tutorial review focuses on the recent findings regarding photoinvolved oxygen cathodes, battery configurations, and the stability of Li-O₂ batteries, aiming to provide a fundamental understanding of photoinvolved Li-O₂ batteries. The challenges and perspectives are discussed in light of the interdisciplinary nature of photochemistry, materials chemistry, electrochemistry, computation, spectroscopy, and surface science.

Photoelectrochemical energy storage materials: design principles and functional devices towards direct solar to electrochemical energy storage

Advanced solar energy utilization technologies have been booming for carbon-neutral and renewable society development. Photovoltaic cells now hold the highest potential for widespread sustainable electricity production and photo(electro)catalytic cells could supply various chemicals. However, both of them require the connection of energy storage devices or matter to compensate for intermittent sunlight, suffering from complicated structures and external energy loss. Newly developed photoelectrochemical energy storage (PES) devices can effectively convert and store solar energy in one two-electrode battery, simplifying the configuration and decreasing the external energy loss. Based on PES materials, the PES devices could realize direct solar-to-electrochemical energy storage, which is fundamentally different from photo(electro)catalytic cells (solar-to-chemical energy conversion) and photovoltaic cells (solar-to-electricity energy conversion). This review summarizes a critically selected overview of advanced PES materials, the key to direct solar to electrochemical energy storage technology, with the focus on the research progress in PES processes and design principles. Based on the specific discussions of the performance metrics, the bottlenecks of PES devices, including low efficiency and deteriorative stability, are also discussed. Finally, several perspectives of potential strategies to overcome the bottlenecks and realize practical photoelectrochemical energy storage devices are presented.

A Dual Photoelectrode Photoassisted Fe-Air Battery: The Photo-Electrocatalysis Mechanism Accounting for the Improved Oxygen Evolution Reaction and Oxygen Reduction Reaction of Air Electrodes

Effective utilization of solar energy in battery systems is a promising solution to achieve sustainable and green development. In this work, a photoassisted Fe-air battery (PFAB) with two photoelectrodes of ZnO-TiO₂ heterostructure and polyterthiophene (pTTh)-coated CuO (pTTh-CuO) grown on fluorine-doped tin oxide (FTO) is proposed. The band structure of semiconductors and the charge-transfer mechanism of heterostructure are studied. The electrochemical results show that the photogenerated electrons and holes play key roles in reducing the oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) overpotential in the discharging and charging processes, respectively. The short-circuit current density, the open-circuit voltage, and the maximum power output of the PFAB can reach 34.28 mA cm^-2 , 1.15 V, and 5.69 mW cm^-2 upon illumination, respectively. The photoassisted Fe-air battery exhibits a low charge voltage of 0.64 V for ZnO-TiO₂ as photoelectrode and a discharge voltage of 1.38 V for pTTh-CuO as a photoelectrode at 0.1 mA cm^-2 .

Nonvolatile and Nonflammable Sulfolane-Based Electrolyte Achieving Effective and Safe Operation of the Li-O2 Battery in Open O2 Environment

The Li-O₂ battery should operate effectively/safely in an open O₂ environment for practical applications, but not trapped in sealed/closed atmosphere. However, the typical use of volatile and flammable electrolyte restricts Li-O₂ battery to be able to be running in open O₂ environment. We report herein, for the first time, a highly electrochemical reversible Li-O₂ battery operated in an open O₂ environment, i.e., under the condition of keeping O₂ flowing continuously based on a nonvolatile and nonflammable sulfolane (TMS) solvent. The electrochemical irreversibility of Li₂O₂/O₂ conversion and incompatibility between Li metal anodes and electrolyte have been addressed via dissolving LiNO₃ in concentrated TMS electrolyte. The tuned electrolyte not only enables a stable solid electrolyte interphase (SEI) with conformal inorganic components (including LiF, LiN_xO_y, and Li₂O) that promotes a uniform Li electro-plating/stripping process but also results in a low charge overpotential, a stable discharge terminal plateau, and reversible O₂ generation of the Li-O₂ battery conducted in an open O₂ environment.