Aqueous Lithium–Iodine Solar Flow Battery for the Simultaneous Conversion and Storage of Solar Energy

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.

Toward Sustainable Metal-Iodine Batteries: Materials, Electrochemistry and Design Strategies

Due to the natural abundance of iodine, cost-effective, and sustainability, metal-iodine batteries are competitive for the next-generation energy storage systems with high energy density, and large power density. However, the inherent properties of iodine such as electronic insulation and shuttle behavior of soluble iodine species affect negatively rate performance, cyclability, and self-discharge behavior of metal-iodine batteries, while the dendrite growth and metal corrosion on the anode side brings potential safety hazards and inferior durability. These problems of metal-iodine system still exist and need to be solved urgently. Herein, we summarize the research progress of metal-iodine batteries in the past decades. Firstly, the classification, design strategy and reaction mechanism of iodine electrode are briefly outlined. Secondly, the current development and protection strategy of conventional metal anodes in metal-iodine batteries are highlighted, and some potential anode materials and their design strategies are proposed. Thirdly, the key electrochemical parameters of state-of-art metal-iodine batteries are compared and analyzed to solve critical issues for realizing next-generation iodine-based energy storage systems. Therefore, the aim of this review is to promote the development of metal-iodine batteries and provide guidelines for their design.

Development of flow battery technologies using the principles of sustainable chemistry

Realizing decarbonization and sustainable energy supply by the integration of variable renewable energies has become an important direction for energy development. Flow batteries (FBs) are currently one of the most promising technologies for large-scale energy storage. This review aims to provide a comprehensive analysis of the state-of-the-art progress in FBs from the new perspectives of technological and environmental sustainability, thus guiding the future development of FB technologies. More importantly, we evaluate the current situation and future development of key materials with key aspects of green economy and decarbonization to promote sustainable development and improve the novel energy framework. Finally, we present an analysis of the current challenges and prospects on how to effectively construct low-carbon and sustainable FB materials in the future.

Photoelectrochemical Engineering for Light-Assisted Rechargeable Metal Batteries: Mechanism, Development, and Future

Rechargeable battery devices with high energy density are highly demanded by  our  modern society. The use of metal anodes is extremely attractive for future rechargeable battery devices. However, the notorious metal dendritic and instability of solid electrolyte interface issues pose a series of challenges for metal anodes. Recently, considering the indigestible dynamical behavior of metal anodes, photoelectrochemical engineering of light-assisted metal anodes have been rapidly developed since they efficiently utilize the integration and synergy of oriented crystal engineering and photocatalysis engineering, which provided a potential way to unlock the interface electrochemical mechanism and deposition reaction kinetics of metal anodes. This review starts with the fundamentals of photoelectrochemical engineering and follows with the state-of-art advance of photoelectrochemical engineering for light-assisted rechargeable metal batteries where photoelectrode materials, working principles, types, and practical applications are explained. The last section summarizes the major challenges and some invigorating perspectives for future research on light-assisted rechargeable metal batteries.

Rechargeable Batteries for Grid Scale Energy Storage

Ever-increasing global energy consumption has driven the development of renewable energy technologies to reduce greenhouse gas emissions and air pollution. Battery energy storage systems (BESS) with high electrochemical performance are critical for enabling renewable yet intermittent sources of energy such as solar and wind. In recent years, numerous new battery technologies have been achieved and showed great potential for grid scale energy storage (GSES) applications. However, their practical applications have been greatly impeded due to the gap between the breakthroughs achieved in research laboratories and the industrial applications. In addition, various complex applications call for different battery performances. Matching of diverse batteries to various applications is required to promote practical energy storage research achievement. This review provides in-depth discussion and comprehensive consideration in the battery research field for GSES. The overall requirements of battery technologies for practical applications with key parameters are systematically analyzed by generating standards and measures for GSES. We also discuss recent progress and existing challenges for some representative battery technologies with great promise for GSES, including metal-ion batteries, lead-acid batteries, molten-salt batteries, alkaline batteries, redox-flow batteries, metal-air batteries, and hydrogen-gas batteries. Moreover, we emphasize the importance of bringing emerging battery technologies from academia to industry. Our perspectives on the future development of batteries for GSES applications are provided.

Integrated Photovoltaic Charging and Energy Storage Systems: Mechanism, Optimization, and Future

As an emerging solar energy utilization technology, solar redox batteries (SPRBs) combine the superior advantages of photoelectrochemical (PEC) devices and redox batteries and are considered as alternative candidates for large-scale solar energy capture, conversion, and storage. In this review, a systematic summary from three aspects, including: dye sensitizers, PEC properties, and photoelectronic integrated systems, based on the characteristics of rechargeable batteries and the advantages of photovoltaic technology, is presented. The matching problem of high-performance dye sensitizers, strategies to improve the performance of photoelectrode PEC, and the working mechanism and structure design of multienergy photoelectronic integrated devices are mainly introduced and analyzed. In particular, the devices and improvement strategies of high-performance electrode materials are analyzed from the perspective of different photoelectronic integrated devices (liquid-based and solid-state-based). Finally, future perspectives are provided for further improving the performance of SPRBs. This work will open up new prospects for the development of high-efficiency photoelectronic integrated batteries.

Integrated photoelectrochemical energy storage cells prepared by benchtop ion soft landing

The exceptional photochromic and redox properties of polyoxometalate anions, PW₁₂O₄₀³⁻, have been exploited to develop an integrated photoelectrochemical energy storage cell for conversion and storage of solar energy. Elimination of strongly coordinating cations using benchtop ion soft landing leads to a ∼370% increase in the maximum power output of the device. Additionally, the photocathode displayed a pronounced color change from clear to blue upon irradiation, which warrants the potential application of the IPES cell in advanced smart windows and photochromic lenses.

Soft landing eliminates counter cations from Keggin polyoxometalate-based photocathodes, resulting in a ∼370% increase in maximum power output from a novel device that simultaneously harvests and stores solar energy.

Solar Energy Storage in an All-Vanadium Photoelectrochemical Cell: Structural Effect of Titania Nanocatalyst in Photoanode

Solar energy storage in the form of chemical energy is considered a promising alternative for solar energy utilization. High-performance solar energy conversion and storage significantly rely on the sufficient active surface area and the efficient transport of both reactants and charge carriers. Herein, the structure evolution of titania nanotube photocatalyst during the photoanode fabrication and its effect on photoelectrochemical activity in a microfluidic all-vanadium photoelectrochemical cell was investigated. Experimental results have shown that there exist opposite variation trends for the pore structure and crystallinity of the photocatalyst. With the increase in calcination temperature, the active surface area and pore volume were gradually declined while the crystallinity was significantly improved. The trade-off between the gradually deteriorated sintering and optimized crystallinity of the photocatalyst then determined the photoelectrochemical reaction efficiency. The optimal average photocurrent density and vanadium ions conversion rate emerged at an appropriate calcination temperature, where both the plentiful pores and large active surface area, as well as good crystallinity, could be ensured to promote the photoelectrochemical activity. This work reveals the structure evolution of the nanostructured photocatalyst in influencing the solar energy conversion and storage, which is useful for the structural design of the photoelectrodes in real applications.

Lithiated interface of Pt/TiO2 enables an efficient wire-shaped Zn-Air solar micro-battery

We report a wire-shaped bifunctional oxygen photoelectrode by integrating Li-doped TiO₂ nanotubes and Pt nanoclusters. Conductive nanoshells have been identified at the lithiated interface of Pt/TiO₂, which facilitates the performance of oxygen catalysis. Thus, the assembled Zn-air micro-battery with solar-assisted charging greatly improves the voltage efficiency compared with the conventional state-of-the-art catalyst as the air electrode.

Micro Energy Storage Systems in Energy Harvesting Applications: Analytical Evaluation towards Future Research Improvement

During the last decade, countless advancements have been made in the field of micro-energy storage systems (MESS) and ambient energy harvesting (EH) shows great potential for research and future improvement. A detailed historical overview with analysis, in the research area of MESS as a form of ambient EH, is presented in this study. The top-cited articles in the field of MESS ambient EH were selected from the Scopus database, and based on articles published from 2010 to 2021, and the number of citations. The search for these top-cited articles was conducted in the third week of December 2021. Mostly the manuscripts were technical and contained an experimental setup with algorithm development (65%), whereas 27.23% of the articles were survey-based. One important observation was that the top 20 selected articles, which are the most-cited articles in the different journals, come from numerous countries of origin. This study revealed that the MESS integrated renewable energy sources (RESs) are an enhancement field of research for EH applications. On the basis of this survey, we hope to identify and solve research problems in the field of MESS and RESs integration, and provide suggestions for future developments for EH applications.

Advanced Zn-I2 Battery with Excellent Cycling Stability and Good Rate Performance by a Multifunctional Iodine Host

The rechargeable zinc-iodine (Zn-I₂) battery is a promising energy-storage system due to its low cost and good security, but the practical use of the battery is largely constrained by the shuttle effect and high dissolvability of iodides. Here a multifunctional iodine host, constructed with nitrogen-doped porous carbon nanocages (NCCs) by the polymerization carbonization activation method, is exploited to improve the electrochemical performance and lifespan of the Zn-I₂ battery, achieving a high specific capacity of 259 mAh g^-1, a good rate performance (maintaining 50.6% expanding 50 times), and a high cycle stability (retention of 100% after 1000 cycles). On the basis of the experimental results and theoretical calculations, NCCs via the introduction of N doping and nanosized porous structure can simultaneously provide rich and robust anchoring and catalytic sites to carry out the electrostatic adsorption of iodides and facilitate the reversible conversion between iodine and iodides. This work shows a novel and efficient strategy to develop high-performance and long-life Zn-I₂ batteries.

Iodine Redox Chemistry in Rechargeable Batteries

Halogens have been coupled with metal anodes in a single cell to develop novel rechargeable batteries based on extrinsic redox reactions. Since the commercial introduction of lithium-iodine batteries in 1972, they have shown great potential to match the high-rate performance, large energy density, and good safety of advanced batteries. With the development of metal anodes (e.g. Li, Zn), one of the actual challenges lies in the preparation of electrochemically active and reliable iodine-based cathodes to prevent self-discharge and capacity decay of the rechargeable batteries. Understanding the fundamental reactions of iodine/polyiodide and their underlying mechanisms is still highly desirable to promote the rational design of advanced cathodes for high-performance rechargeable batteries. In this Minireview, recent advances in the development of iodine-based cathodes to fabricate rechargeable batteries are summarized, with a special focus on the basic principles of iodine redox chemistry to correlate with structure-function relationships.

An efficient and stable solar flow battery enabled by a single-junction GaAs photoelectrode

Converting and storing solar energy and releasing it on demand by using solar flow batteries (SFBs) is a promising way to address the challenge of solar intermittency. Although high solar-to-output electricity efficiencies (SOEE) have been recently demonstrated in SFBs, the complex multi-junction photoelectrodes used are not desirable for practical applications. Here, we report an efficient and stable integrated SFB built with back-illuminated single-junction GaAs photoelectrode with an n-p-n sandwiched design. Rational potential matching simulation and operating condition optimization of this GaAs SFB lead to a record SOEE of 15.4% among single-junction SFB devices. Furthermore, the TiO2 protection layer and robust redox couples in neutral pH electrolyte enable the SFB to achieve stable cycling over 408 h (150 cycles). These results advance the utilization of more practical solar cells with higher photocurrent densities but lower photovoltages for high performance SFBs and pave the way for developing practical and efficient SFBs.

All-in-One, Solid-State, Solar-Powered Electrochemical Cell

Solar-powered electrochemical cells (SPECs) have been perceived as a potential strategy for coping with the intermittent nature of solar power. Most of the SPECs reported so far use corrosive/toxic liquid electrolyte and/or need very careful packaging, which is restricted by the scenario of implementation and arises the fabrication cost. Here, we demonstrate an all-in-one, solid-state SPEC with solar-to-output energy conversion efficiency of ca. 2.8% under AM 1.5 G irradiation. In this SPEC, a LiBr/polyacrylamide (PAM) hydrogel serves both as electrolyte, cathode-active mediator, and separator, which is sandwiched between an FTO/BiVO₄ photoanode and an FTO/Prussian blue (PB) anode. The use of solid-state PAM hydrogel promotes the charge-transfer dynamics at the interface of the photoanode and suppressed the undesired side reactions of electrolyte decomposition, representing an effective strategy by interfacial engineering toward the development of high-performance SPECs.

High-performance solar flow battery powered by a perovskite/silicon tandem solar cell

The fast penetration of electrification in rural areas calls for the development of competitive decentralized approaches. A promising solution is represented by low-cost and compact integrated solar flow batteries; however, obtaining high energy conversion performance and long device lifetime simultaneously in these systems has been challenging. Here, we use high-efficiency perovskite/silicon tandem solar cells and redox flow batteries based on robust BTMAP-Vi/N^Me-TEMPO redox couples to realize a high-performance and stable solar flow battery device. Numerical analysis methods enable the rational design of both components, achieving an optimal voltage match. These efforts led to a solar-to-output electricity efficiency of 20.1% for solar flow batteries, as well as improved device lifetime, solar power conversion utilization ratio and capacity utilization rate. The conceptual design strategy presented here also suggests general future optimization approaches for integrated solar energy conversion and storage systems.

Design Principles and Developments of Integrated Solar Flow Batteries

ConspectusDue to the intermittent nature of sunlight, practical round-trip solar energy utilization systems require both efficient solar energy conversion and inexpensive large-scale energy storage. Conventional round-trip solar energy utilization systems typically rely on the combination of two or more separated devices to fulfill such requirements. Integrated solar flow batteries (SFBs) are a new type of device that integrates solar energy conversion and electrochemical storage. In SFBs, the solar energy absorbed by photoelectrodes is converted into chemical energy by charging up redox couples dissolved in electrolyte solutions in contact with the photoelectrodes. To deliver electricity on demand, the reverse redox reactions are carried out to release chemical energy stored in redox couples as one would do in the discharge of a normal redox flow battery (RFB). The integrated design of SFBs enables all the functions demanded by round trip solar energy utilization systems to be realized within a single device. Leveraging rapidly developing parallel technologies of photovoltaic solar cells and RFBs, significant progress in the field of SFBs has been made in the past few years. This Account aims to provide a general reference and tutorial for researchers who are interested in SFBs, and to describe the design principles and thus facilitate the development of this nascent field.The operation principle of SFBs is built on the working mechanism of RFBs and photoelectrochemical (PEC) cells, so we first describe the basic concept and important features of RFBs and redox couples with the emphasis on the quantitative understanding of RFB cell potentials. We also introduce different types of PEC cells and highlight two different photoelectrode designs that are commonly seen in SFB literature: simple semiconductor photoelectrodes and PV cell photoelectrodes. A set of experimental protocols for characterizing the redox couples, RFBs, photoelectrodes, and SFBs are presented to promote comparable assessment and discussion of important figures of merits of SFBs.Solar-to-output electricity efficiency (SOEE) defines the round trip energy efficiency of SFBs and has received substantial research attention. We introduce a quantitative simulation method to find the relationship between the SOEE and cell potential of SFBs and reveal the design principles for highly efficient SFBs. Several other important performance metrics of SFBs are also introduced. Then we review the historical development of SFBs and identify the state-of-the-art demonstrations at each development stage with more emphasis on our own research efforts in developing SFBs built with PV photoelectrodes. Finally, we preview some promising future directions and the challenges for advancing both the scientific understanding and practical applications of SFBs.

Carbon dot-modified mesoporous carbon as a supercapacitor with enhanced light-assisted capacitance

A light-charging energy storage device is a promising approach of utilizing solar energy, and the reasonable design of light-assisted supercapacitors with photosensitive materials is one of the efficient ways to realize solar energy conversion and storage. Here, the electrode material (OPC-CDs-700) prepared by combining procanthocyanins with carbon dots (CDs) can reach the specific capacitance of 312 F g^-1 at 0.1 A g^-1 under visible light, which is an increase of 54.4% compared with that of under dark conditions. Besides, this light-assisted supercapacitor exhibits excellent cyclic stability after 4000 cycles. A series of electrochemical measurements show that CDs could stabilize the charge under light illumination due to its photoactivity, thus increasing the accumulation and storage of charge on the surface of OPC-CDs-700. This study provides new prospects for the progress of photosensitive energy devices and application of solar energy.

Solar energy storage in a Cs2AgBiBr6 halide double perovskite photoelectrochemical cell

Storing solar energy using a stable visible light absorbing Cs₂AgBiBr₆ double perovskite is achieved using a photoelectrochemical (PEC) device with cobalt complexes and methyl viologen redox mediators. Under illumination, a potential gain of nearly 500 mV is achieved for charging. The charge-discharge cycling was carried out, and using in situ emission and FTIR studies, the self-discharge and solvent crossover were investigated.

Integrated Photo-Responsive Batteries for Solar Energy Harnessing: Recent Advances, Challenges, and Opportunities

Photo-responsive batteries that enable the effective combination of solar harvesting and energy conversion/storage functionalities render a potential solution to achieve the large-scale utilization of unlimited and cost-effective solar energy and alleviate the limits of conventional energy storage devices. The internal integration of photo-responsive electrodes into rechargeable batteries with the simplest two-electrode configuration is regarded as a reliable and appealing strategy for highly-efficient and low-cost utilization of solar energy by simplifying the device architecture and improving the energy efficiency. This progress report provides a brief review on photo-responsive batteries with integrated two-electrode configuration that can achieve solar energy conversion/storage in one single device. The basic device architecture, operating principles and practical performance of various photo-responsive systems based on solar energy harvesting in various batteries including Li ion batteries, Li-S batteries, Li-I batteries, dual-liquid redox batteries, Li-O₂ batteries, non-Li anode-O₂ /air batteries are summarized and discussed. Finally, the future opportunities and challenges regarding the two-electrode photo-responsive batteries are proposed.

A photo-assisted rechargeable battery: synergy, compatibility, and stability of a TiO2/dye/Cu2S bifunctional composite electrode

Herein, we present a simple and novel approach to fabricate a bifunctional compatible electrode, which has functionalities of photoelectric conversion and energy storage. The integrated photo-assisted rechargeable battery of the two-electrode system demonstrates potential reduction and impressive performance enhancement with the assistance of illumination. Such a structure of a bifunctional compatible electrode provides more possibilities to develop more practical photo rechargeable battery.

Available photo-charging integrated device constructed with dye-sensitized solar cells and lithium-ion battery

The development of self-chargeable lithium-ion batteries is of great significance for expanding the usable range of the lithium-ion battery and it has received intensive attention from numerous researchers.

Fully Conjugated Phthalocyanine Copper Metal-Organic Frameworks for Sodium-Iodine Batteries with Long-Time-Cycling Durability

Rechargeable sodium-iodine (Na-I₂ ) batteries are attracting growing attention for grid-scale energy storage due to their abundant resources, low cost, environmental friendliness, high theoretical capacity (211 mAh g^-1 ), and excellent electrochemical reversibility. Nevertheless, the practical application of Na-I₂ batteries is severely hindered by their poor cycle stability owing to the serious dissolution of polyiodide in the electrolyte during charge/discharge processes. Herein, the atomic modulation of metal-bis(dihydroxy) species in a fully conjugated phthalocyanine copper metal-organic framework (MOF) for suppression of polyiodide dissolution toward long-time cycling Na-I₂ batteries is demonstrated. The Fe₂ [(2,3,9,10,16,17,23,24-octahydroxy phthalocyaninato)Cu] MOF composited with I₂ (Fe₂ -O₈ -PcCu/I₂ ) serves as a cathode for a Na-I₂ battery exhibiting a stable specific capacity of 150 mAh g^-1 after 3200 cycles and outperforming the state-of-the-art cathodes for Na-I₂ batteries. Operando spectroelectrochemical and electrochemical kinetics analyses together with density functional theory calculations reveal that the square planar iron-bis(dihydroxy) (Fe-O₄ ) species in Fe₂ -O₈ -PcCu are responsible for the binding of polyiodide to restrain its dissolution into electrolyte. Besides the monovalent Na-I₂ batteries in organic electrolytes, the Fe₂ -O₈ -PcCu/I₂ cathode also operates stably in other metal-I₂ batteries like aqueous multivalent Zn-I₂ batteries. Thus, this work offers a new strategy for designing stable cathode materials toward high-performance metal-iodine batteries.

Iodine encapsulated in mesoporous carbon enabling high-efficiency capacitive potassium-Ion storage

The development of potassium-ion batteries (KIBs) are hampered by the lack of appropriate electrode materials allowing for the reversible insertion/de-insertion of the large K-ion. Iodine, as a conversion-type cathode for rechargeable batteries, has high theoretical capacity and excellent electrochemical reversibility, making it a potential cathode material for KIBs. However, due to the defects of iodine with the poor electronic conductivity and easy dissolution in the electrolyte, an intensive quest for iodine-based KIBs enabling high-performance potassium-ion storage is still underway. In this work, a high-efficiency capacitive K-I₂ battery has been successfully achieved by constructing a nanocomposite of iodine encapsulated in mesoporous carbon (CMK-3). The as-prepared CMK-3/iodine nanocomposite exhibites excellent rate performance (89.3 mA h g^-1 at 0.5 A g^-1) and superior cycling stability, which remarkably exceeds most of reported KIBs cathode materials. Such a excellent electrochemical performance can be ascribed to the engineered structure of CMK-3/iodine hybridized electrode which can alleviate the impact of the shuttle phenomenon, improve electronic conductivity and facilitate ion diffusion. As a consequence, iodine within the conductive protecting CMK-3 can afford an extraordinary pseudo-capacitive potassium-ion storage, which sheds light on the development prospect of conversion-type electrode materials to meet urgent demand for advanced KIBs.

Solar-Driven Rechargeable Lithium-Sulfur Battery

Solar cells and rechargeable batteries are two key technologies for energy conversion and storage in modern society. Here, an integrated solar-driven rechargeable lithium-sulfur battery system using a joint carbon electrode in one structure unit is proposed. Specifically, three perovskite solar cells are assembled serially in a single substrate to photocharge a high energy lithium-sulfur (Li-S) battery, accompanied by direct conversion of the solar energy to chemical energy. In the subsequent discharge process, the chemical energy stored in the Li-S battery is further converted to electrical energy. Therefore, the newly designed battery is capable of achieving solar-to-chemical energy conversion under solar-driven conditions, and subsequently delivering electrical energy from the stored chemical energy. With an optimized structure design, a high overall energy conversion efficiency of 5.14% is realized for the integrated battery. Moreover, owing to the self-adjusting photocharge advantage, the battery system can retain high specific capacity up to 762.4 mAh g-1 under a high photocharge rate within 30 min, showing an effective photocharging feature.

Effect of standard light illumination on electrolyte's stability of lithium-ion batteries based on ethylene and di-methyl carbonates

Combining energy conversion and storage at a device and/or at a molecular level constitutes a new research field raising interest. This work aims at investigating how prolonged standard light exposure (A.M. 1.5G) interacts with conventional batteries electrolyte, commonly used in the photo-assisted or photo-rechargeable batteries, based on 1 mol.L^-1 LiPF₆ EC/DMC electrolyte. We demonstrate the intrinsic chemical robustness of this class of electrolyte in absence of any photo-electrodes. However, based on different steady-state and time-resolved spectroscopic techniques, it is for the first time highlighted that the solvation of lithium and hexafluorophosphate ions by the carbonates are modified by light exposure leading to absorbance and ionic conductivity modifications without detrimental effects onto the electrochemical properties.

Glyme–Li salt equimolar molten solvates with iodide/triiodide redox anions

Room-temperature-fused Li salt solvates that exhibit ionic liquid-like behaviour can be formed using particular combinations of multidentate glymes and lithium salts bearing weakly coordinating anions, and are now deemed a subset of ionic liquids, viz. solvate ionic liquids (SILs). Herein, we report redox-active glyme–Li salt molten solvates consisting of tetraethyleneglycol ethylmethyl ether (G4Et) and lithium iodide/triiodide, [Li(G4Et)]I and [Li(G4Et)]I₃. The coordination structure of the complex ions and the thermal, transport, and electrochemical properties of these molten Li salt solvates were investigated to diagnose whether they can be categorized as SILs. [Li(G4Et)]⁺ and I₃⁻ were found to remain stable as discrete ions and exist as well-dissociated forms in the liquid state, indicating that [Li(G4Et)]I₃ can be classified as a good SIL. This study also clarified that the I⁻ and I₃⁻ counter anions exhibit an electrochemical redox reaction in the highly concentrated molten Li salt solvates. The redox-active molten Li solvates were further studied as a highly concentrated catholyte for use in rechargeable semi-liquid lithium batteries. Although the cell constructed using [Li(G4Et)]I₃ failed to charge after the initial discharge step, the cell containing [Li(G4Et)]I demonstrates reversible charge–discharge behaviour with a high volumetric energy density of 180 W h L⁻¹ based on the catholyte volume.

Redox-active glyme–Li salt equimolar molten solvates based on a I⁻/I₃⁻ couple could be employed as a highly concentrated catholyte for semi-liquid rechargeable lithium batteries.

A Li–urine battery based on organic/aqueous hybrid electrolytes

Urea/urine as a renewable energy source is attracting extensive attention, which represents a promising prospect toward ensuring a clean environment and energy utilization.

Solar-driven capacity enhancement of aqueous redox batteries with a vertically oriented tin disulfide array as both the photo-cathode and battery-anode

A dual-electrode, photo-enhanced aqueous redox battery synergizing photo-cathode and battery-anode is constructed via directly growing vertically oriented SnS₂ on Ti mesh.

Rechargeable redox flow batteries: flow fields, stacks and design considerations

Rechargeable redox flow batteries are being developed for medium and large-scale stationary energy storage applications. Flow batteries could play a significant role in maintaining the stability of the electrical grid in conjunction with intermittent renewable energy. However, they are significantly different from conventional batteries in operating principle. Recent contributions on flow batteries have addressed various aspects, including electrolyte, electrode, membrane, cell design, etc. In this review, we focus on the less-discussed practical aspects of devices, such as flow fields, stack and design considerations for developing high performance large-scale flow batteries. Finally, we provide suggestions for further studies on developing advanced flow batteries and large-scale flow battery stacks.

Ultra-stable binder-free rechargeable Li/I2 batteries enabled by "Betadine" chemical interaction

An activated carbon cloth/polymer-iodine (ACC/PVP-I2) composite was prepared by the "Betadine" method and employed as a high-performance cathode for rechargeable Li/I2 batteries. Due to the synergistic effect of ACC and PVP-I2, Li/I2 cells with ACC/PVP-I2 as the cathode exhibited superior electrochemical performance.

Integrated Photoelectrochemical Solar Energy Conversion and Organic Redox Flow Battery Devices

Building on regenerative photoelectrochemical solar cells and emerging electrochemical redox flow batteries (RFBs), more efficient, scalable, compact, and cost-effective hybrid energy conversion and storage devices could be realized. An integrated photoelectrochemical solar energy conversion and electrochemical storage device is developed by integrating regenerative silicon solar cells and 9,10-anthraquinone-2,7-disulfonic acid (AQDS)/1,2-benzoquinone-3,5-disulfonic acid (BQDS) RFBs. The device can be directly charged by solar light without external bias, and discharged like normal RFBs with an energy storage density of 1.15 Wh L^-1 and a solar-to-output electricity efficiency (SOEE) of 1.7 % over many cycles. The concept exploits a previously undeveloped design connecting two major energy technologies and promises a general approach for storing solar energy electrochemically with high theoretical storage capacity and efficiency.

Efficient Solar Energy Harvesting and Storage through a Robust Photocatalyst Driving Reversible Redox Reactions

Simultaneous solar energy conversion and storage is receiving increasing interest for better utilization of the abundant yet intermittently available sunlight. Photoelectrodes driving nonspontaneous reversible redox reactions in solar-powered redox cells (SPRCs), which can deliver energy via the corresponding reverse reactions, present a cost-effective and promising approach for direct solar energy harvesting and storage. However, the lack of photoelectrodes having both high conversion efficiency and high durability becomes a bottleneck that hampers practical applications of SPRCs. Here, it is shown that a WO₃ -decorated BiVO₄ photoanode, without the need of extra electrocatalysts, can enable a single-photocatalyst-driven SPRC with a solar-to-output energy conversion efficiency as high as 1.25%. This SPRC presents stable performance over 20 solar energy storage/delivery cycles. The high efficiency and stability are attributed to the rapid redox reactions, the well-matched energy level, and the efficient light harvesting and charge separation of the prepared BiVO₄ . This demonstrated device system represents a potential alternative toward the development of low-cost, durable, and easy-to-implement solar energy technologies.

TiO2-Photoanode-Assisted Direct-Solar-Energy Harvesting and Storage in a Solar-Powered Redox Cell Using Halides as Active Materials

The rapid deployment of renewable energy is resulting in significant energy security, climate change mitigation, and economic benefits. We demonstrate here the direct solar-energy harvesting and storage in a rechargeable solar-powered redox cell, which can be charged solely by solar irradiation. The cell follows a conventional redox-flow cell design with one integrated TiO₂ photoanode in the cathode side. Direct charging of the cell by solar irradiation results in the conversion of solar energy in to chemical energy. Whereas discharging the cell leads to the release of chemical energy in the form of electricity. The cell integrates energy conversion and storage processes in a single device, making the solar energy directly and efficiently dispatchable. When using redox couples of Br₂/Br^- and I₃^-/I^- in the cathode side and anode side, respectively, the cell can be directly charged upon solar irradiation, yielding a discharge potential of 0.5 V with good round-trip efficiencies. This design is expected to be a potential alternative toward the development of affordable, inexhaustible, and clean solar-energy technologies.

A Dual-Stimuli-Responsive Sodium-Bromine Battery with Ultrahigh Energy Density

Stimuli-responsive energy storage devices have emerged for the fast-growing popularity of intelligent electronics. However, all previously reported stimuli-responsive energy storage devices have rather low energy densities (<250 Wh kg^-1 ) and single stimuli-response, which seriously limit their application scopes in intelligent electronics. Herein, a dual-stimuli-responsive sodium-bromine (Na//Br₂ ) battery featuring ultrahigh energy density, electrochromic effect, and fast thermal response is demonstrated. Remarkably, the fabricated Na//Br₂ battery exhibits a large operating voltage of 3.3 V and an energy density up to 760 Wh kg^-1 , which outperforms those for the state-of-the-art stimuli-responsive electrochemical energy storage devices. This work offers a promising approach for designing multi-stimuli-responsive and high-energy rechargeable batteries without sacrificing the electrochemical performance.

Toward an Aqueous Solar Battery: Direct Electrochemical Storage of Solar Energy in Carbon Nitrides

Graphitic carbon nitrides have emerged as an earth-abundant family of polymeric materials for solar energy conversion. Herein, a 2D cyanamide-functionalized polyheptazine imide (NCN-PHI) is reported, which for the first time enables the synergistic coupling of two key functions of energy conversion within one single material: light harvesting and electrical energy storage. Photo-electrochemical measurements in aqueous electrolytes reveal the underlying mechanism of this "solar battery" material: the charge storage in NCN-PHI is based on the photoreduction of the carbon nitride backbone and charge compensation is realized by adsorption of alkali metal ions within the NCN-PHI layers and at the solution interface. The photoreduced carbon nitride can thus be described as a battery anode operating as a pseudocapacitor, which can store light-induced charge in the form of long-lived, "trapped" electrons for hours. Importantly, the potential window of this process is not limited by the water reduction reaction due to the high intrinsic overpotential of carbon nitrides for hydrogen evolution, potentially enabling new applications for aqueous batteries. Thus, the feasibility of light-induced electrical energy storage and release on demand by a one-component light-charged battery anode is demonstrated, which provides a sustainable solution to overcome the intermittency of solar radiation.

Recent Progress in the Design of Advanced Cathode Materials and Battery Models for High-Performance Lithium-X (X = O2 , S, Se, Te, I2 , Br2 ) Batteries

Recent advances and achievements in emerging Li-X (X = O₂ , S, Se, Te, I₂ , Br₂ ) batteries with promising cathode materials open up new opportunities for the development of high-performance lithium-ion battery alternatives. In this review, we focus on an overview of recent important progress in the design of advanced cathode materials and battery models for developing high-performance Li-X (X = O₂ , S, Se, Te, I₂ , Br₂ ) batteries. We start with a brief introduction to explain why Li-X batteries are important for future renewable energy devices. Then, we summarize the existing drawbacks, major progress and emerging challenges in the development of cathode materials for Li-O₂ (S) batteries. In terms of the emerging Li-X (Se, Te, I₂ , Br₂ ) batteries, we systematically summarize their advantages/disadvantages and recent progress. Specifically, we review the electrochemical performance of Li-Se (Te) batteries using carbonate-/ether-based electrolytes, made with different electrode fabrication techniques, and of Li-I₂ (Br₂ ) batteries with various cell designs (e.g., dual electrolyte, all-organic electrolyte, with/without cathode-flow mode, and fuel cell/solar cell integration). Finally, the perspective on and challenges for the development of cathode materials for the promising Li-X (X = O₂ , S, Se, Te, I₂ , Br₂ ) batteries is presented.

Light-assisted delithiation of lithium iron phosphate nanocrystals towards photo-rechargeable lithium ion batteries

Recently, intensive efforts are dedicated to convert and store the solar energy in a single device. Herein, dye-synthesized solar cell technology is combined with lithium-ion materials to investigate light-assisted battery charging. In particular we report the direct photo-oxidation of lithium iron phosphate nanocrystals in the presence of a dye as a hybrid photo-cathode in a two-electrode system, with lithium metal as anode and lithium hexafluorophosphate in carbonate-based electrolyte; a configuration corresponding to lithium ion battery charging. Dye-sensitization generates electron-hole pairs with the holes aiding the delithiation of lithium iron phosphate at the cathode and electrons utilized in the formation of a solid electrolyte interface at the anode via oxygen reduction. Lithium iron phosphate acts effectively as a reversible redox agent for the regeneration of the dye. Our findings provide possibilities in advancing the design principles for photo-rechargeable lithium ion batteries.

An All-vanadium Continuous-flow Photoelectrochemical Cell for Extending State-of-charge in Solar Energy Storage

Greater levels of solar energy storage provide an effective solution to the inherent nature of intermittency, and can substantially improve reliability, availability, and quality of the renewable energy source. Here we demonstrated an all-vanadium (all-V) continuous-flow photoelectrochemical storage cell (PESC) to achieve efficient and high-capacity storage of solar energy, through improving both photocurrent and photocharging depth. It was discovered that forced convective flow of electrolytes greatly enhanced the photocurrent by 5 times comparing to that with stagnant electrolytes. Electrochemical impedance spectroscopy (EIS) study revealed a great reduction of charge transfer resistance with forced convective flow of electrolytes as a result of better mass transport at U-turns of the tortuous serpentine flow channel of the cell. Taking advantage of the improved photocurrent and diminished charge transfer resistance, the all-V continuous-flow PESC was capable of producing ~20% gain in state of charge (SOC) under AM1.5 illumination for ca. 1.7 hours without any external bias. This gain of SOC was surprisingly three times more than that with stagnant electrolytes during a 25-hour period of photocharge.

Photoinduced Electron Transfer Process Visualized on Single Silver Nanoparticles

Understanding the photoinduced electron transfer (PET) mechanism is vital to improving the photoelectric conversion efficiency for solar energy materials and photosensitization systems. Herein, we visually demonstrate the PET process by real-time monitoring the photoinduced chemical transformation of p-aminothiophenol (p-ATP), an important SERS signal molecule, to 4,4'-dimercaptoazobenzene on single silver nanoparticles (AgNPs) with a localized surface plasmon resonance (LSPR) spectroscopy coupled dark-field microscopy. The bidirectional LSPR scattering spectral shifts bathochromically at first and hypsochromically then, which are caused by the electron transfer delay of p-ATP, disclose the PET path from p-ATP to O₂ through AgNPs during the reaction, and enable us to digitalize the corresponding electron loss and gain on the surface of AgNP at different time periods. This visualized PET process could provide a simple and efficient approach to explore the nature of PET and help to interpret the SERS mechanism in terms of p-ATP.