Liquid Metal Electrodes for Energy Storage Batteries

The increasing demands for integration of renewable energy into the grid and urgently needed devices for peak shaving and power rating of the grid both call for low‐cost and large‐scale energy storage technologies. The use of secondary batteries is considered one of the most effective approaches to solving the intermittency of renewables and smoothing the power fluctuations of the grid. In these batteries, the states of the electrode highly affect the performance and manufacturing process of the battery, and therefore leverage the price of the battery. A battery with liquid metal electrodes is easy to scale up and has a low cost and long cycle life. In this progress report, the state‐of‐the‐art overview of liquid metal electrodes (LMEs) in batteries is reviewed, including the LMEs in liquid metal batteries (LMBs) and the liquid sodium electrode in sodium‐sulfur (Na–S) and ZEBRA (Na–NiCl₂) batteries. Besides the LMEs, the development of electrolytes for LMEs and the challenge of using LMEs in the batteries, and the future prospects of using LMEs are also discussed.     

Photo‐Rechargeable Electric Energy Storage Systems

The global energy demand is increasing at the same time as fossil fuel resources are dwindling. Consequently, the search for alternative energy sources is a major topic worldwide. Solar energy is one of the most promising, effective and emission‐free energy sources. However, the energy has to be stored to compensate the fluctuating availability of the sun and the actual energy demand. Photo‐rechargeable electric energy storage systems may solve this problem by immediately storing the generated electricity. Different combinations of solar cells and storage devices are possible. High efficiencies can be achieved by the combination of dye‐sensitized solar cells (DSSC) and capacitors. However, other hybrid devices including DSSCs or organic photovoltaic systems and redox flow batteries, lithium ion batteries and metal air batteries are playing an increasing role in this research field. This Progress Report reviews the state of the art research of photo‐rechargeable batteries based on organic solar cells, as well as storage modules.     

Flexible Solar Thermal Fuel Devices: Composites of Fabric and a Photoliquefiable Azobenzene Derivative

Controllable storage and release of solar energy has always been a highlighted scientific issue for its benefit of mankind. Solar thermal fuels (STFs) supply a closed cycle and renewable energy‐storage strategy by transforming solar energy into chemical energy stored in the conformation of molecular isomers, such as cis/trans‐azobenzene, and releasing it as heat under various stimuli. Although the potential high energy density of the STFs which are based on the hybrids of azobenzene derivatives and carbon nanomaterials has been reported the solvent‐assistant charging hinders their practicability. In this study, a solid‐state STF device is designed and fabricated by compositing one photoliquefiable azobenzene (PLAZ) derivative with a flexible fabric template. The photoinduced phase transition of the PLAZ derivative enables the charging of the flexible STFs to be totally solvent‐free. Interestingly, the energy‐storage capacity (energy density ≈201 J g^?1) of flexible PLAZ STFs has been improved by the soft fabric template. The exothermic situation is monitored with one infrared camera, which shows 4 °C temperature difference between charged and discharged samples under blue light stimulus. The flexible STFs are may be used in practice as heating equipment.     

Long‐Lasting Nanophosphors Applied to UV‐Resistant and Energy Storage Perovskite Solar Cells

Recently, considerable progress is achieved in lab prototype perovskite solar cells (PSCs); however, the stability of outdoor applications of PSCs remains a challenge due to the high sensitivity of perovskite material under moist and ultraviolet (UV) light conditions. In this work, the UV photostability of PSC devices is improved by incorporating a photon downshifting layer—SrAl₂O₄: Eu²⁺, Dy³⁺ (SAED)—prepared using the pulsed laser deposition approach. Light‐induced deep trap states in the photoactive layer are depressed, and UV light‐induced device degradation is inhibited after the SAED modification. Optimized power conversion efficiency (PCE) of 17.8% is obtained through the enhanced light harvesting and reduced carrier recombination provided by SAED. More importantly, a solar energy storage effect due to the long‐persistent luminescence of SAED is obtained after light illumination is turned off. The introduction of downconverting material with long‐persistent luminescence in PSCs not only represents a new strategy to improve PCE and light stability by photoconversion from UV to visible light but also provides a new paradigm for solar energy storage.     

Ionic Liquid Redox Catholyte for High Energy Efficiency,Low‐Cost Energy Storage

An approach to energy storage using ionic liquids as joint ion‐conducting medium and redox active catholyte material is described. The earth‐abundant ferric ion is incorporated as an oxidizing agent in the form of the low‐melting NaFeCl₄ in a 1:1 mixture with ethylmethylimidazolium tetrachloraluminate, an ambient temperature ionic liquid. Different possible anode types are considered, and the most obvious one involving liquid sodium (with special wetting of a sodium ion‐conducting ceramic separator) is tested. The high voltage >3.2 V predicted for this cell is verified, and its cyclability is confirmed. Operating at 180 °C, an unexpectedly high energy efficiency >96%, is recorded. This establishes this type of cell as an attractive candidate for energy storage. For optimum energy storage, high energy efficiency is mandated for thermal management, as well as economic reasons. The theoretical capacity of the cell is 288 Wh kg^?1 (418 Wh L^?1) of which 73% is realized. The cell is shown to be fail‐safe against internal shorts. As there are many degrees of freedom for developing this type of cell, it is suggested as a promising area of future research effort in the energy storage area.     

High‐Performance Relaxor Ferroelectric Materials for Energy Storage Applications

Relaxor ferroelectrics usually possess low remnant polarizations and slim hystereses, which can provide high saturated polarizations and superior energy conversion efficiencies, thus receiving increasing interest as energy storage materials with high discharge energy densities and fast discharge ability. In this study, a relaxor ferroelectric multilayer energy storage ceramic capacitor (MLESCC) based on 0.87BaTiO₃‐0.13Bi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃ (BT‐BZNT) with inexpensive Ag/Pd inner electrodes is prepared by the tape casting method. The MLESCC with two dielectric layers (layer thicknesses of 5 µm) sintered by a two‐step sintering method exhibits excellent energy storage properties with a record‐high discharge energy density of 10.12 J cm^?3, a high energy efficiency of 89.4% achieved at an electric field of 104.7 MV m^?1, a high temperature stability of the energy storage density (with minimal variation of <±5%), and energy efficiency (>90%) over a range of ?75 to 150 °C at 40 MV m^?1. These results suggest that the BT‐BZNT relaxor ferroelectric ceramic material can provide realistic solutions for high‐power energy storage capacitors.     

Opening Two‐Dimensional Materials for Energy Conversion and Storage: A Concept

The development of two‐dimensional (2D) materials is experiencing a renaissance since the adventure of graphene. 2D materials typically exhibit strong in‐plane covalent bonding and weak out‐of‐plane van der Waals interactions through the interlayer gap. Opening 2D materials is an effective way to alter the physical and chemical properties, such as band gap, conductivity, optical property, thermoelectric property, photovoltaic property and superconductivity. A larger interlayer distance means more accessible active sites for catalysis, an ion‐accessible surface in the interlayer space, which may greatly enhance the performance of 2D materials for energy conversion and storage. Moreover, opening 2D materials by intercalation can change the band filling state and the Fermi level. This review mainly focuses on the opening of 2D materials and their subsequent applications in energy conversion and storage fields, expecting to promote the development of such a new class of materials, namely expanded 2D materials. The exciting progresses of these expanded materials made in both energy conversion and storage devices including solar cells, thermoelectric devices, electrocatalyst, supercapacitors and rechargeable batteries, is presented and discussed in depth. Furthermore, prospects and further developments in these exciting fields of the expanded 2D materials are also commented.     

Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment

Research on the luminescent solar concentrator (LSC) over the past thirty‐odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon‐to‐electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic‐based selective mirrors which allow sunlight in but reflect luminophore‐emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus‐stop roofs, awnings, windows, paving, or siding tiles.     

Alkaline Benzoquinone Aqueous Flow Battery for Large‐Scale Storage of Electrical Energy

An aqueous flow battery based on low‐cost, nonflammable, noncorrosive, and earth‐abundant elements is introduced. During charging, electrons are stored in a concentrated water solution of 2,5‐dihydroxy‐1,4‐benzoquinone, which rapidly receives electrons with inexpensive carbon electrodes without the assistance of any metal electrocatalyst. Electrons are withdrawn from a second water solution of a food additive, potassium ferrocyanide. When these two solutions flow along opposite sides of a cation‐conducting membrane, this flow battery delivers a cell potential of 1.21 V, a peak galvanic power density of 300 mW cm^?2, and a coulombic efficiency exceeding 99%. Continuous cell cycling at 100 mA cm^?2 shows a capacity retention rate of 99.76% cycle^?1 over 150 cycles. Various molecular modifications involving substitution for hydrogens on the aryl ring are implemented to block decomposition by nucleophilic attack of hydroxide ions. These modifications result in increased capacity retention rates of up to 99.96% cycle^?1 over 400 consecutive cycles, accompanied by changes in voltage, solubility, kinetics, and cell resistance. Quantum chemistry calculations of a large number of organic compounds predict a number of related structures that should have even higher performance and stability. Flow batteries based on alkaline‐soluble dihydroxybenzoquinones and derivatives are promising candidates for large‐scale, stationary storage of electrical energy.     

Solvent-Mediated Synthesis of Functional Powder Materials from Deep Eutectic Solvents for Energy Storage and Conversion: A Review

Developing advanced electrochemical energy storage and conversion (ESC) technologies based on renewable clean energy can alleviate severe global environmental pollution and energy crisis. The efficient preparation of functional electrode materials via a simple, green, and safe synthesis process is the key to the commercial feasibility of these ESC systems. Deep eutectic solvents (DESs) with easy-tunable solvent properties and recyclable features have emerged as novel solvent systems for designing and synthesizing various functional powder materials for ESC devices. In this paper, the application of DESs in the synthesis of energy-related functional powder materials is systematically reviewed. After briefly introducing the classification and synthesis of DESs, their critical roles in synthesizing powder materials are discussed. Then, the recent advances of DES-derived powder materials in ESC, including batteries, fuel cells, supercapacitors, and water splitting, are described in detail from the perspective of preparation-structure-activity. Finally, some challenges and development directions of the DESs-mediated synthesis of powder materials with high electrochemical performance for ESC applications are outlined.     

Solar and Thermal Radiation-Modulation Materials for Building Applications

Regulation of solar and thermal radiation of building envelope shows huge energy-saving potentials. Existing reviews mainly focus on the materials with fixed solar and thermal optical properties. Although there are reviews reporting the materials with modulated optical properties (e.g., radiative cooling materials with modulating thermal emissivity while maintaining high solar reflectance), they merely focus on either solar or thermal radiation modulation, which can provide limited or even negative building energy savings and thus may mislead the researchers to design low energy-efficient materials in practice. To help gain a holistic understanding of the state-of-the-art solar and thermal radiation-modulation materials (STRMMs) and guide researchers to develop more effective STRMMs for maximum building energy savings, here the STRMMs are reviewed in three categories, solar radiation modulation, thermal radiation modulation, and synergetic solar and thermal radiation modulation. In the former two categories, only single solar or thermal optical property is modulated while in the third category both solar and thermal optical properties are modulated. For STRMMs in each category, their working principles, representative examples, potential applications and future perspectives are compared and elaborated.     

Gel Polymer Electrolytes for Electrochemical Energy Storage

With the booming development of flexible and wearable electronics, their safety issues and operation stabilities have attracted worldwide attentions. Compared with traditional liquid electrolytes, gel polymer electrolytes (GPEs) are preferred due to their higher safety and adaptability to the design of flexible energy storage devices. This review summarizes the recent progress of GPEs with enhanced physicochemical properties and specified functionalities for the application in electrochemical energy storage. Functional GPEs that are capable to achieve unity lithium‐ion transference number and offer additional pseudocapacitance to the overall capacitance are carefully discussed. The smart GPEs with self‐protection, thermotolerant, and self‐healing abilities are particularly highlighted. To close, the future directions and remaining challenges of the GPEs for application in electrochemical energy storages are summarized to provide clues for the following development.     

Recent Advances in Rechargeable Magnesium‐Based Batteries for High‐Efficiency Energy Storage

Benefiting from higher volumetric capacity, environmental friendliness and metallic dendrite‐free magnesium (Mg) anodes, rechargeable magnesium batteries (RMBs) are of great importance to the development of energy storage technology beyond lithium‐ion batteries (LIBs). However, their practical applications are still limited by the absence of suitable electrode materials, the sluggish kinetics of Mg²⁺ insertion/extraction and incompatibilities between electrodes and electrolytes. Herein, a systematic and insightful review of recent advances in RMBs, including intercalation‐based cathode materials and conversion reaction‐based compounds is presented. The relationship between microstructures with their electrochemical performances is comprehensively elucidated. In particular, anode materials are discussed beyond metallic Mg for RMBs. Furthermore, other Mg‐based battery systems are also summarized, including Mg–air batteries, Mg–sulfur batteries, and Mg–iodine batteries. This review provides a comprehensive understanding of Mg‐based energy storage technology and could offer new strategies for designing high‐performance rechargeable magnesium batteries.     

Holey 2D Nanomaterials for Electrochemical Energy Storage

2D nanomaterials provide numerous fascinating properties, such as abundant active surfaces and open ion diffusion channels, which enable fast transport and storage of lithium ions and beyond. However, decreased active surfaces, prolonged ion transport pathway, and sluggish ion transport kinetics caused by self‐restacking of 2D nanomaterials during electrode assembly remain a major challenge to build high‐performance energy storage devices with simultaneously maximized energy and power density as well as long cycle life. To address the above challenge, porosity (or hole) engineering in 2D nanomaterials has become a promising strategy to enable porous 2D nanomaterials with synergetic features combining both 2D nanomaterials and porous architectures. Herein, recent important progress on porous/holey 2D nanomaterials for electrochemical energy storage is reviewed, starting with the introduction of synthetic strategies of porous/holey 2D nanomaterials, followed by critical discussion of design rule and their advantageous features. Thereafter, representative work on porous/holey 2D nanomaterials for electrochemical capacitors, lithium‐ion and sodium‐ion batteries, and other emerging battery technologies (lithium‐sulfur and metal‐air batteries) are presented. The article concludes with perspectives on the future directions for porous/holey 2D nanomaterial in energy storage and conversion applications.