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Lithium-sulfur (Li-S) battery is one of the most promising next-generation rechargeable batteries. Lots of fundamental research has been done for the problems during cycling like capacity fading and columbic efficiency reducing owing to severe diffusion and migration of polysulfide intermediates. In the early stage, a wide variety of carbon materials are used as host materials for sulfur to enhance electrical conductivity and adsorb soluble polysulfides. Beyond carbon materials, metal based polar compounds are introduced as host materials for sulfur because of their strong catalytic activity and adsorption ability to suppress the shuttle effect. In addition, relatively high density of metal compounds is helpful for increasing volumetric energy density of Li-S batteries. This review focuses on crystalline multi-metal compounds as host materials in sulfur cathodes. The multi-metal compounds involve not only transition metal composite oxides with specific crystalline structures, binary metal chalcogenides, double or complex salts, but also the metal compounds doped or partially substituted by other metal ions. Generally, for the multi-metal compounds, microstructure and morphologies in micro-nano scale are very significant for mass transfer in electrodes; moreover, adsorption and catalytic ability for polysulfides make fast kinetics in the electrode processes., (© 2021 Wiley-VCH GmbH.)

The organic-inorganic hybrid perovskite solar cells present a rapid improvement on power conversion efficiency from 3.8% to 25.5% in the past decades. Owing to the tuneable bandgaps, low-cost, and ease of fabrication, perovskites become ideal candidate materials for fabricating tandem solar cells, especially for efficient and high-voltage monolithic two-terminal devices. In this review, an overview of recent advances in various monolithic perovskite-based tandem solar cells with a focus on the key challenges is provided. Subsequently, the recombination layer materials, construction of wide-bandgap perovskite layer, stability of narrow-bandgap, and current matching principle in tandems are highlighted in order to optimize the output voltage and conversion efficiency of tandem solar cells. Finally, the recent progress is summarized with a focus on potential applications of tandem solar cells for energy conversion and storage, including hydrogen production by water splitting, CO 2 reduction, supercapacitors, and rechargeable batteries, benefiting from the adjustable output voltage of tandem solar cells. It is hoped that this work can offer a feasible strategy to explore more possibilities for fabricating new two-terminal tandem solar cells with high voltage and high conversion efficiency for energy conversion and storage., (© 2021 Wiley-VCH GmbH.)

Lithium-sulfur (Li-S) batteries hold the promise of the next generation energy storage system beyond state-of-the-art lithium-ion batteries. Despite the attractive gravimetric energy density (W G ), the volumetric energy density (W V ) still remains a great challenge for the practical application, based on the primary requirement of Small and Light for Li-S batteries. This review highlights the importance of cathode density, sulfur content, electroactivity in achieving high energy densities. In the first part, key factors are analyzed in a model on negative/positive ratio, cathode design, and electrolyte/sulfur ratio, orientated toward energy densities of 700 Wh L -1 /500 Wh kg -1 . Subsequently, recent progresses on enhancing W V for coin/pouch cells are reviewed primarily on cathode. Especially, the "Three High One Low" (THOL) (high sulfur fraction, high sulfur loading, high density host, and low electrolyte quantity) is proposed as a feasible strategy for achieving high W V , taking high W G into consideration simultaneously. Meanwhile, host materials with desired catalytic activity should be paid more attention for fabricating high performance cathode. In the last part, key engineering technologies on manipulating the cathode porosity/density are discussed, including calendering and dry electrode coating. Finally, a future outlook is provided for enhancing both W V and W G of the Li-S batteries., (© 2020 Wiley-VCH GmbH.)

For high-energy lithium-sulfur batteries, the poor volumetric energy density is a bottleneck as compared with lithium-ion batteries, due to the low density of both the sulfur active material and sulfur host. Herein, in order to enhance the volumetric energy density of sulfur cathode, a universal approach is proposed to fabricate a compact sulfur cathode with dense materials as sulfur host, instead of the old-fashioned lightweight carbon nanomaterials. Based on this strategy, heavy lanthanum strontium manganese oxide (La 0.8 Sr 0.2 MnO 3 ), with a high theoretical density of up to 6.5 g cm -3 , is introduced as sulfur host. Meanwhile, the La 0.8 Sr 0.2 MnO 3 host also acts as an efficient electrocatalyst to accelerate the diffusion, adsorption, and redox dynamics of lithium polysulfides in the charge-discharge processes. As a result, such S/La 0.8 Sr 0.2 MnO 3 cathode presents high gravimetric/volumetric capacity and outstanding cycling stability. Moreover, an ultra-high volumetric energy density of 2727 Wh L -1 -cathode is achieved based on the densification effect with higher density (1.69 g cm -3 ), which is competitive to the Ni-rich oxide cathode (1800-2160 Wh L -1 ) of lithium-ion batteries. The current study opens up a path for constructing high volumetric capacity sulfur cathode with heavy and catalytic host toward practical applications of lithium-sulfur batteries., Competing Interests: The authors declare no conflict of interest., (© 2020 The Authors. Published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.)

It is undoubtable that the use of solar energy will continue to increase. Solar cells that convert solar energy directly to electricity are one of the most convenient and important photoelectric conversion devices. Though silicon-based solar cells and thin-film solar cells have been commercialized, developing low-cost and highly efficient solar cells to meet future needs is still a long-term challenge. Some emerging solar-cell types, such as dye-sensitized and perovskite, are approaching acceptable performance levels, but their costs remain too high. To obtain a higher performance-price ratio, it is necessary to find new low-cost counter materials to replace conventional precious metal electrodes (Pt, Au, and Ag) in these emerging solar cells. In recent years, the number of counter-electrode materials available, and their scope for further improvement, has expanded for dye-sensitized and perovskite solar cells. Generally regular patterns in the intrinsic features and structural design of counter materials for emerging solar cells, in particular from an electrochemical perspective and their effects on cost and efficiency, are explored. It is hoped that this recapitulative analysis will help to make clear what has been achieved and what still remains for the development of cost-effective counter-electrode materials in emerging solar cells., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., Competing Interests: The authors declare no conflict of interest.

Storable sunshine, reusable rays: A solar rechargeable redox flow battery is proposed based on the photoregeneration of I(3)(-)/I(-) and [Fe(C(10)H(15))(2)](+)/Fe(C(10)H(15))(2) soluble redox couples, which can be regenerated by flowing from a discharged redox flow battery (RFB) into a dye-sensitized solar cell (DSSC) and then stored in tanks for subsequent RFB applications This technology enables effective solar-to-chemical energy conversion., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Layered titanate nanofibers can absorb bivalent ions from waste water via an ion exchange process. The sorption induces a considerable deformation of the layered structure, thus trapping the cations in the fibers permanently. Therefore, the fibers are desirable sorbents for the removal of toxic, radioactive Ra(2+) and Sr(2+) ions from water and subsequent safe disposal thereof., (Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)