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2022 Volume 38 Issue 11
2022, 38(11): 211100
doi: 10.3866/PKU.WHXB202111003
Abstract:
Because of the advantages of high safety, environment-friendliness, affordability, and ease of processing, aqueous rechargeable zinc batteries (ARZBs) are promising candidates for next-generation large-scale energy storage systems. In recent years, various cathode materials based on vanadium/manganese/cobalt oxides, Prussian blue analogs, and organic compounds have been reported. Among them, manganese dioxide (MnO2) is widely used in ARZBs due to their outstanding advantages of low toxicity, eco-friendliness, and high capacity (616 mAh∙g−1 based on two-electron transfer). However, the diversity of the crystal structures of MnO2 and the unpredictability of the electrochemical reaction make it difficult to investigate the specific internal storage mechanism, which impedes further development of the optimal modification strategies. To date, the main recognized energy storage mechanisms are (de)intercalation and dissolution-deposition mechanisms. In the traditional (de)intercalation mechanism, the predominant issues related to MnO2 during the cycling process include Mn dissolution, irreversible phase transformation, structural collapse, and sluggish ion diffusion kinetics. On the other hand, the detailed reaction path for the dissolution-deposition mechanism, which was developed in recent years, remains controversial. In addition, the incomplete dissolution-deposition of MnO2 and the highly acidic environment inevitably leads to corrosion and hydrogen evolution of the zinc anode, as well as low Coulombic efficiency. Accordingly, optimization strategies for different reaction mechanisms have been proposed to make zinc-manganese batteries more competitive. For the (de)intercalation mechanism, modification of composite materials and nanostructure optimization strategies can be adopted to inhibit the dissolution of MnO2 and increase the number of highly active reaction sites, thus enhancing the electrochemical performance. Moreover, the guest pre-intercalation strategy can help optimize the crystal structure of MnO2, preventing the collapse of the internal structure during cycling. Besides, defect engineering and element doping strategies focus on regulating the distribution of the electronic structure for affecting the properties of MnO2, resulting in lowering the energy barrier of zinc insertion. For the dissolution-deposition mechanism, the introduction of a neutral acetate and a halide mediator can effectively facilitate the dissolution-deposition of MnO2. Meanwhile, metal element catalysis can accelerate the reaction kinetics of the MnO2 dissolution-deposition, so that high-rate performance can be achieved. Furthermore, the decoupling battery system can separate the cathodic and anodic electrolytes to restrain the hydrogen and oxygen evolution reactions and enhance the potential difference. The flow battery system can effectively eliminate the influence of concentration polarization and stabilize the ion concentration in the electrolytes, thus leading to a large capacity (> 100 mAh). Undoubtedly, MnO2 as a high-capacity, high-voltage cathode material has broad development prospects for ARZBs. Here, we systematically summarize the crystal structures and reaction mechanisms of MnO2. We also discuss the optimization strategies toward advanced MnO2 cathode materials for resolving the highlighted issues in zinc-manganese batteries, which are expected to provide research directions for the design and development of high-performance ARZBs. ![]()
Because of the advantages of high safety, environment-friendliness, affordability, and ease of processing, aqueous rechargeable zinc batteries (ARZBs) are promising candidates for next-generation large-scale energy storage systems. In recent years, various cathode materials based on vanadium/manganese/cobalt oxides, Prussian blue analogs, and organic compounds have been reported. Among them, manganese dioxide (MnO2) is widely used in ARZBs due to their outstanding advantages of low toxicity, eco-friendliness, and high capacity (616 mAh∙g−1 based on two-electron transfer). However, the diversity of the crystal structures of MnO2 and the unpredictability of the electrochemical reaction make it difficult to investigate the specific internal storage mechanism, which impedes further development of the optimal modification strategies. To date, the main recognized energy storage mechanisms are (de)intercalation and dissolution-deposition mechanisms. In the traditional (de)intercalation mechanism, the predominant issues related to MnO2 during the cycling process include Mn dissolution, irreversible phase transformation, structural collapse, and sluggish ion diffusion kinetics. On the other hand, the detailed reaction path for the dissolution-deposition mechanism, which was developed in recent years, remains controversial. In addition, the incomplete dissolution-deposition of MnO2 and the highly acidic environment inevitably leads to corrosion and hydrogen evolution of the zinc anode, as well as low Coulombic efficiency. Accordingly, optimization strategies for different reaction mechanisms have been proposed to make zinc-manganese batteries more competitive. For the (de)intercalation mechanism, modification of composite materials and nanostructure optimization strategies can be adopted to inhibit the dissolution of MnO2 and increase the number of highly active reaction sites, thus enhancing the electrochemical performance. Moreover, the guest pre-intercalation strategy can help optimize the crystal structure of MnO2, preventing the collapse of the internal structure during cycling. Besides, defect engineering and element doping strategies focus on regulating the distribution of the electronic structure for affecting the properties of MnO2, resulting in lowering the energy barrier of zinc insertion. For the dissolution-deposition mechanism, the introduction of a neutral acetate and a halide mediator can effectively facilitate the dissolution-deposition of MnO2. Meanwhile, metal element catalysis can accelerate the reaction kinetics of the MnO2 dissolution-deposition, so that high-rate performance can be achieved. Furthermore, the decoupling battery system can separate the cathodic and anodic electrolytes to restrain the hydrogen and oxygen evolution reactions and enhance the potential difference. The flow battery system can effectively eliminate the influence of concentration polarization and stabilize the ion concentration in the electrolytes, thus leading to a large capacity (> 100 mAh). Undoubtedly, MnO2 as a high-capacity, high-voltage cathode material has broad development prospects for ARZBs. Here, we systematically summarize the crystal structures and reaction mechanisms of MnO2. We also discuss the optimization strategies toward advanced MnO2 cathode materials for resolving the highlighted issues in zinc-manganese batteries, which are expected to provide research directions for the design and development of high-performance ARZBs.
2022, 38(11): 220102
doi: 10.3866/PKU.WHXB202201021
Abstract:
Industrialization undoubtedly boosts economic development and improves the standard of living; however, it also leads to some serious problems, including the energy crisis, environmental pollution, and global warming. These problems are associated with or caused by the high carbon dioxide (CO2) and sulfur dioxide (SO2) emissions from the burning of fossil fuels such as coal, oil, and gas. Photocatalysis is considered one of the most promising technologies for eliminating these problems because of the possibility of converting CO2 into hydrocarbon fuels and other valuable chemicals using solar energy, hydrogen (H2) production from water (H2O) electrolysis, and degradation of pollutants. Among the various photocatalysts, silicon carbide (SiC) has great potential in the fields of photocatalysis, photoelectrocatalysis, and electrocatalysis because of its good electrical properties and photoelectrochemistry. This review is divided into six sections: introduction, fundamentals of nanostructured SiC, synthesis methods for obtaining nanostructured SiC photocatalysts, strategies for improving the activity of nanostructured SiC photocatalysts, applications of nanostructured SiC photocatalysts, and conclusions and prospects. The fundamentals of nanostructured SiC include its physicochemical characteristics. It possesses a range of unique physical properties, such as extreme hardness, high mechanical stability at high temperatures, a low thermal expansion coefficient, wide bandgap, and superior thermal conductivity. It also possesses exceptional chemical characteristics, such as high oxidation and corrosion resistance. The synthesis methods for obtaining nanostructured SiC have been systematically summarized as follows: Template growth, sol-gel, organic precursor pyrolysis, solvothermal synthesis, arc discharge, carbon thermal reduction, and electrospinning. These synthesis methods require high temperatures, and the reaction mechanism involves SiC formation via the reaction between carbon and silicon oxide. In the section of the review involving the strategies for improving the activity of nanostructured SiC photocatalysts, seven strategies are discussed, viz., element doping, construction of Z-scheme (or S-scheme) systems, supported co-catalysts, visible photosensitization, construction of semiconductor heterojunctions, supported carbon materials, and construction of nanostructures. All of these strategies, except element doping and visible photosensitization, concentrate on enhancing the separation of holes and electrons, while suppressing their recombination, thus improving the photocatalytic performance of the nanostructured SiC photocatalysts. Regarding the element doping and visible photosensitization strategies, element doping can narrow the bandgap of SiC, which generates more holes and electrons to improve photocatalytic activity. On the other hand, the principle of visible photosensitization is that photo-induced electrons move from photosensitizers to the conduction band of SiC to participate in the reaction, thus enhancing the photocatalytic performance. In the section on the applications of nanostructured SiC, photocatalytic H2 production, pollutant degradation, CO2 reduction, photoelectrocatalytic, and electrocatalytic applications will be discussed. The mechanism of a photocatalytic reaction requires the SiC photocatalyst to produce photo-induced electrons and holes during irradiation, which participate in the photocatalytic reaction. For example, photo-induced electrons can transform protons into H2, as well as CO2 into methane, methanol, or formic acid. Furthermore, photo-induced holes can convert organic waste into H2O and CO2. For photoelectrocatalytic and electrocatalytic applications, SiC is used as a catalyst under high temperatures and highly acidic or basic environments because of its remarkable physicochemical characteristics, including low thermal expansion, superior thermal conductivity, and high oxidation and corrosion resistance. The last section of the review will reveal the major obstacles impeding the industrial application of nanostructured SiC photocatalysts, such as insufficient visible absorption, slow reaction kinetics, and hard fabrication, as well as provide some ideas on how to overcome these obstacles. ![]()
Industrialization undoubtedly boosts economic development and improves the standard of living; however, it also leads to some serious problems, including the energy crisis, environmental pollution, and global warming. These problems are associated with or caused by the high carbon dioxide (CO2) and sulfur dioxide (SO2) emissions from the burning of fossil fuels such as coal, oil, and gas. Photocatalysis is considered one of the most promising technologies for eliminating these problems because of the possibility of converting CO2 into hydrocarbon fuels and other valuable chemicals using solar energy, hydrogen (H2) production from water (H2O) electrolysis, and degradation of pollutants. Among the various photocatalysts, silicon carbide (SiC) has great potential in the fields of photocatalysis, photoelectrocatalysis, and electrocatalysis because of its good electrical properties and photoelectrochemistry. This review is divided into six sections: introduction, fundamentals of nanostructured SiC, synthesis methods for obtaining nanostructured SiC photocatalysts, strategies for improving the activity of nanostructured SiC photocatalysts, applications of nanostructured SiC photocatalysts, and conclusions and prospects. The fundamentals of nanostructured SiC include its physicochemical characteristics. It possesses a range of unique physical properties, such as extreme hardness, high mechanical stability at high temperatures, a low thermal expansion coefficient, wide bandgap, and superior thermal conductivity. It also possesses exceptional chemical characteristics, such as high oxidation and corrosion resistance. The synthesis methods for obtaining nanostructured SiC have been systematically summarized as follows: Template growth, sol-gel, organic precursor pyrolysis, solvothermal synthesis, arc discharge, carbon thermal reduction, and electrospinning. These synthesis methods require high temperatures, and the reaction mechanism involves SiC formation via the reaction between carbon and silicon oxide. In the section of the review involving the strategies for improving the activity of nanostructured SiC photocatalysts, seven strategies are discussed, viz., element doping, construction of Z-scheme (or S-scheme) systems, supported co-catalysts, visible photosensitization, construction of semiconductor heterojunctions, supported carbon materials, and construction of nanostructures. All of these strategies, except element doping and visible photosensitization, concentrate on enhancing the separation of holes and electrons, while suppressing their recombination, thus improving the photocatalytic performance of the nanostructured SiC photocatalysts. Regarding the element doping and visible photosensitization strategies, element doping can narrow the bandgap of SiC, which generates more holes and electrons to improve photocatalytic activity. On the other hand, the principle of visible photosensitization is that photo-induced electrons move from photosensitizers to the conduction band of SiC to participate in the reaction, thus enhancing the photocatalytic performance. In the section on the applications of nanostructured SiC, photocatalytic H2 production, pollutant degradation, CO2 reduction, photoelectrocatalytic, and electrocatalytic applications will be discussed. The mechanism of a photocatalytic reaction requires the SiC photocatalyst to produce photo-induced electrons and holes during irradiation, which participate in the photocatalytic reaction. For example, photo-induced electrons can transform protons into H2, as well as CO2 into methane, methanol, or formic acid. Furthermore, photo-induced holes can convert organic waste into H2O and CO2. For photoelectrocatalytic and electrocatalytic applications, SiC is used as a catalyst under high temperatures and highly acidic or basic environments because of its remarkable physicochemical characteristics, including low thermal expansion, superior thermal conductivity, and high oxidation and corrosion resistance. The last section of the review will reveal the major obstacles impeding the industrial application of nanostructured SiC photocatalysts, such as insufficient visible absorption, slow reaction kinetics, and hard fabrication, as well as provide some ideas on how to overcome these obstacles.
2022, 38(11): 220404
doi: 10.3866/PKU.WHXB202204049
Abstract:
Na-ion batteries (SIBs) are promising alternatives for Li-ion batteries owing to the natural abundance of sodium resources and similar energy storage mechanisms. Although significant progress has been achieved in research on SIBs, there remain several challenges to be addressed. One of the major challenges in the construction of high-performance SIBs is the development of suitable anode materials with a large reversible capacity, high cycling stability, and good rate performance. Alloying anode materials mainly composed of elements from Groups IVA and VA, as well as their alloys, have attracted widespread attention because of their low working voltage, high cost-effectiveness, and large theoretical capacity. Alloying-type anode materials can be alloyed with metallic Na to achieve large reversible capacities, ensuring a high energy density. Antimony is a promising anode material for SIBs owing to its high theoretical specific capacity (660 mAh·g−1, corresponding to the full sodiation Na3Sb alloy), small degree of electrode polarization (~0.25 V), appropriate Na+ deintercalation potential (0.5–0.75 V), low price, and environmental friendliness. However, an important challenge for using Sb-based anode materials is that the high specific capacity is accompanied by large volume changes during cycling. Such changes lead to the pulverization of the active materials and their falling off from the collector, which significantly limit their large-scale application in the field of sodium-ion batteries. Therefore, mitigating the volume expansion issue of Sb-based anode materials in the charge-discharge process is very important for the design of high-performance SIBs. In recent years, researchers have attempted to address this issue by designing special structures to prepare various composites, and substantial progress has been achieved in improving the electrochemical performance of SIBs. In this review, the relationship between the structure and properties of Sb-based materials and their applications in SIBs are presented and discussed in detail. The latest research progress on using Sb-based anode materials for SIBs in redox reaction mechanisms along with their morphology design, structure-performance relationship, etc. have been reviewed. The main objective of this review is to explore the determining factors of the performance of Sb-based anode materials to propose suitable modification strategies for improving their reversible capacity and cycle stability. Finally, future developments, challenges, and prospects of Sb-based anode materials for SIBs are discussed. Despite several challenges, Sb-based materials are very promising anode materials for SIBs with alloying reaction mechanisms. To further improve the large-scale application of Sb-based anode materials, it is necessary to optimize the binder, electrode structure, and electrolyte composition. The combination of in-depth studies on the electrochemical reaction mechanisms and advanced characterization technologies is important for the development and construction of advanced Sb-based anode materials for SIBs. Finally, to achieve extensive large-scale applications, it is necessary to further explore environmentally friendly, low-cost, and controllable synthetic technologies to prepare high-performance Sb-based anode materials. This review provides specific perspectives for the construction and optimization of Sb-based anode materials and suggests scope for future work on Sb-based anode materials, thereby promoting the rapid development and practical application of SIBs. ![]()
Na-ion batteries (SIBs) are promising alternatives for Li-ion batteries owing to the natural abundance of sodium resources and similar energy storage mechanisms. Although significant progress has been achieved in research on SIBs, there remain several challenges to be addressed. One of the major challenges in the construction of high-performance SIBs is the development of suitable anode materials with a large reversible capacity, high cycling stability, and good rate performance. Alloying anode materials mainly composed of elements from Groups IVA and VA, as well as their alloys, have attracted widespread attention because of their low working voltage, high cost-effectiveness, and large theoretical capacity. Alloying-type anode materials can be alloyed with metallic Na to achieve large reversible capacities, ensuring a high energy density. Antimony is a promising anode material for SIBs owing to its high theoretical specific capacity (660 mAh·g−1, corresponding to the full sodiation Na3Sb alloy), small degree of electrode polarization (~0.25 V), appropriate Na+ deintercalation potential (0.5–0.75 V), low price, and environmental friendliness. However, an important challenge for using Sb-based anode materials is that the high specific capacity is accompanied by large volume changes during cycling. Such changes lead to the pulverization of the active materials and their falling off from the collector, which significantly limit their large-scale application in the field of sodium-ion batteries. Therefore, mitigating the volume expansion issue of Sb-based anode materials in the charge-discharge process is very important for the design of high-performance SIBs. In recent years, researchers have attempted to address this issue by designing special structures to prepare various composites, and substantial progress has been achieved in improving the electrochemical performance of SIBs. In this review, the relationship between the structure and properties of Sb-based materials and their applications in SIBs are presented and discussed in detail. The latest research progress on using Sb-based anode materials for SIBs in redox reaction mechanisms along with their morphology design, structure-performance relationship, etc. have been reviewed. The main objective of this review is to explore the determining factors of the performance of Sb-based anode materials to propose suitable modification strategies for improving their reversible capacity and cycle stability. Finally, future developments, challenges, and prospects of Sb-based anode materials for SIBs are discussed. Despite several challenges, Sb-based materials are very promising anode materials for SIBs with alloying reaction mechanisms. To further improve the large-scale application of Sb-based anode materials, it is necessary to optimize the binder, electrode structure, and electrolyte composition. The combination of in-depth studies on the electrochemical reaction mechanisms and advanced characterization technologies is important for the development and construction of advanced Sb-based anode materials for SIBs. Finally, to achieve extensive large-scale applications, it is necessary to further explore environmentally friendly, low-cost, and controllable synthetic technologies to prepare high-performance Sb-based anode materials. This review provides specific perspectives for the construction and optimization of Sb-based anode materials and suggests scope for future work on Sb-based anode materials, thereby promoting the rapid development and practical application of SIBs.
2022, 38(11): 220405
doi: 10.3866/PKU.WHXB202204057
Abstract:
Driven by the excessive environmental pollution caused by the over-use of non-renewable fossil-derived energy, renewable energy and electrochemical energy storage devices have made great progress in the past decades. Electrochemical energy storage devices, such as lithium-ion batteries, have the advantages of high capacity, long life cycle, and good safety performance; therefore, they have been used in various applications. For example, economical and environment-friendly electric vehicles have recently taken up increasing market share. However, when compared with vehicles propelled using fossil-derived energy, the slow charging speed of electric vehicles has always restricted their further promotion. The realization of rapid charging for electric vehicles can alleviate the high-pressure usage of charging piles as well as increase the application and market share of electric vehicles. Therefore, it is important to develop high-performance lithium-ion batteries with rapid charge and discharge capacities. The fast-charging capacity of lithium-ion batteries is limited by the slow migration of lithium ions in the electrode and the electrode/electrolyte interface. Therefore, the key to developing fast-charging lithium-ion batteries lies in the successful design of suitable electrode materials. Because of its low cost and excellent electrochemical performance, graphite has been widely used to develop the cathode of lithium-ion batteries. However, the migration of lithium ions in graphite is slow, resulting in large polarization during the high-current charge and discharge processes. In addition, the low lithium intercalation potential of graphite leads to lithium precipitation during fast charging, which can decrease the electrochemical performance and cause potential safety hazards. Therefore, graphite must be improved to meet the needs of such fast-charging devices. In this article, we systematically introduce the research progress made in recent years within the scope of rapid-charging improvement of graphite(-based) cathodes and then highlight the modification strategies for graphite with the goal of achieving functional coating, desired morphological and structural design, optimized electrolyte properties, and an improved charging protocol. Additionally, this article evaluates the advantages and disadvantages of the modification strategies as well as their application prospects. The scheme of functional coating for modifying graphite must simplify the process and improve production efficiency to meet the needs of industrial development. Morphology design should ensure satisfactory initial Coulomb efficiency, while the improvement of the electrolyte properties and optimization of the charging protocol need to consider the commercialization costs. Finally, this paper proposes further evaluation of the effects of the modification strategies based on soft-pack or cylindrical batteries to strengthen the commercialization prospect of the modification strategies. ![]()
Driven by the excessive environmental pollution caused by the over-use of non-renewable fossil-derived energy, renewable energy and electrochemical energy storage devices have made great progress in the past decades. Electrochemical energy storage devices, such as lithium-ion batteries, have the advantages of high capacity, long life cycle, and good safety performance; therefore, they have been used in various applications. For example, economical and environment-friendly electric vehicles have recently taken up increasing market share. However, when compared with vehicles propelled using fossil-derived energy, the slow charging speed of electric vehicles has always restricted their further promotion. The realization of rapid charging for electric vehicles can alleviate the high-pressure usage of charging piles as well as increase the application and market share of electric vehicles. Therefore, it is important to develop high-performance lithium-ion batteries with rapid charge and discharge capacities. The fast-charging capacity of lithium-ion batteries is limited by the slow migration of lithium ions in the electrode and the electrode/electrolyte interface. Therefore, the key to developing fast-charging lithium-ion batteries lies in the successful design of suitable electrode materials. Because of its low cost and excellent electrochemical performance, graphite has been widely used to develop the cathode of lithium-ion batteries. However, the migration of lithium ions in graphite is slow, resulting in large polarization during the high-current charge and discharge processes. In addition, the low lithium intercalation potential of graphite leads to lithium precipitation during fast charging, which can decrease the electrochemical performance and cause potential safety hazards. Therefore, graphite must be improved to meet the needs of such fast-charging devices. In this article, we systematically introduce the research progress made in recent years within the scope of rapid-charging improvement of graphite(-based) cathodes and then highlight the modification strategies for graphite with the goal of achieving functional coating, desired morphological and structural design, optimized electrolyte properties, and an improved charging protocol. Additionally, this article evaluates the advantages and disadvantages of the modification strategies as well as their application prospects. The scheme of functional coating for modifying graphite must simplify the process and improve production efficiency to meet the needs of industrial development. Morphology design should ensure satisfactory initial Coulomb efficiency, while the improvement of the electrolyte properties and optimization of the charging protocol need to consider the commercialization costs. Finally, this paper proposes further evaluation of the effects of the modification strategies based on soft-pack or cylindrical batteries to strengthen the commercialization prospect of the modification strategies.
2022, 38(11): 220500
doi: 10.3866/PKU.WHXB202205005
Abstract:
Lithium metal batteries, which use lithium metal as the anode, have attracted tremendous research interest in recent years, owing to their high energy density and potential for future energy storage applications. Despite their advantages such as high energy density, the safety concerns and short lifespan significantly impede their practical applications in transportation and electronic devices. Tremendous efforts have been devoted to overcoming these problems, including materials design, interface modification, and electrolyte engineering. Among these strategies, electrolyte regulation plays a key role in improving the efficiency, stability, and safety of lithium metal anodes. As an important class of electrolyte components, fluorinated solvents, which can decompose to form LiF-rich interphase layers on both anode and cathode, have been proven to enhance the stability of lithium metal anodes and improve the oxidative stability of the electrolytes. Meanwhile, the spatial structure of fluorinated solvents, such as the number and sites of fluorine atoms, can influence the physicochemical properties of the electrolytes and the compositions/structure of the solid-electrolyte interphase, which eventually dictates the cycling performance of Li metal batteries. Recently, many fluorinated solvents with different molecular structures have been designed to regulate the solvation structure of electrolytes, and these solvents exhibit novel electrochemical properties in lithium metal batteries. However, there are few comprehensive reviews that summarize the fluorinated solvents used in Li metal batteries and discuss their functions in electrolytes and their physicochemical properties. This review summarizes the novel fluorinated solvents used in lithium metal batteries in recent years, which have been classified into three parts: diluents, traditional solvents, and novel molecules, based on their functions in the electrolytes. In every part, the understanding of the interactions between fluorinated solvents and Li ions, the decomposition mechanism of fluorinated solvents at the interface of the electrode, the functions of fluorinated solvents in the electrolytes, and the structure-activity relationship between the fluorinated solvents and battery performance have been comprehensively summarized and discussed. Moreover, the advantages and disadvantages of fluorinated solvents have been discussed, and the importance of precisely controlling the number of fluorine atoms and the structure of fluorinated solvents has been emphasized. At the end of this review, a perspective for designing new fluorinated solvents has been proposed. We believe that this review can provide insights on designing novel fluorinated solvents for high-performance Li metal batteries. ![]()
Lithium metal batteries, which use lithium metal as the anode, have attracted tremendous research interest in recent years, owing to their high energy density and potential for future energy storage applications. Despite their advantages such as high energy density, the safety concerns and short lifespan significantly impede their practical applications in transportation and electronic devices. Tremendous efforts have been devoted to overcoming these problems, including materials design, interface modification, and electrolyte engineering. Among these strategies, electrolyte regulation plays a key role in improving the efficiency, stability, and safety of lithium metal anodes. As an important class of electrolyte components, fluorinated solvents, which can decompose to form LiF-rich interphase layers on both anode and cathode, have been proven to enhance the stability of lithium metal anodes and improve the oxidative stability of the electrolytes. Meanwhile, the spatial structure of fluorinated solvents, such as the number and sites of fluorine atoms, can influence the physicochemical properties of the electrolytes and the compositions/structure of the solid-electrolyte interphase, which eventually dictates the cycling performance of Li metal batteries. Recently, many fluorinated solvents with different molecular structures have been designed to regulate the solvation structure of electrolytes, and these solvents exhibit novel electrochemical properties in lithium metal batteries. However, there are few comprehensive reviews that summarize the fluorinated solvents used in Li metal batteries and discuss their functions in electrolytes and their physicochemical properties. This review summarizes the novel fluorinated solvents used in lithium metal batteries in recent years, which have been classified into three parts: diluents, traditional solvents, and novel molecules, based on their functions in the electrolytes. In every part, the understanding of the interactions between fluorinated solvents and Li ions, the decomposition mechanism of fluorinated solvents at the interface of the electrode, the functions of fluorinated solvents in the electrolytes, and the structure-activity relationship between the fluorinated solvents and battery performance have been comprehensively summarized and discussed. Moreover, the advantages and disadvantages of fluorinated solvents have been discussed, and the importance of precisely controlling the number of fluorine atoms and the structure of fluorinated solvents has been emphasized. At the end of this review, a perspective for designing new fluorinated solvents has been proposed. We believe that this review can provide insights on designing novel fluorinated solvents for high-performance Li metal batteries.
2022, 38(11): 220103
doi: 10.3866/PKU.WHXB202201039
Abstract:
From the industrial perspective, poly(3-hexylthiophene) (P3HT) is one of the most attractive donor materials in organic photovoltaics. The large bandgap in P3HT makes it particularly promising for efficient indoor light harvesting, a unique advantage of organic photovoltaic (PV) devices, and this has started to gain considerable attention in the field of PV technology. In addition, the up-scalability and long material stability associated with the simple chemical structure make P3HT one of the most promising materials for the mass production of organic solar cells. However, the solar cells based on P3HT has a low power conversion efficiency (PCE), which is less than 11%, mainly due to significant voltage losses. In this study, we identified the origin of the high quantum efficiency and voltage losses in the P3HT: non-fullerene based solar cells, and we proposed a strategy to reduce the losses. More specifically, we observed that: 1) the non-radiative decay rate of the charge transfer (CT) states formed at the donor–acceptor interfaces was much higher for the P3HT: non-fullerene solar cells than that for the P3HT: fullerene solar cells, which was the main reason for the more severely limited photovoltage; 2) the origin of the high non-radiative decay rate in the P3HT: non-fullerene solar cell could be ascribed to the short packing distance between the P3HT and non-fullerene acceptor molecules at the donor–acceptor interfaces (DA distance), which is a rarely studied interfacial structural property, highly important in determining the decay rate of CT states; 3) the lower voltage loss in the state-of-the-art P3HT solar cell based on the 2, 2'-((12, 13-bis(2-butyldecyl)-3, 9-diundecyl-12, 13-dihydro-[1, 2, 5]-thiadiazolo[3, 4-e]thieno[2'', 3'': 4', 5']thieno[2', 3': 4, 5]p-yrolo[3, 2-g]thieno[2', 3': 4, 5]thieno[3, 2-b]indole-2, 10-diyl)bis(methanelylidene))bis(5, 6-dichloro-1H-indene-1, 3(2H)-dion-e) (ZY-4Cl) acceptor could be associated with the better alignment of the energy levels of the active materials and the longer DA distance, compared to those based on the commonly used acceptors. However, the DA distance was still very short, limiting the device voltage. Thus, improving the performance of the P3HT based solar cells requires a further increase in the DA distance. Our findings are expected to pave the way for breaking the performance bottleneck of the P3HT based solar cells.![]()
From the industrial perspective, poly(3-hexylthiophene) (P3HT) is one of the most attractive donor materials in organic photovoltaics. The large bandgap in P3HT makes it particularly promising for efficient indoor light harvesting, a unique advantage of organic photovoltaic (PV) devices, and this has started to gain considerable attention in the field of PV technology. In addition, the up-scalability and long material stability associated with the simple chemical structure make P3HT one of the most promising materials for the mass production of organic solar cells. However, the solar cells based on P3HT has a low power conversion efficiency (PCE), which is less than 11%, mainly due to significant voltage losses. In this study, we identified the origin of the high quantum efficiency and voltage losses in the P3HT: non-fullerene based solar cells, and we proposed a strategy to reduce the losses. More specifically, we observed that: 1) the non-radiative decay rate of the charge transfer (CT) states formed at the donor–acceptor interfaces was much higher for the P3HT: non-fullerene solar cells than that for the P3HT: fullerene solar cells, which was the main reason for the more severely limited photovoltage; 2) the origin of the high non-radiative decay rate in the P3HT: non-fullerene solar cell could be ascribed to the short packing distance between the P3HT and non-fullerene acceptor molecules at the donor–acceptor interfaces (DA distance), which is a rarely studied interfacial structural property, highly important in determining the decay rate of CT states; 3) the lower voltage loss in the state-of-the-art P3HT solar cell based on the 2, 2'-((12, 13-bis(2-butyldecyl)-3, 9-diundecyl-12, 13-dihydro-[1, 2, 5]-thiadiazolo[3, 4-e]thieno[2'', 3'': 4', 5']thieno[2', 3': 4, 5]p-yrolo[3, 2-g]thieno[2', 3': 4, 5]thieno[3, 2-b]indole-2, 10-diyl)bis(methanelylidene))bis(5, 6-dichloro-1H-indene-1, 3(2H)-dion-e) (ZY-4Cl) acceptor could be associated with the better alignment of the energy levels of the active materials and the longer DA distance, compared to those based on the commonly used acceptors. However, the DA distance was still very short, limiting the device voltage. Thus, improving the performance of the P3HT based solar cells requires a further increase in the DA distance. Our findings are expected to pave the way for breaking the performance bottleneck of the P3HT based solar cells.
2022, 38(11): 220404
doi: 10.3866/PKU.WHXB202204048
Abstract:
In view of the continuously worsening environmental problems, fossil fuels will not be able to support the development of human life in the future. Hence, it is of great importance to work on the efficient utilization of cleaner energy resources. In this case, cheap, reliable, and eco-friendly grid-scale energy storage systems can play a key role in optimizing our energy usage. When compared with lithium-ion and lead-acid batteries, the excellent safety, environmental benignity, and low toxicity of aqueous Zn-based batteries make them competitive in the context of large-scale energy storage. Among the various Zn-based batteries, due to a high open-circuit voltage and excellent rate performance, Zn-Ni batteries have great potential in practical applications. Nevertheless, the intrinsic obstacles associated with the use of Zn anodes in alkaline electrolytes, such as dendrite, shape change, passivation, and corrosion, limit their commercial application. Hence, we have focused our current efforts on inhibiting the corrosion and dissolution of Zn species. Based on a previous study from our research group, the failure of the Zn-Ni battery was caused by the shape change of the Zn anode, which stemmed from the dissolution of Zn and uneven current distribution on the anode. Therefore, for the current study, we selected K3[Fe(CN)6] as an electrolyte additive that would help minimize the corrosion and dissolution of the Zn anode. In the alkaline electrolyte, [Fe(CN)6]3– was reduced to [Fe(CN)6]4– by the metallic Zn present in the Zn-Ni battery. Owing to its low solubility in the electrolyte, K4[Fe(CN)6] adhered to the active Zn anode, thereby inhibiting the aggregation and corrosion of Zn. Ultimately, the shape change of the anode was effectively eliminated, which improved the cycling life of the Zn-Ni battery by more than three times (i.e., from 124 cycles to more than 423 cycles). As for capacity retention, the Zn-Ni battery with the pristine electrolyte only exhibited 40% capacity retention after 85 cycles, while the Zn-Ni battery with the modified electrolyte (i.e., containing K3[Fe(CN)6]) showed 72% capacity retention. Moreover, unlike conventional organic additives that increase electrode polarization, the addition of K3[Fe(CN)6] not only significantly reduced the charge-transfer resistance in a simplified three-electrode system, but also improved the discharge capacity and rate performance of the Zn-Ni battery. Importantly, considering that this strategy was easy to achieve and minimized additional costs, K3[Fe(CN)6], as an electrolyte additive with almost no negative effect, has tremendous potential in commercial Zn-Ni batteries.![]()
In view of the continuously worsening environmental problems, fossil fuels will not be able to support the development of human life in the future. Hence, it is of great importance to work on the efficient utilization of cleaner energy resources. In this case, cheap, reliable, and eco-friendly grid-scale energy storage systems can play a key role in optimizing our energy usage. When compared with lithium-ion and lead-acid batteries, the excellent safety, environmental benignity, and low toxicity of aqueous Zn-based batteries make them competitive in the context of large-scale energy storage. Among the various Zn-based batteries, due to a high open-circuit voltage and excellent rate performance, Zn-Ni batteries have great potential in practical applications. Nevertheless, the intrinsic obstacles associated with the use of Zn anodes in alkaline electrolytes, such as dendrite, shape change, passivation, and corrosion, limit their commercial application. Hence, we have focused our current efforts on inhibiting the corrosion and dissolution of Zn species. Based on a previous study from our research group, the failure of the Zn-Ni battery was caused by the shape change of the Zn anode, which stemmed from the dissolution of Zn and uneven current distribution on the anode. Therefore, for the current study, we selected K3[Fe(CN)6] as an electrolyte additive that would help minimize the corrosion and dissolution of the Zn anode. In the alkaline electrolyte, [Fe(CN)6]3– was reduced to [Fe(CN)6]4– by the metallic Zn present in the Zn-Ni battery. Owing to its low solubility in the electrolyte, K4[Fe(CN)6] adhered to the active Zn anode, thereby inhibiting the aggregation and corrosion of Zn. Ultimately, the shape change of the anode was effectively eliminated, which improved the cycling life of the Zn-Ni battery by more than three times (i.e., from 124 cycles to more than 423 cycles). As for capacity retention, the Zn-Ni battery with the pristine electrolyte only exhibited 40% capacity retention after 85 cycles, while the Zn-Ni battery with the modified electrolyte (i.e., containing K3[Fe(CN)6]) showed 72% capacity retention. Moreover, unlike conventional organic additives that increase electrode polarization, the addition of K3[Fe(CN)6] not only significantly reduced the charge-transfer resistance in a simplified three-electrode system, but also improved the discharge capacity and rate performance of the Zn-Ni battery. Importantly, considering that this strategy was easy to achieve and minimized additional costs, K3[Fe(CN)6], as an electrolyte additive with almost no negative effect, has tremendous potential in commercial Zn-Ni batteries.
2022, 38(11): 220501
doi: 10.3866/PKU.WHXB202205017
Abstract:
Rechargeable potassium-ion batteries (PIBs), with their low cost and the abundant K reserves, have been promising candidates for energy storage and conversion. Among all anode materials for PIBs, metal sulfides (MSs) show superiority owing to their high theoretical capacity and variety of material species. Nevertheless, the battery performance of MSs is hindered by many factors such as poor conductivity, low ion diffusivity, sluggish interfacial/surface transfer kinetics, and drastic volume changes. In this review, the electrochemical reaction mechanisms, challenges, and synthesis methods of MSs for PIBs are summarized and discussed. In particular, the most common synthesis methods of MSs for PIBs are highlighted, including template synthesis, hydro/solvothermal synthesis, solid-phase chemical synthesis, electrospinning synthesis, and ion-exchange synthesis. During the potassium storage process, the two-dimensional layered MSs follow the intercalation/extraction mechanism, and the MSs with inactive metal undergo the conversion reaction, whereas the metal-active MSs follow the conversion-alloying reaction mechanism. Given the inherent properties of MSs and the reactions they undergo during cycling, when used as anodes for PIBs, such materials experience a series of problems, including poor ion-/electron-transport kinetics, structural instability, and loss of active material caused by the dissolution of discharged polysulfide products and the occurrence of side reactions. These problems can be solved by optimizing the methods for synthesizing MSs with an ideal composition and structure. The template method can precisely prepare porous or hollow-structured materials, the hydro/solvothermal method can alter the thickness or size of the material by adjusting certain synthesis parameters, and the one-dimensional-structured material obtained via electrospinning often has a large specific surface area, all of which can shorten the transport pathway for potassium ions, thereby improving the performance of the battery. The ion-exchange method affords difficult-to-synthesize MSs via anion- or cation-exchange, in which the product inherits the structure of the starting material. The solid-phase synthesis method makes it possible to combine MSs with other materials. Combinations with materials such as carbon or other MSs helps to provide sufficient buffer space for the volume expansion of MSs during cycling, while promoting electron transport and improving the potassium-storage properties of the anodes. Therefore, this review aims to highlight the current defects of MS anodes and explore the construction of their ideal architecture for high-performance PIBs by optimizing the synthesis methods. Ultimately, we propose the possible future advancement of MSs for PIBs. ![]()
Rechargeable potassium-ion batteries (PIBs), with their low cost and the abundant K reserves, have been promising candidates for energy storage and conversion. Among all anode materials for PIBs, metal sulfides (MSs) show superiority owing to their high theoretical capacity and variety of material species. Nevertheless, the battery performance of MSs is hindered by many factors such as poor conductivity, low ion diffusivity, sluggish interfacial/surface transfer kinetics, and drastic volume changes. In this review, the electrochemical reaction mechanisms, challenges, and synthesis methods of MSs for PIBs are summarized and discussed. In particular, the most common synthesis methods of MSs for PIBs are highlighted, including template synthesis, hydro/solvothermal synthesis, solid-phase chemical synthesis, electrospinning synthesis, and ion-exchange synthesis. During the potassium storage process, the two-dimensional layered MSs follow the intercalation/extraction mechanism, and the MSs with inactive metal undergo the conversion reaction, whereas the metal-active MSs follow the conversion-alloying reaction mechanism. Given the inherent properties of MSs and the reactions they undergo during cycling, when used as anodes for PIBs, such materials experience a series of problems, including poor ion-/electron-transport kinetics, structural instability, and loss of active material caused by the dissolution of discharged polysulfide products and the occurrence of side reactions. These problems can be solved by optimizing the methods for synthesizing MSs with an ideal composition and structure. The template method can precisely prepare porous or hollow-structured materials, the hydro/solvothermal method can alter the thickness or size of the material by adjusting certain synthesis parameters, and the one-dimensional-structured material obtained via electrospinning often has a large specific surface area, all of which can shorten the transport pathway for potassium ions, thereby improving the performance of the battery. The ion-exchange method affords difficult-to-synthesize MSs via anion- or cation-exchange, in which the product inherits the structure of the starting material. The solid-phase synthesis method makes it possible to combine MSs with other materials. Combinations with materials such as carbon or other MSs helps to provide sufficient buffer space for the volume expansion of MSs during cycling, while promoting electron transport and improving the potassium-storage properties of the anodes. Therefore, this review aims to highlight the current defects of MS anodes and explore the construction of their ideal architecture for high-performance PIBs by optimizing the synthesis methods. Ultimately, we propose the possible future advancement of MSs for PIBs.
2022, 38(11): 220602
doi: 10.3866/PKU.WHXB202206020
Abstract:
Through the combustion of fossil fuels and other human activities, large amounts of CO2 gas have been emitted into the atmosphere, causing many environmental problems, such as the greenhouse effect and global warming. Thus, developing and utilizing renewable clean energy is crucial to reduce CO2 emission and achieve carbon neutrality. The electrochemical CO2 reduction reaction (CO2RR) has been considered as an effective approach to obtain high value-added chemicals and fuels, which can store intermittent renewable energy and achieve the artificial carbon cycle. In addition, due to its multiple advantages, such as mild reaction conditions, tunable products, and simple implementation, electrochemical CO2RR has attracted extensive attention. Electrochemical CO2RR involves multiple electron–proton transfer steps to obtain multitudinous products, such as C1 products (CO, HCOOH, CH4, etc.) and C2 products (C2H4, C2H5OH, etc.). The intermediates, among which *CO is usually identified as the key intermediate, and reaction pathways of different products intersect, resulting in an extremely complex reaction mechanism. Currently, copper has been widely proven to be the only metal catalyst that can efficiently reduce CO2 to hydrocarbons and oxygenates due to its suitable adsorption energy for *CO. However, the low product selectivity, poor stability, and high overpotential of pure Cu hinder its use for the production of industrial-grade multi-carbon products. Tandem catalysts with multiple types of active sites can sequentially reduce CO2 molecules into desired products. When loaded onto a co-catalyst that can efficiently convert CO2 to *CO (such as Au and Ag), Cu acts as an electron donor owing to its high electrochemical potential. *CO species generated from the substrate can spillover onto the surface of electron-poor Cu due to the stronger adsorption and be further reduced to C2+ products. The use of Cu-based tandem catalysts for electrochemical CO2RR is a promising strategy for improving the performance of CO2RR and thus, has become a research hotspot in recent years. In this review, we first introduce the reaction routes and tandem mechanisms of electrochemical CO2RR. Then, we systematically summarize the recent research progress of Cu-based tandem catalysts for electrochemical CO2RR, including Cu-based metallic materials (alloys, heterojunction, and core-shell structures) as well as Cu-based framework materials, carbon materials, and polymer-modified materials. Importantly, the preparation methods of various Cu-based tandem catalysts and their structure–activity relationship in CO2RR are discussed and analyzed in detail. Finally, the challenges and opportunities of the rational design and controllable synthesis of advanced tandem catalysts for electrochemical CO2RR are proposed.![]()
Through the combustion of fossil fuels and other human activities, large amounts of CO2 gas have been emitted into the atmosphere, causing many environmental problems, such as the greenhouse effect and global warming. Thus, developing and utilizing renewable clean energy is crucial to reduce CO2 emission and achieve carbon neutrality. The electrochemical CO2 reduction reaction (CO2RR) has been considered as an effective approach to obtain high value-added chemicals and fuels, which can store intermittent renewable energy and achieve the artificial carbon cycle. In addition, due to its multiple advantages, such as mild reaction conditions, tunable products, and simple implementation, electrochemical CO2RR has attracted extensive attention. Electrochemical CO2RR involves multiple electron–proton transfer steps to obtain multitudinous products, such as C1 products (CO, HCOOH, CH4, etc.) and C2 products (C2H4, C2H5OH, etc.). The intermediates, among which *CO is usually identified as the key intermediate, and reaction pathways of different products intersect, resulting in an extremely complex reaction mechanism. Currently, copper has been widely proven to be the only metal catalyst that can efficiently reduce CO2 to hydrocarbons and oxygenates due to its suitable adsorption energy for *CO. However, the low product selectivity, poor stability, and high overpotential of pure Cu hinder its use for the production of industrial-grade multi-carbon products. Tandem catalysts with multiple types of active sites can sequentially reduce CO2 molecules into desired products. When loaded onto a co-catalyst that can efficiently convert CO2 to *CO (such as Au and Ag), Cu acts as an electron donor owing to its high electrochemical potential. *CO species generated from the substrate can spillover onto the surface of electron-poor Cu due to the stronger adsorption and be further reduced to C2+ products. The use of Cu-based tandem catalysts for electrochemical CO2RR is a promising strategy for improving the performance of CO2RR and thus, has become a research hotspot in recent years. In this review, we first introduce the reaction routes and tandem mechanisms of electrochemical CO2RR. Then, we systematically summarize the recent research progress of Cu-based tandem catalysts for electrochemical CO2RR, including Cu-based metallic materials (alloys, heterojunction, and core-shell structures) as well as Cu-based framework materials, carbon materials, and polymer-modified materials. Importantly, the preparation methods of various Cu-based tandem catalysts and their structure–activity relationship in CO2RR are discussed and analyzed in detail. Finally, the challenges and opportunities of the rational design and controllable synthesis of advanced tandem catalysts for electrochemical CO2RR are proposed.
2022, 38(11): 220702
doi: 10.3866/PKU.WHXB202207024
Abstract:
The combustion of fossil fuels increases atmospheric carbon dioxide (CO2) concentrations, leading to adverse impacts on the planetary radiation balance and, consequently, on the climate. Fossil fuel utilization has contributed to a marked rise in global temperatures, now at least 1.2 ℃ above 'pre-industrial' levels. To meet the 2015 Paris Agreement target of 1.5 ℃ above pre-industrial levels, considerable efforts are required to efficiently capture and utilize CO2. Among the different strategies developed for converting CO2, electrochemical CO2 reduction (ECR) to valuable chemicals using renewable energy is expected to revolutionize the manufacture of sustainable "green" chemicals, thereby achieving a closed anthropogenic carbon cycle. However, CO2 is a thermodynamically stable and kinetically inert molecule that requires high electrical energy to bend the linear O＝C＝O bond by attacking the C atom. To facilitate the ECR with good energy efficiency, it is essential to lower the reaction overpotential as well as maintain a high current density and desirable product selectivity; therefore, the design and development of advanced electrocatalysts are crucial. A plethora of heterogeneous and homogeneous materials has been explored in the ECR. Among these materials, single-atom catalysts (SACs) have been the focus of most extensive research in the context of ECR. A SAC with isolated metal atoms dispersed on a supporting host exhibits a unique electronic structure, well-defined coordination environment, and an extremely high atom utilization maximum; thus, SACs have emerged as promising materials over the last two decades. Single-atom catalysis has covered the periodic table from d-block and ds-block metals to p-block metals. The types of support materials for SACs, ranging from metal oxides to tailored carbon materials, have also expanded. The adsorption strength and catalytic activity of SACs can be effectively tuned by modulating the central metal and local coordination structure of the SACs. In this article, we discuss the progress made to date in the field of single-atom catalysis for promoting ECR. We provide a comprehensive review of state-of-the-art SACs for the ECR in terms of product distribution, selectivity, partial current density, and performance stability. Special attention is paid to the modification of SACs to improve the ECR efficiency. This includes tailoring the coordination of the heteroatom, constructing bimetallic sites, engineering the morphologies and surface defects of supports, and regulating surface functional groups. The correlation of the coordination structure of SACs and metal-support interactions with ECR performance is analyzed. Finally, development opportunities and challenges for the application of SACs in the ECR, especially to form multi-carbon products, are presented.![]()
The combustion of fossil fuels increases atmospheric carbon dioxide (CO2) concentrations, leading to adverse impacts on the planetary radiation balance and, consequently, on the climate. Fossil fuel utilization has contributed to a marked rise in global temperatures, now at least 1.2 ℃ above 'pre-industrial' levels. To meet the 2015 Paris Agreement target of 1.5 ℃ above pre-industrial levels, considerable efforts are required to efficiently capture and utilize CO2. Among the different strategies developed for converting CO2, electrochemical CO2 reduction (ECR) to valuable chemicals using renewable energy is expected to revolutionize the manufacture of sustainable "green" chemicals, thereby achieving a closed anthropogenic carbon cycle. However, CO2 is a thermodynamically stable and kinetically inert molecule that requires high electrical energy to bend the linear O＝C＝O bond by attacking the C atom. To facilitate the ECR with good energy efficiency, it is essential to lower the reaction overpotential as well as maintain a high current density and desirable product selectivity; therefore, the design and development of advanced electrocatalysts are crucial. A plethora of heterogeneous and homogeneous materials has been explored in the ECR. Among these materials, single-atom catalysts (SACs) have been the focus of most extensive research in the context of ECR. A SAC with isolated metal atoms dispersed on a supporting host exhibits a unique electronic structure, well-defined coordination environment, and an extremely high atom utilization maximum; thus, SACs have emerged as promising materials over the last two decades. Single-atom catalysis has covered the periodic table from d-block and ds-block metals to p-block metals. The types of support materials for SACs, ranging from metal oxides to tailored carbon materials, have also expanded. The adsorption strength and catalytic activity of SACs can be effectively tuned by modulating the central metal and local coordination structure of the SACs. In this article, we discuss the progress made to date in the field of single-atom catalysis for promoting ECR. We provide a comprehensive review of state-of-the-art SACs for the ECR in terms of product distribution, selectivity, partial current density, and performance stability. Special attention is paid to the modification of SACs to improve the ECR efficiency. This includes tailoring the coordination of the heteroatom, constructing bimetallic sites, engineering the morphologies and surface defects of supports, and regulating surface functional groups. The correlation of the coordination structure of SACs and metal-support interactions with ECR performance is analyzed. Finally, development opportunities and challenges for the application of SACs in the ECR, especially to form multi-carbon products, are presented.
