Login In
2022 Volume 38 Issue 6
2022, 38(6): 210102
doi: 10.3866/PKU.WHXB202101028
Abstract:
The electrocatalytic carbon dioxide reduction reaction (E-CO2RR) has attracted attention in recent years for its ability to effectively alleviate the environmental problems caused by the rapid increase of CO2 in the atmosphere and transform CO2 into high value-added fuels or chemicals (e.g., CO, HCOOH, CH4, CH3OH, C2H4, C2H5OH, etc.) under mild conditions. In addition, clean energy sources, such as solar and wind energy, can provide electrical energy for the electrochemical CO2 conversion technology used in large-scale industrial applications. One limitation of the E-CO2RR is that CO2 is a thermodynamically stable linear molecule with a slow kinetic reaction rate. In addition, the E-CO2RR involves complex processes, such as gas diffusion and multi-electron transfer, making its selectivity problematic. Therefore, constructing highly efficient and stable catalytic electrodes has become a core research topic in the field of E-CO2RR. Unfortunately, the traditional method of coating electrodes with binders (e.g., Nafion, polyvinylidene fluoride, and polytetrafluoroethylene) usually results in a low utilization ratio of active sites due to the easy aggregation of the catalysts themselves. This could result in the severe embedding of active sites and limited mass transfer. Moreover, the dissolution of the catalyst layer during the electrocatalytic process also reduces the activity and stability of the electrodes, making it difficult to reuse. Therefore, it is necessary to regulate the electrode reaction interface to improve the utilization ratio of active sites. The integrated electrodes, where the catalyst is grown directly on the current collector, can avoid the use of binders to facilitate the exposure of active sites and transfer of electrons. The integrated structure can also enhance the bonding strength between the active material and current collector and improve the cycling stability of the electrodes. Meanwhile, the micro-environment (e.g., pH, concentration of CO2, and intermediates) at the three-phase interface can be effectively controlled on the integrated electrodes, which can enhance the performance of the E-CO2RR. In recent years, encouraging progress has been achieved in the study of the E-CO2RR. However, current reviews of the E-CO2RR mainly focus on the regulation of the intrinsic activity of catalysts; discussions and reviews from the perspective of the electrodes are rarely reported. This article reviews the latest research of the integrated electrodes for the E-CO2RR with a focus on the application of different types of integrated electrodes (e.g., metal, alloy, metal oxide, metal sulfide/phosphide, and metal single atom). It also analyzes the effects of morphology, surface, and interface regulation on the electrocatalytic performance of the E-CO2RR. Finally, it highlights the challenges that still exist in this field and discusses the future development of the integrated electrodes. ![]()
The electrocatalytic carbon dioxide reduction reaction (E-CO2RR) has attracted attention in recent years for its ability to effectively alleviate the environmental problems caused by the rapid increase of CO2 in the atmosphere and transform CO2 into high value-added fuels or chemicals (e.g., CO, HCOOH, CH4, CH3OH, C2H4, C2H5OH, etc.) under mild conditions. In addition, clean energy sources, such as solar and wind energy, can provide electrical energy for the electrochemical CO2 conversion technology used in large-scale industrial applications. One limitation of the E-CO2RR is that CO2 is a thermodynamically stable linear molecule with a slow kinetic reaction rate. In addition, the E-CO2RR involves complex processes, such as gas diffusion and multi-electron transfer, making its selectivity problematic. Therefore, constructing highly efficient and stable catalytic electrodes has become a core research topic in the field of E-CO2RR. Unfortunately, the traditional method of coating electrodes with binders (e.g., Nafion, polyvinylidene fluoride, and polytetrafluoroethylene) usually results in a low utilization ratio of active sites due to the easy aggregation of the catalysts themselves. This could result in the severe embedding of active sites and limited mass transfer. Moreover, the dissolution of the catalyst layer during the electrocatalytic process also reduces the activity and stability of the electrodes, making it difficult to reuse. Therefore, it is necessary to regulate the electrode reaction interface to improve the utilization ratio of active sites. The integrated electrodes, where the catalyst is grown directly on the current collector, can avoid the use of binders to facilitate the exposure of active sites and transfer of electrons. The integrated structure can also enhance the bonding strength between the active material and current collector and improve the cycling stability of the electrodes. Meanwhile, the micro-environment (e.g., pH, concentration of CO2, and intermediates) at the three-phase interface can be effectively controlled on the integrated electrodes, which can enhance the performance of the E-CO2RR. In recent years, encouraging progress has been achieved in the study of the E-CO2RR. However, current reviews of the E-CO2RR mainly focus on the regulation of the intrinsic activity of catalysts; discussions and reviews from the perspective of the electrodes are rarely reported. This article reviews the latest research of the integrated electrodes for the E-CO2RR with a focus on the application of different types of integrated electrodes (e.g., metal, alloy, metal oxide, metal sulfide/phosphide, and metal single atom). It also analyzes the effects of morphology, surface, and interface regulation on the electrocatalytic performance of the E-CO2RR. Finally, it highlights the challenges that still exist in this field and discusses the future development of the integrated electrodes.
2022, 38(6): 210305
doi: 10.3866/PKU.WHXB202103052
Abstract:
Owing to the rapid development of scientific technology, the demand for energy storage equipment is increasing in modern society. Among the current energy storage devices, lithium-ion batteries (LIBs) have been widely used in portable electronics, handy electric tools, medical electronics, and other fields owing to their high energy density, high power density, long lifespan, low self-discharge rate, wide operating temperature range, and environmental friendliness. However, in recent years, with rapid development in various technological fields, such as mobile electronics and electric vehicles, the demand for batteries with much higher energy densities than the current ones has been increasing. Hence, the development of LIBs with a high energy density, prolonged cycle life, and high safety has become a focal interest in this field. To achieve the above objectives, it is important to strategically use novel anode materials with relatively high specific capacities. At present, artificial graphite is commonly used as an anode material for commercialized traditional LIBs, which can only deliver a practical capacity of 360–365 mAh·g-1. Therefore, LIBs using graphite anodes have limited room for improvement in energy density. In the past two decades, considerable efforts have been devoted to silicon-based anode materials, which belong to the same family as carbon. To date, common silicon anode materials primarily include nano-silicon (nano-Si), silicon monoxide (SiO), suboxidized SiO (SiOx), and amorphous silicon metal alloy (amorphous SiM). Among them, SiO has attracted the most attention for use as a negative electrode material for LIBs. As an anode for lithium-ion batteries (LIBs), silicon monoxide (SiO) has a high specific capacity (~2043 mAh·g-1) and suitable charge (delithiation) potential (< 0.5 V). In addition, with the abundance of its raw material resource, low manufacturing cost, and environmental friendliness, SiO is considered a promising candidate for next-generation high-energy-density LIBs. Based on the testing of existing commercialized SiO materials, the reversible specific capacity of pure SiO can reach 1300–1700 mAh·g-1. However, when acting as the anode for LIBs, SiO undergoes a severe volume change (~200%) during the lithiation/delithiation process, which can result in severe pulverization and detachment of the anode material. Meanwhile, lithium silicate and lithium oxide are irreversibly formed during the initial discharge–charge cycle. Moreover, the electrical conductivity of SiO is relatively low (6.7 × 10-4 S·cm-1). These shortcomings seriously impact the interfacial stability and electrochemical performance of SiO-based LIBs, leading to a low initial Coulombic efficiency and poor long-term cycling stability, which has significantly restricted its commercial application. In recent years, substantial efforts have been made on structural optimization and interfacial modification of SiO anodes. However, there is still a lack of a more comprehensive summary of these important developments. Therefore, this review aims to introduce the research work in this area for readers interested in this emerging field and to summarize in detail the research work on the performance optimization of SiO in recent years. Based on the structural characteristics of the SiO anode material, this review expounds the main challenges facing the material, and then summarizes the structural and interfacial modification strategies from the perspectives of SiO structure optimization, SiO/carbon composites, and SiO/metal composites. The methods and their features in all the studies are concisely introduced, the electrochemical performances are demonstrated, and their correlations are compared and discussed. Finally, we propose the development of the structural and interfacial optimization of the SiO anode in the future. ![]()
Owing to the rapid development of scientific technology, the demand for energy storage equipment is increasing in modern society. Among the current energy storage devices, lithium-ion batteries (LIBs) have been widely used in portable electronics, handy electric tools, medical electronics, and other fields owing to their high energy density, high power density, long lifespan, low self-discharge rate, wide operating temperature range, and environmental friendliness. However, in recent years, with rapid development in various technological fields, such as mobile electronics and electric vehicles, the demand for batteries with much higher energy densities than the current ones has been increasing. Hence, the development of LIBs with a high energy density, prolonged cycle life, and high safety has become a focal interest in this field. To achieve the above objectives, it is important to strategically use novel anode materials with relatively high specific capacities. At present, artificial graphite is commonly used as an anode material for commercialized traditional LIBs, which can only deliver a practical capacity of 360–365 mAh·g-1. Therefore, LIBs using graphite anodes have limited room for improvement in energy density. In the past two decades, considerable efforts have been devoted to silicon-based anode materials, which belong to the same family as carbon. To date, common silicon anode materials primarily include nano-silicon (nano-Si), silicon monoxide (SiO), suboxidized SiO (SiOx), and amorphous silicon metal alloy (amorphous SiM). Among them, SiO has attracted the most attention for use as a negative electrode material for LIBs. As an anode for lithium-ion batteries (LIBs), silicon monoxide (SiO) has a high specific capacity (~2043 mAh·g-1) and suitable charge (delithiation) potential (< 0.5 V). In addition, with the abundance of its raw material resource, low manufacturing cost, and environmental friendliness, SiO is considered a promising candidate for next-generation high-energy-density LIBs. Based on the testing of existing commercialized SiO materials, the reversible specific capacity of pure SiO can reach 1300–1700 mAh·g-1. However, when acting as the anode for LIBs, SiO undergoes a severe volume change (~200%) during the lithiation/delithiation process, which can result in severe pulverization and detachment of the anode material. Meanwhile, lithium silicate and lithium oxide are irreversibly formed during the initial discharge–charge cycle. Moreover, the electrical conductivity of SiO is relatively low (6.7 × 10-4 S·cm-1). These shortcomings seriously impact the interfacial stability and electrochemical performance of SiO-based LIBs, leading to a low initial Coulombic efficiency and poor long-term cycling stability, which has significantly restricted its commercial application. In recent years, substantial efforts have been made on structural optimization and interfacial modification of SiO anodes. However, there is still a lack of a more comprehensive summary of these important developments. Therefore, this review aims to introduce the research work in this area for readers interested in this emerging field and to summarize in detail the research work on the performance optimization of SiO in recent years. Based on the structural characteristics of the SiO anode material, this review expounds the main challenges facing the material, and then summarizes the structural and interfacial modification strategies from the perspectives of SiO structure optimization, SiO/carbon composites, and SiO/metal composites. The methods and their features in all the studies are concisely introduced, the electrochemical performances are demonstrated, and their correlations are compared and discussed. Finally, we propose the development of the structural and interfacial optimization of the SiO anode in the future.
2022, 38(6): 210600
doi: 10.3866/PKU.WHXB202106003
Abstract:
The storage and conversion of renewable energy through electrocatalysis is of considerable significance for improving the energy structure, protecting the ecological environment, and achieving the national strategy of carbon peaking and carbon neutrality. The development of low-cost and high-efficiency electrocatalysts has become a major scientific challenge worldwide. Microorganisms are widely found in nature and are characterized by their rich structure, composition and metabolism. These properties facilitate their use as intelligent templates for electrocatalyst structures and as sources of non-metallic elements such as carbon, phosphorus, sulfur, as well as metallic elements. The use of microorganisms in electrocatalyst production has become a new trend owing to the advantages of non-toxicity, reproducible production, and ease of scaling up. Thus, this paper reviews the development of microbial "intelligence" guided preparation of electrocatalysts and their current applications in the fields of hydrogen evolution reactions, oxygen evolution reactions, oxygen reduction reactions, carbon dioxide reductions and lithium batteries. In order to achieve the function of "intelligent" guidance of microorganisms, four aspects need to be addressed: (1) the selection of suitable microbial species and the culture and activation conditions, which significantly helps in tailoring the microbial properties for specific applications; (2) the exploration of microbial species that can accumulate metal species from their living environment and thus produce metal nanoparticles, which will help obtain nanocomposites with desired properties; (3) the selection of compounds with good catalytic properties, high stability, and compatibility with microbial substrates; and (4) the development of highly controllable nanocatalysts through modern molecular biology and genetic engineering to regulate microbial life processes such as metabolic proliferation and apoptosis. With the resolution of these issues, we believe that the application of microbial intelligent templates guided electrocatalysts can be further extended to other electrocatalytic reactions such as ethanol oxidation reactions (EOR), nitrogen reduction reactions (NRR), and to other applications in fields such as electronics, sensing, imaging, and biomedicine. The goal of this review is to promote a deeper understanding of the correlations among microbial metabolism, catalyst micro-nano structures and structure-activity relationships. Furthermore, the challenges associated with such materials and the prospects for future development are discussed herein. ![]()
The storage and conversion of renewable energy through electrocatalysis is of considerable significance for improving the energy structure, protecting the ecological environment, and achieving the national strategy of carbon peaking and carbon neutrality. The development of low-cost and high-efficiency electrocatalysts has become a major scientific challenge worldwide. Microorganisms are widely found in nature and are characterized by their rich structure, composition and metabolism. These properties facilitate their use as intelligent templates for electrocatalyst structures and as sources of non-metallic elements such as carbon, phosphorus, sulfur, as well as metallic elements. The use of microorganisms in electrocatalyst production has become a new trend owing to the advantages of non-toxicity, reproducible production, and ease of scaling up. Thus, this paper reviews the development of microbial "intelligence" guided preparation of electrocatalysts and their current applications in the fields of hydrogen evolution reactions, oxygen evolution reactions, oxygen reduction reactions, carbon dioxide reductions and lithium batteries. In order to achieve the function of "intelligent" guidance of microorganisms, four aspects need to be addressed: (1) the selection of suitable microbial species and the culture and activation conditions, which significantly helps in tailoring the microbial properties for specific applications; (2) the exploration of microbial species that can accumulate metal species from their living environment and thus produce metal nanoparticles, which will help obtain nanocomposites with desired properties; (3) the selection of compounds with good catalytic properties, high stability, and compatibility with microbial substrates; and (4) the development of highly controllable nanocatalysts through modern molecular biology and genetic engineering to regulate microbial life processes such as metabolic proliferation and apoptosis. With the resolution of these issues, we believe that the application of microbial intelligent templates guided electrocatalysts can be further extended to other electrocatalytic reactions such as ethanol oxidation reactions (EOR), nitrogen reduction reactions (NRR), and to other applications in fields such as electronics, sensing, imaging, and biomedicine. The goal of this review is to promote a deeper understanding of the correlations among microbial metabolism, catalyst micro-nano structures and structure-activity relationships. Furthermore, the challenges associated with such materials and the prospects for future development are discussed herein.
2022, 38(6): 210703
doi: 10.3866/PKU.WHXB202107030
Abstract:
The design functions of lithium-ion batteries are tailored to meet the needs of specific applications. It is crucial to obtain an in-depth understanding of the design, preparation/ modification, and characterization of the separator because structural modifications of the separator can effectively modulate the ion diffusion and dendrite growth, thereby optimizing the electrochemical performance and high safety of the battery. Moreover, the development and utilization of various characterization techniques are critical and essential in bridging the intrinsic properties of separators and their impacts on the electrochemical performance, which guide the functional modification of the separators. In this review, we systematically summarized the recent progress in the separator modification approaches, primarily focusing on its effects on the batteries' electrochemical performance and the related characterization techniques. Herein, we provide a brief introduction on the separators' classification that mainly includes (modified) microporous membranes, nonwoven mats, and composite membranes; thereafter, we discuss the basic requirements that facilitate the use of membranes as separators, such as good wettability with electrolyte, high permeability for ions, and several intrinsic properties including good thermal stability, electronic insulation, excellent (electro)chemical stability, high mechanical strength, and appropriate thickness/porosity. We then highlight the factors that affect the batteries' performance from the viewpoints of ion diffusion, dendrite growth, and safety, along with the modification approaches. Specifically, the separator should possess high ionic conductivity and uniform ion transmission, which can be achieved by adjusting its composition and through surface modifications. The severe dendrite growth, especially in lithium-metal batteries, could be inhibited by controlling the pore structures, increasing affinity between separator and metal anode, constructing artificial solid electrolyte interphase (SEI), adopting high strength separator, as well as smart design of the separator. The safety issue, which is a major concern that limits battery applications, could be mitigated by increasing the separator's mechanical strength, thermal stability, and shutting the batteries down below thermal runaway temperature through various functionalization approaches. More importantly, the characterizations of the separators' structure, and their mechanical, thermal, and electrochemical properties are systematically summarized, including scanning electron microscope (SEM)/atomic force microscope (AFM) for surface morphology observation, focused ion beam scanning electron microscopic (FIB-SEM)/X-ray tomography (X-ray CT) for 3D structure detection, mercury intrusion porosimetry (MIP)/Brunauer-Emmett-Teller (BET)/Gurley number measurement for pore structure analysis, contact angle and climbing behavior of electrolyte in separators for wettability measurements, characterizations of the separator's tensile behavior, puncture behavior and compression behavior, thermo-gravimetric analysis (TGA)/differential scanning calorimetry (DSC)/infrared thermography (FLIR) for thermal properties test, and the electrochemical methods for determining the separator's electrochemical stability, ionic conductivity, internal resistance, lithium-ion transference number, cycle/rate performance, as well as self-discharge characteristic. These characterizations provide theoretical and practical basis for the rational design of functional separators and optimization of the electrochemical performance of lithium-ion batteries. Finally, we provide the perspectives on several related issues that need to be further explored in this research field. ![]()
The design functions of lithium-ion batteries are tailored to meet the needs of specific applications. It is crucial to obtain an in-depth understanding of the design, preparation/ modification, and characterization of the separator because structural modifications of the separator can effectively modulate the ion diffusion and dendrite growth, thereby optimizing the electrochemical performance and high safety of the battery. Moreover, the development and utilization of various characterization techniques are critical and essential in bridging the intrinsic properties of separators and their impacts on the electrochemical performance, which guide the functional modification of the separators. In this review, we systematically summarized the recent progress in the separator modification approaches, primarily focusing on its effects on the batteries' electrochemical performance and the related characterization techniques. Herein, we provide a brief introduction on the separators' classification that mainly includes (modified) microporous membranes, nonwoven mats, and composite membranes; thereafter, we discuss the basic requirements that facilitate the use of membranes as separators, such as good wettability with electrolyte, high permeability for ions, and several intrinsic properties including good thermal stability, electronic insulation, excellent (electro)chemical stability, high mechanical strength, and appropriate thickness/porosity. We then highlight the factors that affect the batteries' performance from the viewpoints of ion diffusion, dendrite growth, and safety, along with the modification approaches. Specifically, the separator should possess high ionic conductivity and uniform ion transmission, which can be achieved by adjusting its composition and through surface modifications. The severe dendrite growth, especially in lithium-metal batteries, could be inhibited by controlling the pore structures, increasing affinity between separator and metal anode, constructing artificial solid electrolyte interphase (SEI), adopting high strength separator, as well as smart design of the separator. The safety issue, which is a major concern that limits battery applications, could be mitigated by increasing the separator's mechanical strength, thermal stability, and shutting the batteries down below thermal runaway temperature through various functionalization approaches. More importantly, the characterizations of the separators' structure, and their mechanical, thermal, and electrochemical properties are systematically summarized, including scanning electron microscope (SEM)/atomic force microscope (AFM) for surface morphology observation, focused ion beam scanning electron microscopic (FIB-SEM)/X-ray tomography (X-ray CT) for 3D structure detection, mercury intrusion porosimetry (MIP)/Brunauer-Emmett-Teller (BET)/Gurley number measurement for pore structure analysis, contact angle and climbing behavior of electrolyte in separators for wettability measurements, characterizations of the separator's tensile behavior, puncture behavior and compression behavior, thermo-gravimetric analysis (TGA)/differential scanning calorimetry (DSC)/infrared thermography (FLIR) for thermal properties test, and the electrochemical methods for determining the separator's electrochemical stability, ionic conductivity, internal resistance, lithium-ion transference number, cycle/rate performance, as well as self-discharge characteristic. These characterizations provide theoretical and practical basis for the rational design of functional separators and optimization of the electrochemical performance of lithium-ion batteries. Finally, we provide the perspectives on several related issues that need to be further explored in this research field.
2022, 38(6): 210502
doi: 10.3866/PKU.WHXB202105024
Abstract:
In previous decades, lithium-ion batteries (LIBs) were the most commonly used energy storage systems for powering portable electronic devices because LIBs exhibit reliable cyclability. However, owing to the low specific capacity of graphite used in the anode, further increase in the energy density of LIBs was limited. The Li metal anode is promising for the construction of next-generation high-energy-density batteries because of its ultrahigh theoretical capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode). However, the high activity of Li causes dendritic growth during cycling, which leads to cracking of the solid-electrolyte interphase (SEI), increase in side reactions, and formation of dead Li. Several strategies have been proposed to address these issues, including use of electrolyte additives, high-concentration electrolytes, protection of the Li metal surface with various coatings, use of solid-state electrolytes, and design of a three-dimensional (3D) "Li host" for regulating the nucleation and deposition of Li metal. Among them, the design of a 3D "Li host" has proven to be a simple and effective strategy. However, the commonly used 3D "Li hosts" include nanostructured carbons, which are lithiophobic and, thus, provide limited interaction sites with Li+ ions, leading to the deposition of Li metal on the "Li host" surface. Therefore, it is necessary to design a 3D "Li host" with enhanced interaction with Li+ ions to achieve uniform deposition. Herein, we develop a soft-hard templating route to synthesize 3D macro-/mesoporous C-TiC (denoted as 3DMM C-TiC) nanocomposites, which has been used in Li metal batteries. The as-synthesized materials possess high surface areas (~510 m2·g-1), ordered structures, large pore volumes, and excellent conductivity. The continuous plating and stripping of Li metal and the formation of the hierarchically porous structure with sufficient volume to allow uniform Li deposition result in the alleviation of the volume change. The high specific surface area significantly decreases the local current density and suppresses dendrite growth. Consequently, ultrasmall TiC nanoparticles are uniformly distributed in the 3D macro-/mesoporous framework, which improves conductivity, enhances their interaction with Li+ ions, and promotes the uniform deposition of Li metal. Therefore, the fabricated 3DMM C-TiC||Li battery displays stable cycling performance with improved Coulombic efficiency (98%) over 300 cycles. Moreover, when the 3DMM C-TiC based Li metal anode is assembled with a LiFePO4 (LFP) cathode, the resultant full cells exhibit high specific capacity and excellent cycling stability. This study provides insight for the effective design of a 3D "Li host" for dendrite-free Li metal anodes. ![]()
In previous decades, lithium-ion batteries (LIBs) were the most commonly used energy storage systems for powering portable electronic devices because LIBs exhibit reliable cyclability. However, owing to the low specific capacity of graphite used in the anode, further increase in the energy density of LIBs was limited. The Li metal anode is promising for the construction of next-generation high-energy-density batteries because of its ultrahigh theoretical capacity (3860 mAh·g-1) and low redox potential (-3.04 V vs. standard hydrogen electrode). However, the high activity of Li causes dendritic growth during cycling, which leads to cracking of the solid-electrolyte interphase (SEI), increase in side reactions, and formation of dead Li. Several strategies have been proposed to address these issues, including use of electrolyte additives, high-concentration electrolytes, protection of the Li metal surface with various coatings, use of solid-state electrolytes, and design of a three-dimensional (3D) "Li host" for regulating the nucleation and deposition of Li metal. Among them, the design of a 3D "Li host" has proven to be a simple and effective strategy. However, the commonly used 3D "Li hosts" include nanostructured carbons, which are lithiophobic and, thus, provide limited interaction sites with Li+ ions, leading to the deposition of Li metal on the "Li host" surface. Therefore, it is necessary to design a 3D "Li host" with enhanced interaction with Li+ ions to achieve uniform deposition. Herein, we develop a soft-hard templating route to synthesize 3D macro-/mesoporous C-TiC (denoted as 3DMM C-TiC) nanocomposites, which has been used in Li metal batteries. The as-synthesized materials possess high surface areas (~510 m2·g-1), ordered structures, large pore volumes, and excellent conductivity. The continuous plating and stripping of Li metal and the formation of the hierarchically porous structure with sufficient volume to allow uniform Li deposition result in the alleviation of the volume change. The high specific surface area significantly decreases the local current density and suppresses dendrite growth. Consequently, ultrasmall TiC nanoparticles are uniformly distributed in the 3D macro-/mesoporous framework, which improves conductivity, enhances their interaction with Li+ ions, and promotes the uniform deposition of Li metal. Therefore, the fabricated 3DMM C-TiC||Li battery displays stable cycling performance with improved Coulombic efficiency (98%) over 300 cycles. Moreover, when the 3DMM C-TiC based Li metal anode is assembled with a LiFePO4 (LFP) cathode, the resultant full cells exhibit high specific capacity and excellent cycling stability. This study provides insight for the effective design of a 3D "Li host" for dendrite-free Li metal anodes.
Voltage-Sensitive Polytriphenylamine-Modified Separator for Over-Charge Protection in Li-S Batteries
2022, 38(6): 210700
doi: 10.3866/PKU.WHXB202107009
Abstract:
In the past decade, lithium-sulfur batteries have attracted increasing attention owing to their high energy density and are considered to be one of the key options for the next generation of commercial high energy density batteries. However, for a practical battery system, both high energy density and good safety are important. The safety shortcomings of lithium-sulfur batteries have hindered their development and commercial application. Overcharging is a common battery safety problem. In the case of lithium-sulfur batteries, overcharging triggers the rapid growth of lithium dendrites, which can break through the separator and cause internal short-circuiting, leading to dangerous accidents such as thermal runaway and explosions. In practice, an electronic control device is typically installed in a battery to monitor its charging voltage and avoid overcharging avoid overcharging. However, this method increases the cost, weight, and size of the battery system, and reduces the energy density. For the self-protection of lithium-sulfur batteries in the case of overcharging, many polymerizable aromatic compounds are used as additives to improve the overcharge tolerance of lithium batteries. When the electrode surface is covered by a polymer film formed by employing electropolymerization, the cell dies permanently; thus the overcharge protection works only once. In contrast, electroactive polymers having reversible electrochemical doping/dedoping properties can be used to inhibit the overcharging of lithium-sulfur batteries is a more attractive approach. In this study, voltage-sensitive polytriphenylamine (PTPAn) was prepared by the chemical oxidation of triphenylamine as a raw material and successfully applied to lithium-sulfur battery separator. The conductivity test results showed that the PTPAn/polypropylene (PP) separator has an ionic conductivity of 1.56 mS·cm-1. The cyclic voltammogram (CV) test results showed that the PTPAn/PP separator has a redox peak in the range of 3.5?.2 V. At a charge/discharge rate of 0.1C, the lithium-sulfur batteries with the PTPAn/PP separator and blank PP separator had a discharge specific capacity of 424.8 and 407.2 mAh·g-1, respectively after 200 cycles, with Coulombic efficiencies of 99.38% and 98.59%, respectively. Further, the rate (0.1C, 0.2C, 0.5C, 1C) tests showed that the lithium-sulfur batteries with PTPAn/PP separator had higher discharge specific capacities at different rates than the lithium-sulfur batteries with the blank PP separator. Moreover, when the lithium-sulfur battery with the PTPAn/PP separator was overcharged at the 4th cycle, the charge specific capacity was 843.1 mAh·g-1 and the discharge specific capacity was 839.8 mAh·g-1. The charging specific capacity was 690.2 mAh·g-1 and the discharging specific capacity was 669.2 mAh·g-1 at the 10th cycle of overcharging. At the 16th cycle of overcharging, the battery had a charge specific capacity of 538.7 mAh·g-1 and a discharge specific capacity of 512.9 mAh·g-1. The overcharge test showed that lithium-sulfur batteries with the PTPAn/PP separator continued to work well after different overcharge rates. At an overcharging rate of 1C, the battery voltage remained stable at 3.9 V, with a charge specific capacity of 349.8 mAh·g-1 and a discharge specific capacity of 328.7 mAh·g-1. ![]()
In the past decade, lithium-sulfur batteries have attracted increasing attention owing to their high energy density and are considered to be one of the key options for the next generation of commercial high energy density batteries. However, for a practical battery system, both high energy density and good safety are important. The safety shortcomings of lithium-sulfur batteries have hindered their development and commercial application. Overcharging is a common battery safety problem. In the case of lithium-sulfur batteries, overcharging triggers the rapid growth of lithium dendrites, which can break through the separator and cause internal short-circuiting, leading to dangerous accidents such as thermal runaway and explosions. In practice, an electronic control device is typically installed in a battery to monitor its charging voltage and avoid overcharging avoid overcharging. However, this method increases the cost, weight, and size of the battery system, and reduces the energy density. For the self-protection of lithium-sulfur batteries in the case of overcharging, many polymerizable aromatic compounds are used as additives to improve the overcharge tolerance of lithium batteries. When the electrode surface is covered by a polymer film formed by employing electropolymerization, the cell dies permanently; thus the overcharge protection works only once. In contrast, electroactive polymers having reversible electrochemical doping/dedoping properties can be used to inhibit the overcharging of lithium-sulfur batteries is a more attractive approach. In this study, voltage-sensitive polytriphenylamine (PTPAn) was prepared by the chemical oxidation of triphenylamine as a raw material and successfully applied to lithium-sulfur battery separator. The conductivity test results showed that the PTPAn/polypropylene (PP) separator has an ionic conductivity of 1.56 mS·cm-1. The cyclic voltammogram (CV) test results showed that the PTPAn/PP separator has a redox peak in the range of 3.5?.2 V. At a charge/discharge rate of 0.1C, the lithium-sulfur batteries with the PTPAn/PP separator and blank PP separator had a discharge specific capacity of 424.8 and 407.2 mAh·g-1, respectively after 200 cycles, with Coulombic efficiencies of 99.38% and 98.59%, respectively. Further, the rate (0.1C, 0.2C, 0.5C, 1C) tests showed that the lithium-sulfur batteries with PTPAn/PP separator had higher discharge specific capacities at different rates than the lithium-sulfur batteries with the blank PP separator. Moreover, when the lithium-sulfur battery with the PTPAn/PP separator was overcharged at the 4th cycle, the charge specific capacity was 843.1 mAh·g-1 and the discharge specific capacity was 839.8 mAh·g-1. The charging specific capacity was 690.2 mAh·g-1 and the discharging specific capacity was 669.2 mAh·g-1 at the 10th cycle of overcharging. At the 16th cycle of overcharging, the battery had a charge specific capacity of 538.7 mAh·g-1 and a discharge specific capacity of 512.9 mAh·g-1. The overcharge test showed that lithium-sulfur batteries with the PTPAn/PP separator continued to work well after different overcharge rates. At an overcharging rate of 1C, the battery voltage remained stable at 3.9 V, with a charge specific capacity of 349.8 mAh·g-1 and a discharge specific capacity of 328.7 mAh·g-1.
2022, 38(6): 210600
doi: 10.3866/PKU.WHXB202106002
Abstract:
Owing to their advantages such as safe operation, high power density, long cycle life, and low self-discharge rate, lithium-ion batteries (LIBs) have attracted attention for applications ranging from portable electronics to electric vehicles (EVs)/hybrid EVs (HEVs). However, the striking exothermic reaction and growth of lithium dendrites during lithiation-delithiation cycles for commercial graphite anodes are hidden safety risks associated with LIBs. Titanium dioxide (TiO2) is considered as an important material for LIBs because of its high safety and excellent cycling stability. In addition, TiO2 anode used in lithium-ion storage system has a relatively high voltage (~1.5 V vs. Li/Li+), and thus, it meets the strict safety standards of commercial LIBs. However, the unsatisfactory conductivity and ion diffusion rate prevent the further application of TiO2 in LIBs. To date, the combination of graphene, carbon nanotubes (CNTs), carbon quantum dots (QDs) and porous carbon with TiO2 has attracted significant research attention. Nevertheless, it is still challenging to introduce a unique nanostructure design by organically compounding TiO2 with N-doped porous carbon matrix. Herein, N-doped porous carbon incorporating fine TiO2 nanoparticles (NPs) with a flower-like structure (denoted as FL-TiO2/NPC) is successfully prepared using flower-like NH2-MIL-125(Ti) as the hard template. The as-prepared Ti-based framework shows a flower-like structure, which is assembled with two-dimensional (2D) corrugated porous nanosheets. On the one hand, the corrugated carbon nanosheets incorporating fine TiO2 particles can offer a magnifying contact area between electrode matrix and electrolyte. On the other hand, the N-doped porous carbon plays a crucial role in improving the conductivity and structural integrity of the whole matrix. Therefore, the as-prepared FL-TiO2/NPC can deliver an excellent reversible lithium storage capacity of 384.2 mAh·g-1 at the current density of 0.5 A·g-1 after 300 cycles and 279.1 mAh·g-1 at 1 A·g-1 after 500 cycles. Furthermore, even when tested at 2 A·g-1, FL-TiO2/NPC can deliver a reversible capacity of 256.5 mAh·g-1 with a coulombic efficiency of 100% after 2000 cycles. The superior electrochemical performance and the structural toughness of LIBs originate from the unique flower-like structure. We believe that the proposed synthesis strategy will provide a new idea for the preparation of metal oxides/N-doped porous carbon composites with high lithium storage performance. ![]()
Owing to their advantages such as safe operation, high power density, long cycle life, and low self-discharge rate, lithium-ion batteries (LIBs) have attracted attention for applications ranging from portable electronics to electric vehicles (EVs)/hybrid EVs (HEVs). However, the striking exothermic reaction and growth of lithium dendrites during lithiation-delithiation cycles for commercial graphite anodes are hidden safety risks associated with LIBs. Titanium dioxide (TiO2) is considered as an important material for LIBs because of its high safety and excellent cycling stability. In addition, TiO2 anode used in lithium-ion storage system has a relatively high voltage (~1.5 V vs. Li/Li+), and thus, it meets the strict safety standards of commercial LIBs. However, the unsatisfactory conductivity and ion diffusion rate prevent the further application of TiO2 in LIBs. To date, the combination of graphene, carbon nanotubes (CNTs), carbon quantum dots (QDs) and porous carbon with TiO2 has attracted significant research attention. Nevertheless, it is still challenging to introduce a unique nanostructure design by organically compounding TiO2 with N-doped porous carbon matrix. Herein, N-doped porous carbon incorporating fine TiO2 nanoparticles (NPs) with a flower-like structure (denoted as FL-TiO2/NPC) is successfully prepared using flower-like NH2-MIL-125(Ti) as the hard template. The as-prepared Ti-based framework shows a flower-like structure, which is assembled with two-dimensional (2D) corrugated porous nanosheets. On the one hand, the corrugated carbon nanosheets incorporating fine TiO2 particles can offer a magnifying contact area between electrode matrix and electrolyte. On the other hand, the N-doped porous carbon plays a crucial role in improving the conductivity and structural integrity of the whole matrix. Therefore, the as-prepared FL-TiO2/NPC can deliver an excellent reversible lithium storage capacity of 384.2 mAh·g-1 at the current density of 0.5 A·g-1 after 300 cycles and 279.1 mAh·g-1 at 1 A·g-1 after 500 cycles. Furthermore, even when tested at 2 A·g-1, FL-TiO2/NPC can deliver a reversible capacity of 256.5 mAh·g-1 with a coulombic efficiency of 100% after 2000 cycles. The superior electrochemical performance and the structural toughness of LIBs originate from the unique flower-like structure. We believe that the proposed synthesis strategy will provide a new idea for the preparation of metal oxides/N-doped porous carbon composites with high lithium storage performance.
2022, 38(6): 210700
doi: 10.3866/PKU.WHXB202107005
Abstract:
Bifunctional electrocatalysts in alkaline media play an important role in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), owing to the considerable influence of water splitting in the green energy sector. Herein, we present surface-modified NiCo2O4 nanowires (NWs) with rich defects as a highly efficient overall water splitting electrocatalyst in alkaline media, where the surface modification is accomplished using organic ligands. X-ray photoelectron spectroscopy reveals that the increase in the Co2+/Co3+ ratio is responsible for the excellent bifunctional electrocatalytic performance of the surface-modified NiCo2O4 NWs. As expected, benefiting from the organic ligand-dominated surface modification, the optimized NiCo2O4 NWs can display an overpotential of only 83 mV for the HER and 280 mV for the OER, with a current density of 10 mA·cm-2 in 1.0 mol·L-1 KOH solution. More importantly, the NiCo2O4 NWs surface-modified using organic ligands exhibit outstanding performance for overall water splitting, with a voltage of 2.1 V and current density of 100 mA·cm-2, and also maintain their activity for at least 15 h. The present work highlights the importance of increasing the content of Co2+ in the spinel structure of NiCo2O4 NWs for enhancing their performance in overall water splitting. ![]()
Bifunctional electrocatalysts in alkaline media play an important role in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), owing to the considerable influence of water splitting in the green energy sector. Herein, we present surface-modified NiCo2O4 nanowires (NWs) with rich defects as a highly efficient overall water splitting electrocatalyst in alkaline media, where the surface modification is accomplished using organic ligands. X-ray photoelectron spectroscopy reveals that the increase in the Co2+/Co3+ ratio is responsible for the excellent bifunctional electrocatalytic performance of the surface-modified NiCo2O4 NWs. As expected, benefiting from the organic ligand-dominated surface modification, the optimized NiCo2O4 NWs can display an overpotential of only 83 mV for the HER and 280 mV for the OER, with a current density of 10 mA·cm-2 in 1.0 mol·L-1 KOH solution. More importantly, the NiCo2O4 NWs surface-modified using organic ligands exhibit outstanding performance for overall water splitting, with a voltage of 2.1 V and current density of 100 mA·cm-2, and also maintain their activity for at least 15 h. The present work highlights the importance of increasing the content of Co2+ in the spinel structure of NiCo2O4 NWs for enhancing their performance in overall water splitting.
2022, 38(6): 210600
doi: 10.3866/PKU.WHXB202106008
Abstract:
Redox flow batteries (RFBs) have been widely recognized as the primary choice for large-scale energy storage due to their high energy efficiency, low cost, and versatile design of decoupled energy storage and power output. However, the broad deployment of RFBs in the power grid has long been plagued by the high cost and low energy density of existing inorganic metal-based electrodes. Redox-active organic molecules (ROMs), on the other hand, have recently been extensively explored as the potentials electrodes in RFBs for their potential low cost, abundant resources, and highly tunable structure. The energy density of RFBs is proportional to the number of electrons transferred per unit reaction, the concentration of active materials, and the cell voltage. Therefore, strategies to improve the energy density of RFBs could be categorized into three classes: (1) expanding the cell voltage; (2) maximizing the practical concentration of active materials; (3) realizing multi-redox process. Benefited by the highly tunable structure and properties of ROMs, the cell voltage of RFBs could be realized by lowering the redox potentials of anolytes or/and increasing the redox potentials of catholytes. To fully exploit the low-potential anolytes and high-potential catholytes, non-aqueous electrolytes with wider electrochemical potential windows (EPWs) are preferred over the aqueous systems. However, the solubility of most ROMs in commonly used non-aqueous electrolytes is very limited. Several effective strategies to improve the practical concentrations of ROMs have been proposed: (1) the solubility of ROMs could be easily tailored by adjusting the intermolecular interactions between ROMs and the interactions between ROMs and electrolytes via molecular engineering; (2) the redox-active eutectic systems remain liquid at or near room temperature, allowing us to reduce or completely remove the inactive solvent used in the conventional electrolyte of RFBs, which leads to an enhanced practical concentration of the redox-active components; (3) the semi-solid suspension achieves a high practical concentration of ROMs by combining the advantages of solid ROMs with high energy density and liquid electrolytes with flowability; (4) the redox-targeting approach breaks the solubility limitation by realizing remote charge exchange between the solid active materials deposited in the tanks and the current collectors of the electrochemical stacks via ROMs dissolved in electrolytes. The first three strategies employ a homogeneous flowing redox-active fluid which suffers from deteriorated physical and electrochemical properties as the practical concentration of ROMs increase, e.g., high viscosity, phase separation, and salt precipitation. The redox-targeting approach uses a hybrid flowing liquid/static solid system, which avoids the aforementioned unfavorable changes in electrolyte properties, however, this design introduces additional chemical reactions between the ROMs and the solid active materials, which may reduce the power output. Another efficient method to improve the energy density of RFBs without affecting the properties of the electrolyte is achieved by realizing the multi-redox process of ROMs, however, the generated high valence state ROMs are highly reactive. Further optimization of the structure of these ROMs is required to improve their lifetime at high valence states. In this perspective, we summarize the general working principle of the RFBs, highlight the recent developments of the ROMs in non-aqueous redox flow batteries (NRFBs), with an emphasis on the strategies to improve the energy density of NRFBs, and discuss the remaining challenges and future directions of the non-aqueous organic redox flow batteries (NORFBs). ![]()
Redox flow batteries (RFBs) have been widely recognized as the primary choice for large-scale energy storage due to their high energy efficiency, low cost, and versatile design of decoupled energy storage and power output. However, the broad deployment of RFBs in the power grid has long been plagued by the high cost and low energy density of existing inorganic metal-based electrodes. Redox-active organic molecules (ROMs), on the other hand, have recently been extensively explored as the potentials electrodes in RFBs for their potential low cost, abundant resources, and highly tunable structure. The energy density of RFBs is proportional to the number of electrons transferred per unit reaction, the concentration of active materials, and the cell voltage. Therefore, strategies to improve the energy density of RFBs could be categorized into three classes: (1) expanding the cell voltage; (2) maximizing the practical concentration of active materials; (3) realizing multi-redox process. Benefited by the highly tunable structure and properties of ROMs, the cell voltage of RFBs could be realized by lowering the redox potentials of anolytes or/and increasing the redox potentials of catholytes. To fully exploit the low-potential anolytes and high-potential catholytes, non-aqueous electrolytes with wider electrochemical potential windows (EPWs) are preferred over the aqueous systems. However, the solubility of most ROMs in commonly used non-aqueous electrolytes is very limited. Several effective strategies to improve the practical concentrations of ROMs have been proposed: (1) the solubility of ROMs could be easily tailored by adjusting the intermolecular interactions between ROMs and the interactions between ROMs and electrolytes via molecular engineering; (2) the redox-active eutectic systems remain liquid at or near room temperature, allowing us to reduce or completely remove the inactive solvent used in the conventional electrolyte of RFBs, which leads to an enhanced practical concentration of the redox-active components; (3) the semi-solid suspension achieves a high practical concentration of ROMs by combining the advantages of solid ROMs with high energy density and liquid electrolytes with flowability; (4) the redox-targeting approach breaks the solubility limitation by realizing remote charge exchange between the solid active materials deposited in the tanks and the current collectors of the electrochemical stacks via ROMs dissolved in electrolytes. The first three strategies employ a homogeneous flowing redox-active fluid which suffers from deteriorated physical and electrochemical properties as the practical concentration of ROMs increase, e.g., high viscosity, phase separation, and salt precipitation. The redox-targeting approach uses a hybrid flowing liquid/static solid system, which avoids the aforementioned unfavorable changes in electrolyte properties, however, this design introduces additional chemical reactions between the ROMs and the solid active materials, which may reduce the power output. Another efficient method to improve the energy density of RFBs without affecting the properties of the electrolyte is achieved by realizing the multi-redox process of ROMs, however, the generated high valence state ROMs are highly reactive. Further optimization of the structure of these ROMs is required to improve their lifetime at high valence states. In this perspective, we summarize the general working principle of the RFBs, highlight the recent developments of the ROMs in non-aqueous redox flow batteries (NRFBs), with an emphasis on the strategies to improve the energy density of NRFBs, and discuss the remaining challenges and future directions of the non-aqueous organic redox flow batteries (NORFBs).
2022, 38(6): 210601
doi: 10.3866/PKU.WHXB202106010
Abstract:
Hydrogen (H2) is an important component in the framework of carbon-neutral energy, and the scalable production of H2 from seawater electrolysis offers a feasible route to address global energy challenges. With abundant seawater reserves, seawater electrolysis, especially when powered by renewable electricity sources, has great prospects. However, chloride ions (Cl-) in seawater can participate in the anodic reaction and accelerate the corrosion of electrode materials during electrolysis. Although the oxygen evolution reaction (OER) is thermodynamically favorable, the chlorine evolution reaction is highly competitive because fewer electrons are involved (2e-). These two problems are compounded by the dearth of corrosion-resistant electrode materials, which hinders the practical applications of seawater electrolysis. Therefore, intensive research efforts have been devoted to optimizing electrode materials using fundamental theories for practical applications. This review summarizes the recent progress in advanced electrode materials with an emphasis on their selectivity and anti-corrosivity. Practical materials with improved selectivity for oxygen generation, such as mixed metal oxides, Ni/Fe/Co-based composites, and manganese oxide (MnOx)-coated heterostructures, are reviewed in detail. Theoretically, alkaline environments (pH > 7.5) are preferred for OER as a constant potential gap (480 mV) exists in the high pH region. Nevertheless, corrosion of both the cathode and anode from ubiquitous Cl- is inevitable. Only a few materials with good corrosion resistance are capable of sustained operation in seawater systems; these include metal titanium and carbon-based materials. The corrosion process is usually accompanied by the formation of a passivated layer on the surface, but the aggressive penetration of Cl- can damage the whole electrode. Therefore, the selective inhibition of Cl- transport in the presence of a robust layer is critical to prevent continuous corrosion. Advances in anti-corrosion engineering, which encompasses inherently anti-corrosive materials, extrinsically protective coating, and in situ generated resistive species, are systematically discussed. Rational design can impart the material with good catalytic activity, stability, and corrosion resistance. Finally, we propose the following opportunities for future research: 1) screening of selective and anti-corrosive materials; 2) mechanism of competitive reactions and corrosion; 3) evaluation of anti-corrosive materials; 4) industrial-scale electrolysis with high current density; 5) optimization of experimental conditions; and 6) development of integrated electrolyzer devices. This review provides insights for the development of strategies aimed at tackling chlorine-related issues in seawater electrolysis. ![]()
Hydrogen (H2) is an important component in the framework of carbon-neutral energy, and the scalable production of H2 from seawater electrolysis offers a feasible route to address global energy challenges. With abundant seawater reserves, seawater electrolysis, especially when powered by renewable electricity sources, has great prospects. However, chloride ions (Cl-) in seawater can participate in the anodic reaction and accelerate the corrosion of electrode materials during electrolysis. Although the oxygen evolution reaction (OER) is thermodynamically favorable, the chlorine evolution reaction is highly competitive because fewer electrons are involved (2e-). These two problems are compounded by the dearth of corrosion-resistant electrode materials, which hinders the practical applications of seawater electrolysis. Therefore, intensive research efforts have been devoted to optimizing electrode materials using fundamental theories for practical applications. This review summarizes the recent progress in advanced electrode materials with an emphasis on their selectivity and anti-corrosivity. Practical materials with improved selectivity for oxygen generation, such as mixed metal oxides, Ni/Fe/Co-based composites, and manganese oxide (MnOx)-coated heterostructures, are reviewed in detail. Theoretically, alkaline environments (pH > 7.5) are preferred for OER as a constant potential gap (480 mV) exists in the high pH region. Nevertheless, corrosion of both the cathode and anode from ubiquitous Cl- is inevitable. Only a few materials with good corrosion resistance are capable of sustained operation in seawater systems; these include metal titanium and carbon-based materials. The corrosion process is usually accompanied by the formation of a passivated layer on the surface, but the aggressive penetration of Cl- can damage the whole electrode. Therefore, the selective inhibition of Cl- transport in the presence of a robust layer is critical to prevent continuous corrosion. Advances in anti-corrosion engineering, which encompasses inherently anti-corrosive materials, extrinsically protective coating, and in situ generated resistive species, are systematically discussed. Rational design can impart the material with good catalytic activity, stability, and corrosion resistance. Finally, we propose the following opportunities for future research: 1) screening of selective and anti-corrosive materials; 2) mechanism of competitive reactions and corrosion; 3) evaluation of anti-corrosive materials; 4) industrial-scale electrolysis with high current density; 5) optimization of experimental conditions; and 6) development of integrated electrolyzer devices. This review provides insights for the development of strategies aimed at tackling chlorine-related issues in seawater electrolysis.
2022, 38(6): 210902
doi: 10.3866/PKU.WHXB202109020
Abstract:
