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2024 Volume 40 Issue 9
2024, 40(9): 230600
doi: 10.3866/PKU.WHXB202306007
Abstract:
Electrocatalytic water splitting driven by renewable energy is a potential approach to obtain green hydrogen. However, the relatively high overpotential of anodic oxygen evolution reaction (OER) is one of the main obstacles hindering the widespread popularity of water electrocatalysis technology. To this end, electrochemical hydrogen-evolution coupled with the oxidation of biomass derived platforms, such as replacing OER with thermodynamically favorable 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR), provides an efficient strategy to lower energy utilization and co-producing valuable organic oxygenates. For instance, 2,5-furandicarboxylic acid (FDCA) is emerging as an important and value-added industrial chemical obtained from HMFOR, which can be used as the monomer of various sustainable bioplastics (e.g., polyesters, polyamides). Great efforts have been devoted to this arena on electrocatalyst engineering for better activity and product selectivity. However, less work has focused on the process scalability of HMFOR to FDCA. Here, we report a simple hydrothermal method to fabricate an array-structured nickel-vanadium layered double hydroxides (NiV-LDH) growth on nickel foam matrix, demonstrating large-sized (6 cm × 10 cm) synthesis of self-supported electrode. The as-prepared material is active and efficient for HMFOR, achieving 100 mA∙cm−2 of current density at 1.52 V vs. RHE (reversible hydrogen electrode) with 94.6% of Faradaic efficiency and 89.1% of yield to FDCA. Compared to traditional water splitting, replacing OER with HMFOR improves the counterpart hydrogen production rate by two-times. As proof-of-concept, we demonstrate the continuous and scalable HMFOR using a low-cost and membrane-free flow reactor system with electrode area of 49.5 cm2. Under a constant current of 10 A, this system achieves high HMF single-pass conversion (94.8%), high FDCA concentration (~186.8 mmol∙L−1), and high FDCA selectivity (98.5%) using 200 mmol∙L−1 of HMF feedstock at a flow rate of 3.62 mL∙min−1. Finally, gram-scale FDCA (119.5 g) can be obtained with hydrogen production using water electrolysis technology. This work highlights that catalyst design and system engineering should be coupled in the future rather than continuing in parallel directions.
Electrocatalytic water splitting driven by renewable energy is a potential approach to obtain green hydrogen. However, the relatively high overpotential of anodic oxygen evolution reaction (OER) is one of the main obstacles hindering the widespread popularity of water electrocatalysis technology. To this end, electrochemical hydrogen-evolution coupled with the oxidation of biomass derived platforms, such as replacing OER with thermodynamically favorable 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR), provides an efficient strategy to lower energy utilization and co-producing valuable organic oxygenates. For instance, 2,5-furandicarboxylic acid (FDCA) is emerging as an important and value-added industrial chemical obtained from HMFOR, which can be used as the monomer of various sustainable bioplastics (e.g., polyesters, polyamides). Great efforts have been devoted to this arena on electrocatalyst engineering for better activity and product selectivity. However, less work has focused on the process scalability of HMFOR to FDCA. Here, we report a simple hydrothermal method to fabricate an array-structured nickel-vanadium layered double hydroxides (NiV-LDH) growth on nickel foam matrix, demonstrating large-sized (6 cm × 10 cm) synthesis of self-supported electrode. The as-prepared material is active and efficient for HMFOR, achieving 100 mA∙cm−2 of current density at 1.52 V vs. RHE (reversible hydrogen electrode) with 94.6% of Faradaic efficiency and 89.1% of yield to FDCA. Compared to traditional water splitting, replacing OER with HMFOR improves the counterpart hydrogen production rate by two-times. As proof-of-concept, we demonstrate the continuous and scalable HMFOR using a low-cost and membrane-free flow reactor system with electrode area of 49.5 cm2. Under a constant current of 10 A, this system achieves high HMF single-pass conversion (94.8%), high FDCA concentration (~186.8 mmol∙L−1), and high FDCA selectivity (98.5%) using 200 mmol∙L−1 of HMF feedstock at a flow rate of 3.62 mL∙min−1. Finally, gram-scale FDCA (119.5 g) can be obtained with hydrogen production using water electrolysis technology. This work highlights that catalyst design and system engineering should be coupled in the future rather than continuing in parallel directions.
2024, 40(9): 230601
doi: 10.3866/PKU.WHXB202306010
Abstract:
Improving the thermal stability, chemical stability, and mechanical strength of battery separators is crucial to prevent safety incidents like thermal runaway in batteries. This significantly enhances the overall safety performance of batteries. Among various options, polyimide (PI) stands out as an ideal choice due to its outstanding thermal stability, excellent chemical stability, and high mechanical strength. However, existing preparation methods of PI separators, such as non-solvent induced phase separation (NIPS), template method, and electrospinning, often suffer from issues like inadequate mechanical strength. Therefore, this study focused on investigating a novel method to prepare thermoplastic PI porous films with thermally closed pores and enhanced mechanical strength. Several characterization techniques, including scanning electron microscopy (SEM), in situ Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analyzer (TGA)-FTIR coupling, were employed to understand the pore-forming mechanism of PI porous films. The findings revealed that the temperature range of triethylamine (TEA) removal was consistent with the main stage of the imidization reaction and pore formation. This indicated that the pore structure was formed in situ during the thermal imidization process when TEA was stripped out of the PI film. PI films with varying TEA contents were prepared to investigate the impact on pore structure, showing that pore size could be regulated by TEA content. A more regular reticulated small pore structure on the macroporous pore wall was observed when TEA content was ≥ 100%. SEM analysis showed that the films were thermally self-closed at a heat treatment temperature of 300 °C. Additionally, TGA indicated that the thermal decomposition temperature of PI porous film reached 580 °C. The mechanical strength of the PI films before and after pore closure was investigated, demonstrating excellent mechanical strength of approximately 120 MPa. The novel in situ pore formation method for PI porous films through the salt-formation method of poly (amic acid) (PAA) with the organic base TEA, followed by TEA release during thermal imidization, resulted in PI porous films with outstanding thermal stability and high mechanical strength. The self-closure of the PI porous film at high temperatures effectively isolates material and heat transport, providing robust safety assurance for batteries. This advancement has the potential to significantly improve battery safety and performance.
Improving the thermal stability, chemical stability, and mechanical strength of battery separators is crucial to prevent safety incidents like thermal runaway in batteries. This significantly enhances the overall safety performance of batteries. Among various options, polyimide (PI) stands out as an ideal choice due to its outstanding thermal stability, excellent chemical stability, and high mechanical strength. However, existing preparation methods of PI separators, such as non-solvent induced phase separation (NIPS), template method, and electrospinning, often suffer from issues like inadequate mechanical strength. Therefore, this study focused on investigating a novel method to prepare thermoplastic PI porous films with thermally closed pores and enhanced mechanical strength. Several characterization techniques, including scanning electron microscopy (SEM), in situ Fourier transform infrared spectroscopy (FTIR), and thermal gravimetric analyzer (TGA)-FTIR coupling, were employed to understand the pore-forming mechanism of PI porous films. The findings revealed that the temperature range of triethylamine (TEA) removal was consistent with the main stage of the imidization reaction and pore formation. This indicated that the pore structure was formed in situ during the thermal imidization process when TEA was stripped out of the PI film. PI films with varying TEA contents were prepared to investigate the impact on pore structure, showing that pore size could be regulated by TEA content. A more regular reticulated small pore structure on the macroporous pore wall was observed when TEA content was ≥ 100%. SEM analysis showed that the films were thermally self-closed at a heat treatment temperature of 300 °C. Additionally, TGA indicated that the thermal decomposition temperature of PI porous film reached 580 °C. The mechanical strength of the PI films before and after pore closure was investigated, demonstrating excellent mechanical strength of approximately 120 MPa. The novel in situ pore formation method for PI porous films through the salt-formation method of poly (amic acid) (PAA) with the organic base TEA, followed by TEA release during thermal imidization, resulted in PI porous films with outstanding thermal stability and high mechanical strength. The self-closure of the PI porous film at high temperatures effectively isolates material and heat transport, providing robust safety assurance for batteries. This advancement has the potential to significantly improve battery safety and performance.
2024, 40(9): 230603
doi: 10.3866/PKU.WHXB202306031
Abstract:
MXenes are two-dimensional metal carbides, nitrides, and carbonitrides that are typically achieved by selectively etching the A-site elements from their corresponding MAX phase precursors. Thanks to the merits including high mechanical stability, excellent conductivity, and a high specific surface area, MXenes have attracted widespread attention in the field of energy storage and conversion. By far, most studies are focus on the synthesis and applications of Ti- or V-based MXenes. Mo-based MXenes, while less investigated due to the difficulty of synthesis, have shown significant potential in various fields, including electrochemical biomolecular sensing, electrocatalysis, and energy storage. The conventional method of preparing Mo-based MXenes involves etching precursors with hazardous HF-containing solutions, which is not only time-consuming but also poses safety risks. In this study, we present a Lewis molten salt synthesis approach to prepare Mo2CTx MXene by etching Mo2Ga2C precursor that eliminates the need for hazardous HF and significantly reduces the synthesis time. The impact of etching temperature and time on the phase and microstructure of Mo2CTx MXene were carefully investigated, and our findings indicate that the Mo2Ga2C precursor can be almost fully etched at 600 °C for just 30 min using the molten salt method, which is a challenging feat to achieve using HF etching. Furthermore, it is found that Mo2CTx MXene can be obtained in a wide temperature range from 600 to 800 °C with excellent structural stability. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the selective etching of Ga atoms from Mo2Ga2C and the successful preparation of Mo2CTx MXene, and X-ray photoelectron spectroscopy (XPS) suggests the preservation of Mo-C bonds in the Mo2CTx layered structure. The hydrogen evolution reaction (HER) performance of Mo2CTx MXene prepared by the molten salt method was investigated in alkaline electrolytes. The molten salt derived Mo2CTx MXene displayed exceptional catalytic performance for the HER, maintaining long-term stability in alkaline conditions, and exhibiting a low overpotential of only 114 mV and a Tafel slope of 124 mV∙dec−1 at 10 mA∙cm−2. The much larger double layer capacitance of molten salt derived Mo2CTx MXene as compare to the Mo2Ga2C precursor suggests that accordion-like structure can greatly increase the electrochemical active sites and thus plays a key role in boosting the catalytic performance.
MXenes are two-dimensional metal carbides, nitrides, and carbonitrides that are typically achieved by selectively etching the A-site elements from their corresponding MAX phase precursors. Thanks to the merits including high mechanical stability, excellent conductivity, and a high specific surface area, MXenes have attracted widespread attention in the field of energy storage and conversion. By far, most studies are focus on the synthesis and applications of Ti- or V-based MXenes. Mo-based MXenes, while less investigated due to the difficulty of synthesis, have shown significant potential in various fields, including electrochemical biomolecular sensing, electrocatalysis, and energy storage. The conventional method of preparing Mo-based MXenes involves etching precursors with hazardous HF-containing solutions, which is not only time-consuming but also poses safety risks. In this study, we present a Lewis molten salt synthesis approach to prepare Mo2CTx MXene by etching Mo2Ga2C precursor that eliminates the need for hazardous HF and significantly reduces the synthesis time. The impact of etching temperature and time on the phase and microstructure of Mo2CTx MXene were carefully investigated, and our findings indicate that the Mo2Ga2C precursor can be almost fully etched at 600 °C for just 30 min using the molten salt method, which is a challenging feat to achieve using HF etching. Furthermore, it is found that Mo2CTx MXene can be obtained in a wide temperature range from 600 to 800 °C with excellent structural stability. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the selective etching of Ga atoms from Mo2Ga2C and the successful preparation of Mo2CTx MXene, and X-ray photoelectron spectroscopy (XPS) suggests the preservation of Mo-C bonds in the Mo2CTx layered structure. The hydrogen evolution reaction (HER) performance of Mo2CTx MXene prepared by the molten salt method was investigated in alkaline electrolytes. The molten salt derived Mo2CTx MXene displayed exceptional catalytic performance for the HER, maintaining long-term stability in alkaline conditions, and exhibiting a low overpotential of only 114 mV and a Tafel slope of 124 mV∙dec−1 at 10 mA∙cm−2. The much larger double layer capacitance of molten salt derived Mo2CTx MXene as compare to the Mo2Ga2C precursor suggests that accordion-like structure can greatly increase the electrochemical active sites and thus plays a key role in boosting the catalytic performance.
2024, 40(9): 230700
doi: 10.3866/PKU.WHXB202307008
Abstract:
Carbon dioxide (CO2) serves as a non-toxic, abundant, cheap, and renewable C1 feedstock in synthetic chemistry. The synthesis of high value-added fine chemicals, such as organic carboxylic acids, using CO2, is always a focal point of research. Due to the thermodynamic stability and kinetic inertness of carbon dioxide, traditional carboxylation reactions utilizing CO2 often require harsh reaction conditions. However, organic electrochemical synthesis, which employs electrons as clean reagents to drive the reaction and avoids additional chemical oxidants or reductants, has emerged as a safer, more economical, highly selective, sustainable, and environmentally friendly method for preparing fine chemicals. Electrocarboxylation, which leverages organic electrochemical synthesis to catalytically transform CO2, provides a milder and more efficient route for CO2 utilization. Among these approaches, electrocarboxylation of organic halides or pseudohalides containing C―X bonds with CO2 has been extensively investigated as a means to access value-added carboxylic acids. Phosphates, known for their good leaving group properties, find extensive applications in organic synthesis. Under reductive conditions, the radical anion generated by benzyl phosphate easily dissociates into a benzyl radical and a phosphate anion. Hence, it can serve as an attractive substrate for participating in electrocarboxylation reactions. In this study, we report the highly efficient electrocarboxylation of benzylic phosphate and phosphinate derivatives using CO2 as the carboxyl source. The constant current reaction took place in an undivided cell, employing glassy carbon as the cathode, and magnesium as the sacrificial anode, in a mixed solvent of DMF and THF. Additionally, this mild electrolysis can be carried out under nonsacrificial anode conditions, using cheap carbon felt electrode as both the nonsacrificial anode and cathode and N,N-diisopropylethylamine as an external reductant, therefore provided operationally simple and highly efficient synthetic method toward aryl acetic acids in moderate to good yield. The broad substrate scope, simple operation, facile scalability, and highly efficient transformation of phosphates into high value-added aryl acetic acids under mild conditions demonstrate the potential applicability of this reaction. To gain insight into the possible reaction mechanism, several control experiments were conducted. Isotope-labeling 13CO2 experiment, cyclic voltammetry experiments, radical trapping reactions, and deuterium-labeling experiment indicated that cathodically generated benzylic radical and benzylic anion were key intermediates. Moreover, the single electron reduction of CO2 to CO2•− might also occur during the reaction.
Carbon dioxide (CO2) serves as a non-toxic, abundant, cheap, and renewable C1 feedstock in synthetic chemistry. The synthesis of high value-added fine chemicals, such as organic carboxylic acids, using CO2, is always a focal point of research. Due to the thermodynamic stability and kinetic inertness of carbon dioxide, traditional carboxylation reactions utilizing CO2 often require harsh reaction conditions. However, organic electrochemical synthesis, which employs electrons as clean reagents to drive the reaction and avoids additional chemical oxidants or reductants, has emerged as a safer, more economical, highly selective, sustainable, and environmentally friendly method for preparing fine chemicals. Electrocarboxylation, which leverages organic electrochemical synthesis to catalytically transform CO2, provides a milder and more efficient route for CO2 utilization. Among these approaches, electrocarboxylation of organic halides or pseudohalides containing C―X bonds with CO2 has been extensively investigated as a means to access value-added carboxylic acids. Phosphates, known for their good leaving group properties, find extensive applications in organic synthesis. Under reductive conditions, the radical anion generated by benzyl phosphate easily dissociates into a benzyl radical and a phosphate anion. Hence, it can serve as an attractive substrate for participating in electrocarboxylation reactions. In this study, we report the highly efficient electrocarboxylation of benzylic phosphate and phosphinate derivatives using CO2 as the carboxyl source. The constant current reaction took place in an undivided cell, employing glassy carbon as the cathode, and magnesium as the sacrificial anode, in a mixed solvent of DMF and THF. Additionally, this mild electrolysis can be carried out under nonsacrificial anode conditions, using cheap carbon felt electrode as both the nonsacrificial anode and cathode and N,N-diisopropylethylamine as an external reductant, therefore provided operationally simple and highly efficient synthetic method toward aryl acetic acids in moderate to good yield. The broad substrate scope, simple operation, facile scalability, and highly efficient transformation of phosphates into high value-added aryl acetic acids under mild conditions demonstrate the potential applicability of this reaction. To gain insight into the possible reaction mechanism, several control experiments were conducted. Isotope-labeling 13CO2 experiment, cyclic voltammetry experiments, radical trapping reactions, and deuterium-labeling experiment indicated that cathodically generated benzylic radical and benzylic anion were key intermediates. Moreover, the single electron reduction of CO2 to CO2•− might also occur during the reaction.
2024, 40(9): 230800
doi: 10.3866/PKU.WHXB202308005
Abstract:
Glycerol carbonylation with CO2 to synthesize glycerol carbonate is a promising approach for CO2 utilization. This reaction can be achieved through a thermally-driven catalytic pathway, but it is constrained by thermodynamic equilibrium. In the present study, we introduced solar energy into the reaction system to enable a photo-thermal synergistic catalytic reaction, breaking through the thermodynamic limitations. We developed a series of xAu/20Co3O4-ZnO catalysts, where Co3O4-ZnO, a composite of p-type semi-conductor Co3O4 and n-type semi-conductor ZnO, exhibited a heterojunction structure, and Au nanoparticles loaded onto the surface of Co3O4-ZnO revealed the localized surface plasmon resonance (LSPR). We investigated the ability of xAu/Co3O4-ZnO to absorb visible light absorption, the efficiency of separating photo-generated hole-electron pairs, and the impact of Au on the photothermal synergistic catalytic performances of Au/Co3O4-ZnO catalysts. We also examined the effects of Au doping on the bulk and surface properties, including crystalline structures, morphologies, specific surface areas and pore structures, the binding energies of the elements, surface acid-base sites, and reduction behaviors of xAu/Co3O4-ZnO. Our findings revealed that the heterojunction structure of Au/20Co3O4-ZnO facilitated visible light absorption and hole-electron pair separation. The size of Au nano-particles (NPs) loaded on Co3O4-ZnO surface was approximately 50 nm. The loading of Au altered the electron density of Co and Zn, improved the reducibility of Co species, and enhanced the presence of oxygen vacancies on Co3O4-ZnO surface. The LSPR of Au NPs further enhanced the visible light absorption capacity of Au/20Co3O4-ZnO, and improved the separating of photogenerated hole-electron pairs, thus enhancing the photothermal catalytic performances. With the optimizing conditions (150 °C, 5 MPa, 6 h, and 225 W visible light irradiation), the 2%Au/20Co3O4-ZnO catalyst demonstrated excellent performances, yielding a glycerol carbonate yield of 6.5%. This study is expected to serve as a reference for the rational design of improved photothermal catalysts for glycerol carbonylation with CO2 to produce glycerol carbonate in the future.
Glycerol carbonylation with CO2 to synthesize glycerol carbonate is a promising approach for CO2 utilization. This reaction can be achieved through a thermally-driven catalytic pathway, but it is constrained by thermodynamic equilibrium. In the present study, we introduced solar energy into the reaction system to enable a photo-thermal synergistic catalytic reaction, breaking through the thermodynamic limitations. We developed a series of xAu/20Co3O4-ZnO catalysts, where Co3O4-ZnO, a composite of p-type semi-conductor Co3O4 and n-type semi-conductor ZnO, exhibited a heterojunction structure, and Au nanoparticles loaded onto the surface of Co3O4-ZnO revealed the localized surface plasmon resonance (LSPR). We investigated the ability of xAu/Co3O4-ZnO to absorb visible light absorption, the efficiency of separating photo-generated hole-electron pairs, and the impact of Au on the photothermal synergistic catalytic performances of Au/Co3O4-ZnO catalysts. We also examined the effects of Au doping on the bulk and surface properties, including crystalline structures, morphologies, specific surface areas and pore structures, the binding energies of the elements, surface acid-base sites, and reduction behaviors of xAu/Co3O4-ZnO. Our findings revealed that the heterojunction structure of Au/20Co3O4-ZnO facilitated visible light absorption and hole-electron pair separation. The size of Au nano-particles (NPs) loaded on Co3O4-ZnO surface was approximately 50 nm. The loading of Au altered the electron density of Co and Zn, improved the reducibility of Co species, and enhanced the presence of oxygen vacancies on Co3O4-ZnO surface. The LSPR of Au NPs further enhanced the visible light absorption capacity of Au/20Co3O4-ZnO, and improved the separating of photogenerated hole-electron pairs, thus enhancing the photothermal catalytic performances. With the optimizing conditions (150 °C, 5 MPa, 6 h, and 225 W visible light irradiation), the 2%Au/20Co3O4-ZnO catalyst demonstrated excellent performances, yielding a glycerol carbonate yield of 6.5%. This study is expected to serve as a reference for the rational design of improved photothermal catalysts for glycerol carbonylation with CO2 to produce glycerol carbonate in the future.
2024, 40(9): 230801
doi: 10.3866/PKU.WHXB202308019
Abstract:
Zeolites with short microporous channels offer advantages in the diffusion of guest molecules, leading to significant improvements in their adsorption and catalytic performance, as well as a reduction in coke formation during catalytic reactions. However, preparing zeolite L (LTL) with an ultrashort length (20–50 nm) along the c-axis has proven challenging due to its preferential growth behavior along the one-dimensional microporous channel direction. Additionally, the conventional synthesis method of zeolite L struggles to achieve both low aspect ratio and short length along the c-axis due to the coupling of nucleation and growth stages during crystallization. In this study, we present an innovative approach by utilizing seeds of nanorod-cluster zeolite L, pre-prepared under high alkalinity conditions, to synthesize a novel morphology of zeolite L mesocrystals. The resulting zeolite L product exhibits a unique cluster structure composed of a series of disc nanocrystals with an ultrashort c-axis length (approximately 29 nm), and the entire crystallization process is completed within just 4 h in a low alkaline system without the need for additional additives. This intentionally designed seed-induced synthesis method effectively decouples the nucleation and growth stages of zeolite L, enabling precise control of each stage to achieve the desired morphology. By analyzing the time-resolved evolution of mesoscopic nuclei and microscopic building units in the synthetic system, we find that the ring-cage structures dissolved from seeds exist as four-membered rings and eight-membered rings. These structures accelerate gel ordering and shorten the induction period. Meanwhile, the reserved part of the seeds provides densely-distributed nuclei for growth, resulting in the formation of the novel disc-cluster structures. Furthermore, by controlling growth conditions, we confirm the assembly of worm-like precursor particles during the growth period, allowing for precise regulation of the length along the c-axis of each disc within the range of 18 to 55 nm. Moreover, we extensively demonstrate the significantly enhanced adsorption and diffusion properties of zeolite L with an ultrashort c-axis for a range of model molecules, spanning sizes from 0.43 to 4.5 nm, in both gaseous and liquid phase systems. Our typical sample exhibits advantages in the diffusion rate of small molecules and the adsorption capacity of large molecules in the gaseous phase. It holds great potential for practical applications in the adsorption and separation of aromatic hydrocarbons, as well as the adsorption of dyes and proteins.
Zeolites with short microporous channels offer advantages in the diffusion of guest molecules, leading to significant improvements in their adsorption and catalytic performance, as well as a reduction in coke formation during catalytic reactions. However, preparing zeolite L (LTL) with an ultrashort length (20–50 nm) along the c-axis has proven challenging due to its preferential growth behavior along the one-dimensional microporous channel direction. Additionally, the conventional synthesis method of zeolite L struggles to achieve both low aspect ratio and short length along the c-axis due to the coupling of nucleation and growth stages during crystallization. In this study, we present an innovative approach by utilizing seeds of nanorod-cluster zeolite L, pre-prepared under high alkalinity conditions, to synthesize a novel morphology of zeolite L mesocrystals. The resulting zeolite L product exhibits a unique cluster structure composed of a series of disc nanocrystals with an ultrashort c-axis length (approximately 29 nm), and the entire crystallization process is completed within just 4 h in a low alkaline system without the need for additional additives. This intentionally designed seed-induced synthesis method effectively decouples the nucleation and growth stages of zeolite L, enabling precise control of each stage to achieve the desired morphology. By analyzing the time-resolved evolution of mesoscopic nuclei and microscopic building units in the synthetic system, we find that the ring-cage structures dissolved from seeds exist as four-membered rings and eight-membered rings. These structures accelerate gel ordering and shorten the induction period. Meanwhile, the reserved part of the seeds provides densely-distributed nuclei for growth, resulting in the formation of the novel disc-cluster structures. Furthermore, by controlling growth conditions, we confirm the assembly of worm-like precursor particles during the growth period, allowing for precise regulation of the length along the c-axis of each disc within the range of 18 to 55 nm. Moreover, we extensively demonstrate the significantly enhanced adsorption and diffusion properties of zeolite L with an ultrashort c-axis for a range of model molecules, spanning sizes from 0.43 to 4.5 nm, in both gaseous and liquid phase systems. Our typical sample exhibits advantages in the diffusion rate of small molecules and the adsorption capacity of large molecules in the gaseous phase. It holds great potential for practical applications in the adsorption and separation of aromatic hydrocarbons, as well as the adsorption of dyes and proteins.
2024, 40(9): 230804
doi: 10.3866/PKU.WHXB202308045
Abstract:
Urea electrolysis is critically important for the advancement of sustainable and clean energy conversion technologies, addressing global energy shortages and environmental concerns. The urea oxidation reaction (UOR) poses a significant challenge due to its unfavorable thermodynamics, making it a pivotal step in urea splitting. The 6-electron transfer process of UOR presents a bottleneck due to its sluggish kinetics. Consequently, the development of efficient urea oxidation electrocatalysts and gaining insights into the electronic configuration of the central metal ion are of paramount significance in achieving high-performance urea-based energy conversion technologies. In this study, we report the successful synthesis of hierarchical Ni2P nanosheets@nanorods (P-Ni2P HNNs) as promising catalysts to enhance UOR efficiency. This catalyst is designed and constructed using a hexamethylenetetramine-hydrolytic coprecipitation-oxidation process and a straightforward phosphorus-substituted method. X-ray absorption fine structure spectroscopy indicates that the presence of P-modified metal centers is responsible for the elevated UOR activity of P-Ni2P HNNs, with the electronic structure of Nin+ significantly enhancing Ni―O―O bond coupling for rapid UOR kinetics. Thanks to the highly exposed Nin+ centers and the well-designed architecture, P-Ni2P HNNs exhibit superior UOR activity and stability, with a low overpotential of 132 mV at 10 mA∙cm−2, a small Tafel slope of 33.7 mV∙dec−1, and sustained durability for 6 h at 10 mA∙cm−2. Furthermore, a two-electrode cell for overall urea electrolysis is assembled with a P-Ni2P HNNs-2/NF anode, yielding a low potential of 1.411 V at 10 mA∙cm−2 and a high current density of 100 mA∙cm−2 at 1.595 V. This study presents an effective and viable approach for designing and synthesizing high-efficiency nickel-based phosphide electrocatalysts, which could pave the way for cost-effective and energy-efficient electrochemical hydrogen production, and advance phosphide research for various energy-related applications.
Urea electrolysis is critically important for the advancement of sustainable and clean energy conversion technologies, addressing global energy shortages and environmental concerns. The urea oxidation reaction (UOR) poses a significant challenge due to its unfavorable thermodynamics, making it a pivotal step in urea splitting. The 6-electron transfer process of UOR presents a bottleneck due to its sluggish kinetics. Consequently, the development of efficient urea oxidation electrocatalysts and gaining insights into the electronic configuration of the central metal ion are of paramount significance in achieving high-performance urea-based energy conversion technologies. In this study, we report the successful synthesis of hierarchical Ni2P nanosheets@nanorods (P-Ni2P HNNs) as promising catalysts to enhance UOR efficiency. This catalyst is designed and constructed using a hexamethylenetetramine-hydrolytic coprecipitation-oxidation process and a straightforward phosphorus-substituted method. X-ray absorption fine structure spectroscopy indicates that the presence of P-modified metal centers is responsible for the elevated UOR activity of P-Ni2P HNNs, with the electronic structure of Nin+ significantly enhancing Ni―O―O bond coupling for rapid UOR kinetics. Thanks to the highly exposed Nin+ centers and the well-designed architecture, P-Ni2P HNNs exhibit superior UOR activity and stability, with a low overpotential of 132 mV at 10 mA∙cm−2, a small Tafel slope of 33.7 mV∙dec−1, and sustained durability for 6 h at 10 mA∙cm−2. Furthermore, a two-electrode cell for overall urea electrolysis is assembled with a P-Ni2P HNNs-2/NF anode, yielding a low potential of 1.411 V at 10 mA∙cm−2 and a high current density of 100 mA∙cm−2 at 1.595 V. This study presents an effective and viable approach for designing and synthesizing high-efficiency nickel-based phosphide electrocatalysts, which could pave the way for cost-effective and energy-efficient electrochemical hydrogen production, and advance phosphide research for various energy-related applications.
2024, 40(9): 230805
doi: 10.3866/PKU.WHXB202308053
Abstract:
The Li/CrOx battery has gained attention in the construction of smart cities, aerospace, and national defense and military applications due to its high energy density and excellent rate performance. Developing a Li/CrOx battery with high specific capacity, high energy density, excellent magnification performance, long storage life, and low cost is a primary goal. In this pursuit, the role of the electrolyte in battery performance for Li/CrOx primary batteries cannot be underestimated. However, current research on Li/CrOx primary batteries has primarily focused on electrode materials, with limited attention given to the electrolyte. Propylene carbonate (PC) solvent possesses a wide temperature range for melting and boiling points (−48.8 to 242 °C) and a high dielectric constant of 64.92. As a result, it is frequently used as a key component in electrolytes that operate under extreme temperatures and high rates. Nevertheless, its use in Li/CrOx batteries remains limited. Developing electrolyte systems based on PC with a wide temperature range and high dielectric constant is crucial for the advancement of high-power and environmentally robust lithium primary batteries. In this study, we investigated the discharge behavior of CrOx in PC-based electrolytes and identified suitable electrolyte systems for high-current discharge, specifically a 1 mol∙L−1 LiTFSI PC : DOL (1,3-dioxolane) = 1 : 2 ratio. We also demonstrated that the coordination number of solvent molecules in the solvation sheath layer around Li+ ions and the solvated structure involved in coordination significantly influence the rate performance of Li/CrOx battery systems in PC-based electrolytes. Reducing the coordination number of solvent molecules facilitates the desolvation behavior of solvated Li+, thereby enhancing the desolvation process on the material surface. Furthermore, lowering the coordination number of solvent molecules promotes the involvement of anions in the solvated sheath structure. When the coordination number of solvent molecules falls below 3, it tends to form a solvated coordination structure involving anions with a higher lowest unoccupied molecular orbital (LUMO) level. This enables anions to participate in forming a solid electrolyte interface (SEI), resulting in a thinner and denser SEI film that significantly improves battery performance. Ultimately, modifying the coordination number for PC-based electrolytes is a practical and effective approach to enhance the rate performance of solvated sheath structures. The coordination number and the solvated sheath structure of Li+ in PC-based electrolytes have a profound impact on the high-current-discharge performance of the Li/CrOx battery system. A lower coordination number and the participation of anions in the solvated sheath structure effectively accommodate the high-rate discharge characteristics of the Li/CrOx battery. Among several selected electrolyte solvents, an electrolyte with DOL (a cyclic ether) and PC reduces the solvent’s coordination number to less than four, thereby enabling high-rate discharge. Understanding these principles is crucial for advancing the application of PC-based electrolytes in high-rate Li/CrOx battery systems.
The Li/CrOx battery has gained attention in the construction of smart cities, aerospace, and national defense and military applications due to its high energy density and excellent rate performance. Developing a Li/CrOx battery with high specific capacity, high energy density, excellent magnification performance, long storage life, and low cost is a primary goal. In this pursuit, the role of the electrolyte in battery performance for Li/CrOx primary batteries cannot be underestimated. However, current research on Li/CrOx primary batteries has primarily focused on electrode materials, with limited attention given to the electrolyte. Propylene carbonate (PC) solvent possesses a wide temperature range for melting and boiling points (−48.8 to 242 °C) and a high dielectric constant of 64.92. As a result, it is frequently used as a key component in electrolytes that operate under extreme temperatures and high rates. Nevertheless, its use in Li/CrOx batteries remains limited. Developing electrolyte systems based on PC with a wide temperature range and high dielectric constant is crucial for the advancement of high-power and environmentally robust lithium primary batteries. In this study, we investigated the discharge behavior of CrOx in PC-based electrolytes and identified suitable electrolyte systems for high-current discharge, specifically a 1 mol∙L−1 LiTFSI PC : DOL (1,3-dioxolane) = 1 : 2 ratio. We also demonstrated that the coordination number of solvent molecules in the solvation sheath layer around Li+ ions and the solvated structure involved in coordination significantly influence the rate performance of Li/CrOx battery systems in PC-based electrolytes. Reducing the coordination number of solvent molecules facilitates the desolvation behavior of solvated Li+, thereby enhancing the desolvation process on the material surface. Furthermore, lowering the coordination number of solvent molecules promotes the involvement of anions in the solvated sheath structure. When the coordination number of solvent molecules falls below 3, it tends to form a solvated coordination structure involving anions with a higher lowest unoccupied molecular orbital (LUMO) level. This enables anions to participate in forming a solid electrolyte interface (SEI), resulting in a thinner and denser SEI film that significantly improves battery performance. Ultimately, modifying the coordination number for PC-based electrolytes is a practical and effective approach to enhance the rate performance of solvated sheath structures. The coordination number and the solvated sheath structure of Li+ in PC-based electrolytes have a profound impact on the high-current-discharge performance of the Li/CrOx battery system. A lower coordination number and the participation of anions in the solvated sheath structure effectively accommodate the high-rate discharge characteristics of the Li/CrOx battery. Among several selected electrolyte solvents, an electrolyte with DOL (a cyclic ether) and PC reduces the solvent’s coordination number to less than four, thereby enabling high-rate discharge. Understanding these principles is crucial for advancing the application of PC-based electrolytes in high-rate Li/CrOx battery systems.
2024, 40(9): 230901
doi: 10.3866/PKU.WHXB202309011
Abstract:
Catassembly is a newly developed concept concerning the process of molecular assembly improved by a catalyst-assembler (catassembler). However, it has not been visualized in detail at the molecular level. To achieve the formation of highly complex structures with high efficiency and selectivity, a deeper understanding of catassembly is essential. In this study, we present the scanning tunneling microscopy (STM) characterization of a catassembly process within host-guest assembly. We utilize a metastable self-assembled network of 1,3,5-tris(4-carboxyphenyl)-benzene (BTB) at the liquid-solid interface between 1-octanoic acid and highly oriented pyrolytic graphite (HOPG). Different adsorption behaviors of low-concentration guest molecules (copper phthalocyanine (CuPc), and coronene (COR)) are contrastively analyzed during the host-guest assembly in both single-guest (COR/BTB or CuPc/BTB) and multi-guest molecule (COR&CuPc/BTB) systems. The spontaneous phase transition from a hexagonal to an oblique structure of BTB monolayers (high concentration, approximately 500 μmol∙L−1 in octanoic acid) provides an ideal metastable phase for studying the dynamic assembly process. In the host-guest assembly, the metastable BTB hexagonal phase serves as a host network and can be stabilized by co-assembling guest molecules under a negative bias voltage. However, the stability of the metastable phase varies with different guest molecules. We observe that the BTB metastable phase is more robust with COR guest molecules than with CuPc. In the CuPc/BTB system, we find that low-concentration CuPc (approximately 1.5 μmol∙L−1 in octanoic acid) can hardly co-assemble with BTB, leading to the gradual collapse of the metastable BTB networks into the oblique phase. The different stability of BTB metastable phase in the host-guest assembly is attributed to differences in the kinetics of trapping guest molecules. Guest COR molecules exhibit kinetic advantages over CuPc when assembling with host BTB networks under a negative sample bias. The lower trapping rate of CuPc hinders the formation of co-assembled BTB/CuPc networks. These differences in the dynamic behavior of the guest molecules are further explored in the research of catassembly. In a multi-guest molecule system (COR&CuPc/BTB), COR molecules are preferentially trapped by BTB hexagonal networks and can gradually be replaced by CuPc during continuous scanning. The more energetically stable structure of CuPc/BTB compared to COR/BTB rationalizes the exchange of the guest molecule and the evolution of the assembly phase. The involvement of COR significantly increases both the efficiency and quality of the CuPc/BTB assembly, serving as a catassembler. This observation provides insights into a complete catassembly process at the molecular level, enabling further investigations into the selectivity and efficiency of host-guest phenomena for potential applications in analysis and separation. Additionally, this work serves as a prototype for constructing highly complex 2D assembled monolayers.
Catassembly is a newly developed concept concerning the process of molecular assembly improved by a catalyst-assembler (catassembler). However, it has not been visualized in detail at the molecular level. To achieve the formation of highly complex structures with high efficiency and selectivity, a deeper understanding of catassembly is essential. In this study, we present the scanning tunneling microscopy (STM) characterization of a catassembly process within host-guest assembly. We utilize a metastable self-assembled network of 1,3,5-tris(4-carboxyphenyl)-benzene (BTB) at the liquid-solid interface between 1-octanoic acid and highly oriented pyrolytic graphite (HOPG). Different adsorption behaviors of low-concentration guest molecules (copper phthalocyanine (CuPc), and coronene (COR)) are contrastively analyzed during the host-guest assembly in both single-guest (COR/BTB or CuPc/BTB) and multi-guest molecule (COR&CuPc/BTB) systems. The spontaneous phase transition from a hexagonal to an oblique structure of BTB monolayers (high concentration, approximately 500 μmol∙L−1 in octanoic acid) provides an ideal metastable phase for studying the dynamic assembly process. In the host-guest assembly, the metastable BTB hexagonal phase serves as a host network and can be stabilized by co-assembling guest molecules under a negative bias voltage. However, the stability of the metastable phase varies with different guest molecules. We observe that the BTB metastable phase is more robust with COR guest molecules than with CuPc. In the CuPc/BTB system, we find that low-concentration CuPc (approximately 1.5 μmol∙L−1 in octanoic acid) can hardly co-assemble with BTB, leading to the gradual collapse of the metastable BTB networks into the oblique phase. The different stability of BTB metastable phase in the host-guest assembly is attributed to differences in the kinetics of trapping guest molecules. Guest COR molecules exhibit kinetic advantages over CuPc when assembling with host BTB networks under a negative sample bias. The lower trapping rate of CuPc hinders the formation of co-assembled BTB/CuPc networks. These differences in the dynamic behavior of the guest molecules are further explored in the research of catassembly. In a multi-guest molecule system (COR&CuPc/BTB), COR molecules are preferentially trapped by BTB hexagonal networks and can gradually be replaced by CuPc during continuous scanning. The more energetically stable structure of CuPc/BTB compared to COR/BTB rationalizes the exchange of the guest molecule and the evolution of the assembly phase. The involvement of COR significantly increases both the efficiency and quality of the CuPc/BTB assembly, serving as a catassembler. This observation provides insights into a complete catassembly process at the molecular level, enabling further investigations into the selectivity and efficiency of host-guest phenomena for potential applications in analysis and separation. Additionally, this work serves as a prototype for constructing highly complex 2D assembled monolayers.
2024, 40(9): 230703
doi: 10.3866/PKU.WHXB202307034
Abstract:
One of the crucial directions in the pursuit of high-energy-density lithium batteries involves pairing Ni-rich cathodes with lithium metal anodes (LMAs). However, battery systems with high energy density often suffer from issues such as poor phase structure stability and inadequate interface compatibility. These problems are exacerbated under the actual operating conditions with high cut-off voltages and wide temperature ranges. Interface degradation, in such cases, accelerates the destruction of phase structure, leading to rapid performance deterioration of electrode materials. Compared to methods like ion doping and surface coating, an approach centered around electrolyte-induced interface reconstruction modification through solvent-lithium salt optimization or functional additives shows promise. This approach allows for simultaneous electrochemical cyclic modification of both high-energy-density cathode and anode materials, and it can be easily integrated into large-scale industrial production. Ester-based electrolytes, while possessing greater voltage stability compared to ether-based electrolytes, still exhibit side reactions at the interface between high Ni-content cathodes and the electrolyte, as well as between Li metal anodes and the electrolyte. In the absence of effective cathode-electrolyte interface (CEI) and solid-electrolyte interface (SEI) protection, persistent side reactions occur, ultimately leading to electrode failure. To address these challenges and simultaneously enhance electrode/electrolyte interface compatibility while regulating electrolyte solvation structure, functional additives are employed to modify the electrochemical behavior of the high-energy-density battery interface. Traditional ether electrolytes often employ lithium hexafluorophosphate (LiPF6) as the primary salt. However, LiPF6 suffers from poor thermal stability. Its decomposition or hydrolysis generates hydrogen fluoride (HF), which corrodes the cathode. Moreover, LiPF6 decomposition releases phosphorus pentafluoride (PF5), triggering the ring-opening of ethylene carbonate (EC), leading to electrolyte failure. PF5 can also react with water to produce acidic compounds, further deteriorating battery performance. The extraction of Li+ ions in the cathode reduces oxygen binding energy, facilitating the release of lattice oxygen. This can lead to side reactions between reactive oxygen species and the electrolyte, increasing interface impedance. To tackle these issues, choosing electrolyte additives with diverse functions can expand the potential of electrolytes. By leveraging various functional electrolyte additives, it becomes possible to inhibit irreversible structural transformations in the cathode, prevent O2/CO2 precipitation, suppress interface side reactions, and facilitate the removal of acid-water impurities. This comprehensive study delves into the impact of different functional electrolyte additives on interface film reconstruction, interfacial adsorption stability, synergy on high-energy-density anode interface, and acid-water impurity removal in Ni-rich cathode and anode materials. The research opens up new avenues for the identification and design of specific functionalized additives, paving the way for achieving stable cycling in high-energy-density Ni-rich lithium batteries.
One of the crucial directions in the pursuit of high-energy-density lithium batteries involves pairing Ni-rich cathodes with lithium metal anodes (LMAs). However, battery systems with high energy density often suffer from issues such as poor phase structure stability and inadequate interface compatibility. These problems are exacerbated under the actual operating conditions with high cut-off voltages and wide temperature ranges. Interface degradation, in such cases, accelerates the destruction of phase structure, leading to rapid performance deterioration of electrode materials. Compared to methods like ion doping and surface coating, an approach centered around electrolyte-induced interface reconstruction modification through solvent-lithium salt optimization or functional additives shows promise. This approach allows for simultaneous electrochemical cyclic modification of both high-energy-density cathode and anode materials, and it can be easily integrated into large-scale industrial production. Ester-based electrolytes, while possessing greater voltage stability compared to ether-based electrolytes, still exhibit side reactions at the interface between high Ni-content cathodes and the electrolyte, as well as between Li metal anodes and the electrolyte. In the absence of effective cathode-electrolyte interface (CEI) and solid-electrolyte interface (SEI) protection, persistent side reactions occur, ultimately leading to electrode failure. To address these challenges and simultaneously enhance electrode/electrolyte interface compatibility while regulating electrolyte solvation structure, functional additives are employed to modify the electrochemical behavior of the high-energy-density battery interface. Traditional ether electrolytes often employ lithium hexafluorophosphate (LiPF6) as the primary salt. However, LiPF6 suffers from poor thermal stability. Its decomposition or hydrolysis generates hydrogen fluoride (HF), which corrodes the cathode. Moreover, LiPF6 decomposition releases phosphorus pentafluoride (PF5), triggering the ring-opening of ethylene carbonate (EC), leading to electrolyte failure. PF5 can also react with water to produce acidic compounds, further deteriorating battery performance. The extraction of Li+ ions in the cathode reduces oxygen binding energy, facilitating the release of lattice oxygen. This can lead to side reactions between reactive oxygen species and the electrolyte, increasing interface impedance. To tackle these issues, choosing electrolyte additives with diverse functions can expand the potential of electrolytes. By leveraging various functional electrolyte additives, it becomes possible to inhibit irreversible structural transformations in the cathode, prevent O2/CO2 precipitation, suppress interface side reactions, and facilitate the removal of acid-water impurities. This comprehensive study delves into the impact of different functional electrolyte additives on interface film reconstruction, interfacial adsorption stability, synergy on high-energy-density anode interface, and acid-water impurity removal in Ni-rich cathode and anode materials. The research opens up new avenues for the identification and design of specific functionalized additives, paving the way for achieving stable cycling in high-energy-density Ni-rich lithium batteries.
2024, 40(9): 230705
doi: 10.3866/PKU.WHXB202307059
Abstract:
Hydrogen fuel has long been considered a promising and practical alternative to conventional fossil fuels for shaping the future of our energy landscape. The electrocatalytic water-splitting technique, a sustainable and eco-friendly technology, provides a viable solution for efficiently and abundantly producing high-purity hydrogen on a large scale. However, practical applications of this technology require continuous improvement in the reaction kinetics for the hydrogen evolution reaction (HER) at the anode and the oxygen evolution reaction (OER) at the cathode. Additionally, ongoing optimization of the catalyst’s catalytic activity and structural stability is crucial for the practical implementation of this technology. The selection of suitable catalysts is of paramount importance in water splitting. As a result, two-dimensional (2D) nanomaterials have become a focal point in water electrolysis due to their unique physicochemical properties and abundant active sites. The atomic thinness of 2D materials makes their electronic structure easily adjustable, allowing for the precise control of electrocatalytic performance through morphological modifications, defect engineering, phase transitions, cocatalyst deposition, and element doping. However, the complex system structure design and the potentially mutual interference of various chemical components could hinder further improvements in hydrogen evolution performance. Fortunately, the distinctive physicochemical characteristics of 2D materials can synergize with external physical fields, leading to enhanced electrocatalytic performance through distinct effects. For example, magnetic fields, electric fields, and light fields can induce thermal effects, effectively reducing charge transfer resistance and bubble coverage on the catalyst surface. Strain can regulate the d-band center, thus controlling adsorption energy. Moreover, the superposition of multiple physical fields and the multiple effects of a single physical field offer enormous potential for enhancing electrocatalytic performance. It is evident that the regulation of electrocatalytic performance through physical fields holds significant untapped potential. Consequently, the roles and mechanisms of external physical field assistance in HER and OER have garnered increasing attention. External fields such as electric fields, magnetic fields, strain, light, temperature, and ultrasound can be applied to synthesis and electrocatalysis. This paper first provides a summary of research on the synthesis of physical field-assisted electrolytic water catalysts. It then classifies studies on field-assisted HER and OER based on different mechanisms. Finally, it outlines the key challenges and prospects in this rapidly evolving research field.
Hydrogen fuel has long been considered a promising and practical alternative to conventional fossil fuels for shaping the future of our energy landscape. The electrocatalytic water-splitting technique, a sustainable and eco-friendly technology, provides a viable solution for efficiently and abundantly producing high-purity hydrogen on a large scale. However, practical applications of this technology require continuous improvement in the reaction kinetics for the hydrogen evolution reaction (HER) at the anode and the oxygen evolution reaction (OER) at the cathode. Additionally, ongoing optimization of the catalyst’s catalytic activity and structural stability is crucial for the practical implementation of this technology. The selection of suitable catalysts is of paramount importance in water splitting. As a result, two-dimensional (2D) nanomaterials have become a focal point in water electrolysis due to their unique physicochemical properties and abundant active sites. The atomic thinness of 2D materials makes their electronic structure easily adjustable, allowing for the precise control of electrocatalytic performance through morphological modifications, defect engineering, phase transitions, cocatalyst deposition, and element doping. However, the complex system structure design and the potentially mutual interference of various chemical components could hinder further improvements in hydrogen evolution performance. Fortunately, the distinctive physicochemical characteristics of 2D materials can synergize with external physical fields, leading to enhanced electrocatalytic performance through distinct effects. For example, magnetic fields, electric fields, and light fields can induce thermal effects, effectively reducing charge transfer resistance and bubble coverage on the catalyst surface. Strain can regulate the d-band center, thus controlling adsorption energy. Moreover, the superposition of multiple physical fields and the multiple effects of a single physical field offer enormous potential for enhancing electrocatalytic performance. It is evident that the regulation of electrocatalytic performance through physical fields holds significant untapped potential. Consequently, the roles and mechanisms of external physical field assistance in HER and OER have garnered increasing attention. External fields such as electric fields, magnetic fields, strain, light, temperature, and ultrasound can be applied to synthesis and electrocatalysis. This paper first provides a summary of research on the synthesis of physical field-assisted electrolytic water catalysts. It then classifies studies on field-assisted HER and OER based on different mechanisms. Finally, it outlines the key challenges and prospects in this rapidly evolving research field.
2024, 40(9): 230805
doi: 10.3866/PKU.WHXB202308051
Abstract:
Over the past three decades, significant advancements in lithium-ion battery technology have greatly improved human convenience, particularly in today's thriving electric vehicle industry. Further enhancements in the energy density, cycle life, and safety of lithium-ion batteries are crucial for the widespread adoption of electric vehicles. In recent years, transition metal layered oxides have garnered significant attention in the industrial power battery sector due to their advantages, including high specific capacity, commendable low-temperature performance, and cost-effectiveness. Increasing the nickel content and adjusting the charging cut-off voltage are recognized as effective means to enhance the energy density of transition metal layered oxides. However, these strategies tend to degrade cycling stability and thermal safety in conventional polycrystalline layered cathode materials. Benefiting from the mechanical stability of intact primary particles, the single-crystal structure of layered cathode materials can effectively mitigate intergranular cracking issues associated with high charging voltages. Nevertheless, due to the intrinsic structural properties of layered materials, single-crystal structures still face challenges related to sluggish Li+ transport kinetics, heterogeneous state of charge, anisotropic changes in lattice parameters, cation mixing, and chemo-mechanical degradation. The temporal and spatial evolution of the physicochemical properties within the internal microstructure of materials still requires comprehensive analysis using advanced operando characterization techniques. Currently, there is limited understanding of the intricate interplay between thermodynamics and kinetics in the synthesis process of single-crystal cathode materials. A more profound exploration of the structural degradation and synthesis mechanisms of single-crystal materials will serve as a fundamental basis for targeted modification strategies. Regrettably, existing single-crystal synthesis processes and modification approaches still fall short of market expectations. This shortfall is especially noticeable in future applications in solid-state batteries, where interface issues related to solid-state-electrolyte and cathode material are serious. Addressing these challenges necessitates the precise regulation of the microstructure of composite cathodes. Therefore, this review systematically analyzes and summarizes common issues related to the failure of both polycrystal and single-crystal structures, taking into account the intrinsic structural evolution at various temporal and spatial scales. We also outline strategies for regulating the synthesis process, element doping, and surface-interface modification of single-crystal nickel-rich layered cathode materials from the perspective of coherent structural design. We also intent to elucidate the essential connection between structural design and electrochemical performance. The microstructural design of single-crystal nickel-rich cathode materials should emphasize the alignment of lattice parameters between heterostructures and layered oxides, as well as the modulation of their spatial distribution, thereby ensuring the long-term efficacy of element doping and surface-interface modification. Finally, we offer a perspective on the future development of single-crystal nickel-rich cathode materials, highlighting their potential success in the realm of power batteries.
Over the past three decades, significant advancements in lithium-ion battery technology have greatly improved human convenience, particularly in today's thriving electric vehicle industry. Further enhancements in the energy density, cycle life, and safety of lithium-ion batteries are crucial for the widespread adoption of electric vehicles. In recent years, transition metal layered oxides have garnered significant attention in the industrial power battery sector due to their advantages, including high specific capacity, commendable low-temperature performance, and cost-effectiveness. Increasing the nickel content and adjusting the charging cut-off voltage are recognized as effective means to enhance the energy density of transition metal layered oxides. However, these strategies tend to degrade cycling stability and thermal safety in conventional polycrystalline layered cathode materials. Benefiting from the mechanical stability of intact primary particles, the single-crystal structure of layered cathode materials can effectively mitigate intergranular cracking issues associated with high charging voltages. Nevertheless, due to the intrinsic structural properties of layered materials, single-crystal structures still face challenges related to sluggish Li+ transport kinetics, heterogeneous state of charge, anisotropic changes in lattice parameters, cation mixing, and chemo-mechanical degradation. The temporal and spatial evolution of the physicochemical properties within the internal microstructure of materials still requires comprehensive analysis using advanced operando characterization techniques. Currently, there is limited understanding of the intricate interplay between thermodynamics and kinetics in the synthesis process of single-crystal cathode materials. A more profound exploration of the structural degradation and synthesis mechanisms of single-crystal materials will serve as a fundamental basis for targeted modification strategies. Regrettably, existing single-crystal synthesis processes and modification approaches still fall short of market expectations. This shortfall is especially noticeable in future applications in solid-state batteries, where interface issues related to solid-state-electrolyte and cathode material are serious. Addressing these challenges necessitates the precise regulation of the microstructure of composite cathodes. Therefore, this review systematically analyzes and summarizes common issues related to the failure of both polycrystal and single-crystal structures, taking into account the intrinsic structural evolution at various temporal and spatial scales. We also outline strategies for regulating the synthesis process, element doping, and surface-interface modification of single-crystal nickel-rich layered cathode materials from the perspective of coherent structural design. We also intent to elucidate the essential connection between structural design and electrochemical performance. The microstructural design of single-crystal nickel-rich cathode materials should emphasize the alignment of lattice parameters between heterostructures and layered oxides, as well as the modulation of their spatial distribution, thereby ensuring the long-term efficacy of element doping and surface-interface modification. Finally, we offer a perspective on the future development of single-crystal nickel-rich cathode materials, highlighting their potential success in the realm of power batteries.
