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2023 Volume 39 Issue 8
2023, 39(8): 220501
doi: 10.3866/PKU.WHXB202205012
Abstract:
All-solid-state batteries (ASSBs) have been considered a promising candidate for the next-generation electrochemical energy storage because of their high theoretical energy density and inherent safety. Lithium superionic conductors with high lithium-ion transference number and good processability are imperative for the development of practical ASSBs. However, the lithium superionic conductors currently available are predominantly limited to hard ceramics. Practical lithium superionic conductors employing flexible polymers are yet to be realized. The rigid and brittle nature of inorganic ceramic electrolytes limits their application in high-performance ASSBs. In this study, we demonstrate a novel design of a ternary random copolymer single-ion superionic conductor (SISC) through the radical polymerization of three different organic monomers that uses an anion-trapping borate ester as a crosslinking agent to copolymerize with vinylene carbonate and methyl vinyl sulfone. The proposed SISC contains abundant solvation sites for lithium-ion transport and anion receptors to immobilize the corresponding anions. Furthermore, the copolymerization of the three different monomers results in a low crystallinity and low glass transition temperature, which facilitates superior chain segment motion and results in a small activation energy for lithium-ion transport. The ionic conductivity and lithium-ion transference number of the SISC are 1.29 mS·cm−1 and 0.94 at room temperature, respectively. The SISC exhibits versatile processability and favorable Young's modulus (3.4 ± 0.4 GPa). The proposed SISC can be integrated into ASSBs through in situ polymerization, which facilitates the formation of suitable electrode/electrolyte contacts. Solid-state symmetric Li||Li cells employing in situ polymerized SISCs show excellent lithium stripping/plating reversibility for more than 1000 h at a current density of 0.25 mA·cm−2. This indicates that the interface between the SISC and lithium metal anode is electrochemically stable. The ASSBs that employ in situ polymerized SISCs coupled with a lithium metal anode and various cathodes, including LiFePO4, LiCoO2, and sulfurized polyacrylonitrile (SPAN), exhibit acceptable electrochemical stability, including high rate performance and good cyclability. In particular, the Li||LiFePO4 ASSBs retained ~ 70% of the discharge capacity when the charge/discharge rate was increased from 1 to 8C. They also demonstrate long-term cycling stability (> 700 cycles at 0.5C rate) at room temperature. A capacity retention of 90% was achieved even at a high rate of 2C after 300 cycles at room temperature. Furthermore, the SISCs have been applied to Li||LiFePO4 pouch cells and exhibit exceptional flexibility and safety. This work provides a novel design principle for the fabrication of polymer-based superionic conductors and is valuable for the development of practical ambient-temperature ASSBs.![]()
All-solid-state batteries (ASSBs) have been considered a promising candidate for the next-generation electrochemical energy storage because of their high theoretical energy density and inherent safety. Lithium superionic conductors with high lithium-ion transference number and good processability are imperative for the development of practical ASSBs. However, the lithium superionic conductors currently available are predominantly limited to hard ceramics. Practical lithium superionic conductors employing flexible polymers are yet to be realized. The rigid and brittle nature of inorganic ceramic electrolytes limits their application in high-performance ASSBs. In this study, we demonstrate a novel design of a ternary random copolymer single-ion superionic conductor (SISC) through the radical polymerization of three different organic monomers that uses an anion-trapping borate ester as a crosslinking agent to copolymerize with vinylene carbonate and methyl vinyl sulfone. The proposed SISC contains abundant solvation sites for lithium-ion transport and anion receptors to immobilize the corresponding anions. Furthermore, the copolymerization of the three different monomers results in a low crystallinity and low glass transition temperature, which facilitates superior chain segment motion and results in a small activation energy for lithium-ion transport. The ionic conductivity and lithium-ion transference number of the SISC are 1.29 mS·cm−1 and 0.94 at room temperature, respectively. The SISC exhibits versatile processability and favorable Young's modulus (3.4 ± 0.4 GPa). The proposed SISC can be integrated into ASSBs through in situ polymerization, which facilitates the formation of suitable electrode/electrolyte contacts. Solid-state symmetric Li||Li cells employing in situ polymerized SISCs show excellent lithium stripping/plating reversibility for more than 1000 h at a current density of 0.25 mA·cm−2. This indicates that the interface between the SISC and lithium metal anode is electrochemically stable. The ASSBs that employ in situ polymerized SISCs coupled with a lithium metal anode and various cathodes, including LiFePO4, LiCoO2, and sulfurized polyacrylonitrile (SPAN), exhibit acceptable electrochemical stability, including high rate performance and good cyclability. In particular, the Li||LiFePO4 ASSBs retained ~ 70% of the discharge capacity when the charge/discharge rate was increased from 1 to 8C. They also demonstrate long-term cycling stability (> 700 cycles at 0.5C rate) at room temperature. A capacity retention of 90% was achieved even at a high rate of 2C after 300 cycles at room temperature. Furthermore, the SISCs have been applied to Li||LiFePO4 pouch cells and exhibit exceptional flexibility and safety. This work provides a novel design principle for the fabrication of polymer-based superionic conductors and is valuable for the development of practical ambient-temperature ASSBs.
2023, 39(8): 221003
doi: 10.3866/PKU.WHXB202210039
Abstract:
Environment-friendly energy storage and conversion technologies, such as metal–air batteries and fuel cells, are considered promising approaches to address growing environmental concerns. The oxygen reduction reaction (ORR) is the core of renewable energy conversion technology and plays an irreplaceable role in this fundamental issue. However, the complex multi-reaction process of the ORR presents a bottleneck that limits efforts to accelerate its kinetics. Traditionally, Pt and Pt-based catalysts are regarded as a good choice to improve the sluggish kinetics of the ORR. However, because Pt-based catalysts are expensive and have low durability, their use to resolve the energy crisis and current environmental challenges is impractical. Hence, exploring low-cost, highly active, and durable ORR catalysts as potential alternatives to commercial Pt/C is an urgent undertaking. Atomic cluster catalysts (ACCs) may be suitable alternatives to commercial Pt/C catalysts owing to their ultra-high atomic utilization efficiency, unique electronic structure, and stable nanostructures. However, despite the significant progress achieved in recent years, ACCs remain unusable for practical applications. In this study, a facile plasma bombing method combined with an acid washing strategy is proposed to fabricate an atomic Co cluster-decorated porous carbon supports catalyst (CoAC/NC) showing improved ORR performance. The typical atomic cluster features of the resultant CoAC/NC catalyst are confirmed using comprehensive characterization techniques. The CoAC/NC catalyst exhibits considerable ORR activity with a half-wave potential of as high as 0.887 V (versus a reversible hydrogen electrode (RHE)), which is much higher than that of a commercial Pt/C catalyst. More importantly, the CoAC/NC catalyst displays excellent battery performance when applied to a Zn-air battery, showing a peak power density of 181.5 mW∙cm−2 and long discharge ability (over 67 h at a discharge current density of 5 mA∙cm−2). The desirable ORR performance of the fabricated CoAC/NC catalyst could be mainly attributed to the high atom utilization efficiency and stable active sites endowed by the unique Co atomic clusters, as well as synergistic effects between the neighboring Co atoms of these clusters. Moreover, the high specific surface area and wide pore distribution of the catalyst offer abundant accessible active sites for the ORR. This work not only provides an outstanding alternative to commercial Pt catalysts for the ORR but also offers new insights into the rational design and practical application of ACCs.![]()
Environment-friendly energy storage and conversion technologies, such as metal–air batteries and fuel cells, are considered promising approaches to address growing environmental concerns. The oxygen reduction reaction (ORR) is the core of renewable energy conversion technology and plays an irreplaceable role in this fundamental issue. However, the complex multi-reaction process of the ORR presents a bottleneck that limits efforts to accelerate its kinetics. Traditionally, Pt and Pt-based catalysts are regarded as a good choice to improve the sluggish kinetics of the ORR. However, because Pt-based catalysts are expensive and have low durability, their use to resolve the energy crisis and current environmental challenges is impractical. Hence, exploring low-cost, highly active, and durable ORR catalysts as potential alternatives to commercial Pt/C is an urgent undertaking. Atomic cluster catalysts (ACCs) may be suitable alternatives to commercial Pt/C catalysts owing to their ultra-high atomic utilization efficiency, unique electronic structure, and stable nanostructures. However, despite the significant progress achieved in recent years, ACCs remain unusable for practical applications. In this study, a facile plasma bombing method combined with an acid washing strategy is proposed to fabricate an atomic Co cluster-decorated porous carbon supports catalyst (CoAC/NC) showing improved ORR performance. The typical atomic cluster features of the resultant CoAC/NC catalyst are confirmed using comprehensive characterization techniques. The CoAC/NC catalyst exhibits considerable ORR activity with a half-wave potential of as high as 0.887 V (versus a reversible hydrogen electrode (RHE)), which is much higher than that of a commercial Pt/C catalyst. More importantly, the CoAC/NC catalyst displays excellent battery performance when applied to a Zn-air battery, showing a peak power density of 181.5 mW∙cm−2 and long discharge ability (over 67 h at a discharge current density of 5 mA∙cm−2). The desirable ORR performance of the fabricated CoAC/NC catalyst could be mainly attributed to the high atom utilization efficiency and stable active sites endowed by the unique Co atomic clusters, as well as synergistic effects between the neighboring Co atoms of these clusters. Moreover, the high specific surface area and wide pore distribution of the catalyst offer abundant accessible active sites for the ORR. This work not only provides an outstanding alternative to commercial Pt catalysts for the ORR but also offers new insights into the rational design and practical application of ACCs.
2023, 39(8): 221203
doi: 10.3866/PKU.WHXB202212037
Abstract:
In recent years, increased attention has been paid to aqueous Zn-ion batteries (ZIBs) owing to their low cost, inherent safety, and environmental benignity, which enable them to become promising electrochemical energy storage systems for grid-scale applications. However, some critical issues in zinc metal anode, like zinc dendrites growth, corrosion, and side reactions, act as obstacles to developing aqueous ZIBs. Exploring zinc-storage anodes to replace zinc-metal anodes is proposed as an effective strategy to promote the practical application of ZIBs. Therefore, several transition metal oxides, sulfides, and conductive polymers have been intensively studied as zinc-metal-free anodes. Two-dimensional metal dichalcogenides (TMDs), especially TiX2 (X = S, Se), are the most appealing candidates because of their sizeable interlayer space and facile 2D ion-transport channels. However, the reaction mechanism of these TiX2 in ZIBs still needs fundamental study. In this work, density functional theory (DFT) calculations are performed to investigate the behavior of zinc intercalation reaction in TiX2 systematically. First, the most stable interlayer configurations of zinc-intercalated TiX2 are characterized by group theory. We define a supercell-dependent group that only involves translation and rotation operation and find that the subgroup of the group that describes the symmetry of the most stable interlayer configurations possesses the highest order. So, the most stable configuration can be fast screened out among thousands of candidates. Calculations based on a series of those stable configurations at different discharge depths reveal the low open circuit voltage (OCV) of < 0.5 V for both ZnxTiS2 and ZnxTiSe2. The density of states (DOS) result suggests the good electronic conductivity of TiX2, and the partial density of states (PDOS) result indicates the closed-shell Ti(IV) is reduced to open-shell Ti(III), and the formation of Zn―X bonds as zinc ions intercalate into the interlayer of TiX2. Interestingly, Bader charge analysis demonstrates that X anions also participate in the redox process during charge and discharge because X gains more negative charge than Ti as zinc intercalates into TiX2. The climbing image nudged elastic band (CINEB) calculations reveal the low zinc ion diffusion barriers (0.333 and 0.338 eV for TiS2 and TiSe2, respectively). This study proves that TiX2 is suitable as zinc intercalating anode materials for ZIBs and provides new insights into the DFT investigation of other TMDs as high-performance battery materials.![]()
In recent years, increased attention has been paid to aqueous Zn-ion batteries (ZIBs) owing to their low cost, inherent safety, and environmental benignity, which enable them to become promising electrochemical energy storage systems for grid-scale applications. However, some critical issues in zinc metal anode, like zinc dendrites growth, corrosion, and side reactions, act as obstacles to developing aqueous ZIBs. Exploring zinc-storage anodes to replace zinc-metal anodes is proposed as an effective strategy to promote the practical application of ZIBs. Therefore, several transition metal oxides, sulfides, and conductive polymers have been intensively studied as zinc-metal-free anodes. Two-dimensional metal dichalcogenides (TMDs), especially TiX2 (X = S, Se), are the most appealing candidates because of their sizeable interlayer space and facile 2D ion-transport channels. However, the reaction mechanism of these TiX2 in ZIBs still needs fundamental study. In this work, density functional theory (DFT) calculations are performed to investigate the behavior of zinc intercalation reaction in TiX2 systematically. First, the most stable interlayer configurations of zinc-intercalated TiX2 are characterized by group theory. We define a supercell-dependent group that only involves translation and rotation operation and find that the subgroup of the group that describes the symmetry of the most stable interlayer configurations possesses the highest order. So, the most stable configuration can be fast screened out among thousands of candidates. Calculations based on a series of those stable configurations at different discharge depths reveal the low open circuit voltage (OCV) of < 0.5 V for both ZnxTiS2 and ZnxTiSe2. The density of states (DOS) result suggests the good electronic conductivity of TiX2, and the partial density of states (PDOS) result indicates the closed-shell Ti(IV) is reduced to open-shell Ti(III), and the formation of Zn―X bonds as zinc ions intercalate into the interlayer of TiX2. Interestingly, Bader charge analysis demonstrates that X anions also participate in the redox process during charge and discharge because X gains more negative charge than Ti as zinc intercalates into TiX2. The climbing image nudged elastic band (CINEB) calculations reveal the low zinc ion diffusion barriers (0.333 and 0.338 eV for TiS2 and TiSe2, respectively). This study proves that TiX2 is suitable as zinc intercalating anode materials for ZIBs and provides new insights into the DFT investigation of other TMDs as high-performance battery materials.
2023, 39(8): 221205
doi: 10.3866/PKU.WHXB202212051
Abstract:
All-solid-state thin film lithium batteries (TFBs) are regarded as the ideal power source for microelectronics in the upcoming era of the Internet of Things, owing to their solid-state architecture, flexible size and shape, long cycle life, low self-discharge rate, and facile miniaturization. Even though tremendous improvements have been made in TFBs in the last decades, recycling of TFBs, which is supposed to be a serious issue in the future, is rarely studied. With continuous TFB market expansion, the sustainable development of TFBs is becoming an increasingly important research topic. To date, Li anode failure has been regarded as the most common reason for the failure of TFBs due to the following aspects of Li metal anode: strong reactivity with moisture, large volume change during cycling, and inevitable dendrite growth during Li plating. In this work, a facile recycling strategy is developed based on the most commonly used TFBs of LiCoO2 (LCO)/lithium phosphorus oxynitride (LiPON)/Li (F-TFB). Our findings indicate that the Li anode in F-TFB is partially oxidized during cycling with noticeable surface morphology change, which leads to an obvious increase in Li/LiPON interfacial resistance associated with rapid capacity loss. To directly recycle the F-TFB, we developed a simple method to remove the spent Li anode by dissolving the metallic Li metal of the F-TFB in an ethanol solution. The efficient dissolution of metallic Li allows the oxidized Li residues to be easily wiped away from the surface of the LiPON electrolyte film with the assistance of a dust-free cloth, resulting in the rapid recycling of the underlying LCO/LiPON film. Structural and surface characterization results indicate that the obtained LCO/LiPON which was part of the failed F-TFB remains in a good condition without structural degradation, which enables its direct reuse in fabricating new TFBs. Consequently, a recycled LCO/LiPON/Li TFB (R-TFB) is constructed by sequentially depositing new LiPON and Li films on the used LCO/LiPON film, exhibiting a small interfacial resistance, recovered Li anode morphology and surface, and restorative electrochemical performance. Specifically, the R-TFB delivers a specific capacity of 0.223 mAh·cm−2 at 0.1 mA·cm−2, acceptable rate performance (0.120 mAh·cm−2 at 0.8 mA·cm−2), and good cycle performance (77.3% capacity retention after 500 cycles), which are very close to those of a newly fabricated TFB, demonstrating the feasibility of this direct recycling approach. Such a simple yet efficient recycling approach may effectively extend the lifespan of solid-state batteries and provide important insights to develop sustainable TFBs for microelectronic devices.![]()
All-solid-state thin film lithium batteries (TFBs) are regarded as the ideal power source for microelectronics in the upcoming era of the Internet of Things, owing to their solid-state architecture, flexible size and shape, long cycle life, low self-discharge rate, and facile miniaturization. Even though tremendous improvements have been made in TFBs in the last decades, recycling of TFBs, which is supposed to be a serious issue in the future, is rarely studied. With continuous TFB market expansion, the sustainable development of TFBs is becoming an increasingly important research topic. To date, Li anode failure has been regarded as the most common reason for the failure of TFBs due to the following aspects of Li metal anode: strong reactivity with moisture, large volume change during cycling, and inevitable dendrite growth during Li plating. In this work, a facile recycling strategy is developed based on the most commonly used TFBs of LiCoO2 (LCO)/lithium phosphorus oxynitride (LiPON)/Li (F-TFB). Our findings indicate that the Li anode in F-TFB is partially oxidized during cycling with noticeable surface morphology change, which leads to an obvious increase in Li/LiPON interfacial resistance associated with rapid capacity loss. To directly recycle the F-TFB, we developed a simple method to remove the spent Li anode by dissolving the metallic Li metal of the F-TFB in an ethanol solution. The efficient dissolution of metallic Li allows the oxidized Li residues to be easily wiped away from the surface of the LiPON electrolyte film with the assistance of a dust-free cloth, resulting in the rapid recycling of the underlying LCO/LiPON film. Structural and surface characterization results indicate that the obtained LCO/LiPON which was part of the failed F-TFB remains in a good condition without structural degradation, which enables its direct reuse in fabricating new TFBs. Consequently, a recycled LCO/LiPON/Li TFB (R-TFB) is constructed by sequentially depositing new LiPON and Li films on the used LCO/LiPON film, exhibiting a small interfacial resistance, recovered Li anode morphology and surface, and restorative electrochemical performance. Specifically, the R-TFB delivers a specific capacity of 0.223 mAh·cm−2 at 0.1 mA·cm−2, acceptable rate performance (0.120 mAh·cm−2 at 0.8 mA·cm−2), and good cycle performance (77.3% capacity retention after 500 cycles), which are very close to those of a newly fabricated TFB, demonstrating the feasibility of this direct recycling approach. Such a simple yet efficient recycling approach may effectively extend the lifespan of solid-state batteries and provide important insights to develop sustainable TFBs for microelectronic devices.
2023, 39(8): 221003
doi: 10.3866/PKU.WHXB202210032
Abstract:
All-solid-state batteries are a promising energy storage technology owing to their high energy density and safety. Exploring solid electrolytes with high room-temperature ionic conductivity, good electrochemical stability, and excellent cathode/anode compatibility is key to realizing the practical application of all-solid-state batteries. Lithium metal halide solid electrolytes have attracted extensive research attention because of their excellent electrochemical windows, high positive electrode stabilities, and acceptable room-temperature Li-ion conductivities of up to 10−3 S·cm−1. In this paper, the chemical compositions, structural details, lithium-ion conduction pathways, and synthesis routes of lithium metal halide solid electrolytes are reviewed based on recently published papers and our studies. The lithium metal halide Lia-M-X6 can be classified as Lia-M-Cl6, Lia-M-Cl4, and Lia-M-Cl8 based on the substitution of the Li ions with different transition metal elements. Among these, the Lia-M-X6 and Lia-M-X4 electrolytes have been widely investigated because of their high ionic conductivities of up to 10−3 S·cm−1. Lia-M-X6 electrolytes exhibit three types of structure: trigonal, orthorhombic, and monoclinic. Li+ diffusion in lithium metal halide electrolytes with different structures follows a vacancy mechanism. When transition metal cations with larger ionic radii and higher valances are used to substitute Li+ in the structure, vacancies are generated and larger Li+ transport channels are produced, both of which are helpful for achieving faster Li-ion conductivities in the modified electrolytes. The typical synthetic route for lithium metal halide electrolytes is mechanical milling and subsequent sintering. Moreover, recent studies have reported that a pure phase with high conductivity can be obtained via water-mediated synthesis, which is a promising method for mass production. The electrochemical stability of lithium metal halide electrolytes with temperature, humidity, and active electrode materials is also summarized herein. Some lithium halide electrolytes suffer from a low phase-transition temperature close to room temperature, making it difficult to prepare the pure phase and limiting their applications. Owing to the high sensitivity of halides to moisture, lithium halide electrolytes suffer poor stability during storage and operation in the open air. The wide electrochemical window and excellent stability of high-voltage cathode materials of lithium metal halide electrolytes enable the construction of all-solid-state lithium batteries with a high energy density and long lifespan. Moreover, this property makes it possible to introduce carbon conductive additives into the cathode without a surface coating layer on the active materials, which is helpful for designing highly conductive frameworks for thick electrodes used in solid-state batteries. However, lithium metal halide electrolytes exhibit poor stability with bare lithium metal or lithium alloys because of their high reduction potentials. Therefore, another solid electrolyte layer requires the isolation of the direct contact between the lithium metal halide electrolytes and Li-related anodes. Finally, this review summarizes the application of these electrolytes in all-solid-state batteries in recent years and highlights the challenges and research directions of lithium halide electrolytes. ![]()
All-solid-state batteries are a promising energy storage technology owing to their high energy density and safety. Exploring solid electrolytes with high room-temperature ionic conductivity, good electrochemical stability, and excellent cathode/anode compatibility is key to realizing the practical application of all-solid-state batteries. Lithium metal halide solid electrolytes have attracted extensive research attention because of their excellent electrochemical windows, high positive electrode stabilities, and acceptable room-temperature Li-ion conductivities of up to 10−3 S·cm−1. In this paper, the chemical compositions, structural details, lithium-ion conduction pathways, and synthesis routes of lithium metal halide solid electrolytes are reviewed based on recently published papers and our studies. The lithium metal halide Lia-M-X6 can be classified as Lia-M-Cl6, Lia-M-Cl4, and Lia-M-Cl8 based on the substitution of the Li ions with different transition metal elements. Among these, the Lia-M-X6 and Lia-M-X4 electrolytes have been widely investigated because of their high ionic conductivities of up to 10−3 S·cm−1. Lia-M-X6 electrolytes exhibit three types of structure: trigonal, orthorhombic, and monoclinic. Li+ diffusion in lithium metal halide electrolytes with different structures follows a vacancy mechanism. When transition metal cations with larger ionic radii and higher valances are used to substitute Li+ in the structure, vacancies are generated and larger Li+ transport channels are produced, both of which are helpful for achieving faster Li-ion conductivities in the modified electrolytes. The typical synthetic route for lithium metal halide electrolytes is mechanical milling and subsequent sintering. Moreover, recent studies have reported that a pure phase with high conductivity can be obtained via water-mediated synthesis, which is a promising method for mass production. The electrochemical stability of lithium metal halide electrolytes with temperature, humidity, and active electrode materials is also summarized herein. Some lithium halide electrolytes suffer from a low phase-transition temperature close to room temperature, making it difficult to prepare the pure phase and limiting their applications. Owing to the high sensitivity of halides to moisture, lithium halide electrolytes suffer poor stability during storage and operation in the open air. The wide electrochemical window and excellent stability of high-voltage cathode materials of lithium metal halide electrolytes enable the construction of all-solid-state lithium batteries with a high energy density and long lifespan. Moreover, this property makes it possible to introduce carbon conductive additives into the cathode without a surface coating layer on the active materials, which is helpful for designing highly conductive frameworks for thick electrodes used in solid-state batteries. However, lithium metal halide electrolytes exhibit poor stability with bare lithium metal or lithium alloys because of their high reduction potentials. Therefore, another solid electrolyte layer requires the isolation of the direct contact between the lithium metal halide electrolytes and Li-related anodes. Finally, this review summarizes the application of these electrolytes in all-solid-state batteries in recent years and highlights the challenges and research directions of lithium halide electrolytes.
2023, 39(8): 221003
doi: 10.3866/PKU.WHXB202210036
Abstract:
Proton-exchange membrane fuel cells (PEMFCs) are an efficient and clean energy conversion technology with the advantage of zero pollution for transportation applications. The oxygen reduction reaction (ORR) is the key step in the energy conversion at the cathode, but the slow kinetics requires a high content of expensive platinum-group-metal (PGM) catalysts. Therefore, research on high-performance and inexpensive catalysts to replace PGM-based catalysts are essential to promote the commercialization of fuel cells. Single-atom catalysts (SACs) with highly active sites that are atomically dispersed on substrates exhibit unique advantages, such as maximum atomic utilization, abundant chemical structures, and extraordinary catalytic performances for multiple important reactions. Inspired by macrocyclic compounds with MN4 active centers, the application of pyrolyzed M-Nx/C type SACs (M = Fe, Co, Mn, Ru, Cr, Zn, etc.) in the ORR has significantly progressed within the last ten years. Particularly, single-atom Fe-N-C catalysts have been extensively investigated, demonstrating high ORR activity, which indicates that the initial electrochemistry and fuel cell performance are similar to that of conventional Pt/C catalysts. However, in the oxidizing and acidic PEMFC cathode, Fe-N-C catalysts are degraded rapidly, which hinders the application of these nonprecious metal M-Nx/C-type catalysts. Several degradation mechanisms have been proposed over the past few years, such as carbon oxidation, demetallation, and waterflooding. However, the degradation mechanisms remain unknown and require further investigation of the underlying causes of the mechanism, degradation process, and coping strategies. To achieve the future commercialization of high-performance M-Nx/C catalysts, several key challenges are summarized with potential research guidelines proposed to overcome bottlenecks. This review summarizes the development history and state-of-the-art research progress on nonprecious metal M-Nx/C-type catalysts in PEMFCs. First, we introduce the basic theory of the ORR and the methods of advanced characterization techniques for active site identification and reaction mechanism analysis to gain a comprehensive understanding of the structure–performance relationship. Subsequently, the representative studies and recent advancements in M-Nx/C-type catalysts by experimental and theoretical calculations are presented. Additionally, we analyze the root cause of the stability problems and propose the corresponding solution strategies to promote the intrinsic electrocatalytic ORR activity and durability, including regulating the electronic structure and coordination environment, as well as altering the central metal atoms and guest groups. Finally, we propose that the future direction of M-Nx/C-type catalysts is the rational design of catalysts with a high site density and high stability. Moreover, improving the lifetime of nonprecious metal catalysts remains essential for feasible applications in the future. ![]()
Proton-exchange membrane fuel cells (PEMFCs) are an efficient and clean energy conversion technology with the advantage of zero pollution for transportation applications. The oxygen reduction reaction (ORR) is the key step in the energy conversion at the cathode, but the slow kinetics requires a high content of expensive platinum-group-metal (PGM) catalysts. Therefore, research on high-performance and inexpensive catalysts to replace PGM-based catalysts are essential to promote the commercialization of fuel cells. Single-atom catalysts (SACs) with highly active sites that are atomically dispersed on substrates exhibit unique advantages, such as maximum atomic utilization, abundant chemical structures, and extraordinary catalytic performances for multiple important reactions. Inspired by macrocyclic compounds with MN4 active centers, the application of pyrolyzed M-Nx/C type SACs (M = Fe, Co, Mn, Ru, Cr, Zn, etc.) in the ORR has significantly progressed within the last ten years. Particularly, single-atom Fe-N-C catalysts have been extensively investigated, demonstrating high ORR activity, which indicates that the initial electrochemistry and fuel cell performance are similar to that of conventional Pt/C catalysts. However, in the oxidizing and acidic PEMFC cathode, Fe-N-C catalysts are degraded rapidly, which hinders the application of these nonprecious metal M-Nx/C-type catalysts. Several degradation mechanisms have been proposed over the past few years, such as carbon oxidation, demetallation, and waterflooding. However, the degradation mechanisms remain unknown and require further investigation of the underlying causes of the mechanism, degradation process, and coping strategies. To achieve the future commercialization of high-performance M-Nx/C catalysts, several key challenges are summarized with potential research guidelines proposed to overcome bottlenecks. This review summarizes the development history and state-of-the-art research progress on nonprecious metal M-Nx/C-type catalysts in PEMFCs. First, we introduce the basic theory of the ORR and the methods of advanced characterization techniques for active site identification and reaction mechanism analysis to gain a comprehensive understanding of the structure–performance relationship. Subsequently, the representative studies and recent advancements in M-Nx/C-type catalysts by experimental and theoretical calculations are presented. Additionally, we analyze the root cause of the stability problems and propose the corresponding solution strategies to promote the intrinsic electrocatalytic ORR activity and durability, including regulating the electronic structure and coordination environment, as well as altering the central metal atoms and guest groups. Finally, we propose that the future direction of M-Nx/C-type catalysts is the rational design of catalysts with a high site density and high stability. Moreover, improving the lifetime of nonprecious metal catalysts remains essential for feasible applications in the future.
2023, 39(8): 221101
doi: 10.3866/PKU.WHXB202211017
Abstract:
With the rapid development of electric vehicles and intelligent electronics, Li-based batteries are required to have a higher specific capacity and better safety. To develop batteries with higher energy densities, Li may be used as an anode material owing to its higher theoretical capacity (3860 mAh·g−1, 10 times higher than graphite) and low redox potential (−3.04 V vs. the standard hydrogen electrode). However, uncontrolled Li dendrite growth may occur during electrochemical Li plating/stripping in the liquid electrolyte and may penetrate the separator, resulting in a short circuit of the battery. In addition, the conventional liquid organic electrolyte is flammable and easy to leak, posing safety concerns regarding fire and explosion risks. To address these issues, solid-state electrolytes are considered as a particularly ideal alternative because of their desirable mechanical properties, highly reduced flammability, and reduced risk of leakage. Such properties are expected to prevent Li dendrite growth, mitigate structural damage of the Li anode, and improve battery safety. Nonetheless, it is still a great challenge to manufacture solid-state batteries with high areal capacity and good rate performance stems from the high interfacial resistance between the electrolyte and electrode, which hinders Li-ion transport. Therefore, understanding and addressing the general interface issues in solid-state batteries is key to manufacturing high-performance solid-state lithium batteries. Interface issues in solid-state batteries are highly complex and may be broadly categorized into chemical/electrochemical interface and physical interface problems. The chemical/electrochemical interface problem comprises the narrow electrochemical stability window, elemental interdiffusion, and space charge layers, while the physical interface problem can be divided into rigid interfacial contact, volume change during cycling, and fracture and pulverization caused by stress accumulation. Previous reports represent a relatively comprehensive summary of the methods to solve the chemical/electrochemical interface problems but do not discuss in detail the influence of physical interfaces in solid-state batteries of different structures and the related addressing strategies. First, this review will briefly introduce the chemical/electrochemical interface problems and their solutions. Then, solid-state lithium batteries are divided into divided into the sandwich structure, powder composite structure, and 3D integrated structure, according to the key structural characteristics; the physical interface characteristics and optimization strategies of different battery structures are further analyzed in detail, and the advantages and disadvantages of each system are compared and analyzed. Finally, the future research direction of the electrode/electrolyte interface in solid-state lithium batteries is presented. ![]()
With the rapid development of electric vehicles and intelligent electronics, Li-based batteries are required to have a higher specific capacity and better safety. To develop batteries with higher energy densities, Li may be used as an anode material owing to its higher theoretical capacity (3860 mAh·g−1, 10 times higher than graphite) and low redox potential (−3.04 V vs. the standard hydrogen electrode). However, uncontrolled Li dendrite growth may occur during electrochemical Li plating/stripping in the liquid electrolyte and may penetrate the separator, resulting in a short circuit of the battery. In addition, the conventional liquid organic electrolyte is flammable and easy to leak, posing safety concerns regarding fire and explosion risks. To address these issues, solid-state electrolytes are considered as a particularly ideal alternative because of their desirable mechanical properties, highly reduced flammability, and reduced risk of leakage. Such properties are expected to prevent Li dendrite growth, mitigate structural damage of the Li anode, and improve battery safety. Nonetheless, it is still a great challenge to manufacture solid-state batteries with high areal capacity and good rate performance stems from the high interfacial resistance between the electrolyte and electrode, which hinders Li-ion transport. Therefore, understanding and addressing the general interface issues in solid-state batteries is key to manufacturing high-performance solid-state lithium batteries. Interface issues in solid-state batteries are highly complex and may be broadly categorized into chemical/electrochemical interface and physical interface problems. The chemical/electrochemical interface problem comprises the narrow electrochemical stability window, elemental interdiffusion, and space charge layers, while the physical interface problem can be divided into rigid interfacial contact, volume change during cycling, and fracture and pulverization caused by stress accumulation. Previous reports represent a relatively comprehensive summary of the methods to solve the chemical/electrochemical interface problems but do not discuss in detail the influence of physical interfaces in solid-state batteries of different structures and the related addressing strategies. First, this review will briefly introduce the chemical/electrochemical interface problems and their solutions. Then, solid-state lithium batteries are divided into divided into the sandwich structure, powder composite structure, and 3D integrated structure, according to the key structural characteristics; the physical interface characteristics and optimization strategies of different battery structures are further analyzed in detail, and the advantages and disadvantages of each system are compared and analyzed. Finally, the future research direction of the electrode/electrolyte interface in solid-state lithium batteries is presented.
2023, 39(8): 230101
doi: 10.3866/PKU.WHXB202301019
Abstract:
Lithium-sulfur batteries are one of the prospective next-generation power sources that can replace commercial lithium-ion batteries owing to their high theoretical energy density, eco-friendliness, and low cost. However, the insulating nature of the charge–discharge products, the shuttle effect of soluble lithium polysulfides, the volume expansion of the sulfur cathode, and the uncontrollable growth of lithium dendrites severely affect the actual capacity and cycling stability of lithium-sulfur batteries. Replacing the inorganic sulfur (S8) cathode with an organosulfur-based cathode is a promising strategy for resolving the aforementioned issues. By modulating the fundamental units of the organosulfur compound, including the sulfur chain, carbon chain, and their interactions, the electrochemical reaction process can be altered, the ion/electron conductivity can be increased, and the shuttle effect can be effectively suppressed. In addition, organosulfur compounds as electrolyte additives can regulate the reaction process of the sulfur cathode and protect the lithium anode by forming a stable solid electrolyte interface, and as polymer electrolyte segments, they can accelerate the conduction of lithium ions. This review provides a detailed outline of the research progress and application of organosulfur compounds as cathodes, electrolyte additives, and solid-state electrolytes in lithium-sulfur batteries. The structure, reaction mechanism, and electrochemical properties of organosulfur compounds are correlated to provide comprehensive insights that can help address the prevailing issues of lithium-sulfur batteries. Finally, future prospects, including the challenges and potential solutions, are presented to guide the design, synthesis, and mechanistic studies of high-performance organosulfur compounds to realize a practical lithium-sulfur battery. ![]()
Lithium-sulfur batteries are one of the prospective next-generation power sources that can replace commercial lithium-ion batteries owing to their high theoretical energy density, eco-friendliness, and low cost. However, the insulating nature of the charge–discharge products, the shuttle effect of soluble lithium polysulfides, the volume expansion of the sulfur cathode, and the uncontrollable growth of lithium dendrites severely affect the actual capacity and cycling stability of lithium-sulfur batteries. Replacing the inorganic sulfur (S8) cathode with an organosulfur-based cathode is a promising strategy for resolving the aforementioned issues. By modulating the fundamental units of the organosulfur compound, including the sulfur chain, carbon chain, and their interactions, the electrochemical reaction process can be altered, the ion/electron conductivity can be increased, and the shuttle effect can be effectively suppressed. In addition, organosulfur compounds as electrolyte additives can regulate the reaction process of the sulfur cathode and protect the lithium anode by forming a stable solid electrolyte interface, and as polymer electrolyte segments, they can accelerate the conduction of lithium ions. This review provides a detailed outline of the research progress and application of organosulfur compounds as cathodes, electrolyte additives, and solid-state electrolytes in lithium-sulfur batteries. The structure, reaction mechanism, and electrochemical properties of organosulfur compounds are correlated to provide comprehensive insights that can help address the prevailing issues of lithium-sulfur batteries. Finally, future prospects, including the challenges and potential solutions, are presented to guide the design, synthesis, and mechanistic studies of high-performance organosulfur compounds to realize a practical lithium-sulfur battery.
2023, 39(8): 230102
doi: 10.3866/PKU.WHXB202301027
Abstract:
All-solid-state lithium batteries (ASSB) have emerged as key components in energy storage applications owing to their superior safety characteristics and high energy density. The use of sulfide solid electrolytes has considerably promoted the development of all-solid-state lithium batteries because of advantages such as a high ionic conductivity, formability, and good interface compatibility with electrodes. In this review, we first discuss the issues hindering the use of sulfide-based all-solid-state lithium batteries, focusing on aspects related to the cathode/electrolyte interface, sulfide solid electrolytes, and the anode/electrolyte interface. At the cathode/electrolyte interface, interfacial side reactions inherently occur due to the narrow electrochemical window of sulfide electrolytes when used with high-voltage cathode materials, which degrades the battery performance. In addition, owing to the chemical potential difference between cathode materials and sulfide solid electrolytes, the space-charge layer generated due to the formation of a lithium depletion layer is also detrimental to the cell performance. To overcome these difficulties, inert coatings, replacing sulfide solid electrolytes with halide solid electrolytes, and replacing frequently used transitional metal oxide cathode materials with other materials that are better suited for sulfide solid electrolytes to modify the composite cathode have been explored. Improvements in the ionic conductivity and air stability are imperative for sulfide solid electrolytes. Strategies to optimize the solid electrolyte have mainly focused on doping or adjusting the synthesis routes of the sulfide solid electrolyte, which have resulted in notable improvements. At the anode/electrolyte interface, lithium dendrite formation and interfacial reactions between lithium metal and the sulfide solid electrolyte are the most notable challenges. Using artificial solid electrolyte interfaces with a low electronic conductivity, employing an alloy anode, and synthesizing composite electrolytes are typical approaches for overcoming these problems. In addition, from the perspective of the practical production of sulfide-based all-solid-state lithium batteries, electrode/electrolyte membrane-forming technology and the assembly of pouch cells are introduced. Membrane-forming technology has gained extensive attention with the aim of fabricating thin and mechanically stronger solid electrolyte membranes. High-loading cathode membranes as well as solid electrolyte membranes, dry processing, and wet processing are reviewed. Moreover, the improvement in the solid-solid contact of pouch cells, the design of high-loading cathodes, and the low-cost and scaled up production of sulfide solid electrolytes are introduced. Finally, we also propose research directions and future development trends for sulfide-based all-solid-state lithium batteries. ![]()
All-solid-state lithium batteries (ASSB) have emerged as key components in energy storage applications owing to their superior safety characteristics and high energy density. The use of sulfide solid electrolytes has considerably promoted the development of all-solid-state lithium batteries because of advantages such as a high ionic conductivity, formability, and good interface compatibility with electrodes. In this review, we first discuss the issues hindering the use of sulfide-based all-solid-state lithium batteries, focusing on aspects related to the cathode/electrolyte interface, sulfide solid electrolytes, and the anode/electrolyte interface. At the cathode/electrolyte interface, interfacial side reactions inherently occur due to the narrow electrochemical window of sulfide electrolytes when used with high-voltage cathode materials, which degrades the battery performance. In addition, owing to the chemical potential difference between cathode materials and sulfide solid electrolytes, the space-charge layer generated due to the formation of a lithium depletion layer is also detrimental to the cell performance. To overcome these difficulties, inert coatings, replacing sulfide solid electrolytes with halide solid electrolytes, and replacing frequently used transitional metal oxide cathode materials with other materials that are better suited for sulfide solid electrolytes to modify the composite cathode have been explored. Improvements in the ionic conductivity and air stability are imperative for sulfide solid electrolytes. Strategies to optimize the solid electrolyte have mainly focused on doping or adjusting the synthesis routes of the sulfide solid electrolyte, which have resulted in notable improvements. At the anode/electrolyte interface, lithium dendrite formation and interfacial reactions between lithium metal and the sulfide solid electrolyte are the most notable challenges. Using artificial solid electrolyte interfaces with a low electronic conductivity, employing an alloy anode, and synthesizing composite electrolytes are typical approaches for overcoming these problems. In addition, from the perspective of the practical production of sulfide-based all-solid-state lithium batteries, electrode/electrolyte membrane-forming technology and the assembly of pouch cells are introduced. Membrane-forming technology has gained extensive attention with the aim of fabricating thin and mechanically stronger solid electrolyte membranes. High-loading cathode membranes as well as solid electrolyte membranes, dry processing, and wet processing are reviewed. Moreover, the improvement in the solid-solid contact of pouch cells, the design of high-loading cathodes, and the low-cost and scaled up production of sulfide solid electrolytes are introduced. Finally, we also propose research directions and future development trends for sulfide-based all-solid-state lithium batteries.
2023, 39(8): 221205
doi: 10.3866/PKU.WHXB202212053
Abstract:
The realization of carbon and nitrogen cycles is an urgent requirement for the development of human society, and is also a hot research topic in the field of catalysis. Electrocatalysis driven by renewable energy has attracted considerable attention, and the target products can be obtained by varying the applied potentials. Accordingly, electrocatalysis is considered to be an effective strategy to alleviate the current energy crisis and environmental problems and is of great significance in realizing carbon neutrality. Electrocatalytic CO2 reduction reaction (CO2RR) and N2 reduction reaction (N2RR) are also promising strategies for the conversion of small molecules. However, the high dissociation energies of the C＝O and N≡N bonds in the linear molecules of CO2 and N2, respectively, lead to their high chemical inactivity. In addition, the large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) further results in high chemical stability. Besides, the low proton affinity of CO2 and N2 makes direct protonated difficult. However, because of the similar redox potentials of CO2RR, N2RR, and hydrogen evolution reaction (HER), HER competes with CO2RR and N2RR, affecting the CO2RR and N2RR performance. Therefore, both CO2RR and N2RR still face challenges, such as high overpotential and low Faradaic efficiency. To overcome these bottlenecks, considerable efforts have been made to improve the performance of the CO2RR and N2RR electrocatalysts. The electrocatalytic process primarily occurs on the catalyst surface and involves mass diffusion and electron transfer; thus, the performance of the catalysts is closely related to their mass and electron transfer abilities. Modulating the catalyst surface structure can regulate the mass and electron transfer behavior of the active sites during the electrocatalytic process. Defect and interface engineering of electrocatalysts is important for enhancing the adsorption of gas, inhibiting HER, enriching the gas, stabilizing the intermediates, and modifying the electronic structure by engineering the surface atoms. To date, various defective and composite electrocatalysts have shown great potential to enhance the CO2RR and N2RR performance. Herein, recent advances in defect and interface engineering for CO2RR and N2RR are reviewed. The effects of four different defects (vacancy, high-index facet, lattice stain, and lattice disorder) on the CO2RR and N2RR performance are discussed. Then, the main roles of interface engineering of polymer-inorganic composite catalysts are further reviewed, and representative examples are presented. Finally, the opportunities and challenges for defect and interface engineering in the electroreduction of CO2 and N2 are also proposed, suggesting directions for the future development of highly efficient CO2RR and N2RR catalysts. ![]()
The realization of carbon and nitrogen cycles is an urgent requirement for the development of human society, and is also a hot research topic in the field of catalysis. Electrocatalysis driven by renewable energy has attracted considerable attention, and the target products can be obtained by varying the applied potentials. Accordingly, electrocatalysis is considered to be an effective strategy to alleviate the current energy crisis and environmental problems and is of great significance in realizing carbon neutrality. Electrocatalytic CO2 reduction reaction (CO2RR) and N2 reduction reaction (N2RR) are also promising strategies for the conversion of small molecules. However, the high dissociation energies of the C＝O and N≡N bonds in the linear molecules of CO2 and N2, respectively, lead to their high chemical inactivity. In addition, the large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) further results in high chemical stability. Besides, the low proton affinity of CO2 and N2 makes direct protonated difficult. However, because of the similar redox potentials of CO2RR, N2RR, and hydrogen evolution reaction (HER), HER competes with CO2RR and N2RR, affecting the CO2RR and N2RR performance. Therefore, both CO2RR and N2RR still face challenges, such as high overpotential and low Faradaic efficiency. To overcome these bottlenecks, considerable efforts have been made to improve the performance of the CO2RR and N2RR electrocatalysts. The electrocatalytic process primarily occurs on the catalyst surface and involves mass diffusion and electron transfer; thus, the performance of the catalysts is closely related to their mass and electron transfer abilities. Modulating the catalyst surface structure can regulate the mass and electron transfer behavior of the active sites during the electrocatalytic process. Defect and interface engineering of electrocatalysts is important for enhancing the adsorption of gas, inhibiting HER, enriching the gas, stabilizing the intermediates, and modifying the electronic structure by engineering the surface atoms. To date, various defective and composite electrocatalysts have shown great potential to enhance the CO2RR and N2RR performance. Herein, recent advances in defect and interface engineering for CO2RR and N2RR are reviewed. The effects of four different defects (vacancy, high-index facet, lattice stain, and lattice disorder) on the CO2RR and N2RR performance are discussed. Then, the main roles of interface engineering of polymer-inorganic composite catalysts are further reviewed, and representative examples are presented. Finally, the opportunities and challenges for defect and interface engineering in the electroreduction of CO2 and N2 are also proposed, suggesting directions for the future development of highly efficient CO2RR and N2RR catalysts.
