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Layered transition-metal oxides have attracted intensive interest for cathode materials of sodium-ion batteries. However, they are hindered by the limited capacity and inferior phase transition due to the gliding of transition-metal layers upon Na+ extraction and insertion in the cathode materials. Here, we report that the large-sized K+ is riveted in the prismatic Na+ sites of P2-Na0.612K0.056MnO2 to enable more thermodynamically favorable Na+ vacancies. The Mn-O bonds are reinforced to reduce phase transition during charge and discharge. 0.901 Na+ per formula are reversibly extracted and inserted, in which only the two-phase transition of P2 ↔ P'2 occurs at low voltages. It exhibits the highest specific capacity of 240.5 mAh g-1 and energy density of 654 Wh kg-1 based on the redox of Mn3+/Mn4+, and a capacity retention of 98.2% after 100 cycles. This investigation will shed lights on the tuneable chemical environments of transition-metal oxides for advanced cathode materials and promote the development of sodium-ion batteries.

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3 Co1/3 Mn1/3 O2 . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Redox reactions of oxygen have been considered critical in controlling the electrochemical properties of lithium-excessive layered-oxide electrodes. However, conventional electrode materials without overlithiation remain the most practical. Typically, cationic redox reactions are believed to dominate the electrochemical processes in conventional electrodes. Herein, we show unambiguous evidence of reversible anionic redox reactions in LiNi1/3 Co1/3 Mn1/3 O2 . The typical involvement of oxygen through hybridization with transition metals is discussed, as well as the intrinsic oxygen redox process at high potentials, which is 75 % reversible during initial cycling and 63 % retained after 10 cycles. Our results clarify the reaction mechanism at high potentials in conventional layered electrodes involving both cationic and anionic reactions and indicate the potential of utilizing reversible oxygen redox reactions in conventional layered oxides for high-capacity lithium-ion batteries.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn–Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na0.833[Li0.25Mn0.75]O2 can not only suppress the Jahn–Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn–Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na0.833[Li0.25Mn0.75]O2 can not only suppress the Jahn–Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

This work reports the fabrication of a three-dimensional (3D) zinc oxide/zinc cobaltite (ZnO/ZnCo2O4) hybrid with a hierarchical macroporous honeycomb-like structure using highly uniform cobalt glycerate spheres as a self-sacrificial template. In the proposed method, the conversion of the template cobalt glycerate nanospheres into a 3D hierarchical macroporous honeycomb-like ZnO/ZnCo2O4 hybrid is achieved via a facile room-temperature reaction with aqueous zinc nitrate solution, followed by calcination in air at 350 °C. The proposed method offers several benefits including (i) the attainment of the ZnO/ZnCo2O4 hybrid in one step without additional or separate coating steps, (ii) the achievement of a unique 3D hierarchical macroporous honeycomb-like structure with interconnecting nanosheets and macropores which are assembled from smaller mesopores, leading to higher surface area and good interparticle separation, (iii) the relatively low calcination temperature required to obtain the ZnO/ZnCo2O4 hybrid (350 °C) and (iv) potential generalization for the creation of other macroporous honeycomb-like cobalt-based oxide nanostructures (including Al–Co and Cu–Co systems). When evaluated as a sensing material for carbon monoxide (CO), the hierarchical honeycomb-like ZnO/ZnCo2O4 hybrid sensor displays a higher sensing response with enhanced selectivity and stability towards CO gas at 300 °C compared to both ZnO hierarchical spheres and ZnCo2O4 nanospheres. The enhanced sensing performance of the hierarchical honeycomb-like ZnO/ZnCo2O4 hybrid is derived from the synergistic cooperation of the formed p–n hetero

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn–Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na0.833[Li0.25Mn0.75]O2 can not only suppress the Jahn–Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

Manganese based layered oxides have received increasing attention as cathode materials for sodium ion batteries due to their high theoretical capacities and good sodium ion conductivities. However, the Jahn–Teller distortion arising from the manganese (III) centers destabilizes the host structure and deteriorates the cycling life. Herein, we report that zinc-doped Na0.833[Li0.25Mn0.75]O2 can not only suppress the Jahn–Teller effect but also reduce the inherent phase separations. The reduction of manganese (III) amount in the zinc-doped sample, as predicted by first-principles calculations, has been confirmed by its high binding energies and the reduced octahedral structural variations. In the viewpoint of thermodynamics, the zinc-doped sample has lower formation energy, more stable ground states, and fewer spinodal decomposition regions than those of the undoped sample, all of which make it charge or discharge without any phase transition. Hence, the zinc-doped sample shows superior cycling performance, demonstrating that zinc doping is an effective strategy for developing high-performance layered cathode materials.

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe 3 C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m 2 g -1 ), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe 3 C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe 3 C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H 2 O 2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe 3 C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Low efficiency, short lifetimes, and limited kinds of catalysts are still three fundamental shortcomings that have plagued electrochemical water splitting. Herein, we rationally synthesized a cost-effective Co 3 S 4 @MoS 2 hetero-structured catalyst that has proven to be a highly active and stable bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The heterostructure was obtained via a first hydrothermal approach to obtain hollow Co 3 S 4 nanoboxes based on the ionic exchange reaction between Fe(CN) 6 3− of Co-Fe Prussian blue analogue (PBA) and S 2− at 120 °C, and the subsequent in situ growth of MoS 2 nanosheets on the surface of Co 3 S 4 nanoboxes at an elevated temperature of 200 °C. The synergistic effects between the active and stable HER catalyst of MoS 2 and the efficient OER catalyst of Co 3 S 4 , as well as the morphological superiority of hollow and core-shell structures, endow Co 3 S 4 @MoS 2 with remarkable electrocatalytic performance and robust durability toward overall water splitting. As a result, the designed non-noble electrocatalyst of Co 3 S 4 @MoS 2 exhibits a low overpotential of 280 mV for OER and 136 mV for HER at a current density of 10 mA cm −2 in an alkaline solution. Meanwhile, a low cell voltage of 1.58 V is achieved by using the heterostructure as both anode and cathode catalysts. This work paves the way to the design and construction of other prominent electrocatalysts for overall water splitting.

Dual ion batteries based on Na+and PF6-received considerable attention due to their high operating voltage and the abundant Na resources. Here, cheap and easily obtained graphite that served as a cathode material for dual ion battery delivered a very high average discharge platform (4.52 V vs Na+/Na) by using sodium hexafluorophosphate in propylene carbonate as electrolyte. Moreover, the all-carbon dual ion batteries with graphite as cathode and hard carbon as anode exhibited an ultrahigh discharge voltage of 4.3 V, and a reversible capacity of 62 mAh·g-1at 40 mA·g-1. Phase changes have been investigated in detail through in situ X-ray diffraction and in situ Raman characterizations. The stable structure provides long life cycling performance, and the pseudocapacitance behavior also demonstrates its benefits to the rate capability. Thus, dual ion batteries based on sodium chemistry are very promising to find their applications in future.

Since their successful commercialization in 1990s, lithium-ion batteries (LIBs) have been widely applied in portable digital products. The energy density and power density of LIBs are inadequate, however, to satisfy the continuous growth in demand. Considering the cost distribution in battery system, it is essential to explore cathode/anode materials with excellent rate capability and long cycle life. Nanometer-sized electrode materials could quickly take up and store numerous Li + ions, afforded by short diffusion channels and large surface area. Unfortunately, low thermodynamic stability of nanoparticles results in electrochemical agglomeration and raises the risk of side reactions on electrolyte. Thus, micro/nano and hetero/hierarchical structures, characterized by ordered assembly of different sizes, phases, and/or pores, have been developed, which enable us to effectively improve the utilization, reaction kinetics, and structural stability of electrode materials. This review summarizes the recent efforts on electrode materials with hierarchical structures, and discusses the effects of hierarchical structures on electrochemical performance in detail. Multidimensional self-assembled structures can achieve integration of the advantages of materials with different sizes. Core/yolk-shell structures provide synergistic effects between the shell and the core/yolk. Porous structures with macro-, meso-, and micropores can accommodate volume expansion and facilitate electrolyte infiltration.

Hexacyanoferrate (Prussian blue, PB)/reduced graphene oxide (PB-RGO) composites with a synergistic structure (graphene/PB/graphene) and a chemical bond are fabricated using a facile one-step method that does not require any external chemical reducing agent. Here, Na4Fe(CN)6 is decomposed in an acidic solution to produce Fe2+ ions, which anchor onto the electronegative graphene oxide (GO) layers by electrostatic interaction and then reduce the GO. The formation of an FeOC chemical bond in the composite results in an excellent rate capability of the PB-RGO composite at room temperature, delivering capacities of 78.1, 68.9, and 46.0 mAh g−1 even at the high rates of 10, 20, and 50 C, with a capacity retention of 70.2%, 63.4%, and 41.0%, respectively. The composite also shows an unprecedentedly outstanding cycling stability, retaining ≈90% of the initial capacity after 600 cycles.

Dual ion batteries based on Na+and PF6-received considerable attention due to their high operating voltage and the abundant Na resources. Here, cheap and easily obtained graphite that served as a cathode material for dual ion battery delivered a very high average discharge platform (4.52 V vs Na+/Na) by using sodium hexafluorophosphate in propylene carbonate as electrolyte. Moreover, the all-carbon dual ion batteries with graphite as cathode and hard carbon as anode exhibited an ultrahigh discharge voltage of 4.3 V, and a reversible capacity of 62 mAh·g-1at 40 mA·g-1. Phase changes have been investigated in detail through in situ X-ray diffraction and in situ Raman characterizations. The stable structure provides long life cycling performance, and the pseudocapacitance behavior also demonstrates its benefits to the rate capability. Thus, dual ion batteries based on sodium chemistry are very promising to find their applications in future.

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe 3 C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m 2 g -1 ), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe 3 C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe 3 C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H 2 O 2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe 3 C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Hexacyanoferrate (Prussian blue, PB)/reduced graphene oxide (PB-RGO) composites with a synergistic structure (graphene/PB/graphene) and a chemical bond are fabricated using a facile one-step method that does not require any external chemical reducing agent. Here, Na4Fe(CN)6 is decomposed in an acidic solution to produce Fe2+ ions, which anchor onto the electronegative graphene oxide (GO) layers by electrostatic interaction and then reduce the GO. The formation of an FeOC chemical bond in the composite results in an excellent rate capability of the PB-RGO composite at room temperature, delivering capacities of 78.1, 68.9, and 46.0 mAh g−1 even at the high rates of 10, 20, and 50 C, with a capacity retention of 70.2%, 63.4%, and 41.0%, respectively. The composite also shows an unprecedentedly outstanding cycling stability, retaining ≈90% of the initial capacity after 600 cycles.

Low efficiency, short lifetimes, and limited kinds of catalysts are still three fundamental shortcomings that have plagued electrochemical water splitting. Herein, we rationally synthesized a cost-effective Co 3 S 4 @MoS 2 hetero-structured catalyst that has proven to be a highly active and stable bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The heterostructure was obtained via a first hydrothermal approach to obtain hollow Co 3 S 4 nanoboxes based on the ionic exchange reaction between Fe(CN) 6 3− of Co-Fe Prussian blue analogue (PBA) and S 2− at 120 °C, and the subsequent in situ growth of MoS 2 nanosheets on the surface of Co 3 S 4 nanoboxes at an elevated temperature of 200 °C. The synergistic effects between the active and stable HER catalyst of MoS 2 and the efficient OER catalyst of Co 3 S 4 , as well as the morphological superiority of hollow and core-shell structures, endow Co 3 S 4 @MoS 2 with remarkable electrocatalytic performance and robust durability toward overall water splitting. As a result, the designed non-noble electrocatalyst of Co 3 S 4 @MoS 2 exhibits a low overpotential of 280 mV for OER and 136 mV for HER at a current density of 10 mA cm −2 in an alkaline solution. Meanwhile, a low cell voltage of 1.58 V is achieved by using the heterostructure as both anode and cathode catalysts. This work paves the way to the design and construction of other prominent electrocatalysts for overall water splitting.

Since their successful commercialization in 1990s, lithium-ion batteries (LIBs) have been widely applied in portable digital products. The energy density and power density of LIBs are inadequate, however, to satisfy the continuous growth in demand. Considering the cost distribution in battery system, it is essential to explore cathode/anode materials with excellent rate capability and long cycle life. Nanometer-sized electrode materials could quickly take up and store numerous Li + ions, afforded by short diffusion channels and large surface area. Unfortunately, low thermodynamic stability of nanoparticles results in electrochemical agglomeration and raises the risk of side reactions on electrolyte. Thus, micro/nano and hetero/hierarchical structures, characterized by ordered assembly of different sizes, phases, and/or pores, have been developed, which enable us to effectively improve the utilization, reaction kinetics, and structural stability of electrode materials. This review summarizes the recent efforts on electrode materials with hierarchical structures, and discusses the effects of hierarchical structures on electrochemical performance in detail. Multidimensional self-assembled structures can achieve integration of the advantages of materials with different sizes. Core/yolk-shell structures provide synergistic effects between the shell and the core/yolk. Porous structures with macro-, meso-, and micropores can accommodate volume expansion and facilitate electrolyte infiltration.

Low efficiency, short lifetimes, and limited kinds of catalysts are still three fundamental shortcomings that have plagued electrochemical water splitting. Herein, we rationally synthesized a cost-effective Co 3 S 4 @MoS 2 hetero-structured catalyst that has proven to be a highly active and stable bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The heterostructure was obtained via a first hydrothermal approach to obtain hollow Co 3 S 4 nanoboxes based on the ionic exchange reaction between Fe(CN) 6 3− of Co-Fe Prussian blue analogue (PBA) and S 2− at 120 °C, and the subsequent in situ growth of MoS 2 nanosheets on the surface of Co 3 S 4 nanoboxes at an elevated temperature of 200 °C. The synergistic effects between the active and stable HER catalyst of MoS 2 and the efficient OER catalyst of Co 3 S 4 , as well as the morphological superiority of hollow and core-shell structures, endow Co 3 S 4 @MoS 2 with remarkable electrocatalytic performance and robust durability toward overall water splitting. As a result, the designed non-noble electrocatalyst of Co 3 S 4 @MoS 2 exhibits a low overpotential of 280 mV for OER and 136 mV for HER at a current density of 10 mA cm −2 in an alkaline solution. Meanwhile, a low cell voltage of 1.58 V is achieved by using the heterostructure as both anode and cathode catalysts. This work paves the way to the design and construction of other prominent electrocatalysts for overall water splitting.

Dual ion batteries based on Na+and PF6-received considerable attention due to their high operating voltage and the abundant Na resources. Here, cheap and easily obtained graphite that served as a cathode material for dual ion battery delivered a very high average discharge platform (4.52 V vs Na+/Na) by using sodium hexafluorophosphate in propylene carbonate as electrolyte. Moreover, the all-carbon dual ion batteries with graphite as cathode and hard carbon as anode exhibited an ultrahigh discharge voltage of 4.3 V, and a reversible capacity of 62 mAh·g-1at 40 mA·g-1. Phase changes have been investigated in detail through in situ X-ray diffraction and in situ Raman characterizations. The stable structure provides long life cycling performance, and the pseudocapacitance behavior also demonstrates its benefits to the rate capability. Thus, dual ion batteries based on sodium chemistry are very promising to find their applications in future.

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe 3 C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m 2 g -1 ), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe 3 C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe 3 C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H 2 O 2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe 3 C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Hexacyanoferrate (Prussian blue, PB)/reduced graphene oxide (PB-RGO) composites with a synergistic structure (graphene/PB/graphene) and a chemical bond are fabricated using a facile one-step method that does not require any external chemical reducing agent. Here, Na4Fe(CN)6 is decomposed in an acidic solution to produce Fe2+ ions, which anchor onto the electronegative graphene oxide (GO) layers by electrostatic interaction and then reduce the GO. The formation of an FeOC chemical bond in the composite results in an excellent rate capability of the PB-RGO composite at room temperature, delivering capacities of 78.1, 68.9, and 46.0 mAh g−1 even at the high rates of 10, 20, and 50 C, with a capacity retention of 70.2%, 63.4%, and 41.0%, respectively. The composite also shows an unprecedentedly outstanding cycling stability, retaining ≈90% of the initial capacity after 600 cycles.

Dual ion batteries based on Na+and PF6-received considerable attention due to their high operating voltage and the abundant Na resources. Here, cheap and easily obtained graphite that served as a cathode material for dual ion battery delivered a very high average discharge platform (4.52 V vs Na+/Na) by using sodium hexafluorophosphate in propylene carbonate as electrolyte. Moreover, the all-carbon dual ion batteries with graphite as cathode and hard carbon as anode exhibited an ultrahigh discharge voltage of 4.3 V, and a reversible capacity of 62 mAh·g-1at 40 mA·g-1. Phase changes have been investigated in detail through in situ X-ray diffraction and in situ Raman characterizations. The stable structure provides long life cycling performance, and the pseudocapacitance behavior also demonstrates its benefits to the rate capability. Thus, dual ion batteries based on sodium chemistry are very promising to find their applications in future.

Dual ion batteries based on Na+and PF6-received considerable attention due to their high operating voltage and the abundant Na resources. Here, cheap and easily obtained graphite that served as a cathode material for dual ion battery delivered a very high average discharge platform (4.52 V vs Na+/Na) by using sodium hexafluorophosphate in propylene carbonate as electrolyte. Moreover, the all-carbon dual ion batteries with graphite as cathode and hard carbon as anode exhibited an ultrahigh discharge voltage of 4.3 V, and a reversible capacity of 62 mAh·g-1at 40 mA·g-1. Phase changes have been investigated in detail through in situ X-ray diffraction and in situ Raman characterizations. The stable structure provides long life cycling performance, and the pseudocapacitance behavior also demonstrates its benefits to the rate capability. Thus, dual ion batteries based on sodium chemistry are very promising to find their applications in future.

Low efficiency, short lifetimes, and limited kinds of catalysts are still three fundamental shortcomings that have plagued electrochemical water splitting. Herein, we rationally synthesized a cost-effective Co 3 S 4 @MoS 2 hetero-structured catalyst that has proven to be a highly active and stable bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The heterostructure was obtained via a first hydrothermal approach to obtain hollow Co 3 S 4 nanoboxes based on the ionic exchange reaction between Fe(CN) 6 3− of Co-Fe Prussian blue analogue (PBA) and S 2− at 120 °C, and the subsequent in situ growth of MoS 2 nanosheets on the surface of Co 3 S 4 nanoboxes at an elevated temperature of 200 °C. The synergistic effects between the active and stable HER catalyst of MoS 2 and the efficient OER catalyst of Co 3 S 4 , as well as the morphological superiority of hollow and core-shell structures, endow Co 3 S 4 @MoS 2 with remarkable electrocatalytic performance and robust durability toward overall water splitting. As a result, the designed non-noble electrocatalyst of Co 3 S 4 @MoS 2 exhibits a low overpotential of 280 mV for OER and 136 mV for HER at a current density of 10 mA cm −2 in an alkaline solution. Meanwhile, a low cell voltage of 1.58 V is achieved by using the heterostructure as both anode and cathode catalysts. This work paves the way to the design and construction of other prominent electrocatalysts for overall water splitting.

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe 3 C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m 2 g -1 ), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe 3 C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe 3 C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H 2 O 2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe 3 C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Hexacyanoferrate (Prussian blue, PB)/reduced graphene oxide (PB-RGO) composites with a synergistic structure (graphene/PB/graphene) and a chemical bond are fabricated using a facile one-step method that does not require any external chemical reducing agent. Here, Na4Fe(CN)6 is decomposed in an acidic solution to produce Fe2+ ions, which anchor onto the electronegative graphene oxide (GO) layers by electrostatic interaction and then reduce the GO. The formation of an FeOC chemical bond in the composite results in an excellent rate capability of the PB-RGO composite at room temperature, delivering capacities of 78.1, 68.9, and 46.0 mAh g−1 even at the high rates of 10, 20, and 50 C, with a capacity retention of 70.2%, 63.4%, and 41.0%, respectively. The composite also shows an unprecedentedly outstanding cycling stability, retaining ≈90% of the initial capacity after 600 cycles.

Hexacyanoferrate (Prussian blue, PB)/reduced graphene oxide (PB-RGO) composites with a synergistic structure (graphene/PB/graphene) and a chemical bond are fabricated using a facile one-step method that does not require any external chemical reducing agent. Here, Na4Fe(CN)6 is decomposed in an acidic solution to produce Fe2+ ions, which anchor onto the electronegative graphene oxide (GO) layers by electrostatic interaction and then reduce the GO. The formation of an FeOC chemical bond in the composite results in an excellent rate capability of the PB-RGO composite at room temperature, delivering capacities of 78.1, 68.9, and 46.0 mAh g−1 even at the high rates of 10, 20, and 50 C, with a capacity retention of 70.2%, 63.4%, and 41.0%, respectively. The composite also shows an unprecedentedly outstanding cycling stability, retaining ≈90% of the initial capacity after 600 cycles.

Sub-50 nm iron-nitrogen-doped hollow carbon sphere-encapsulated iron carbide nanoparticles (Fe 3 C-Fe,N/C) are synthesized by using a triblock copolymer of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) as a soft template. Their typical features, including a large surface area (879.5 m 2 g -1 ), small hollow size (≈16 nm), and nitrogen-doped mesoporous carbon shell, and encapsulated Fe 3 C nanoparticles generate a highly active oxygen reduction reaction (ORR) performance. Fe 3 C-Fe,N/C hollow spheres exhibit an ORR performance comparable to that of commercially available 20 wt% Pt/C in alkaline electrolyte, with a similar half-wave potential, an electron transfer number close to 4, and lower H 2 O 2 yield of less than 5%. It also shows noticeable ORR catalytic activity under acidic conditions, with a high half-wave potential of 0.714 V, which is only 59 mV lower than that of 20 wt% Pt/C. Moreover, Fe 3 C-Fe,N/C has remarkable long-term durability and tolerance to methanol poisoning, exceeding Pt/C regardless of the electrolyte.

Since their successful commercialization in 1990s, lithium-ion batteries (LIBs) have been widely applied in portable digital products. The energy density and power density of LIBs are inadequate, however, to satisfy the continuous growth in demand. Considering the cost distribution in battery system, it is essential to explore cathode/anode materials with excellent rate capability and long cycle life. Nanometer-sized electrode materials could quickly take up and store numerous Li + ions, afforded by short diffusion channels and large surface area. Unfortunately, low thermodynamic stability of nanoparticles results in electrochemical agglomeration and raises the risk of side reactions on electrolyte. Thus, micro/nano and hetero/hierarchical structures, characterized by ordered assembly of different sizes, phases, and/or pores, have been developed, which enable us to effectively improve the utilization, reaction kinetics, and structural stability of electrode materials. This review summarizes the recent efforts on electrode materials with hierarchical structures, and discusses the effects of hierarchical structures on electrochemical performance in detail. Multidimensional self-assembled structures can achieve integration of the advantages of materials with different sizes. Core/yolk-shell structures provide synergistic effects between the shell and the core/yolk. Porous structures with macro-, meso-, and micropores can accommodate volume expansion and facilitate electrolyte infiltration.

Since their successful commercialization in 1990s, lithium-ion batteries (LIBs) have been widely applied in portable digital products. The energy density and power density of LIBs are inadequate, however, to satisfy the continuous growth in demand. Considering the cost distribution in battery system, it is essential to explore cathode/anode materials with excellent rate capability and long cycle life. Nanometer-sized electrode materials could quickly take up and store numerous Li + ions, afforded by short diffusion channels and large surface area. Unfortunately, low thermodynamic stability of nanoparticles results in electrochemical agglomeration and raises the risk of side reactions on electrolyte. Thus, micro/nano and hetero/hierarchical structures, characterized by ordered assembly of different sizes, phases, and/or pores, have been developed, which enable us to effectively improve the utilization, reaction kinetics, and structural stability of electrode materials. This review summarizes the recent efforts on electrode materials with hierarchical structures, and discusses the effects of hierarchical structures on electrochemical performance in detail. Multidimensional self-assembled structures can achieve integration of the advantages of materials with different sizes. Core/yolk-shell structures provide synergistic effects between the shell and the core/yolk. Porous structures with macro-, meso-, and micropores can accommodate volume expansion and facilitate electrolyte infiltration.

Low efficiency, short lifetimes, and limited kinds of catalysts are still three fundamental shortcomings that have plagued electrochemical water splitting. Herein, we rationally synthesized a cost-effective Co 3 S 4 @MoS 2 hetero-structured catalyst that has proven to be a highly active and stable bifunctional catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in an alkaline environment. The heterostructure was obtained via a first hydrothermal approach to obtain hollow Co 3 S 4 nanoboxes based on the ionic exchange reaction between Fe(CN) 6 3− of Co-Fe Prussian blue analogue (PBA) and S 2− at 120 °C, and the subsequent in situ growth of MoS 2 nanosheets on the surface of Co 3 S 4 nanoboxes at an elevated temperature of 200 °C. The synergistic effects between the active and stable HER catalyst of MoS 2 and the efficient OER catalyst of Co 3 S 4 , as well as the morphological superiority of hollow and core-shell structures, endow Co 3 S 4 @MoS 2 with remarkable electrocatalytic performance and robust durability toward overall water splitting. As a result, the designed non-noble electrocatalyst of Co 3 S 4 @MoS 2 exhibits a low overpotential of 280 mV for OER and 136 mV for HER at a current density of 10 mA cm −2 in an alkaline solution. Meanwhile, a low cell voltage of 1.58 V is achieved by using the heterostructure as both anode and cathode catalysts. This work paves the way to the design and construction of other prominent electrocatalysts for overall water splitting.

The non-aqueous lithium oxygen battery has been considered as one of the most promising energy storage systems owing to its potentially high energy density, exceeding that of any other existing storage system for storing sustainable and clean energy. The success of Li-O2 batteries could efficiently reduce greenhouse gas emissions and the consumption of non-renewable fossil fuels. How to achieve high round-trip efficiency, high capacity, and satisfactory cycling performance, however, is still a great challenge for the lithium oxygen battery. Thus, this report will point out what factors will affect the electrochemical performance and will aim to describe how to increase the electrochemical performance by air electrode optimization. Based on recent achievements, several approaches to solving this problem, such as synthesizing electrocatalysts with high catalytic activity and designing appropriate air electrode structures, are described in details. Also, recent progress associated with novel air electrode designs and understanding the reaction process is discussed.

Developing hierarchically layered cathode materials with high performance is key to further improving advanced lithium-ion batteries, but it is challenging to synthesize such systems with controllable shape on a large scale. We used a high-shear mixer to continuously prepare flower-like hydroxide precursors via a co-precipitation method in a continuous stirred-tank reactor (CSTR). After the lithiation reaction, hierarchical lithium transition-metal oxides with predominantly exposed electrochemically active {010} planes are produced. The electrochemical properties of these peculiar materials (i.e. LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.6 Co 0.2 Mn 0.2 O 2 and Li 1.2 Ni 0.2 Mn 0.6 O 2 ) are demonstrated to be excellent. Particularly, LiNi 1/3 Co 1/3 Mn 1/3 O 2 exhibits a durable high-rate capability with a capacity retention of 98.2% after 500 cycles at 20C between 2.7 and 4.3 V. This work provides a new technology to fabricate two-dimensional nanoarchitectured electrode materials. Moreover, the outstanding electrochemical properties of this hierarchically structured cathode material in combination with the scalable method make it highly interesting for commercialization.

Even though Na 3 V 2 (PO 4 ) 3 (NVP) is regarded as one of the next-generation cathode materials in sodium-ion batteries (SIBs), its undesirable rate performance due to its inherently low electrical conductivity has limited its application in demanding fields such as electric vehicles. Motivated by this fact, the present study profitably employed a conductive carbon grown in situ to obtain an NVP@C composite with a pomegranate-like structure by a simple sol-gel assisted hydrothermal technique. The as-prepared NVP@C composite consists of small carbon-coated NVP particles (¿200 nm) embedded in a conductive carbon matrix, which ensures short ion diffusion distances, percolating electron/ion conduction pathways and stable structural integrity. As a result, the pomegranate-structured NVP@C composite displayed remarkable overall electrochemical performance: a high discharge capacity (110 mA h g -1 at 1C), excellent rate capability (92 mA h g -1 even at 50C) and impressive cycling stability (capacity retention of 91.3% over 2000 cycles at 10C). Such a feasible and beneficial design provides a good strategy for other materials that require both size reduction and high electronic/ionic conductivity.

A hydroxide co-precipitation method is used to synthesize transition metal hydroxide (Ni1/3Co1/3Mn1/3(OH)2), which is the precursor for layer-structured LiNi1/3Co1/3Mn1/3O2. The optimum pH range for the preparation of Ni1/3Co1/3Mn1/3(OH)2 using a continuous stirred-tank reactor is calculated by taking into account the underlying chemical equilibria. The entire growth process of the Ni1/3Co1/3Mn1/3(OH)2 particles is investigated by monitoring the structure, morphology, particle size distribution, and tap density as a function of the reaction time. The results confirm that the co-precipitation reaction in the presence of ammonia started with the formation of crystal nuclei and (001) plane dominated nanosheets. The reaction ended with spherical and dense hydroxide precursors. The crystal growth mechanism was interpreted during the co-precipitation process, which involved the quick nucleation of primary particles followed by its slow aggregation and crystallization. The electrochemical properties of the final cathode materials with different morphologies are also studied. The results show that the electrochemical performances of the final LiNi1/3Co1/3Mn1/3O2 are strongly affected by the hierarchical structure of Ni1/3Co1/3Mn1/3(OH)2..

The non-aqueous lithium oxygen battery has been considered as one of the most promising energy storage systems owing to its potentially high energy density, exceeding that of any other existing storage system for storing sustainable and clean energy. The success of Li-O2 batteries could efficiently reduce greenhouse gas emissions and the consumption of non-renewable fossil fuels. How to achieve high round-trip efficiency, high capacity, and satisfactory cycling performance, however, is still a great challenge for the lithium oxygen battery. Thus, this report will point out what factors will affect the electrochemical performance and will aim to describe how to increase the electrochemical performance by air electrode optimization. Based on recent achievements, several approaches to solving this problem, such as synthesizing electrocatalysts with high catalytic activity and designing appropriate air electrode structures, are described in details. Also, recent progress associated with novel air electrode designs and understanding the reaction process is discussed.

Developing hierarchically layered cathode materials with high performance is key to further improving advanced lithium-ion batteries, but it is challenging to synthesize such systems with controllable shape on a large scale. We used a high-shear mixer to continuously prepare flower-like hydroxide precursors via a co-precipitation method in a continuous stirred-tank reactor (CSTR). After the lithiation reaction, hierarchical lithium transition-metal oxides with predominantly exposed electrochemically active {010} planes are produced. The electrochemical properties of these peculiar materials (i.e. LiNi 1/3 Co 1/3 Mn 1/3 O 2 , LiNi 0.6 Co 0.2 Mn 0.2 O 2 and Li 1.2 Ni 0.2 Mn 0.6 O 2 ) are demonstrated to be excellent. Particularly, LiNi 1/3 Co 1/3 Mn 1/3 O 2 exhibits a durable high-rate capability with a capacity retention of 98.2% after 500 cycles at 20C between 2.7 and 4.3 V. This work provides a new technology to fabricate two-dimensional nanoarchitectured electrode materials. Moreover, the outstanding electrochemical properties of this hierarchically structured cathode material in combination with the scalable method make it highly interesting for commercialization.

Even though Na 3 V 2 (PO 4 ) 3 (NVP) is regarded as one of the next-generation cathode materials in sodium-ion batteries (SIBs), its undesirable rate performance due to its inherently low electrical conductivity has limited its application in demanding fields such as electric vehicles. Motivated by this fact, the present study profitably employed a conductive carbon grown in situ to obtain an NVP@C composite with a pomegranate-like structure by a simple sol-gel assisted hydrothermal technique. The as-prepared NVP@C composite consists of small carbon-coated NVP particles (¿200 nm) embedded in a conductive carbon matrix, which ensures short ion diffusion distances, percolating electron/ion conduction pathways and stable structural integrity. As a result, the pomegranate-structured NVP@C composite displayed remarkable overall electrochemical performance: a high discharge capacity (110 mA h g -1 at 1C), excellent rate capability (92 mA h g -1 even at 50C) and impressive cycling stability (capacity retention of 91.3% over 2000 cycles at 10C). Such a feasible and beneficial design provides a good strategy for other materials that require both size reduction and high electronic/ionic conductivity.

A hydroxide co-precipitation method is used to synthesize transition metal hydroxide (Ni1/3Co1/3Mn1/3(OH)2), which is the precursor for layer-structured LiNi1/3Co1/3Mn1/3O2. The optimum pH range for the preparation of Ni1/3Co1/3Mn1/3(OH)2 using a continuous stirred-tank reactor is calculated by taking into account the underlying chemical equilibria. The entire growth process of the Ni1/3Co1/3Mn1/3(OH)2 particles is investigated by monitoring the structure, morphology, particle size distribution, and tap density as a function of the reaction time. The results confirm that the co-precipitation reaction in the presence of ammonia started with the formation of crystal nuclei and (001) plane dominated nanosheets. The reaction ended with spherical and dense hydroxide precursors. The crystal growth mechanism was interpreted during the co-precipitation process, which involved the quick nucleation of primary particles followed by its slow aggregation and crystallization. The electrochemical properties of the final cathode materials with different morphologies are also studied. The results show that the electrochemical performances of the final LiNi1/3Co1/3Mn1/3O2 are strongly affected by the hierarchical structure of Ni1/3Co1/3Mn1/3(OH)2..

A hydroxide co-precipitation method is used to synthesize transition metal hydroxide (Ni1/3Co1/3Mn1/3(OH)2), which is the precursor for layer-structured LiNi1/3Co1/3Mn1/3O2. The optimum pH range for the preparation of Ni1/3Co1/3Mn1/3(OH)2 using a continuous stirred-tank reactor is calculated by taking into account the underlying chemical equilibria. The entire growth process of the Ni1/3Co1/3Mn1/3(OH)2 particles is investigated by monitoring the structure, morphology, particle size distribution, and tap density as a function of the reaction time. The results confirm that the co-precipitation reaction in the presence of ammonia started with the formation of crystal nuclei and (001) plane dominated nanosheets. The reaction ended with spherical and dense hydroxide precursors. The crystal growth mechanism was interpreted during the co-precipitation process, which involved the quick nucleation of primary particles followed by its slow aggregation and crystallization. The electrochemical properties of the final cathode materials with different morphologies are also studied. The results show that the electrochemical performances of the final LiNi1/3Co1/3Mn1/3O2 are strongly affected by the hierarchical structure of Ni1/3Co1/3Mn1/3(OH)2..