Observation of Pseudocapacitive Effect and Fast Ion Diffusion in Bimetallic Sulfides as an Advanced Sodium‐Ion Battery Anode

Sodium‐ion batteries (SIBs) are promising next‐generation alternatives due to the low cost and abundance of sodium sources. Yet developmental electrodes in SIBs such as transition metal sulfides have huge volume expansion, sluggish Na⁺ diffusion kinetics, and poor electrical conductivity. Here bimetallic sulfide (Co₉S₈/ZnS) nanocrystals embedded in hollow nitrogen‐doped carbon nanosheets are demonstrated with a high sodium diffusion coefficient, pseudocapacitive effect, and excellent reversibility. Such a unique composite structure is designed and synthesized via a facile sulfidation of the CoZn‐MOFs followed by calcination and is highly dependant on the reaction time and temperature. The optimized Co₁Zn₁‐S(600) electrode exhibits excellent sodium storage performance, including a high capacity of 542 mA h g^?1 at 0.1 A g^?1, good rate capability at 10 A g^?1, and excellent cyclic stability up to 500 cycles for half‐cell. It also shows potential in full‐cell configuration. Such capabilities will accelerate the adoption of sodium‐ion batteries for electrical energy applications.     

Synthesis and Operando Sodiation Mechanistic Study of Nitrogen‐Doped Porous Carbon Coated Bimetallic Sulfide Hollow Nanocubes as Advanced Sodium Ion Battery Anode

Due to the obvious advantages of utilizing naturally abundant and low cost sodium resources, sodium ion batteries (SIBs) show great potential for large‐scale energy storage applications. And the high theoretical capacities of transition metal sulfides (TMSs) make them appealing anode materials for SIBs; however, structural collapse caused by the severe volume change during de/sodiation processes results in poor capacity retention and rate capabilities. Compared to the development of new materials and the improvement of their electrochemical performance, the studies on their reaction mechanisms are still rare, especially the operando characterizations. Herein, the synthesis, anode application, and the operando observation of the de/sodiation mechanism of a nitrogen‐doped porous carbon coated nickel cobalt bimetallic sulfide hollow nanocube ((Ni_0.5Co_0.5)₉S₈@NC) composite are reported. Such a material is synthesized via facile sulfidation of phenol formaldehyde coated Ni₃[Co(CN)₆]₂ metal–organic framework precursors with Na₂S followed by calcination. The nanocomposite displays a remarkable specific capacity of 752 mAh g^?1 at 100 mA g^?1 after 100 cycles and outstanding rate capability due to the synergistic effect of several appealing features. Particularly, the pseudocapacitive effect appears to substantially contribute to the sodium storage capability. Operando X‐ray diffraction reveals the conversion reaction mechanism of (Ni_0.5Co_0.5)₉S₈@NC, forming Ni, Co, Na₂S, and Na₂S₅.     

In Situ Formation of Co9S8 Quantum Dots in MOF‐Derived Ternary Metal Layered Double Hydroxide Nanoarrays for High‐Performance Hybrid Supercapacitors

Layered double hydroxides (LDHs) are promising cathode materials for supercapacitors because of the enhanced flow efficiency of ions in the interlayers. However, the limited active sites and monotonous metal species further hinder the improvement of the capacity performance. Herein, cobalt sulfide quantum dots (Co₉S₈‐QDs) are effectively created and embedded within the interlayer of metal‐organic‐frameworks‐derived ternary metal LDH nanosheets based on in situ selective vulcanization of Co on carbon fibers. The hybrid CF@NiCoZn‐LDH/Co₉S₈‐QD retains the lamellar structure of the ternary metal LDH very well, inheriting low transfer impedance of interlayer ions. Significantly, the selectively generated Co₉S₈‐QDs expose more abundant active sites, effectively improving the electrochemical properties, such as capacitive performance, electronic conductivity, and cycling stability. Due to the synergistic relationship, the hybrid material delivers an ultrahigh electrochemical capacity of 350.6 mAh g^?1 (2504 F g^?1) at 1 A g^?1. Furthermore, hybrid supercapacitors fabricated with CF@NiCoZn‐LDH/Co₉S₈‐QD and carbon nanosheets modified by single‐walled carbon nanotubes display an outstanding energy density of 56.4 Wh kg^?1 at a power density of 875 W kg^?1, with an excellent capacity retention of 95.3% after 8000 charge–discharge cycles. Therefore, constructing hybrid electrode materials by in situ‐created QDs in multimetallic LDHs is promising.     

Nanoconfined Carbon‐Coated Na3V2(PO4)3 Particles in Mesoporous Carbon Enabling Ultralong Cycle Life for Sodium‐Ion Batteries

Na₃V₂(PO₄)₃ (denoted as NVP) has been considered as a promising cathode material for room temperature sodium ion batteries. Nevertheless, NVP suffers from poor rate capability resulting from the low electronic conductivity. Here, the feasibility to approach high rate capability by designing carbon‐coated NVP nanoparticles confined into highly ordered mesoporous carbon CMK‐3 matrix (NVP@C@CMK‐3) is reported. The NVP@C@CMK‐3 is prepared by a simple nanocasting technique. The electrode exhibits superior rate capability and ultralong cyclability (78 mA h g^?1 at 5 C after 2000 cycles) compared to carbon‐coated NVP and pure NVP cathode. The improved electrochemical performance is attributed to double carbon coating design that combines a variety of advantages: very short diffusion length of Na⁺/e^? in NVP, easy access of electrolyte, and short transport path of Na⁺ through carbon toward the NVP nanoparticle, high conductivity transport of electrons through the 3D interconnected channels of carbon host. The optimum design of the core–shell nanostructures with double carbon coating permits fast kinetics for both transported Na⁺ ions and electrons, enabling high‐power performance.     

A 3D Trilayered CNT/MoSe2/C Heterostructure with an Expanded MoSe2 Interlayer Spacing for an Efficient Sodium Storage

Freestanding composite structures with embedded transition metal dichalcogenides (TMDCs) as the active material are highly attractive in the development of advanced electrodes for energy storage devices. Most 3D electrodes consist of a bilayer design involving a core–shell combination. To further enhance the gravimetric and areal capacities, a 3D trilayer design is proposed that has MoSe₂ as the TMDC sandwiched in‐between an inner carbon nanotube (CNT) core and an outer carbon layer to form a CNT/MoSe₂/C framework. The CNT core creates interconnected pathways for the e^?/Na⁺ conduction, while the conductive inert carbon layer not only protects the corrosive environment between the electrolyte and MoSe₂ but also is fully tunable for an optimized Na⁺ storage. This unique heterostructure is synthesized via a solvothermal‐carbonization approach. Due to annealing under a confined structural configuration, MoSe₂ interlayer spaces are expanded to facilitate a faster Na⁺ diffusion. It is shown that an ≈3 nm thick carbon layer yielded an optimized anode for a sodium‐ion battery. The 3D porosity of the heterostructure remains intact after an intense densification process to produce a high areal capacity of 4.0 mAh cm^?2 and a high mass loading of 13.9 mg cm^?2 with a gravimetric capacity of 347 mAh g^?1 at 500 mA g^?1 after 500 cycles.     

Tailored Plum Pudding‐Like Co2P/Sn Encapsulated with Carbon Nanobox Shell as Superior Anode Materials for High‐Performance Sodium‐Ion Capacitors

Sodium‐ion capacitors (SICs) are emerging energy storage devices with high energy, high power, and durable life. Sn is a promising anode material for lithium storage, but the poor conductivity of the a‐NaSn phase upon sodaition hinders its implementation in SICs. Herein, a superior Sn‐based anode material consisting of plum pudding‐like Co₂P/Sn yolk encapsulated with nitrogen‐doped carbon nanobox (Co₂P/Sn@NC) for high‐performance SICs is reported. The 8–10 nm metallic nanoparticles produced in situ are uniformly dispersed in the amorphous Sn matrix serving as conductive fillers to facilitate electron transfer in spite of the formation of electrically resistive a‐NaSn phase during cycling. Meanwhile, the carbon shell buffers the large expansion of active Sn and provides a stable electrode–electrolyte interface. Owing to these merits, the yolk–shell Co₂P/Sn@NC demonstrates a large capacity of 394 mA h g^?1 at 100 mA g^?1, high rate capability of 168 mA h g^?1 at 5000 mA g^?1, and excellent cyclability with 87% capacity retention after 10 000 cycles. By integrating the Co₂P/Sn@NC anode with a peanut shell‐derived carbon cathode in the SIC, high energy densities of 112.3 and 43.7 Wh kg^?1 at power densities of 100 and 10 000 W kg^?1 are achieved.     

Ultrathin and Porous Ni3S2/CoNi2S4 3D‐Network Structure for Superhigh Energy Density Asymmetric Supercapacitors

3D‐networked, ultrathin, and porous Ni₃S₂/CoNi₂S₄ on Ni foam (NF) is successfully designed and synthesized by a simple sulfidation process from 3D Ni–Co precursors. Interestingly, the edge site‐enriched Ni₃S₂/CoNi₂S₄/NF 3D‐network is realized by the etching‐like effect of S^2? ions, which made the surfaces of Ni₃S₂/CoNi₂S₄/NF with a ridge‐like feature. The intriguing structural/compositional/componental advantages endow 3D‐networked‐free‐standing Ni₃S₂/CoNi₂S₄/NF electrodes better electrochemical performance with specific capacitance of 2435 F g^?1 at a current density of 2 A g^?1 and an excellent rate capability of 80% at 20 A g^?1. The corresponding asymmetric supercapacitor achieves a high energy density of 40.0 W h kg^?1 at an superhigh power density of 17.3 kW kg^?1, excellent specific capacitance (175 F g^?1 at 1A g^?1), and electrochemical cycling stability (92.8% retention after 6000 cycles) with Ni₃S₂/CoNi₂S₄/NF as the positive electrode and activated carbon/NF as the negative electrode. Moreover, the temperature dependences of cyclic voltammetry curve polarization and specific capacitances are carefully investigated, and become more obvious and higher, respectively, with the increase of test temperature. These can be attributed to the components' synergetic effect assuring rich redox reactions, high conductivity as well as highly porous but robust architectures. This work provides a general, low‐cost route to produce high performance electrode materials for portable supercapacitor applications on a large scale.     

Metal–Organic Frameworks‐Derived Tunnel Structured Co3(PO4)2@C as Cathode for New Generation High‐Performance Al‐Ion Batteries

Despite great progress in aluminum ion batteries (AIBs), the commercialization and performance improvement of AIBs‐based carbon cathodes is greatly impeded by sluggish intercalation/extraction and redox kinetics due to large‐sized AlCl₄^? anions. Phosphates with tunnel channels and much larger d‐spacing than the radius of Al³⁺ could be an alternative candidate as a cathode for potential high‐performance AIBs. Herein, elaborately designed porous tunnel structured Co₃(PO₄)₂@C composites derived from ZIF‐67 as AIBs cathodes are demonstrated, showing increased active sites, high ionic mobility, and high Al³⁺ ion diffusion coefficient, leading to remarkably enhanced discharge–charge redox reaction kinetics. Furthermore, the carbon shell and porous structure performs as armor to alleviate volume change and maintain the structure integrity of the cathodes. As expected, the rationally constructed Co₃(PO₄)₂@C composite exhibits a superior capacity of 111 mA h g^?1 at a high current density of 6 A g^?1 and 151 mA h g^?1 at 2 A g^?1 after 500 cycles with capacity decay of 0.02% per cycle. This innovative strategy could be a big step forward for long‐term cycle stable AIBs and reveals significant insights into the redox reaction mechanism for high‐performance AIBs based on Al³⁺ rather than large‐sized AlCl₄^?.     

Fractal (NixCo1−x)9Se8 Nanodendrite Arrays with Highly Exposed () Surface for Wearable,All‐Solid‐State Supercapacitor

Hierarchical nanostructures with highly exposed active surfaces for high‐performance pseudocapacitors have attracted considerable attention. Herein, a one‐step growth of (Ni _xCo_1?x)₉Se₈ solid solution series in various conductive substrates as advanced electrodes for flexible, foldable supercapacitors is developed. The formation of (Ni_xCo_1?x)₉Se₈ solid solution is confirmed by Vegard's law. Interestingly, the as‐grown (Ni_xCo_1?x)₉Se₈ solid solution series spontaneously crystallized into nanodendrite arrays with hierarchical morphology and fractal feature. The optimized (Ni_0.1Co_0.9)₉Se₈ nanodendrites deliver a specific capacitance of 3762 F g^?1 at a current density of 5 A g^?1 and remains 94.8% of the initial capacitance after 5000 cycles, owing to the advantage from fractal feature with numerous exposed () surface as well as fast ion diffusion. The as‐assembled flexible (Ni_0.1Co_0.9)₉Se₈@carbon fiber cloth (CFC)//PVA/KOH//reduced graphene oxide@CFC device exhibits an ultrahigh energy density of 17.0 Wh kg^?1@ 3.1 kW kg^?1, outperforming recently reported pseudocapacitors based on nickel‐cobalt sulfide and selenide counterparts. This study provides rational guidance toward the design of fractal feature with superior electrochemical performances due to the significantly increased electrochemical active sites. The resulting device can be easily folded, pulled, and twisted, enabling potential applications in high‐performance wearable and gadget devices.     

Radially Oriented Single‐Crystal Primary Nanosheets Enable Ultrahigh Rate and Cycling Properties of LiNi0.8Co0.1Mn0.1O2 Cathode Material for Lithium‐Ion Batteries

Ni‐rich Li[Ni_xCo_yMn_1?x?y]O₂ (x ≥ 0.8) layered oxides are the most promising cathode materials for lithium‐ion batteries due to their high reversible capacity of over 200 mAh g^?1. Unfortunately, the anisotropic properties associated with the α‐NaFeO₂ structured crystal grains result in poor rate capability and insufficient cycle life. To address these issues, a micrometer‐sized Ni‐rich LiNi_0.8Co_0.1Mn_0.1O₂ secondary cathode material consisting of radially aligned single‐crystal primary particles is proposed and synthesized. Concomitant with this unique crystallographic texture, all the exposed surfaces are active {010} facets, and 3D Li⁺ ion diffusion channels penetrate straightforwardly from surface to center, remarkably improving the Li⁺ diffusion coefficient. Moreover, coordinated charge–discharge volume change upon cycling is achieved by the consistent crystal orientation, significantly alleviating the volume‐change‐induced intergrain stress. Accordingly, this material delivers superior reversible capacity (203.4 mAh g^?1 at 3.0–4.3 V) and rate capability (152.7 mAh g^?1 at a current density of 1000 mA g^?1). Further, this structure demonstrates excellent cycling stability without any degradation after 300 cycles. The anisotropic morphology modulation provides a simple, efficient, and scalable way to boost the performance and applicability of Ni‐rich layered oxide cathode materials.     

Controllable Chain‐Length for Covalent Sulfur–Carbon Materials Enabling Stable and High‐Capacity Sodium Storage

Room temperature sodium–sulfur batteries have emerged as promising candidate for application in energy storage. However, the electrodes are usually obtained through infusing elemental sulfur into various carbon sources, and the precipitation of insoluble and irreversible sulfide species on the surface of carbon and sodium readily leads to continuous capacity degradation. Here, a novel strategy is demonstrated to prepare a covalent sulfur–carbon complex (SC‐BDSA) with high covalent‐sulfur concentration (40.1%) that relies on ? SO₃H (Benzenedisulfonic acid, BDSA) and SO₄^2? as the sulfur source rather than elemental sulfur. Most of the sulfur is exists in the form of O? S/C? S bridge‐bonds (short/long‐chain) whose features ensure sufficient interfacial contact and maintain high ionic/electronic conductivities of the sulfur–carbon cathode. Meanwhile, the carbon mesopores resulting from the thermal‐treated salt bath can confine a certain amount of sulfur and localize the diffluent polysulfides. Furthermore, the C? S_x? C bridges can be electrochemically broken at lower potential (<0.6 V vs Na/Na⁺) and then function as a capacity sponsor. And the R‐SO units can anchor the initially generated S_x^2? to form insoluble surface‐bound intermediates. Thus SC‐BDSA exhibits a specific capacity of 696 mAh g^?1 at 2500 mA g^?1 and excellent cycling stability for 1000 cycles with 0.035% capacity decay per cycle.     

Metal–Organic Framework Derived Honeycomb Co9S8@C Composites for High‐Performance Supercapacitors

Unique nanostructures always lead to extraordinary electrochemical energy storage performance. Here, the authors report a new strategy for using Metal‐organic frameworks (MOFs) derived cobalt sulfide in a carbon matrix with a 3D honeycombed porous structure, resulting in a high‐performance supercapacitor with unrivalled capacity of ≈1887 F g^‐1 at the current density of 1 A g^‐1. The honeycomb‐like structure of Co₉S₈@C composite is loosely adsorbed, with plentiful surface area and high conductivity, leading to improved Faradaic processes across the interface and enhanced redox reactions at active Co₉S₈ sites. Therefore, the heterostructure‐fabricated hybrid supercapacitor, using activated carbon as the counter electrode, demonstrates a high energy density of 58 Wh kg^‐1 at the power density of 1000 W kg^‐1. Even under an ultrahigh power density of 17 200 W kg^‐1, its energy density maintains ≈38 Wh kg^‐1. The hybrid supercapacitor also exhibits suitable cycling stability, with ≈90% capacity retention after 10 000 continuous cycles at the current density of 5 A g^‐1. This work presents a practical method for using MOFs as sacrificial templates to synthesize metal‐sulfides for highly efficient electrochemical energy storage.     

Revealing Pseudocapacitive Mechanisms of Metal Dichalcogenide SnS2/Graphene‐CNT Aerogels for High‐Energy Na Hybrid Capacitors

SnS₂ nanoplatelet electrodes can offer an exceptionally high pseudocapacitance in an organic Na⁺ ion electrolyte system, but their underlying mechanisms are still largely unexplored, hindering the practical applications of pseudocapacitive SnS₂ anodes in Na‐ion batteries (SIBs) and Na hybrid capacitors (SHCs). Herein, SnS₂ nanoplatelets are grown directly on SnO₂/C composites to synthesize SnS₂/graphene‐carbon nanotube aerogel (SnS₂/GCA) by pressurized sulfidation where the original morphology of carbon framework is preserved. The composite electrode possessing a large surface area delivers a remarkable specific capacity of 600.3 mA h g^?1 at 0.2 A g^?1 and 304.8 mA h g^?1 at an ultrahigh current density of 10 A g^?1 in SIBs. SHCs comprising a SnS₂/GCA composite anode and an activated carbon cathode present exceptional energy densities of 108.3 and 26.9 W h kg^?1 at power densities of 130 and 6053 W kg^?1, respectively. The in situ transmission electron microscopy and the density functional theory calculations reveal that the excellent pseudocapacitance originates from the combination of Na adsorption on the surface/Sn edge of SnS₂ nanoplatelets and ultrafast Na⁺ ion intercalation into the SnS₂ layers.     

Highly Reversible Na Storage in Na3V2(PO4)3 by Optimizing Nanostructure and Rational Surface Engineering

Sodium‐ion batteries (NIBs) have attracted more and more attention as economic alternatives for lithium‐ion batteries (LIBs). Sodium super ionic conductor (NASICON) structure materials, known for high conductivity and chemical diffusion coefficient of Na⁺ (≈10^?14 cm² s^?1), are promising electrode materials for NIBs. However, NASICON structure materials often suffer from low electrical conductivity (<10^?4 S cm^?1), which hinders their electrochemical performance. Here high performance sodium storage performance in Na₃V₂(PO₄)₃ (NVP) is realized by optimizing nanostructure and rational surface engineering. A N, B codoped carbon coated three‐dimensional (3D) flower‐like Na₃V₂(PO₄)₃ composite (NVP@C‐BN) is designed to enable fast ions/electrons transport, high‐surface controlled energy storage, long‐term structural integrity, and high‐rate cycling. The conductive 3D interconnected porous structure of NVP@C‐BN greatly releases mechanical stress from Na⁺ extraction/insertion. In addition, extrinsic defects and active sites introduced by the codoping heteroatoms (N, B) both enhance Na⁺ and e^? diffusion. The NVP@C‐BN displays excellent electrochemical performance as the cathode, delivering reversible capacity of 70% theoretical capacity at 100 C after 2000 cycles. When used as anode, the NVP@C‐BN also shows super long cycle life (38 mA h g^?1 at 20 C after 5000 cycles). The design provides a novel approach to open up possibilities for designing high‐power NIBs.     

From Water Oxidation to Reduction: Homologous Ni–Co Based Nanowires as Complementary Water Splitting Electrocatalysts

A homologous Ni–Co based nanowire system, consisting of both nickel cobalt oxide and nickel cobalt sulfide nanowires, is developed for efficient, complementary water splitting. The spinel‐type nickel cobalt oxide (NiCo₂O₄) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo₂O₄ nanowires leads to the formation of pyrite‐type nickel cobalt sulfide (Ni_0.33Co_0.67S₂) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni_0.33Co_0.67S₂ nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS₂ or NiS₂ nanostructures synthesized under similar methods. The Ni_0.33Co_0.67S₂ nanowires exhibit low onset potential of ?65, ?39, and ?50 mV versus reversible hydrogen electrode, Tafel slopes of 44, 68, and 118 mV dec^?1 at acidic, neutral, and basic conditions, respectively, and excellent stability, comparable to the best reported non‐noble metal‐based HER catalysts. Furthermore, the homologous Ni_0.33Co_0.67S₂ nanowires and NiCo₂O₄ nanowires are assembled into an all‐nanowire based water splitting electrolyzer with a current density of 5 mA cm^?2 at a voltage as 1.65 V, thus suggesting a unique homologous, earth abundant material system for water splitting.     

Slurry‐Fabricable Li+‐Conductive Polymeric Binders for Practical All‐Solid‐State Lithium‐Ion Batteries Enabled by Solvate Ionic Liquids

For mass production of all‐solid‐state lithium‐ion batteries (ASLBs) employing highly Li⁺ conductive and mechanically sinterable sulfide solid electrolytes (SEs), the wet‐slurry process is imperative. Unfortunately, the poor chemical stability of sulfide SEs severely restrict available candidates for solvents and in turn polymeric binders. Moreover, the binders interrupt Li⁺‐ionic contacts at interfaces, resulting in the below par electrochemical performance. In this work, a new scalable slurry fabrication protocol for sheet‐type ASLB electrodes made of Li⁺‐conductive polymeric binders is reported. The use of intermediate‐polarity solvent (e.g., dibromomethane) for the slurry allows for accommodating Li₆PS₅Cl and solvate‐ionic‐liquid‐based polymeric binders (NBR‐Li(G3)TFSI, NBR: nitrile?butadiene rubber, G3: triethylene glycol dimethyl ether, LiTFSI: lithium bis(trifluoromethanesulfonyl)imide) together without suffering from undesirable side reactions or phase separation. The LiNi_0.6Co_0.2Mn_0.2O₂ and Li₄Ti₅O₁₂ electrodes employing NBR‐Li(G3)TFSI show high capacities of 174 and 160 mA h g^?1 at 30 °C, respectively, which are far superior to those using conventional NBR (144 and 76 mA h g^?1). Moreover, high areal capacity of 7.4 mA h cm^?2 is highlighted for the LiNi_0.7Co_0.15Mn_0.15O₂ electrodes with ultrahigh mass loading of 45 mg cm^?2. The facilitated Li⁺‐ionic contacts at interfaces paved by NBR‐Li(G3)TFSI are evidenced by the complementary analysis from electrochemical and ⁷Li nuclear magnetic resonance measurements.     

Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium‐Based Compounds

A high‐rate of oxygen redox assisted by cobalt in layered sodium‐based compounds is achieved. The rationally designed Na_0.6[Mg_0.2Mn_0.6Co_0.2]O₂ exhibits outstanding electrode performance, delivering a discharge capacity of 214 mAh g^?1 (26 mA g^?1) with capacity retention of 87% after 100 cycles. High rate performance is also achieved at 7C (1.82 A g^?1) with a capacity of 107 mAh g^?1. Surprisingly, the Na_0.6[Mg_0.2Mn_0.6Co_0.2]O₂ compound is able to deliver capacity for 1000 cycles at 5C (at 1.3 A g^?1), retaining 72% of its initial capacity of 108 mAh g^?1. X‐ray absorption spectroscopy analysis of the O K‐edge indicates the oxygen‐redox species (O^2?/1?) is active during cycling. First‐principles calculations show that the addition of Co reduces the bandgap energy from ≈2.65 to ≈0.61 eV and that overlapping of the Co 3d and O 2p orbitals facilitates facile electron transfer, enabling the long‐term reversibility of the oxygen redox, even at high rates. To the best of the authors' knowledge, this is the first report on high‐rate oxygen redox in sodium‐based cathode materials, and it is believed that the findings will open a new pathway for the use of oxygen‐redox‐based materials for sodium‐ion batteries.     

Mn‐Rich P′2‐Na0.67[Ni0.1Fe0.1Mn0.8]O2 as High‐Energy‐Density and Long‐Life Cathode Material for Sodium‐Ion Batteries

Herein, P′2‐type Na_0.67[Ni_0.1Fe_0.1Mn_0.8]O₂ is introduced as a promising new cathode material for sodium‐ion batteries (SIBs) that exhibits remarkable structural stability during repetitive Na⁺ de/intercalation. The O? Ni? O? Mn? O? Fe? O bond in the octahedra of transition‐metal layers is used to suppress the elongation of the Mn? O bond and to improve the electrochemical activity, leading to the highly reversible Na storage mechanism. A high discharge capacity of ≈220 mAh g^?1 (≈605 Wh kg^?1) is delivered at 0.05 C (13 mAg^?1) with a high reversible capacity of ≈140 mAh g^?1 at 3 C and excellent capacity retention of 80% over 200 cycles. This performance is associated with the reversible P′2–OP4 phase transition and small volume change upon charge and discharge (≈3%). The nature of the sodium storage mechanism in a full cell paired with a hard carbon anode reveals an unexpectedly high energy density of ≈542 Wh kg^?1 at 0.2 C and good capacity retention of ≈81% for 500 cycles at 1 C (260 mAg^?1).     

P2‐K0.75[Ni1/3Mn2/3]O2 Cathode Material for High Power and Long Life Potassium‐Ion Batteries

Rationally designed P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is introduced as a novel cathode material for potassium‐ion batteries (KIBs). P2‐K_0.75[Ni_1/3Mn_2/3]O₂ cathode material designed through electrochemical ion‐exchange from P2‐Na_2/3[Ni_1/3Mn_2/3]O₂ exhibits satisfactory electrode performances; 110 mAh g^?1 (20 mA g^?1) retaining 86% of capacity for 300 cycles and unexpectedly high reversible capacity of about 91 mAh g^?1 (1400 mA g^?1) with excellent capacity retention of 83% over 500 cycles. According to theoretical and experimental investigations, the overall potassium storage mechanism of P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is revealed to be a single‐phase reaction with small lattice change upon charge and discharge, presenting the Ni^4+/2+ redox couple reaction. Such high power capability is possible through the facile K⁺ migration in the K_0.75[Ni_1/3Mn_2/3]O₂ structure with a low activation barrier energy of ≈210 meV. These findings indicate that P2‐K_0.75[Ni_1/3Mn_2/3]O₂ is a promising candidate cathode material for high‐rate and long‐life KIBs.     

A Dual‐Insertion Type Sodium‐Ion Full Cell Based on High‐Quality Ternary‐Metal Prussian Blue Analogs

Prussian blue analogs (PBAs) are especially investigated as superior cathodes for sodium‐ion batteries (SIBs) due to high theoretical capacity (≈170 mA h g^?1) with 2‐Na storage and low cost. However, PBAs suffer poor cyclability due to irreversible phase transition in deep charge/discharge states. PBAs also suffer low crystallinity, with considerable [Fe(CN)₆] vacancies, and coordinated water in crystal frameworks. Presently, a new chelating agent/surfactant coassisted crystallization method is developed to prepare high‐quality (HQ) ternary‐metal Ni_xCo_1?x[Fe(CN)₆] PBAs. By introducing inactive metal Ni to suppress capacity fading caused by excessive lattice distortion, these PBAs have tunable limits on depth of charge/discharge. HQ‐Ni_xCo_1?x[Fe(CN)₆] (x = 0.3) demonstrates the best reversible Na‐storage behavior with a specific capacity of ≈145 mA h g^?1 and a remarkably improved cycle performance, with ≈90% capacity retention over 600 cycles at 5 C. Furthermore, a dual‐insertion full cell on the cathode and NaTi₂(PO₄)₃ anode delivers reversible capacity of ≈110 mA h g^?1 at a current rate of 1.0 C without capacity fading over 300 cycles, showing promise as a high‐performance SIB for large‐scale energy‐storage systems. The ultrastable cyclability achieved in the lab and explained herein is far beyond that of any previously reported PBA‐based full cells.