Blowing Iron Chalcogenides into Two-Dimensional Flaky Hybrids with Superior Cyclability and Rate Capability for Potassium-Ion Batteries

Engineering sulfur defective Bi2S3@C with remarkably enhanced electrochemical kinetics of lithium-ion batteries

Introducing the appropriate vacancies to augment the active sites and improve the electrochemical kinetics while maintaining high cyclability is a major challenge for its widespread application in electrochemical energy storage. Here, core-shell structured Bi2S3@C with sulfur vacancies was prepared by hydrothermal method and one-step carbonization/sulfuration process, which significantly improves the intrinsic electrical conductivity and ion transport efficiency of Bi2S3. Additionally, the uniform protective carbon layer around surface of composite maintains structural stability and effectively alleviates volume expansion during alloying/dealloying. As a result, the BSC-500 anode exhibits a brilliant reversible capacity of 636 mAh/g at 0.2 A/g and a long-term stable capacity of 524 mAh/g for 500 cycles at a high current density of 3 A/g in lithium-ion batteries. In addition, the assembled Bi2S3@C//LiCoO2 full cell delivered a capacity of 184 mAh/g at 1 A/g and excellent cyclability (125 mAh/g after 1000 cycles). The proposed strategy of combining sulfur vacancies with a core-shell structure to improve the electrochemical kinetics of Bi2S3 in lithium-ion batteries off the prospect for practical applications of transition metal sulfide anodes.

Chloro-Functionalized Ether-Based Electrolyte for High-Voltage and Stable Potassium-Ion Batteries

Ether-based electrolyte is beneficial to obtaining good low-temperature performance and high ionic conductivity in potassium ion batteries. However, the dilute ether-based electrolytes usually result in ion-solvent co-intercalation of graphite, poor cycling stability, and hard to withstand high voltage cathodes above 4.0 V. To address the aforementioned issues, an electron-withdrawing group (chloro-substitution) was introduced to regulate the solid-electrolyte interphase (SEI) and enhance the oxidative stability of ether-based electrolytes. The dilute (~0.91 M) chloro-functionalized ether-based electrolyte not only facilitates the formation of homogeneous dual halides-based SEI, but also effectively suppress aluminum corrosion at high voltage. Using this functionalized electrolyte, the K||graphite cell exhibits a stability of 700 cycles, the K||Prussian blue (PB) cell (4.3 V) delivers a stability of 500 cycles, and the PB||graphite full-cell reveals a long stability of 6000 cycles with a high average Coulombic efficiency of 99.98 %. Additionally, the PB||graphite full-cell can operate under a wide temperature range from -5 °C to 45 °C. This work highlights the positive impact of electrolyte functionalization on the electrochemical performance, providing a bright future of ether-based electrolytes application for long-lasting, wide-temperature, and high Coulombic efficiency PIBs and beyond.

A bimetallic sulfide FeCoS4@rGO hybrid as a high-performance anode for potassium-ion batteries

We synthesized a low metal-to-sulfur atomic ratio (0.5) FeCoS4, exhibiting high reversible specific capacity. Reduced graphene oxide was covered on the surface to improve the cycling stability and rate performance further. Density functional theory calculations show that composite materials can effectively increase the adsorption energy and enhance the diffusion kinetics.

Thermodynamic Origin-Based In Situ Electrochemical Construction of Reversible p-n Heterojunctions for Optimal Stability in Potassium Ion Storage

Heterojunctions in electrode materials offer diverse improvements during the cycling process of energy storage devices, such as volume change buffering, accelerated ion/electron transfer, and better electrode structure integrity, however, obtaining optimal heterostructures with nanoscale domains remains challenging within constrained materials. A novel in situ electrochemical method is introduced to develop a reversible CuSe/PSe p-n heterojunction (CPS-h) from Cu3PSe4 as starting material, targeting maximum stability in potassium ion storage. The CPS-h formation is thermodynamically favorable, characterized by its superior reversibility, minimized diffusion barriers, and enhanced conversion post K+ interaction. Within CPS-h, the synergy of the intrinsic electric field and P-Se bonds enhance electrode stability, effectively countering the Se shuttling phenomenon. The specific orientation between CuSe and PSe leads to a 35° lattice mismatch generates large space at the interface, promoting efficient K ion migration. The Mott-Schottky analysis validates the consistent reversibility of CPS-h, underlining its electrochemical reliability. Notably, CPS-h demonstrates a negligible 0.005% capacity reduction over 10,000 half-cell cycles and remains stable through 2,000 and 4,000 cycles in full cells and hybrid capacitors, respectively. This study emphasizes the pivotal role of electrochemical dynamics in formulating highly stable p-n heterojunctions, representing a significant advancement in potassium-ion battery (PIB) electrode engineering.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Efficient Self-Assembly Preparation of 3D Carbon-Supported Ti3 C2 Tx Hollow Spheres for High-Performance Potassium Ion Batteries

MXenes are considered a promising negative electrode material for potassium ion batteries (PIBs) in view of their low potassium ion diffusion barrier and excellent electrical conductivity. However, the stacking phenomenon in practical applications severely reduces their active surface and leads to slow K+ diffusion. Herein, a facile composite template method is proposed to construct stacking-resistance 3D carbon-supported Ti3 C2 Tx (3D-C@Ti3 C2 Tx ) hollow spheres. Due to the unique structure, when used as a negative electrode material, as-prepared 3D-C@Ti3 C2 Tx hollow spheres show not only improved rate capability with 160.4 mAh g-1 at 100 mA g-1 and 133.7 mAh g-1 at 500 mA g-1 , but also stable cycling performance with 142.5 mAh g-1 specific capacity remained at 2 A g-1 after 4200 cycles. Furthermore, the full cells with 3D-C@Ti3 C2 Tx anode can operate stably for 1000 cycles at 100 mA g-1 . Moreover, the linear fit analysis demonstrates that 3D-C@Ti3 C2 Tx hollow spheres have a fast and stable capacitive potassium storage mechanism. This method is simple and easy to implement, which provide a feasible path to solve the stacking problem of 2D materials.

Oxygen Vacancy-Tailored Schottky Heterojunction Activates Interface Dipole Amplification and Carrier Inversion for High-Performance Potassium-Ion Batteries

An oxygen vacancy-tailored Schottky heterostructure composed of polyvinylpyrrolidone-assisted Bi2 Sn2 O7 (PVPBSO) nanocrystals and moderate work function graphene (mWFG, WF = 4.36 eV) is designed, which in turn intensifies the built-in voltage and interface dipole across the space charge region (SCR), leading to the inversion of majority carriers for facilitating K+ transport/diffusion behaviors. Thorough band-alignment experiments and interface simulations reveal the dynamics between deficient BSO and mWFG, and how charge redistribution within the SCR leads to carrier inversion, demonstrating the impact of different defect engineering degrees on the amplification of Schottky junctions. The ordered transport of bipolar carriers can boost electrons and K ions easily passing through the inner and outer surfaces of the heterostructure. With high activity and low resistance in electrochemical reactions, the PVPBSO/mWFG exhibits an attractive capacity of 430 mA h g-1 and a rate capability exceeding 2000 mA g-1 , accompanied by minimal polarization and efficient utilization of conversion-alloying reactions. The substantial cell capacity and high-redox plateau of PVPBSO/mWFG//PB contribute to the practical feasibility of high-energy full batteries, offering long-cycle retention and high-voltage output. This study emphasizes the direct importance of interface and junction engineering in improving the efficiency of diverse electrochemical kinetic and diffusion processes for potassium-ion batteries.

Bowl-shaped hollow carbon wrapped in graphene grown in situ by chemical vapor deposition as an advanced anode material for sodium-ion batteries

Sodium-ion batteries (SIBs) are expected to be ideal alternatives to lithium-ion batteries (LIBs) in the future due to their abundant and low-cost resource advantages. A key challenge in SIBs is the development of anodes capable of insertion/extraction of sodium ions (Na⁺) with large radii. Here, hollow bowl-shaped porous carbon materials are uniformly modified with vertically grown graphene (denoted as HBC/VGSs) demonstrating a large specific surface area and three-dimensional structure, which are employed as a viable high-performance anode for SIBs. HBC/VGSs anodes are highly effective at storing sodium because of their structural features. As a result, the HBC/VGSs electrodes provide a high reversible capacity of 409 mAh g^-1 after 100 cycles at 0.1 A g^-1, as well as outstanding rate capability (301.6 mAh g^-1 at 5 A g^-1). Moreover, it also shows extraordinary cycling stability (230.3 mAh g^-1 after 2500 cycles at a high current density of 5 A g^-1) that is significantly better than the pristine hollow bowl-shaped porous carbon (HBC). Cyclic Voltammetry (CV) and Galvanostatic Intermittent Titration Technique (GITT) were used to analyze the pseudocapacitance and sodium storage kinetics. It was found that high electrical conductivity and large surface area can improve Na⁺ adsorption and diffusion, enhance the electronic conductivity, and deliver superior capacity and rate. The results, taken as a whole, provide new insight into the creation of long-lasting carbon anodes that deliver optimal performance in SIBs.

Graphene supported FeS2 nanoparticles with sandwich structure as a promising anode for High-Rate Potassium-Ion batteries

Pyrite FeS₂ now emerges as a promising anode for potassium-ion batteries (PIBs) due to its low cost and high theoretical capacity. However, the significant volume expansion, low electrical conductivity, and the ambiguous mechanism related to potassium storage severely hinder its development for PIBs anodes. Herein, FeS₂ nanostructures are skillfully dispersed on the graphene surface layer by layer (FeS₂@C-rGO) to form a sandwich structure by using Fe-based metal organic framework (Fe-MOF) as precursors. The unique structural design can improve the transfer kinetics of K⁺ and effectively buffer the volume expansion during cycling, thereby enhancing the potassium storage performance. As a result, the FeS₂@C-rGO delivers a high capacity of 550 mAh/g at a current density of 0.1 A/g. At a high rate of 2 A/g, the capacity can maintain 171 mAh/g even after 500 cycles. Moreover, the electrochemical reaction mechanism and potassium storage behavior are revealed by in-situ X-ray diffractionand density functional theory calculations. This work not only provides a novel insight into the structural design of electrode materials for high-performance PIBs, but also proposes a valuable understanding of the potassium storage mechanism of the FeS₂-based anode.

Two dimensional MnPSe3 layer stacking composites with superior storage performance for alkali metal-ion batteries

Two-dimensional MnPSe₃ Van der Waals stacked in an ABC sequence has abundant and low-cost Mn resources, but has yet to exhibit expected performance as an electrode material in battery devices. Here, we report a 2D/2D composite consisting of few layer MnPSe₃ nanosheets and graphite through a high energy ball milling method for uses on the anodes alkali metal-ion batteries, including lithium ion battery (LIB) and potassium ion battery (PIB). These unique 2D/2D layer nanostructures, with MnPSe₃ layers hierarchically stacked in graphite, can successfully overcome the severe aggregation due to restacking during charge/discharge cycles. Moreover, density functional theory (DFT) calculations show that the band gap of the MnPSe₃/graphite hybrid is as low as 0.07 eV, confirming that the combination of MnPSe₃ and graphite efficiently reduces the ion migration energy barrier. As a result, MnPSe₃/graphite stacking composites achieve a discharge capacity of 488.1 mA h g^-1 after 500 cycles at 2000 mA g^-1 in LIB, and 236.7 mA h g^-1 after 700 cycles at 250 mA g^-1 in PIB. Moreover, the analysis of electrochemical, kinetics, reactions mechanism, DFT, and full cell applications were investigated deeply. This work strongly supports the possibility of MnPSe₃/graphite hybrid as a promising candidate for alkali ion batteries, and makes important improvements for the application of two-dimensional MPCh₃ layer materials in storage systems.

Electrochemical Self-Healing Nanocrystal Electrodes for Ultrastable Potassium-Ion Storage

The unique properties of self-healing materials hold great potential in battery systems, which can exhibit excellent deformability and return to its original shape after cycling. Herein, a Cu₃ BiS₃ anode material with self-healing mechanisms is proposed for use in ultrastable potassium-ion battery (PIB) and potassium-ion hybrid capacitor (PIHC). Different from the binder design, Cu₃ BiS₃ anode can exhibit the dual advantages of phase and morphological reversibility, further remaining original property after potassiation/depotassiation and exhibiting ultrastable cycling performance. The reversible electrochemical reconstruction during the continuous charge/discharge processes is beneficial to maintain the structure and function of the material. Furthermore, the conversion reactions during the charge and discharge process produce two advantages: i) suppressing the shuttle effect due to the formation of the heterostructure interface between Cu (111) and Bi (012); ii) Cu can avoid the agglomeration of Bi nanoparticles (NPs), further improving the electrochemical performance and long-cycle stability of the Cu₃ BiS₃ electrode. As a result, the Cu₃ BiS₃ electrode not only exhibits a long cycle life in half cells, but also 2000 cycles and 12000 cycles in PIB and PIHC full cells, respectively.

Pillar strategy enhanced ion transport and structural stability toward ultra-stable KVPO4F cathode for practical potassium-ion batteries

KVPO₄F (KVPF) is a promising cathode material for potassium-ion batteries (PIBs) because of its high operating voltage, high energy density, and excellent thermal stability. Nevertheless, the low kinetics and large volume change have been the major hurdles causing irreversible structural damage, high inner resistance, and poor cycle stability. Herein, a pillar strategy of Cs⁺ doping in KVPO₄F is introduced to reduce the energy barrier for ion diffusion and volume change during potassiation/depotassiation, which significantly enhances the K⁺ diffusion coefficient and stabilizes the crystal structure of the material. Consequently, the K_0.95Cs_0.05VPO₄F (Cs-5-KVPF) cathode exhibits an excellent discharge capacity of 104.5 mAh g^-1 at 20 mA g^-1 and a capacity retention rate of 87.9% after 800 cycles at 500 mA g^-1. Importantly, Cs-5-KVPF//graphite full cells attain an energy density of 220 Wh kg^-1 (based on the cathode and anode weight) with a high operating voltage of 3.93 V and 79.1% capacity retention after 2000 cycles at 300 mA g^-1. The Cs-doped KVPO₄F cathode successfully innovates the ultra-durable and high-performance cathode materials for PIBs, demonstrating its considerable potential for practical applications.

Spatially Guided Assembly of Polyoxometalate Superlattices and Their Derivatives as High-Power Sodium-Ion Battery Anodes

The use of polyoxometalate clusters (POMs) with multitudinous structures and surface properties as building blocks has sparked the development of cluster-assembled materials with many prospective applications. In comparison to classic molecules and assembly processes, control over the steric interactions and linkage of large POMs to achieve superlattices with multiple levels of organization remains a great challenge. This work presents a universal approach to modulate the spatial coordination behavior and configurations, and achieves a class of cluster superlattice architectures formed by linear alignment and two-dimensional arrangement of POM units. The formation mechanism is explained as a stepwise co-assembly pathway in which POMs can intervene and dictate a typical stripping-restacking combination mode with the lamellar mediator. These cluster superlattices with long-range POMs ordering impart distinct merits to their derivatives by sulfuration, for which we demonstrate the substantially promoted power and cycling life of these POM derivatives applied as sodium-ion battery anodes.

Thermodynamic Transformation of Crystalline Organic Hybrid Iron Selenide to FexSey@CN Microrods for Sodium Ion Storage

Carbon-coated metal chalcogenide composites have been demonstrated as one type of promising anode material for sodium-ion batteries (SIBs). However, combining carbon materials with micronanoparticles of metal chalcogenide always involve complicated processes, such as polymer coating, carbonization, and sulfidation/selenization. To address this issue, herein, we reported a series of carbon-coated FexSey@CN (FexSey = FeSe2, Fe3Se4, Fe7Se8) composites prepared via the thermodynamic transformation of a crystalline organic hybrid iron selenide [Fe(phen)2](Se4) (phen = 1,10-phenanthroline). By pyrolyzing the bulk crystals of [Fe(phen)2](Se4) at different temperatures, FexSey microrods were formed in situ, where the nitrogen-doped carbon layers were coated on the surface of the microrods. Moreover, all the as-prepared FexSey@CN composites exhibited excellent sodium-ion storage capabilities as anode materials in SIBs. This work proves that crystalline organic hybrid metal chalcogenides can be used as a novel material system for the in situ formation of carbon-coated metal chalcogenide composites, which could have great potential in the application of electrochemical energy storage.

Tailoring the Boron Configurations in B-doped Na3 V2 (PO4 )3 @Carbon for Fast and Durable Sodium Storage

Na₃ V₂ (PO₄ )₃ (NVP) is a widely studied cathode material for sodium-ion batteries because of its high ionic conductivity and attractive charge/discharge plateau (3.4 V vs. Na/Na⁺ ). However, its poor electronic conductivity and severe volume expansion during sodium storage need to be addressed before its intensive application could be realized. Herein, boron-doped NVP was synthesized through a facile electrospinning method. By adding boric acid into the reaction mixture during electrospinning followed by carbonization, boron could be directly inserted into the carbon matrix, giving rise to B-doped carbon nanofiber wrapped NVP. By tuning the doping amount, the boron-containing configurations could be facilely manipulated, playing different roles in promoting the sodium storage properties of the composite. Based on the calculation results, BC₂ O enhanced sodium diffusion by lowering the energy barrier, while BCO₂ improved the structural stability. Due to these specific functionalities of the configurations, the as-prepared composite with a balanced amount of BC₂ O and BCO₂ demonstrated superior sodium storage capacity of 113 mAh g^-1 at 1 C, outstanding long cycling performance of 103 mAh g^-1 at 10 C, and retained 91 mAh g^-1 after 1500 cycles. This gave rise to a capacity loss of only 0.08‰ per cycle, much better than the undoped counterpart.

Emerging carbon-based flexible anodes for potassium-ion batteries: Progress and opportunities

In recent years, carbon-based flexible anodes for potassium-ion batteries are increasingly investigated owing to the low reduction potential and abundant reserve of K and the simple preparation process of flexible electrodes. In this review, three main problems on pristine carbon-based flexible anodes are summarized: excessive volume change, repeated SEI growth, and low affinity with K⁺, which thus leads to severe capacity fade, sluggish K⁺ diffusion dynamics, and limited active sites. In this regard, the recent progress on the various modification strategies is introduced in detail, which are categorized as heteroatom-doping, coupling with metal and chalcogenide nanoparticles, and coupling with other carbonaceous materials. It is found that the doping of heteroatoms can bring the five enhancement effects of increasing active sites, improving electrical conductivity, expediting K⁺ diffusion, strengthening structural stability, and enlarging interlayer spacing. The coupling of metal and chalcogenide nanoparticles can largely offset the weakness of the scarcity of K⁺ storage sites and the poor wettability of pristine carbon-based flexible electrodes. The alloy nanoparticles consisting of the electrochemically active and inactive metals can concurrently gain a stable structure and high capacity in comparison to mono-metal nanoparticles. The coupling of the carbonaceous materials with different characteristics can coordinate the advantages of the nanostructure from graphite carbon, the defects and vacancies from amorphous carbon, and the independent structure from support carbon. Finally, the emerging challenges and opportunities for the development of carbon-based flexible anodes are presented.

Fundamental Understanding and Research Progress on the Interfacial Behaviors for Potassium-Ion Battery Anode

Potassium-ion batteries (PIBs) exhibit a considerable application prospect for energy storage systems due to their low cost, high operating voltage, and superior ionic conductivity. As a vital configuration in PIBs, the two-phase interface, which refers to K-ion diffusion from the electrolyte to the electrode surface (solid-liquid interface) and K-ion migration between different particles (solid-solid interface), deeply determines the diffusion/reaction kinetics and structural stability, thus significantly affecting the rate performance and cyclability. However, researches on two-phase interface are still in its infancy and need further attentions. This review first starts from the fundamental understanding of solid-liquid and solid-solid interfaces to in-depth analyzing the effect mechanism of different improvement strategies on them, such as optimization of electrolyte and binders, heterostructure design, modulation of interlayer spacing, etc. Afterward, the research progress of these improvement strategies is summarized comprehensively. Finally, the major challenges are proposed, and the corresponding solving strategies are presented. This review is expected to give an insight into the importance of two-phase interface on diffusion/reaction kinetics, and provides a guidance for developing other advanced anodes in PIBs.

Doping-Induced Electronic/Ionic Engineering to Optimize the Redox Kinetics for Potassium Storage: A Case Study of Ni-Doped CoSe2

Heteroatom doping effectively tunes the electronic conductivity of transition metal selenides (TMSs) with rapid K⁺ accessibility in potassium ion batteries (PIBs). Although considerable efforts are dedicated to investigating the relationship between the doping strategy and the resulting electrochemistry, the doping mechanisms, especially in view of the ion and electronic diffusion kinetics upon cycling, are seldom elucidated systematically. Herein, the crystal structure stability, charge/ion state, and bandgap of the active materials are found to be precisely modulated by favorable heteroatom doping, resulting in intrinsically fast kinetics of the electrode materials. Based on the combined mechanisms of intercalation and conversion reactions, electron and K⁺ ion transfer in Ni-doped CoSe₂ embedded in carbon nanocomposites (Ni-CoSe₂ @NC) can be significantly enhanced via electronic engineering. Benefiting from the synthetic controlled Ni grains, the heterointerface formed by the intermediate products of electrochemical reactions in Ni-CoSe₂ @NC strengthens the conversion kinetics and interdiffusion process, developing a low-barrier mesophase with optimized potassium storage. Overall, an electronic tuning strategy can offer deeper atomic insights into the conversion reaction of TMSs in PIBs.

Ultrathin-Walled Bi2 S3 Nanoroll/MXene Composite toward High Capacity and Fast Lithium Storage

It is extremely important to develop a high energy density power source with rapid charge-discharge rate to meet people's growing needs. Hence, the development of advanced electrode materials is the top priority. Herein, a simple yet elaborate vacuum-assisted room-temperature phase transfer method is reported to transform MXene nanosheets from water into organic solution. Subsequently, an in-situ growth strategy is employed to deposit ultrathin-walled bismuth sulfide (Bi₂ S₃ ) nanorolls on MXene surface to prepare Bi₂ S₃ /MXene composite as an efficient and high-performance anode material for lithium-ion batteries. Attributed to the unique nanoroll-like structure and the strong synergistic effect, the Bi₂ S₃ /MXene-10 composite can deliver the high discharge capacities of 849 and 541 mAh g^-1 at 0.1 and 5 A g^-1 , respectively. The Bi₂ S₃ /MXene-10 electrode can deliver a high specific capacity of 541 mAh g^-1 even after 600 cycles at a large current density of 1 A g^-1 , proving the superb cycling stability of the Bi₂ S₃ /MXene-10 composite. Additionally, the simple vacuum-assisted room-temperature phase transfer strategy can enlighten researchers to expand the potential application of MXene. Furthermore, the formation mechanism of Bi₂ S₃ nanorolls is also proposed, which may open a new avenue to design and fabricate other nanoroll-like structures.

Hierarchical Nanocapsules of Cu-Doped MoS2@H-Substituted Graphdiyne for Magnesium Storage

Hierarchical nanocomposites, which integrate electroactive materials into carbonaceous species, are significant in addressing the structural stability and electrical conductivity of electrode materials in post-lithium-ion batteries. Herein, a hierarchical nanocapsule that encapsulates Cu-doped MoS₂ (Cu-MoS₂) nanopetals with inner added skeletons in an organic-carbon-rich nanotube of hydrogen-substituted graphdiyne (HsGDY) has been developed for rechargeable magnesium batteries (RMB). Notably, both the incorporation of Cu in MoS₂ and the generation of the inner added nanoboxes are developed from a dual-template of Cu-cysteine@HsGDY hybrid nanowire; the synthesis involves two morphology/composition evolutions by CuS@HsGDY intermediates both taking place sequentially in one continuous process. These Cu-doped MoS₂ nanopetals with stress-release skeletons provide abundant active sites for Mg²⁺ storage. The microporous HsGDY enveloped with an extended π-conjugation system offers more effective electron and ion transfer channels. These advantages work together to make this nanocapsule an effective cathode material for RMB with a large reversible capacity and superior rate and cycling performance.

Puffing Up Hollow Carbon Nanofibers with High-Energy Metal-Organic Frameworks for Capacitive-Dominated Potassium-Ion Storage

Nitrogen-doped carbon materials with abundant defects and strong potassium adsorption ability have been recognized as potential anodes for potassium ion batteries (PIBs). However, the limited content and uncontrolled type of nitrogen-doped sites hinder the further performance improvement of PIBs. Herein, this work proposes nitrogen phosphorous co-doped hollow carbon nanofibers (PNCNFs) derived from high-energy metal-organic frameworks (MOFs) with an ultra-high nitrogen content of 19.52 wt% and a high portion of more reactive pyridinic N sites. Furthermore, the highly open architecture exploded by released gases from high-energy MOFs provides sufficient edge sites to settle the N atoms and further form pyridinic N sites induced by phosphorous dopants. The resulting PNCNFs achieve excellent potassium ion storage performance with high reversible capacity (466.2 mAh g^-1 ), superb rate capability (244.4 mAh g^-1 at 8 A g^-1 ), and excellent cycling performance (294.6 mAh g^-1 after 3250 cycles). The density functional theory calculation reveals that the N/P defects enhance the potassium adsorption ability and improve the conductivity.

Design of Flexible Films Based on Kinked Carbon Nanofibers for High Rate and Stable Potassium-Ion Storage

With the emergence of wearable electronics, flexible energy storage materials have been extensively studied in recent years. However, most studies focus on improving the electrochemical properties, ignoring the flexible mechanism and structure design for flexible electrode materials with high rate capacities and long-time stability. In this study, porous, kinked, and entangled network structures are designed for highly flexible fiber films. Based on theoretical analysis and finite element simulation, the bending degree of the porous structure (30% porosity) increased by 192% at the micro-level. An appropriate increase in kinking degree at the meso-level and contact points in entanglement network at the macro-level are beneficial for the flexibility of fiber films. Therefore, a porous and entangled network of sulfur-/nitrogen-co-doped kinked carbon nanofibers (S/N-KCNFs) is synthesized. The nanofiber films synthesized from melamine as nitrogen sources and segmented vulcanization exhibited a porous, kinked, and entangled network structure, and the stretching degree increased several times. The flexible S/N-KCNFs anode delivered a higher rate performance of 270 mAh g^-1 at a current density of 2000 mA g^-1 and a higher capacity retention rate of 93.3% after 2000 cycles. Moreover, the foldable pouch cell assembled by potassium-ion hybrid supercapacitor operated safely at large-angle bending and showed long-time stability of 88% capacity retention after 4000 cycles. This study provides a new idea and strategy for the flexible structure design of high-performance potassium-ion storage materials.

Coordination compound-derived La-doped FeS2/N-doped carbon (NC) as an efficient electrocatalyst for oxygen evolution reaction

By annealing/sulfuring of coordination compound (CC) precursors, [LaxFe1-x(H2O)8Fe(CN)6]·2hmt (x = 0, 0.33 and 0.5) (hmt = hexamethylenetetramine), it was synthesized FeS2/N-doped carbon (NC), La-doped FeS2/NC-0.33 and La-doped FeS2/NC-0.5. All of...

Prospects of Electrode Materials and Electrolytes for Practical Potassium-Based Batteries

Potassium-ion batteries (PIBs) have attracted tremendous attention because of their high energy density and low-cost. As such, much effort has focused on developing electrode materials and electrolytes for PIBs at the material levels. This review begins with an overview of the high-performance electrode materials and electrolytes, and then evaluates their prospects and challenges for practical PIBs to penetrate the market. The current status of PIBs for safe operation, energy density, power density, cyclability, and sustainability is discussed and future studies for electrode materials, electrolytes, and electrode-electrolyte interfaces are identified. It is anticipated that this review will motivate research and development to fill existing gaps for practical potassium-based full batteries so that they may be commercialized in the near future.

Fe-Triazole coordination compound-derived Fe2O3 nanoparticles anchored on Fe-MOF/N-doped carbon nanosheets for efficient electrocatalytic oxygen evolution reaction

Using FeCl₃·6H₂O and 1,2,4-triazole (Htrz) as starting materials, an Fe coordination compound (CC), [FeCl₃(Htrz)₃]·H₂O, was synthesized at room temperature. Fe-CC can be partially transformed into an Fe metal-organic framework (MOF), [FeCl₂(Htrz)], via low-temperature annealing. After sulfurization at 250, 300, and 400 °C, S-doped Fe₂O₃/N-doped carbon (denoted as NC)/Fe-MOF, FeS₂/NC/Fe-MOF, and FeS₂/NC were obtained, respectively. S-doped Fe₂O₃/NC/Fe-MOF shows the best oxygen evolution reaction (OER) catalytic activity in 1 M KOH solution, with overpotentials (η) of 185, 232, and 258 mV required to reach current densities of 10, 30, and 50 mA cm^-2, respectively, outperforming commercial RuO₂ and most transition-metal oxides reported to date; this high performance is associated with the Fe₂O₃ nanoparticles (NPs) anchored on the Fe-MOF/NC nanosheets. The Fe-MOF/NC matrix can act as a support to prevent the agglomeration of Fe₂O₃ NPs. In addition, S-doped Fe₂O₃/NC/Fe-MOF exhibits long-term OER activity at 20 mA cm^-2, which is related to the partial transformation of Fe₂O₃/Fe-MOF into FeOOH. In addition, density functional theory (DFT) calculations show that the rate-determining step of the OER process at the Fe sites of both Fe₂O₃ and FeS₂ is the formation of Fe*-OH, and the Fe₂O₃ sites display a lower Gibbs free energy (ΔG_max) of 1.674 eV and a smaller η value of ∼0.444 V. Bader charge, differential charge density mapping, and density of states (DOS) analysis all reveal more charge accumulation at the Fe sites of FeS₂ than at the Fe sites of Fe₂O₃, which is due to the lower electronegativity of S than of O. As a result, the Fe sites of FeS₂ show weaker affinity for -OH intermediates, giving rise to inferior OER performance compared with Fe₂O₃.

Boosting potassium-storage performance via confining highly dispersed molybdenum dioxide nanoparticles within N-doped porous carbon nano-octahedrons

The development of durable and stable metal oxide anodes for potassium ion batteries (PIBs) has been hampered by poor electrochemical performance and ambiguous reaction mechanisms. Herein, we design and fabricate molybdenum dioxide (MoO₂)@N-doped porous carbon (NPC) nano-octahedrons through metal-organic frameworks derived strategy for PIBs with MoO₂ nanoparticles confined within NPC nano-octahedrons. Benefiting from the synergistic effect of nanoparticle level of MoO₂ and N-doped carbon porous nano-octahedrons, the MoO₂@NPC electrode exhibits superior electron/ion transport kinetics, excellent structural integrity, and impressive potassium-ion storage performance with enhanced cyclic stability and high-rate capability. The density functional theory calculations and experiment test proved that MoO₂@NPC has a higher affinity of potassium and higher conductivity than MoO₂ and N-doped carbon electrodes. Kinetics analysis revealed that surface pseudocapacitive contributions are greatly enhanced for MoO₂@NPC nano-octahedrons. In-situ and ex-situ analysis confirmed an intercalation reaction mechanism of MoO₂@NPC for potassium ion storage. Furthermore, the assembled MoO₂@NPC//perylenetetracarboxylic dianhydride (PTCDA) full cell exhibits good cycling stability with 72.6 mAh g^-1 retained at 100 mA g^-1 over 200 cycles. Therefore, this work present here not only evidences an effective and viable structural engineering strategy for enhancing the electrochemical behavior of MoO₂ material in PIBs, but also gives a comprehensive insight of kinetic and mechanism for potassium ion interaction with metal oxide.

Synthesis, structure, and magnetic properties of the quaternary oxysulfides Ln 5V3O7S6 (Ln = La, Ce)

Two rare earth oxysulfides Ln 5V3O7S6 (Ln = La, Ce) have been synthesized and their structures determined. The two isostructural compounds crystallize in the orthorhombic space group Pmmn (no. 59). The structure features one-dimensional edge-sharing VS4O2 octahedron chains parallel to the b axis. The bonding between V and S/O is covalent, and between Ln 3+ and the rest of the matrix ionic. Magnetic susceptibility measurement revealed that V is in a mixed valence state of V3+ and V4+. Its magnetic behavior follows the Curie-Weiss law.

Realizing Fast Diffusion Kinetics Based on Three-Dimensional Ordered Macroporous Cu9S5@C for Potassium-Ion Batteries

Recently, potassium-ion batteries (PIBs) have been deemed to be a potential next-generation energy storage system for large-scale application because of the similar metal-ion storage mechanism as lithium-ion batteries and rich potassium resources. However, the large-sized potassium ion will cause sluggish reaction kinetics of K⁺ during charge/discharge processes, hindering the development of high-performance PIBs. In this work, copper sulfide embedded in three-dimensional ordered macroporous carbon framework (3DOM Cu₉S₅@C) was prepared through a sulfidation and subsequent ion exchange strategy with 3D ordered macropore Zn-based metal-organic frameworks as a precursor for an advanced PIBs anode. In particular, the interconnected 3D ordered macroporous structure can provide rapid transport channels for the large potassium ions and create a sufficient contact area for solid electrode materials and the liquid electrolyte, which is conducive to improve the ionic diffusion kinetics of batteries. Consequently, when the prepared 3DOM Cu₉S₅@C composite was used as a PIBs anode material, it shows a remarkable potassium storage rate capacity of 170 mA h g^-1 at 2.0 A g^-1 and an excellent cycling stability of 316 mA h g^-1 at 100 mA g^-1 after 200 cycles.