Unraveling the Nature of Excellent Potassium Storage in Small-Molecule Se@Peapod-Like N-Doped Carbon Nanofibers

Carbon Nanofibers-Based Anodes for Potassium-Ion Battery

In recent years, with the global warming getting worse and increasing demand for energy, countries around the world are trying to develop new energy storage technologies to solve this problem. Currently, potassium-ion batteries (PIBs) have attracted tremendous attention from researchers as low-cost and high-performance energy storage devices. However, due to the huge ionic radius of K+, PIBs face significant volume expansion during cycling, which can easily lead to the collapse of electrode structures. In addition, the poor diffusion kinetics of K+ seriously affect the electrochemical performance of the battery. Carbon nanofibers (CNFs)-based materials (including CNFs, metal/CNFs composites, chalcogenide/CNFs composites, and other CNFs-based materials) are widely used as PIBs electrode anode materials due to their three-dimensional conductive network, heteroatom doping and excellent mechanical properties. This review discusses in detail the research progress of CNFs-based materials in PIBs, including material preparation, structural design, and performance optimization. On this basis, this article explores the key issues faced by CNFs-based materials and future development directions, and proposes improvement suggestions for providing new ideas for the development of CNFs-based materials.

Optimization of Electrospun TORLON® 4000 Polyamide-Imide (PAI) Nanofibers: Bridging the Gap to Industrial-Scale Production

Polyamide-imide (PAI) is an exceptional polymer known for its outstanding mechanical, chemical, and thermal resistance. This makes it an ideal choice for applications that require excellent durability, such as those in the aerospace sector, bearings, gears, and the oil and gas industry. The current study explores the optimization of TORLON® 4000 T HV polyamide-imide nanofibers utilizing needleless electrospinning devices, ranging from laboratory-scale to industrial-scale production, for the first time. The PAI polymer has been dispersed in several solvent systems at varying concentrations. The diameter of the electrospun PAI nanofibers ranged from 65.8 nanometers to 1.52 μm. Their filtering efficiency was above 90% for particles with a size of 0.3 microns. The TGA results proved that PAI nanofibers have excellent resistance to high temperatures up to 450 °C. The PAI nanofibers are ideal for hot air intake filtration and fire-fighter personal protection equipment applications.

In-situ confining selenium within bubble - like carbon nanoshells for ultra-stable Li-Se batteries

Lithium-selenium (Li-Se) batteries are promising energy storage devices. However, the long-term durability and high-rate performance of the Se cathode have been limited by significant volume expansion and the troublesome shuttle effect of polyselenides during repeated charging/discharging processes. To revolutionize these issues, we applied a top-down strategy through the in-situ trapping of amorphous Se within bubble-like carbon (BLC) frameworks, which can radically minimize the presence of surface-absorbed Se while enhancing Se loading capacity. This ingenious technique successfully encapsulates all Se species within carbon nanoshells, creating a distinct half-filled core-shell structure known as Se@void@BLC. This in-situ trapping approach ensures the efficient management of Se volume changes during repeated discharge and charge cycles. Moreover, an extraordinary Se loading capacity of up to 65.6 wt% is reached. Using the Se@void@BLC as cathode for Li-Se battery, we achieve a high initial Columbic efficiency of 84.2 %, a high reversible capacity of 585 mAh g-1, and an ultralow capacity decay of only 0.0037 % per cycle during 4000 cycles at 10 A g-1.

SAXS unveils porous anodes for potassium-ion batteries: dynamic evolution of pore structures in Fe@Fe2O3/PCNFs composite nanofibers

The porous structure of composite nanofibers plays a key role in improving their electrochemical performance. However, the dynamic evolution of pore structures and their action during ion intercalation/extraction processes for negative electrodes are not clear. Herein, porous carbon composite nanofibers (Fe@Fe2O3/PCNFs) were prepared as negative electrode materials for potassium-ion batteries. Electrochemical test findings revealed that the composites had good electrochemical characteristics, and the porous structure endowed composite electrodes with pseudo-capacitive behaviors. After 1500 discharge/charge cycles at a current density of 1000 mA g-1, the specific capacity of the potassium-ion batteries was 144.8 mAh g-1. We innovatively used synchrotron small-angle X-ray scattering (SAXS) technique to systematically investigate the kinetic process of potassium formation in composites and showed that the kinetic process of potassium reaction in composites can be divided into four stages, and the pores with smaller average diameter distribution are more sensitive to changes in the reaction process. This work paves a new way to study the deposition kinetics of potassium in porous materials, which facilitates the design of porous structures and realizes the development of alkali metal ion-anode materials with high energies.

Boosting Potassium Storage via Multifunctional Interface with High Lattice-Matching

Atomic-scale interface engineering is a prominent strategy to address the large volume expansions and sluggish redox kinetics for reinforcing K-storage. Here, to accelerate charge transport and lower the activation energy, dual carbon-modified interfacial regions are synthesized with high lattice-matching degree, which is formed from a CoSe2 /FeSe2 heterostructure coated onto hollow carbon fibers. State-of-the-art characterization techniques and theoretical analysis, including ex-situ soft X-ray absorption spectroscopy, synchrotron X-ray tomography, ultrasonic transmission mapping, and density functional theory, are conducted to probe local atomic structure evolution, mechanical degradation mechanisms, and ion/electron migration pathways. The results suggest that the heterostructure composed of the same crystal system and space group can sharply regulate the redox kinetics of transition metal selenium and dual carbon-modified approach can tailor physicochemical degradation. Overall, this work presents the design of a stable heterojunction synergistic superior hollow carbon substrate, inspiring a pathway of interface engineering strategy toward high-performance electrode.

Carbon Electrode Materials for Advanced Potassium-Ion Storage

Tremendous progress has been made in the field of electrochemical energy storage devices that rely on potassium-ions as charge carriers due to their abundant resources and excellent ion transport properties. Nevertheless, future practical developments not only count on advanced electrode materials with superior electrochemical performance, but also on competitive costs of electrodes for scalable production. In the past few decades, advanced carbon materials have attracted great interest due to their low cost, high selectivity, and structural suitability and have been widely investigated as functional materials for potassium-ion storage. This article provides an up-to-date overview of this rapidly developing field, focusing on recent advanced and mechanistic understanding of carbon-based electrode materials for potassium-ion batteries. In addition, we also discuss recent achievements of dual-ion batteries and conversion-type K-X (X=O2 , CO2 , S, Se, I2 ) batteries towards potential practical applications as high-voltage and high-power devices, and summarize carbon-based materials as the host for K-metal protection and possible directions for the development of potassium energy-related devices as well. Based on this, we bridge the gaps between various carbon-based functional materials structure and the related potassium-ion storage performance, especially provide guidance on carbon material design principles for next-generation potassium-ion storage devices.

Oxygen Defect Engineering toward Zero-Strain V2O2.8@Porous Reticular Carbon for Ultrastable Potassium Storage

Potassium-ion batteries (KIBs) are promising candidates for large-scale energy storage devices due to their high energy density and low cost. However, the large potassium-ion radius leads to its sluggish diffusion kinetics during intercalation into the lattice of the electrode material, resulting in electrode pulverization and poor cycle stability. Herein, vanadium trioxide anodes with different oxygen vacancy concentrations (V2O2.9, V2O2.8, and V2O2.7 determined by the neutron diffraction) are developed for KIBs. The V2O2.8 anode is optimal and exhibits excellent potassium storage performance due to the realization of expanded interlayer spacing and efficient ion/electron transport. In situ X-ray diffraction indicates that V2O2.8 is a zero-strain anode with a volumetric strain of 0.28% during the charge/discharge process. Density functional theory calculations show that the impacts of oxygen defects are embodied in reducing the band gap, increasing electron transfer ability, and lowering the diffusion energy barriers for potassium ions. As a result, the electrode of nanosized V2O2.8 embedded in porous reticular carbon (V2O2.8@PRC) delivers high reversible capacity (362 mAh g-1 at 0.05 A g-1), ultralong cycling stability (98.8% capacity retention after 3000 cycles at 2 A g-1), and superior pouch-type full-cell performance (221 mAh g-1 at 0.05 A g-1). This work presents an oxygen defect engineering strategy for ultrastable KIBs.

Biochar-Derived Hierarchical Porous Carbon as Tellurium Host for High-Performance Potassium-Tellurium Batteries

Potassium-ion battery is promising for its high abundance and low redox potential. As a conversion cathode, Te possesses high conductivity and theoretical volumetric capacity to couple with potassium. The stubborn issues of K-Te battery focus on the large volume change and rapid structure degradation of Te. Herein, we produce biomass carbon from mangosteen shell in a facile method, and obtain a hierarchical porous host with abundance of micropores and mesopores, which is obviously beneficial for hosting Te during K+ storage in K-Te battery. The specific capacity reach to 560 mAh g-1 in the initial cycle at 0.1 A g-1 , and remained 83.8 % after 200 cycles. Impressively, at a high current density of 2.0 A g-1 , the specific capacity still remained 62.6 % after 5000 cycle. These results endow such strategy an efficient way for the development of K-Te batteries.

Construction of Synchronous-Deposition K Metal Anodes Via Modulation of Ion/Electron Transport Kinetics for High-Rate and Low-Temperature K Metal Batteries

The construction of conductive scaffolds is demonstrated to be an ideal strategy to alleviate the volume expansion and dendrite growth of K metal anodes. Nevertheless, the heterogeneous top-bottom deposition behavior caused by incompatible electronic/ionic conductivity of three-dimensional (3D) skeleton severely hinders its application. Here, a K2 Se/Cu conducting layer is fabricated on the Cu foam so as to enhance ionic transport and weaken electronic conductivity of the skeleton. Then, an excellent simultaneous deposition behavior of K metal inside the host is obtained for the first time via tuning fast ionic transport and low electronic conductivity. The simultaneous deposition mode can not only utilize the entire 3D structure to accommodate the volume expansion during K deposition but also avoid the formation of K dendrites at high current and ultra-low temperature. Consequently, the symmetric cells present a long cycle lifespan over 1000 h with a low deposition overpotential of 80 mV at 1 mA cm-2 . Furthermore, the full cell matching with the perylene-tetracarboxylic dianhydride (PTCDA) cathode presents an outstanding cycle lifespan over 600 cycles at 5 C at -20°C. The proposed simultaneous deposition strategy provides a new design direction for the construction of dendrite-free K metal anodes.

High-Performance Alkali Metal Ion Storage in Bi2Se3 Enabled by Suppression of Polyselenide Shuttling Through Intrinsic Sb-Substitution Engineering

Bismuth selenide holds great promise as a kind of conversion-alloying-type anode material for alkali metal ion storage because of its layered structure with large interlayer spacing and high theoretical specific capacity. Nonetheless, its commercial development has been significantly hammered by the poor kinetics, severe pulverization, and polyselenide shuttle during the charge/discharge process. Herein, Sb-substitution and carbon encapsulation strategies are simultaneously employed to synthesize Sb_xBi_2-xSe₃ nanoparticles decorated on Ti₃C₂T_x MXene with encapsulation of N-doped carbon (Sb_xBi_2-xSe₃/MX⊂NC) as anodes for alkali metal ion storage. The superb electrochemical performances could be assigned to the cationic displacement of Sb³⁺ that effectively inhibits the shuttling effect of soluble polyselenides and the confinement engineering that alleviates the volume change during the sodiation/desodiation process. When used as anodes for sodium- and lithium-ion batteries, the Sb_0.4Bi_1.6Se₃/MX⊂NC composite exhibits superior electrochemical performances. This work offers valuable guidance to suppress the shuttling of polyselenides/polysulfides in high-performance alkali metal ion batteries with conversion/alloying-type transition metal sulfide/selenide anode materials.

Cobalt selenide with ordered cation vacancies for efficient oxygen reduction and frigostable Al-air batteries

Aluminum-air batteries are still inhibited by the sluggish cathodic oxygen reduction reactions, especially in low temperature conditions. Thus, it is urgent to develop efficient electrocatalysts for Al-air batteries to allow their use in extreme weather conditions. In this work, hexagonal Co0.85Se-decorated N,Se co-doped carbon nanofibers (Co0.85Se@N,Se-CNFs) were synthesized via facile carbonization/selenization of electrospun ZIF-67 nanocubes. The as-prepared metallic Co0.85Se with ordered structural cation vacancies endows Co0.85Se@N,Se-CNFs with remarkable oxygen reduction reaction activity, including high onset and half-wave potentials (0.93 V and 0.87 V vs. RHE, respectively). Consequently, the corresponding Al-air battery exhibits superior performance in a wide range of operating temperatures (-40-50 °C). For instance, this Al-air battery exhibits a voltage from 0.15-1.2 V with a peak power density of about 0.7 mW cm-2 at -40 °C. It is expected that TMSe-decorated N,Se co-doped carbon nanofibers could be applied in extensive energy fields.

Anion Defects Engineering of Ternary Nb-Based Chalcogenide Anodes Toward High-Performance Sodium-Based Dual-Ion Batteries

Highlights

We developed an efficient and extensible strategy to produce the single-phase ternary NbSSe nanohybrids with defect-enrich microstructure. The anionic-Se doping play a key role in effectively modulating the electronic structure and surface chemistry of NbS2 phase, including the increased interlayers distance (0.65 nm), the enhanced intrinsic electrical conductivity (3.23 × 103 S m-1) and extra electroactive defect sites. The NbSSe/NC composite as anode exhibits rapid Na+ diffusion kinetics and increased capacitance behavior for Na+ storage, resulting in high reversible capacity and excellent cycling stability.

Abstract

Sodium-based dual-ion batteries (SDIBs) have gained tremendous attention due to their virtues of high operating voltage and low cost, yet it remains a tough challenge for the development of ideal anode material of SDIBs featuring with high kinetics and long durability. Herein, we report the design and fabrication of N-doped carbon film-modified niobium sulfur–selenium (NbSSe/NC) nanosheets architecture, which holds favorable merits for Na+ storage of enlarged interlayer space, improved electrical conductivity, as well as enhanced reaction reversibility, endowing it with high capacity, high-rate capability and high cycling stability. The combined electrochemical studies with density functional theory calculation reveal that the enriched defects in such nanosheets architecture can benefit for facilitating charge transfer and Na+ adsorption to speed the electrochemical kinetics. The NbSSe/NC composites are studied as the anode of a full SDIBs by pairing the expanded graphite as cathode, which shows an impressively cyclic durability with negligible capacity attenuation over 1000 cycles at 0.5 A g−1, as well as an outstanding energy density of 230.6 Wh kg−1 based on the total mass of anode and cathode.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40820-023-01070-0.

3D space-confined Co0.85Se architecture with effective interfacial stress relaxation as anode material reveals robust and highly loading potassium-ion batteries

Conversion-type transition metal chalcogenide anodes could bring relatively high specific capacity in potassium ion storage due to multiple electron transport reactions, but often accompanying huge volume changes and resulting in low cycle life and rapid capacity fading.While electrode materials are closely packed, the contact at the interface during potassiation/depotassiation is similar to point-to-point contact, generating strong stress to make self-aggregation occur. In this work, we constructed a 3D carbon framework to confine Co_0.85Se nanocrystals in three-dimensional space, both fulfilling the requirements of the material's size in the nano-scale and providing the largest contact area for releasing stress. With this optimization, nitrogen-doped carbon confined Co_0.85Se nanocrystals (Co_0.85Se@NC) reach an ultra-stable cycle life over 4000 times with a specific capacity of 190.9 mA h g^-1 at 500 mA g^-1 and provide 155.6 mA h g^-1 at 10 A g^-1 in the rate capability test. It also renders the areal capacity up to 1.03 mA h cm^-2 at 500 mA g^-1 in the high-mass loading test. Furthermore, based on the finite element analysis, the 3D confinement strategy has the lowest interfacial stress, ensuring Co_0.85Se nanocrystals with high structural integrity. This strategy can relieve the stress issue in the conversion-type anode and demonstrate superior electrochemical performance even at high-loading mass electrodes.

Bond modulation of MoSe2+x driving combined intercalation and conversion reactions for high-performance K cathodes

The urgent demand for large-scale global energy storage systems and portable electronic devices is driving the need for considerable energy density and stable batteries. Here, Se atoms are introduced between MoSe₂ layers (denoted as MoSe_2+x ) by bond modulation to produce a high-performance cathode for potassium-ion batteries. The introduced Se atoms form covalent Se-Se bonds with the Se in MoSe₂, and the advantages of bond modulation are as follows: (i) the interlayer spacing is enlarged which increases the storage space of K⁺; (ii) the system possesses a dual reaction mechanism, and the introduced Se can provide an additional conversion reaction when discharged to 0.5 V, which improves the capacity further; (iii) the Se atoms confined between MoSe₂ layers do not give rise to the shuttle effect. MoSe_2+x is compounded with rGO (MoSe_2+x -rGO) as a cathode for potassium-ion batteries and displays an ultrahigh capacity (235 mA h g^-1 at 100 mA g^-1), a long cycle life (300 cycles at 100 mA g^-1) and an extraordinary rate performance (135 mA h g^-1 at 1000 mA g^-1 and 89 mA h g^-1 at 2000 mA g^-1). Pairing the MoSe_2+x -rGO cathode with graphite, the full cell delivers considerable energy density compared to other K cathode materials. The MoSe_2+x -rGO cathode also exhibits excellent electrochemical performance for lithium-ion batteries. This study on bond modulation driving combined intercalation and conversion reactions offers new insights into the design of high-performance K cathodes.

Electrospun carbon-based nanomaterials for next-generation potassium batteries

Rechargeable potassium (K) batteries that are of low cost, with high energy densities and long cycle lives have attracted tremendous interest in affordable and large-scale energy storage. However, the large size of the K-ion leads to sluggish reaction kinetics and causes a large volume variation during the ion insertion/extraction processes, thus hindering the utilization of active electrode materials, triggering a serious structural collapse, and deteriorating the cycling performance. Therefore, the exploration of suitable materials/hosts that can reversibly and sustainably accommodate K-ions and host K metals are urgently needed. Electrospun carbon-based materials have been extensively studied as electrode/host materials for rechargeable K batteries owing to their designable structures, tunable composition, hierarchical pores, high conductivity, large surface areas, and good flexibility. Here, we present the recent developments in electrospun CNF-based nanomaterials for various K batteries (e.g., K-ion batteries, K metal batteries, K-chalcogen batteries), including their fabrication methods, structural modulation, and electrochemical performance. This Feature Article is expected to offer guidelines for the rational design of novel electrospun electrodes for the next-generation K batteries.

Confined WS2 Nanosheets Tubular Nanohybrid as High-Kinetic and Durable Anode for Sodium-Based Dual Ion Batteries

Sodium based dual-ion battery (SDIB) has been regarded as one of the promising batteries technologies thanks to its high working voltage and natural abundance of sodium source, its practical application yet faces critical issues of low capacity and sluggish kinetics of intercalation-type graphite anode. Here, a tubular nanohybrid composed of building blocks of carbon-film wrapped WS₂ nanosheets on carbon nanotube (WS₂ /C@CNTs) was reported. The expanded (002) interlayer and dual-carbon confined structure endowed WS₂ nanosheets with fast charge transportation and excellent structural stability, and thus WS₂ /C@CNTs showed highly attractive electrochemical properties for Na⁺ storage with high reversible capacity, fast kinetic, and robust durability. The full sodium-based dual ion batteries by coupling WS₂ /C@CNTs anode with graphite cathode full cell presented a high reversible capacity (210 mAh g^-1 at 0.1 A g^-1 ), and excellent rate performance with a high capacity of 137 mAh g^-1 at 5.0 A g^-1 .

Embedding Atomically Dispersed Iron Sites in Nitrogen-Doped Carbon Frameworks-Wrapped Silicon Suboxide for Superior Lithium Storage

Silicon suboxide (SiO_x ) has attracted widespread interest as Li-ion battery (LIB) anodes. However, its undesirable electronic conductivity and apparent volume effect during cycling impede its practical applications. Herein, sustainable rice husks (RHs)-derived SiO₂ are chosen as a feedstock to design SiO_x /iron-nitrogen co-doped carbon (Fe-N-C) materials. Using a facile electrospray-carbonization strategy, SiO_x nanoparticles (NPs) are encapsulated in the nitrogen-doped carbon (N-C) frameworks decorating atomically dispersed iron sites. Systematic characterizations including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) verify the existence of Fe single atoms and typical coordination environment. Benefiting from its structural and compositional merits, the SiO_x /Fe-N-C anode delivers significantly improved discharge capacity of 799.1 mAh g^-1 , rate capability, and exceptional durability, compared with pure SiO₂ and SiO_x /N-C, which has been revealed by the density functional theory (DFT) calculations. Additionally, the electrochemical tests and in situ X-ray diffraction (XRD) analysis reveal the oxidation of Li_x Si phase and the storage mechanism. The synthetic strategy is universal for the design and synthesis of metal single atoms/clusters dispersed N-C frameworks encapsulated SiO_x NPs. Meanwhile, this work provides impressive insights into developing various LIB anode materials suffering from inferior conductivity and huge volume fluctuations.

Integrating antimony/amorphous vanadium oxide composite structure into electrospun carbon nanofibers for synergistically enhanced lithium/sodium-ion storage

Development of advanced anode material is highly desired in the energy storage field, not only for the current dominant lithium ion battery (LIB), but also for the sodium ion battery (SIB) with high potential in large-scale and low-cost stationary energy storage systems. Herein, we present a sea cucumber-like hybrid (Sb/VO_x-CNFs) which integrates Sb and amorphous VO_x composite structure into carbon fibers through electrospinning and sequential annealing treatment. With the specific structural and composition advantages brought synergistically by the Sb/VO_x composite structure and the carbon fiber skeleton, the as-prepared Sb/VO_x-CNFs delivers a high rate capacity and long-cycle life for both Na⁺ (∼337 mAh g^-1 after 1000 cycles at 1 A g^-1) and Li⁺ (∼554 mAh g^-1 after 300 cycles at 0.5 A g^-1) when applied as anode materials in the assembled SIB and LIB coin cells due to the improved charge transfer, enlarged active sites and enhanced structural stabilities. The practical applicability of Sb/VO_x-CNFs is also demonstrated by the assembly and tests of Sb/VO_x-CNFs//Na₃V₂(PO₄)₃ full cell where the commercial Na₃V₂(PO₄)₃ is employed as the cathode material. Importantly, the presented strategy with favorable synergistic effect could provide expansion opportunities for the design of composite structured nanomaterials in the energy storage fields.

Insights into the Potassium Ion Storage Behavior and Phase Evolution of a Tailored Yolk-Shell SnSe@C Anode

Tin chalcogenides are regarded as promising anode materials for potassium ion batteries (PIBs) due to their considerable specific capacity. However, the severe volume effect, limited electronic conductivity, and the shuttle effect of the potassiation product restrict the application prospect. Herein, based on the metal evaporation reaction, a facile structural engineering strategy for yolk-shell SnSe encapsulated in carbon shell (SnSe@C) is proposed. The internal void can accommodate the volume change of the SnSe core and the carbon shell can enhance the electronic conductivity. Combining qualitative and quantitative electrochemical analyses, the distinguished electrochemical performance of SnSe@C anode is attributed to the contribution of enhanced capacitive behavior. Additionally, first-principles calculations elucidate that the heteroatomic doped carbon exhibits a preferable affinity toward potassium ions and the potassiation product K₂ Se, boosting the rate performance and capacity retention consequently. Furthermore, the phase evolution of SnSe@C electrode during the potassiation/depotassiation process is clarified by in situ X-ray diffraction characterization, and the crystal transition from the SnSe Pnma(62) to Cmcm(63) point group is discovered unpredictably. This work demonstrates a pragmatic avenue to tailor the SnSe@C anode via a facile structural engineering strategy and chemical regulation, providing substantial clarification for the phase evolution mechanism of SnSe-based anode for PIBs.

Amorphous Tellurium-Embedded Hierarchical Porous Carbon Nanofibers as High-Rate and Long-Life Electrodes for Potassium-Ion Batteries

Tellurium (Te) is a promising electrode active material for potassium-ion batteries due to its intrinsic electrical conductivity and ultra-high theoretical volumetric capacity. Nevertheless, Te-based electrodes usually exhibit low capacity at high rates and poor cycling stability caused by the large volume expansion and severe polytellurides dissolution. Herein, hierarchical porous carbon nanofibers (HPCNFs) film is utilized as a multifunctional Te substrate. The free-standing Te@HPCNFs electrode renders an outstanding K-ion storage performance with a high-rate capacity of 1294.4 mAh cm^-3 (207.1 mAh g^-1 _Te ) at 14C and ultra-long lifespan for 4500 cycles at 7C, and K-ion full batteries coupled with KSn alloy anode also exhibit good cyclability. Such a superior performance benefits from the space confinement of HPCNFs to load amorphous Te in the micropores for accommodating the volume change, where the interconnected conductive frameworks and residual hierarchical pores enable fast ion/electron diffusion kinetics. In situ UV-vis absorption spectra confirm that the detachment of polytellurides and K₂ Te from the electrode is effectively suppressed, and ex situ X-ray photoelectron spectroscopy analysis reveals the conversion of Te into K₅ Te₃ and K₂ Te. This work presents the significance of porous structure design of carbon matrix to construct high performance Te electrodes, which will be instructive for chalcogens-based energy-storage materials.

In Situ Atomic-Scale Observation of Electrochemical (De)potassiation in Te Nanowires

Potassium-ion batteries (PIBs) have great potential in energy storage due to their high abundance and low cost of potassium resources. Tellurium (Te) is a promising PIB cathode due to its high volumetric capacity and good electronic conductivity. However, the electrochemical (de)potassiation mechanism of Te remains elusive due to the lack of an effective method of directly observing the dynamic reaction at atomic resolution. Here, the phase transformations of single crystal Te on (de)potassiation are clearly revealed by in situ high-resolution transmission electron microscopy and electron diffraction. Te undergoes a consecutive phase transformation during potassiation: from Te to K₂ Te₃ in the initial potassiation, and then part of the K₂ Te₃ to K₅ Te₃ on further potassiation. The reaction has extremely high reversibility in the following depotassiation. By atomic-scale observation, an anisotropic reaction mechanism where K⁺ intercalates into Te crystalline lattice preferentially through the (001) plane (having a large d-spacing) is established during potassiation. While in the depotassiation process, K ions extract from the polycrystalline K_x Te along the same diffusion path to form single crystal Te, indicating the potassium storage is highly reversible. The strong orientation-dependent (de)potassiation mechanism revealed by this work provides implications for the future design of nanostructured cathodes for high-performance PIBs.

In Situ Fabrication of Porous Cox P Hierarchical Nanostructures on Carbon Fiber Cloth with Exceptional Performance for Sodium Storage

Superior high-rate performance and ultralong cycling life have been constantly pursued for rechargeable sodium-ion batteries (SIBs). In this work, a facile strategy is employed to successfully synthesize porous Co_x P hierarchical nanostructures supported on a flexible carbon fiber cloth (Co_x P@CFC), constructing a robust architecture of ordered nanoarrays. Via such a unique design, porous and bare structures can thoroughly expose the electroactive surfaces to the electrolyte, which is favorable for ultrafast sodium-ion storage. In addition, the CFC provides an interconnected 3D conductive network to ensure firm electrical connection of the electrode materials. Besides the inherent flexibility of the CFC, the integration of the hierarchical structures of Co_x P with the CFC, as well as the strong synergistic effect between them, effectively help to buffer the mechanical stress caused by repeated sodiation/desodiation, thereby guaranteeing the structural integrity of the overall electrode. Consequently, Co_x P@CFC as an anode shows a record-high capacity of 279 mAh g^-1 at 5.0 A g^-1 with almost no capacity attenuation after 9000 cycles.

Iron Selenide-Based Heterojunction Construction and Defect Engineering for Fast Potassium/Sodium-Ion Storage

Suitable anode materials with high capacity and long cycling stability, especially capability at high current densities, are urgently needed to advance the development of potassium ion batteries (PIBs) and sodium ion batteries (SIBs). Herein, a porous Ni-doped FeSe₂ /Fe₃ Se₄ heterojunction encapsulated in Se-doped carbon (NF₁₁ S/C) is designed through selenization of MOFs precursor. The porous composite possesses enriched active sites and facilitates transport for both ion and electron. Ni-doping is adopted to enrich the lattice defects and active sites. The Se-C bond and carbon framework endow integrity of the composite and hamper aggregation of selenide nano-particles during potassiation/de-potassiation. The NF₁₁ S/C exhibits exceptional rate performance and ultra-long cycling stability (177.3 mA h g^-1 after 3050 cycles at 2 A g^-1 for PIBs and 208.8 mA h g^-1 after 2000 cycles at 8 A g^-1 for SIBs). The potassiation/de-potassiation mechanism is investigated via ex-situ X-ray powder diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectrocopy and Raman analysis. PTCDA//NF₁₁ S/C full cell stably cycles for 1200 cycles at 200 mA g^-1 with a capacity of 103.7 mA h g^-1 , indicating the high application potential of the electrode for highly stable rechargeable batteries.

Electrolyte formulation strategies for potassium-based batteries

Potassium (K)-based batteries are viewed as the most promising alternatives to lithium-based batteries, owing to their abundant potassium resource, lower redox potentials (-2.97 V vs. SHE), and low cost. Recently, significant achievements on electrode materials have boosted the development of potassium-based batteries. However, the poor interfacial compatibility between electrode and electrolyte hinders their practical. Hence, rational design of electrolyte/electrode interface by electrolytes is the key to develop K-based batteries. In this review, the principles for formulating organic electrolytes are comprehensively summarized. Then, recent progress of various liquid organic and solid-state K⁺ electrolytes for potassium-ion batteries and beyond are discussed. Finally, we offer the current challenges that need to be addressed for advanced K-based batteries.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Bimetallic Selenide Decorated Nanoreactor Synergizing Confinement and Electrocatalysis of Se Species for 3D-Printed High-Loading K-Se Batteries

The potassium-selenium (K-Se) battery has been considered an appealing candidate for next-generation energy storage systems owing to the high energy and low cost. Nonetheless, its development is plagued by the tremendous volume expansion and sluggish reaction kinetics of the Se cathode. Moreover, implementing favorable areal capacity and longevous cycling of a high-loading K-Se battery remains a daunting challenge facing commercial applications. Herein, we devise a Se and CoNiSe₂ coembedded nanoreactor (Se/CoNiSe₂-NR) affording low carbon content as an advanced cathode for K-Se batteries. We systematically uncover the enhanced K₂Se₂/K₂Se adsorption and promoted K⁺ diffusion behavior with the incorporation of Co throughout theoretical simulation and electrokinetic analysis. As a result, Se/CoNiSe₂-NR harvests high cycling stability with a capacity decay rate of 0.038% per cycle over 950 cycles at 1.0 C. More encouragingly, equipped with a 3D-printed Se/CoNiSe₂-NR electrode with tunable Se loadings, K-Se full batteries enable steady cycling at an elevated Se loading of 3.8 mg cm^-2. Our endeavor ameliorates the capacity and lifetime performance of the emerging K-Se device, thereby offering a meaningful tactic in pursuing its practical application.

Toward Theoretical Capacity and Superhigh Power Density for Potassium-Selenium Batteries via Facilitating Reversible Potassiation Kinetics

Potassium-selenium (K-Se) batteries attract tremendous attention because of the two-electron transfer of the selenium cathode. Nonetheless, practical K-Se cells normally display selenium underutilization and unsatisfactory rate capability. Herein, we employ a synergistic spatial confinement and architecture engineering strategy to establish selenium cathodes for probing the effect of K⁺ diffusion kinetics on K-Se battery performance and improving the charge transfer efficiency at ultrahigh rates. By impregnating selenium into hollow and solid carbon spheres with similar diameters and porous structures, the obtained parallel Se/C composites possess nearly identical selenium loadings, molecular structures, and heterogeneous interfaces but enormously different paths for K⁺ diffusion. Remarkably, as the solid-state K⁺ diffusion distance is significantly reduced, the K-Se cell achieves 96% of 2e^- transfer capacity (647.1 mA h g^-1). Reversible capacities of 283.5 and 224.1 mA h g^-1 are obtained at 7.5 and 15C, respectively, corresponding to an unprecedented high power density of 8777.8 W kg^-1. Quantitative kinetic analysis demonstrated a twofold higher capacitive charge storage contribution and a 1 order of magnitude higher K⁺ diffusion coefficient due to the short K⁺ diffusion path. By combining the determination of potassiation products by ex situ characterization and density functional theory (DFT) calculations, it is identified that the kinetic factor is decisive for K-Se battery performances.

Rational Design of Tungsten Selenide @ N-Doped Carbon Nanotube for High-Stable Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are deemed as one of the most promising energy storage systems due to their high energy density and low cost. However, their commercial application is far away from satisfactory because of limited suitable electrode materials. Herein, core-shell structured WSe₂ @N-doped C nanotubes are rationally designed and synthesized via selenizing WO₃ @ polypyrrole for the first time. The large interlayer spacing of WSe₂ can facilitate the intercalation/deintercalation of K⁺ . Meanwhile, the core-shell structured nanotube provides favorable interior void space to accommodate the volume expansion of WSe₂ during cycling. Thus, the obtained electrode exhibits superb electrochemical performance with a high capacity of 301.7 mAh g^-1 at 100 mA g^-1 over 120 cycles, and 122.1 mAh g^-1 can remain at 500 mA g^-1 even after 1300 cycles. Ex-situ X-ray diffraction analysis reveals the K-ion storage mechanism of WSe₂ @N-doped C includes intercalation and conversion reaction. Density function theory (DFT) calculation demonstrates the reasonable diffusion pathway of K⁺ . In addition, the obtained WSe₂ @N-doped C nanotubes have been used as anode material for lithium-ion batteries, which also show good rate performance and high cycle stability. Therefore, this work offers a new methodology for the ration design of new structure electrode materials with long cycle stability.

Se/N co-doped carbon nanorods for potassium ion storage

Herein, we make use of the large-sized Se atom to regulate the local structure of the graphitic lattice to optimize N-doping species to strengthen the adsorption of K+, which can greatly boost the capacitive capacity for PIBs.

Carbon-Coated Three-Dimensional MXene/Iron Selenide Ball with Core-Shell Structure for High-Performance Potassium-Ion Batteries

Two-dimensional (2D) MXenes are promising as electrode materials for energy storage, owing to their high electronic conductivity and low diffusion barrier. Unfortunately, similar to most 2D materials, MXene nanosheets easily restack during the electrode preparation, which degrades the electrochemical performance of MXene-based materials. A novel synthetic strategy is proposed for converting MXene into restacking-inhibited three-dimensional (3D) balls coated with iron selenides and carbon. This strategy involves the preparation of Fe₂O₃@carbon/MXene microspheres via a facile ultrasonic spray pyrolysis and subsequent selenization process. Such 3D structuring effectively prevents interlayer restacking, increases the surface area, and accelerates ion transport, while maintaining the attractive properties of MXene. Furthermore, combining iron selenides and carbon with 3D MXene balls offers many more sites for ion storage and enhances the structural robustness of the composite balls. The resultant 3D structured microspheres exhibit a high reversible capacity of 410 mAh g^-1 after 200 cycles at 0.1 A g^-1 in potassium-ion batteries, corresponding to the capacity retention of 97% as calculated based on 100 cycles. Even at a high current density of 5.0 A g^-1, the composite exhibits a discharge capacity of 169 mAh g^-1.

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.

A Low-Strain Phosphate Cathode for High-Rate and Ultralong Cycle-Life Potassium-Ion Batteries

Most potassium-ion battery (PIB) cathode materials have deficient structural stability because of the huge radius of potassium ion, leading to inferior cycling performance. We report the controllable synthesis of a novel low-strain phosphate material K₃ (VO)(HV₂ O₃ )(PO₄ )₂ (HPO₄ ) (denoted KVP) nanorulers as an efficient cathode for PIBs. The as-synthesized KVP nanoruler cathode exhibits an initial reversible capacity of 80.6 mAh g^-1 under 20 mA g^-1 , with a large average working potential of 4.11 V. It also manifests an excellent rate property of 54.4 mAh g^-1 under 5 A g^-1 , with a high capacity preservation of 92.1 % over 2500 cycles. The outstanding potassium storage capability of KVP nanoruler cathode originates from a low-strain K⁺ uptake/removal mechanism, inherent semiconductor characteristic, and small K⁺ migration energy barrier. The high energy density and prolonged cyclic stability of KVP nanorulers//polyaniline-intercalated layered titanate full battery verifies the superiority of KVP nanoruler cathode in PIBs.

Mechanistic Insights into the Structural Modulation of Transition Metal Selenides to Boost Potassium Ion Storage Stability

Atomic-level structure engineering is an effective strategy to reduce mechanical degradation and boost ion transport kinetics for battery anodes. To address the electrode failure induced by large ionic radius of K⁺ ions, herein we synthesized Mn-doped ZnSe with modulated electronic structure for potassium ion batteries (PIBs). State-of-the-art analytical techniques and theoretical calculations were conducted to probe crystalline structure changes, ion/electron migration pathways, and micromechanical stresses evolution mechanisms. We demonstrate that the heterogeneous adjustment of the electronic structure can relieve the potassiumization-induced internal strain and improve the structural stability of battery anodes. Our work highlights the importance of the correlation between doping chemistry and mechanical stability, inspiring a pathway of structural engineering strategy toward a highly stable PIBs.

Metal-Organic-Framework-Derived Nanostructures as Multifaceted Electrodes in Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) are considered up-and-coming future-generation energy storage systems because of their prominent theoretical energy density. However, the practical applications of MSBs are still hampered by several critical challenges, i.e., the shuttle effects, sluggish redox kinetics, and low conductivity of sulfur species. Recently, benefiting from the high surface area, regulated networks, molecular/atomic-level reactive sites, the metal-organic frameworks (MOFs)-derived nanostructures have emerged as efficient and durable multifaceted electrodes in MSBs. Herein, a timely review is presented on recent advancements in designing MOF-derived electrodes, including fabricating strategies, composition management, topography control, and electrochemical performance assessment. Particularly, the inherent charge transfer, intrinsic polysulfide immobilization, and catalytic conversion on designing and engineering of MOF nanostructures for efficient MSBs are systematically discussed. In the end, the essence of how MOFs' nanostructures influence their electrochemical properties in MSBs and conclude the future tendencies regarding the construction of MOF-derived electrodes in MSBs is exposed. It is believed that this progress review will provide significant experimental/theoretical guidance in designing and understanding the MOF-derived nanostructures as multifaceted electrodes, thus offering promising orientations for the future development of fast-kinetic and robust MSBs in broad energy fields.

A General Self-Sacrifice Template Strategy to 3D Heteroatom-Doped Macroporous Carbon for High-Performance Potassium-Ion Hybrid Capacitors

Potassium-ion hybrid capacitors (PIHCs) tactfully combining capacitor-type cathode with battery-type anode have recently attracted increasing attentions due to their advantages of decent energy density, high power density, and low cost; the mismatches of capacity and kinetics between capacitor-type cathode and battery-type anode in PIHCs yet hinder their overall performance output. Herein, based on prediction of density functional theory calculations, we find Se/N co-doped porous carbon is a promising candidate for K⁺ storage and thus develop a simple and universal self-sacrifice template method to fabricate Se and N co-doped three-dimensional (3D) macroporous carbon (Se/N-3DMpC), which features favorable properties of connective hierarchical pores, expanded interlayer structure, and rich activity site for boosting pseudocapacitive activity and kinetics toward K⁺ storage anode and enhancing capacitance performance for the reversible anion adsorption/desorption cathode. As expected, the as-assembled PIHCs full cell with a working voltage as high as 4.0 V delivers a high energy density of 186 Wh kg^-1 and a power output of 8100 W kg^-1 as well as excellent long service life. The proof-of-concept PIHCs with excellent performance open a new avenue for the development and application of high-performance hybrid capacitors.

An Emerging Energy Storage System: Advanced Na-Se Batteries

Sodium-selenium (Na-Se) batteries have aroused enormous attention due to the large abundance of the element sodium as well as the high electronic conductivity and volumetric capacity of selenium. In this battery system, some primary advances in electrode materials have been achieved, mainly involving the design of Se-based cathode materials. In this Review, the electrochemical mechanism is discussed, thus revealing the main challenges in Na-Se batteries. Then, the advances in the design of Se-based cathode materials for Na-ion storage are systemically summarized, classified, and discussed, including Se/carbon composite, Se/polar material/carbon composites, and hybrid Se_xS_y alloys. Some potential strategies enabling the improvement of crucial challenges and enhancement of electrochemical performance are also proposed to provide guidelines for the enhancements of Na-ion storage. An outlook for future valuable research directions is proposed to understand more deeply the electrochemical mechanism of Na-Se batteries and promote their further developments in full cell performance and commercialization.

Confined Selenium in N-Doped Mesoporous Carbon Nanospheres for Sodium-Ion Batteries

In this paper, we have adopted a simple and etching-free method to prepare mesoporous carbon spheres in one step. Selenium can be deposited in the internal cavity, which can avoid pulverization due to the combined effect of volume expansion and a solid-electrolyte interphase (SEI) film while charging and discharging. Therefore, the as-prepared selenium and nitrogen codoped mesoporous carbon nanosphere (Se@NMCS) composites can deliver an outstanding sodium-storage performance of 336.6 mAh g^-1 at a present density of 200 mA g^-1 and great long-cycling performance. For a further understanding of the Na⁺ storage mechanism of the Se@NMCS anode in sodium-ion batteries (SIBs), the phase evolution of the Se@NMCS anode has been explored during the charge/discharge process by conducting in situ Raman investigation.