Direct Synthesis of Few-Layer F-Doped Graphene Foam and Its Lithium/Potassium Storage Properties

Surface Charge Regulation of Graphene by Fluorine and Chlorine Co-Doping for Constructing Ultra-Stable and Large Energy Density Micro-Supercapacitors

Settling the structure stacking of graphene (G) nanosheets to maintain the high dispersity has been an intense issue to facilitate their practical application in the microelectronics-related devices. Herein, the co-doping of the highest electronegative fluorine (F) and large atomic radius chlorine (Cl) into G via a one-step electrochemical exfoliation protocol is engineered to actualize the ultralong cycling stability for flexible micro-supercapacitors (MSCs). Density functional theoretical calculations unveiled that the F into G can form the "ionic" C─F bond to increase the repulsive force between nanosheets, and the introduction of Cl can enlarge the layer spacing of G as well as increase active sites by accumulating the charge on pore defects. The co-doping of F and Cl generates the strong synergy to achieve high reversible capacitance and sturdy structure stability for G. The as-constructed aqueous gel-based MSC exhibited the superb cycling stability for 500,000 cycles with no capacitance loss and structure stacking. Furthermore, the ionic liquid gel-based MSC demonstrated a high energy density of 113.9 mW h cm-3 under high voltage of up to 3.5 V. The current work enlightens deep insights into the design and scalable preparation of high-performance co-doped G electrode candidate in the field of flexible microelectronics.

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.

On the Road to the Frontiers of Lithium-Ion Batteries: A Review and Outlook of Graphene Anodes

Graphene has long been recognized as a potential anode for next-generation lithium-ion batteries (LIBs). The past decade has witnessed the rapid advancement of graphene anodes, and considerable breakthroughs are achieved so far. In this review, the aim is to provide a research roadmap of graphene anodes toward practical LIBs. The Li storage mechanism of graphene is started with and then the approaches to improve its electrochemical performance are comprehensively summarized. First, morphologically engineered graphene anodes with porous, spheric, ribboned, defective and holey structures display improved capacity and rate performance owing to their highly accessible surface area, interconnected diffusion channels, and sufficient active sites. Surface-modified graphene anodes with less aggregation, fast electrons/ions transportation, and optimal solid electrolyte interphase are discussed, demonstrating the close connection between the surface structure and electrochemical activity of graphene. Second, graphene derivatives anodes prepared by heteroatom doping and covalent functionalization are outlined, which show great advantages in boosting the Li storage performances because of the additionally introduced defect/active sites for further Li accommodation. Furthermore, binder-free and free-standing graphene electrodes are presented, exhibiting great prospects for high-energy-density and flexible LIBs. Finally, the remaining challenges and future opportunities of practically available graphene anodes for advanced LIBs are highlighted.

Single-step salt-template-based scalable production of 2D carbon sheets heterostructured with nickel nanocatalysts for lowering overpotential of hydrogen evolution reaction

Non-precious metals have been considered as suitable alternatives for high-performance hydrogen evolution reactions (HER). Although the incorporation of carbon substances is shown to improve the number of active sites, electron transfer pathways, and long-term stability, there have been rare reports on their single-step scalable production. Herein, we realize free-standing two-dimensional (2D) carbon sheets heterostructured with nickel (Ni) nanocatalysts by pyrolyzing ultrathin layers of acetate tetrahydrate (i.e. the single precursor for both Ni and C sources) over water-soluble salt crystals. Such a salt-templated methodology is environmentally friendly and readily scalable without the implementation of sophisticated equipment. The resulting 2D carbon sheets exhibit an average small thickness of ∼ 3 nm and lateral dimensions with tens of micrometers, where a large number of nano-sized Ni particles with an average diameter of 14 nm are uniformly dispersed. Such 2D Ni-C sheets demonstrate a small overpotential of 111 mV at 10 mA/cm2 and a low Tafel slope of 86 mV/dec for HER in 1 M KOH, which is significantly improved over those of reported non-precious metals composited with carbon substances. This work offers new insight into the design and practical production of non-precious metal matrixes for economical HER.

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.

Electrosynthesis of H2O2 through a two-electron oxygen reduction reaction by carbon based catalysts: From mechanism, catalyst design to electrode fabrication

Hydrogen peroxide (H2O2) is an efficient oxidant with multiple uses ranging from chemical synthesis to wastewater treatment. The in-situ H2O2 production via a two-electron oxygen reduction reaction (ORR) will bring H2O2 beyond its current applications. The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts in order to provide a full and comprehensive picture of the current state of the art treatments and inspire new research in this area. Herein, the most up-to-date findings in theoretical predictions, synthetic methodologies, and experimental investigations of carbon-based catalysts are systematically summarized. Various electrode fabrication and modification methods were also introduced and compared, along with our original research on the air-breathing cathode and three-phase interface theory inside a porous electrode. In addition, our current understanding of the challenges, future directions, and suggestions on the carbon-based catalyst designs and electrode fabrication are highlighted.

Insight into a Nitrogen-Doping Mechanism in a Hard-Carbon-Microsphere Anode Material for the Long-Term Cycling of Potassium-Ion Batteries

To investigate the alternatives to lithium-ion batteries, potassium-ion batteries have attracted considerable interest due to the cost-efficiency of potassium resources and the relatively lower standard redox potential of K⁺/K. Among various alternative anode materials, hard carbon has the advantages of extensive resources, low cost, and environmental protection. In the present study, we synthesize a nitrogen-doping hard-carbon-microsphere (N-SHC) material as an anode for potassium-ion batteries. N-SHC delivers a high reversible capacity of 248 mAh g⁻¹ and a promoted rate performance (93 mAh g⁻¹ at 2 A g⁻¹). Additionally, the nitrogen-doping N-SHC material also exhibits superior cycling long-term stability, where the N-SHC electrode maintains a high reversible capacity at 200 mAh g⁻¹ with a capacity retention of 81% after 600 cycles. DFT calculations assess the change in K ions’ absorption energy and diffusion barriers at different N-doping effects. Compared with an original hard-carbon material, pyridinic-N and pyrrolic-N defects introduced by N-doping display a positive effect on both K ions’ absorption and diffusion.

Recent Progress of Carbon-Based Anode Materials for Potassium Ion Batteries

With the increasing demand for clean energy, rechargeable batteries with K⁺ as carriers have attracted wide attention due to their advantages of expandability and low cost. High-performance anode materials are the key to the development of potassium ion batteries (PIBs), improving their competitiveness and feasibility. Carbon materials have become promising anodes for PIBs due to their abundant resources, low cost, non-toxicity and electrochemical diversity. This article reviews the research progress of carbon based anode materials in recent years. Firstly, the unique characteristics of carbon as a competitive anode for advanced PIBs are discussed, which provides guidance for optimal design and exploration. Then, various carbon materials as the anodes towards PIBs are summarized in detail, and the involved problems and corresponding solutions are analyzed. Finally, the future development and perspective of advanced carbons for next-generation PIBs are proposed.

High-performance K-ion half/full batteries with superb rate capability and cycle stability

SignificanceThe present work might be significant for exploring advanced K-ion batteries with superb rate capability and cycle stability toward practical applications. The as-assembled K-ion half cell exhibits an excellent rate capability of 428 mA h g-1 at 100 mA g-1 and a high reversible specific capacity of 330 mA h g-1 with 120% specific capacity retention after 2,000 cycles at 2,000 mA g-1, which is the best among those based on carbon materials. The as-constructed full cell delivers 98% specific capacity retention over 750 cycles at 500 mA g-1, superior to most of those based on carbon materials that have been reported thus far.

N, P Codoped Hollow Carbon Nanospheres Decorated with MoSe2 Ultrathin Nanosheets for Efficient Potassium-Ion Storage

Potassium-ion batteries (KIBs) are gradually being considered as an alternative for lithium-ion batteries because of their non-negligible advantages such as abundance and low expenditure of K, as well as higher electrochemical potential than another alternative─sodium-ion batteries. Nevertheless, when the electrode materials are inserted and extracted with large-sized K⁺ ions, the tremendous volume change will cause the collapse of the microstructures of electrodes and make the charging/discharging process irreversible, thus disapproving their extended application. In response to this, we put forward a feasible strategy to realize the in situ assembly of layered MoSe₂ nanosheets onto N, P codoped hollow carbon nanospheres (MoSe₂/NP-HCNSs) through thermal annealing and heteroatom doping strategies, and the resulting nanoengineered material can function well as an anode for KIBs. This cleverly designed nanostructure of MoSe₂/NP-HCNS can broaden the interlayer spacing of MoSe₂ to boost the efficiency of the insertion/extraction of K ions and also can accommodate large volume change-caused mechanical strain, facilitate electrolyte penetration, and prevent the aggregation of MoSe₂ nanosheets. This synthetic method generates abundant defects to increase the amounts of active sites, as well as conductivity. The hierarchical nanostructure can effectively increase the proportion of pseudo-capacitance and promote interfacial electronic transfer and K⁺ diffusion, thus imparting great electrochemical performance. The MoSe₂/NP-HCNS anode exhibits a high reversible capacity of 239.9 mA h g^-1 at 0.1 A g^-1 after 200 cycles and an ultralong cycling life of 161.1 mA h g^-1 at 1 A g^-1 for a long period of 1000 cycles. This nanoengineering method opens up new insights into the development of promising anode materials for KIBs.

PVDF-triggered multicolor fluorine-doped graphene quantum dots for water detection and anti-counterfeiting

Fluorescent fluorine-doped graphene quantum dots (F-GQDs) have been synthesized via the hydrothermal method using long-chain polymer polyvinylidene fluoride (PVDF) as the precursor. Due to the unique molecular structure of PVDF, a possible synthesis process of F-GQDs has been put forward. F-GQDs have adjustable emission wavelength by simply adjusting the concentration of the solution. As the concentration increases, the emission wavelength of F-GQDs gradually red shifts from 455 nm (blue) to 551 nm (yellow-green). In addition, F-GQDs also exhibit a sensitive fluorescence response to water content in organic solvents, and the ultralow detections limit are 0.056% in ethanol and 0.124% in DMF. Besides, due to strong UV absorption capacity, a photothermal film is fabricated by embedding F-GQDs in PDMS. The temperature of F-GQDs/PDMS polymer film can reach 33.4 ^oC under simulated sunlight, while the maximum temperature of blank PDMS film only reach 29.4 ^oC. Based on this phenomenon, a new type of anti-counterfeiting device is designed by combining F-GQDs/PDMS film with temperature change ink.

Carbon-Based Electrocatalysts for Efficient Hydrogen Peroxide Production

Hydrogen peroxide (H₂ O₂ ) is an environment-friendly and efficient oxidant with a wide range of applications in different industries. Recently, the production of hydrogen peroxide through direct electrosynthesis has attracted widespread research attention, and has emerged as the most promising method to replace the traditional energy-intensive multi-step anthraquinone process. In ongoing efforts to achieve highly efficient large-scale electrosynthesis of H₂ O₂ , carbon-based materials have been developed as 2e^- oxygen reduction reaction catalysts, with the benefits of low cost, abundant availability, and optimal performance. This review comprehensively introduces the strategies for optimizing carbon-based materials toward H₂ O₂ production, and the latest advances in carbon-based hybrid catalysts. The active sites of the carbon-based materials and the influence of coordination heteroatom doping on the selectivity of H₂ O₂ are extensively analyzed. In particular, the appropriate design of functional groups and understanding the effect of the electrolyte pH are expected to further improve the selective efficiency of producing H₂ O₂ via the oxygen reduction reaction. Methods for improving catalytic activity by interface engineering and reaction kinetics are summarized. Finally, the challenges carbon-based catalysts face before they can be employed for commercial-scale H₂ O₂ production are identified, and prospects for designing novel electrochemical reactors are proposed.

Electrochemical removal of tetrabromobisphenol A by fluorine-doped titanium suboxide electrochemically reactive membrane

This study reports fluorine-doped titanium suboxide anode for electrochemical mineralization of hydrophobic micro-contaminant, tetrabromobisphenol A. Fluorinated TiSO anode promoted electro-generated hydroxyl radicals (•OH) with higher selectivity and activity, due to increased O₂ evolution potential and more loosely interaction with hydrophobic electrode surface. For electro-oxidation process, fluorine doping had an insignificant impact on outer-sphere reaction and exerted inhibition on inner-sphere reaction, as indicated by cyclic voltammogram performed on Ru(NH₃)₆^3+/2+, Fe(CN)₆^3-/4- and Fe^3+/2+ redox couple. This facilitated electrochemical conversion of TBBPA and intermediates via more efficient outer-sphere reaction and hydroxylation route. Additionally, generated O₂ micro-bubbles could be stabilized on hydrophobic F-doped TiSO anode, which extended the three-phase boundary available for interfacial enrichment of TBBPA and subsequent mineralization. Under action of these comprehensive factors, 0.5% F-doped TiSO electrochemically reactive membrane could achieve 99.7% mineralization of TBBPA upon energy consumption of 0.52 kWh m^-3 at current density of 7.8 ± 0.24 mA cm^-2 (3.75 V vs SHE) and flow rate of 1628 LHM based on flow-through electrolysis. The modified anode exhibited superior performances compared with un-modified one with more efficient TBBPA removal, less toxic intermediate accumulation and lower energy consumption. The results may have important implications for electrochemical removal and detoxification of hydrophobic micro-pollutants.

Nitrogen and Fluorine Dual Doping of Soft Carbon Nanofibers as Advanced Anode for Potassium Ion Batteries

Potassium-ion batteries (PIBs) are recognized as promising alternatives for lithium-ion batteries as the next-generation energy storage systems. However, the larger radius of K⁺ hinders the K⁺ insertion into the conventional carbon electrode and results in sluggish potassiation kinetics and poor cycling stability. Here, nitrogen and fluorine dual doping of soft carbon nanotubes (NFSC) anode are synthesized in one pot, achieving extraordinary electrochemical performance for PIBs. It is demonstrated that NFSC with a doping dose of 5.6 at% nitrogen and 1.3 at% fluorine together exhibits the highest reversible capacity of 238 mAh g^-1 at 0.2 A g^-1 and cycling stability of 186 mAh g^-1 after 1000 cycles at 1 A g^-1 . The extraordinary electrochemical performance can be attributed to the hollow structure, expanded interlayer distance, nitrogen and fluorine dual doping, and the binding ability of abundant defect sites. Moreover, density functional theory shows that the extra fluorine modification can dramatically enhance the conventional nitrogen doping effect and reduces the formation energy which makes a great contribution to the improvement of electrical conduction and K-ions insert. This work may promote the development of low-cost and sustainable carbon-based materials for PIBs and other advanced energy storage devices.

Hierarchical Porous Carbon Nanofibers with Tunable Geometries and Porous Structures Fabricated by a Scalable Electrospinning Technique

Porous carbon nanofibers (PCNFs) have rich channels for transporting ions, molecules, and nanoparticles, but the control over their porous structures is a challenge. Here, we report a scalable electrospinning technique by using poly(tetrafluoroethylene) as a pore template, boric acid as a cross-linking agent, and polyvinyl alcohol and polyurethane as dual carbon precursors to fabricate flexible PCNFs with tunable geometries and macro/meso/microporous structures. In the water solvent, the negatively charged template cross-links with the positively charged carbon precursors to form a stable sol for electrospinning. By varying the mass ratios of these precursors, the electrospun hybrid nanofibers are directly transformed into B-F-N-O doped PCNFs with tunable macro-, meso-, and micropores after carbonization. The porosity of an individual PCNF is as high as ∼85%, and the pore volume can be tuned from 0.23 to 0.58 cm³·g^-1. When constructing high-sulfur-content (86 wt %) electrodes with the freestanding PCNF films, the porous structures with rich electroactive sites provide rapid pathways for poly-anions and have strong chemisorption of poly-sulfides, leading to a great electrochemical performance. The reported strategy offers a new perspective for synthesizing hierarchical PCNFs with appealing applications.

High-Performance Potassium-Ion Batteries with Robust Stability Based on N/S-Codoped Hollow Carbon Nanocubes

Currently, a big challenge for the practical use of potassium-ion batteries (PIBs) is their intrinsically poor cycling stability, due to the relatively large radius of K⁺ and sluggish kinetics for intercalation/deintercalation. Here we report the scalable fabrication of N/S-codoped hollow carbon nanocubes (NSHCCs), which have the potential as an electrode for advanced PIBs with robust stability. Their discharge and charge specific capacities are ∼560 mA h g^-1 and 310 mA h g^-1 at a current density of 50 mA g^-1, respectively. Meanwhile, they exhibit 100% specific capacity retention after 620 cycles over 9 months at a low current density of 50 mA g^-1, which is state-of-the-art among carbon materials. Moreover, they demonstrate nearly no sacrifice in specific capacities with 99.9% retention after 3000 cycles over 4 months under a high current density of 1000 mA g^-1, superior to most carbon analogues for potassium storage previously reported. The improved electrochemical performance of NSHCC can be mainly attributed to the unique hollow carbon nanocubes with incorporated N and S dopants, which can expand the carbon layer spacing, facilitate K⁺ adsorption, and relieve the volume change during the intercalation/deintercalation of K⁺ ions.

Carbon defects applied to potassium-ion batteries: a density functional theory investigation

Functionalized carbon nanomaterials are potential candidates for use as anode materials in potassium-ion batteries (PIBs). The inevitable defect sites in the architectures significantly affect the physicochemical properties of the carbon nanomaterials, thus defect engineering has recently become a vital research area for carbon-based electrodes. However, one of the major issues holding back its further development is the lack of a complete understanding of the effects accounting for the potassium (K) storage of different carbon defects, which have remained elusive. Owing to pressing research demands, the construction strategies, adsorption difficulties, and structure-activity relationships of the carbon defect-involved reaction centers for the K adsorption are systematically summarized using first principles calculations. Carbon defects affect the ability to trap K by affecting the geometry, charge distribution, and conductive behavior of the carbon surface. The results show that carbon doping with pyridinic-N, pyrrolic-N, and P defect sites tend to act as trapping K sites because of electron-deficient sites. However, graphite-N and sulfur doping are less capable of trapping K. In addition, it has been proved using calculations that the defects can inhibit the growth of the K dendrite. Finally, using the molten salt method, we prepared the undoped and nitrogen-doped carbon materials for comparison, verifying the results of the calculation.

Smart fibers for energy conversion and storage

Fibers have played a critical role in the long history of human development. They are the basic building blocks of textiles. Synthetic fibers not only make clothes stronger and more durable, but are also customizable and cheaper. The growth of miniature and wearable electronics has promoted the development of smart and multifunctional fibers. Particularly, the incorporation of functional semiconductors and electroactive materials in fibers has opened up the field of fiber electronics. The energy supply system is the key branch for fiber electronics. Herein, after a brief introduction on the history of smart and functional fibers, we review the current state of advanced functional fibers for their application in energy conversion and storage, focusing on nanogenerators, solar cells, supercapacitors and batteries. Subsequently, the importance of the integration of fiber-shaped energy conversion and storage devices via smart structure design is discussed. Finally, the challenges and future direction in this field are highlighted. Through this review, we hope to inspire scientists with different research backgrounds to enter this multi-disciplinary field to promote its prosperity and development and usher in a truly new era of smart fibers.

"Self-doping" defect engineering in SnP3@gamma-irradiated hard carbon anode for rechargeable sodium storage

Reasonable design of defect engineering in the electrode materials for sodium-ion batteries (SIBs) can significantly optimize battery performance. Here, compared with the traditional "foreign-doping" defects method, we report an innovative gamma-irradiation technique to introduce the "self-doping" defects in the popcorn hard carbon (HC). Considering the advantages of adsorption-intercalation-alloying sodium storage mechanism, the defect-rich HC-coated alloy structure (SnP₃@HC-γ) was integrated. Due to the high energy and strong penetrability of γ-rays, the constructed "self-doping" defect engineering effectively expands the interlayer structure of HC and forms the irregular ring structure. Simultaneously, the exposed large number of coordination unsaturated sites can accelerate the reaction kinetics on the surface. Based on the synergistic effect of the SnP₃@HC-γ, the composites exhibit an excellent reversible capacity of 668 mAh g^-1 at 0.1 A g^-1 in SIBs. Even, after 400 cycles at 1.0 A g^-1, an exceptional cyclability with 88% capacity retention (430 mAh g^-1) can be maintained. We envision that the γ-irradiation technology used in this research not only overturns the general perception that "self-doping" defects will reduce performance, but also provides reliable technical support for large-scale construction of high-defect, high-capacity and stable sodium-ion anode materials.

3D tremella-like nitrogen-doped carbon encapsulated few-layer MoS2 for lithium-ion batteries

MoS₂ is regarded as an attractive anode material for lithium-ion batteries due to its layered structure and high theoretical specific capacity. Its unsatisfied conductivity and the considerable volume change during the charge and discharge process, however, limits its rate performance and cycling stability. Herein, 3D tremella-like nitrogen-doped carbon encapsulated few-layer MoS₂ (MoS₂@NC) hybrid is obtained via a unique strategy with simultaneously poly-dopamine carbonization, and molybdenum oxide specifies sulfurization. The three-dimensional porous nitrogen-doped carbon served both as a mechanical supporting structure for stabilization of few-layers MoS₂ and a good electron conductor. The MoS₂@NC exhibits enhanced high rate performance with a specific capacity of 208.7 mAh g^-1 at a current density of 10 A g^-1 and stable cycling performance with a capacity retention rate of 85.7% after 1000 cycles at 2 A g^-1.

From Micropores to Ultra-micropores inside Hard Carbon: Toward Enhanced Capacity in Room-/Low-Temperature Sodium-Ion Storage

HIGHLIGHTS

Hard-carbon anode dominated with ultra-micropores (< 0.5 nm) was synthesized for sodium-ion batteries via a molten diffusion-carbonization method. The ultra-micropores dominated carbon anode displays an enhanced capacity, which originates from the extra sodium-ion storage sites of the designed ultra-micropores. The thick electrode (~ 19 mg cm-2) with a high areal capacity of 6.14 mAh cm-2 displays an ultrahigh cycling stability and an outstanding low-temperature performance. Pore structure of hard carbon has a fundamental influence on the electrochemical properties in sodium-ion batteries (SIBs). Ultra-micropores (< 0.5 nm) of hard carbon can function as ionic sieves to reduce the diffusion of slovated Na+ but allow the entrance of naked Na+ into the pores, which can reduce the interficial contact between the electrolyte and the inner pores without sacrificing the fast diffusion kinetics. Herein, a molten diffusion-carbonization method is proposed to transform the micropores (> 1 nm) inside carbon into ultra-micropores (< 0.5 nm). Consequently, the designed carbon anode displays an enhanced capacity of 346 mAh g-1 at 30 mA g-1 with a high ICE value of ~ 80.6% and most of the capacity (~ 90%) is below 1 V. Moreover, the high-loading electrode (~ 19 mg cm-2) exhibits a good temperature endurance with a high areal capacity of 6.14 mAh cm-2 at 25 °C and 5.32 mAh cm-2 at - 20 °C. Based on the in situ X-ray diffraction and ex situ solid-state nuclear magnetic resonance results, the designed ultra-micropores provide the extra Na+ storage sites, which mainly contributes to the enhanced capacity. This proposed strategy shows a good potential for the development of high-performance SIBs.

Recent progress in electrochemical performance of binder-free anodes for potassium-ion batteries

Potassium ion batteries (PIBs) are regarded as one of the most promising candidates for large-scale stationary energy storage beyond lithium-ion batteries (LIBs), owing to the abundance of potassium resources and low cost. Unfortunately, the practical application of PIBs is severely restricted by their poor rate capacity and unsatisfactory cycle performance. In traditional electrodes, a binder usually plays an important role in integrating individual active materials with conductive additives. Nevertheless, binders are not only generally electrochemically inactive but also insulating, which is unfavorable for improving overall energy density and cycling stability. To this end, in terms of both improved electronic conductivity and electrochemical reaction reversibility, binder-free electrodes offer great potential for high-performance PIBs. Moreover, the anode is a crucial configuration to determine full cell electrochemical performance. Therefore, this review analyzes in detail the electrochemical properties of the different type binder-free anodes, including carbon-based substrates (graphene, carbon nanotubes, carbon nanofibers, and so on), MXene-based substrates and metal-based substrates (Cu and Ni). More importantly, the recent progress, critical issues, challenges, and perspectives in binder-free electrodes for PIBs are further discussed. This review will provide theoretical guidance for the synthesis of high-performance anode materials and promote the further development of PIBs.

MOF-derived fluorine and nitrogen co-doped porous carbon for an integrated membrane in lithium–sulfur batteries

The F and N co-doped porous carbon derived from ZIF-8 is used as a membrane in Li–S batteries with enhanced capacity and cycling stability.

Recent progress of defect chemistry on 2D materials for advanced battery anodes

The rational design of anode materials plays a significant factor in harnessing energy storage. With an in-depth insight into the relationships and mechanisms that underlie the charge and discharge process of two-dimensional (2D) anode materials. The efficiency of rechargeable batteries has significantly been improved through the implementation of defect chemistry on anode materials. This mini review highlights the recent progress achieved in defect chemistry on 2D materials for advanced rechargeable battery electrodes, including vacancies, chemical functionalization, grain boundary, Stone Wales defects, holes and cracks, folding and wrinkling, layered von der Waals (vdW) heterostructure in 2D materials. The defect chemistry on 2D materials provides numerous features such as a more active adsorption sites, great adsorption energy, better ions-diffusion and therefore higher ion storage, which enhances the efficiency of the battery electrode.

Novel fluorescence probe based on bright emitted carbon dots for ClO- detection in real water samples and living cells

Low-toxic and environmentally friendly carbon dots (CDs) have been extensively applied in various fields. CDs usually demonstrate excellent selectivity and high sensitivity, especially in ion detection. However, the most commonly used CDs are excited by ultraviolet (UV) light and emit weak fluorescence light, limiting their application in some fields. Herein, novel fluorine and nitrogen codoped carbon dots (FNCDs) were prepared by a simple hydrothermal method and used as a fluorescent probe for ion detection. The FNCDs were excited by blue light and emitted strong green fluorescence, and the photoluminescence quantum yield was as high as 56.7%. The fluorescence of the FNCDs could be rapidly quenched by ClO^- ions, indicating their potential application for ClO^- detection. The fluorescence of the FNCDs was quenched by ClO^- ions in less than 1 min, and the intensity of the fluorescence decreased linearly as the ClO^- concentration increased from 0 to 20 μM. The detection limit was calculated to be as low as 8.2 nM, indicating high sensitivity of the FNCDs probe. The quench effect of the ClO^- ions on the FNCDs probe fluorescence was not affected by other ions, demonstrating excellent selectivity of the FNCDs probe. Because of their excellent biological compatibility, the FNCDs were also successfully used to identify exogenous ClO^- in living cells. These FNCDs have promising prospects as novel sensitive and inexpensive probes for the detection of pollutants and in the pathological studies of clinical diseases.

Boosting potassium-ion storage in large-diameter carbon nanotubes/MoP hybrid

Potassium-ion batteries (KIBs) as a substitute for lithium ion batteries have attracted tremendous attention in recent years thanks to the cost-effectiveness and abundance of potassium resources. However, the current lack of suitable electrode materials is a major obstacle against the practical application of KIBs. Hence, design and preparation of capable anode materials are critical for the development of KIBs. In this study, a promising electrode based on N, P-codoped large diameter hollow carbon nanotubes decorated with ultrasmall MoP nanoparticles (MoP@NP-HCNTs) were prepared. The hollow carbon nanotubes facilitate the rapid electron and ion transfer, and release the huge volume expansion during discharge/charge. The MoP@NP-HCNT electrode delivers high initial capacity of 485, 482 and 463 mAh g^-1 corresponding to 100, 200 and 1000 mA g^-1, respectively. The discharge specific capacity still maintains 300 mAh g^-1 at 100 mA g^-1 after over 80 cycles. It still shows ultralong cycling stability with a discharge capacity of 255 mAh g^-1 at a high current density of 1000 mA g^-1 after 120 cycles. This study opens up a new routine to develop high reversible capacity and promising electrode materials for KIBs.

Interface Engineering between the Metal-Organic Framework Nanocrystal and Graphene toward Ultrahigh Potassium-Ion Storage Performance

The potassium-ion battery (PIB) has been recognized as a promising low-cost and high-energy battery; however, it suffers from a relatively low capacity and inferior cycling performance compared with current electrode materials. Herein, we report an effective interface engineering strategy to prepare metal-organic framework (MOF) nanocrystals tightly encapsulated by reduced graphene oxide (rGO) via strong chemical interaction as a free-standing anode for PIB. Based on experimental analysis and theoretical calculations, we systematically investigated the effect of the chemical-bonded interface between MOF nanocrystals and conductive rGO and revealed that the strong chemical interface can substantially enhance the adsorption energy and ion transport kinetics of the potassium ion within the MOF nanocrystals compared to the physical mixture of MOF and rGO with almost the same microscopic morphologies. As a result, such an MOF-rGO hybrid with strong interfacial chemical couplings delivered an ultrahigh reversible capacity of 422 mAh g^-1 at 0.1 A g^-1, superior rate performance (202 mAh g^-1 at 5 A g^-1), and outstanding long-term cycling performance (an ultralow decay rate of 0.013% per cycle after 2000 cycles at 2 A g^-1), which are not only significantly better than those of the physical mixture of MOF/rGO but also among the best for anodes for PIB reported thus far.

Enhancing the Cycling Stability by Tuning the Chemical Bonding between Phosphorus and Carbon Nanotubes for Potassium-Ion Battery Anodes

Phosphorus/carbon (P/C) composites as promising potassium-ion storage materials have been extensively investigated for its compound superiorities of high specific capacity and favorable electronic conductivity. However, the effects of different chemical bonding states between P and the carbon matrix for potassium-ion storage and cycling performance still need to be investigated. Herein, three P/C composites with different chemical bonding states were successfully fabricated through simply ball-milling red P with carboxylic group carbon nanotubes (CGCNTs), carbon nanotubes (CNTs), and reduced carboxylic group carbon nanotubes (RCGCNTs), respectively. When used as potassium-ion battery (PIB) anodes, the red P and CGCNT (P-CGCNT) composite deliver the most outstanding cycling stability (402.6 mAh g^-1 over 110 cycles) with a favorable capacity retention of 68.26% at a current density of 0.1 A g^-1, much higher than that of the phosphorus-CNT (P-CNT) composite (297.5 mAh g^-1 and 50.40%). Based on the results of X-ray photoelectron spectroscopy and electrochemical performance, we propose that the existence of a carboxyl functional group will be instrumental for the formation of the P-O-C bond. More importantly, when compared with the P-C bond, the P-O-C bond can lead to a higher reversible capacity and a better long-term cycling stability as a result of the more robust bonding energy of the P-O-C bond (585 KJ mol^-1) than that of the P-C bond (264 kJ mol^-1). This work provides some insights into designing high-performance P anodes for PIBs.

Exploring doped or vacancy-modified graphene-based electrodes for applications in asymmetric supercapacitors

We report here density functional theory calculations and molecular dynamics atomistic simulations to determine the total capacitance of graphene-modified supercapacitors.

Three-dimensional nitrogen–sulfur codoped layered porous carbon nanosheets with sulfur-regulated nitrogen content as a high-performance anode material for potassium-ion batteries

Through the gel and nitrogen-sulfur co-doping process, a three-dimensional nitrogen-sulfur co-doped layered porous carbon nanosheet with adjusted nitrogen content was constructed as a high-performance anode material for potassium ion batteries.

Nanocubes composed of FeS2@C nanoparticles as advanced anode materials for K-ion storage

The unique core–shell structural FeS₂@C nanocubes display outstanding K-storage performance with impressive specific capacity, excellent cycling stability and superior rate capability with 73% capacity retention at 2 A g⁻¹.

Reversible Alloying of Phosphorene with Potassium and Its Stabilization Using Reduced Graphene Oxide Buffer Layers

High specific capacity materials that can store potassium (K) are essential for next-generation K-ion batteries. One such candidate material is phosphorene (the 2D allotrope of phosphorus (P)), but the potassiation capability of phosphorene has not yet been established. Here we systematically investigate the alloying of few-layer phosphorene (FLP) with K. Unlike lithium (Li) and sodium (Na), which form Li₃P and Na₃P, FLP alloys with K to form K₄P₃, which was confirmed by ex situ X-ray characterization as well as density functional theory calculations. The formation of K₄P₃ results in high specific capacity (∼1200 mAh g^-1) but poor cyclic stability (only ∼9% capacity retention in subsequent cycles). We show that this capacity fade can be successfully mitigated by the use of reduced graphene oxide (rGO) as buffer layers to suppress the pulverization of FLP. We studied the performance of rGO and single-walled carbon nanotubes (sCNTs) as buffer materials and found that rGO being a 2D material can better encapsulate and protect FLP relative to 1D sCNTs. The half-cell performance of FLP/rGO could also be successfully reproduced in a full-cell configuration, indicating the possibility of high-performance K-ion batteries that could offer a sustainable and low-cost alternative to Li-ion technology.

Capacity Contribution Induced by Pseudo-Capacitance Adsorption Mechanism of Anode Carbonaceous Materials Applied in Potassium-ion Battery

The intrinsic bottleneck of graphite intercalation compound mechanism in potassium-ion batteries necessitates the exploitation of novel potassium storage strategies. Hence, utmost efforts have been made to efficiently utilize the extrinsic pseudo-capacitance, which offers facile routes by employing low-cost carbonaceous anodes to improve the performance of electrochemical kinetics, notably facilitating the rate and power characteristics for batteries. This mini-review investigates the methods to maximize the pseudo-capacitance contribution based on the size control and surface activation in recent papers. These methods employ the use of cyclic voltammetry for kinetics analysis, which allows the quantitative determination on the proportion of diffusion-dominated vs. pseudo-capacitance by verifying a representative pseudo-capacitive material of single-walled carbon nanotubes. Synergistically, additional schemes such as establishing matched binder–electrolyte systems are in favor of the ultimate purpose of high-performance industrialized potassium-ion batteries.

Two-Dimensional T-NiSe2 as a Promising Anode Material for Potassium-Ion Batteries with Low Average Voltage, High Ionic Conductivity, and Superior Carrier Mobility

Potassium-ion batteries (KIBs) have attracted great attention due to their unique advantages including abundant resources, low redox potential of K, and feasible usage of cheap aluminum current collector in battery assembly. In the present work, through first-principles calculations, we find that the recently synthesized two-dimensional T-NiSe₂ is a promising anode material of KIBs. It possesses a large capacity (247 mAh/g), small diffusion barrier (0.05 eV), and low average voltage (0.49 V), rendering T-NiSe₂ a high-performance KIB anode candidate. In addition, we analyze the carrier mobility of T-NiSe₂, and the results demonstrate that it possesses a superior carrier mobility of 1685 cm²/(V s), showing the potential for applications in nanoelectronic devices.

Bio-Derived Hierarchical Multicore-Shell Fe2N-Nanoparticle-Impregnated N-Doped Carbon Nanofiber Bundles: A Host Material for Lithium-/Potassium-Ion Storage

Despite the significant progress in the fabrication of advanced electrode materials, complex control strategies and tedious processing are often involved for most targeted materials to tailor their compositions, morphologies, and chemistries. Inspired by the unique geometric structures of natural biomacromolecules together with their high affinities for metal species, we propose the use of skin collagen fibers for the template crafting of a novel multicore-shell Fe2N-carbon framework anode configuration, composed of hierarchical N-doped carbon nanofiber bundles firmly embedded with Fe2N nanoparticles (Fe2N@N-CFBs). In the resultant heterostructure, the Fe2N nanoparticles firmly confined inside the carbon shells are spatially isolated but electronically well connected by the long-range carbon nanofiber framework. This not only provides direct and continuous conductive pathways to facilitate electron/ion transport, but also helps cushion the volume expansion of the encapsulated Fe2N to preserve the electrode microstructure. Considering its unique structural characteristics, Fe2N@N-CFBs as an advanced anode material exhibits remarkable electrochemical performances for lithium- and potassium-ion batteries. Moreover, this bio-derived structural strategy can pave the way for novel low-cost and high-efficiency syntheses of metal-nitride/carbon nanofiber heterostructures for potential applications in energy-related fields and beyond.

Nitrogen and Phosphorus Codoped Vertical Graphene/Carbon Cloth as a Binder-Free Anode for Flexible Advanced Potassium Ion Full Batteries

With the fast development in flexible electronic technology, power supply devices with high performance, low-cost, and flexibility are becoming more and more important. Potassium ion batteries (KIBs) have a brilliant prospect for applications benefiting from high voltage, lost cost, as well as similar electrochemistry to lithium ion batteries (LIBs). Although carbon materials have been studied as KIBs anodes, their rate capability and cycling stability are still unsatisfactory due to the large-size potassium ions. Herein, a nitrogen (N) and phosphorus (P) dual-doped vertical graphene (N, P-VG) uniformly grown on carbon cloth (N, P-VG@CC) is reported as a binder-free anode for flexible KIBs. With the combined advantages of rich active sites, highly accessible surface, highly conductive network, larger interlayer spacing as well as robust structural stability, this binder-free N, P-VG@CC anode exhibits high capacity (344.3 mAh g^-1 ), excellent rate capability (2000 mA g^-1 ; 46.5% capacity retention), and prominent long-term cycling stability (1000 cycles; 82% capacity retention), outperforming most of the recently reported carbonaceous anodes. Moreover, a potassium ion full cell is successfully assembled on the basis of potassium Prussian blue (KPB)//N, P-VG@CC, exhibiting a large energy density of 232.5 Wh kg^-1 and outstanding cycle stability.

3D Sulfur and Nitrogen Codoped Carbon Nanofiber Aerogels with Optimized Electronic Structure and Enlarged Interlayer Spacing Boost Potassium-Ion Storage

Carbonaceous materials are promising anodes for potassium-ion batteries (PIBs). However, it is hard for large K ions (1.38 Å) to achieve long-distance diffusion in pristine carbonaceous materials. In this work, the following are synthesized: S/N codoped carbon nanofiber aerogels (S/N-CNFAs) with optimized electronic structure by S/N codoping, enhanced interlayer spacing by S doping, and a 3D interconnected porous structure of aerogel, through a pyrolysis sustainable seaweed (Fe-alginate) aerogel strategy. Specifically, the S/N-CNFAs electrode delivers high reversible capacities of 356 and 112 mA h g^-1 at 100 and 5000 mA g^-1 , respectively. The capacity reaches 168 mA h g^-1 at 2000 mA g^-1 after 1000 cycles. A full cell with a S/N-CNFAs anode and potassium prussian blue cathode displays a specific capacity of 198 mA h g^-1 at 200 mA g^-1 . Density functional theory calculations indicate that S/N codoping is beneficial to synergistically improve K ions storage of S/N-CNFAs by enhancing the adsorption of K ions and reducing the diffusion barrier of K ions. This work offers a facile heteroatom doping paradigm for designing new carbonaceous anodes for high-performance PIBs.

Approaching high-performance potassium-ion batteries via advanced design strategies and engineering

Potassium-ion batteries (PIBs) have attracted tremendous attention due to their low cost, fast ionic conductivity in electrolyte, and high operating voltage. Research on PIBs is still in its infancy, however, and achieving a general understanding of the drawbacks of each component and proposing research strategies for overcoming these problems are crucial for the exploration of suitable electrode materials/electrolytes and the establishment of electrode/cell assembly technologies for further development of PIBs. In this review, we summarize our current understanding in this field, classify and highlight the design strategies for addressing the key issues in the research on PIBs, and propose possible pathways for the future development of PIBs toward practical applications. The strategies and perspectives summarized in this review aim to provide practical guidance for an increasing number of researchers to explore next-generation and high-performance PIBs, and the methodology may also be applicable to developing other energy storage systems.