Rational design of metal organic framework-derived FeS2 hollow nanocages@reduced graphene oxide for K-ion storage

Core-shell structured Fe2O3@C hollow nanospheres as a high-performance negative material for potassium-ion batteries

Fe2O3 is considered a promising electrode for potassium-ion batteries (PIBs) applications due to their natural abundance, low cost and high theoretical capacity. However, Fe2O3 suffers from capacity decay and sluggish reaction kinetic during the electrochemical process. Herein, the unique core-shell Fe2O3@C featured with hollow nanospheres Fe2O3 as core and amorphous carbon layer as protect shell, the optimal framework of Fe2O3@C is proposed to improve the structural stability and promote K+ charge transport. Accordingly, the Fe2O3@C delivers an extraordinary cycling performance (437 mAh g-1 after 200 cycles at 0.2 A/g), which is a remarkable electrochemical performance in Fe-based oxides negative electrode. To further elucidate the K-ion storage mechanisms of Fe2O3, an in-depth characterization of Fe2O3 phase transition and changes in the coordination environment of iron atoms using in operando synchrotron techniques. Results from this study proved that K+/Fe2+/3+ displacement/reordering occurs in the Fe2O3@C electrode, which leads to inverse Fe2O3 (maghemite and hematite) phase and turning into KxFe2O3 (0 < x < 2, intermediate rock-salt-like phase), finally converting into K2O and metallic Fe. Furthermore, the Mg0.008K0.51V2O5 (MKVO)//Fe2O3@C full cell was assembled to investigate the practical application. These results may provide theoretical support for modifying Fe-base metal oxides of PIBs.

Natural Pitch-Derived Carbon Networks Induced Lattice Strain Engineering in Nickel-Based Heterostructures Enables Efficient Anodes for Sodium-Ion Batteries

The development of high-performance sodium-ion batteries (SIBs) relies on enhancing the electrochemical properties of the electrodes, particularly the transition metal compounds (TMCs) through effective carbon coatings. Herein, a straightforward approach using polymerized natural pitch-derived carbon (PNPC) via step-growth polymerization regulates the lattice strain in Ni3S2-NiO heterostructures (NSNO) on nickel foam (NF). This method replaces the complex multistep carbon coatings with a cost-effective liquid-phase application of PNPC, followed by pyrolysis to create PNPC@NSNO/NF. Comparative analysis shows that PNPC effectively modulates lattice strain, achieving 3.50% tensile strain compared to 5.60% for non-polymerized carbon. The optimized PNPC@NSNO/NF electrode exhibits exceptional high areal capacity of 2.72 mAh cm-2@1 mA cm-2, impressive rate capability, and 97.28% capacity retention after 200 cycles. The enhanced contact area and electrical conductivity provided by the PNPC improve charge transfer kinetics and overall performance. Theoretical analyses confirm that the PNPC@NSNO/NF electrode with 3.50% lattice strain lowers the Na⁺ diffusion barrier, enhances charge transfer, and improves charge distribution, boosting the electrode performance. This work establishes a straightforward method for synthesizing lattice-strained SIB anodes, highlighting its potential for advancing SIB technology.

Prussian Blue Analogue-Templated Nanocomposites for Alkali-Ion Batteries: Progress and Perspective

Lithium-ion batteries (LIBs) have dominated the portable electronic and electrochemical energy markets since their commercialisation, whose high cost and lithium scarcity have prompted the development of other alkali-ion batteries (AIBs) including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Owing to larger ion sizes of Na+ and K+ compared with Li+, nanocomposites with excellent crystallinity orientation and well-developed porosity show unprecedented potential for advanced lithium/sodium/potassium storage. With enticing open rigid framework structures, Prussian blue analogues (PBAs) remain promising self-sacrificial templates for the preparation of various nanocomposites, whose appeal originates from the well-retained porous structures and exceptional electrochemical activities after thermal decomposition. This review focuses on the recent progress of PBA-derived nanocomposites from their fabrication, lithium/sodium/potassium storage mechanism, and applications in AIBs (LIBs, SIBs, and PIBs). To distinguish various PBA derivatives, the working mechanism and applications of PBA-templated metal oxides, metal chalcogenides, metal phosphides, and other nanocomposites are systematically evaluated, facilitating the establishment of a structure-activity correlation for these materials. Based on the fruitful achievements of PBA-derived nanocomposites, perspectives for their future development are envisioned, aiming to narrow down the gap between laboratory study and industrial reality.

Design of heterostructured hydrangea-like FeS2/MoS2 encapsulated in nitrogen-doped carbon as high-performance anode for potassium-ion capacitors

The means of structural hybridization such as heterojunction construction and carbon-coating engineering for facilitating charge transfer and electron transport are considered viable strategies to address the challenges associated with the low rate capability and poor cycling stability of sulfide-based anodes in potassium-ion batteries (PIBs). Motivated by these concepts, we have successfully prepared a hydrangea-like bimetallic sulfide heterostructure encapsulated in nitrogen-doped carbon (FMS@NC) using a simple solvothermal method, followed by poly-dopamine wrapping and a one-step sulfidation/carbonization process. When served as an anode for PIBs, this FMS@NC demonstrates a high specific capacity (585 mAh g-1 at 0.05 A/g) and long cycling stability. Synergetic effects of mitigated volume expansions and enhanced conductivity that are responsbile for such high performance have been verified to originate from the heterostructured sulfides and the N-doped carbon matrix. Meanwhile, comprehensive characterization reveals existence of an intercalation-conversion hybrid K-ion storage mechanism in this material. Impressively, a K-ion capacitor with the FMS@NC anode and a commercial activated carbon cathode exhibits a superior energy density of up to 192 Wh kg-1, a high power density, and outstanding cycling stability. This study provides constructive guidance for designing high-performance and durable potassium-ion storage anodes for next-generation energy storage devices.

Thermal Induced Conversion of CoFe Prussian Blue Analogs Nanocubes Wrapped by Doped Carbon Network Exhibiting Fast and Stable Potassium Ion Storage as Anode

Prussian blue analogs (PBAs) show great promise as anode materials for potassium-ion batteries (PIBs) due to their high specific capacity. However, PBAs still suffer from the drawbacks of low electronic conductivity and poor structural stability, leading to inadequate rate and cyclic performance. To address these limitations, CoFe PBA nanocubes wrapped with N/S doped carbon network (CoFe PBA@NSC) as anode for PIBs is designed by using thermal-induced in situ conversion strategy. As expected, the structural advantages of nanosized PBA cubes, such as abundant interfaces and large surface area, enable the CoFe PBA@NSC electrode to demonstrate superior rate properties (557 and 131 mAh g-1 at 0.05 and 10 A g-1) and low capacity degradation (0.093% per cycle over 1000 cycles at 0.5 A g-1). Furthermore, several ex situ characterizations revealed the K-ion storage mechanism. Fe+ and Co0 are generated during potassicization, followed by a completely reversible chemical state of iron while some cobalt monomers remained during depotassication. Additionally, the as-built potassium-ion hybrid capacitor based on CoFe PBA@NSC anode exhibits a high energy density of 118 Wh kg-1. This work presents an alternative but promising synthesis route for Prussian blue analogs, which is significant for the advancement of PIBs and other related energy storage devices.

Electrolyte and Interphase Engineering of Aqueous Batteries Beyond "Water-in-Salt" Strategy

Aqueous batteries are promising alternatives to non-aqueous lithium-ion batteries due to their safety, environmental impact, and cost-effectiveness. However, their energy density is limited by the narrow electrochemical stability window (ESW) of water. The "Water-in-salts" (WIS) strategy is an effective method to broaden the ESW by reducing the "free water" in the electrolyte, but the drawbacks (high cost, high viscosity, poor low-temperature performance, etc.) also compromise these inherent superiorities. In this review, electrolyte and interphase engineering of aqueous batteries to overcome the drawbacks of the WIS strategy are summarized, including the developments of electrolytes, electrode-electrolyte interphases, and electrodes. First, the main challenges of aqueous batteries and the problems of the WIS strategy are comprehensively introduced. Second, the electrochemical functions of various electrolyte components (e.g., additives and solvents) are summarized and compared. Gel electrolytes are also investigated as a special form of electrolyte. Third, the formation and modification of the electrolyte-induced interphase on the electrode are discussed. Specifically, the modification and contribution of electrode materials toward improving the WIS strategy are also introduced. Finally, the challenges of aqueous batteries and the prospects of electrolyte and interphase engineering beyond the WIS strategy are outlined for the practical applications of aqueous batteries.

Dual carbon engineering enabling 1T/2H MoS2 with ultrastable potassium ion storage performance

Potassium-ion batteries (PIBs) as a promising and low-cost battery technology offer the advantage of utilizing abundant and cost-effective K-salt sources. However, the effective adoption of PIBs necessitates the identification of suitable electrode materials. The 1T phase of MoS2 exhibits enhanced electronic conductivity and greater interlayer spacing compared to the 2H phase, leading to a capable potassium ion storage ability. Herein, we fabricated dual carbon engineered 1T/2H MoS2via a secure and straightforward ammonia-assisted hydrothermal method. The 1T/2H MoS2@rGO@C structure demonstrated an expanded interlayer spacing (9.3 Å). Additionally, the sandwich-like structural design not only enhanced material conductivity but also effectively curbed the agglomeration of nanosheets. Remarkably, 1T/2H MoS2@rGO@C exhibited impressive potassium storage ability, delivering capacities of 351.0 mA h g-1 at 100 mA g-1 and 233.8 mA h g-1 at 1000 mA g-1 following 100 and 1000 cycles, respectively. Moreover, the construction of a K-ion full cell was successfully achieved, utilizing perylene tetracarboxylic dianhydride (PTCDA) as the cathode, and manifesting a capacity of 294.3 mA h g-1 at 100 mA g-1 after 160 cycles. This underscores the substantial potential of employing the 1T/2H MoS2@rGO@C electrode material for PIBs.

Sulfur-Decorated Ti3 C2 TX MXene for High-Performance Sodium/Potassium-Ion Batteries

As post-lithium ion batteries, both sodium-ion batteries (SIBs) and potassium ion batteries (PIBs) possess great potential for large scale energy storage. However, the application of both SIBs and PIBs are hindered by the lack of suitable electrode materials. Here, we synthesized the sulfur decorated Ti3 C2 Tx (S-T3 C2 Tx ) MXene as electrode material for SIBs and PIBs. Thanks to the sulfur functional group and the formation of Ti-S bond, which facilitates the sodium in-/desertion and strengthens the potassium ion adsorption ability, as well as enhances ion reaction kinetics and improved structure stability, the S-T3 C2 Tx exhibit excellent sodium/potassium storage performance, high reversible capacities of 151 and 101 mAh g-1 at 0.1 mA g-1 were achieved for SIBs and PIBs, respectively. Moreover, the S-T3 C2 Tx exhibits remarkable long-term capacity stability at a high density of 500 mA g-1 , providing an impressive storage of 88 mAh g-1 for SIBs and 41 mAh g-1 for PIBs even after 2000 cycles. This work could give a deep comprehension of the heteroatom modification influence on the MXene-based framework and promote the application of MXene electrodes.

Durable modulation of Zn(002) plane deposition via reproducible zincophilic carbon quantum dots towards low N/P ratio zinc-ion batteries

Aqueous zinc-ion batteries (ZIBs) are promising candidates for next-generation energy storage systems due to their intrinsic safety, environmental friendliness, and low cost. However, the uncontrollable Zn dendrite growth during cycling is still a critical challenge for the long-term operation of ZIBs, especially under harsh lean-Zn conditions. Herein, we report nitrogen and sulfur-codoped carbon quantum dots (N,S-CDs) as zincophilic electrolyte additives to regulate the Zn deposition behaviors. The N,S-CDs with abundant electronegative groups can attract Zn2+ ions and co-deposit with Zn2+ ions on the anode surface, inducing a parallel orientation of the (002) crystal plane. The deposition of Zn preferentially along the (002) crystal direction fundamentally avoids the formation of Zn dendrites. Moreover, the co-depositing/stripping feature of N,S-CDs under an electric field force ensures the reproducible and long-lasting modulation of the Zn anode stability. Benefiting from these two unique modulation mechanisms, stable cyclability of the thin Zn anodes (10 and 20 μm) at a high depth of discharge (DOD) of 67% and high Zn||Na2V6O16·3H2O (NVO, 11.52 mg cm-2) full-cell energy density (144.98 W h Kg-1) at a record-low negative/positive (N/P) capacity ratio of 1.05 are achieved using the N,S-CDs as an additive in ZnSO4 electrolyte. Our findings not only offer a feasible solution for developing actual high-energy density ZIBs but also provide in-depth insights into the working mechanism of CDs in regulating Zn deposition behaviors.

In-situ catalytic mechanism coupling quantum dot effect for achieving high-performance sulfide anode in potassium-ion batteries

Though numerous framework structures have been constructed to strengthen the reaction kinetics and durability, the inevitable generation of polysulfide dissolution during conversion-process can cause irreparable destruction to ion-channel and crystal structure integrality, which has become a huge obstacle to the application of metal-sulfide in potassium-ion batteries. Herein, the quantum dot structure with catalytic conversion capability is synchronously introduced into the design of FeS₂ anode materials to heighten its K⁺-storage performance. The constructed quantum dot structure anchored by the graphene with space-confinement effect can shorten the ion diffusion path and enlarge the active area, thus accelerating the K⁺-ions transmission kinetics and absorption action, respectively. The intermediate phase of formed Fe-nanoclusters possesses high-active catalysis ability, which can effectively suppress the polysulfide shuttle combined with the enhanced absorption effect, fully guaranteeing the structure stability and cycling reversibility. Predictably, the fabricated quantum dot FeS₂ can express a prominent advantage in rapid potassiation/depotassiation processes (518.1 mAh g^-1, 10 A g^-1) and a superior cycling lifespan with gratifying reversible capacity at superhigh rate (177.7 mAh g^-1, 9000 cycles, 5 A g^-1). Therefore, engineering quantum dot structure with self-induced catalysis action for detrimental polysulfide is an achievable strategy to implement high-performance sulfide anode materials for K-ions accommodation.

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

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

Multilevel spatial confinement of transition metal selenides porous microcubes for efficient and stable potassium storage

Recently, potassium-ion batteries (PIBs) have been considered as one of the most promising energy storage systems; however, the slow kinetics and large volume variation induced by the large radius of potassium ions (K⁺) during chemical reactions lead to inferior structural stability and weak electrochemical activity for most potassium storage anodes. Herein, a multilevel space confinement strategy is proposed for developing zinc-cobalt bimetallic selenide (ZnSe/Co_0.85Se@NC@C@rGO) as high-efficient anodes for PIBs by in-situ carbonizing and subsequently selenizing the resorcinol-formaldehyde (RF)-coated zeolitic imidazolate framework-8/zeolitic imidazolate framework-67 (ZIF-8/ZIF-67) encapsulated into 2D graphene. The highly porous carbon microcubes derived from ZIF-8/ZIF-67 and carbon shell arising from RF provide rich channels for ion/electron transfer, present a rigid skeleton to ensure the structural stability, offer space for accommodating the volume change, and minimize the agglomeration of active material during the insertion/extraction of large-radius K⁺. In addition, the three-dimensional (3D) carbon network composed of graphene and RF-derived carbon-coated microcubes accelerates the electron/ion transfer rate and improves the electrochemical reaction kinetics of the material. As a result, the as-synthesized ZnSe/Co_0.85Se@NC@C@rGO as the anode of PIBs possesses the excellent rate capability of 203.9 mA h g^-1 at 5 A g^-1 and brilliant long-term cycling performance of 234 mA h g^-1 after 2,000 cycles at 2 A g^-1. Ex-situ X-ray diffraction (Ex-situ XRD) diffraction reveals that the intercalation/de-intercalation of K⁺ proceeds through the conversion-alloying reaction. The proposed strategy based on the spatial confinement engineering is highly effective to construct high-performance anodes for PIBs.

Ferric ion substitution renders cadmium metal-organic framework derivatives for modulated Li storage based on local oxidation active centers

In this work, a novel anionic Cd-MOF ([(CH₃)₂NH₂]_n[Cd(HL)DMF]_n·2nH₂O·nDMF, H₄L = 1,2,4,5-tetrakis[(4-carboxy)phenoxymethyl]benzene) was synthesized for the first time. As a precursor, it was utilized to obtain Fe@Cd-MOF crystals via the substitution of Fe³⁺ ions due to a negatively charged framework and free-coordinated carboxyl group. Fe₃O₄/Fe-embedded carbon-based materials (Fe@Cd-MOFD) were further constructed by deriving Fe@Cd-MOF at high temperatures. The derived Fe@Cd-MOFD showed a structure resembling a central city with metal redox centers embedded into a carbon matrix. The introduced Fe³⁺ ions formed a local nano-sized metal oxide upon annealing, and these derived carbon materials offered high electronic conductivity. These pushed Fe@Cd-MOFD to remarkable electrochemical performance with an initial discharge capacity of 1703.8 mA h g^-1. This work offers new insights into the fabrication of novel MOF-derived iron oxide hybrids for lithium storage.

Bifunctional Alloy/Solid-Electrolyte Interphase Layer for Enhanced Potassium Metal Batteries Via Prepassivation

Potassium (K) metal batteries have attracted great attention owing to their low price, widespread distribution, and comparable energy density. However, the arbitrary dendrite growth and side reactions of K metal are attributed to high environmental sensitivity, which is the Achilles' heel of its commercial development. Interface engineering between the current collector and K metal can tailor the surface properties for K-ion flux accommodation, dendrite growth inhibition, parasitic reaction suppression, etc. We have designed bifunctional layers via prepassivation, which can be recognized as an O/F-rich Sn-K alloy and a preformed solid-electrolyte interphase (SEI) layer. This Sn-K alloy with high substrate-related binding energy and Fermi level demonstrates strong potassiophilicity to homogeneously guide K metal deposition. Simultaneously, the preformed SEI layer can effectually eliminate side reactions initially, which is beneficial for the spatially and temporally KF-rich SEI layer on K metal. K metal deposition and protection can be implemented by the bifunctional layers, delivering great performance with a low nucleation overpotential of 0.066 V, a high average Coulombic efficiency of 99.1%, and durable stability of more than 900 h (1 mA cm-2, 1 mAh cm-2). Furthermore, the high-voltage platform, energy, and power densities of K metal batteries can be realized with a conventional Prussian blue analogue cathode. This work provides a paradigm to passivate fragile interfaces for alkali metal anodes.

Graphene-Based Metal-Organic Framework Hybrids for Applications in Catalysis, Environmental, and Energy Technologies

Current energy and environmental challenges demand the development and design of multifunctional porous materials with tunable properties for catalysis, water purification, and energy conversion and storage. Because of their amenability to de novo reticular chemistry, metal-organic frameworks (MOFs) have become key materials in this area. However, their usefulness is often limited by low chemical stability, conductivity and inappropriate pore sizes. Conductive two-dimensional (2D) materials with robust structural skeletons and/or functionalized surfaces can form stabilizing interactions with MOF components, enabling the fabrication of MOF nanocomposites with tunable pore characteristics. Graphene and its functional derivatives are the largest class of 2D materials and possess remarkable compositional versatility, structural diversity, and controllable surface chemistry. Here, we critically review current knowledge concerning the growth, structure, and properties of graphene derivatives, MOFs, and their graphene@MOF composites as well as the associated structure-property-performance relationships. Synthetic strategies for preparing graphene@MOF composites and tuning their properties are also comprehensively reviewed together with their applications in gas storage/separation, water purification, catalysis (organo-, electro-, and photocatalysis), and electrochemical energy storage and conversion. Current challenges in the development of graphene@MOF hybrids and their practical applications are addressed, revealing areas for future investigation. We hope that this review will inspire further exploration of new graphene@MOF hybrids for energy, electronic, biomedical, and photocatalysis applications as well as studies on previously unreported properties of known hybrids to reveal potential "diamonds in the rough".

NiFe-LDH/MXene nano-array hybrid architecture for exceptional capacitive lithium storage

Layered double hydroxides (LDHs) have great advantages in the domain of energy storage because of their exchangeable anions and large specific surface area. Nevertheless, the shortcomings of their poor electrical conductivity, easy stacking of nanosheets, and large volume variation in the cycling processes lead to unsatisfactory cycling stability and rate performance, which severely limits their further application. Therefore, we generated homogeneous nanoarrays of NiFe-LDH on the surface of Ti₃C₂T_x-MXene by a refluxing process. The resulting NiFe-LDH/MXene-500 hybrid material was applied as an anode of a lithium-ion battery (LIB) and exhibited a discharge capacity of 894.8 mA h g^-1 at 200 mA g^-1 (over 300 cycles) and could maintain a reversible capacity of 547.1 mA h g^-1 even at 1 A g^-1. With the addition of MXene, the volume increases of the NiFe-LDH/MXene hybrid materials were also significantly alleviated. The thickness of the NiFe-LDH/MXene-500 electrode only increased by 31% after 50 cycles, which was far better than the prepared NiFe-LDH electrode. On the hand, the synergistic interaction of NiFe-LDH and MXene could stabilize the structure, reduce the activation barrier of ion/electron diffusion, and promote electron transfer in the electrode. MXene with high conductivity can be used as electrical and ionic conductance media to promote the transformation reaction of NiFe-LDH. According to the detailed kinetic analysis, the capacitance control behavior is the main electrochemical reaction of NiFe-LDH/MXene electrodes.

Self-Healing of Prussian Blue Analogues with Electrochemically Driven Morphological Rejuvenation

Maintaining the morphology of electrode materials with high invertibility contributes to the prolonged cyclic stability of battery systems. However, the majority of electrode materials tend to degrade during the charge-discharge process owing to the inevitable increase in entropy. Herein, a self-healing strategy is designed to promote morphology rejuvenation in Prussian blue analogue (PBA) cathodes by cobalt doping. Experimental characterization and theoretical calculations demonstrate that a trace amount of cobalt can decelerate the crystallization process and restore the cracked areas to ensure perfect cubic structures of PBA cathodes. The electric field controls the kinetic dynamics, rather than the conventional thermodynamics, to realize the "electrochemically driven dissolution-recrystallization process" for the periodic self-healing phenomenon. The properties of electron transportation and ion diffusion in bulk PBA are also improved by the doping strategy, thus boosting the cyclability with 4000 cycles in a diluent electrolyte. This discovery provides a new paradigm for the construction of self-healing electrodes for cathodes.

Electrochemical performance of CoSe2 with mixed phases decorated with N-doped rGO in potassium-ion batteries

Potassium-ion batteries (PIBs) have received much attention as next-generation energy storage systems because of their abundance, low cost, and slightly lower standard redox potential than lithium-ion batteries (LIBs). Nevertheless, they still face great challenges in the design of the best electrode materials for applications. Herein, we have successfully synthesized nano-sized CoSe₂ encapsulated by N-doped reduced graphene oxide (denoted as CoSe₂@N-rGO) by a direct one-step hydrothermal method, including both orthorhombic and cubic CoSe₂ phases. The CoSe₂@N-rGO anodes exhibit a high reversible capacity of 599.3 mA h g⁻¹ at 0.05 A g⁻¹ in the initial cycle, and in particular, they also exhibit a cycling stability of 421 mA h g⁻¹ after 100 cycles at 0.2 A g⁻¹. Density functional theory (DFT) calculations show that CoSe₂ with N-doped carbon can greatly accelerate electron transfer and enhance the rate performance. In addition, the intrinsic causes of the higher electrochemical performance of orthorhombic CoSe₂ than that of cubic CoSe₂ are also discussed.

Potassium-ion batteries (PIBs) have received much attention as next-generation energy storage systems because of their abundance, low cost, and slightly lower standard redox potential than lithium-ion batteries (LIBs).

Design of reduced graphene oxide coating carbon sub-microspheres hierarchical nanostructure for ultra-stable potassium storage performance

The development of high-performance carbon-based anode materials is still a significant challenge for K-ion storage. In our work, we designed reduced graphene oxide coating carbon sub-microspheres hierarchical nanostructure (CS@RGO) hierarchical nanostructure via a simple freeze-drying and subsequent pyrolysis as anode for K-ion batteries (KIBs), which presented an excellent electrochemical performance for K-ion storage, with a reversible specific capacity of 295 mAh g^-1 after 100 cycles at 100 mAh g^-1. Even at a high current density of 1 A g^-1, our CS@RGO still achieves ultra-stable K-ion storage of 200 mAh g^-1 at 1 A g^-1 after 5000 cycles almost without capacity fade. According to the galvanostatic intermittent titration technique result, the CS@RGO hybrid receives a high average diffusion coefficient of 7.35 × 10^-8 cm² s^-1, contributing to the rapid penetration of K-ion, which facilitates the enhancement of electrochemical performance for KIBs. Besides, we also use Raman spectra to investigate the electrochemical behavior of our CS@RGO hybrid for K-ion storage and confirm the reaction process. We believe that our work will offer the opportunity to enable ultra-stable carbon-based materials by the structure design in the K-ion battery field.

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.

Ultrahigh Rate and Ultralong Life Span Sodium Storage of FePS3 Enabled by the Space Confinement Effect of Layered Expanded Graphite

Metal phosphorus trichalcogenides have been regarded as promising high-capacity anode materials for sodium-ion batteries (SIBs) owing to their high reversible capacity. Nevertheless, their practical application is plagued by poor diffusion kinetics and dramatic volume fluctuations during the charge-discharge process, resulting in no satisfactory rate and life span so far. Herein, we propose a space-confinement strategy to remarkably promote the cycling stability and rate capacity by embedding FePS₃ particles in the interlayer of expanded graphite (EG), which are derived from in situ transformation of graphite intercalation compounds. The layered EG not only greatly alleviates the volume fluctuations of FePS₃ by the space confinement effect so as to maintain the stability of the electrode microstructure, but it also ensures rapid Na⁺ and electron transfer during cycling. When acting as an anode for SIBs, the hybrid electrode delivers a highly reversible capacity of 312.5 mAh g^-1 at an ultrahigh rate of 50 A g^-1 while retaining an ultralong life span of 1300 cycles with a retention of 82.4% at 10 A g^-1. Moreover, the excellent performance of the assembled full battery indicates the practical application potential of FPS/EG.

Carbon-Free Crystal-like Fe1-xS as an Anode for Potassium-Ion Batteries

Potassium-ion batteries (PIBs) as a new electrochemical energy storage system have been considered as a desirable candidate in the post-lithium-ion battery era. Nevertheless, the study on this realm is in its infancy; it is urgent to develop electrode materials with high electrochemical performance and low cost. Iron sulfides as anode materials have aroused wide attention by virtue of their merits of high theoretical capacities, environmental benignity, and cost competitiveness. Herein, we constructed carbon-free crystal-like Fe_1-xS and demonstrated its feasibility as a PIB anode. The unique structural feature endows the prepared Fe_1-xS with plentiful active sites for electrochemical reactions and short transmission pathways for ions/electrons. The Fe_1-xS electrode retained capacities of 420.8 mAh g^-1 after 100 cycles at 0.1 A g^-1 and 212.9 mAh g^-1 after 250 cycles at 1.0 A g^-1. Even at a high rate of 5.0 A g^-1, an average capacity of 167.6 mAh g^-1 was achieved. In addition, a potassium-ion full cell is assembled by employing Fe_1-xS as an anode and potassium Prussian blue as a cathode; it delivered a discharge capacity of 127.6 mAh g^-1 at 100 mA g^-1 after 50 cycles.

In Situ Monitoring the Potassium-Ion Storage Enhancement in Iron Selenide with Ether-Based Electrolyte

As one of the promising anode materials, iron selenide has received much attention for potassium-ion batteries (KIBs). Nevertheless, volume expansion and sluggish kinetics of iron selenide result in the poor reversibility and stability during potassiation-depotassiation process. In this work, we develop iron selenide composite matching ether-based electrolyte for KIBs, which presents a reversible specific capacity of 356 mAh g^-1 at 200 mA g^-1 after 75 cycles. According to the measurement of mechanical properties, it is found that iron selenide composite also exhibits robust and elastic solid electrolyte interphase layer in ether-based electrolyte, contributing to the improvement in reversibility and stability for KIBs. To further investigate the electrochemical enhancement mechanism of ether-based electrolyte in KIBs, we also utilize in situ visualization technique to monitor the potassiation-depotassiation process. For comparison, iron selenide composite matching carbonate-based electrolyte presents vast morphology change during potassiation-depotassiation process. When changing to ether-based electrolyte, a few minor morphology changes can be observed. This phenomenon indicates an occurrence of homogeneous electrochemical reaction in ether-based electrolyte, which results in a stable performance for potassium-ion (K-ion) storage. We believe that our work will provide a new perspective to visually monitor the potassium-ion storage process and guide the improvement in electrode material performance.

Synthesis of Fe1-xS Nanoparticles with Various Superstructures by a Simple Thermal Decomposition Route and Their Magnetic Properties

Pyrrhotite nanoparticles with 5C and 3C superstructures were synthesized via a simple one-step thermal decomposition method in which hexadecylamine was used as a solvent at various reaction temperatures (TR). Structural analysis showed that at TR = 360 °C, almost uniform in size and shape Fe7S8 nanoparticles with 3C superstructure are formed, and an increase in the reaction temperature leads to the formation of Fe9S10 nanoparticles (5C superstructure), herewith a significant increase in the size of nanoparticles is observed. High-temperature magnetic measurements in 5 repeated heating-cooling cycles revealed that after the first heating branch in the Fe9S10 samples, the λ-Peak transition disappears, and the magnetization has a Weiss-type behavior characteristic of the Fe7S8 sample. The change in the behavior of magnetization can be explained by the redistribution of iron vacancies, which changes the initial phase composition of nanoparticles.

Ultra-Stable Potassium Ion Storage of Nitrogen-Doped Carbon Nanofiber Derived from Bacterial Cellulose

As a promising energy storage system, potassium (K) ion batteries (KIBs) have received extensive attention due to the abundance of potassium resource in the Earth's crust and the similar properties of K to Li. However, the electrode always presents poor stability for K-ion storage due to the large radius of K-ions. In our work, we develop a nitrogen-doped carbon nanofiber (N-CNF) derived from bacterial cellulose by a simple pyrolysis process, which allows ultra-stable K-ion storage. Even at a large current density of 1 A g-1, our electrode exhibits a reversible specific capacity of 81 mAh g-1 after 3000 cycles for KIBs, with a capacity retention ratio of 71%. To investigate the electrochemical enhancement performance of our N-CNF, we provide the calculation results according to density functional theory, demonstrating that nitrogen doping in carbon is in favor of the K-ion adsorption during the potassiation process. This behavior will contribute to the enhancement of electrochemical performance for KIBs. In addition, our electrode exhibits a low voltage plateau during the potassiation-depotassiation process. To further evaluate this performance, we calculate the "relative energy density" for comparison. The results illustrate that our electrode presents a high "relative energy density", indicating that our N-CNF is a promising anode material for KIBs.

Recent progress in metal-organic framework/graphene-derived materials for energy storage and conversion: design, preparation, and application

Graphene or chemically modified graphene, because of its high specific surface area and abundant functional groups, provides an ideal template for the controllable growth of metal-organic framework (MOF) particles. The nanocomposite assembled from graphene and MOFs can effectively overcome the limitations of low stability and poor conductivity of MOFs, greatly widening their application in the field of electrochemistry. Furthermore, it can also be utilized as a versatile precursor due to the tunable structure and composition for various derivatives with sophisticated structures, showing their unique advantages and great potential in many applications, especially energy storage and conversion. Therefore, the related studies have been becoming a hot research topic and have achieved great progress. This review summarizes comprehensively the latest methods of synthesizing MOFs/graphene and their derivatives, and their application in energy storage and conversion with a detailed analysis of the structure-property relationship. Additionally, the current challenges and opportunities in this field will be discussed with an outlook also provided.

Branched In2O3 Mesocrystal of Ordered Architecture Derived from the Oriented Alignment of a Metal-Organic Framework for Accelerated Hydrogen Evolution over In2O3-ZnIn2S4

It is fascinating yet challenging to assemble anisotropic nanowires into ordered architectures of high complexity and intriguing functions. We exploited a facile strategy involving oriented etching of a metal-organic fragment (MOF) to advance the rational design of highly ordered nanostructures. As a proof of concept, a microscale MIL-68(In) single crystal was etched with a K₃[Co(CN)₆] solution to give a microtube composed of aligned MIL-68(In) nanorods. Annealing such a MIL-68(In) microtube readily created an unprecedented branched In₂O₃ mesocrystal by assembly of In₂O₃ nanorods aligned in order. The derived ordered-In₂O₃-ZnIn₂S₄ is more efficient in catalyzing visible-light-driven H₂ evolution (8753 μmol h^-1 g^-1) outperforming the disordered-In₂O₃-ZnIn₂S₄ counterpart (2700 μmol h^-1 g^-1) as well as many other state-of-the-art ZnIn₂S₄-based photocatalysts. The ordered architecture significantly boosts the short-range electron transfer in an In₂O₃-ZnIn₂S₄ heterojunction but has a negligible impact on the long-range electron transfer among In₂O₃ mesocrystals. The density functional theory (DFT) calculation reveals that the oriented etching is achieved by the selective binding of the [Co(CN)₆]^3- etchant on the (110) plane of MIL-68(In), which can drag the In atoms out of the framework in order. Our findings could broaden the technical sense toward advanced photocatalyst design and impose scientific impacts on unveiling how ordered photosystems operate.

A review on metal-organic framework-derived anode materials for potassium-ion batteries

In recent years, metal-organic frameworks (MOFs) have been widely used in the field of electrochemical energy storage and conversion because of their excellent properties, such as high specific surface area, adjustable pore size, high porosity, structural diversity, and functional controllability. This paper reviews the applications of metal-organic framework-derived composites such as nitrogen-doped carbon, transition metal sulfides, transition metal selenides, transition metal phosphides and metal selenium compound modifications in potassium ion batteries (PIBs) as anode electrode materials. A variety of MOF-derived composites with different structures and morphologies based on several types of ligands, including 2-methylimidazole, aromatic carboxylic acids, and ferricyanide, have been discussed. Moreover, the current challenges faced by MOF-derived materials and possible countermeasures are proposed.

Te-S covalent bond induces 1T&2H MoS2 with improved potassium-ion storage performance

The modulation of the characteristics of an MoS2 anode via substitutional doping, particularly N, P and Se, is vital for promoting the potassium-ion storage performances. However, these traditional chalcogen doping can only take the place of a sulfur element and not essentially change the inherent electrical nature of MoS2. Herein, novel Te-MoS2 materials have been synthesized via a simple hydrothermal process under Te doping. A half-metallic Te occupies the position of an Mo atom to form Te-S bonds, which is different from the same group Se element. After theoretical modeling and electrochemical measurements, it was observed that the formation of Te-S bonds can increase the electrical conductivity (about 530 times increment) and mitigate the mechanical stress to ensure the whole structural stability during the repeated insertion/extraction of K-ions. Moreover, the insertion of Te into the lattice of MoS2 generated the fractional phase transformation from 2H to the 1T phase of MoS2 and 1T&2H in-plane hetero-junction. Benefiting from these advantages, the 1T&2H Te-MoS2 anode delivered high capacities of 718 and 342 mA h g-1 at 50 and 5000 mA g-1, respectively, and an ultra-stable cycling performance (88.1% capacity retention after 1000 cycles at 2 A g-1). Moreover, the potassium-ion full cell assembled with K2Fe[Fe(CN)6] as the cathode demonstrates its practical application.

Nitrogen and sulfur co-doped vanadium carbide MXene for highly reversible lithium-ion storage

As an emerging group of two-dimensional (2D) layered material, MXenes have received significant attention in the direction of energy storage. However, the restacking of MXene flakes severely hinders the ion transport within electrodes, which limits their application for lithium-ion batteries (LIBs). To address this issue, herein, we rationally designed and optimized the structure of N, S co-doped V₂CT_x MXene, which exhibits excellent electrochemical performance with a high reversible capacity of 590 mAh g^-1 after 100 cycles at 0.1 A g^-1 when used as anode of LIBs. Even at a high current density of 2 A g^-1, a reversible capacity of 298 mAh g^-1 is obtained after 300 cycles, which outperforms most of the V₂CT_x-based anode materials reported so far. The lithium-ion storage mechanism of N, S co-doped V₂CT_x MXene was studied by a series of characterizations. The results show that the significant improvement of electrochemical performance should be attributed to the facilitated charge transfer after N and S co-doping in V₂CT_x MXene, which can effectively improve the ion transfer kinetics during the lithiation-delithiation process. Furthermore, the expanded interlayer spacing of N, S co-doped V₂CT_x provides more active sites for the adsorption of lithium ions, promoting the insertion capacity of lithium ions. This work indicates that the N, S co-doped 2D V₂CT_x MXene should be a promising anode material for high-performance LIBs.

Pseudocapacitive Crystalline MnCo2O4.5 and Amorphous MnCo2S4 Core/Shell Heterostructure with Graphene for High-Performance K-Ion Hybrid Capacitors

Potassium-ion capacitors (KICs) have received a surge of interest because of their higher reserves and lower costs of potassium than lithium. However, the cycle performance and capacity of potassium devices have been reported to be unsatisfactory. Herein, a unique crystalline MnCo₂O_4.5 and amorphous MnCo₂S₄ core/shell nanoscale flower structure grown on graphene (MCO@MCS@rGO) was synthesized by a two-step hydrothermal process and demonstrated in KICs. The MCO@MCS@rGO exhibits improved electrical conductivity and excellent structural integrity during the charging and discharging process. The reasons could be attributed to the cavity structure of MCO, the mechanical buffer and high electrolyte diffusion rate of MCS, and the auxiliary effect of graphene. The electrical conductivity of MCO@MCS shows a specific capacity of 272.3 mA h g^-1 after 400 cycles at 1 A g^-1 and a capacity of 125.6 mA h g^-1 at 2 A g^-1. Besides, the MCO@MCS@rGO and high-surface-area activated carbon in KICs exhibit a relative energy density of 85.3 W h kg^-1 and a power density of 9000 W kg^-1 and outstanding cycling stability with a capacity retention of 76.6% after 5000 cycles. Moreover, the reaction mechanism of MCO@MCS@rGO in the K-ion cell was investigated systematically using X-ray diffraction and transmission electron microscopy, providing guidance on the further development of pseudocapacitive materials.

Bottom-Up Synthesis of MoS2 /CNTs Hollow Polyhedron with 1T/2H Hybrid Phase for Superior Potassium-Ion Storage

Enslaved to the large-size K-ions, the construction of suitable anode materials with superior and stable potassium-ion storage properties is a major challenge. 1T phase MoS₂ possesses higher conductivity, bigger interlayer distance, and more electrochemically active sites than the 2H phase, which offers intriguing benefits for energy-related applications. In this work, the 1T/2H-phase hybrid MoS₂ nanosheets are successfully anchored in the N-doped carbon nanotube hollow polyhedron (1T/2H-MoS₂ /NCNHP) by a bottom-up solvothermal method. For the synthesized 1T/2H-MoS₂ /NCNHP, the fewer-layer 1T/2H-MoS₂ nanosheets are embedded in an N-doped carbon nanotube hollow polyhedron, with an enlarged interlayer spacing of 0.96 nm. When evaluated as anode material for potassium-ion batteries, the 1T/2H-MoS₂ /NCNHP hybrid presents outstanding potassium storage performance. It delivers a high-specific capacity of 519.2 mAh g^-1 at 50 mA g^-1 and maintains 281.2 mAh g^-1 at 1 A g^-1 over 500 cycles. The good potassium-ion electrochemical performance is attributed to the rational structural design and the synergistic effect of the components. Moreover, the 1T-MoS₂ nanosheet has excellent electrical conductivity and its enlarged interlayer spacing reduces the barrier for the embedding and stripping of K ions. Finally, the practical application of the 1T/2H-MoS₂ /NCNHP electrode material is also evaluated by assembled K-ion full cells.

Potassium Nickel Iron Hexacyanoferrate as Ultra-Long-Life Cathode Material for Potassium-Ion Batteries with High Energy Density

The abundant reserve and low price of potassium resources promote K-ion batteries (KIBs) becoming a promising alternative to Li-ion batteries, while the large ionic radius of K-ions creates a formidable challenge for developing suitable electrodes. Here Ni-substituted Prussian blue analogues (PBAs) are investigated comprehensively as cathodes for KIBs. The synthesized K_1.90Ni_0.5Fe_0.5[Fe(CN)₆]_0.89·0.42H₂O (KNFHCF-1/2) takes advantage of the merits of high capacity from electrochemically active Fe-ions, outstanding electrochemical kinetics induced by decreased band gap and K-ion diffusion activation energy, and admirable structure stability from inert Ni-ions. Therefore, a high first capacity of 81.6 mAh·g^-1 at 10 mA·g^-1, an excellent rate property (53.4 mAh·g^-1 at 500 mA·g^-1), and a long-term lifespan over 1000 cycles with the lowest fading rate of 0.0177% per cycle at 100 mA·g^-1 can be achieved for KNFHCF-1/2. The K-ion intercalation/deintercalation proceeds through a facile solid solution mechanism, allowing 1.5-electron transfer based on low- and high-spins Fe^II/Fe^III couples, which is verified by ex situ XRD, XPS, and DFT calculations. The K-ion full battery is also demonstrated using a graphite anode with a high energy density of 282.7 Wh·kg^-1. This work may promote more studies on PBA electrodes and accelerate the development of KIBs.

Bi-MOF derived micro/meso-porous Bi@C nanoplates for high performance lithium-ion batteries

Micro/meso-porous Bi@C nanoplates are synthesized by pyrolyzing Bi-based metal-organic frameworks (MOFs) prepared by a microwave-assisted hydrothermal method to overcome huge volume expansion and pulverization of anode materials during battery operation. The Bi@C nanoplates are composed of ∼10-50 nm Bi nanoparticles in an amorphous carbon shell. The material shows very high capacity (556 mA h g-1) after 100 cycles at 100 mA g-1 and good cycling performance. Moreover, the Bi@C nanoplates perform well at high current densities and have excellent cyclic stability; their capacity is 308 mA h g-1 after 50 cycles and 200 mA h g-1 after 1000 cycles at 3000 mA g-1. The outstanding performance of this anode is due to the nanosized Bi and amorphous carbon shell. The nanosized Bi reduces the diffusion length of Li ions, while the amorphous carbon shell improves the electrical conductivity of the anode and also restrains the pulverization and aggregation of the metal during cycling. The proposed hierarchical micro/meso-porous materials derived from MOFs are a new type of nanostructures that can aid the development of novel Bi-based anodes for LIBs.

Boosting Potassium Storage Performance of the Cu2S Anode via Morphology Engineering and Electrolyte Chemistry

Transition metal sulfides (TMSs) have been demonstrated as attractive anodes for potassium-ion batteries (KIBs) due to the high capacity, abundant resource, and excellent redox reversibility. Unfortunately, practical implementation of TMSs to KIBs is still hindered by the unsatisfactory cyclability and rate performance which result from the vast volume variation during charge/discharge processes. Herein, a uniform nitrogen-doped carbon coated Cu₂S hollow nanocube (Cu₂S@NC) is designed as an anode material for the KIB, which displays an outstanding cycle performance (317 mAh g^-1 after 1200 cycles at 1 A g^-1) and excellent rate capacity (257 mAh g^-1 at 6 A g^-1) in a half-cell. The hollow nanosized structure can both shorten the diffusion length of potassium ions/electrons and buffer the volume expansion upon cycling. Besides, the high concentration electrolyte is beneficial to form the stable solid electrolyte interphase (SEI) film, reducing the interface impedance and enhancing the cycling stability. Ex situ transmission electron microscopy (TEM) and ex situ X-ray diffraction (XRD) reveal the reaction mechanism of Cu₂S@NC.

Two Birds with One Stone: FeS2@C Yolk-Shell Composite for High-Performance Sodium-Ion Energy Storage and Electromagnetic Wave Absorption

Cost-effective material with a rational design is significant for both sodium-ion batteries (SIBs) and electromagnetic wave (EMW) absorption. Herein, we report an elaborate yolk-shell FeS₂@C nanocomposite as a promising material for application in both SIBs and EMW absorption. When applied as an anode material in SIBs, the yolk-shell structure not only facilitates a fast electron transport and shortens Na ion diffusion paths but also eases the huge volume change of FeS₂ during repeated discharge/charge processes. The as-developed FeS₂@C exhibits a high specific capacity of 616 mA h g^-1 after 100 cycles at 0.1 A g^-1 with excellent rate performance. Furthermore, owing to the significant cavity and interfacial effects enabled by yolk-shell structuring, the FeS₂@C nanocomposite delivers excellent EMW absorption properties with a strong reflection loss (-45 dB with 1.45 mm matching thickness) and a broad 15.4 GHz bandwidth. This work inspires the development of high-performance bifunctional materials.

Environment-Stable CoxNiy Encapsulation in Stacked Porous Carbon Nanosheets for Enhanced Microwave Absorption

Magnetic/dielectric@porous carbon composites, derived from metal-organic frameworks (MOFs) with adjustable composition ratio, have attracted wide attention due to their unique magnetoelectric properties. In addition, MOFs-derived porous carbon-based materials can meet the needs of lightweight feature. This paper reports a simple process for synthesizing stacked CoxNiy@C nanosheets derived from CoxNiy-MOFs nanosheets with multiple interfaces, which is good to the microwave response. The CoxNiy@C with controllable composition can be obtained by adjusting the ratio of Co2+ and Ni2+. It is supposed that the increased Co content is benefit to the dielectric and magnetic loss. Additionally, the bandwidth of CoNi@C nanosheets can take up almost the whole Ku band. Moreover, this composite has better environmental stability in air, which characteristic provides a sustainable potential for the practical application.

Amidation-Dominated Re-Assembly Strategy for Single-Atom Design/Nano-Engineering: Constructing Ni/S/C Nanotubes with Fast and Stable K-Storage

An amidation-dominated re-assembly strategy is developed to prepare uniform single atom Ni/S/C nanotubes. In this re-assembly process, a single-atom design and nano-structured engineering are realized simultaneously. Both the NiO₅ single-atom active centers and nanotube framework endow the Ni/S/C ternary composite with accelerated reaction kinetics for potassium-ion storage. Theoretical calculations and electrochemical studies prove that the atomically dispersed Ni could enhance the convention kinetics and decrease the decomposition energy barrier of the chemically-absorbed small-molecule sulfur in Ni/S/C nanotubes, thus lowering the electrode reaction overpotential and resistance remarkably. The mechanically stable nanotube framework could well accommodate the volume variation during potassiation/depotassiation process. As a result, a high K-storage capacity of 608 mAh g^-1 at 100 mA g^-1 and stable cycling capacity of 330.6 mAh g^-1 at 1000 mA g^-1 after 500 cycles are achieved.

Engineering Hollow Porous Carbon-Sphere-Confined MoS2 with Expanded (002) Planes for Boosting Potassium-Ion Storage

Potassium-ion batteries (PIBs) are emerging as promising next-generation electrochemical storage systems for their abundant and low-cost potassium resource. The key point of applying PIBs is to exploit stable K-host materials to accommodate the large-sized potassium ion. In this work, a yolk-shell structured MoS₂@hollow porous carbon-sphere composite (MoS₂@HPCS) assembled by engineering HPCS-confined MoS₂ with expanded (002) planes is proposed for boosting potassium-ion storage. When used as a PIB anode, the as-synthesized MoS₂@HPCS composite shows superior potassium storage performance. It delivers a reversible capacity of 254.9 mAh g^-1 at 0.5 A g^-1 after 100 discharge/charge cycles and maintains 126.2 mAh g^-1 at 1 A g^-1 over 500 cycles. The superior potassium-ion storage performance is ascribed to the elaborate yolk-shell nanoarchitecture and the expanded interlayer of the MoS₂ nanosheet, which could shorten the transport distance, enhance the electronic conductivity, relieve the volume variation, prevent the self-aggregation of MoS₂, facilitate the electrolyte penetration, and boost the intercalation/deintercalation of K⁺. Moreover, the potential application of the MoS₂@HPCS composite is also evaluated by assembled K-ion full cells with a perylenetetracarboxylic dianhydride cathode. Accordingly, the as-developed synthetic strategy can be extended to manufacture other host materials for PIBs and beyond.

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.

Pyrrhotite Fe1-x S microcubes as a new anode material in potassium-ion batteries

Potassium-ion batteries are an emerging energy storage technology that could be a promising alternative to lithium-ion batteries due to the abundance and low cost of potassium. Research on potassium-ion batteries has received considerable attention in recent years. With the progress that has been made, it is important yet challenging to discover electrode materials for potassium-ion batteries. Here, we report pyrrhotite Fe_1-x S microcubes as a new anode material for this exciting energy storage technology. The anode delivers a reversible capacity of 418 mAh g^-1 with an initial coulombic efficiency of ~70% at 50 mA g^-1 and a great rate capability of 123 mAh g^-1 at 6 A g^-1 as well as good cyclability. Our analysis shows the structural stability of the anode after cycling and reveals surface-dominated K storage at high rates. These merits contribute to the obtained electrochemical performance. Our work may lead to a new class of anode materials based on sulfide chemistry for potassium storage and shed light on the development of new electrochemically active materials for ion storage in a wider range of energy applications.