Metallic Octahedral CoSe2 Threaded by N-Doped Carbon Nanotubes: A Flexible Framework for High-Performance Potassium-Ion Batteries

Constructing Bipolar Dual-Active Sites through High-Entropy-Induced Electric Dipole Transition for Decoupling Oxygen Redox

It remains a significant challenge to construct active sites to break the trade-off between oxidation and reduction processes occurring in battery cathodes with conversion mechanism, especially for the oxygen reduction and evolution reactions (ORR/OER) involved in the zinc-air batteries (ZABs). Here, using a high-entropy-driven electric dipole transition strategy to activate and stabilize the tetrahedral sites is proposed, while enhancing the activity of octahedral sites through orbital hybridization in a FeCoNiMnCrO spinel oxide, thus constructing bipolar dual-active sites with high-low valence states, which can effectively decouple ORR/OER. The FeCoNiMnCrO high-entropy spinel oxide with severe lattice distortion, exhibits a strong 1s→4s electric dipole transition and intense t2g(Co)/eg(Ni)-2p(OL) orbital hybridization that regulates the electronic descriptors, eg and t2g, which leads to the formation of low-valence Co tetrahedral sites (Coth) and high-valence Ni octahedral sites (Nioh), resulting in a higher half-wave potential of 0.87 V on Coth sites and a lower overpotential of 0.26 V at 10 mA cm-2 on Nioh sites as well as a superior performance of ZABs compared to low/mild entropy spinel oxides. Therefore, entropy engineering presents a distinctive approach for designing catalytic sites by inducing novel electromagnetic properties in materials across various electrocatalytic reactions, particularly for decoupling systems.

Construction of Sugar-Gourd-Shaped Carbon Nanofibers Embedded with Heterostructured Zinc-Cobalt Selenide Nanocages for Superior Potassium-Ion Storage

Transition metal selenides are considered as promising anode materials for potassium-ion batteries (PIBs) due to their high theoretical capacities. However, their applications are limited by low conductivity and large volume expansion. Herein, sugar-gourd-shaped carbon nanofibers embedded with heterostructured ZnCo-Se nanocages are prepared via a facile template-engaged method combined with electrospinning and selenization process. In this hierarchical ZnCo-Se@NC/CNF, abundant phase boundaries of CoSe2/ZnSe heterostructure can promote interfacial electron transfer and chemical reactivity. The interior porous ZnCo-Se@NC nanocage structure relieves volume expansion and maintains structural integrity during K+ intercalation and deintercalation. The exterior spinning carbon nanofibers connect the granular nanocages in series, which prevents the agglomeration, shortens the electron transport distance and enhances the reaction kinetics. As a self-supporting anode material, ZnCo-Se@NC/CNF delivers a high capacity (362 mA h g-1 at 0.1 A g-1 after 100 cycles) with long-term stability (95.9% capacity retention after 1000 cycles) and shows superior reaction kinetics with high-rate K-storage. Energy level analysis and DFT calculations illustrate heterostructure facilitates the adsorption of K+ and interfacial electron transfer. The K+ storage mechanism is revealed by ex situ XRD and EIS analyses. This work opens a novel avenue in designing high-performance heterostructured anode materials with ingenious structure for PIBs.

Transition metal chalcogenides for next-generation energy storage

Transition-metal chalcogenide nanostructures provide a unique material platform to engineer next-generation energy storage devices such as lithium-ion, sodium-ion, and potassium-ion batteries and flexible supercapacitors. The transition-metal chalcogenide nanocrystals and thin films have enhanced electroactive sites for redox reactions and hierarchical flexibility of structure and electronic properties in the multinary compositions. They also consist of more earth-abundant elements. These properties make them attractive and more viable new electrode materials for energy storage devices compared to the traditional materials. This review highlights the recent advances in chalcogenide-based electrodes for batteries and flexible supercapacitors. The viability and structure-property relation of these materials are explored. The use of various chalcogenide nanocrystals supported on carbonaceous substrates, two-dimensional transition metal chalcogenides, and novel MXene-based chalcogenide heterostructures as electrode materials to improve the electrochemical performance of lithium-ion batteries is discussed. The sodium-ion and potassium-ion batteries offer a more viable alternative to lithium-ion technology as they consist of readily available source materials. Application of various transition metal chalcogenides such as MoS2, MoSe2, VS2, and SnSx, composite materials, and heterojunction bimetallic nanosheets composed of multi-metals as electrodes to enhance the long-term cycling stability, rate capability, and structural strength to counteract the large volume expansion during the ion intercalation/deintercalation processes is highlighted. The promising performances of layered chalcogenides and various chalcogenide nanowire compositions as electrodes for flexible supercapacitors are also discussed in detail. The review also details the progress made in new chalcogenide nanostructures and layered mesostructures for energy storage applications.

Boosting cobalt ditelluride quantum-rods anode materials for excellent potassium-ion storage via hierarchical physicochemical encapsulation

The exploration of anode materials that can store large-sized K-ion to solve the poor kinetics and large volume expansion issues has become the key scientific bottlenecks hindering the development of potassium-ion batteries (PIBs). Herein, ultrafine CoTe₂ quantum rods physiochemically encapsulated by graphene and nitrogen-doped carbon (CoTe₂@rGO@NC) are regarded as anode electrodes for PIBs. Dual physicochemical confinement and quantum size effect not only enhance electrochemical kinetics but also restrain large lattice stress during repeated K-ion insertion/extraction process. Superior electronic conductivity, K-ion adsorption, and diffusion ability can be acquired for CoTe₂@rGO@NC, confirmed through first-principles calculations and kinetics study. K-ion insertion/extraction proceeds via a typical conversion mechanism relying on Co as the redox site, where the robust chemical bond of COCo plays an important role in maintaining the electrode stability. Accordingly, CoTe₂@rGO@NC contributes a high initial capacity of 237.6 mAh·g^-1 at 200 mA·g^-1, a long lifetime over 500 cycles with low-capacity decay of 0.10% per cycle. This research will lay the materials science foundation for the construction of quantum-rod electrodes.

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.

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).

Hierarchical hollow superstructure cobalt selenide bird nests for high-performance lithium storage

The inferior cycling performance caused by large volume variation is the main problem that restricts the application of cobalt selenides in lithium-ion batteries. Herein, we synthesize raspberry-like Co-ethylene glycol precursor. It is further selenized into the hierarchical hollow superstructure CoSe₂/CoSe bird nests that are assembled by the hollow nanosphere units of CoSe₂ and CoSe nanocrystalline. CoSe₂/CoSe bird nests achieve excellent cycling performance, high reversible capacity and satisfactory rate capability (1361 mAh/g at 1 A/g after 1000 cycles, 579 mAh/g at 2 A/g after 2000 cycles, 315 mAh/g at 5 A/g after 1000 cycles). Electrochemical kinetics analyses and ex-situ material characterization reveal that the surface capacitive behavior controls the electrochemical reaction, and the composite has low reaction impedance, fast and stable Li⁺ diffusion, and superior structural stability. The superior lithium storage performance is attributed to the unique superstructure bird nest. Large specific surface area, abundant hierarchical pores and the opening mouth result in high electrochemical activity, which induces high reversible capacity. The small hollow nanosphere units, the sufficiently thick hierarchical porous superstructure shell and the large hollow interior bring about the strong synergistic effect to improve cycling performance. The intimately coupling of CoSe₂/CoSe nanocrystalline and the hollow nanosphere units guarantees high conductivity. This work has greatly enriched the understanding of structure design of high-performance cobalt selenide anodes.

Phosphorus/Phosphide-Based Materials for Alkali Metal-Ion Batteries

Phosphorus- and phosphide-based materials with remarkable physicochemical properties and low costs have attracted significant attention as the anodes of alkali metal (e.g., Li, Na, K, Mg, Ca)-ion batteries (AIBs). However, the low electrical conductivity and large volume expansion of these materials during electrochemical reactions inhibit their practical applications. To solve these problems, various promising solutions have been explored and utilized. In this review, the recent progress in AIBs using phosphorus- and phosphide-based materials is summarized. Thereafter, the in-depth working principles of diverse AIBs are discussed and predicted. Representative works with design concepts, construction approaches, engineering strategies, special functions, and electrochemical results are listed and discussed in detail. Finally, the existing challenges and issues are concluded and analyzed, and future perspectives and research directions are given. This review can provide new guidance for the future design and practical applications of phosphorus- and phosphide-based materials used in AIBs.

Rationally Designed Three-Layered TiO2 @amorphous MoS3 @Carbon Hierarchical Microspheres for Efficient Potassium Storage

Amorphous MoS₃ has been an attractive electrode material for sodium-ion batteries and lithium-sulfur batteries. However, the potassium storage capability of amorphous MoS₃ remains unreported. Herein, the construction of hybrid hierarchical microspheres composed of amorphous MoS₃ nanosheets dual-confined with TiO₂ core, and nitrogen-doped carbon shell layer (denoted as TiO₂ @A-MoS₃ @NC) via a self-templating method, combined with a low-temperature sulfurization process as a new anode material for potassium-ion batteries (PIBs), is reported. Benefitting from the unique structural merits including unique 1D chain structure, disordered arrangement of atoms and a large number of defects of amorphous MoS₃ , more active heterointerfacial sites, effectively mitigated volume change, good electrical contact, and easy K⁺ ion migration, the TiO₂ @A-MoS₃ @NC microspheres exhibit excellent potassium-storage performance with high specific capacity, superior rate capability, and cycling stability.

Foldable batteries: from materials to devices

Wearable electronics is a growing field that has important applications in advanced human-integrated systems with high performance and mechanical deformability, especially foldable characteristics. Although foldable electronics such as rollable TVs (LG signature OLED R) or foldable smartphones (Samsung Galaxy Z fold/flip series) have been successfully established in the market, these devices are still powered by rigid and stiff batteries. Therefore, to realize fully wearable devices, it is necessary to develop state-of-the-art foldable batteries with high performance and safety in dynamic deformation states. In this review, we cover the recent progress in developing materials and system designs for foldable batteries. The Materials section is divided into three sections aimed at helping researchers choose suitable materials for their systems. Several foldable battery systems are discussed and the combination of innovative materials and system design that yields successful devices is considered. Furthermore, the basic analysis process of electrochemical and mechanical properties is provided as a guide for researchers interested in the evaluation of foldable battery systems. The current challenges facing the practical application of foldable batteries are briefly discussed. This review will help researchers to understand various aspects (from material preparation to battery configuration) of foldable batteries and provide a brief guideline for evaluating the performance of these batteries.

Highly catalytically active CoSe2 supported on nitrogen-doped three dimensional porous carbon as a cathode for high-stability lithium-sulfur battery

Lithium-sulfur batteries, promising secondary energy storage devices, were mainly limited by its unsatisfactory cyclability owing to inefficient reversible conversion of sulfur and lithium sulfide on the cathode during the discharge/charging process. In this study, nitrogen-doped three-dimensional porous carbon material loaded with CoSe 2 nanoparticles (CoSe 2 -PNC) is developed as a cathode for lithium-sulfur battery application. A combination of CoSe 2 and nitrogen-doped porous carbon can efficiently improve the cathode activity and its conductivity, resulting in enhanced redox kinetics of the charge/discharge process. The obtained electrode exhibits a high discharge specific capacity of 1139.6 mAh g -1 at a current density of 0.2 C. After 100 cycles, its capacity remained at 865.7 mAh g -1 corresponding to a capacity retention of 75.97%. In a long-term cycling test, a discharge specific capacity of 546.7 mAh g -1 was observed after 300 cycles performed at a current density of 1 C.

Cobalt metal organic framework (Co-MOF) derived CoSe2/C hybrid nanostructures for the electrochemical hydrogen evolution reaction supported by DFT studies

Through a single step and facile approach, CoSe2/C is synthesized from a HER inactive pristine MOF without adding any external carbon source, which efficiently catalyses HER in acidic medium which was also supported by DFT studies.

Confining MoSe2 Nanosheets into N-Doped Hollow Porous Carbon Microspheres for Fast-Charged and Long-Life Potassium-Ion Storage

The potassium-ion battery (PIB) is the most promising alternative to a lithium-ion battery (LIB). Exploitation of a suitable electrode material is crucial to promote the development of PIBs. The MoSe₂ material has attracted much attention due to its high theoretical capacity, unique layered structure, and good conductivity. However, the potassium storage property of MoSe₂ has been suffering from structural fragmentation and sluggish reaction kinetic caused by large potassium ions upon insertion/extraction, which needs to be further improved. Herein, the MoSe₂ nanosheets are confined into N-doped hollow porous carbon microspheres (MoSe₂@N-HCS) by spray drying and high-temperature selenization. It delivers a superior rate performance of 113.7 mAh g^-1 at 10 A g^-1 and remains at a high capacity of 158.3 mAh g^-1 at 2 A g^-1 even after 16 700 cycles for PIBs. The excellent electrochemical performance can be attributed to unique structure, N-doping, and robust chemical bonds. The storage mechanism of MoSe₂ for potassium ions was explored. The outstanding properties of MoSe₂@N-HCS make it a promising anode material for PIBs.

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

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

Orthorhombic Cobalt Ditelluride with Te Vacancy Defects Anchoring on Elastic MXene Enables Efficient Potassium-Ion Storage

The fast and reversible potassiation/depotassiation of anode materials remains an elusive yet intriguing goal. Herein, a class of the P-doping-induced orthorhombic CoTe₂ nanowires with Te vacancy defects supported on MXene (o-P-CoTe₂ /MXene) is designed and prepared, taking advantage of the synergistic effects of the conductive o-P-CoTe₂ arrays with rich Te vacancy defects and the elastic MXene sheets with self-autoadjustable function. Consequently, the o-P-CoTe₂ /MXene superstructure exhibits boosted potassium-storage performance, in terms of high reversible capacity (373.7 mAh g^-1 at 0.2 A g^-1 after 200 cycles), remarkable rate capability (168.2 mAh g^-1 at 20 A g^-1 ), and outstanding long-term cyclability (0.011% capacity decay per cycle over 2000 cycles at 2 A g^-1 ), representing the best performance in transition-metal-dichalcogenides-based anodes to date. Impressively, the flexible full battery with o-P-CoTe₂ /MXene anode achieves a satisfying energy density of 275 Wh kg^-1 and high bending stability. The kinetics analysis and first-principles calculations reveal superior pseudocapacitive property, high electronic conductivity, and favorable K⁺ ion adsorption and diffusion capability, corroborating fast K⁺ ion storage. Especially, ex situ characterizations confirm o-P-CoTe₂ /MXene undergoes reversible evolutions of initially proceeding with the K⁺ ion insertion, followed by the conversion reaction mechanism.

Robust Pseudocapacitive Sodium Cation Intercalation Induced by Cobalt Vacancies at Atomically Thin Co1-x Se2 /Graphene Heterostructure for Sodium-Ion Batteries

Electronic structure engineering on electrode materials could bring in a new mechanism to achieve high energy and high power densities in sodium ion batteries. Herein, we design and create Co vacancies at the interface of atomically thin CoSe₂ /graphene heterostructure and obtain Co_1-x Se₂ /graphene heterostructure electrode materials that facilitate significant Na⁺ intercalation pseudocapacitance. Density functional theory (DFT) calculation suggests that the Na⁺ adsorption energy is dramatically increased, and the Na⁺ diffusion barrier is remarkably reduced due to the introduction of Co vacancy. The optimized electrode delivers a superior capacity of 673.6 mAh g^-1 at 0.1 C, excellent rate capability of 576.5 mAh g^-1 at 2.0 C and ultra-long life up to 2000 cycles. Kinetics analysis indicates that the enhanced Na⁺ storage is mainly attributed to the intercalation pseudocapacitance induced by Co vacancies. This work suggests that the creation of cation vacancy could bestow heterostructured electrode materials with pseudocapacitive Na⁺ intercalation for high-capacity and high-rate energy storage.

Dual Confinement of CoSe2 Nanorods with Polyphosphazene-Derived Heteroatom-Doped Carbon and Reduced Graphene Oxide for Potassium-Ion Batteries

High-capacity and highly stable anode materials are some of the keys to the realization of the application of potassium-ion batteries (PIBs). Cobalt diselenide (CoSe₂) has been regarded as a high-potential anode material for PIBs. However, solving the problems of sluggish kinetics and large volumetric expansion during intercalation/deintercalation of K⁺ ions is always very challenging in terms of cobalt diselenide-based anode materials. Herein, reduced graphene oxide-encapsulated polyphosphazene-derived S, P, and N codoped carbon (SPNC)-coated CoSe₂ nanorods (CoSe₂⊂SPNC⊂rGO) were designed as PIB anode materials. CoSe₂⊂SPNC⊂rGO delivers an excellent reversible capacity of 287.2 mAh g^-1 at 100 mA g^-1. Benefiting from the coating of heteroatom-doped carbon and encapsulation of rGO, the CoSe₂⊂SPNC⊂rGO anodes exhibit a remarkable rate capability (100-1500 mA g^-1 current density) and high stability (208.8 mAh g^-1 after 500 cycles at 500 mA g^-1). The results demonstrate that S, P, and N codoping in carbon layers provides active sites for K⁺ ion storage and increases the electrical conductivity. More importantly, the dual confinement of CoSe₂ nanorods with carbon layers and rGO significantly reduced the volume expansion and kept the electrode structural integrity with repeating intercalation/deintercalation of K⁺ ions.

Co3 Se4 Quantum Dots as an Ultrastable Host Material for Potassium-Ion Intercalation

Potassium-ion batteries (KIBs) are receiving increased attention due to their cost-effective and similar energy-storage mechanism to lithium-ion batteries. However, the lack of appropriate electrode materials is still hampered for their development, which is mainly caused by the large size of the potassium ions (1.38 Å) including low structural stability and poor electrochemical redox reaction kinetics. Herein, Co₃ Se₄ quantum dots (QD) encapsulated by N-doped carbon (CSC) are reported as an anode material for KIBs, in which a morphology change process occurs. Benefiting from the unique uniform nanostructure reducing the ion-diffusion length, the improved electronic conductivity, and the enhanced protective effect of N-doped carbon (NC) alleviating volume fluctuation, the CSC demonstrates excellent electrochemical performance. The core-shell-like CSC composite demonstrates remarkable discharge capacity (410 mA h g^-1 at 0.1 A g^-1 after 550 cycles, 360 mA h g^-1 at 0.5 A g^-1 after 3200 cycles) and excellent cyclic performance over 10 000 cycles at 1 A g^-1 . Density functional theory calculations show a larger reaction energy of Co₃ Se₄ QD than bulk Co₃ Se₄ , a lower barrier of K atom migration in Co₃ Se₄ QD than bulk Co₃ Se₄ , and also favor the intercalation reaction rather than replacement reaction. In situ X-ray diffraction and ex situ transmission electron microscopy are further used to evaluate potassiation/depotassiation phenomena.

Fast and Durable Potassium Storage Enabled by Constructing Stress-Dispersed Co3Se4 Nanocrystallites Anchored on Graphene Sheets

Transition metal dichalcogenides are regarded as promising anode materials for potassium-ion batteries (PIBs) because of their high theoretical capacities. However, due to the large atomic radius of K⁺, the structural damage caused by the huge volume expansion upon potassiation is much more severe than that of their lithium counterparts. In this research, a stress-dispersed structure with Co₃Se₄ nanocrystallites orderly anchored on graphene sheets is achieved through a two-step hydrothermal treatment to alleviate the structural deterioration. The ability to reduce the contact stress by the well-dispersed Co₃Se₄ nanocrystallites during K⁺ intercalation, together with the highly conductive graphene matrix, provides a more reliable and efficient anode architecture than its two agminated counterparts. Given these advantages, the optimized electrode delivers excellent cycling stability (301.8 mA h g^-1 after 500 cycles at 1 A g^-1), as well as an outstanding rate capacity (203.8 mA h g^-1 at 5 A g^-1). Further in situ and ex situ characterizations and density functional theory calculations elucidate the potassium storage mechanism of Co₃Se₄ during the conversion reaction and reveal the fast electrochemical kinetics of the rationally designed electrode. This work provides a practical approach for constructing stable metal-selenide anodes with long cycle life and high-rate performance for PIBs.

Yolk-shell structured CoSe2/C nanospheres as multifunctional anode materials for both full/half sodium-ion and full/half potassium-ion batteries

Transition metal selenides (TMSs) are suitable for SIBs and PIBs owing to their satisfactory theoretical capacity and superior electrical conductivity. However, the large radius of Na+/K+ easily leads to sluggish kinetics and poor conductivity, which hinder the development of SIBs and PIBs. Structure design is an effective method to solve these obstacles. In this study, Co2+ ions combined with glycerol molecules to form self-assembled nanospheres at first, and then they were in situ converted into CoSe2 nanoparticles embedded in a carbon matrix during the selenization process. This structure has three-dimensional ion diffusion channels that can effectively hamper the aggregation of metal compound nanoparticles. Meanwhile, the CoSe2/C of the yolk-shell structure and a large number of pores help alleviate volume expansion and facilitate electrolyte wettability. These structural advantages of CoSe2/C endow it with remarkable electrochemical performances for full/half SIBs and full/half PIBs. The obtained CoSe2/C exhibits superior stability and excellent performance (312.1 mA h g-1 at 4 A g-1 after 1600 cycles) for SIBs. When it is used as an anode material for PIBs, 369.2 mA h g-1 can be retained after 200 cycles at 50 mA g-1 and 248.1 mA h g-1 can be retained after 200 cycles at 500 mA g-1; in addition, CoSe2/C also shows superior rate capacity (186.4 mA h g-1 at 1000 mA g-1). A series of ex situ XRD measurements were adapted to explore the possible conversion mechanism of CoSe2/C as the anode for PIBs. It is worth noting that the full-cell of CoSe2/C//Na3V2(PO4)3@rGO for SIBs and the full-cell of CoSe2/C//PTCDA-450 for PIBs were successfully assembled. The relationship between the structure and performance of CoSe2/C was investigated through density functional theory (DFT).

Self-assembly of carbon nanotubes on a hollow carbon polyhedron to enhance the potassium storage cycling stability of metal organic framework-derived metallic selenide anodes

Potassium-ion batteries (PIBs) is increasingly studied because of their suitable redox potential and high natural abundance. However, potential anode materials with long-term cycling stability are still in high demand because of the large radius of K⁺. Herein, an MOF-derived hierarchical carbon structure and the self-assembly of CNTs on hollow carbon polyhedrons are used as carbon matrices to disperse and stabilize metal selenides(Co-Se@CNNCP). When the hybrid is utilized in PIBs, it displays a specific capacity of 410 mA h g^-1 at 0.1 A g^-1 after 80 cycles and 253 mA h g^-1 at 0.5 A g^-1 after 200 cycles with a capacity retention of 100%, while the metal selenides dispersed on hollow carbon polyhedrons without CNTs (Zn-Co-Se@NCP) lose 86% of their capacity after 200 cycles. The superior cycling stability of the hybrid is mainly attributed to the large amounts of CNTs suppressing the agglomeration of the metal selenide nanoparticles on the surface, and the hollow carbon polyhedrons cause a high structural integrity during the repreated K⁺ insertion and extraction process. This work offers a feasible route to design a hierarchical carbon matrix for use as the anode materials of PIBs with long-term cycling stability.

Reshaping two-dimensional MoS2 for superior magnesium-ion battery anodes

Few-atom-thick two-dimensional (2D) molybdenum disulfide (MoS₂) monolayers possess numerous crucial applications in energy storage. Usually, the strategy of activating interfacial electron transfer was employed to promote their performance. Herein, we reshape the structure of materials to excite their subinterfacial and interfacial electron transfer for superior metal-ion batteries. As an example, we rationally design and reconfigure the structure of 2D MoS₂ and propose a new stable structure, B-MoS₂, which has an S-Mo-S sandwich structure with a buckled square lattice. The B-MoS₂ monolayer is a promising anode material for magnesium-ion batteries (MgIBs) with a high capacity (921.3 mA h g^-1) and a low averaged open circuit voltage (0.154 V). Multiscale underlying mechanisms for the storage of Mg and Li ions in MoS₂ are provided. Based on the electronic level, the high capacity is ascribed to the occurrence of interfacial and subinterfacial electron transfer between metal ions and B-MoS₂. Based on the atomic level, the insertion-adsorption mechanism or adsorption-insertion mechanism is determined for different ion storage at B-MoS₂. The intrinsic metallic property of B-MoS₂ and the enhanced electronic conductivity of Mg/B-MoS₂ systems as well as low migration barriers (∼0.604 eV) of Mg ions at MoS₂ suggest that the B-MoS₂ anode has fast charge/discharge rates. This work offers novel concepts (i.e. subinterfacial electron transfer and its activation) for superior energy storage materials, and proposes new multiscale underlying mechanisms for ion storage in the MoS₂ family.

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.

Expanded MoSe2 Nanosheets Vertically Bonded on Reduced Graphene Oxide for Sodium and Potassium-Ion Storage

The cost-efficient and plentiful Na and K resources motivate the research on ideal electrodes for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Here, MoSe₂ nanosheets perpendicularly anchored on reduced graphene oxide (rGO) are studied as an electrode for SIBs and PIBs. Not only does the graphene network serves as a nucleation substrate for suppressing the agglomeration of MoSe₂ nanosheets to eliminate the electrode fracture but also facilitates the electrochemical kinetics process and provides a buffer zone to tolerate the large strain. An expanded interplanar spacing of 7.9 Å is conducive to fast alkaline ion diffusion, and the formed chemical bondings (C-Mo and C-O-Mo) promote the structure integrity and the charge transfer kinetics. Consequently, MoSe₂@5%rGO exhibits a reversible specific capacity of 458.3 mAh·g^-1 at 100 mA·g^-1, great cyclability with a retention of 383.6 mAh·g^-1 over 50 cycles, and excellent rate capability (251.3 mAh·g^-1 at 5 A·g^-1) for SIBs. For PIBs, a high first specific capacity of 365.5 mAh·g^-1 at 100 mA·g^-1 with a low capacity fading of 51.5 mAh·g^-1 upon 50 cycles and satisfactory rate property are acquired for MoSe₂@10%rGO composite. Ex situ measurements validate that the discharge products are Na₂Se for SIBs and K₅Se₃ for PIBs, and robust chemical bonds boost the structure stability for Na- and K-ion storage. The full batteries are successfully fabricated to verify the practical feasibility of MoSe₂@5%rGO composite.

Hollow Biphase Cobalt Nickel Perselenide Spheres Derived from Metal Glycerol Alkoxides for High-Performance Hybrid Supercapacitors

Transition-metal selenides (TMSe) incorporate reversible multielectron Faradaic reactions that can deliver high specific capacitance. Unfortunately, they usually exhibit actual capacitance lower than their theoretical value and suffer from sluggish kinetics, which do not satisfy the demands of hybrid supercapacitors (HSCs), due to poor electron-transmission capability and inferior ion-transport rate. Herein, a kind of hollow biphase and bimetal cobalt nickel perselenide composed of metastable marcasite-type CoSe₂ (m-CoSe₂) and stable pyrite-type NiCoSe₄ (p-NiCoSe₄) is synthesized with metal glycerol alkoxide as precursors by regulating the Ni/Co ratios. This unique hollow biphase structure and bimetallic synergistic effect serves to boost electron-transmission capability and accelerate the ion/electron transfer rate, delivering an excellent specific capacitance of 1008 F g^-1 at 0.5 A g^-1 and a high discharge rate capability of 859 F g^-1 at 20 A g^-1. The capacitance remains around 80% of the initial capacitance after 5000 cycles. Consequently, a HSC based on the cobalt nickel perselenide cathode and a hierarchical porous carbon anode reveals a maximum energy density of 34.8 W h kg^-1 and a maximum power density of 7272 W kg^-1. This polymorphic bimetallic phase engineering provides an advanced and effective guidance for TMSe with high electrochemical properties.

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

Chalcogenide-based anodes are receiving increasing attention for rechargeable potassium-ion batteries (PIBs) due to their high theoretical capacities. However, they usually exhibit poor electrochemical performance due to poor structural stability, low conductivity, and severe electrolyte decomposition on the reactive surface. Herein, a method analogous to "blowing bubbles with gum" is used to confine FeS₂ and FeSe₂ in N-doped carbon for PIB anodes with ultrahigh cyclic stability and enhanced rate capability (over 5000 cycles at 2 A g^-1). Several theoretical and experimental methods are employed to understand the electrodes' performance. The density functional theory calculations showed high affinity for potassium adsorption on the FeS₂ and FeSe₂. The in situ XRD and ex situ TEM analysis confirmed the formation of several intermediate phases of the general formula K_xFeS₂. These phases have high conductivity and large interlayer distance, which promote reversible potassium insertion and facilitate the charge transfer. Also, the calculated potassium diffusion coefficient during charge/discharge further proves the enhanced kinetics. Furthermore, The FeS₂@NC anode in a full cell also exhibits high cyclic stability (88% capacity retention after 120 cycles with 99.9% Coulombic efficiency). Therefore, this work provides not only an approach to overcome several challenges in PIB anodes but also a comprehensive understanding of the mechanism and kinetics of the potassium interaction with chalcogenides.

Self-Assembled FeSe2 Microspheres with High-Rate Capability and Long-Term Stability as Anode Material for Sodium- and Potassium-Ion Batteries

Sodium- and potassium-ion batteries have attracted intensive attention recently as low-cost alternatives to lithium-ion batteries with naturally abundant resources. However, the large ionic radii of Na⁺ and K⁺ render their slow mobility, leading to sluggish diffusion in host materials. Herein, hierarchical FeSe₂ microspheres assembled by closely packed nano/microrods are rationally designed and synthesized through a facile solvothermal method. Without carbonaceous material incorporation, the electrode delivers a reversible Na⁺ storage capacity of 559 mA h g^-1 at a current rate of 0.1 A g^-1 and a remarkable rate performance with a capacity of 525 mA h g^-1 at 20 A g^-1 . As for K⁺ storage, the FeSe₂ anode delivers a high reversible capacity of 393 mA h g^-1 at 0.4 A g^-1 . Even at a high current rate of 5 A g^-1 , a discharge capacity of 322 mA h g^-1 can be achieved, which is among the best high-rate anodes for K⁺ storage. The excellent electrochemical performance can be attributed to the favorable morphological structure and the use of an ether-based electrolyte during cycling. Moreover, quantitative study suggests a strong pseudocapacitive contribution, which boosts fast kinetics and interfacial storage.

CoSe@N-Doped Carbon Nanotubes as a Potassium-Ion Battery Anode with High Initial Coulombic Efficiency and Superior Capacity Retention

Potassium-ion batteries (KIBs) have gained significant interest in recent years from the battery research community because potassium is an earth-abundant and redox-active metal, thus having the potential to replace lithium-ion batteries for sustainable energy storage. However, the current development of KIBs is critically challenged by the lack of competitive electrode materials that can reversibly store large amounts of K⁺ and electrolyte systems that are compatible with the electrode materials. Here, we report that cobalt monochalcogenide (CoSe) nanoparticles confined in N-doped carbon nanotubes (CoSe@NCNTs) can be used as a K⁺-storing electrode. The CoSe@NCNT composite exhibits a high initial Columbic efficiency (95%), decent capacity (435 mAh g^-1 at 0.1 A g^-1), and stability (282 mAh g^-1 2.0 A g^-1 after 500 cycles) in a 1 M KPF₆-DME electrolyte with K as the anode over the voltage range from 0.01 to 3.0 V. A full KIB cell consisting of this anode and a Prussian blue cathode also shows excellent electrochemical performance (228 mAh g^-1 at 0.5 A g^-1 after 200 cycles). We show that the NCNT shell is effective not only in providing high electronic conductivity for fast charge transfer but also in accommodating the volume changes during cycling. We also provide experimental and theoretical evidence that KPF₆ in the electrolyte plays a catalytic role in promoting the formation of a polymer-like film on the CoSe surface during the initial activation process, and this amorphous film is of critical importance in preventing the dissolution of polyselenide intermediates into the electrolyte, stabilizing the Co⁰/K₂Se interface, and realizing the reversibility of Co⁰/K₂Se conversion.

Hierarchical interlayer-expanded MoSe2/N–C nanorods for high-rate and long-life sodium and potassium-ion batteries

The hierarchical structure and N-doped carbon coating buffer the volume expansion and maintain the structure integrity of MoSe₂, which enables the long cycle performance and high rate capability for both sodium and potassium storage.

Hierarchical 3D Oxygenated Cobalt Vanadium Selenide Nanosheets as Advanced Electrode for Flexible Zinc-Cobalt and Zinc-Air Batteries

Highly flexible quasi solid-state batteries are promising in next-generation energy storage sectors due to their high energy density, power density, and low manufacturing cost. However, poor cycle life seriously limits their application in industrial sectors. Herein, a novel strategy is established to design the oxygenated cobalt vanadium selenide (O-Co_x V_{1- x} Se₂ ) nanostructures for high-performance quasi solid-state (QSS) zinc-cobalt batteries (ZCBs) and zinc-air batteries (ZABs). Density functional theory (DFT) calculation reveals that the doping effect of Co²⁺ into O-VSe₂ nanostructure could increase the density of states near the edge of the conduction band, demonstrating ultrafast electron transport kinetics. Most interestingly, the optimal O-Co_0.33 V_0.67 Se₂ cathode-based QSS-ZCB exhibits an ultrahigh specific capacity of 422.7 mAh g^-1 at a current density of 1 A g^-1 , excellent energy density of 186.4 Wh kg^-1 , tremendous power density of 5.65 kW kg^-1 , and ultralong cycle life (86.9% capacity retention after 3000 cycles). Furthermore, O-Co_0.33 V_0.67 Se₂ air-cathode based QSS-ZAB delivers a peak power density of 162 mW cm^-2 and ultralong cycle life over 100 h. These experimental and theoretical studies indicate that the electrochemically induced, cobalt stabilizes the vanadium is essential to boost the energy storage properties and cycle life of both ZCBs and ZABs.

Advanced Anode Materials of Potassium Ion Batteries: from Zero Dimension to Three Dimensions

Potassium ion batteries (PIBs) with the prominent advantages of sufficient reserves and economical cost are attractive candidates of new rechargeable batteries for large-grid electrochemical energy storage systems (EESs). However, there are still some obstacles like large size of K⁺ to commercial PIBs applications. Therefore, rational structural design based on appropriate materials is essential to obtain practical PIBs anode with K⁺ accommodated and fast diffused. Nanostructural design has been considered as one of the effective strategies to solve these issues owing to unique physicochemical properties. Accordingly, quite a few recent anode materials with different dimensions in PIBs have been reported, mainly involving in carbon materials, metal-based chalcogenides (MCs), metal-based oxides (MOs), and alloying materials. Among these anodes, nanostructural carbon materials with shorter ionic transfer path are beneficial for decreasing the resistances of transportation. Besides, MCs, MOs, and alloying materials with nanostructures can effectively alleviate their stress changes. Herein, these materials are classified into 0D, 1D, 2D, and 3D. Particularly, the relationship between different dimensional structures and the corresponding electrochemical performances has been outlined. Meanwhile, some strategies are proposed to deal with the current disadvantages. Hope that the readers are enlightened from this review to carry out further experiments better.

MOF-Derived CoSe2@N-Doped Carbon Matrix Confined in Hollow Mesoporous Carbon Nanospheres as High-Performance Anodes for Potassium-Ion Batteries

In this work, a novel vacuum-assisted strategy is proposed to homogenously form Metal-organic frameworks within hollow mesoporous carbon nanospheres (HMCSs) via a solid-state reaction. The method is applied to synthesize an ultrafine CoSe2 nanocrystal@N-doped carbon matrix confined within HMCSs (denoted as CoSe2@NC/HMCS) for use as advanced anodes in high-performance potassium-ion batteries (KIBs). The approach involves a solvent-free thermal treatment to form a Co-based zeolitic imidazolate framework (ZIF-67) within the HMCS templates under vacuum conditions and the subsequent selenization. Thermal treatment under vacuum facilitates the infiltration of the cobalt precursor and organic linker into the HMCS and simultaneously transforms them into stable ZIF-67 particles without any solvents. During the subsequent selenization process, the "dual confinement system", composed of both the N-doped carbon matrix derived from the organic linker and the small-sized pores of HMCS, can effectively suppress the overgrowth of CoSe2 nanocrystals. Thus, the resulting uniquely structured composite exhibits a stable cycling performance (442 mAh g-1 at 0.1 A g-1 after 120 cycles) and excellent rate capability (263 mAh g-1 at 2.0 A g-1) as the anode material for KIBs.

CoSe2/N-doped carbon porous nanoframe as an anode material for potassium-ion storage

Transition metal selenides (TMS), on account of their relatively high theoretical capacity, unique electrical properties, easy compositing and low cost, are considered to be a candidate anode material for potassium-ion batteries. However, the cycling stability of TMS is unsatisfactory owing to the large intercalation/deintercalation of K ions. Herein, a CoSe₂/N-doped carbon porous frame (CoSe₂@NC) is successfully synthesized through a simple mixing and sintering approach and displays excellent potassium storage performance. Plentiful C-N bonds in the precursor can induce the formation of homogeneous N-doped carbon matrix and C-N-Co bonds, thus endowing robust structure and high electronic conductivity for superior cycling stability. Therefore, the unique porous nanoframe suppresses volume expansion and provides more diffusion paths for K ions. After 1000 cycles at 50 mA g^-1, a high capacity of 311.3 mA h g^-1 is acquired. When the current density increases to 500 mA g^-1, the CoSe₂@NC can still maintain a capacity of 184.5 mA h g^-1 after 1000 cycles. The high performance, easy compositing and low cost of the CoSe₂@NC make it a favorable material for application in KIBs.

Conversion Reaction Mechanism of Ultrafine Bimetallic Co-Fe Selenides Embedded in Hollow Mesoporous Carbon Nanospheres and Their Excellent K-Ion Storage Performance

Potassium-ion batteries (KIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundance and affordability of potassium. However, the development of suitable electrode materials that can stably store large-sized K ions remains a challenge. This study proposes a facile impregnation method for synthesizing ultrafine cobalt-iron bimetallic selenides embedded in hollow mesoporous carbon nanospheres (HMCSs) as superior anodes for KIBs. This involves loading metal precursors into HMCS templates using a repeated "drop and drying" process followed by selenization at various temperatures, facilitating not only the preparation of bimetallic selenide/carbon composites but also controlling their structures. HMCSs serve as structural skeletons, conductive templates, and vehicles to restrain the overgrowth of bimetallic selenide particles during thermal treatment. Various analysis strategies are employed to investigate the charge-discharge mechanism of the new bimetallic selenide anodes. This unique-structured composite exhibits a high discharge capacity (485 mA h g^-1 at 0.1 A g^-1 after 200 cycles) and enhanced rate capability (272 mA h g^-1 at 2.0 A g^-1 ) as a promising anode material for KIBs. Furthermore, the electrochemical properties of various nanostructures, from hollow to frog egg-like structures, obtained by adjusting the selenization temperature, are compared.

Porous-Carbon Aerogels with Tailored Sub-Nanopores for High Cycling Stability and Rate Capability Potassium-Ion Battery Anodes

Developing advanced electrode materials for potassium-ion batteries (PIBs) is an emerging research area in recent years; so far, several strategies such as heteroatom doping into carbon, increasing interlayer spacing, or creating amorphous region in graphite have been investigated. Here, we studied the effect of sub-nanopores in a porous-carbon aerogel with a pore size distribution centered at around 0.8 nm and achieved outstanding PIB performance including long cycling stability (particularly at small current densities for prolonged charge/discharge period) and high rate capability with enhanced retentions. Mechanism studies reveal very high contribution from surface capacitive potassium (K)-ion storage (more than 90%) to the total capacity, and theoretical calculations show that 0.8 nm sub-nanopores lead to substantially low barrier for K-ion transport and storage, with ultrasmall diffusion energy and negligible lattice change. Sub-nanopore engineering, as demonstrated here, may be adopted to develop highly efficient and stable porous-carbon-based structures for applications in advanced energy storage systems and electrochemical catalysis.

Tellurium: A High-Volumetric-Capacity Potassium-Ion Battery Electrode Material

Currently, exploring high-volumetric-capacity electrode materials that allow for reversible (de-)insertion of large-size K⁺ ions remains challenging. Tellurium (Te) is a promising alternative electrode for storage of K⁺ ions due to its high volumetric capacity, confirmed in lithium-/sodium-ion batteries, and the intrinsic good electronic conductivity. However, the charge storage capability and mechanism of Te in potassium-ion batteries (KIBs) have not been unveiled until now. Here, a novel K-Te battery is constructed, and the K⁺ -ion storage mechanism of Te is revealed to be a two-electron conversion-type reaction of 2K + Te ↔ K₂ Te, resulting in a high theoretical volumetric capacity of 2619 mAh cm^-3 . Consequently, the rationally fabricated tellurium/porous carbon electrodes deliver an ultrahigh reversible volumetric capacity of 2493.13 mAh cm^-3 at 0.5 C (based on Te), a high-rate capacity of 783.13 mAh cm^-3 at 15 C, and superior long-term cycling stability for 1000 cycles at 5 C. This excellent electrochemical performance proves the feasibility of utilizing Te as a high-volumetric-capacity active material for storage of K⁺ ions and will advance the practical application of KIBs.

Heterostructures of 2D Molybdenum Dichalcogenide on 2D Nitrogen-Doped Carbon: Superior Potassium-Ion Storage and Insight into Potassium Storage Mechanism

Constructing 2D heterostructure materials by stacking different 2D materials can combine the merits of the individual building blocks while eliminating their shortcomings. Dichalcogenides are attractive anodes for potassium-ion batteries (KIBs) due to their high theoretical capacity. However, the practical application of dichalcogenide is greatly hampered by the poor electrochemical performance due to sluggish kinetics of K⁺ insertion and the electrode structure collapse resulting from the large K⁺ insertion. Herein, heterostructures of 2D molybdenum dichalcogenide on 2D nitrogen-doped carbon (MoS₂ , MoSe₂ -on-NC) are prepared to boost their potassium storage performance. The unique 2D heterostructures possess built-in heterointerfaces, facilitating K⁺ diffusion. The robust chemical bonds (CS, CSe, CMo bonds) enhance the mechanical strength of electrodes, thus suppressing the volume expansion. The 2D N-doped carbon nanosheets interconnected as a 3D structure offer a fast diffusion path for electrons. Benefitting from these merits, both the MoS₂ -on-NC and the MoSe₂ -on-NC exhibit unprecedented cycle life. Moreover, the electrochemical reaction mechanism of MoSe₂ is revealed during the process of potassiation and depotassiation.

In-Situ Fabrication of Bone-Like CoSe2 Nano-Thorn Loaded on Porous Carbon Cloth as a Flexible Electrode for Na-Ion Storage

Sodium-ion batteries (SIBs) based on flexible electrode materials are being investigated recently for improving sluggish kinetics and developing energy density. Transition metal selenides present excellent conductivity and high capacity; nevertheless, their low conductivity and serious volume expansion raise challenging issues of inferior lifespan and capacity fading. Herein, an in-situ construction method through carbonization and selenide synergistic effect is skillfully designed to synthesize a flexible electrode of bone-like CoSe₂ nano-thorn coated on porous carbon cloth. The designed flexible CoSe₂ electrode with stable structural feature displays enhanced Na-ion storage capabilities with good rate performance and outstanding cycling stability. As expected, the designed SIBs with flexible BL-CoSe₂ /PCC electrode display excellent reversible capacity with 360.7 mAh g^-1 after 180 cycles at a current density of 0.1 A g^-1 .

Construction of Bimetallic Selenides Encapsulated in Nitrogen/Sulfur Co-Doped Hollow Carbon Nanospheres for High-Performance Sodium/Potassium-Ion Half/Full Batteries

Metallic selenides have been widely investigated as promising electrode materials for metal-ion batteries based on their relatively high theoretical capacity. However, rapid capacity decay and structural collapse resulting from the larger-sized Na⁺ /K⁺ greatly hamper their application. Herein, a bimetallic selenide (MoSe₂ /CoSe₂ ) encapsulated in nitrogen, sulfur-codoped hollow carbon nanospheres interconnected reduced graphene oxide nanosheets (rGO@MCSe) are successfully designed as advanced anode materials for Na/K-ion batteries. As expected, the significant pseudocapacitive charge storage behavior substantially contributes to superior rate capability. Specifically, it achieves a high reversible specific capacity of 311 mAh g^-1 at 10 A g^-1 in NIBs and 310 mAh g^-1 at 5 A g^-1 in KIBs. A combination of ex situ X-ray diffraction, Raman spectroscopy, and transmission electron microscopy tests reveals the phase transition of rGO@MCSe in NIBs/KIBs. Unexpectedly, they show quite different Na⁺ /K⁺ insertion/extraction reaction mechanisms for both cells, maybe due to more sluggish K⁺ diffusion kinetics than that of Na⁺ . More significantly, it shows excellent energy storage properties in Na/K-ion full cells when coupled with Na₃ V₂ (PO₄ )₂ O₂ F and PTCDA@450 °C cathodes. This work offers an advanced electrode construction guidance for the development of high-performance energy storage devices.

Hierarchical Co3O4@N-Doped Carbon Composite as an Advanced Anode Material for Ultrastable Potassium Storage

Cobalt oxide (Co₃O₄) delivers a poor capacity when applied in large-sized alkali metal-ion systems such as potassium-ion batteries (KIBs). Our density functional theory calculation suggests that this is due to poor conductivity, high diffusion barrier, and weak potassium interaction. N-doped carbon can effectively attract potassium ions, improve conductivity, and reduce diffusion barriers. Through interface engineering, the properties of Co₃O₄ can be tuned via composite design. Herein, a Co₃O₄@N-doped carbon composite was designed as an advanced anode for KIBs. Due to the interfacial design of the composite, K⁺ were effectively transported through the Co₃O₄@N-C composite via multiple ionic pathways. The structural design of the composite facilitated increased Co₃O₄ spacing, a nitrogen-doped carbon layer reduced K-ion diffusion barrier, and improved conductivity and protected the electrode from damage. Based on the entire composite, a superior capacity of 448.7 mAh/g was delivered at 50 mA/g after 40 cycles, and moreover, 213 mAh/g was retained after 740 cycles when cycled at 500 mA/g. This performance exceeds that of most metal-oxide-based KIB anodes reported in literature.

Amorphous carbon coated SnO2 nanohseets on hard carbon hollow spheres to boost potassium storage with high surface capacitive contributions

Potassium-ion batteries (KIBs) have becoming a prospective energy storage technique, due to the abundant potassium resources in the earth crust, approximate redox potential and similar electrochemical behavior of potassium and lithium. However, the insufficient capacity, poor stability and volume expansion of electrode materials during charge and discharge are main factors restricting the further development of KIBs. This work reports an amorphous carbon coated SnO₂ nanohseets on hard carbon hollow spheres (AC/SnO₂@HCHS) anode with enhanced potassium storage performance. The HCHS acts as a carrier for SnO₂ nanosheets, providing high electrical conductivity and stable skeleton. The self-assembled SnO₂ nanosheets with high surface area ensures sufficient contact with the electrolyte. Amorphous carbon wrapping can not only relieve SnO₂ volume expansion but also provide surface-induced capacitive capacity. As a consequence, the AC/SnO₂@HCHS anode presents excellent potassium-ion storage performance with high discharge capacity of 346 mAh g^-1 at 0.1 A g^-1 over 200 cycles, ultra-long cycling lifetime and outstanding rate capability (236 mAh g^-1 at 1 A g^-1 over 1000 cycles).