Facile Synthesis of Hierarchical Hollow CoP@C Composites with Superior Performance for Sodium and Potassium Storage

Construction of N-doped carbon encapsulated CoP hollow nanofibers as multifunctional electrode materials for potassium-ion and lithium-sulfur batteries

Herein, a composite of N-doped carbon coated phosphating cobalt hollow nanofibers (N/C@CoP-HNFs) was synthesized by electrospinning, phosphating, and carbon coating processes. When employed as multifunctional electrode materials for potassium-ion batteries (PIBs) and lithium-sulfur (Li-S) batteries, the N/C@CoP-HNFs demonstrated notable electrochemical properties. Specifically, it delivered an initial specific capacity of 420.4 mA h g-1 at a current density of 100 mA g-1, with a sustained capacity of 190.8 mA h g-1 after 200 cycles in PIBs, and a specific capacity of 1448 mA h g-1 at a current density of 0.5C in Li-S batteries, which is considered relatively high for these types of battery technology. This good performance may due to the combination of the carbon nitrogen layer and cobalt phosphide bilayer hollow tube structure, which is conducive to telescoping the diffusion length of ions and electrons and buffer volume variation, and effectively inhibits the shuttle effect. Density functional theory (DFT) calculations were also used to explore the energy storage mechanism of the material. The possible adsorption sites and corresponding adsorption energy of K+ were analyzed, and the advantages of the material were explored by calculating the diffusion barrier and state density. The theoretical simulations further validated the strong adsorption capability of CoP for polysulfides. This work is expected to provide new ideas for new energy storage materials.

Vacancies-Induced Delocalized States Cobalt Phosphide for Binder-Free Anode Toward Stable and High-Rate Sodium-Ion Batteries

Metal phosphides with easy synthesis, controllable morphology, and high capacity are considered as potential anodes for sodium-ion batteries (SIBs). However, the inherent shortcomings of metal phosphating materials, such as conductivity, kinetics, volume strain, etc are not satisfactory, which hinders their large-scale application. Here, a CoP@carbon nanofibers-composite containing rich Co─N─C heterointerface and phosphorus vacancies grown on carbon cloth (CoP1-x@MEC) is synthesized as SIB anode to accomplish extraordinary capacity and ultra-long cycle life. The hybrid composite nanoreactor effectively impregnates defective CoP as active reaction center while offering Co─N─C layer to buffer the volume expansion during charge-discharge process. These vast active interfaces, favored electrolyte infiltration, and a well-structured ion-electron transport network synergistically improve Na+ storage and electrode kinetics. By virtue of these superiorities, CoP1-x@MEC binder-free anode delivers superb SIBs performance including a high areal capacity (2.47 mAh cm-2@0.2 mA cm-2), high rate capability (0.443 mAh cm-2@6 mA cm-2), and long cycling stability (300 cycles without decay), thus holding great promise for inexpensive binder-free anode-based SIBs for practical applications.

Peapod-Like Structured B/N Co-Doped Carbon Nanotube Array Encapsulating MxPy (M = Fe, Co, and Ni) Nanoparticles for High-Rate Potassium Storage

Potassium-ion batteries (PIBs) have become the desirable alternatives for lithium-ion batteries (LIBs) originating from abundant reserves and appropriate redox potential, while the considerable radius size of K+ leading to poor reaction kinetics and huge volume expansion limits the practical application of PIBs. Hybridization of transition-metal phosphides and carbon substrates can effectively optimize the obstacles of poor conductivity, sluggish kinetics, and huge volume variation. Thus, the peapod-like structural MxPy@BNCNTs (M = Fe, Co, and Ni) composites as anode materials for PIBs were synthesized through a facile strategy. Notably, the unique architecture of B/N codoped carbon nanotube array as fast ion/electron transfer pathways effectively improves the electronic conductivity of composites. The MxPy nanoparticles (NPs) are encapsulated in BNCNTs with an amorphous carbon layer (5-10 nm), which discernibly alleviate the volume changes during potassiation/depotassiation. In conclusion, the composites show a commendable cycling performance, possessing reversible capacities of 111, 152, and 122 mA h g-1 after 1000 cycles at 1.0 A g-1 with a negligible capacity loss for FeP@BNCNT, CoP/Co2P@BNCNT, and Ni2P@BNCNT electrodes, respectively. Especially, after 1000 cycles at 2.0 A g-1, the CoP/Co2P@BNCNT electrode still possesses a capacity of 87.9 mA h g-1, demonstrating excellent rate performance and long-term life. This work may offer an innovative and viable route to construct a stable architecture for solving the issue of poor stability of TMP-based anodes at a high current density.

Synergy Between Surface Confinement and Heterointerfacial Regulations with Fast Electron/Ion Migration in InSe-PPy for Sodium-Ion Storage

Layered indium selenide (InSe) is a new 2D semiconductor material with high carrier mobility, widely adjustable bandgap, and high ductility. However, its ion storage behavior and related electrochemical reaction mechanism are rarely reported. In this study, InSe nanoflakes encapsulated in conductive polypyrrole (InSe@PPy) are designed in consideration of restraining the severe volume change in the electrochemical reaction and increasing conductivity via in situ chemical oxidation polymerization. Density functional theory calculations demonstrate that the construction of heterostructure can generate an internal electric field to accelerate electron transfer via additional driving forces, offering synergistically enhanced structural stability, electrical conductivity, and Na+ diffusion process. The resulting InSe@PPy composite shows outstanding electrochemical performance in the sodium ion batteries system, achieving a high reversible capacity of 336.4 mA h g-1 after 500 cycles at 1 A g-1 and a long-term cyclic stability with capacity of 274.4 mA h g-1 after 2800 cycles at 5 A g-1 . In particular, the investigation of capacity fluctuation within the first cycling reveals the alternating significance of intercalation and conversion reactions and evanescent alloying reaction. The combined reaction mechanism of insertion, conversion, and alloying of InSe@PPy is revealed by in situ X-ray diffraction, ex situ electrochemical impedance spectroscopy, and transmission electron microscopy.

Loss-free pulverization by confining copper oxide inside hierarchical nitrogen-doped carbon nanocages toward superb potassium-ion batteries

Taking the advantages of hierarchical nitrogen-doped carbon nanocages (hNCNCs) with nanocavities for encapsulation and multiscale micro-meso-macropores/high conductivity for mass/electron synergistic transportation, a conversion-type CuO anode material is confined inside hNCNCs for potassium storage. The so-obtained yolk-shelled CuO@hNCNC hybrids have tunable CuO contents in the range of 11.7-63.7 wt%. The unique architecture leads to the loss-free pulverization of the active components during charge/discharge, which increases the surface-controlled charge storage, shortens the K+ solid diffusion lengths with an enlarged K+ diffusion coefficient, and meanwhile enhances the rate capability and durability. Consequently, the optimized CuO@hNCNC delivers a high specific capacity of 498 mA h g-1 at 0.1 A g-1 and 194 mA h g-1 at 10.0 A g-1 based on the total mass of CuO@hNCNC, and a long-term stability. The capacity based on the CuO active component reaches a record-high 522 mA h g-1 at 1.0 A g-1 after 2000 cycles, which is ca. 2.5 times the state-of-the-art value in the literature. The evolution of the cycling performance with CuO loading is well understood based on the loss-free pulverization. This study demonstrates a new strategy to turn the generally harmful pulverization of active components into a beneficial factor for K+ storage, which paves the way for exploring high-performance anodes for rechargeable batteries.

Solvation Structure Modulation of High-Voltage Electrolyte for High-Performance K-Based Dual-Graphite Battery

Due to the advantages of dual-ion batteries (DIBs) and abundant resources, potassium-based dual-carbon batteries (K-DCBs) have wide application prospects. However, conventional carbonate ester-based electrolyte systems have obvious drawbacks such as poor oxidation resistance and difficulty in sustaining the anion intercalation process at high voltages, which seriously affect the capacity and cycle performance of K-DCBs. Therefore, a rational design of more efficient novel electrolyte systems is urgently required to realize high-performance K-DCBs. Herein, a solvation structure modulation strategy for the K-DCB electrolyte systems is reported. Consequently, substantial K⁺ ion storage improvement at the graphite anode and enhanced bis(fluorosulfonyl)imide anion (FSI^- ) intercalation capacity at the graphite cathode are successfully realized simultaneously. As a proof-of-concept, the assembled K-DCB exhibited a discharge capacity of 103.4 mAh g^-1 , and after 400 cycles, ≈90% capacity retention is observed. Moreover, the energy density of the K-DCB full cell reached 157.6 Wh kg^-1 , which is the best performance in reported K-DCBs till date. This study demonstrates the effectiveness of solvation modulation in improving the performance of K-DCBs.

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.

Low volume expansion hierarchical porous sulfur-doped Fe2O3@C with high-rate capability for superior lithium storage

Ingenious morphology design and doping engineering have remarkable effects on enhancing conductivity and reducing volume expansion, which need to be improved by transition metal oxides serving as anode materials for lithium-ion batteries. Herein, S_0.15-Fe₂O₃@C nano-spindles with a hierarchical porous structure are obtained by carbonizing MIL-88B@PDA and subsequent high-temperature S-doping. Kinetic analysis showed that S-doping increases capacitive contribution, enhances charge transfer capability and accelerates Li⁺ diffusion rate. Therefore, the S_0.15-Fe₂O₃@C electrode exhibits superior lithium storage performance with a remarkable specific capacity of 1014.4 mA h g^-1 at 200 mA g^-1, ultrahigh rate capability of 513.1 mA h g^-1 at 5.0 A g^-1, and excellent cycling stability of 842.3 mA h g^-1 at 1.0 A g^-1 after 500 cycles. Moreover, the size of S_0.15-Fe₂O₃@C particles barely changed after 50 cycles, indicating an extremely low volume expansion, related to the carbon shell, fine Fe₂O₃ nanoparticles, abundant voids inside, and improved kinetics. This strategy can be applied to other metal oxides for synthesizing anodes with high-rate capability and low volume expansion.

Ni3 S2 -Ni Hybrid Nanospheres with Intra-Core Void Structure Encapsulated in N-Doped Carbon Shells for Efficient and Stable K-ion Storage

Iron group metals chalcogenides, especially NiS, are promising candidates for K-ion battery anodes due to their high theoretical specific capacity and abundant reserves. However, the practical application of NiS-based anodes is hindered by slow electrochemical kinetics and unstable structure. Herein, a novel structure of Ni₃ S₂ -Ni hybrid nanosphere with intra-core voids encapsulated by N-doped carbon shells (Ni₃ S₂ -Ni@NC-AE) is constructed, based on the first electrodeposited NiS nanosphere particles, dopamine coating outer layer, oxygen-free annealing treatment to form Ni₃ S₂ -Ni core and N-doped carbon shell, and selective etching of the Ni phase to form intra-core void. The electron/K⁺ transport and K⁺ storage reaction kinetics are enhanced due to shortened diffusion pathways, increased active sites, generation of built-in electric field, high K⁺ adsorption energies, and large electronic density of states at Fermi energy level, resulting from the multi-structures synergistic effect of Ni₃ S₂ -Ni@NC-AE. Simultaneously, the volume expansion is alleviated due to the sufficient buffer space and strong chemical bonding provided by intra-core void and yolk-shell structure. Consequently, the Ni₃ S₂ -Ni@NC-AE exhibits excellent specific capacity (438 mAh g^-1 at 0.1 A g^-1 up to 150 cycles), outstanding rate performances, and ultra-stable long-cycle performance (176.4 mAh g^-1 at 1 A g^-1 up to 5000 cycles) for K-ion storage.

Honeycomb-like 3D carbon skeletons with embedded phosphorus-rich phosphide nanoparticles as advanced anodes for lithium-ion batteries

Phosphorus-rich iron phosphides (FeP₂) have been regarded as excellent anode candidates for lithium storage owing to their low cost, high natural abundance, high theoretical capacity, and reasonable redox potential. However, FeP₂ suffers from a few challenging problems such as low reversibility, fast capacity degradation, and big volume variation. Herein, we have designed and synthesized a 3D honeycomb-like carbon skeleton with embedded FeP₂ nanoparticles (denoted as FeP₂ NPs@CK), which can significantly promote the kinetics and maintain the structural stability during the cycling, resulting in an excellent electrochemical performance reflected by high reversibility and long-term cycling stability. FeP₂ NPs@CK shows high reversibility, delivering a reversible capacity as high as 938 mA h g^-1 at 0.5 A g^-1. It also shows excellent cycling stability, delivering a capacity of 620 mA h g^-1 after 500 cycles at 1 A g^-1. Moreover, the fast kinetics and lithium storage mechanism of FeP₂ NPs@CK are investigated by quantitative analysis and in situ X-ray diffraction. Such superior performance demonstrates that FeP₂ NPs@CK could be a promising and attractive anode candidate for lithium storage.

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

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

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.

Coupling of Metallic VSe2 and Conductive Polypyrrole for Boosted Sodium-Ion Storage by Reinforced Conductivity Within and Outside

Although transitional metal dichalcogenides have been regarded as appealing electrodes for sodium/potassium-ion batteries (SIBs/PIBs) owing to their high theoretical capacity, it is a key challenge to realize dichalcogenide anodes with long-period cycling performance and high-rate capability because of their poor conductivity and large volumetric change. Herein, polypyrrole-encapsulated VSe₂ nanoplates (VSe₂@PPy) were prepared by the selenization of VOOH hollow nanospheres and subsequent in situ polymerization and coating by pyrrole. Benefiting from the inherent metallicity of VSe₂, the improvement in the conductivity and the structural protection provided by the PPy layer, the VSe₂@PPy nanoplates exhibited enhanced sodium/potassium-storage performances, delivering a superior rate capability with a capacity of 260.0 mA h g^-1 at 10 A g^-1 in SIBs and 148.6 mA h g^-1 at 5 A g^-1 in PIBs, as well as revealing an ultrastability in cycling of 324.6 mA h g^-1 after 2800 cycles at 4 A g^-1 in SIBs. Moreover, the insertion and conversion mechanisms of VSe₂@PPy in SIBs with intermediates of Na_0.6VSe₂, NaVSe₂, and VSe were elucidated by in situ/ex situ X-ray diffraction combined with ex situ transmission electron microscopy observation and in situ potentio-electrochemical impedance spectroscopy during the sodiation and desodiation processes. Density functional theory calculations show that the strong coupling between VSe₂ and PPy not only causes it to have a stronger total density of states and a built-in electric field, leading to an increased electrical conductivity, but also effectively decreases the ion diffusion barrier.

Hierarchical Core-Shell Co2 N/CoP Embedded in N, P-doped Carbon Nanotubes as Efficient Oxygen Reduction Reaction Catalysts for Zn-air Batteries

Projecting a cost-effective and highly efficient electrocatalyst for the oxygen reaction reduction (ORR) counts a great deal for Zn-air batteries. Herein, a hierarchical core-shell ORR catalyst (Co₂ N/CoP@PNCNTs) is developed by embedding cobalt phosphides and/or cobalt nitrides as the core into N, P-doped carbon nanotubes (PNCNTs) as the shell via one-step carbonization, nitridation, and phosphorization of pyrolyzing Co-MOF precursor. The globally N, P-doped structure of Co₂ N/CoP@PNCNTs demonstrates an outstanding electrocatalytic activity in the alkaline solution with the onset and half-wave potentials of 1.07 and 0.85 V respectively. Moreover, a Zn-air battery assembled from Co₂ N/CoP@PNCNTs as the air cathode delivers an open circuit potential of 1.49 V, a maximum power density of 151.1 mW cm^-2 and a specific capacity of 823.8 mAh kg^-1 . It is reflected that Co₂ N/CoP@PNCNTs provides a long-term durability with a slight decline of 15 h in the chronoamperometry measurement and an excellent charge-discharge stability with negligible voltage decay for 150 h at 10 mA cm^-2 in Zn-air batteries. The results reveal that Co₂ N/CoP@PNCNTs has superiority over most Co-N_x -C or Co_x P@C catalysts reported so far. The excellent catalytic properties and stability of Co₂ N/CoP@PNCNTs derive from synergistic effects between Co₂ N/CoP and mesoporous N, P-doped carbon nanotubes.

Creating Unidirectional Fast Ion Diffusion Channels in G/NiS2 -MoS2 Heterostructures for High-Performance Sodium-Ion Batteries

Exploring novel electrode composites and their unique interface physics plays a significant role in tuning electrochemical properties for boosting the performance of sodium-ion batteries (SIBs). Herein, mixed-dimensional G/NiS₂ -MoS₂ heterostructures are synthesized in a low-cost meteorological vulcanization process. The stable graphene supporting layer and nanowire heterostructure guarantee an outstanding structural stability to tolerate certain volume changes during the charge/discharge process. The rational construction of NiS₂ -MoS₂ heterostructures induces abundant interfaces and unique ion diffusion channels, which render fast electrochemical kinetics and superior reversible capacities for high-performance SIBs. Interestingly, theoretical studies reveal that the anisotropic diffusion barriers create unidirectional "high-speed" channels, which can lead to ordered and fast Na⁺ insertion/extraction in designed heterostructures. G/NiS₂ -MoS₂ anode exhibits a high capacity of 509.6 mA h g^-1 after 500 cycles and a coulombic efficiency >99% at 0.5 A g^-1 , which also displays excellent cycling performance with the capacity of 383.8 mA h g^-1 after the 1000 cycles at 5 A g^-1 . Furthermore, full cells are constructed exhibiting a high capacity of 70 mA h g^-1 at 0.1 A g^-1 after 150 cycles and applied to light LEDs. This study provides a feasible strategy of constructing mixed-dimensional heterostructures for SIBs with excellent performance and a long service lifetime.

Zeolitic Imidazolate Framework 67-Derived Ce-Doped CoP@N-Doped Carbon Hollow Polyhedron as High-Performance Anodes for Lithium-Ion Batteries

Zeolitic Imidazolate Framework 67 (ZIF-67) and its derivates have attracted extensive interest for lithium-ion batteries (LIBs). Here, Cerium-doped cobalt phosphide@nitrogen-doped carbon (Ce-doped CoP@NC) with hollow polyhedron structure materials were successfully synthesized via ionic-exchange with Co and Ce ions using the ZIF-67 as a template followed with a facile low-temperature phosphorization treatment. Benefitting from the well-designed hollow polyhedron, steady carbon network, and Ce-doping structural merits, the as-synthesized Ce-doped CoP@NC electrode demonstrated superior performance as the anode in LIBs: a superior cyclability (400 mA h g−1 after 500 cycles) and outstanding rate-capability (590 mA h g−1, reverted to 100 mA g−1). These features not only produced more lithium-active sites for LIBs anode and a shorter Li-ion diffusion pathway to expedite the charge transfer, but also the better tolerance against volume variation of CoP during the repeated lithiation/delithiation process and greater electronic conductivity properties. These results provide a methodology for the design of well-organized ZIFs and rare earth element-doped transition metal phosphate with a hollow polyhedron structure.

Implantation of Fe7S8 nanocrystals into hollow carbon nanospheres for efficient potassium storage

As a desirable candidate for lithium-ion batteries, potassium-ion batteries (PIBs) have aroused great interest because of their low cost and high power and energy densities. However, the insertion/extraction of K⁺ with a large radius (1.38 Å) usually bring about the destruction of the electrode materials. Here, ultrafine Fe₇S₈ nanocrystals are successfully implanted into hollow carbon nanospheres (Fe₇S₈@HCSs) via a facile solvothermal method and subsequent novel low-temperature sulfurization, which avoid the aggregation of Fe₇S₈ nanoparticles produced during high-temperature sulfidation. The ultrafine Fe₇S₈ nanoparticles and hollow carbon spheres can not only buffer the severe expansion/shrinkage of electrode materials caused by the repeated insertion/extraction of K⁺, but also provide additional accessible pathways for the high-rate K⁺ transmission. When tested as an anode material for PIBs, Fe₇S₈@HCSs achieve impressive K⁺ storage capacity of 523.2 mAh g^-1 at 0.1 A g^-1 after 100 cycles and remarkable rate capacity of 176.3 mAh g^-1 at 5 A g^-1. Further, assembling this anode with a K₂NiFe(CN)₆ cathode yields stable cycling performance, revealing its great potential for large-scale energy storage applications.

Heterojunction-Promoted Sodium Ion Storage of Bimetallic Selenides Encapsulated in a Carbon Sheath with Boosted Ion Diffusion and Stable Structure

Although metallic chalcogenides are deemed as attractive sodium anode materials recently, the electrochemical performance is severely confined by the liability of structural collapse and sluggish ion diffusion kinetics. Herein a composite of carbon-encapsulated bimetallic selenides MoSe₂-Sb₂Se₃ was prepared by a hydrothermal method on the basis of abundant reaction sites, high activity, an extra built-in electric field generated from heterointerfaces, and synergistic effects between the different components. Equally important, the carbon coating is effective to support the structural stability by restraining the vast volumetric variation to achieve the purpose of improving the cycling performance. The density functional theory calculation results indicate that the band gap is narrowed and that the work function is decreased on the interface of the MoSe₂-Sb₂Se₃ heterojunction, leading to an additional driving force stemming from the introduction of the built-in electric field and the formation of the Sb-Se (Se from MoSe₂) bond. Therefore, the resultant composite presents increased reaction kinetics and good electrochemical properties by acquiring a capacity of 376.0 mA h g^-1 over 580 cycles at 2.0 A g^-1 for the half-cell and 276 mA h g^-1 over 750 cycles at 2 A g^-1 for the full-cell. This work highlights bimetallic selenides with facilitated ion transferability with high performance.

Constructing Heterostructured Bimetallic Selenides on an N-Doped Carbon Nanoframework as Anodes for Ultrastable Na-Ion Batteries

Transition-metal selenides have been recognized as a class of promising anode materials for sodium-ion batteries (SIBs) on account of their high capacity. Nevertheless, the sluggish conversion kinetics and rapid capacity decay caused by insufficient conductivity and volume change restrain their applications. Herein, hollow heterostructured bimetallic selenides embedded in an N-doped carbon nanoframework (H-CoSe₂/ZnSe@NC) were prepared via a facile template-engaged method. Benefiting from the rich defect at the phase boundary of the CoSe₂/ZnSe heterostructure, pre-reserved cavity, and enhanced structure rigidity, the abovementioned issues are resolved at once, and the accelerated charge transportation kinetics traced by spectroscopy techniques and theoretical calculations certify the interface effect in the capacity release. In addition, ex situ X-ray photoelectron spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy all confirm the high-reversible electrochemical conversion mechanism in H-CoSe₂/ZnSe@NC. Together with a reasonable structural architecture and the highly reversible conversion reaction, H-CoSe₂/ZnSe@NC displays a prominent rate capacity (244.8 mA h g^-1 at 10 A g^-1) as well as an ultralong lifespan (10,000 cycles at 10 A g^-1), highlighting the significance of structure control in fabricating high-performance anodes for SIBs.

Versatile Preparation of Mesoporous Single-Layered Transition-Metal Sulfide/Carbon Composites for Enhanced Sodium Storage

Transition-metal sulfides are promising electrochemical energy storage materials due to their abundant active sites, large interlayer space, and high theoretical capacities, especially for sodium storage. However, the low conductivity and poor cycling stability at high current densities hamper their applications. Herein, a versatile dual-template method is reported to elaborate ordered mesoporous single-layered MoS₂ /carbon composite with high specific area, uniform pore size, and large pore volume. The single-layered MoS₂ is confined in the carbon matrix. The mesopores between the composite nanorods provide fast electrolyte diffusion. The obtained nanocomposite shows a high sodium-storage capability, excellent rate capacity, and very good cycling performance. A capacity of 310 mAh g^-1 can remain at 5.0 A g^-1 after 2500 cycles. Furthermore, a sodium-ion battery (SIB) full cell composed of the MoS₂ /carbon composite anode and a Na₃ V₂ (PO₄ )₃ (NVP) cathode maintains a specific capacity of 330 mAh g^-1 at 1.0 A g^-1 during 100 cycles. The mechanism is investigated by in situ and ex situ characterizations as well as density functional theory (DFT) calculations.

Bismuth Nanoparticle Embedded in Carbon Skeleton as Anode for High Power Density Potassium-Ion Batteries

Bismuth is a promising anode for potassium-ion batteries (PIBs) due to its suitable redox potential, large theoretical capacity, and superior electronic conductivity. Herein, we report a Bi@C (Bi nanoparticles uniformly embedded in a carbon skeleton) composite anode which delivers a superior rate performance of 244.3 mA h g⁻¹ at 10.0 A g⁻¹ and a reversible capacity of 255.6 mA h g⁻¹ after 200 cycles in an optimized ether-based electrolyte. The outstanding electrochemical performance results from its robust structural design with fast reaction kinetics, which are confirmed by both experimental characterization studies and first-principles calculations. The reversible potassium storage mechanism of the Bi@C composite was also investigated by in situ X-ray diffraction. In addition, the full PIB cell assembled with a Bi@C composite anode and nickel-based Prussian blue analogue cathode exhibits high discharge voltage (3.18 V), remarkable power density (>10 kW kg⁻¹), and an excellent capacity retention of 87.8% after 100 cycles. The results demonstrate that the PIBs with Bi anodes are promising candidates for power-type energy storage devices.

An ultrahigh power density (>10 kW kg⁻¹) potassium-ion full cell was fabricated by using a designed Bi@C composite as the anode. This workproves that potassium-ion batteries are promising candidates for power-type large-scale energy storage devices.

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.

In-Depth Mechanism Understanding for Potassium-Ion Batteries by Electroanalytical Methods and Advanced In Situ Characterization Techniques

The advancement of potassium ion batteries (PIBs) stimulated by the dearth of lithium resources is accelerating. Major progresses on the electrochemical properties are based on the optimization of electrode materials, electrolytes, and other components. More significantly, the prerequisites for optimizing these key compositions are in-depth and comprehensive exploration of electrochemical reaction processes, including the evolution of morphology and structure, phase transition, interface behaviors, and K⁺ movement, etc. As a result, the obtained K⁺ storage mechanism via analyzing aforementioned reaction processes sheds light on furthering practical application of PIBs. Typical electrochemical analysis methods are capable of obtaining physical and chemical characteristics. The advent of in situ electrochemical measurements enables dynamic observation and monitoring, thereby gaining extensive insights into the intricate mechanism of capacity degradation and interface kinetics. By coupling with these powerful electrochemical characterization techniques, inspiring works in PIBs will burgeon into wide realms of energy storage fields. In this review, some typical electroanalytical tests and in situ hyphenated measurements are described with the main concentration on how these techniques play a role in investigating the potassium storage mechanism for PIBs and achieving encouraging results.

Binders for sodium-ion batteries: progress, challenges and strategies

Binders as a bridge in electrodes can bring various components together thus guaranteeing the integrity of electrodes and electronic contact during battery cycling. In this review, we summarize the recent progress of traditional binders and novel binders in the different electrodes of SIBs. The challenges faced by binders in terms of bond strength, wettability, thermal stability, conductivity, cost, and environment are also discussed in details. Correspondingly, the designing principle and advanced strategies of future research on SIB binders are also provided. Moreover, a general conclusion and perspective on the development of binder design for SIBs in the future are presented.

Co-Salen Complex-Derived CoP Nanoparticles Confined in N-Doped Carbon Microspheres for Stable Sodium Storage

The poor rate and cycle performance rooting from the inferior electrical conductivity and large volume change are bottlenecks for further application of the potential anode material in sodium-ion batteries. To address this problem, homogeneous CoP nanoparticles enwrapped in the N-doped carbon (CoP/NC) microspheres are synthesized by the simultaneous carbonization and phosphorization of Co-salen complex microspheres for the first time. The N-doped carbon enhances its conductivity and diminishes the volume stress, and the dispersed CoP nanoparticles in carbon provide more reaction sites, resulting in a superior sodium storage performance. CoP/NC microspheres exhibit the capacity of 373 mA h g^-1 at 0.1 A g^-1 after 100 cycles. Even at 2 A g^-1 for 2000 cycles, the capacity of 195 mA h g^-1 is also achieved. This work provides an excellent reference for the design and synthesis of sulfide, selenide, and other transition-metal composites. It is also beneficial to expand the application of salen complexes in the design and synthesis of catalysts and energy storage materials.

Cage-Confinement Pyrolysis Strategy to Synthesize Hollow Carbon Nanocage-Coated Copper Phosphide for Stable and High-Capacity Potassium-Ion Storage

Metal phosphides with a high theoretical capacity and low redox potential have been proposed as promising anodes for potassium-ion batteries (PIBs). A reasonable configuration design and introduction of a hollow structure with adequate internal void spaces are effective strategies to overcome the volume expansion of metal phosphides in potassium-ion batteries. Herein, we report a cage-confinement pyrolysis strategy to obtain hollow nanocage-structured nitrogen/phosphorus dual-doped carbon-coated copper phosphide (Cu3P/CuP2@NPC), which exhibits a high initial charge capacity (409 mA h g-1 at 100 mA g-1) and an outstanding cycle performance (100 mA h g-1 after 5000 cycles at 1000 mA g-1) as an anode material for PIBs. The novel hollow nanocage structure could prevent volume expansion during cycling and reduce the electron/ion diffusion distance. Besides, the nitrogen/phosphorus dual-doped carbon-coated layer could promote electronic conductivity. In situ X-ray diffraction (XRD) measurements are conducted to study the potassiation/depotassiation mechanism of Cu3P/CuP2@NPC and reveal the structure stability during the cycle process, which further proves that the design ideas of the conductive carbon layer and the hollow structure with adequate internal void spaces are successful.

Stable Rooted Solid Electrolyte Interphase for Lithium-Ion Batteries

Metal oxide-based materials are attractive anode candidates for lithium-ion batteries (LIBs) because of their high theoretical capacity. However, these materials suffer from large volume expansion and poor stability of solid electrolyte interphase (SEI) during the charge-discharge process, casusing rapid capacity degradation. Herein, we report that Li₃PO₄-rooted and intact SEI in situ formed on the phosphate-modified SnO₂/CNFs during cycling. The phosphate anions in the anode, could serve as the root to form Li₃PO₄ by bonding with Li ions and participate in the formation of the SEI, thus firmly anchoring and stabilizing the SEI layer. The rooted Li₃PO₄ and enriched LiF in the SEI could synergistically enhance the Li-ion diffusion, significantly reduce the volume expansion, and lead to ultrastable cycling performance over 1100 charge-discharge cycles at 1 A g^-1. This work provides a new avenue for forming stable SEI rooted into the anode and inspires the development of interface engineering toward electrochemical energy storage.

Realizing high-performance and low-cost lithium-ion capacitor by regulating kinetic matching between ternary nickel cobalt phosphate microspheres anode with ultralong-life and super-rate performance and watermelon peel biomass-derived carbon cathode

Lithium-ion capacitors (LICs) are emerging as one of the most advanced energy storage devices by combining the virtues of both supercapacitors (SCs) and lithium-ion batteries (LIBs). However, the kinetic and capacity mismatch between anode and cathode is the main obstacle to wide applications of LICs. Therefore, the effective strategy of constructing a high-performance LIC is to improve the rate and cycle performance of the anode and the specific capacity of the cathode. Herein, the nickel cobalt phosphate (NiCoP) microspheres anode is demonstrated with robust structural integrity, high electrical conductivity, and fast kinetic feature. Simultaneously, the watermelon-peel biomass-derived carbon (WPBC) cathode is demonstrated a sustainable synthesis strategy with high specific capacity. As expected, the NiCoP exhibits high specific capacities (567 mAh g^-1 at 0.1 A g^-1), superior rate performance (300 mAh g^-1 at 1A g^-1), and excellent cycle stability (58 mAh g^-1 at 5 A g^-1 after 15,000 cycles). The WPBC possesses a high specific surface area (SSA) of 3303.6 m² g^-1 and a high specific capacity of 226 mAh g^-1 at 0.1 A g^-1. Encouragingly, the NiCoP//WPBC-6 LIC device can deliver high energy density (ED) of 127.4 ± 3.3 and 67 ± 3.8Wh kg^-1 at power density (PD) of 190 and 18240 W kg^-1 (76.4% capacity retention after 7000 cycles), respectively.

Activity Promotion of Core and Shell in Multifunctional Core-Shell Co2 P@NC Electrocatalyst by Secondary Metal Doping for Water Electrolysis and Zn-Air Batteries

Developing cost-efficient multifunctional electrocatalysts is highly critical for the integrated electrochemical energy-conversion systems such as water electrolysis based on hydrogen/oxygen evolution reactions (HER/OER) and metal-air batteries based on OER/oxygen reduction reactions (ORR). The core-shell structured materials with transition metal phosphide as the core and nitrogen-doped carbon (NC) as the shell have been known as promising HER electrocatalysts. However, their oxygen-related electrocatalytic activities still remain unsatisfactory, which severely limits their further applications. Herein an effective strategy to improve the core and shell performances of core-shell Co₂ P@NC electrocatalysts through secondary metal (e.g., Fe, Ni, Mo, Al, Mn) doping (termed M-Co₂ P@M-N-C) is reported. The as-synthesized M-Co₂ P@M-N-C electrocatalysts show multifunctional HER/OER/ORR activities and good integrated capabilities for overall water splitting and Zn-air batteries. Among the M-Co₂ P@M-N-C catalysts, Fe-Co₂ P@Fe-N-C electrocatalyst exhibits the best catalytic activities, which is closely related to the configuration of highly active species (Fe-doping Co₂ P core and Fe-N-C shell) and their subtle synergy, and a stable carbon shell for outstanding durability. Combination of electrochemical-based in situ Fourier transform infrared spectroscopy with extensive experimental investigation provides deep insights into the origin of the activity and the underlying electrocatalytic mechanisms at the molecular level.

Recent progress in electrochemical performance of carbon-based anodes for potassium-ion batteries based on first principles calculations

Carbonaceous materials and the composite materials of transition metals compounds in carbon matrix were widely used as anode for potassium-ion batteries (PIBs). During the research of these anode materials, first-principles calculations based on adsorption energy, density of states (DOSs) as well as diffusion energy barriers was regarded as an effectively approach to investigate their potassium storage mechanism. The underlying reasons for the improvement of electrochemical performance could be well illustrated via the corresponding calculations. Moreover, first-principles calculations also played a vital role to predict the material properties of electrodes before conducting experimental analysis. Hence, this review is to analyze in-depth the effect mechanism of K-adsorption energy, DOSs as well as diffusion energy barrier and so on for electrochemical performance of carbon-based anode materials. We summarized the corresponding research progress, the challenges of first principles calculations in PIBs, and proposed the corresponding strategies along with future perspectives for further development of carbon-based anode materials. This work not only can provide theoretical guidance for the development of anode materials with excellent physical and chemical properties, but also have reference significance for other energy storage systems.

3D Carbon Networks: Design and Applications in Sodium Ion Batteries

As the key component of a new generation for low-cost energy storage systems, sodium-ion batteries (SIBs) have attracted enormous attention and research due to its promising potentiality in large-scale electrochemical energy storage. For practical application of SIBs, carbonaceous materials have been considered to be one of the best choices for electrodes in virtue of their abundant reserves, low cost, easy availability, and environmental friendliness. 3D carbon network (3D-carbon) is of particular interests, which has displayed outstanding features, including abundant active sites, interconnected multi-level pore structures, high electronic conductivity, and excellent mechanical stability. Herein, we review the structural advantages of 3D-carbon and its preparation methods, and then discuss recent progress in 3D carbon materials and their composites for SIBs. The superior functionalities of 3D-carbon are emphasized as support templates or encapsulation shell membranes. Finally, we summarize and outline the challenges and future prospects of 3D-carbon in SIBs.

Robust Electrodes for Flexible Energy Storage Devices Based on Bimetallic Encapsulated Core-Multishell Structures

Developing flexible electrodes with high active materials loading and excellent mechanical stability is of importance to flexible electronics, yet remains challenging. Herein, robust flexible electrodes with an encapsulated core-multishell structure are developed via a spraying-hydrothermal process. The multilayer electrode possesses an architecture of substrate/reduced graphene oxide (rGO)/bimetallic complex/rGO/bimetallic complex/rGO from the inside to the outside, where the cellulosic fibers serve as the substrate, namely, the core; and the multiple layers of rGO and bimetallic complex, are used as active materials, namely, the shells. The inner two rGO interlayers function as the cement that chemically bind to two adjacent layers, while the two outer rGO layers encapsulate the inside structure effectively protecting the electrode from materials detachment or electrolyte corrosion. The electrodes with a unique core-multishell structure exhibit excellent cycle stability and exceptional temperature tolerance (-25 to 40 °C) for lithium and sodium storage. A combination of experimental and theoretical investigations are carried out to gain insights into the synergetic effects of cobalt-molybdenum-sulfide (CMS) materials (the bimetallic complex), which will provide guidance for future exploration of bimetallic sulfides. This strategy is further demonstrated in other substrates, showing general applicability and great potential in the development of flexible energy storage devices.

Regulating Solvent Molecule Coordination with KPF6 for Superstable Graphite Potassium Anodes

Graphite is one of the most attractive anode materials due to its low cost, environmental friendliness, and high energy density for potassium ion batteries (PIBs). However, the severe capacity fade of graphite anodes in traditional KPF₆-based electrolyte hinders its practical applications. Here, we demonstrate that the cycling stability of graphite anodes can be significantly improved by regulating the coordination of solvent molecules with KPF₆ via a high-temperature precycling step. In addition to the solvents being electrochemically stable against reduction, a stable and uniform organic-rich passivation layer also forms on the graphite anodes after high-temperature precycling. Consequently, the PIBs with graphite anodes could operate for more than 500 cycles at 50 mA g^-1 with a reversible capacity of about 220 mAh g^-1 and an average Coulombic efficiency greater than 99%. Furthermore, full batteries based on Prussian blue cathodes and high-temperature precycled graphite anodes also exhibit excellent performance. Molecular dynamics simulations were performed to explore the solvation chemistry of the electrolytes used in this study.

Unlocking Rapid and Robust Sodium Storage Performance of Zinc-Based Sulfide via Indium Incorporation

Zinc sulfide (ZnS) exhibits promise in sodium-ion batteries (SIBs) because of its low operation voltage and high theoretical specific capacity. However, pristine ZnS is not adequate in realizing rapid and robust sodium storage owing to its low reversibility, poor structure stability, and sluggish kinetics. To date, most efforts focus on utilizing carbonaceous incorporation to improve its electrochemical performances. Nevertheless, it remains an arduous challenge for realizing superior rate capability while obtaining stable cycling. Herein, inspired by the crystal structure of hexagonal ZnIn₂S₄, which possesses an intrinsic layered feature with larger unit-cell volume versus that of ZnS, indium incorporation is thus deployed as an immediate remedy. In/ex situ investigations combined with density functional theory calculations are conducted to reveal the superior kinetics, high reversibility, and good structure stability of ZnIn₂S₄. Notably, the formed indium-based derivatives during cycling manifest a Na⁺ (de)intercalation process, thereby exciting a synergetic mechanism to stabilize electrochemical cycling. As a result, the electrochemical performances of Zn-based sulfide are significantly improved via the indium incorporation. Furthermore, a full cell based on the ZnIn₂S₄ anode with the superior electrochemical performance is developed. This work provides an effective tactic of heteroatom incorporation for optimizing structure as well as exciting a complementary reaction process toward developing superior anodes for high-performance alkali-ion batteries.

The Efficient K Ion Storage of M2 P2 O7 /C (M=Fe, Co, Ni) Anode Derived from Organic-Inorganic Phosphate Precursors

Metal phosphates have been widely explored in lithium ion batteries and sodium ion batteries owing to high theoretical capacities, mild toxicity and low cost. However, their potassium ion battery applications are less reported due to the limited conductivity and the slow diffusion kinetics. Considering these drawbacks, novel structured M₂ P₂ O₇ /C (M=Fe, Co, Ni) nanoflake composites are prepared through an organic-phosphors precursor-assisted solvothermal method and a subsequent high temperature annealing process. The designed Co₂ P₂ O₇ /C composite exhibits the highest rate capacity with 502 mAh g^-1 at 0.1 A g^-1 and good cyclability for 900 cycles at 1 A g^-1 and 2 A g^-1 when compared with Ni and Fe based composites. The superior electrochemical performance can be attributed to their unique nanoparticle-assembled nanoflake structure, which can afford enough active sites for K⁺ intercalation. In addition, the robust pyrophosphate crystal structure and the in situ formed carbon composition also have positive effects on enhancing the long-term cycling performance and the electrode's conductivity. Finally, this organic-phosphors precursor induced simple approach can be applied for easy fabrication of other pyrophosphate/carbon hybrids as advanced electrodes.

Facile Synthesis of Birnessite δ-MnO2 and Carbon Nanotube Composites as Effective Catalysts for Li-CO2 Batteries

Li-CO₂ batteries are one type of promising energy storage and conversion devices to capture and utilize the greenhouse gas CO₂, mitigating global temperature rise and climate change. Catalysts that could effectively decompose the discharge product, Li₂CO₃, are essential for high-performance Li-CO₂ batteries. Benefiting from the interconnected porous structure, favorable oxygen vacancy, and the synergistic effects between the carbon nanotube (CNT) and layered birnessite δ-MnO₂, our Li-CO₂ cathodes with the as-prepared CNT@δ-MnO₂ catalyst can efficiently afford a large reaction surface area and abundant active sites, provide sufficient electron/Li⁺ transport pathways, and facilitate electrolyte infiltration and CO₂ diffusion, demonstrating low overpotential and superior cycling stability, which have been proven by both experimental characterization and theoretical computation. It is expected that this work can provide guidance for the design and synthesis of high-performance electrochemical catalysts for Li-CO₂ batteries.

Ultrafine MoP Nanoparticle Splotched Nitrogen-Doped Carbon Nanosheets Enabling High-Performance 3D-Printed Potassium-Ion Hybrid Capacitors

Size engineering is deemed to be an adoptable method to boost the electrochemical properties of potassium-ion storage; however, it remains a critical challenge to significantly reduce the nanoparticle size without compromising the uniformity. In this work, a series of MoP nanoparticle splotched nitrogen-doped carbon nanosheets (MoP@NC) is synthesized. Due to the coordinate and hydrogen bonds in the water-soluble polyacrylamide hydrogel, MoP is uniformly confined in a 3D porous NC to form ultrafine nanoparticles which facilitate the extreme exposure of abundant three-phase boundaries (MoP, NC, and electrolyte) for ionic binding and storage. Consequently, MoP@NC-1 delivers an excellent capacity performance (256.1 mAh g-1 at 0.1 A g-1) and long-term cycling durability (89.9% capacitance retention after 800 cycles). It is further confirmed via density functional theory calculations that the smaller the MoP nanoparticle, the larger the three-phase boundary achieved for favoring competitive binding energy toward potassium ions. Finally, MoP@NC-1 is applied as highly electroactive additive for 3D printing ink to fabricate 3D-printed potassium-ion hybrid capacitors, which delivers high gravimetric energy/power density of 69.7 Wh kg-1/2041.6 W kg-1, as well as favorable areal energy/power density of 0.34 mWh cm-2/9.97 mW cm-2.

Developing better ester- and ether-based electrolytes for potassium-ion batteries

Potassium-ion batteries (PIBs) have attracted extensive attention for next-generation energy storage systems because of the high abundance of potassium resources and low cost. However, the electrochemical performance of PIBs still cannot satisfy the requirements of practical application. One of the most effective strategies to improve the electrochemical performance of PIBs is electrolyte optimization. In this review, we focus on recent advances in ester- and ether-based electrolytes for high-performance PIBs. First, we discuss the requirements and components of organic electrolytes (potassium salts and solvents) for PIBs. Then, the strategies toward optimizing the electrolytes have been summarized, including potassium salt optimization, solvent optimization, electrolyte concentration optimization, and introducing electrolyte additives. In general, the electrolyte optimization methods can adjust the solvation energy, the lowest unoccupied molecular orbital energy level, and the highest occupied molecular orbital energy level, which are beneficial for achieving fast kinetics, stable and highly K⁺-conductive solid-electrolyte interphase layer, and superior oxidation resistance, respectively. Future studies should focus on exploring the effects of composition on electrolyte characteristics and the corresponding laws. This review provides some significant guidance to develop better electrolytes for high-performance PIBs.

A comprehensive summary on how to optimize ester- and ether-based electrolytes for high-performance potassium-ion batteries.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Achieving the robust immobilization of CoP nanoparticles in cellulose nanofiber network-derived carbon via chemical bonding for a stable potassium ion storage

Potassium-ion batteries (KIBs) are currently being investigated as a potential alternative to lithium-ion batteries (LIBs) because of the natural abundance of K resources. Presently, it is crucial yet challenging to explore suitable anode materials for stable K-storage. Herein, a novel robust CoP-carbon composite with highly dispersed CoP nanoparticles (NPs) immobilized in natural cellulose nanofiber network (CNF)-derived carbon (denoted as CoP@CNFC) is synthesized via chemical bonding through a facile hydrothermal and subsequent in situ phosphidation approach. The designed structure can provide diverse merits, including fast reaction kinetics, sufficient active sites and effective accommodation for K⁺ insertion/extraction; thus, CoP@CNFC delivers desired electrochemical performance, including considerable reversible capacity, enhanced rate capability and excellent cycling stability. Additionally, the electrochemical reaction mechanism of CoP@CNFC was clearly revealed by ex situ characterizations and theoretical simulations of cyclic voltammetry (CV) and solid electrolyte interface (SEI) profiles based on first-principles calculations. The achieved deep elucidation of the reversible process of K⁺ insertion and extraction on the surface/interface of the active material during the discharge and charge states clearly highlights its significance for stable K-storage. This work promotes the facile design and deep understanding of nanostructured high-capacity electrodes of transition metal phosphates for rechargeable KIBs.