K-ion and Na-ion storage performances of Co3O4-Fe2O3 nanoparticle-decorated super P carbon black prepared by a ball milling process

A portable magnetic electrochemical sensor for highly efficient Pb(II) detection based on bimetal composites from Fe-on-Co-MOF

The practical, sensitive, and real-time detection of heavy metal ions is an essential and difficult problem. This study presents the design of a unique magnetic electrochemical detection system that can achieve real-time field detection. To enhance the electrochemical performance of the sensor, Fe2O3@C-800, Co/CoO@/C-600, and CoFe2O4@C-600 magnetic composites were synthesized using three MOFs precursors by the solvothermal method. And the morphology structure and electrochemical properties of as-prepared magnetic composites were researched by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), specific surface area and porosity analyzer (BET) and differential pulse voltammetry (DPV). The results shown that these composites improve conductivity and stability while preserving the MOFs basic frame structure. Compared with the monometallic MOFs-derived composites, the synergistic effect of the bimetallic composite CoFe2O4@C-600 can significantly enhance the electrochemical performance of the sensor. The linear range for the detection of lead ions was 0.001-60 μM, and the detection limit was 0.0043 μM with a sensitivity of 22.22 μA μM·cm-2 by differential pulse voltammetry. The sensor has good selectivity, stability, reproducibility and can be used for actual sample testing.

Fe2O3 for stable K-ion storage: mechanism insight into dimensional construction from stress distribution and micro-tomography

Fe2O3 is considered a potential electrode material owing to its high theoretical capacity, low cost, and non-toxic characteristics. However, the significant volume expansion and structural degradation during charging and discharging hinder its application in potassium ion batteries. The electrochemical properties of the electrode material are primarily influenced by the diffusion efficiency of ions and the mechanics of the object. From the construction of a one dimensional structure, a three-dimensional flower-like Fe2O3 with a high specific surface and low-dimensional spherical Fe2O3 were prepared. Considering the convenience and visualization of the research, micron-scale Fe2O3 was prepared, although the larger particle size will lose part of the capacity. Notably, compared with the spherical structure, the specific capacity of the flower structure was increased by about 100%. The von Mises stress distribution on the two structures was simulated by the finite element method, revealing the mechanism of electrode failure induced by volume expansion and confirming the vital role of the multidimensional system in relieving stress concentration and improving electrochemical performance. Furthermore, synchrotron radiation soft X-ray absorption spectrum and X-ray micro-tomography revealed the phase transformation process and reaction mechanism of Fe2O3 in potassium ion batteries. The dimensional structure construction strategy reported here can provide theoretical support for modifying transition metal oxides.

Octahedral/Tetrahedral Vacancies in Fe3 O4 as K-Storage Sites: A Case of Anti-Spinel Structure Material Serving as High-Performance Anodes for PIBs

Potassium-ion batteries (PIBs) have attracted more and more attention as viable alternatives to lithium-ion batteries (LIBs) due to the deficiency and uneven distribution of lithium resources. However, it is shown that potassium storage in some compounds through reaction or intercalation mechanisms cannot effectively improve the capacity and stability of anodes for PIBs. The unique anti-spinel structure of magnetite (Fe3 O4 ) is densely packed with thirty-two O atoms to form a face-centered cubic (fcc) unit cell with tetrahedral/octahedral vacancies in the O-closed packing structure, which can serve as K+ storage sites according to the density functional theory (DFT) calculation results. In this work, carbon-coated Fe3 O4 @C nanoparticles are prepared as high-performance anodes for PIBs, which exhibit high reversible capacity (638 mAh g-1 at 0.05 A g-1 ) and hyper stable cycling performance at ultrahigh current density (150 mAh g-1 after 9000 cycles at 10 A g-1 ). In situ XRD, ex-situ Fe K-edge XAFS, and DFT calculations confirm the storage of K+ in tetrahedral/octahedral vacancies.

Regulated electrochemical performance of manganese oxide cathode for potassium-ion batteries: A combined experimental and first-principles density functional theory (DFT) investigation

Potassium-ion batteries (KIBs) are promising energy storage devices owing to their low cost, environmental-friendly, and excellent K⁺ diffusion properties as a consequence of the small Stoke's radius. The evaluation of cathode materials for KIBs, which are perhaps the most favorable substitutes to lithium-ion batteries, is of exceptional importance. Manganese dioxide (α-MnO₂) is distinguished by its tunnel structures and plenty of electroactive sites, which can host cations without causing fundamental structural breakdown. As a result of the satisfactory redox kinetics and diffusion pathways of K⁺ in the structure, α-MnO₂ nanorods cathode prepared through hydrothermal method, reversibly stores K⁺ at a fast rate with a high capacity and stability. It has a first discharge capacity of 142 mAh/g at C/20, excellent rate execution up to 5C, and a long cycling performance with a demonstration of moderate capacity retention up to 100 cycles. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) simulations confirm that the K⁺ intercalation/deintercalation occurs through 0.46 K movement between Mn^IV/Mn^III redox pairs. First-principles density functional theory (DFT) calculations predict a diffusion barrier of 0.31 eV for K⁺ through the 1D tunnel of α-MnO₂ electrode, which is low enough to promote faster electrochemical kinetics. The nanorod structure of α-MnO₂ facilitates electron conductive connection and provides a strong electrode-electrolyte interface for the cathode, resulting in a very consistent and prevalent execution cathode material for KIBs.

Emerging carbon-based flexible anodes for potassium-ion batteries: Progress and opportunities

In recent years, carbon-based flexible anodes for potassium-ion batteries are increasingly investigated owing to the low reduction potential and abundant reserve of K and the simple preparation process of flexible electrodes. In this review, three main problems on pristine carbon-based flexible anodes are summarized: excessive volume change, repeated SEI growth, and low affinity with K⁺, which thus leads to severe capacity fade, sluggish K⁺ diffusion dynamics, and limited active sites. In this regard, the recent progress on the various modification strategies is introduced in detail, which are categorized as heteroatom-doping, coupling with metal and chalcogenide nanoparticles, and coupling with other carbonaceous materials. It is found that the doping of heteroatoms can bring the five enhancement effects of increasing active sites, improving electrical conductivity, expediting K⁺ diffusion, strengthening structural stability, and enlarging interlayer spacing. The coupling of metal and chalcogenide nanoparticles can largely offset the weakness of the scarcity of K⁺ storage sites and the poor wettability of pristine carbon-based flexible electrodes. The alloy nanoparticles consisting of the electrochemically active and inactive metals can concurrently gain a stable structure and high capacity in comparison to mono-metal nanoparticles. The coupling of the carbonaceous materials with different characteristics can coordinate the advantages of the nanostructure from graphite carbon, the defects and vacancies from amorphous carbon, and the independent structure from support carbon. Finally, the emerging challenges and opportunities for the development of carbon-based flexible anodes are presented.

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

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

Perspectives on Iron Oxide-Based Materials with Carbon as Anodes for Li- and K-Ion Batteries

The necessity for large scale and sustainable energy storage systems is increasing. Lithium-ion batteries have been extensively utilized over the past decades for a range of applications including electronic devices and electric vehicles due to their distinguishing characteristics. Nevertheless, their massive deployment can be questionable due to use of critical materials as well as limited lithium resources and growing costs of extraction. One of the emerging alternative candidates is potassium-ion battery technology due to potassium’s extensive reserves along with its physical and chemical properties similar to lithium. The challenge to develop anode materials with good rate capability, stability and high safety yet remains. Iron oxides are potentially promising anodes for both battery systems due to their high theoretical capacity, low cost and abundant reserves, which aligns with the targets of large-scale application and limited environmental footprint. However, they present relevant limitations such as low electronic conductivity, significant volume changes and inadequate energy efficiency. In this review, we discuss some recent design strategies of iron oxide-based materials for both electrochemical systems and highlight the relationships of their structure performance in nanostructured anodes. Finally, we outline challenges and opportunities for these materials for possible development of KIBs as a complementary technology to LIBs.

Stabilizing V2O3 in carbon nanofiber flexible films for ultrastable potassium storage

Vanadium oxides, such as V2O3 and VO2, are expected to be potential anode materials for potassium-ion batteries (KIBs) on account of their high theoretical capacity, low price and natural abundance....

Hybrid nanostructures for electrochemical potassium storage

The wide availability and low cost of potassium resources have made electrochemical potassium storage a promising energy storage solution for sustainable decarbonisation. Research activities have been rapidly increasing in the last few years to investigate various potassium batteries such as K-ion batteries (KIBs), K-S batteries and K-Se batteries. The electrode materials of these battery technologies are being extensively studied to examine their suitability and performance, and the utilisation of hybrid nanostructures has undoubtedly contributed to the advancement of the performance. This review presents a timely summary of utilising hybrid nanostructures as battery electrodes to address the issues currently existing in potassium batteries via taking advantage of the compositional and structural diversity of hybrid nanostructures. The complex challenges in KIBs and K-S and K-Se batteries are outlined and the role of hybrid nanostructures is discussed in detail regarding the characteristics of intercalation, conversion and alloying reactions that take place to electrochemically store K in hybrid nanostructures, highlighting their multifunctionality in addressing the challenges. Finally, outlooks are given to stimulate new ideas and insights into the future development of hybrid nanostructures for electrochemical potassium storage.

State-of-the-art anodes of potassium-ion batteries: synthesis, chemistry, and applications

The growing demand for green energy has fueled the exploration of sustainable and eco-friendly energy storage systems. To date, the primary focus has been solely on the enhancement of lithium-ion battery (LIB) technologies. Recently, the increasing demand and uneven distribution of lithium resources have prompted extensive attention toward the development of other advanced battery systems. As a promising alternative to LIBs, potassium-ion batteries (KIBs) have attracted considerable interest over the past years owing to their resource abundance, low cost, and high working voltage. Capitalizing on the significant research and technological advancements of LIBs, KIBs have undergone rapid development, especially the anode component, and diverse synthesis techniques, potassiation chemistry, and energy storage applications have been systematically investigated and proposed. In this review, the necessity of exploring superior anode materials is highlighted, and representative KIB anodes as well as various structural construction approaches are summarized. Furthermore, critical issues, challenges, and perspectives of KIB anodes are meticulously organized and presented. With a strengthened understanding of the associated potassiation chemistry, the composition and microstructural modification of KIB anodes could be significantly improved.

Enhanced Potassium Storage Performance for K-Te Batteries via Electrode Design and Electrolyte Salt Chemistry

Potassium batteries are an emerging energy storage technology due to the large abundance of potassium, low cost, and potentially high energy density. However, it remains challenging to find suitable electrode materials with high energy density and good cycling stability due to the structural instability and kinetics issues resulting from large size K⁺. Herein, a durable and high-capacity K-Te battery was developed by rational design of a Te/C electrode and electrolyte salt chemistry. A well-confined Te/C cathode structure was prepared by using a commercially available activated carbon as the Te host via a melt-diffusion method. Compared to bulky Te, the confined Te/C electrode exhibited greatly improved cycling stability, specific capacity, and rate capability in K-Te batteries. Moreover, it was found that the electrolyte salts (KPF₆ and KFSI) had significant impacts on the electrochemical performance of K-Te batteries. The Te/C electrode in the KPF₆-based carbonate electrolyte exhibited higher specific capacity and better rate performance than the Te/C electrode in the KFSI-based one. Mechanism studies revealed that the KPF₆ salt resulted in an organic species-rich solid-electrolyte interphase (SEI) on the Te/C electrode, allowing for fast electron transfer and K-ion diffusion and enhanced K-ion storage performance in K-Te batteries. In contrast, KFSI salt led to the formation of KF-rich SEI layers, which had much higher resistances for electron and K-ion transport and was less effective for the well-confined Te/C electrode. Our work finds that the Te electrode and electrolyte chemistry need to be simultaneously optimized and tailored toward K-ion storage in K-Te batteries. It is expected that the finding reported herein might be inspirable for the future development of K-chalcogen (S/Se/Te) batteries.

Encapsulating V2O3 Nanoparticles in Hierarchical Porous Carbon Nanosheets via C-O-V Bonds for Fast and Durable Potassium-Ion Storage

Vanadium oxide (V₂O₃) has been considered as a promising anode material for potassium-ion batteries (PIBs), but challenging as well for the low electron/ion conductivity and poor structural stability. To tackle these issues, herein, a novel sheetlike hybrid nanoarchitecture constructed by uniformly encapsulating V₂O₃ nanoparticles in amorphous carbon nanosheets (V₂O₃@C) with the generation of C-O-V bonding is presented. Such a subtle architecture effectively facilitates the infiltration of electrolyte, relieves the mechanical strain, and reduces the potassium-ion diffusion distance during the repetitive charging/discharging processes. The generated C-O-V bonding not only accelerated charge transfer across the carbon-V₂O₃ interface but also strengthened the structural stability. Benefiting from the synergistic effects, the as-prepared V₂O₃@C nanosheets display fast and durable potassium storage behaviors with a reversible capacity of 116.6 mAh g^-1 delivered at 5 A g^-1, and a specific capacity of 147.9 mAh g^-1 retained after 1800 cycles at a high current density of 2 A g^-1. Moreover, the insertion/extraction mechanism of V₂O₃@C nanosheets in potassium-ion storage is systematically demonstrated by electrochemical analysis and ex situ technologies. This study will shed light on the fabricating of other metal oxides anodes for high-performance PIBs and beyond.

Diagnosing the SEI Layer in a Potassium Ion Battery Using Distribution of Relaxation Time

Understanding the solid electrolyte interphase (SEI) formation process in novel battery systems is of primary importance. Alongside increasingly powerful in situ techniques, searching for readily accessible, noninvasive, and low-cost tools to probe battery chemistry is highly demanded. Here, we applied distribution of relaxation time analysis to interpret in situ electrochemical impedance spectroscopy results during cycling, which is able to distinguish various electrochemical processes based on their time constants. By building a direct link between the SEI layer and the cell performances, it allows us to track the formation and evolution process of the SEI layer, diagnose the failure of the cell, and unveil the reaction mechanisms. For instance, in a K-ion cell using a SnS₂/N-doped reduced graphene oxide composite electrode, we found that the worsened mass transport in the electrolyte phase caused by the weak SEI layer is the main reason for cell deterioration. In the electrolyte with potassium bis(fluorosulfonyl)imide, the porous structure of the composite electrode was reinforced by rapid formation of a robust SEI layer at the SnS₂/electrolyte interface, and thus, the cell delivers a high capacity and good cyclability. This method lowers the barrier of in situ EIS analysis and helps public researchers to explore high-performance electrode materials.

Hierarchical Triple-Shelled MnCo2 O4 Hollow Microspheres as High-Performance Anode Materials for Potassium-Ion Batteries

Metal oxide anode materials generally possess high theoretical capacities. However, their further development in potassium-ion batteries (KIBs) is limited by self-aggregation and large volume fluctuations during charge/discharge processes. Herein, hierarchical MnCo₂ O₄ hollow microspheres (ts-MCO HSs) with three porous shells that consist of aggregated primary nanoparticles are fabricated as anode materials of KIBs. The porous shells are in favor of reducing the diffusion path of K-ions and electrons, and thus the rate performance can be enhanced. The unique triple-shelled hollow structure is believed to provide sufficient contact between electrolyte and metal oxides, possess additional active storage sites for K-ions, and buffer the volume change during K-ions insertion/extraction. A high specific capacity of 243 mA h g^-1 at 100 mA g^-1 in the 2nd cycle and a highly improved rate performance of 153 mA h g^-1 at 1 A g^-1 are delivered when cycled between 0.01 and 3.0 V. In addition, the transformation of substances during charging/discharging processes are intuitively demonstrated by the in situ X-ray diffraction strategy for the first time, which further proves that the unique structure of ts-MCO HSs with three porous shells can significantly enhance the potassium ions storage performance.

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.

Intercalation-Induced Conversion Reactions Give High-Capacity Potassium Storage

Potassium ion batteries (PIBs) have shown great potential as a next-generation electrochemical energy storage system, due to the natural abundance of potassium and the relatively low redox potential of K ions. To accommodate the large ionic radius of K ions, conversion-type electrode materials are regarded as suitable candidates for K ion storage. However, the triggering mechanism of a conversion reaction in most anode materials of PIBs is unclear, which limits their further development. To reveal the mechanism, in this work, MoSe₂, MoS₂, and MoO₂ were selected as model materials, guided by theoretical calculations, to investigate the K ion storage process. Through ex situ characterization, it was found that intercalation reactions preferentially occur in MoSe₂ and MoS₂, while an adsorption reaction preferentially occurs in MoO₂. This is because of the larger interlayer spacing and lower K ion intercalation barrier in MoSe₂ and MoS₂ than in MoO₂. The preferential intercalation reactions are able to induce a further conversion reaction by reducing the reaction barrier, thereby realizing high K ion storage capacities. As a result, the MoSe₂-rGO and MoS₂-rGO hybrids showed higher reversible capacities than the MoO₂-rGO hybrid. By demonstrating a relationship between intercalation and the conversion reaction and understanding the mechanism, guidance is provided for selecting the electrode materials to obtain PIBs with high performance.

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.

Two-Dimensional Transition Metal Chalcogenides for Alkali Metal Ions Storage

On the heels of exacerbating environmental concerns and ever-growing global energy demand, development of high-performance renewable energy-storage and -conversion devices has aroused great interest. The electrode materials, which are the critical components in electrochemical energy storage (EES) devices, largely determine the energy-storage properties, and the development of suitable active electrode materials is crucial to achieve efficient and environmentally friendly EES technologies albeit the challenges. Two-dimensional transition-metal chalcogenides (2D TMDs) are promising electrode materials in alkali metal ion batteries and supercapacitors because of ample interlayer space, large specific surface areas, fast ion-transfer kinetics, and large theoretical capacities achieved through intercalation and conversion reactions. However, they generally suffer from low electronic conductivities as well as substantial volume change and irreversible side reactions during the charge/discharge process, which result in poor cycling stability, poor rate performance, and low round-trip efficiency. In this Review, recent advances of 2D TMDs-based electrode materials for alkali metal-ion energy-storage devices with the focus on lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), potassium-ion batteries (PIBs), high-energy lithium-sulfur (Li-S), and lithium-air (Li-O₂ ) batteries are described. The challenges and future directions of 2D TMDs-based electrode materials for high-performance LIBs, SIBs, PIBs, Li-S, and Li-O₂ batteries as well as emerging alkali metal-ion capacitors are also discussed.

Research Development on K-Ion Batteries

Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/toxic elements, the low standard electrode potentials of K/K⁺ electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K⁺ ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li⁺ ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

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

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

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

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

A Black Phosphorus-Graphite Composite Anode for Li-/Na-/K-Ion Batteries

Black phosphorus (BP) is a desirable anode material for alkali metal ion storage owing to its high electronic/ionic conductivity and theoretical capacity. In-depth understanding of the redox reactions between BP and the alkali metal ions is key to reveal the potential and limitations of BP, and thus to guide the design of BP-based composites for high-performance alkali metal ion batteries. Comparative studies of the electrochemical reactions of Li⁺ , Na⁺ , and K⁺ with BP were performed. Ex situ X-ray absorption near-edge spectroscopy combined with theoretical calculation reveal the lowest utilization of BP for K⁺ storage than for Na⁺ and Li⁺ , which is ascribed to the highest formation energy and the lowest ion diffusion coefficient of the final potassiation product K₃ P, compared with Li₃ P and Na₃ P. As a result, restricting the formation of K₃ P by limiting the discharge voltage achieves a gravimetric capacity of 1300 mAh g^-1 which retains at 600 mAh g^-1 after 50 cycles at 0.25 A g^-1 .

Engineering of nanonetwork-structured carbon to enable high-performance potassium-ion storage

Potassium-ion batteries (KIBs) have been developed as an emerging electrochemical energy storage device due to the low cost and abundant resource of potassium. However, they suffer insufficient cyclability and poor rate capability caused by the large K⁺, severely limits their further applications. Herein, a nanonetwork-structured carbon (NNSC) is reported to address the issue. Cycling stability with very low decay rate of 0.004% per cycle over 2000 cycles and excellent rate capability (i.e., 261 mAh g^-1 at 100 mA g^-1 and 108 mAh g^-1 at 5000 mA g^-1) are achieved. The superior performance is attributed to the unique structure of NNSC, in which the three-dimensional interconnected hierarchical porous structure with hollow nanosphere as network units not only can effectively alleviate the volume expansion induced by the insertion of large K⁺, but also can offer fast pathways for K⁺ diffusion. In addition, the local graphitized carbon shell of NNSC can promote conductivity of material and reduce the resistance to K⁺ transportation. Thus, the NNSC has great potential in developing stable-structure and high-rate electrodes for next generation KIBs.

In situ carbon coated flower-like VPO4 as an anode material for potassium-ion batteries

Here we design a novel carbon coating method for phosphate-based VPO4 with three different morphologies, via the intercalation of isobutanol to layered VOPO4·2H2O combined with thermal reduction. The well-constructed flower-like VPO4 delivers a high reversible capacity of 400 mA h g-1 with a long cycle-life of more than 500 cycles, proving that the special structure is suitable to accommodate the large volume expansion during the electrochemical charge-discharge process.

CuO Nanoplates for High-Performance Potassium-Ion Batteries

Potassium-ion batteries (KIBs) are promising alternatives to lithium-ion batteries because of the abundance and low cost of K. However, an important challenge faced by KIBs is the search for high-capacity materials that can hold large-diameter K ions. Herein, copper oxide (CuO) nanoplates are synthesized as high-performance anode materials for KIBs. CuO nanoplates with a thickness of ≈20 nm afford a large electrode-electrolyte contact interface and short K⁺ ion diffusion distance. As a consequence, a reversible capacity of 342.5 mAh g^-1 is delivered by the as-prepared CuO nanoplate electrode at 0.2 A g^-1 . Even after 100 cycles at a high current density of 1.0 A g^-1 , the capacity of the electrode remains over 206 mAh g^-1 , which is among the best values for KIB anodes reported in the literature. Moreover, a conversion reaction occurs at the CuO anode. Cu nanoparticles form during the first potassiation process and reoxidize to Cu₂ O during the depotassiation process. Thereafter, the conversion reaction proceeds between the as-formed Cu₂ O and Cu, yielding a reversible theoretical capacity of 374 mAh g^-1 . Considering their low cost, easy preparation, and environmental benignity, CuO nanoplates are promising KIB anode materials.

SnP3/Carbon Nanocomposite as an Anode Material for Potassium-Ion Batteries

New anode materials with large capacity and long cyclability for next-generation potassium-ion batteries (PIBs) are required. PIBs are in the initial stage of investigation and only a few anode materials have been explored. In this study, for the first time, an SnP₃/C nanocomposite with superior cyclability and rate performance was evaluated as an anode for PIBs. The SnP₃/C nanocomposite was synthesized by a facile and cost-effective high-energy ball-milling technique. The SnP₃/C electrode delivered a first reversible capacity of 410 mAh g^-1 and maintained 408 mAh g^-1 after 50 cycles at a specific current of 50 mA g^-1. After 80 cycles at a high specific current of 500 mA g^-1, a high capacity of 225 mAh g^-1 remained. From a crystallographic analysis, it was suggested that the SnP₃/C nanocomposite underwent a sequential and reversible conversion and alloying reactions. The excellent cycling stability and rate capability of the SnP₃/C electrode were attributed to the nanosized SnP₃ particles and carbon buffer layer, which supplied channels for the migration of K-ions and mitigated the stress induced by a large volume change during potassiation/depotassiation. In addition, a full cell composed of the SnP₃/C nanocomposite anode and potassium Prussian blue cathode exhibited a reversible capacity of 305 mAh g^-1 at a specific current of 30 mA g^-1 and retained 71.7% of the original capacity after 30 cycles. These results are important for understanding the electrochemical process of the SnP₃/C nanocomposite and using the SnP₃/C as an anode for PIBs.

Hierarchical Mn3O4/Graphene Microflowers Fabricated via a Selective Dissolution Strategy for Alkali-Metal-Ion Storage

Mn₃O₄ is a potential anode for alkali-metal (Li/Na/K)-ion batteries because of the high capacity, abundant resources, and eco-friendliness. However, its ion storage performance is limited by poor electronic conductivity and large volume expansion during the charging/discharging process. In this study, we presented a facile dissolution strategy to fabricate ultrathin nanosheet-assembled hierarchical Mn₃O₄/graphene microflowers, realizing enhanced alkali-metal-ion storage performance. The synthetic mechanism was proven as the selective dissolution of vanadium via controlled experiments with different reaction times. The as-synthesized composites showed high lithium storage capacity (about 900 mA h g^-1) and superior cyclability (∼400 mA h g^-1 after 500 cycles). In addition, when evaluated as a Na-ion battery anode, the reversible capacity of about 200 mA h g^-1 was attained, which remained at 167 mA h g^-1 after 200 cycles. Moreover, to the best of our knowledge, the potassium storage properties of Mn₃O₄ were evaluated for the first time and a reversible capacity of about 230 mA h g^-1 was achieved. We believe that our findings will be instructive for future investigations of high-capacity anode materials for alkali-metal-ion batteries.

In Situ Alloying Strategy for Exceptional Potassium Ion Batteries

We report an in situ alloying strategy for obtaining homogeneous (Bi,Sb) alloy nanoparticles from (Bi,Sb)₂S₃ nanotubes for the exceptional anode of potassium ion batteries (KIBs). The operando X-ray diffraction results, along with transmission electron microscopy and energy-dispersive X-ray spectroscopy mappings, successfully reveal the phase evolution of this material, which is (Bi,Sb)₂S₃ → (Bi,Sb) → K(Bi,Sb) → K₃(Bi,Sb) during the initial discharge and K₃(Bi,Sb) → K(Bi,Sb) → (Bi,Sb) in the charging process. The in situ alloying strategy produces a synergistic effect and brings an outstanding electrochemical performance. It achieves ultrahigh discharge capacities of 611 mAh g^-1 at 100 mA g^-1 (0.135C) and 300 mAh g^-1 at 1000 mA g^-1 (1.35C) and retains a capacity as high as 353 mAh g^-1 after 1000 cycles at 500 mA g^-1 (0.675C) with a Coulombic efficiency close to 100%. In addition, the KIBs full cell, which is composed of this anode and a perylenetetracarboxylic dianhydride cathode, reaches an initial discharge capacity as high as 276 mAh g^-1 at 500 mA g^-1 and maintains 207 mAh g^-1 after 100 cycles.

Direct Growth of MoO2 /Reduced Graphene Oxide Hollow Sphere Composites as Advanced Anode Materials for Potassium-Ion Batteries

Hollow MoO₂ /reduced graphene oxide (MoO₂ /rGO) sub-microsphere composites have been fabricated through a simple hydrothermal approach followed by a heat treatment process. When employed as an anode material for potassium-ion batteries, the as-synthesized MoO₂ /rGO composite can deliver an initial charge specific capacity of 367.2 mAh g^-1 at 50 mA g^-1 , and its reversible capacity is 218.9 mAh g^-1 after 200 cycles. Even when cycled at 500 mA g^-1 , a high charge specific capacity of 104.2 mAh g^-1 is achieved after 500 cycles. The excellent cycling capability and rate performance may be ascribed to the synergistic effects of the reduced graphene oxide and the hollow MoO₂ spheres, which can increase the electrical conductivity of the composite, as well as resisting the strain arising from the repeated discharge-charge processes. These results indicate that the MoO₂ /rGO hollow sphere composites are promising negative electrode materials for potassium-ion batteries.

Boosting Potassium-Ion Battery Performance by Encapsulating Red Phosphorus in Free-Standing Nitrogen-Doped Porous Hollow Carbon Nanofibers

Potassium-ion batteries (KIBs) are a promising alternative to lithium-ion batteries (LIBs) for large-scale renewable energy storage owning to the natural abundance and low cost of potassium. However, the biggest challenge for KIBs application lies in the lack of suitable electrode materials that can deliver long cycle life and high reversible capacity. In this work, we realized unprecedented long cycle life with high reversible capacity (465 mAh g^-1 at 2 A g^-1 after 800 cycles) as well as outstanding rate capability (342 mAh g^-1 at 5 A g^-1) for KIBs by embedding red P into free-standing nitrogen-doped porous hollow carbon nanofibers (red P@N-PHCNFs). This design circumvents the problems of pulverization and aggregation of P particles. The  in situ transmission electron microscopy (TEM) investigation reveals the structural robustness of the composite fibers during potassiation. The formation of P-C chemical bonds as well as nitrogen doping in the carbon matrix can facilitate the sturdy contact and enhance the adsorption energy of P atoms evidenced by DFT results. In situ Raman and ex situ XRD demonstrate that the final discharge product of the red P@N-PHCNFs is K₄P₃.

Bismuth oxychloride nanoflake assemblies as a new anode for potassium ion batteries

BiOCl is demonstrated as the first bismuth oxyhalide compound serving as a new anode material for potassium ion batteries.