Cathode Materials for Potassium-Ion Batteries: Current Status and Perspective

High-Performance Cathode Materials for Potassium-Ion Batteries: Structural Design and Electrochemical Properties

Due to the obvious advantage in potassium reserves, potassium-ion batteries (PIBs) are now receiving increasing research attention as an alternative energy storage system for lithium-ion batteries (LIBs). Unfortunately, the large size of K⁺ makes it a challenging task to identify suitable electrode materials, particularly cathode ones that determine the energy density of PIBs, capable of tolerating the serious structural deformation during the continuous intercalation/deintercalation of K⁺ . It is therefore of paramount importance that proper design principles of cathode materials be followed to ensure stable electrochemical performance if a practical application of PIBs is expected. Herein, the current knowledge on the structural engineering of cathode materials acquired during the battle against its performance degradation is summarized. The K⁺ storage behavior of different types of cathodes is discussed in detail and the structure-performance relationship of materials sensitive to their different lattice frameworks is highlighted. The key issues facing the future development of different categories of cathode materials are also highlighted and perspectives for potential approaches and strategies to promote the further development of PIBs are provided.

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

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

Metal-Organic Frameworks and Their Derivatives: Designing Principles and Advances toward Advanced Cathode Materials for Alkali Metal Ion Batteries

Metal-organic frameworks (MOFs) and their derivatives have attracted enormous attention in the field of energy storage, due to their high specific surface area, tunable structure, highly ordered pores, and uniform metal sites. Compared with the wide research of MOFs and their related materials on anode materials for alkali metal ion batteries, few works are on cathode materials. In this review, design principles for promoting the electrochemical performance of MOF-related materials in terms of component/structure design, composite fabrication, and morphology engineering are presented. By summarizing the advancement of MOFs and their derivatives, Prussian blue and its analogs, and MOF surface coating, challenges and opportunities for future outlooks of MOF-related cathode materials are discussed.

Rational design of MoSe2 nanosheet-coated MOF-derived N-doped porous carbon polyhedron for potassium storage

For potassium-ion battery (PIB), it remains a huge challenge to develop an appropriate anode material to compensate the large radius of K⁺. MoSe₂ shows great potential for efficient K⁺ insertion/extraction due to its unique lamellar structures with an interlayer spacing of 6.46 Å. However, pure MoSe₂ has low electronic conductivity and agglomerates during long-term cycling. In the present work, MoSe₂ nanosheets were fabricated on the N-doped porous carbon polyhedron (NPCP). The obtained product was designated as NPCP@MoSe₂ and functioned as anode materials for PIBs. NPCP@MoSe₂ displayed a promising reversible capacity (325 mAh/g at 100 mA/g after 80 cycles), long-term cycling performance (128 mAh/g at 500 mA/g after 800 cycles), and superior rate property at 5000 mA/g. The enhanced electrochemical performance of NPCP@MoSe₂ could be attributed to the rational design of hybrid structures. Notably, the hollow NPCP provide a large contact area for the interactions among the electrolytes and electro-active materials as well as partly buffer the volume expansion. The synergistic effects between MoSe₂ and NPCP could mitigate the agglomeration of MoSe₂ nanosheets. Besides, the uniformly doping N elements enhanced the conductivity of the carbon matrix, and the N-group also provided potential binding active sites for K-ion accommodation. This work paves the ideas for the design of novel anode materials with high specific capacity, good cycling stability and outstanding rate capability for PIBs.

Enhanced Potassium Storage Capability of Two-Dimensional Transition-Metal Chalcogenides Enabled by a Collective Strategy

Potassium-ion batteries (PIBs) have been considered as a promising alternative to lithium-ion batteries due to their merits of high safety and low cost. Two-dimensional transition-metal chalcogenides (2D TMCs) with high theoretical specific capacities and unique layered structures have been proven to be amenable materials for PIB anodes. However, some intrinsic properties including severe stacking and unsatisfactory conductivity restrict their electrochemical performance, especially rate capability. Herein, we prepared a heterostructure of high-crystallized ultrathin MoSe₂ nanosheet-coated multiwall carbon nanotubes and investigated its electrochemical properties with a view to demonstrating the enhancement of a collective strategy for K storage of 2D TMCs. In such a heterostructure, the constructive contribution of CNTs not only suppresses the restacking of MoSe₂ nanosheets but also accelerates electron transport. Meanwhile, the MoSe₂ nanosheets loaded on CNTs exhibit an ultrathin feature, which can expose abundant active sites for the electrochemical reaction and shorten K⁺ diffusion length. Therefore, the synergistic effect between ultrathin MoSe₂ and CNTs endows the resulting nanocomposite with superior structural and electrochemical properties. Additionally, the high crystallinity of the MoSe₂ nanosheets further leads to the improvement of electrochemical performance. The composite electrode delivers high-rate capacities of 209.7 and 186.1 mAh g^-1 at high current densities of 5.0 and 10.0 A g^-1, respectively.

Molecular Regulation on Carbonyl-Based Organic Cathodes: Toward High-Rate and Long-Lifespan Potassium-Organic Batteries

Organic redox-active molecules have been identified as promising cathodes for practical usage of potassium-ion batteries (PIBs) but still struggle with serious dissolution problems and sluggish kinetic properties. Herein, we propose a pseudocapacitance-dominated novel insoluble carbonyl-based cathode, [2,6-di[1-(perylene-3,4,9,10-tetracarboxydiimide)]anthraquinone, AQ-diPTCDI], which possesses high reversible capacities of 150 mAh g^-1, excellent cycle stability with capacity retention of 88% over 2000 cycles, and fast kinetic properties. The strong intermolecular interactions of AQ-diPTCDI and in situ formed cathode electrolyte interphase films support it against the dissolution problem. The high capacitive-like contribution in capacities and fast potassium-ion diffusion enhance its reaction kinetics. Moreover, a symmetric organic potassium-ion battery (OPIB) based on AQ-diPTCDI electrodes also exhibits outstanding K-storage capability. These results suggest that AQ-diPTCDI is a promising organic cathode for OPIBs and provide a practicable route to realize high-performance K storage.

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.

Heterostructure Engineering of Core-Shelled Sb@Sb2 O3 Encapsulated in 3D N-Doped Carbon Hollow-Spheres for Superior Sodium/Potassium Storage

In this work, the core-shelled Sb@Sb₂ O₃ heterostructure encapsulated in 3D N-doped carbon hollow-spheres is fabricated by spray-drying combined with heat treatment. The novel core-shelled heterostructures of Sb@Sb₂ O₃ possess a mass of heterointerfaces, which formed spontaneously at the core-shell contact via annealing oxidation and can promote the rapid Na⁺ /K⁺ transfer. The density functional theory calculations revealed the mechanism and significance of Na/K-storage for the core-shelled Sb@Sb₂ O₃ heterostructure, which validated that the coupling between the high-conductivity of Sb and the stability of Sb₂ O₃ can relieve the shortcomings of the individual building blocks, thereby enhancing the Na/K-storage capacity. Furthermore, the core-shell structure embedded in the 3D carbon framework with robust structure can further increase the electrode mechanical strength and thus buffer the severe volume changes upon cycling. As a result, such composite architecture exhibited a high specific capacity of ≈573 mA h g^-1 for sodium-ion battery (SIB) anode and ≈474 mA h g^-1 for potassium-ion battery (PIB) anode at 100 mA g^-1 , and superior rate performance (302 mA h g^-1 at 30 A g^-1 for SIB anode, while 239 mA h g^-1 at 5 A g^-1 for PIB anode).

Manipulating the Solvation Structure of Nonflammable Electrolyte and Interface to Enable Unprecedented Stability of Graphite Anodes beyond 2 Years for Safe Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are attractive for low-cost and large-scale energy storage applications, in which graphite is one of the most promising anodes. However, the large size and the high activity of K⁺ ions and the highly catalytic surface of graphite largely prevent the development of safe and compatible electrolytes. Here, a nonflammable, moderate-concentration electrolyte is reported that is highly compatible with graphite anodes and that consists of fire-retardant trimethyl phosphate (TMP) and potassium bis(fluorosulfonyl)imide (KFSI) in a salt/solvent molar ratio of 3:8. It shows unprecedented stability, as evidenced by its 74% capacity retention over 24 months of cycling (over 2000 cycles) at the 0.2 C current rate. Electrolyte structure and surface analyses show that this excellent cycling stability is due to the nearly 100% solvation of TMP molecules with K⁺ cations and the formation of FSI^- -derived F-rich solid electrolyte interphase (SEI), which effectively suppresses the decomposition of the solvent molecules toward the graphite anode. Furthermore, excellent performance on high-mass loaded graphite electrodes and in a full cell with perylenetetracarboxylic dianhydride cathode is demonstrated. This study highlights the importance of the compatibility of both electrolyte and the interface, and offers new opportunities to design the electrolyte-SEI nexus for safe and practical PIBs.

MXenes for Rechargeable Batteries Beyond the Lithium-Ion

Research on next-generation battery technologies (beyond Li-ion batteries, or LIBs) has been accelerating over the past few years. A key challenge for these emerging batteries has been the lack of suitable electrode materials, which severely limits their further developments. MXenes, a new class of 2D transition metal carbides, carbonitrides, and nitrides, are proposed as electrode materials for these emerging batteries due to several desirable attributes. These attributes include large and tunable interlayer spaces, excellent hydrophilicity, extraordinary conductivity, compositional diversity, and abundant surface chemistries, making MXenes promising not only as electrode materials but also as other components in the cells of emerging batteries. Herein, an overview and assessment of the utilization of MXenes in rechargeable batteries beyond LIBs, including alkali-ion (e.g., Na⁺ , K⁺ ) storage, multivalent-ion (e.g., Mg²⁺ , Zn²⁺ , and Al³⁺ ) storage, and metal batteries are presented. In particular, the synthetic strategies and properties of MXenes that enable MXenes to play various roles as electrodes, metal anode protective layers, sulfur hosts, separator modification layers, and conductive additives in these emerging batteries are discussed. Moreover, a perspective on promising future research directions on MXenes and MXene-based materials, ranging from material design and processing, fundamental understanding of the reaction mechanisms, to device performance optimization strategies is provided.

Stable Potassium Metal Anodes with an All-Aluminum Current Collector through Improved Electrolyte Wetting

This is the first report of successful potassium metal battery anode cycling with an aluminum-based rather than copper-based current collector. Dendrite-free plating/stripping is achieved through improved electrolyte wetting, employing an aluminum-powder-coated aluminum foil "Al@Al," without any modification of the support surface chemistry or electrolyte additives. The reservoir-free Al@Al half-cell is stable at 1000 cycles (1950 h) at 0.5 mA cm^-2 , with 98.9% cycling Coulombic efficiency and 0.085 V overpotential. The pre-potassiated cell is stable through a wide current range, including 130 cycles (2600 min) at 3.0 mA cm^-2 , with 0.178 V overpotential. Al@Al is fully wetted by a 4 m potassium bis(fluorosulfonyl)imide-dimethoxyethane electrolyte (θ_CA  = 0°), producing a uniform solid electrolyte interphase (SEI) during the initial galvanostatic formation cycles. On planar aluminum foil with a nearly identical surface oxide, the electrolyte wets poorly (θ_CA  = 52°). This correlates with coarse irregular SEI clumps at formation, 3D potassium islands with further SEI coarsening during plating/stripping, possibly dead potassium metal on stripped surfaces, and rapid failure. The electrochemical stability of Al@Al versus planar Al is not related to differences in potassiophilicity (nearly identical) as obtained from thermal wetting experiments. Planar Cu foils are also poorly electrolyte-wetted and become dendritic. The key fundamental takeaway is that the incomplete electrolyte wetting of collectors results in early onset of SEI instability and dendrites.

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

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

Boosted Charge Transfer in Twinborn α-(Mn2O3-MnO2) Heterostructures: Toward High-Rate and Ultralong-Life Zinc-Ion Batteries

Aqueous ZIBs are one of the most promising next-generation rechargeable batteries because of the high capacity, high hydrogen evolution overpotential, and chemically stable reversible plating/stripping of the zinc electrode in the mild aqueous electrolyte. However, there are limited cathode materials that can store Zn²⁺ reversibly with superior cycling and rate capability. Herein, hierarchically porous nanorods composed of twinborn α-(Mn₂O₃-MnO₂) heterostructures are proposed as a robust cathode for Zn storage. Thanks to the hierarchically porous nanorod morphology and the abundant interface of the heterostructures involving a built-in electric field, the as-obtained twinborn α-(Mn₂O₃-MnO₂) electrode delivers a high capacity of 170 mA h g^-1 for 2000 cycles at 500 mA g^-1 and shows an excellent rate capability of up to 1.5 A g^-1 with a capacity of 124 mA h g^-1. The inspiring results achieved exhibit the enormous potential of the high-performance heterostructure cathode for fast and stable ZIBs.

High Sulfur-Doped Hard Carbon with Advanced Potassium Storage Capacity via a Molten Salt Method

Owing to the slight volume expansion after potassiation, hard carbon is regarded as a promising anode material for potassium-ion batteries (PIBs). Heteroatom doping (such as sulfur or nitrogen) is a common method to modify hard carbon for high K-storage capacity and long cycling performance. High sulfur-doped hard carbon with a sulfur content of 25.8 wt % is prepared by calcining glucose in molten salt (K₂SO₄@LiCl/KCl). It exhibits high specific capacities of 361.4 mA h g^-1 during the 1st cycle and 317.7 mA h g^-1 during the 100th cycle at 0.05 A g^-1. The high capacity arises from the K-S reaction behavior, which is demonstrated by the cyclic voltammetry test and galvanostatic intermittent titration technique. This work is an effective application of the molten salt method for PIBs, furnishing an understanding to K-storage behaviors of hard carbon- with high sulfur content.

Emerging Potassium Metal Anodes: Perspectives on Control of the Electrochemical Interfaces

ConspectusPotassium metal serves as the anode in emerging potassium metal batteries (KMBs). It also serves as the counter electrode for potassium ion battery (KIB) half-cells, with its reliable performance being critical for assessing the working electrode material. This first-of-its-kind critical Account focuses on the dual challenge of controlling the potassium metal-substrate and the potassium metal-electrolyte interface so as to prevent dendrites. The discussion begins with a comparison of the physical and chemical properties of K metal anodes versus the much oft studied Li and Na metal anodes. Based on established descriptions for root causes of dendrites, the problem should be less severe for K than for Li or Na, while in fact the opposite is observed. The key reason that the K metal surface rapidly becomes dendritic in common electrolytes is its unstable solid electrolyte interphase (SEI). An unstable SEI layer is defined as being non-self-passivating. No SEI is perfectly stable during cycling, and all SEI structures are heterogeneous both vertically and horizontally relative to the electrolyte interface. The difference between a "stable" and an "unstable" SEI may be viewed as the relative degree to which during cycling it thickens and becomes further heterogeneous. The unstable SEI on K metal leads to a number of interrelated problems, such as low cycling Coulombic efficiency (CE), a severe impedance rise, large overpotentials, and possibly electrical shorting, all of which have been reported to occur as early as in the first 10 plating/stripping cycles. Many of the traditional "interface fixes" employed for Li and Na metal anodes, such as various artificial SEIs, surface membranes, barrier layers, secondary separators, etc., have not been attempted or optimized for the case of K. This is an important area for further exploration, with an understanding that success may come harder with K than with Li due to K-based SEI reactivity with both ether and ester solvents.The second critical problem with K metal anodes is that they do not thermally or electrochemically wet a standard (untreated) Cu foil current collector. Published experimental and modeling research directly highlights the weak bonding between the K atoms and a Cu surface. Existing surface treatment approaches that achieve improved K wetting are discussed, along with the general design rules for future studies. Also discussed are geometry-based methods to tune nucleation as well dual approaches where nucleation and SEI structure are tuned through complementary schemes to achieve extended half-cell and full battery stability. We hypothesize that K metal never achieves a planar wetting morphology even at cycle one, making the dendrites "baked-in". We propose that classical thin film growth models, Frank van der Merwe (F-M), Volmer-Weber (V-W), and Stranski-Krastanov (S-K), can be employed to describe early stage plating behavior. It is demonstrated that islandlike V-W growth is the applicable description for the natural plating behavior of K on pristine Cu. Moving forward, there are three inter-related thrusts to be pursued: First, K salt-based electrolyte formulations have to mature and become further tailored to handle the increased reactivity of a metal rather than an ion anode. Second, the K-based SEI structure needs to be further understood and ultimately tuned to be less reactive. Third, the energetics of the K metal-current collector interface must be controlled to promote planar wetting/dewetting throughout cycling.

Dehydration-Triggered Ionic Channel Engineering in Potassium Niobate for Li/K-Ion Storage

Boosting charge transfer in materials is critical for applications involving charge carriers. Engineering ionic channels in electrode materials can create a skeleton to manipulate their ion and electron behaviors with favorable parameters to promote their capacity and stability. Here, tailoring of the atomic structure in layered potassium niobate (K₄ Nb₆ O₁₇ ) nanosheets and facilitating their application in lithium and potassium storage by dehydration-triggered lattice rearrangement is reported. The spectroscopy results reveal that the interatomic distances of the NbO coordination in the engineered K₄ Nb₆ O₁₇ are slightly elongated with increased degrees of disorder. Specifically, the engineered K₄ Nb₆ O₁₇ shows enhanced electrical and ionic conductivity, which can be attributed to the enlarged interlamellar spacing and subtle distortions in the fine atomic arrangements. Moreover, subsequent experimental results and calculations demonstrate that the energy barrier for Li⁺ /K⁺ diffusion is significantly lower than that in pristine K₄ Nb₆ O₁₇ . Interestingly, the diffusion coefficient of K⁺ is one order of magnitude higher than that of Li⁺ , and the engineered K₄ Nb₆ O₁₇ presents superior electrochemical performance for K⁺ to Li⁺ . This work offers an ionic engineering strategy to enable fast and durable charge transfer in materials, holding great promise for providing guidance for the material design of related energy storage systems.

Review of Emerging Potassium-Sulfur Batteries

This is the first review on potassium-sulfur (K-S) batteries (KSBs), which are emerging metal battery (MB) systems. Since KSBs are quite new, there are fundamental questions regarding the electrochemistry of S-based cathode and of K metal anode, as well as the holistic aspects of full-cell performance. The manuscript begins with a critical discussion regarding the potassium-sulfur electrochemistry and on how it differs from the much better-known lithium-sulfur. Cathodes are discussed next, focusing on the role of sulfur structure, carbon host chemistry and porosity, and electrolytes in establishing the reversible potassium sulfide K₂ S_n phase sequence, the parasitic polysulfide shuttle, pulverization-driven capacity fade, etc. Following is a discussion of solid-state electrolytes (SSEs), including of hybrid solid-liquid systems that show much promise. Potassium metal anodes are then critically reviewed, emphasizing electrolyte reactions to form stable versus unstable solid electrolyte interphase (SEI), covering the current understanding of potassium dendrites, and highlighting the deep-eutectic K-Na alloying approaches for room temperature liquid anodes. The manuscript concludes with K-S batteries, focusing on cell architectures and providing quantitative performance comparisons as master plots. Unanswered scientific/technological questions are identified, emerging research opportunities are discussed, and potential experimental and simulation-based studies that can unravel these unknowns are proposed.

Rechargeable Zn-MnO2 batteries: advances, challenges and perspectives

As one type of advanced alternative batteries, zinc-ion batteries (ZIBs) have attracted increasing attention because of their advantages of cost-effectiveness, high safety and environmentally benign features. However, the performance of cathode materials has become a bottleneck for the future application of ZIBs. In recent years, manganese dioxide (MnO₂)-based materials as cathodes for ZIBs have been intensively explored. In this review, recent advances in MnO₂-based cathode materials for ZIBs are comprehensively reviewed with a discussion about the reaction mechanisms for the fundamental understanding of the electrochemical processes. Furthermore, several challenges hindering the technology maturity are also analyzed with corresponding strategies to further improve the electrochemical performance of such Zn-MnO₂ batteries.

Ni-Doped Layered Manganese Oxide as a Stable Cathode for Potassium-Ion Batteries

Potassium-ion batteries (PIBs) are one of the promising alternatives to lithium-ion batteries (LIBs). Layered potassium manganese oxides are more attractive as cathodes for PIBs due to their high capacity, low cost, and simple synthesis method but suffer from the Jahn-Teller effect of Mn³⁺ in material synthesis. Here, a layered P3-type K_0.67Mn_0.83Ni_0.17O₂ material with a suppressed Jahn-Teller effect was successfully synthesized. K_0.67Mn_0.83Ni_0.17O₂ delivers a specific capacity of 122 mAh g^-1 at 20 mA g^-1 in the first discharge, superior rate performance, and good cycling stability (75% capacity retention cycled at a high rate of 500 mA g^-1 after 200 cycles). Besides, the K ion diffusion coefficient of the K_0.67Mn_0.83Ni_0.17O₂ electrode can reach 10^-11 cm² s^-1, which are larger than the Ni-free electrode. The X-ray diffraction and electron diffraction analyses demonstrate that appropriate nickel could suppress the Jahn-Teller effect and reduce the structural deterioration, resulting in more migration pathways for K ions, thus enhancing the rate capability and cycling performance. These results provide a strategy to develop high-performance cathode materials for PIBs and deepen the understanding of structural deterioration in layered manganese-based oxides.

K0.83V2O5: A New Layered Compound as a Stable Cathode Material for Potassium-Ion Batteries

Recently, potassium-ion batteries (PIBs) are being actively investigated. The development of PIBs calls for cathode materials with a rigid framework, reversible electrochemical reactivity, and a high amount of extractable K ions, which is extremely challenging due to the large size of potassium. Herein, a new layered compound K_0.83V₂O₅ is reported as a potential cathode material for PIBs. It delivers an initial depotassiation capacity of 86 mAh g^-1 and exhibits a reversible capacity of 90 mAh g^-1 with a high redox potential of 3.5 V (vs K⁺/K) and a capacity retention of more than 80% after 200 cycles. Experimental investigations combined with theoretical calculation indicate that depotassiation-potassiation is accommodated by contraction-expansion of the interlayer spacing along with unpuckering-puckering of the layers. Additionally, the calculated electronic structure suggests the (semi)metallic feature of K_xV₂O₅ (0 < x ≤ 0.875) and K-ion transport in the material is predicted to be one-dimensional with the experimentally estimated chemical diffusion coefficient in the order of 10^-15-10^-12 cm² s^-1. Finally, a K-ion full cell consisting of the K_0.83V₂O₅ cathode and a graphite anode is demonstrated to deliver an energy density of 136 Wh kg^-1. This study will provide insights for further designing novel layered cathodes with high K-ion content for PIBs.

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.

Mixed anion/cation redox in K0.78Fe1.60S2 for a high-performance cathode in potassium ion batteries

A new cathode material (K_0.78Fe_1.60S₂) shows mixed anion/cation redox (Fe(i) ⇆ Fe(ii) ⇆ Fe(iii) and 2S²⁻ ⇆ S₂²⁻) during charge/discharge for high performance electrodes in potassium ion batteries.

Embracing high performance potassium-ion batteries with phosphorus-based electrodes: a review

The ever-increasing global energy demand and rising price of raw materials adopted in currently prevalent lithium ion batteries (LIBs) have boosted the development of potassium ion batteries (KIBs). Despite the similarity in the working principle to LIBs, it remains a big challenge to select a suitable electrode material for KIBs. Phosphorus (P) and P-based composites have been identified as promising electrodes for LIBs or sodium ion batteries (NIBs) with remarkable electrochemical performance. Yet it was not until recent years that P-based materials have been explored as potential electrodes for KIBs. In this paper, we will try to provide a timely review of the current research progress of the P-based electrode materials, both cathodes and anodes, for KIBs. The synthetic strategies, electrochemical behaviours, and ion storage mechanisms will be discussed in detail. The challenges and future perspectives worth investigating will also been presented. Through timely update of the research progress and presentation of the existing arguments, it is expected that this review will help to clarify the puzzles encountered in current KIBs and benefit their future development and commercialization.

Scalable Synthesis Nano-Perovskite K(Mn0.95Ni0.05)F3 Cathode by Homogeneous Precipitation Method for Potassium-Ion Batteries

Potassium-ion batteries (KIBs) are favored by researchers because of the unique advantages. In this work, KIB cathode material nano-perovskite K(Mn_0.95Ni_0.05)F₃ with concentration gradient was synthesized by EDTA-assisted homogeneous precipitation method for the first time and characterized. The solid solution material was deposited on the multi-walled carbon nanotubes (MWCNTs) to form K(Mn_0.95Ni_0.05)F₃/MWCNT nanocomposites to improve the electron conductivity of the electrode material so as to obtain the excellent electrochemical performance. As expected, the charge and discharge capacities of K(Mn_0.95Ni_0.05)F₃/MWCNTs after the 60th cycle can still reach 106.8 and 98.5 mAh g^-1 over the voltage range 4.2-1.2 V vs. K/K⁺ at the current density of 35 mA g^-1, respectively. Electrochemical performance studies showed that solid solution K(Mn_0.95Ni_0.05)F₃ had the potential applications as the cathode material for KIBs. Electrochemical impedance spectroscopy (EIS) was used to study the transport and reaction processes of ions at the solid-liquid interface. The main factors affecting electrochemical performance could be analyzed from the Nyquist plot of the EIS test.

Surface-Engineered Black Niobium Oxide@Graphene Nanosheets for High-Performance Sodium-/Potassium-Ion Full Batteries

Nanoscale surface-engineering plays an important role in improving the performance of battery electrodes. Nb₂ O₅ is one typical model anode material with promising high-rate lithium storage. However, its modest reaction kinetics and low electrical conductivity obstruct the efficient storage of larger ions of sodium or potassium. In this work, partially surface-amorphized and defect-rich black niobium oxide@graphene (black Nb₂ O_{5- x} @rGO) nanosheets are designed to overcome the above Na/K storage problems. The black Nb₂ O_{5- x} @rGO nanosheets electrodes deliver a high-rate Na and K storage capacity (123 and 73 mAh g^-1 , respectively at 3 A g^-1 ) with long-term cycling stability. Besides, both Na-ion and K-ion full batteries based on black Nb₂ O_{5- x} @rGO nanosheets anodes and vanadate-based cathodes (Na_0.33 V₂ O₅ and K_0.5 V₂ O₅ for Na-ion and K-ion full batteries, respectively) demonstrate promising rate and cycling performance. Notably, the K-ion full battery delivers higher energy and power densities (172 Wh Kg^-1 and 430 W Kg^-1 ), comparable to those reported in state-of-the-art K-ion full batteries, accompanying with a capacity retention of ≈81.3% over 270 cycles. This result on Na-/K-ion batteries may pave the way to next-generation post-lithium batteries.

Room-Temperature Potassium-Sulfur Batteries Enabled by Microporous Carbon Stabilized Small-Molecule Sulfur Cathodes

Potassium-sulfur (K-S) batteries are a promising alternative to lithium ion batteries for large-area energy storage applications, owing to their high capacity and inexpensiveness, but they have been seldom investigated. Here we report room-temperature K-S batteries utilizing a microporous carbon-confined small-molecule sulfur composite cathode. The synergetic effects of the strong confinement of microporous carbon matrix and the small-molecule sulfur structure can effectually eliminate the formation of soluble polysulfides and ensure a reversible capacity of 1198.3 mA h g^-1 and retain 72.5% after 150 cycles with a Coulombic efficiency of ∼97%. The potassium-storage mechanism was investigated by X-ray photoelectron spectroscopy analysis and theoretical calculations. The results suggest that K₂S is the final potassiation product along with the reaction of 2K + S ↔ K₂S, giving a theoretical capacity of 1675 mA h g^-1. Our findings not only provide an effective strategy to fabricate high-performance room-temperature K-S batteries but also offer a basic comprehension of the potassium storage mechanism of sulfur cathode materials.