Low-Cost High-Energy Potassium Cathode

Recent advances in rational design for high-performance potassium-ion batteries

The growing global energy demand necessitates the development of renewable energy solutions to mitigate greenhouse gas emissions and air pollution. To efficiently utilize renewable yet intermittent energy sources such as solar and wind power, there is a critical need for large-scale energy storage systems (EES) with high electrochemical performance. While lithium-ion batteries (LIBs) have been successfully used for EES, the surging demand and price, coupled with limited supply of crucial metals like lithium and cobalt, raised concerns about future sustainability. In this context, potassium-ion batteries (PIBs) have emerged as promising alternatives to commercial LIBs. Leveraging the low cost of potassium resources, abundant natural reserves, and the similar chemical properties of lithium and potassium, PIBs exhibit excellent potassium ion transport kinetics in electrolytes. This review starts from the fundamental principles and structural regulation of PIBs, offering a comprehensive overview of their current research status. It covers cathode materials, anode materials, electrolytes, binders, and separators, combining insights from full battery performance, degradation mechanisms, in situ/ex situ characterization, and theoretical calculations. We anticipate that this review will inspire greater interest in the development of high-efficiency PIBs and pave the way for their future commercial applications.

Inhibiting the Jahn-Teller Effect of Manganese Hexacyanoferrate via Ni and Cu Codoping for Advanced Sodium-Ion Batteries

Manganese (Mn)-based Prussian blue analogs (PBAs) are of great interest as a prospective cathode material for sodium-ion batteries (SIBs) due to their high redox potential, easy synthesis, and low cost. However, the Jahn-Teller effect and low electrical conductivity of Mn-based PBA cause poor structure stability and unsatisfactory performance during the cycling. Herein, a novel nickel- and copper-codoped K2Mn[Fe(CN)6] cathode is developed via a simple coprecipitation strategy. The doping elements improve the electrical conductivity of Mn-based PBA by reducing the bandgap, as well as suppress the Jahn-Teller effect by stabilizing the framework, as verified by the density functional theory calculations. Simultaneously, the substitution of sodium with potassium in the lattice is beneficial for filling vacancies in the PBA framework, leading to higher average operating voltages and superior structural stability. As a result, the as-prepared Mn-based cathode exhibits excellent reversible capacity (116.0 mAh g-1 at 0.01 A g-1) and superior cycling stability (81.8% capacity retention over 500 cycles at 0.1 A g-1). This work provides a profitable doping strategy to inhibit the Jahn-Teller structural deformation for designing stable cathode material of SIBs.

High Capacity and Fast Kinetics Enabled by Metal-Doping in Prussian Blue Analogue Cathodes for Sodium-Ion Batteries

Prussian blue analogues (PBAs) have gained tremendous attention as promising low-cost electrochemically-tunable electrode materials, which can accommodate large Na+ ions with attractive specific capacity and charge-discharge kinetics. However, poor cycling stability caused by lattice strain and volume change remains to be improved. Herein, metal-doping strategy has been demonstrated in FeNiHCF, Na1.40Fe0.90Ni0.10[Fe(CN)6]0.85 ⋅ 1.3H2O, delivering a capacity as high as 148 mAh g-1 at 10 mA g-1. At an exceptionally high rate of 25.6 A g-1, a reversible capacity of ~55 mAh g-1 still can be obtained with a very small capacity decay rate of 0.02 % per cycle for 1000 cycles, considered one of the best among all metal-doped PBAs. This exhibits the stabilizing effect of Ni doping which enhances structural stability and long-term cyclability. In situ synchrotron X-ray diffraction reveals an extremely small (~1 %) change in unit cell parameters. The Ni substitution is found to increase the electronic conductivity and redox activity, especially at the low-spin (LS) Fe center due to inductive effect. This larger capacity contribution from LS Fe2+C6/Fe3+C6 redox couple is responsible for stable high-rate capability of FeNiHCF. The insight gained in this work may pave the way for the design of other high-performance electrode materials for sustainable sodium-ion batteries.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Review on Layered Manganese-Based Metal Oxides Cathode Materials for Potassium-Ion Batteries: From Preparation to Modification

Potassium-ion battery is rich in resources and cheap in price, in the era of lithium-ion battery commercialization, potassium-ion battery is the most likely to replace it. Based on the classification and summary of electrode materials for potassium-ion batteries, this paper focuses on the introduction of manganese-based oxide KxMnO2. The layered KxMnO2 has a large layer spacing and can be embedded with large size potassium-ions. This paper focuses on the preparation and doping of manganese-based cathode materials for potassium-ion batteries, summarizes the main challenges of KxMnO2-based cathode materials in the current stage of research and further looks into its future development direction.

Effect of alkali-metal cation on oxygen adsorption at Pt single-crystal electrodes in non-aqueous electrolytes

The effect of Group 1 alkali-metal cations (Na+, K+, and Cs+) on the oxygen reduction and evolution reactions (ORR and OER) using dimethyl sulfoxide (DMSO)-based electrolytes was investigated. Cyclic voltammetry (CV) utilising different Pt-electrode surfaces (polycrystalline Pt, Pt(111) and Pt(100)) was undertaken to investigate the influence of surface structure upon the ORR and OER. For K+ and Cs+, negligible variation in the CV response (in contrast to Na+) was observed using Pt(111), Pt(100) and Pt(poly) electrodes, consistent with a weak surface-metal/superoxide complex interaction. Indeed, changes in the half-wave potentials (E1/2) and relative intensities of the redox peaks corresponding to superoxy (O2-) and peroxy (O22-) ion formation were consistent with a solution-mediated mechanism for larger cations, such as Cs+. Support for this finding was obtained via in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). During the ORR and in the presence of Cs+, O2- and weakly adsorbed caesium superoxide (CsO2) species were detected. Because DMSO was found to strongly interact with the surface at potentials associated with the ORR, CsO2 was readily displaced at more negative potentials via increased solvent adsorption at the surface. This finding highlights the important impact of the solvent during ORR/OER reactions.

KI-Assisted Formation of Spindle-like Prussian White Nanoparticles for High-Performance Potassium-Ion Battery Cathodes

Prussian white (PW) is considered as a promising cathode material for potassium-ion batteries (KIBs) due to its low cost and high theoretical capacity. However, the high water content and structural defects and the strict synthesis conditions of PW lead to its unsatisfactory cycling performance and low specific capacity, hindering its practical applications. Herein, a template-engaged reduction method is proposed, using MIL-88B(Fe) as a self-template and KI as the reducing agent to prepare K-rich PW with low defects and water content. Furthermore, the hierarchical porous spindle-like morphology can be inherited from the precursor, furnishing sufficient active sites and reducing the ion diffusion path. Consequently, when applied as a KIB cathode material, spindle-like PW (K1.72Fe[Fe(CN)6]0.96·0.342H2O) manifested remarkable potassium storage properties. Notably, a full cell assembled by the spindle-like PW cathode and graphite anode exhibited a large energy density of ∼216.7 Wh kg-1, demonstrating its huge potential for energy storage systems.

Design and Synthesis of Cubic K3-2 x Bax SbSe4 Solid Electrolytes for K-O2 Batteries

Developing K-ion conducting solid-state electrolytes (SSEs) plays a critical role in the safe implementation of potassium batteries. In this work, a chalcogenide-based potassium ion SSE is reported, K3 SbSe4 , which adopts a trigonal structure at room temperature. Single-crystal structural analysis reveals a trigonal-to-cubic phase transition at the low temperature of 50 °C, which is the lowest among similar compounds and thus provides easy access to the cubic phase. The substitution of barium for potassium in K3 SbSe4 leads to the creation of potassium vacancies, expansion of lattice parameters, and a transformation from a trigonal phase to a cubic phase. As a result, the maximum conductivity of K3-2 x Bax SbSe4 reaches around 0.1 mS cm-1 at 40 °C for K2.2 Ba0.4 SbSe4 , which is over two orders of magnitude higher than that of undoped K3 SbSe4 . This novel SSE is successfully employed in a K-O2 battery operating at room temperature where a polymer-laminated K2.2 Ba0.4 SbSe4 pellet serves as a separator between the oxygen cathode and the potassium metal anode. Effective protection of the K metal anode against corrosion caused by O2 is demonstrated.

Ultrathin Cobalt-Based Prussian Blue Analogue Nanosheet-Assembled Nanoboxes Interpenetrated with Carbon Nanotubes as a Fast Electron/Potassium-Ion Conductor for Superior Potassium Storage

Rechargeable potassium-ion batteries (PIBs) are regarded as potential substitutes for industrial lithium-ion batteries in large scale energy storage systems due to the world's abundant potassium supplies. Althogh cobalt hexacyanocobaltate (CoHCC) exhibits broad potential as a PIB anode material, its performance is currently unsatisfactory. Herein, novel 5 nm scale ultrathin CoHCC nanosheet-assembled nanoboxes with interspersed carbon nanotubes (CNTs/CoHCC nanoboxes) are fabricated to realize a highly reactive PIB anode. The ultrathin CoHCC layers substantially accelerate electron conduction and provide numerous active sites, while the connected CNTs provide fast axial electron transport. Consequently, the optimized anode exhibits a remarkable discharge capacity of 580.9 mAh g-1 at 0.1 A g-1 and long-term stability with 71.3% retention over 1000 cycles. In situ and ex situ characterizations and density functional theory calculations are further employed to elucidate the K+ storage process and the reason for the enhanced performance of the CNTs/CoHCC nanoboxes.

Control of Gradient Concentration Prussian White Cathodes for High-Performance Potassium-Ion Batteries

Owing to their abundant resources and low cost, potassium-ion batteries (PIBs) have become a promising alternative to lithium-ion batteries (LIBs). However, the larger ionic radius and higher mass of K+ propose a challenging issue for finding suitable cathode materials. Prussian whites (PWs) have a rigid open framework and affordable synthesis method, but they suffer quick capacity fade due to lattice volume change and structural instability during K+ insertion/extraction. Here, we prepared controllable gradient concentration KxFeaNibMn1-a-b[Fe(CN)6]y·zH2O particles via a facile coprecipitation process, demonstrating high-performance potassium-ion storage. The high-Mn content in the interior can minimize capacity loss caused by electrochemically inert Ni and achieve a high reversible capacity; meanwhile, the high-FeNi content in the exterior can alleviate the volume change of the core material upon cycling, thus enhancing structural stability. Taking the above synergistic effect, the controllable gradient concentration PWs deliver a high reversible capacity of 109.8 mAh g-1 at 100 mA g-1 and good capacity retention of 77.8% after 200 cycles. The gradient concentration PWs can retain structural integrity and stability during long-term cycling. This work provides a prospective strategy to fabricate PWs with stable structure and excellent electrochemical performance for developing high-performance PIBs.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

Highly Crystalline Prussian Blue for Kinetics Enhanced Potassium Storage

Prussian blue analogs (PBAs) are promising cathode materials for potassium-ion batteries (KIBs) owing to their large open framework structure. As the K⁺ migration rate and storage sites rely highly on the periodic lattice arrangement, it is rather important to guarantee the high crystallinity of PBAs. Herein, highly crystalline K₂ Fe[Fe(CN)₆ ] (KFeHCF-E) is synthesized by coprecipitation, adopting the ethylenediaminetetraacetic acid dipotassium salt as a chelating agent. As a result, an excellent rate capability and ultra-long lifespan (5000 cycles at 100 mA g^-1 with 61.3% capacity maintenance) are achieved when tested in KIBs. The highest K⁺ migration rate of 10^-9 cm² s^-1 in the bulk phase is determined by the galvanostatic intermittent titration technique. Remarkably, the robust lattice structure and reversible solid-phase K⁺ storage mechanism of KFeHCF-E are proved by in situ XRD. This work offers a simple crystallinity optimization method for developing high-performance PBAs cathode materials in advanced KIBs.

Advance of Prussian Blue-Derived Nanohybrids in Energy Storage: Current Status and Perspective

Great changes have occurred in the energy storage area in recent years as a result of rapid economic expansion. People have conducted substantial research on sustainable energy conversion and storage systems in order to mitigate the looming energy crisis. As a result, developing energy storage materials is critical. Materials with an open frame structure are known as Prussian blue analogs (PBAs). Anode materials for oxides, sulfides, selenides, phosphides, borides, and carbides have been extensively explored as anode materials in the field of energy conversion and storage in recent years. The advantages and disadvantages of oxides, sulfides, selenides, phosphides, borides, carbides, and other elements, as well as experimental methodologies and electrochemical properties, are discussed in this work. The findings reveal that employing oxides, sulfides, selenides, phosphides, borides, and other electrode materials to overcome the problems of low conductivity, excessive material loss, and low specific volume is ineffective. Therefore, this review intends to address the issues of diverse energy storage materials by combining multiple technologies to manufacture battery materials with low cost, large capacity, and extended service life.

Nanostructured Conducting Polymers and Their Applications in Energy Storage Devices

Due to the energy requirements for various human activities, and the need for a substantial change in the energy matrix, it is important to research and design new materials that allow the availability of appropriate technologies. In this sense, together with proposals that advocate a reduction in the conversion, storage, and feeding of clean energies, such as fuel cells and electrochemical capacitors energy consumption, there is an approach that is based on the development of better applications for and batteries. An alternative to commonly used inorganic materials is conducting polymers (CP). Strategies based on the formation of composite materials and nanostructures allow outstanding performances in electrochemical energy storage devices such as those mentioned. Particularly, the nanostructuring of CP stands out because, in the last two decades, there has been an important evolution in the design of various types of nanostructures, with a strong focus on their synergistic combination with other types of materials. This bibliographic compilation reviews state of the art in this area, with a special focus on how nanostructured CP would contribute to the search for new materials for the development of energy storage devices, based mainly on the morphology they present and on their versatility to be combined with other materials, which allows notable improvements in aspects such as reduction in ionic diffusion trajectories and electronic transport, optimization of spaces for ion penetration, a greater number of electrochemically active sites and better stability in charge/discharge cycles.

PEDOT-intercalated NH4V3O8 nanobelts as high-performance cathode materials for potassium ion batteries

Potassium ion batteries (PIBs) have great potential to replace lithium ion batteries (LIBs) for large-scale energy storage applications because of the low cost and earth abundance of potassium resources. However, it is critically challenging to exploit an appropriate cathode material to accommodate the large size of K⁺. Herein, a conducting polymer (PEDOT) intercalation method is utilized to tailor the interlayer spacing of NH₄V₃O₈ nanobelts from 7.8 Å to 10.8 Å, and afford rich oxygen vacancies inside the vanadate, thus enhancing its electronic conductivity and accelerating the K⁺ insertion/extraction kinetics. Benefiting from these features, PEDOT-intercalated NH₄V₃O₈ (PNVO) nanobelts deliver an improved capacity of 87 mA h g^-1 at 20 mA g^-1, high rate capability of 51 mA h g^-1 at 500 mA g^-1, and a stable cycle life (capacity retention of 92.5 % after 100 cycles at 50 mA g^-1). Even cycled at 200 mA g^-1, PNVO nanobelts feature a long cycle life over 300 cycles with a capacity retention of 71.7 %. This work is of great significance for exploitation of PIBs cathode with improved electrochemical performance through pre-intercalation and defect engineering.

Influence of Vacancies in Manganese Hexacyanoferrate Cathode for Organic Na-Ion Batteries: A Structural Perspective

Manganese hexacyanoferrates (MnHCF) are promising positive electrode materials for non-aqueous batteries, including Na-ion batteries, due to their large specific capacity (>130 mAh g^-1 ), high discharge potential and sustainability. Typically, the electrochemical reaction of MnHCF associates with phase and structural changes, due to the Jahn-Teller (JT) distortion of Mn sites upon the charge process. To understand the effect of the MnHCF structure on its electrochemical performance, two MnHCF materials with different vacancies content are investigated herein. The electrochemical results show that the sample with lower vacancy content (4 %) exhibits relatively higher capacity retention of 99.1 % and 92.6 % at 2^nd and 10^th cycles, respectively, with respect to 97.4 % and 79.3 % in sample with higher vacancy content (11 %). Ex-situ X-ray absorption spectroscopy (XAS) and ex situ X-ray diffraction (XRD) characterization results show that a weaker cooperative JT-distortion effect and relatively smaller crystal structure modification occurred for the material with lower vacancies, which explains the better electrochemical performance in cycled electrodes.

Metal Halides for High-Capacity Energy Storage

High-capacity electrochemical energy storage systems are more urgently needed than ever before with the rapid development of electric vehicles and the smart grid. The most efficient way to increase capacity is to develop electrode materials with low molecular weights. The low-cost metal halides are theoretically ideal cathode materials due to their advantages of high capacity and redox potential. However, their cubic structure and large energy barrier for deionization impede their rechargeability. Here, the reversibility of potassium halides, lithium halides, sodium halides, and zinc halides is achieved through decreasing their dimensionality by the strong π-cation interactions between metal cations and reduced graphene oxide (rGO). Especially, the energy densities of KI-, KBr-, and KCl-based materials are 722.2, 635.0, and 739.4 Wh kg^-1 , respectively, which are higher than those of other cathode materials for potassium-ion batteries. In addition, the full-cell with 2D KI/rGO as cathode and graphite as anode demonstrates a lifespan of over 150 cycles with a considerable capacity retention of 57.5%. The metal halides-based electrode materials possess promising application prospects and are worthy of more in-depth researches.

Solid-solution reaction suppresses the Jahn-Teller effect of potassium manganese hexacyanoferrate in potassium-ion batteries

Potassium manganese hexacyanoferrate (KMnHCF) suffers from poor cycling stability in potassium-ion batteries due to the Jahn-Teller effect, and experiences destabilizing asymmetric expansions and contractions during cycling. Herein, hollow nanospheres consisting of ultrasmall KMnHCF nanocube subunits (KMnHCF-S) are developed by a facile strategy. In situ XRD analysis demonstrates that the traditional phase transition for KMnHCF is replaced by a single-phase solid-solution reaction for KMnHCF-S, which effectively suppresses the Jahn-Teller effect. From DFT calculations, it was found that the calculated reaction energy for K+ extraction in the solid-solution reaction is much lower than that in the phase transition, indicating easier K+ extraction during the solid-solution reaction. KMnHCF-S delivers high capacity, outstanding rate capability, and superior cycling performance. Impressively, the K-ion full cell composed of the KMnHCF-S cathode and graphite anode also displays excellent cycling stability. The solid-solution reaction not only suppresses the Jahn-Teller effect of KMnHCF-S but also provides a strategy to enhance the electrochemical performance of other electrodes which undergo phase transitions.

Active material and interphase structures governing performance in sodium and potassium ion batteries

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

Superconcentrated NaFSA-KFSA Aqueous Electrolytes for 2 V-Class Dual-Ion Batteries

Superconcentrated aqueous electrolytes containing NaN(SO2F)2 and KN(SO2F)2 (for which sodium and potassium bis(fluorosulfonyl)amides (FSA), respectively, are abbreviated) have been developed for 2 V-class aqueous batteries. Based on the eutectic composition of the NaFSA-KFSA (56:44 mol/mol) binary system, the superconcentrated solutions of 35 mol kg-1 Na0.55K0.45FSA/H2O and 33 mol kg-1 Na0.45K0.55FSA/H2O are found to form at 25 °C. As both electrolytes demonstrate a wider potential window of ∼3.5 V compared to that of either saturated 20 mol kg-1 NaFSA or 31 mol kg-1 KFSA solution, we applied the 33 mol kg-1 Na0.45K0.55FSA/H2O to two different battery configurations, carbon-coated Na2Ti2(PO4)3∥K2Mn[Fe(CN)6] and carbon-coated Na3V2(PO4)3∥K2Mn[Fe(CN)6]. The former cell shows highly reversible charge/discharge curves with a mean discharge voltage of 1.4 V. Although the latter cell exhibits capacity degradation, it demonstrates 2 V-class operations. Analysis data of the two cells confirmed that Na+ ions were mainly inserted into the negative electrodes passivated by a Na-rich solid electrolyte interphase, and both Na+ and K+ ions were inserted into the positive electrode. Based upon the observation, we propose new sodium-/potassium-ion batteries using the superconcentrated NaFSA-KFSA aqueous electrolytes.

Resolving the Seeming Contradiction between the Superior Rate Capability of Prussian Blue Analogues and the Extremely Slow Ionic Diffusion

The superior rate capabilities of metal ion battery materials based on Prussian blue analogues (PBAs) are almost exclusively ascribed to the extremely fast solid-state ionic diffusion, which is possible due to structural voids and spacious three-dimensional channels in PBA structures. We performed a detailed electroanalytical study of alkali ion diffusivities in nanosized cation-rich and cation-poor PBAs obtained as particles or electrodeposited films in both aqueous and non-aqueous media, which resulted in a solid conclusion about the exceptionally slow ionic transport. We show that the impressive rate capability of PBA materials is determined solely by the small size of the primary particles of PBAs, while the apparent diffusion coefficients are 3-5 orders of magnitude lower than those reported in earlier studies. Our finding calls for a reconsideration of the apparent facility of ionic transport in PBA materials and deeper analysis of the charge carrier-host interactions in PBAs.

Universal Strategy for Preparing Highly Stable PBA/Ti3C2Tx MXene toward Lithium-Ion Batteries via Chemical Transformation

Prussian blue analogues (PBAs) are believed to be intriguing anode materials for Li⁺ storage because of their tunable composition, designable topologies, and tailorable porous structures, yet they suffer from severe capacity decay and inferior cycling stability due to the volume variation upon lithiation and high electrical resistance. Herein, we develop a universal strategy for synthesizing small PBA nanoparticles hosted on two-dimensional (2D) MXene or rGO (PBA/MX or PBA/rGO) via an in situ transformation from ultrathin layered double hydroxides (LDH) nanosheets. 2D conductive nanosheets allow for fast electron transport and guarantee the full utilization of PBA even at high rates; at the meantime, PBA nanoparticles effectively prevent 2D materials from restacking and facilitate rapid ion diffusion. The optimized Ni_0.8Mn_0.2-PBA/MX as an anode for lithium-ion batteries (LIBs) delivers a capacity of 442 mAh g^-1 at 0.1 A g^-1 and an excellent cycling robustness in comparison with bare PBA bulk crystals. We believe that this study offers an alternative choice for rationally designing PBA-based electrode materials for energy storage.

Electrolyte formulation strategies for potassium-based batteries

Potassium (K)-based batteries are viewed as the most promising alternatives to lithium-based batteries, owing to their abundant potassium resource, lower redox potentials (-2.97 V vs. SHE), and low cost. Recently, significant achievements on electrode materials have boosted the development of potassium-based batteries. However, the poor interfacial compatibility between electrode and electrolyte hinders their practical. Hence, rational design of electrolyte/electrode interface by electrolytes is the key to develop K-based batteries. In this review, the principles for formulating organic electrolytes are comprehensively summarized. Then, recent progress of various liquid organic and solid-state K⁺ electrolytes for potassium-ion batteries and beyond are discussed. Finally, we offer the current challenges that need to be addressed for advanced K-based batteries.

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

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

The Quest for Stable Potassium-Ion Battery Chemistry

Potassium-ion batteries (KIBs) have attracted wide interest for energy storage because of the abundance of the electrode materials involved; however, their electrochemical performances are far behind what can be achieved from lithium-ion batteries (LIBs) or sodium-ion batteries (SIBs). Herein, key promising electrode and electrolyte materials for potassium-ion batteries are identified, the coupled electrochemical reactions in the cell are investigated, and the compatibility between different materials is demonstrated to play the most important role. K₂ Mn[Fe(CN)₆ ] cathode can deliver a high capacity of ≈125 mAh g^-1 and exceptional cycling stability over 61 000 cycles (≈9 months) if the side reactions from the anode can be prevented. Graphite is a good anode material but is subjected to degradation in traditional carbonate electrolytes. New concentrated electrolytes are developed and evaluated. A stable KIB system is demonstrated by coupling a stable K₂ Mn[Fe(CN)₆ ] cathode, a prepotassiated graphite anode with a concentrated electrolyte to achieve a high energy density of ≈260 Wh kg^-1 (based on the active mass of cathode and anode) and good cycling of over 1000 cycles.

Multifunctional Separator Allows Stable Cycling of Potassium Metal Anodes and of Potassium Metal Batteries

This is the first report of a multifunctional separator for potassium-metal batteries (KMBs). Double-coated tape-cast microscale AlF₃ on polypropylene (AlF₃ @PP) yields state-of-the-art electrochemical performance: symmetric cells are stable after 1000 cycles (2000 h) at 0.5 mA cm^-2 and 0.5 mAh cm^-2 , with 0.042 V overpotential. Stability is maintained at 5.0 mA cm^-2 for 600 cycles (240 h), with 0.138 V overpotential. Postcycled plated surface is dendrite-free, while stripped surface contains smooth solid electrolyte interphase (SEI). Conventional PP cells fail rapidly, with dendrites at plating, and "dead metal" and SEI clumps at stripping. Potassium hexacyanoferrate(III) cathode KMBs with AlF₃ @PP display enhanced capacity retention (91% at 100 cycles vs 58%). AlF₃ partially reacts with K to form an artificial SEI containing KF, AlF₃ , and Al₂ O₃ phases. The AlF₃ @PP promotes complete electrolyte wetting and enhances uptake, improves ion conductivity, and increases ion transference number. The higher of K⁺ transference number is ascribed to the strong interaction between AlF₃ and FSI^- anions, as revealed through ¹⁹ F NMR. The enhancement in wetting and performance is general, being demonstrated with ester- and ether-based solvents, with K-, Na-, or Li- salts, and with different commercial separators. In full batteries, AlF₃ prevents Fe crossover and cycling-induced cathode pulverization.

Topotactic Epitaxy Self-Assembly of Potassium Manganese Hexacyanoferrate Superstructures for Highly Reversible Sodium-Ion Batteries

The cycle stability and voltage retention of a Na₂Mn[Fe(CN)₆] (NMF) cathode for sodium-ion batteries (SIBs) has been impeded by the huge distortion from NaMn^II[Fe^III(CN)₆] to Mn^III[Fe^III(CN)₆] caused by the Jahn-Teller (JT) effect of Mn³⁺. Herein, we propose a topotactic epitaxy process to generate K₂Mn[Fe(CN)₆] (KMF) submicron octahedra and assemble them into octahedral superstructures (OSs) by tuning the kinetics of topotactic transformation. As the SIB cathode, the self-assembly behavior of KMF improves the structural stability and decreases the contact area with the electrolyte, thereby inhibiting the transition metal in the KMF cathode from dissolving in the electrolyte. More importantly, the KMF partly transforms into NMF with Na⁺ de/intercalation, and the existing KMF acts as a stabilizer to disrupt the long-range JT order of NMF, thereby suppressing the overall JT distortion. As a result, the electrochemical performances of KMF cathodes outperform NMF with a highly reversible phase transition and outstanding cycling performance, and 80% capacity retention after 1500/1300 cycles at 0.1/0.5 A g^-1. This work not only promotes creative synthetic methodologies but also promotes to explore the relationship between Jahn-Teller structural deformation and cycle stability.

Polypyrrole-Coated K2Mn[Fe(CN)6] Stabilizing Its Interfaces and Inhibiting Irreversible Phase Transition during the Zinc Storage Process in Aqueous Batteries

Prussian blue analogues (PBAs) have been considered as promising cathodes for aqueous zinc-ion batteries because of their open framework for accommodating large ions, tunable valence state, and facile synthesis. Among PBAs, potassium manganese hexacyanoferrate (KMHCF) is favored due to its high working voltage, high specific capacity, and low cost. However, it suffers from severe capacity decay and poor rate capability, which are mainly a result of poor intrinsic conductivity, irreversible phase transition, transition metal dissolution, and structural collapse during charge/discharge cycling. These issues extremely limit its practical application. In order to solve these problems, conductive polypyrrole (PPy) was used to coat KMHCF microcubes to form KMHCF@PPy composites to achieve superior rate capability and prolonged cycle life. With the PPy coating, the KMHCF@PPy composite delivers a discharge capacity of 107.6 mA h g^-1 after 100 cycles at 100 mA g^-1, and even at 500 mA g^-1 after 500 cycles, 64.2 mA h g^-1 still remained. The excellent electrochemical performance can be attributed to the effects from PPy. On the one hand, PPy supplies an effective electronic transmission network for KMHCF to enhance the electronic conductivity. On the other hand, it plays the role of a protective layer to effectively inhibit the dissolution of Mn and the phase transition during the cycling.

A carboxylate- and pyridine-based organic anode material for K-ion batteries

In this work, a new carboxylate- and pyridine-based organic anode material was exploited in K-ion batteries. The results demonstrate the critical role of multiple active centers (CO and CN) in one molecule to enhance the electrochemical performance.

Boosting the K+-adsorption capacity in edge-nitrogen doped hierarchically porous carbon spheres for ultrastable potassium ion battery anodes

Although carbon materials have great potential for potassium ion battery (KIB) anodes due to their structural stability and abundant carbon-containing resources, the limited K⁺-intercalated capacity impedes their extensive applications in energy storage devices. Current research studies focus on improving the surface-induced capacitive behavior to boost the potassium storage capacity of carbon materials. Herein, we designed edge-nitrogen (pyridinic-N and pyrrolic-N) doped carbon spheres with a hierarchically porous structure to achieve high potassium storage properties. The electrochemical tests confirmed that the edge-nitrogen induced active sites were conducive for the adsorption of K⁺, and the hierarchical porous structure promoted the generation of stable solid electrolyte interphase (SEI) films, both of which endow the resulting materials with a high reversible capacity of 381.7 mA h g^-1 at 0.1 A g^-1 over 200 cycles and an excellent rate capability of 178.2 mA h g^-1 at 5 A g^-1. Even at 5 A g^-1, the long-term cycling stability of 5000 cycles was achieved with a reversible capacity of 190.1 mA h g^-1. This work contributes to deeply understand the role of the synergistic effect of edge-nitrogen induced active sites and the hierarchical porous structure in the potassium storage performances of carbon materials.

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.

Challenges and Strategies toward Cathode Materials for Rechargeable Potassium-Ion Batteries

With increasing demand for grid-scale energy storage, potassium-ion batteries (PIBs) have emerged as promising complements or alternatives to commercial lithium-ion batteries owing to the low cost, natural abundance of potassium resources, the low standard reduction potential of potassium, and fascinating K⁺ transport kinetics in the electrolyte. However, the low energy density and unstable cycle life of cathode materials hamper their practical application. Therefore, cathode materials with high capacities, high redox potentials, and good structural stability are required with the advancement toward next-generation PIBs. To this end, understanding the structure-dependent intercalation electrochemistry and recognizing the existing issues relating to cathode materials are indispensable prerequisites. This review summarizes the recent advances of PIB cathode materials, including metal hexacyanometalates, layered metal oxides, polyanionic frameworks, and organic compounds, with an emphasis on the structural advantages of the K⁺ intercalation reaction. Moreover, major current challenges with corresponding strategies for each category of cathode materials are highlighted. Finally, future research directions and perspectives are presented to accelerate the development of PIBs and facilitate commercial applications. It is believed that this review will provide practical guidance for researchers engaged in developing next-generation advanced PIB cathode materials.

Robust Solid Electrolyte Interphases in Localized High Concentration Electrolytes Boosting Black Phosphorus Anode for Potassium-Ion Batteries

Black phosphorus (BP) shows superior capacity toward K ion storage, yet it suffers from poor reversibility and fast capacity degradation. Herein, a BP-graphite (BP/G) composite with a high BP loading of 80 wt % is synthesized and stabilized via the utilization of a localized high concentration electrolyte (LHCE), i.e., potassium bis(fluorosulfonyl)imide in trimethyl phosphate with a fluorinated ether as the diluent. We reveal the benefits of high concentration electrolytes rely on the formation of an inorganic component rich solid electrolyte interphase (SEI), which effectively passivates the electrode from copious parasite reactions. Furthermore, the diluent increases the electrolyte's ionic conductivity for achieving attractive rate capability and homogenizes the elemental distribution in the SEI. The latter essentially improves the SEI's maximum elastic deformation energy for accommodating the volume change, resulting in excellent cyclic performance. This work promotes the application of advanced potassium-ion batteries by adopting high-capacity BP anodes, on the one hand. On the other hand, it unravels the beneficial roles of LHCE in building robust SEIs for stabilizing alloy anodes.

A vanadium-based oxide-phosphate-pyrophosphate framework as a 4 V electrode material for K-ion batteries

K-ion batteries (KIBs) are promising for large-scale electrical energy storage owing to the abundant resources and the electrochemical specificity of potassium. Among the positive electrode materials for KIBs, vanadium-based polyanionic materials are interesting because of their high working voltage and good structural stability which dictates the cycle life. In this study, a potassium vanadium oxide phosphate, K6(VO)2(V2O3)2(PO4)4(P2O7), has been investigated as a 4 V class positive electrode material for non-aqueous KIBs. The material is synthesized through pyrolysis of a single metal-organic molecular precursor, K2[(VOHPO4)2(C2O4)] at 500 °C in air. The material demonstrates a reversible extraction/insertion of 2.7 mol of potassium from/into the structure at a discharge voltage of ∼4.03 V vs. K. Operando and ex situ powder X-ray diffraction analyses reveal that the material undergoes reversible K extraction/insertion during charge/discharge via a two-phase reaction mechanism. Despite the extraction/insertion of large potassium ions, the material demonstrates an insignificant volume change of ∼1.2% during charge/discharge resulting in excellent cycling stability without capacity degradation over 100 cycles in a highly concentrated electrolyte cell. Robustness of the polyanionic framework is proved from identical XRD patterns of the pristine and cycled electrodes (after 100 cycles).

Chemically Self-Charging Aqueous Zinc-Organic Battery

Zn-organic batteries are attracting extensive attention, but their energy density is limited by the low capacity (<400 mAh g^-1) and potential (<1 V vs Zn/Zn²⁺) of organic cathodes. Herein, we propose a long-life and high-rate Zn-organic battery that includes a poly(1,5-naphthalenediamine) cathode and a Zn anode in an alkaline electrolyte, where the cathode reaction is based on the coordination reaction between K⁺ and the C═N group (i.e., C═N/C-N-K conversion). Interestingly, we find that the discharged Zn-organic battery can recover to its initial state quickly with the presence of O₂, and the theoretical calculation demonstrates that the K-N bond in the discharged cathode can be easily broken by O₂ via redox reaction. Accordingly, we design a chemically self-charging aqueous Zn-organic battery. Benefiting from the excellent self-rechargeability, the organic cathode exhibits an accumulated capacity of 16264 mAh g^-1, which enables the Zn-organic battery to show a record high energy density of 625.5 Wh kg^-1.

High-Performance Potassium-Ion Batteries with Robust Stability Based on N/S-Codoped Hollow Carbon Nanocubes

Currently, a big challenge for the practical use of potassium-ion batteries (PIBs) is their intrinsically poor cycling stability, due to the relatively large radius of K⁺ and sluggish kinetics for intercalation/deintercalation. Here we report the scalable fabrication of N/S-codoped hollow carbon nanocubes (NSHCCs), which have the potential as an electrode for advanced PIBs with robust stability. Their discharge and charge specific capacities are ∼560 mA h g^-1 and 310 mA h g^-1 at a current density of 50 mA g^-1, respectively. Meanwhile, they exhibit 100% specific capacity retention after 620 cycles over 9 months at a low current density of 50 mA g^-1, which is state-of-the-art among carbon materials. Moreover, they demonstrate nearly no sacrifice in specific capacities with 99.9% retention after 3000 cycles over 4 months under a high current density of 1000 mA g^-1, superior to most carbon analogues for potassium storage previously reported. The improved electrochemical performance of NSHCC can be mainly attributed to the unique hollow carbon nanocubes with incorporated N and S dopants, which can expand the carbon layer spacing, facilitate K⁺ adsorption, and relieve the volume change during the intercalation/deintercalation of K⁺ ions.

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.

Cell-like-carbon-micro-spheres for robust potassium anode

Large-scale low-cost synthesis methods for potassium ion battery (PIB) anodes with long cycle life and high capacity have remained challenging. Here, inspired by the structure of a biological cell, biomimetic carbon cells (BCCs) were synthesized and used as PIB anodes. The protruding carbon nanotubes across the BCC wall mimicked the ion-transporting channels present in the cell membrane, and enhanced the rate performance of PIBs. In addition, the robust carbon shell of the BCC could protect its overall structure, and the open space inside the BCC could accommodate the volume changes caused by K+ insertion, which greatly improved the stability of PIBs. For the first time, a stable solid electrolyte interphase layer is formed on the surface of amorphous carbon. Collectively, the unique structural characteristics of the BCCs resulted in PIBs that showed a high reversible capacity (302 mAh g-1 at 100 mA g-1 and 248 mAh g-1 at 500 mA g-1), excellent cycle stability (reversible capacity of 226 mAh g-1 after 2100 cycles and a continuous running time of more than 15 months at a current density of 100 mA g-1), and an excellent rate performance (160 mAh g-1 at 1 A g-1). This study represents a new strategy for boosting battery performance, and could pave the way for the next generation of battery-powered applications.