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

'Beyond Li-ion technology'-a status review

Li-ion battery is currently considered to be the most proven technology for energy storage systems when it comes to the overall combination of energy, power, cyclability and cost. However, there are continuous expectations for cost reduction in large-scale applications, especially in electric vehicles and grids, alongside growing concerns over safety, availability of natural resources for lithium, and environmental remediation. Therefore, industry and academia have consequently shifted their focus towards 'beyond Li-ion technologies'. In this respect, other non-Li-based alkali-ion/polyvalent-ion batteries, non-Li-based all solid-state batteries, fluoride-ion/ammonium-ion batteries, redox-flow batteries, sand batteries and hydrogen fuel cells etc. are becoming potential cost-effective alternatives. While there has been notable swift advancement across various materials, chemistries, architectures, and applications in this field, a comprehensive overview encompassing high-energy 'beyond Li-ion' technologies, along with considerations of commercial viability, is currently lacking. Therefore, in this review article, a rationalized approach is adopted to identify notable 'post-Li' candidates. Their pros and cons are comprehensively presented by discussing the fundamental principles in terms of material characteristics, relevant chemistries, and architectural developments that make a good high-energy 'beyond Li' storage system. Furthermore, a concise summary outlining the primary challenges of each system is provided, alongside the potential strategies being implemented to mitigate these issues. Additionally, the extent to which these strategies have positively influenced the performance of these 'post-Li' technologies is discussed.

The Enormous Potential of Sodium/Potassium-Ion Batteries as the Mainstream Energy Storage Technology for Large-Scale Commercial Applications

Cost-effectiveness plays a decisive role in sustainable operating of rechargeable batteries. As such, the low cost-consumption of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) provides a promising direction for "how do SIBs/PIBs replace Li-ion batteries (LIBs) counterparts" based on their resource abundance and advanced electrochemical performance. To rationalize the SIBs/PIBs technologies as alternatives to LIBs from the unit energy cost perspective, this review gives the specific criteria for their energy density at possible electrode-price grades and various battery-longevity levels. The cost ($ kWh-1 cycle-1) advantage of SIBs/PIBs is ascertained by the cheap raw-material compensation for the cycle performance deficiency and the energy density gap with LIBs. Furthermore, the cost comparison between SIBs and PIBs, especially on cost per kWh and per cycle, is also involved. This review explicitly manifests the practicability and cost-effectiveness toward SIBs are superior to PIBs whose commercialization has so far been hindered by low energy density. Even so, the huge potential on sustainability of PIBs, to outperform SIBs, as the mainstream energy storage technology is revealed as long as PIBs achieve long cycle life or enhanced energy density, the related outlook of which is proceeded as the next development directions for commercial applications.

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.

Nanoconfinement of ultra-small Bi2Te3 nanocrystals on reduced graphene oxide: a pathway to high-performance sodium-ion battery anodes

Bismuth telluride (Bi2Te3) nanomaterials have attracted considerable attention owing to their intriguing physicochemical properties and wide-ranging potential applications arising from their distinctive layered structure and nanoscale size effects. However, synthesizing sub-100 nm ultra-small Bi2Te3 nanocrystals remains a formidable challenge. To date, there has been little investigation on the performance of these ultra-small Bi2Te3 nanocrystals in sodium-ion batteries (SIBs). This study presents a general strategy for synthesizing ultra-small Bi2Te3 nanocrystals on reduced graphene oxide (Bi2Te3/rGO) through a nanoconfinement approach. First-principles calculations and electrochemical kinetic studies confirm that the ultra-small Bi2Te3/rGO composite material can effectively mitigate volumetric expansion, preserve electrode integrity, and enhance electron transfer, Na-ion adsorption, and diffusion capacity. As a result, the Bi2Te3/rGO electrode demonstrates a remarkable initial specific capacity of 521 mA h g-1 at 0.1 A g-1, showcasing outstanding rate behaviour and long-lasting cycle life exceeding 800 cycles at 1 A g-1 while preserving exceptional rate properties. The function of the battery is indicated by ex situ TEM and XPS findings, which propose a conventional dual mechanism involving conversion and alloying. This work paves the way for rapid advancements in Bi2Te3-based SIB anodes while contributing to our understanding of sodium ion storage mechanisms.

Defect-Free Prussian Blue Analogue as Zero-Strain Cathode Material for High-Energy-Density Potassium-Ion Batteries

Prussian blue analogues (PBAs) have been widely studied as cathodes for potassium-ion batteries (PIBs) due to their three-dimensional framework structure and easily adjustable composition. However, the phase transition behavior and [Fe(CN)6]4- anionic defects severely deteriorate electrochemical performances. Herein, we propose a defect-free potassium iron manganese hexacyanoferrate (K1.47Fe0.5Mn0.5[Fe(CN)6]·1.26H2O, KFMHCF-1/2) as the cathode material for PIBs. The Fe-Mn binary synergistic and defect-free effects can inhibit the cell volume change and octahedral slip during the K-ion insertion/extraction process, so that the phase transformation behavior (monoclinic ↔ cubic) is effectively inhibited, achieving a zero-strain solid solution mechanism employing Fe and Mn as dual active-sites. Thus, KFMHCF-1/2 contributes the highest initial capacity of 155.3 mAh·g-1 with an energy density of 599.5 Wh·kg-1 at 10 mA·g-1 among the reported PBA cathodes, superior rate capability, and cyclic stability over 450 cycles. The assembled K-ion full battery using K deposited on graphite (K@G) as anode also delivers high reversible specific capacity of 131.1 mAh·g-1 at 20 mA·g-1 and ultralong lifespans over 1000 cycles at 50 mA·g-1 with the lowest capacity decay rate of 0.044% per cycle. This work will promote the rapid application of high-energy-density PIBs.

Potassium-Rich Iron Hexacyanoferrate/Carbon Cloth Electrode for Flexible and Wearable Potassium-Ion Batteries

The fast development of flexible and wearable electronics increases the demand for flexible secondary batteries, and the emerging high-performance K-ion batteries (KIBs) have shown immense promise for the flexible electronics due to the abundant and cost-effective potassium resources. However, the implementation of flexible cathodes for KIBs is hampered by the critical issues of low capacity, rapid capacity decay with cycles, and limited initial Coulombic efficiency. To address these pressing issues, a freestanding K-rich iron hexacyanoferrate/carbon cloth (KFeHCF/CC) electrode is designed and fabricated by cathodic deposition. This innovative binder-free and self-supporting KFeHCF/CC electrode not only provides continuous conductive channels for electrons, but also accelerates the diffusion of potassium ions through the active electrode-electrolyte interface. Moreover, the nanosized potassium iron hexacyanoferrate particles limit particle fracture and pulverization to preserve the structure and stability during cycling. As a result, the K-rich KFeHCF/CC electrode shows a reversible discharging capacity of 110.1 mAh g-1 at 50 mA g-1 after 100 cycles in conjunction with capacity retention of 92.3% after 1000 cycles at 500 mA g-1 . To demonstrate the commercial feasibility, a flexible tubular KIB is assembled with the K-rich KFeHCF/CC electrode, and excellent flexibility, capacity, and stability are observed.

Boosting Potassium Storage via Multifunctional Interface with High Lattice-Matching

Atomic-scale interface engineering is a prominent strategy to address the large volume expansions and sluggish redox kinetics for reinforcing K-storage. Here, to accelerate charge transport and lower the activation energy, dual carbon-modified interfacial regions are synthesized with high lattice-matching degree, which is formed from a CoSe2 /FeSe2 heterostructure coated onto hollow carbon fibers. State-of-the-art characterization techniques and theoretical analysis, including ex-situ soft X-ray absorption spectroscopy, synchrotron X-ray tomography, ultrasonic transmission mapping, and density functional theory, are conducted to probe local atomic structure evolution, mechanical degradation mechanisms, and ion/electron migration pathways. The results suggest that the heterostructure composed of the same crystal system and space group can sharply regulate the redox kinetics of transition metal selenium and dual carbon-modified approach can tailor physicochemical degradation. Overall, this work presents the design of a stable heterojunction synergistic superior hollow carbon substrate, inspiring a pathway of interface engineering strategy toward high-performance electrode.

Boosting Manganese Selenide Anode for Superior Sodium-Ion Storage via Triggering α → β Phase Transition

Sodium-ion batteries (SIBs) have been extensively studied owing to the abundance and low-price of Na resources. However, the infeasibility of graphite and silicon electrodes in sodium-ion storage makes it urgent to develop high-performance anode materials. Herein, α-MnSe nanorods derived from δ-MnO2 (δ-α-MnSe) are constructed as anodes for SIBs. It is verified that α-MnSe will be transferred into β-MnSe after the initial Na-ion insertion/extraction, and δ-α-MnSe undergoes typical conversion mechanism using a Mn-ion for charge compensation in the subsequent charge-discharge process. First-principles calculations support that Na-ion migration in defect-free α-MnSe can drive the lattice distortion to phase transition (alpha → beta) in thermodynamics and dynamics. The formed β-MnSe with robust lattice structure and small Na-ion diffusion barrier boosts great structure stability and electrochemical kinetics. Hence, the δ-α-MnSe electrode contributes excellent rate capability and superior cyclic stability with long lifespan over 1000 cycles and low decay rate of 0.0267% per cycle. Na-ion full batteries with a high energy density of 281.2 Wh·kg-1 and outstanding cyclability demonstrate the applicability of δ-α-MnSe anode.

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.

Bismuth Telluride Nanoplates Hierarchically Confined by Graphene and N-Doped C as Conversion-Alloying Anode Materials for Potassium-Ion Batteries

Potassium-ion batteries (PIBs) have broad application prospects in the field of electric energy storage systems because of its abundant K reserves, and similar "rocking chair" operating principle as lithium-ion batteries (LIBs). Aiming to the large volume expansion and sluggish dynamic behavior of anode materials for storing large sized K-ion, bismuth telluride (Bi2 Te3 ) nanoplates hierarchically encapsulated by reduced graphene oxide (rGO), and nitrogen-doped carbon (NC) are constructed as anodes for PIBs. The resultant Bi2 Te3 @rGO@NC architecture features robust chemical bond of Bi─O─C, tightly physicochemical confinement effect, typical conductor property, and enhanced K-ion adsorption ability, thereby producing superior electrochemical kinetics and outstanding morphological and structural stability. It is visually elucidated via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) that conversion-alloying dual-mechanism plays a significant role in K-ion storage, allowing 12 K-ion transport per formular unit employing Bi as redox site. Thus, the high first reversible specific capacity of 322.70 mAh g-1 at 50 mA g-1 , great rate capability and cyclic stability can be achieved for Bi2 Te3 @rGO@NC. This work lays the foundation for an in-depth understanding of conversion-alloying mechanism in potassium-ion storage.

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.

First principles study of B7N5 as a high capacity electrode material for K-ion batteries

Potassium-ion batteries (KIBs) are considered a promising candidate for energy storage owing to their low cost and similar "rocking chair" mechanism to lithium-ion batteries. However, there is a great lack of suitable and high working performance electrode materials for KIBs. Herein, first principles calculations based on density functional theory (DFT) are applied to evaluate the potential of the B7N5 monolayer as an anode material for KIBs. It is found that B7N5 shows negative adsorption energies for Li/Na/K on its surface. Besides, the B7N5 monolayer can effectively achieve double-layer adsorption for K atoms on both sides of the monolayer surface, while one-layer adsorption for Li and Na. Thus, B7N5 exhibits an ultra-high theoretical capacity of 1471.5 mA h g-1 for KIBs, which is almost the highest value for the anode materials reported in the literature. And the attractive capacity for KIBs is mainly contributed by the multiple empty electron orbitals of the constituting elements of B7N5 and the small distance mismatch. In addition, K atoms display high diffusivities on B7N5 with low energy barriers of 0.10 eV, and the open circuit voltages of 0.14 V for KIBs are also smaller compared with previous research. It is also shown that after adsorbing K, the semiconducting B7N5 monolayer is transformed to a metallic state with good conductivity. Furthermore, despite the large size of K+, the maximum change in the lattice constant of B7N5 is only 1.32%, indicating structural stability and ensuring good cycling stability for KIBs. The above-mentioned results suggest that B7N5 is a potential anode material for KIBs.

Novel Bilayer-Shelled N, O-Doped Hollow Porous Carbon Microspheres as High Performance Anode for Potassium-Ion Hybrid Capacitors

With the advantages of high energy/power density, long cycling life and low cost, dual-carbon potassium ion hybrid capacitors (PIHCs) have great potential in the field of energy storage. Here, a novel bilayer-shelled N, O-doped hollow porous carbon microspheres (NOHPC) anode has been prepared by a self-template method, which is consisted of a dense thin shell and a hollow porous spherical core. Excitingly, the NOHPC anode possesses a high K-storage capacity of 325.9 mA h g-1 at 0.1 A g-1 and a capacity of 201.1 mAh g-1 at 5 A g-1 after 6000 cycles. In combination with ex situ characterizations and density functional theory calculations, the high reversible capacity has been demonstrated to be attributed to the co-doping of N/O heteroatoms and porous structure improved K+ adsorption and intercalation capabilities, and the stable long-cycling performance originating from the bilayer-shelled hollow porous carbon sphere structure. Meanwhile, the hollow porous activated carbon microspheres (HPAC) cathode with a high specific surface area (1472.65 m2 g-1) deriving from etching NOHPC with KOH, contributing to a high electrochemical adsorption capacity of 71.2 mAh g-1 at 1 A g-1. Notably, the NOHPC//HPAC PIHC delivers a high energy density of 90.1 Wh kg-1 at a power density of 939.6 W kg-1 after 6000 consecutive charge-discharge cycles.

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.

Synthesis, crystal structures and semiconducting properties of new hexacyanidometallates

We describe the synthesis, crystal structure and semiconducting properties of a number of hexacyanidometallates with the formula A₂[MFe(CN)₆]·xH₂O (A = Na, K; M = Mg, Ca, Sr and Ba). All crystal structures were studied via single-crystal or powder X-ray diffraction. The unexpectedly low-symmetric structures in these ferrocyanides are described and contrasted with analogous transition-metal compounds which have been reported to be strictly or nearly cubic. The amount of crystal water in the structure for powder samples was determined by the thermogravimetric analysis (TGA), supported by IR and Raman spectroscopy. Electronic-structure calculations of K₂[MgFe(CN)₆] and K₂[CaFe(CN)₆] are compared with experimental UV-Vis measurements. The large band gaps by advanced theory indicate that the smaller experimental band gaps are due to surface effects of impurity states. Mott-Schottky curves of K₂[MgFe(CN)₆], K₂[CaFe(CN)₆] and K₂[BaFe(CN)₆]·3H₂O exhibit positive slopes, which characterizes these compounds as n-type semiconductors.

Pillar strategy enhanced ion transport and structural stability toward ultra-stable KVPO4F cathode for practical potassium-ion batteries

KVPO₄F (KVPF) is a promising cathode material for potassium-ion batteries (PIBs) because of its high operating voltage, high energy density, and excellent thermal stability. Nevertheless, the low kinetics and large volume change have been the major hurdles causing irreversible structural damage, high inner resistance, and poor cycle stability. Herein, a pillar strategy of Cs⁺ doping in KVPO₄F is introduced to reduce the energy barrier for ion diffusion and volume change during potassiation/depotassiation, which significantly enhances the K⁺ diffusion coefficient and stabilizes the crystal structure of the material. Consequently, the K_0.95Cs_0.05VPO₄F (Cs-5-KVPF) cathode exhibits an excellent discharge capacity of 104.5 mAh g^-1 at 20 mA g^-1 and a capacity retention rate of 87.9% after 800 cycles at 500 mA g^-1. Importantly, Cs-5-KVPF//graphite full cells attain an energy density of 220 Wh kg^-1 (based on the cathode and anode weight) with a high operating voltage of 3.93 V and 79.1% capacity retention after 2000 cycles at 300 mA g^-1. The Cs-doped KVPO₄F cathode successfully innovates the ultra-durable and high-performance cathode materials for PIBs, demonstrating its considerable potential for practical applications.

Phase-engineered cathode for super-stable potassium storage

The crystal phase structure of cathode material plays an important role in the cell performance. During cycling, the cathode material experiences immense stress due to phase transformation, resulting in capacity degradation. Here, we show phase-engineered VO₂ as an improved potassium-ion battery cathode; specifically, the amorphous VO₂ exhibits superior K storage ability, while the crystalline M phase VO₂ cannot even store K⁺ ions stably. In contrast to other crystal phases, amorphous VO₂ exhibits alleviated volume variation and improved electrochemical performance, leading to a maximum capacity of 111 mAh g^-1 delivered at 20 mA g^-1 and over 8 months of operation with good coulombic efficiency at 100 mA g^-1. The capacity retention reaches 80% after 8500 cycles at 500 mA g^-1. This work illustrates the effectiveness and superiority of phase engineering and provides meaningful insights into material optimization for rechargeable batteries.

An Amorphous Anode for Proton Battery

Developing advanced electrode materials is crucial for improving the electrochemical performances of proton batteries. Currently, the anodes are primarily crystalline materials which suffer from inferior cyclic stability and high electrode potential. Herein, we propose amorphous electrode materials for proton batteries by using a general ion-exchange protocol to introduce multivalent metal cations for activating the host material. Taking Al3+ as an example, theoretical and experimental analysis demonstrates electrostatic interaction between metal cations and lattice oxygen, which is the primary barrier for direct introduction of the multivalent cations, is effectively weakened through ion exchange between Al3+ and pre-intercalated K+. The as-prepared Al-MoOx anode therefore delivered a remarkable capacity and outstanding cycling stability that outperforms most of the state-of-the-art counterparts. The assembled full cell also achieved a high voltage of 1.37 V. This work opens up new opportunities for developing high-performance electrodes of proton batteries by introducing amorphous materials.

Lithiated Prussian blue analogues as positive electrode active materials for stable non-aqueous lithium-ion batteries

Prussian blue analogues (PBAs) are appealing active materials for post-lithium electrochemical energy storage. However, PBAs are not generally suitable for non-aqueous Li-ion storage due to their instability upon prolonged cycling. Herein, we assess the feasibility of PBAs with various lithium content for non-aqueous Li-ion storage. We determine the crystal structure of the lithiated PBAs via neutron powder diffraction measurements and investigate the influence of water on structural stability and Li-ion migration through operando X-ray diffraction measurements and bond valence simulations. Furthermore, we demonstrate that a positive electrode containing Li_2-xFeFe(CN)₆⋅nH₂O (0 ≤ x ≤ 2) active material coupled with a Li metal electrode and a LiPF₆-containing organic-based electrolyte in coin cell configuration delivers an initial discharge capacity of 142 mAh g^-1 at 19 mA g^-1 and a discharge capacity retention of 80.7% after 1000 cycles at 1.9 A g^-1. By replacing the lithium metal with a graphite-based negative electrode, we also report a coin cell capable of cycling for more than 370 cycles at 190 mA g^-1 with a stable discharge capacity of about 105 mAh g^-1 and a discharge capacity retention of 98% at 25 °C.

Self-Healing of Prussian Blue Analogues with Electrochemically Driven Morphological Rejuvenation

Maintaining the morphology of electrode materials with high invertibility contributes to the prolonged cyclic stability of battery systems. However, the majority of electrode materials tend to degrade during the charge-discharge process owing to the inevitable increase in entropy. Herein, a self-healing strategy is designed to promote morphology rejuvenation in Prussian blue analogue (PBA) cathodes by cobalt doping. Experimental characterization and theoretical calculations demonstrate that a trace amount of cobalt can decelerate the crystallization process and restore the cracked areas to ensure perfect cubic structures of PBA cathodes. The electric field controls the kinetic dynamics, rather than the conventional thermodynamics, to realize the "electrochemically driven dissolution-recrystallization process" for the periodic self-healing phenomenon. The properties of electron transportation and ion diffusion in bulk PBA are also improved by the doping strategy, thus boosting the cyclability with 4000 cycles in a diluent electrolyte. This discovery provides a new paradigm for the construction of self-healing electrodes for cathodes.

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.

Scalable Printing of Prussian Blue Analogue@Au Edge-Rich Microcubes as Flexible Biosensing Microchips Performing Ultrasensitive Sucrose Fermentation Monitoring

Sucrose is one of the most applied carbon sources in the fermentation process, and it directly determines the microbial metabolism with its concentration fluctuation. Meanwhile, sucrose also plays a key role of a protective agent in the production of biological vaccines, especially in the new mRNA vaccines for curing COVID-19. However, rapid and precise detection of sucrose is always desired but unrealized in industrial fermentation and synthetic biology research. In order to address the above issue, we proposed an ultrasensitive biosensor microchip achieving accurate sucrose recognition within only 12 s, relying on the construction of a Prussian blue analogue@Au edge-rich (PBA@AuER) microarchitecture. This special geometric structure was formed through exactly inducing the oriented PBA crystallization toward a certain plane to create more regular and continuous edge features. This composite was further transformed to a screen-printed ink to directly and large-scale fabricate an enzymatic biosensor microchip showing ultrahigh sensitivity, a wide detection range, and a low detection limit to the accurate sucrose recognition. As confirmed in a real alcohol fermentation reaction, the as-prepared microchip enabled us to accurately detect the sucrose and glucose concentrations with outstanding reusability (more than 300 times) during the whole process through proposing a novel analytical strategy for the binary mixture substrate system.

Incorporating Near-Pseudocapacitance Insertion Ni/Co-Based Hexacyanoferrate and Low-Cost Metallic Zn for Aqueous K-Ion Batteries

The limited availability of cathode materials with high specific capacity and significant cycling stability for aqueous K-ion batteries (AKIBs) hinder their further development owing to the large radius of K⁺ (1.38 Å). Prussian blue and its analogs with a three-dimensional frame structure possessing special energy storage mechanism are promising candidates as cathode materials for AKIBs. In this study, K_0.2 Ni_0.68 Co_0.77 Fe(CN)₆  ⋅ 1.8H₂ O (KNCHCF) was prepared as a cathode material for AKIBs. Both the electrochemical activity of Co ions and the near-pseudocapacitance intercalation of KNCHCF enhance K⁺ storage. Therefore, KNCHCF exhibits a superior capacity maintenance rate of 86 % after 1000 cycles at a high current density of 3.0 A g^-1 . The storage mechanism of K⁺ in AKIBs was revealed through ex situ X-ray diffraction, ex situ Fourier transform infrared spectroscopy, and ex situ X-ray photoelectron spectroscopy measurements. Moreover, the assembled K-Zn hybrid battery showed good cycling stability with 93.1 % capacity maintenance at 0.1 A g^-1 after 50 cycles and a high energy density of 96.81 W h kg^-1 . Hence, KNCHCF may be a potential material for the development of AKIBs.

Monoclinic Bimetallic Prussian Blue Analog Cathode with High Capacity and Long Life for Advanced Sodium Storage

Prussian blue analogs (PBAs) are regarded as promising cathode materials for sodium-ion batteries (SIBs), but most of them suffer from an incompatibility between capacity and structural stability. Herein, an innovative disodium ethylenediaminetetraacetate (Na₂EDTA)-assisted hydrothermal method is proposed to synthesize monoclinic Fe-substituted Ni-rich PBA (H-PBA) cathodes for Na-ion storage. The as-designed H-PBA cathode combines the merits of the low strain of a Ni-based PBA framework and the enhanced capacity of N-Fe³⁺/Fe²⁺ redox sites. It can achieve superior sodium-storage performance in terms of capacity, rate capability, and cycle stability. Moreover, ex situ measurements reveal that solid solution (2.0-3.0 V) and phase-transition (3.0-4.0 V) reactions occur during the charge/discharge process to allow almost 1.5 Na⁺ storage in the H-PBA lattice. Meanwhile, the H-PBA//NaTi₂(PO₄)₃@C full cell also delivers remarkable electrochemical properties. Prospectively, this work would promote the practical application of SIBs in grid-scale electric energy storage.

Prussian Blue Analogues for Sodium-Ion Batteries: Past, Present, and Future

Prussian blue analogues (PBAs) have attracted wide attention for their application in the energy storage and conversion field due to their low cost, facile synthesis, and appreciable electrochemical performance. At the present stage, most research on PBAs is focused on their material-level optimization, whereas their properties in practical battery systems are seldom considered. This review aims to first provide an overview of the history and parameters of PBA materials and analyze the fundamental principles toward rational design of PBAs, and then evaluate the prospects and challenges for PBAs for practical sodium-ion batteries, hoping to bridge the gap between laboratory research and commercial reality.

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.

Nitrogen and Oxygen Co-Doped Porous Hard Carbon Nanospheres with Core-Shell Architecture as Anode Materials for Superior Potassium-Ion Storage

The investigation of carbonaceous-based anode materials will promote the fast application of low-cost potassium-ion batteries (PIBs). Here a nitrogen and oxygen co-doped yolk-shell carbon sphere (NO-YS-CS) is constructed as anode material for K-ion storage. The novel architecture, featuring with developed porous structure and high surface specific area, is beneficial to achieving excellent electrochemical kinetics behavior and great electrode stability from buffering the large volume expansion. Furthermore, the N/O heteroatoms co-doping can not only boost the adsorption and intercalation ability of K-ion but also increase the electron transfer capability. It is also demonstrated by experimental results and DFT calculations that K-ion insertion/extraction proceeds through both intercalation and surface capacitive adsorption mechanisms. As expected, the NO-YS-CS electrodes show high initial charge capacity of 473.7 mAh g^-1 at 20 mA g^-1 , ultralong cycling life over 2500 cycles with the retention of 85.8% at 500 mA g^-1 , and superior rate performance (183.3 mAh g^-1 at 1.0 A g^-1 ). The K-ion full cell, with a high energy density of 271.4 Wh kg^-1 and an excellent cyclic stability over 500 cycles, is successfully fabricated with K₂ Fe[Fe(CN)₆ ] cathode. This work will provide new insight on the synthesis and mechanism understanding of high-performance hard carbon anode for PIBs.

Sb2S3-based conversion-alloying dual mechanism anode for potassium-ion batteries

The large volume expansion and sluggish dynamic behavior are the key bottleneck to suppress the development of conversion-alloying dual mechanism anode for potassium-ion batteries (PIBs). Herein, Sb2S3 nanorods encapsulated by reduced graphene oxide and nitrogen-doped carbon (Sb2S3@rGO@NC) are constructed as anodes for PIBs. The synergistic effect of dual physical protection and robust C-Sb chemical bonding boosts superior electrochemical kinetics and great electrode stability. Thus, Sb2S3@rGO@NC exhibits a high initial charge capacity of 505.6 mAh·g-1 at 50 mA·g-1 and a great cycle stability with the lifetime over 200 cycles at 200 mA·g-1. Ex situ XRD, XPS, and TEM characterizations confirm that the electrode undergoes a multielectron transfer process (Sb2S3↔ Sb + K2S ↔ KSb + K3Sb), where K-ion insert into/extract from the material via dual mechanisms of conversion and alloying. This work sheds a light on the construction of high-performance anode materials and the understanding of K-ion storage mechanism.

Decoration of nickel hexacyanoferrate nanocubes onto reduced graphene oxide sheets as high-performance cathode material for rechargeable aqueous zinc-ion batteries

Prussian blue analogues (PBA) have attracted much attention in energy research due to their unique three-dimensional open framework structure, adjustable metal ions, and facile synthesis. However, the application of PBA as a cathode material for aqueous zinc-ion batteries (ZIBs) is restricted by its poor cycling performance and lower capacity. In this paper, we develop a new PBA-based hybrid cathode material for aqueous ZIBs by loading uniform nickel hexacyanoferrate (NiHCF) nanocubes onto reduced graphene oxide (RGO) sheets. In the NiHCF/RGO hybrid, NiHCF nanoparticles are well anchored on the RGO layers, forming a conductive network. The strong synergy between NiHCF and highly conductive RGO effectively increases the specific surface area, accelerates the electron and ion transport, and inhibits the structural collapse of the NiHCF/RGO electrode during the Zn²⁺ insertion/extraction process. Benefiting from the above advantages, the NiHCF/RGO hybrid exhibits a remarkable reversible capacity of 94.5 mAh g^-1 at a current density of 5 mA g^-1, excellent rate performance of 50.1 mAh g^-1 at 200 mA g^-1, and enhanced cycling stability with a capacity retention of 80.3% after 1000 cycles at 200 mA g^-1. This work provides a simple and effective way to improve the electrochemical performance of PBA-based cathodes for aqueous ZIBs application.

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.

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).

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

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

A P3-Type K1/2Mn5/6Mg1/12Ni1/12O2 Cathode Material for Potassium-Ion Batteries with High Structural Reversibility Secured by the Mg-Ni Pinning Effect

Mn-based layered oxides are very attractive as cathodes for potassium-ion batteries (PIBs) due to their low-cost and environmentally friendly precursors. Their transfer to practical application, however, is inhibited by some issues including consecutive phase transitions, sluggish K⁺ deintercalation/intercalation, and serious capacity loss. Herein, Mg-Ni co-substituted K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ is designed as a promising cathode material for PIBs, with suppressed phase transitions that occurred in K_1/2MnO₂ and improved K⁺ storage performance. Part of Mg²⁺ and Ni²⁺ occupies the K⁺ layer, playing the role of a "nailed pillar", which restrains metal oxide layer gliding during the K⁺ (de)intercalation. The "Mg-Ni pinning effect" not only suppresses the phase transitions but also reduces the cell volume variation, leading to the improved cycle performance. Moreover, K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ has low activation barrier energy for K⁺ diffusion and high electron conductivity as demonstrated by first-principles calculations, resulting in better rate capability. In addition, K_1/2Mn_5/6Mg_1/12Ni_1/12O₂ also delivers a higher reversible capacity owing to the participation of the Ni element in electrochemical reactions and the pseudocapacitive contribution. This study provides a basic understanding of structural evolution in layered Mn-based oxides and broadens the strategic design of cathode materials for PIBs.

Ultrafast Potassium Storage in F-Induced Ultra-High Edge-Defective Carbon Nanosheets

Carbonaceous materials have been considered as promising anodes for potassium-ion batteries (PIBs) because of their high electronic conductivity, eco-friendliness, and structural stability. However, the small interlayer spacing and serious volume expansion caused by the repeated insertion/extraction of large K-ions restrict their potassium-ion storage performance. Herein, F and N codoped carbon nanosheets (FNCS) with rich-edge defects are designed to resolve these problems. The F doping is in favor of the formation of more edge defects in the carbon layer, offering strong K⁺ adsorption capability and promoting the K⁺ storage. The ultrathin carbon nanosheets can provide a large contact area for the electrochemical reactions and shorten the transportation pathways for both K-ions and electrons. Consequently, the FNCS anode shows a high reversible capacity (610 mAh g^-1 at 0.1 A g^-1) and ultrastable cyclability over 4000 cycles at 5 A g^-1. Moreover, K-ion full cells (FNCS|K₂FeFe(CN)₆) display excellent cycling stability (128 mAh g^-1 at 1 A g^-1 after 500 cycles) and rate capability (93 mAh g^-1 at 20 A g^-1). This design strategy can be extended to design other electrode materials for high-performance energy storage, such as magnesium-ion batteries, supercapacitors, and electrocatalysis.

A Low-Strain Potassium-Rich Prussian Blue Analogue Cathode for High Power Potassium-Ion Batteries

Most of the cathode materials for potassium ion batteries (PIBs) suffer from poor structural stability due to the large ionic radius of K⁺ , resulting in poor cycling stability. Here we report a low-strain potassium-rich K_1.84 Ni[Fe(CN)₆ ]_0.88 ⋅0.49 H₂ O (KNiHCF) as a cathode material for PIBs. The as-prepared KNiHCF cathode can deliver reversible discharge capacity of 62.8 mAh g^-1 at 100 mA g^-1 , with a high discharge voltage of 3.82 V. It can also achieve a superior rate performance of 45.8 mAh g^-1 at 5000 mA g^-1 , with a capacity retention of 88.6 % after 100 cycles. The superior performance of KNiHCF cathode results from low-strain de-/intercalation mechanism, intrinsic semiconductor property and low potassium diffusion energy barrier. The high power density and long-term stability of KNiHCF//graphite full cell confirmed the feasibility of K-rich KNiHCF cathode in PIBs. This work provides guidance to develop Prussian blue analogues as cathode materials for PIBs.

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.

In Situ Anchoring Polymetallic Phosphide Nanoparticles within Porous Prussian Blue Analogue Nanocages for Boosting Oxygen Evolution Catalysis

The controllable synthesis of metal-based nanoclusters for heterogeneous catalytic reactions has received considerable attention. Nevertheless, manufacturing these architectures, while avoiding aggregation and retaining surface activity, remains challenging. Herein, for the first time we designed NiCoFe-Prussian blue analogue (PBA) nanocages as a support for in situ dispersion and anchoring of polymetallic phosphide nanoparticles (pMP-NPs). Benefiting from the porous surfaces and the synergistic effects between pMP-NPs and the cyano groups in PBA, the NiCoFe-P-NP@NiCoFe-PBA nanocages exhibit a significantly enhanced catalytic activity for oxygen evolution reaction (OER) with an overpotential of 223 mV at 10 mA cm^-2 and a Tafel slope of 78 mV dec^-1, outperforming the NiCoFe-PBA nanocubes, NiCoFe-P nanocages, NiFe-P-NP@NiFe-PBA nanocubes, and CoFe-P-NP@CoFe-PBA nanoboxes. This work not only offers the synthesis strategy of in situ anchoring pMP-NPs on PBA nanocages but also provides a new insight into optimized Gibbs free energy of OER by regulating electron transfer from metallic phosphides to PBA substrate.

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

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