Low-Cost High-Energy Potassium Cathode

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.

Low-defect K2Mn[Fe(CN)6]-reduced graphene oxide composite for high-performance potassium-ion batteries

Here, a low-defect K₂Mn[Fe(CN)₆]-reduced graphene oxide (KMF-RGO) composite is fabricated, which demonstrates excellent cycling stability, fast rate capability, and exceptional air stability as a cathode material for potassium-ion batteries (KIBs). This work provides a practical strategy for the synthesis of high-performance K₂Mn[Fe(CN)₆] cathode materials for KIBs.

Exploring the Role of Crystal Water in Potassium Manganese Hexacyanoferrate as a Cathode Material for Potassium-Ion Batteries

The Prussian Blue analogue K2−δMn[Fe(CN)6]1−ɣ∙nH2O is regarded as a key candidate for potassium-ion battery positive electrode materials due to its high specific capacity and redox potential, easy scalability, and low cost. However, various intrinsic defects, such as water in the crystal lattice, can drastically affect electrochemical performance. In this work, we varied the water content in K2−δMn[Fe(CN)6]1−ɣ∙nH2O by using a vacuum/air drying procedure and investigated its effect on the crystal structure, chemical composition and electrochemical properties. The crystal structure of K2−δMn[Fe(CN)6]1−ɣ∙nH2O was, for the first time, Rietveld-refined, based on neutron powder diffraction data at 10 and 300 K, suggesting a new structural model with the Pc space group in accordance with Mössbauer spectroscopy. The chemical composition was characterized by thermogravimetric analysis combined with mass spectroscopy, scanning transmission electron microscopy microanalysis and infrared spectroscopy. Nanosized cathode materials delivered electrochemical specific capacities of 130–134 mAh g−1 at 30 mA g−1 (C/5) in the 2.5–4.5 V (vs. K+/K) potential range. Diffusion coefficients determined by potentiostatic intermittent titration in a three-electrode cell reached 10−13 cm2 s−1 after full potassium extraction. It was shown that drying triggers no significant changes in crystal structure, iron oxidation state or electrochemical performance, though the water level clearly decreased from the pristine to air- and vacuum-dried samples.

In-situ growth of Cu(2-methylimidazole imidazole)2 on Cu2O polyhedrons for high-performance potassium-ion batteries

In this work, Cu₂O/Cu(2-MeIm)₂ core-shell structure was designed and used as an anode for potassium-ion batteries. The Cu₂O core not only ensured a high energy density through surface redox reactions, but also served as a copper-ion reservoir to keep the stability of skeleton structure of Cu(2-MeIm)₂ shell during electrochemical process. The Cu(2-MeIm)₂ shell, in turn, not only provided high power through rapid K⁺ adsorption/desorption, but also acted as an artificial solid electrolyte interphase layer and accommodated volumetric change during K⁺ intercalation/de-intercalation. The as-designed composite material was studied by X-ray diffraction, thermo gravimetric, Fourier transform infrared, nitrogen adsorption-desorption, X-ray photoelectron spectroscopic and electron microscopic characterizations. As an anode for potassium-ion batteries, it was galvanostatically discharged and charged to study its electrochemical properties, such as Coulombic efficiency, capacity retention and rate performance, and cyclic voltammetry curves were also tested to reveal its K-storage mechanism.

Cubic Manganese Potassium Hexacyanoferrate Regulated by Controlling of the Water and Defects as a High-Capacity and Stable Cathode Material for Rechargeable Aqueous Zinc-Ion Batteries

Aqueous zinc ion batteries (A-ZIBs) have been used as new alternative batteries for grid-scale electrochemical energy storage because of their low cost and environmental protection. Finding a suitable and economical cathode material, which is needed to achieve high energy density and long cycle stability, is one of the most important and arduous challenges at the present stage. Potassium manganese hexacyanoferrate (KMHCF) is a kind of Prussian blue analogue. It has the advantages of a large 3D frame structure that can accommodate the insertion/extraction of zinc ions, and is nontoxic, safe, and easy to prepare. However, regularly synthesized KMHCF has higher water and crystal defects, which reduce the possibility of zinc ions' insertion/extraction, and subsequently the discharge capacity and cycling stability. In this work, a KMHCF material with less water and low defects was obtained by adding polyvinylpyrrolidone during the synthesis process to control the reaction process. The KMHCF serves as the cathode of A-ZIBs and exhibits an excellent electrochemical performance providing a specific capacity of 140 mA h g^-1 for the initial cycle at a current density of 100 mA g^-1 (1 C). In particular, it can maintain a reversible capacity of 85 mA h g^-1, even after 400 cycles at 1 C. Moreover, unlike the traditional zinc storage mechanism of A-ZIBs, we found that the KMHCF electrode undergoes a phase transition process when the KMHCF electrode was activated by a small current density, which is attributed to part of the Mn on the lattice site being replaced by Zn, thus forming a new stable phase to participate in the subsequent electrochemical reaction.

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.

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

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

Rational construction of K0.5V2O5 nanobelts/CNTs flexible cathode for multi-functional potassium-ion batteries

Potassium-ion battery (KIB) is one of the emerging electrochemical energy storage technologies due to the abundance, low cost, and low redox potential of K. One of the most promising cathodes of KIBs is a layered vanadium-based compound, but it often suffers from fast capacity decay during repeated cycling. Herein, a K0.5V2O5/CNTs hybrid film composed of K0.5V2O5 nanobelt and carbon nanotube (CNT) network was synthesized by an electrostatic self-assembly and vacuum filtration process, and further used as the cathode in KIBs. The K0.5V2O5/CNTs cathode possessed a flexible and interconnected network structure, which not only offered fast kinetics for electron transfer and ion transportation, but also provided an elastic medium to buffer the large volume change of the K0.5V2O5 nanobelts during cycling. As a cathode for KIBs, the K0.5V2O5/CNTs electrode showed a reversible discharge capacity of ∼90 mA h g-1 at 50 mA g-1 and exhibited good cycling stability (88.8% capacity retention for 100 cycles at 50 mA g-1, 82.2% capacity retention for 300 cycles at 500 mA g-1) and excellent rate performance of ∼62 mA h g-1 at 500 mA g-1. K-Ion full battery testing further confirmed its good electrochemical performance by presenting a high reversible discharge capacity (68 mA h g-1 at 50 mA g-1) and long-term retention (>80% after 80 cycles). Interestingly, a cable-shaped KIB with the flexible K0.5V2O5/CNTs film as the cathode electrode was assembled and showed its further application potential as a power source for wearable electronics.

Defect-free potassium manganese hexacyanoferrate cathode material for high-performance potassium-ion batteries

Potassium-ion batteries (KIBs) are promising electrochemical energy storage systems because of their low cost and high energy density. However, practical exploitation of KIBs is hampered by the lack of high-performance cathode materials. Here we report a potassium manganese hexacyanoferrate (K₂Mn[Fe(CN)₆]) material, with a negligible content of defects and water, for efficient high-voltage K-ion storage. When tested in combination with a K metal anode, the K₂Mn[Fe(CN)₆]-based electrode enables a cell specific energy of 609.7 Wh kg⁻¹ and 80% capacity retention after 7800 cycles. Moreover, a K-ion full-cell consisting of graphite and K₂Mn[Fe(CN)₆] as anode and cathode active materials, respectively, demonstrates a specific energy of 331.5 Wh kg⁻¹, remarkable rate capability, and negligible capacity decay for 300 cycles. The remarkable electrochemical energy storage performances of the K₂Mn[Fe(CN)₆] material are attributed to its stable frameworks that benefit from the defect-free structure.

Potassium-ion battery is a promising candidate for post-Li-ion energy storage but the lack of cathode materials hinders practical exploitation. Here the authors investigate defect-free potassium manganese hexacyanoferrate as cathode active material for high energy and long lifespan K-based cells.

Effect of Particle Size and Anion Vacancy on Electrochemical Potassium Ion Insertion into Potassium Manganese Hexacyanoferrates

Potassium manganese hexacyanoferrate (KMnHCF) can be used as a positive electrode for potassium-ion batteries because of its high energy density. The effect of particle size and [Fe(CN)₆ ]^n- vacancies on the electrochemical potassium insertion of KMnHCFs was examined through experimental data and theoretical calculations. When nearly stoichiometric KMnHCF was synthesized and tested, smaller particle sizes were found to be important for achieving superior electrochemical performance in terms of capacity and rate capability. However, even in the case of larger particles, introducing a suitable number of anion vacancies enabled KMnHCF to exhibit comparable electrode performance. Electrochemical tests and density functional theory calculations indicated that anion vacancies contribute to the enhancement of K⁺ ion diffusion, which realizes good electrochemical performance. Structural design, including crystal vacancies and particle size, is the key to their high performance as a positive electrode.

Understanding the Effect of Interplanar Space and Preintercalated Cations of Vanadate Cathode Materials on Potassium-Ion Battery Performance

Nonaqueous potassium-ion batteries (KIBs) have been regarded as a promising alternative energy system to lithium-ion batteries, due to the abundance of the K resource and unique electrochemical properties. However, exploring suitable KIB cathode materials remains a great challenge, owing to the much larger size of the K ion than that of the Li ion. Here, a series of layered vanadates have been developed as cathodes for KIBs to elucidate the key factors that determine the electrochemical performance of KIBs, including the interlayer distance between adjacent (100) planes (d₁₀₀) and preintercalated cations. Compared to NH₄V₃O₈ nanowires with a d₁₀₀ of 7.80 Å, (NH₄)_0.5V₂O₅ nanowires with a wider d₁₀₀ of 9.52 Å show a faster K⁺ diffusion and much higher reversible capacity. The preintercalation of potassium ions into V-O slabs is also crucial to the stability of the structure of vanadates, which leads to better electrochemical cycling stability in K_0.5V₂O₅ than that in (NH₄)_0.5V₂O₅ and NH₄V₃O₈ nanowires. These findings reveal the great potential of the vanadate cathode in future KIBs and provide a new direction to rationally design a stable layered intercalation compound for practical KIBs.

Locally Ordered Graphitized Carbon Cathodes for High-Capacity Dual-Ion Batteries

Dual-ion batteries (DIBs) inherently suffer from limited energy density. Proposed here is a strategy to effectively tackle this issue by employing locally ordered graphitized carbon (LOGC) cathodes. Quantum mechanical modeling suggests that strong anion-anion repulsions and severe expansion at the deep-charging stage raise the anion intercalation voltage, therefore only part of the theoretical anion storage sites in graphite is accessible. The LOGC interconnected with disordered carbon is predicted to weaken the interlaminar van der Waals interactions, while disordered carbons not only interconnect the dispersed nanographite but also partially buffer severe anion-anion repulsion and offer extra capacitive anion storage sites. As a proof-of-concept, ketjen black (KB) with LOGC was used as a model cathode for a potassium-based DIB (KDIB). The KDIB delivers an unprecedentedly high specific capacity of 232 mAh g^-1 at 50 mA g^-1 , a good rate capability of 110 mAh g^-1 at 2000 mA g^-1 , and excellent cycling stability of 1000 cycles without obvious capacity fading.

Electrolytes and Interphases in Potassium Ion Batteries

Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost-effectiveness, high-voltage, and high-power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid-state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.

Turning waste into treasure: Reuse of contaminant-laden adsorbents (Cr(vi)-Fe3O4/C) as anodes with high potassium-storage capacity

In this study, we try to find possible solutions to synchronously solving energy and environmental problems. In our design, orange peel is used as a carbon source to synthesize low-cost Fe₃O₄/C composites, which are employed as adsorbents to purify Cr(vi)-contaminated water. After that, these Cr(vi)-laden Fe₃O₄/C composites are used and tested as anodes in potassium-ion batteries. It is found that their K-storage capacity is more than 300 mAh g^-1 at a current density of 0.1 A g^-1, depending on the mass content of Fe₃O₄. The more Fe₃O₄ component in composite, the more adsorbed Cr(vi) species through chemisorption, and the larger K-storage capacity. The good electrical conductivity of cabon-based anodes endows them with superior rate performance. At current densities of 0.1, 0.5, 1 and 2 A g^-1, K-storage capacity amounts to 357.8, 316.3, 276.3 and 236.8 mAh g^-1, respectively. The reuse of contaminant-laden adsorbents as anodes will shed new light on the disposal of exhausted adsorbents after water treatment and development of anode materials for secondary batteries.

Challenges and Strategies for High-Energy Aqueous Electrolyte Rechargeable Batteries

Aqueous rechargeable batteries are becoming increasingly important to the development of renewable energy sources, because they promise to meet cost-efficiency, energy and power demands for stationary applications. Over the past decade, efforts have been devoted to the improvement of electrode materials and their use in combination with highly concentrated aqueous electrolytes. Here the latest ground-breaking advances in using such electrolytes to construct aqueous battery systems efficiently storing electrical energy, i.e., offering improved energy density, cyclability and safety, are highlighted. This Review aims to timely provide a summary of the strategies proposed so far to overcome the still existing hurdles limiting the present aqueous batteries technologies employing concentrated electrolytes. Emphasis is placed on aqueous batteries for lithium and post-lithium chemistries, with potentially improved energy density, resulting from the unique advantages of concentrated electrolytes.

CNT-modified two-phase manganese hexacyanoferrate as a superior cathode for sodium-ion batteries

The two-phase KNa-MnFe(CN)₆@CNT material was synthesized via a facile concentration-gradient coprecipitation method. The outstanding electrochemical performance was achieved for KNa-MnFe(CN)₆@CNT material with the addition of CNT.

A high-performance potassium metal battery using safe ionic liquid electrolyte

Potassium secondary batteries are contenders of next-generation energy storage devices owing to the much higher abundance of potassium than lithium. However, safety issues and poor cycle life of K metal battery have been key bottlenecks. Here we report an ionic liquid electrolyte comprising 1-ethyl-3-methylimidazolium chloride/AlCl₃/KCl/potassium bis(fluorosulfonyl) imide for safe and high-performance batteries. The electrolyte is nonflammable and exhibits a high ionic conductivity of 13.1 mS cm^-1 at room temperature. A 3.6-V battery with K anode and Prussian blue/reduced graphene oxide cathode delivers a high energy and power density of 381 and 1,350 W kg^-1, respectively. The battery shows an excellent cycling stability over 820 cycles, retaining ∼89% of the original capacity with high Coulombic efficiencies of ∼99.9%. High cyclability is also achieved at elevated temperatures up to 60 °C. Uniquely, robust K, Al, F, and Cl-containing passivating interphases are afforded with this electrolyte, which is key to superior battery cycling performances.

Organic Electrode Materials for Non-aqueous K-Ion Batteries

The demands for high-performance and low-cost batteries make K-ion batteries (KIBs) considered as promising supplements or alternatives for Li-ion batteries (LIBs). Nevertheless, there are only a small amount of conventional inorganic electrode materials that can be used in KIBs, due to the large radius of K+ ions. Differently, organic electrode materials (OEMs) generally own sufficiently interstitial space and good structure flexibility, which can maintain superior performance in K-ion systems. Therefore, in recent years, more and more investigations have been focused on OEMs for KIBs. This review will comprehensively cover the researches on OEMs in KIBs in order to accelerate the research and development of KIBs. The reaction mechanism, electrochemical behavior, etc., of OEMs will all be summarized in detail and deeply. Emphasis is placed to overview the performance improvement strategies of OEMs and the characteristic superiority of OEMs in KIBs compared with LIBs and Na-ion batteries.

Post-Lithium-Ion Battery Era: Recent Advances in Rechargeable Potassium-Ion Batteries

Lithium shortage and the growing demand for electricity storage has encouraged researchers to look for new alternative energy-storage materials. Due to abundant potassium resources, similar redox potential to lithium metal, and low cost, potassium-ion batteries (PIBs), as one of the promising alternatives, have been applied in energy-storage research recently. However, PIBs do not have adequate competition in their electrochemical efficiency because the molar volume of potassium ions is higher than those in lithium and sodium ions. Therefore, for better application and development of PIBs, finding suitable anode and cathode materials is currently the most important task. The latest developments in electrode materials for PIBs have been outlined in depth in this review. It focuses on the structural design and synthetic methods for novel electrode materials, ingenious optimization and tuning strategies, and explains the intrinsic reaction mechanism. The effects of organic electrolytes and aqueous electrolytes on battery systems are compared and clarified. Finally, theoretical and viable insights are given to the challenges posed by the creation and practical application of PIBs in the future.

The Features and Progress of Electrolyte for Potassium Ion Batteries

Nowadays, Li-ion batteries have achieved great success and are widely used in various fields. However, the scarcity and uneven distribution of lithium resources together with the increasing cost may hamper the sustainable development of Li-ion batteries in the future. Hence, many researchers have turned to potassium ion batteries due to their abundant raw materials, low price, and high energy density. Although great progress has been made in recent years, there are still existing many challenges, especially the severe side reaction between electrolyte and K metal, which leads to an unstable solid-liquid interface and low coulombic efficiency. Hence, an excellent electrolyte may be the key to development of K-ion batteries in the future. Unfortunately, no systematic research has been conducted to study the electrolyte and its role on the performance yet. In order to compensate for this limitation, in this paper, the status and progress of electrolytes for K-ion batteries are reviewed, the issues and challenges existing in the development of electrolyte are clarified, and the future development is prospected.

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

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

Plum pudding model inspired KVPO4F@3DC as high-voltage and hyperstable cathode for potassium ion batteries

The investigation on the cathode material of potassium ion batteries (PIBs), one of the most promising alternatives to lithium ion batteries, is of great significance. Potassium vanadium fluorophosphate (KVPO₄F) with a high working voltage is an appealing cathode candidate for PIBs, while the poor cycling performance and low electronic conductivity dramatically hinder the application. Herein, a plum pudding model inspired three-dimensional amorphous carbon network modified KVPO₄F composite (KVPO₄F@3DC) is successfully designed in this study to tackle these problems. In the composite, KVPO₄F particles are homogeneously wrapped by a layer of amorphous carbon and bridged by cross-linked large area carbon sheets. As the cathode for PIBs, the KVPO₄F@3DC composite exhibits a high average operating voltage about 4.10 V with a super-high discharge capacity of 102.96 mAh g^-1 at 20 mA g^-1. An excellent long cycle stability with a capacity retention of 85.4% over 550 cycles at 500 mA g^-1 is achieved. In addition, it maintains 83.6% of its initial capacity at 50 mA g^-1 after 100 cycles at 55 °C. The design of KVPO₄F@3DC with plum pudding structure provides facilitative electron conductive network and stable electrode/electrode interface for electrode, successfully innovating an ultra-stable and high-performance cathode material for potassium ion batteries.

Development of KPF6/KFSA Binary-Salt Solutions for Long-Life and High-Voltage K-Ion Batteries

A series of binary-salt electrolytes of KPF₆/KN(SO₂F)₂ (KFSA) in carbonate ester solvents have been developed for high-voltage K-ion batteries by clarifying the effect of salt ratio and different solvents on the physical properties of the electrolyte solutions and electrochemical performance of K-ion batteries. The KPF₆/KFSA carbonate ester solutions, such as KPF₆/KFSA ethylene carbonate (EC)/diethyl carbonate (DEC), exhibit higher ionic conductivity than single-salt KPF₆ one, and higher KFSA content results in higher ionic conductivity. The KPF₆-rich binary-salt electrolytes with KPF₆/KFSA ratios of ≥3 (mol/mol) provide enough oxidation stability and passivation against Al corrosion at 4.6 V over 100 h, ensuring reversible operation of a 4 V class positive electrode, K₂Mn[Fe(CN)₆] in half-cell. Graphite negative electrodes exhibit higher Coulombic efficiency and better rate performance in 0.75 mol kg^-1 K(PF₆)_0.9(FSA)_0.1/EC/DEC and 1 mol kg^-1 K(PF₆)_0.75(FSA)_0.25/EC/DEC electrolytes than those in the KPF₆ one. Surface analysis by hard X-ray photoelectron spectroscopy reveals that the decomposition product of N(SO₂F)₂^- anion contributes to stabilizing solid electrolyte interphase on a graphite electrode. From comparing different solvents of EC/DEC, EC/ethyl methyl carbonate, and EC/propylene carbonate (PC), the K₂Mn[Fe(CN)₆] electrode demonstrates the highest Coulombic efficiency in the EC/PC binary electrolyte, while graphite electrodes exhibit no significant difference. Based on the half-cell tests, we successfully achieve the 3.6 V class full cell of graphite|K(PF₆)_0.75(FSA)_0.25/EC/PC|K₂Mn[Fe(CN)₆] showing excellent cyclability over 500 cycles, which is far superior to that of the conventional KPF₆/EC/DEC electrolyte cell.

Strain engineering of two-dimensional multilayered heterostructures for beyond-lithium-based rechargeable batteries

Beyond-lithium-ion batteries are promising candidates for high-energy-density, low-cost and large-scale energy storage applications. However, the main challenge lies in the development of suitable electrode materials. Here, we demonstrate a new type of zero-strain cathode for reversible intercalation of beyond-Li+ ions (Na+, K+, Zn2+, Al3+) through interface strain engineering of a 2D multilayered VOPO4-graphene heterostructure. In-situ characterization and theoretical calculations reveal a reversible intercalation mechanism of cations in the 2D multilayered heterostructure with a negligible volume change. When applied as cathodes in K+-ion batteries, we achieve a high specific capacity of 160 mA h g-1 and a large energy density of ~570 W h kg-1, presenting the best reported performance to date. Moreover, the as-prepared 2D multilayered heterostructure can also be extended as cathodes for high-performance Na+, Zn2+, and Al3+-ion batteries. This work heralds a promising strategy to utilize strain engineering of 2D materials for advanced energy storage applications.

Nano-size porous carbon spheres as a high-capacity anode with high initial coulombic efficiency for potassium-ion batteries

Hard carbon materials have been recognized as a promising family of anode materials for potassium ion batteries (PIBs), but their practical application is severely hindered due to the inferior initial coulombic efficiency (ICE) and low capacity. Herein, we report our findings in simultaneously improved potassium storage capacity and ICE through the design of nano-size and porous structure and the appropriate selection of electrolytes. Benefiting from the high specific surface area, stable electrode|electrolyte interface, and fast potassium ion and electron transfer, the optimized electrode exhibits a high ICE of up to 68.2% and an outstanding reversible capacity of 232.6 mA h g^-1 at 200 mA g^-1. In particular, superior cycling stability of 165.2 mA h g^-1 at 1000 mA g^-1 and 129.7 mA h g^-1 at 2000 mA g^-1 can be retained after 1500 cycles, respectively. Quantitative analysis reveals that this optimized structure leads to an enhanced surface-controlled contribution, resulting in fast potassiation kinetics and electronic|ionic conductivities, which are regarded as essential features for potassium storage. Our findings in this work provide an efficient strategy to significantly improve potassium storage capacity while maintaining a high ICE for hard carbon electrodes.

Two-Dimensional Transition Metal Chalcogenides for Alkali Metal Ions Storage

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

Titanium-based potassium-ion battery positive electrode with extraordinarily high redox potential

The rapid progress in mass-market applications of metal-ion batteries intensifies the development of economically feasible electrode materials based on earth-abundant elements. Here, we report on a record-breaking titanium-based positive electrode material, KTiPO4F, exhibiting a superior electrode potential of 3.6 V in a potassium-ion cell, which is extraordinarily high for titanium redox transitions. We hypothesize that such an unexpectedly major boost of the electrode potential benefits from the synergy of the cumulative inductive effect of two anions and charge/vacancy ordering. Carbon-coated electrode materials display no capacity fading when cycled at 5C rate for 100 cycles, which coupled with extremely low energy barriers for potassium-ion migration of 0.2 eV anticipates high-power applications. Our contribution shows that the titanium redox activity traditionally considered as "reducing" can be upshifted to near-4V electrode potentials thus providing a playground to design sustainable and cost-effective titanium-containing positive electrode materials with promising electrochemical characteristics.

A fluoroxalate cathode material for potassium-ion batteries with ultra-long cyclability

Potassium-ion batteries are a compelling technology for large scale energy storage due to their low-cost and good rate performance. However, the development of potassium-ion batteries remains in its infancy, mainly hindered by the lack of suitable cathode materials. Here we show that a previously known frustrated magnet, KFeC2O4F, could serve as a stable cathode for potassium ion storage, delivering a discharge capacity of ~112 mAh g-1 at 0.2 A g-1 and 94% capacity retention after 2000 cycles. The unprecedented cycling stability is attributed to the rigid framework and the presence of three channels that allow for minimized volume fluctuation when Fe2+/Fe3+ redox reaction occurs. Further, pairing this KFeC2O4F cathode with a soft carbon anode yields a potassium-ion full cell with an energy density of ~235 Wh kg-1, impressive rate performance and negligible capacity decay within 200 cycles. This work sheds light on the development of low-cost and high-performance K-based energy storage devices.

An amalgam route to stabilize potassium metal anodes over a wide temperature range

Potassium metal anodes are very appealing due to their high theoretical capacity (687 mA h g^-1), but violent reactivity and severe dendrite issues hinder their practical application. Here we report the in situ fabrication of an amalgam layer to stabilize potassium anodes over a wide temperature range of -20-40 °C.

A Flexible Potassium-Ion Hybrid Capacitor with Superior Rate Performance and Long Cycling Life

Potassium-ion batteries are promising candidates for large-scale energy storage applications owing to their merits of abundant resources, low cost, and high working voltage. However, the unsatisfying rate performance and cycling stability caused by sluggish K⁺ diffusion kinetics and dramatic volume expansion hinder the development of potassium-ion batteries. In this study, we design a flexible potassium-ion hybrid capacitor (PIHC) by combining the K-Sn alloying mechanism on the Sn anode and the fast capacitive behavior on the AC cathode with high surface area and mesoporous structure. After optimization, the fabricated Sn||AC PIHC achieves both a high energy density of 120 W h kg^-1 and high power density of 2850 W kg^-1, much better than other similar hybrid devices. Moreover, a gel polymer electrolyte with a 3D porous structure and high ionic conductivity was employed to improve the structural stability of the Sn anode, which not only realizes good flexibility but also achieves long cycling stability with a capacity retention of nearly 100% for 2000 cycles at a high current density of 3.0 A g^-1.

Research Development on K-Ion Batteries

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

Influence of sodium content on the thermal behavior of low vacancy Prussian white cathode material

Rechargeable sodium-ion batteries are the most attractive substitutes for lithium-ion batteries in large-scale energy storage devices due to wide spread reserves and low-cost of sodium resources and the similarities between sodium and lithium chemistry. However, finding a suitable cathode material is still a hurdle to be overcome. To date, Prussian white (PW), NaxFe[Fe(CN)6]y·nH2O has stood out as one of the most promising Na-host materials due to its low cost, facile synthesis and competitive electrochemical capacity. Despite this, there are concerns that this material will thermally decompose at relatively low temperatures to form cyanogen gas, which is a safety hazard. Thus, low vacancy NaxFe[Fe(CN)6]y·nH2O (x = 1.5, 1, 0.5 and 0) has been synthesized, and the influence of x on its thermal behavior systematically investigated. It is demonstrated that the thermal decomposition temperature, water content and moisture sensitivity of the samples strongly depend on the sodium content. The sample with x = 1.5 is found to be the most thermally stable and has the highest water content under the same experimental conditions. In addition, the sodium-rich samples (x = 1.5, 1 and 0.5) have higher surface water than the sodium-deficient one (x = 0). The local structure for this sample is also very different to the sodium-rich ones. Our findings offer new insights into the profound implications of proper material handling and safer operating conditions for practical Na-ion batteries and may be extended to analogous systems.

A highly-effective nitrogen-doped porous carbon sponge electrode for advanced K–Se batteries

A rechargeable K–Se battery is emerging as an energy storage system because of its much higher specific capacity than that of the traditional alkali metal-ion batteries, but is facing some critical issues and challenges.