Potassium Manganese Hexacyanoferrate Research Articles

Cesium-doped manganese-based Prussian blue analogue as a high-efficiency cathode material for potassium-ion batteries

Prussian blue analogues (PBAs) are regarded as promising cathode materials for potassium-ion batteries (PIBs) owing to their low cost and high reversible capacity. Compared to other PBAs, potassium manganese hexacyanoferrate (KMnHCF) stands out for its superior capacity and operating voltage. However, Jahn-Teller effect of Mn3+ and the structural collapse caused by potassium ion insertion/extraction still affect the structural stability and electrochemical performance of this material. Herein, a green and efficient synthesis method is adopted to substitute potassium ions in KMnHCF with an appropriate amount of cesium ions to form a column effect. Cesium-doped KMnHCF (Cs-KMnHCF) mitigates the irreversible structural damage caused by potassiation/depotassiation and the Jahn-Teller effect, thereby improving the cycling stability. In addition, it widens the lattice channels, reduces the diffusion barrier of potassium ions, and optimizes the diffusion kinetics. By rationally controlling the doping amount of Cs+, the obtained K1.71Cs0.05Mn[Fe(CN)6]0.95·0.05·0.88H2O exhibits remarkable electrochemical performance, with an initial discharge capacity of 137.6 mA h g−1 at a current density of 20 mA g−1 and a capacity retention of 89.6% after 600 cycles at 200 mA g−1. More importantly, when assembled with a pitch-derived soft carbon anode, the full cell manifests excellent cycle stability and rate performance. This work is expected to provide a highly efficient cathode material for the practical application of PIBs.

Gel biopolymer electrolyte for high-voltage, durable, and flexible Zn/K dual-ion pouch cells

Zinc-ion rechargeable battery (ZIRB) is a promising candidate for future energy storage systems due to its cost-effectiveness, operational safety, and environmental friendliness. However, the commercial-scale application of ZIRB is hindered by its insufficient energy efficiency, poor rate capability, and inferior cycling stability. Here, we introduce a gel biopolymer electrolyte (GBE) empowered Zn/K dual-ion battery with exceptional cycling and rate performance. Calling for sustainability, this cellulose-encapsulated bisalt electrolyte presents strong mechanical robustness that inhibits Zn dendrite growth and promotes anodic plating/stripping reversibility. Sandwiching the GBE between a potassium manganese hexacyanoferrate (KMnHCF) cathode and a Zn anode, the coin-cell configuration of KMnHCF|GBE|Zn demonstrates a high voltage discharge plateau of 1.80 V, an initial capacity of up to 65 mAh g−1, and excellent cycling stability with capacity retention of 92 % after 1,000 cycles. Furthermore, the superior durability, flexibility, safety, and endurance with extreme conditions (e.g., bending, puncturing, and cutting) demonstrated by KMnHCF|GBE|Zn pouch-cell enables its future application in wearable electronics.

Interior-Confined Vacancy in Potassium Manganese Hexacyanoferrate for Ultra-Stable Potassium-Ion Batteries.

Metal hexacyanoferrates (HCFs) are viewed as promising cathode materials for potassium-ion batteries (PIBs) because of their high theoretical capacities and redox potentials. However, the development of an HCF cathode with high cycling stability and voltage retention is still impeded by the unavoidable Fe(CN)6 vacancies (VFeCN) and H2O in the materials. Here, a repair method is proposed that significantly reduces the VFeCN content in potassium manganese hexacyanoferrate (KMHCF) enabled by the reducibility of sodium citrate and removal of ligand H2O at high temperature (KMHCF-H). The KMHCF-H obtained at 90 °C contains only 2% VFeCN, and the VFeCN is concentrated in the lattice interior. Such an integrated Fe-CN-Mn surface structure of the KMHCF-H cathode with repaired surface VFeCN allows preferential decomposition of potassium bis(fluorosulfonyl)imide (KFSI) in the electrolyte, which constitutes a dense anion-dominated cathode electrolyte interphase (CEI) , inhibiting effectively Mn dissolution into the electrolyte. Consequently, the KMHCF-H cathode exhibits excellent cycling performance for both half-cell (95.2 % at 0.2 Ag-1 after 2000 cycles) and full-cell (99.4 % at 0.1 Ag-1 after 200 cycles). This thermal repair method enables scalable preparation of KMHCF with a low content of vacancies, holding substantial promise for practical applications of PIBs.

The Role of Transition Metals on Chemo‐Mechanical Instabilities in Prussian Blue Analogues For K‐Ion Batteries: The Case Study on KNHCF Versus KMHCF

AbstractPrussian blue analogues (PBAs) cathodes can host diverse monovalent and multivalent metal ions due to their tunable structure. However, their electrochemical performance suffers from poor cycle life associated with chemo‐mechanical instabilities. This study investigates the driving forces behind chemo‐mechanical instabilities in Ni‐ and Mn‐based PBAs cathodes for K‐ion batteries by combining electrochemical analysis, digital image correlation, and spectroscopy techniques. Capacity retention in Ni‐based PBA is 96% whereas it is 91.5% for Mn‐based PBA after 100 cycles at C/5 rate. During charge, the potassium nickel hexacyanoferrate (KNHCF) electrode experiences a positive strain generation whereas the potassium manganese hexacyanoferrate (KMHCF) electrode undergoes initially positive strain generation followed by a reduction in strains at a higher state of charge. Overall, both cathodes undergo similar reversible electrochemical strains in each charge–discharge cycle. There is ~0.80% irreversible strain generation in both cathodes after 5 cycles. XPS studies indicated richer organic layer compounds in the cathode‐electrolyte interface (CEI) layer formed on KMHCF cathodes compared to the KNHCF ones. Faster capacity fades in Mn‐based PBA, compared to Ni‐based ones, is attributed to the formation of richer organic compounds in CEI layers, rather than mechanical deformations. Understanding the driving forces behind instabilities provides a guideline to develop material‐based strategies for better electrochemical performance.

Tailoring Interface to Boost the High‐Performance Aqueous Al Ion Batteries

AbstractElectrolytes are highly important for aqueous Al ion batteries (AAIBs). However, Al metal anode usually has poor reversibility of plating/tripping, resulting in low Coulombic efficiency (CE) and poor cycling stability by using traditional Al ion electrolyte. Herein, a novel type of aqueous Al ion electrolyte with polyethylene glycol (PEG) as the primary skeleton for solvated Al ions is proposed, named as PEG‐Al@H. Outstandingly, an Al electrolyte interface (AEI) is generated on the Al metal surface during galvanostatic charging process by polymerization of PEG. It is proved that AEI, acting as the protective layer, effectively mitigates the side reaction caused by the rapid kinetic in aqueous electrolyte and prevents Al metal anode from deeply corroding. Moreover, PEG disrupts the hydrogen bond of the solvent in the electrolyte, extending the working temperature of AAIBs. The design that a full‐cell consisting of PEG‐Al@H as the electrolyte, potassium manganese hexacyanoferrate as the cathode, Al metal as anode can run over 20 000 cycles at 500 mA g−1, with an average CE higher than 95%. More importantly, even at −5 °C, for the first time, an initial capacity of 16 mAh in a pouch‐cell is achieved and maintain roughly 87% of capacity after 5500 cycles.

Ultra‐Thin Single‐Particle‐Layer Sodium Beta‐Alumina‐Based Composite Polymer Electrolyte Membrane for Sodium‐Metal Batteries

AbstractInorganic/organic composite polymer electrolytes (CPEs) with good flexibility and electrode contact have been pursued for solid−state sodium‐metal batteries. However, the application of CPEs for high energy density solid−state sodium‐metal batteries is still limited by the low Na+ conductivity, large thickness, and low ion transference number. Herein, an ultra‐thin single‐particle‐layer (UTSPL) composite polymer electrolyte membrane with a thickness of ≈20 µm straddled by a sodium beta−alumina ceramic electrolyte (SBACE) is presented. A ceramic Na+‐ion electrolyte that bridges or percolates across an ultra‐thin and flexible polymer membrane provides: 1) the strength and flexibility from the polymer membrane, 2) excellent electrolyte/electrode interfacial contact, and 3) a percolation path for Na+‐ion transfer. Owing to this novel design, the obtained UTSPL‐35SBACE membrane exhibits a high Na+‐ion conductivity of 0.19 mS cm−1 and a transference number of 0.91 at room temperature, contributing to long−term cycling stability of symmetric sodium cells with a small overpotential. The assembled quasi‐solid‐state cell with the as−prepared UTSPL‐35SBACE membrane displays superior cycling performance with a discharge capacity of 105 mAh g−1 at 0.5 °C rate after 100 cycles and excellent rate performance (82 mAh g−1 at 5 °C rate) at room temperature with the potassium manganese hexacyanoferrate (KMHCF)@CNTs/CNFs cathode, where KMHCF refers to potassium manganese hexacyanoferrate.

Constructing Nonaqueous Rechargeable Zinc-Ion Batteries with Zinc Trifluoroacetate

Rechargeable zinc-ion batteries (RZIBs) are recognized as promising alternative energy storage techniques for large-scale application. Using organic electrolytes with wide electrochemical stability windows to construct nonaqueous RZIBs enables high operating voltage and energy density. However, the selection of zinc salts with cost-effectiveness and excellent electrochemical performance for nonaqueous electrolytes is limited. In this work, the electrochemical properties of a competitive zinc salt, zinc trifluoroacetate (Zn(TFA)2), are investigated in acetonitrile and triethyl phosphate. It is found that the Zn(TFA)2 is highly soluble in the organic solvents, reaching 15 mol·kg–1 in acetonitrile. The Zn(TFA)2 electrolytes show a wide electrochemical stability window of 2.2 V, good conductivity of 6.14 mS·cm–1 at 1.0 mol·kg–1, and good compatibility with the Zn metal anode. Zn(TFA)2 tends to form ion pairs or aggregates in triethyl phosphate, which improve the electrochemical stability but partially expense the ionic conductivity. Full batteries are fabricated with the Zn(TFA)2 electrolytes using potassium manganese hexacyanoferrate as the cathode. The batteries with Zn(TFA)2-AN-TEP (19%) electrolyte deliver a high specific density of 181.9 Wh·kg–1 at 0.1 A·g–1 and excellent cyclic stability with a capacity retention rate of 71.7% after 1000 cycles at 0.5 A·g–1. Our results suggest that the Zn(TFA)2 is a promising candidate of zinc salts in the application of nonaqueous RZIBs.

High-cycling Stability of Zn/MnHCF Batteries Boosted by Non-aqueous Ethanol Electrolyte

Potassium manganese hexacyanoferrate (MnHCF) is a promising cathode material in aqueous zinc-ion batteries (ZIBs) due to its high capacity and operating potential. This study improves its electrochemical performance by using ethanol (EA) as the electrolyte solvent, which can effectively inhibit the dissolution of Mn ions and crystalline phase transformation of MnHCF. This study is of great significance for improving the cycling stability of ZIBs.

(Digital Presentation) Prussian Blue Analogs – a Wide Variety of Promising Cathode Materials with Peculiar Electrochemical Properties

Today, post-lithium energy storage technologies are now rapidly progressing due to the high price of a net Li-ion battery, which also depends on the desired capacity and power. Among sodium, potassium, calcium, magnesium, and even aluminum-based alternatives, young potassium-ion batteries demonstrate high capacity and energy density, notable ionic transport in electrolytes, the possibility to employ graphite anodes, and a wide variety of possible electrode materials: layered oxides, polyanionic, organic compounds, Prussian Blue analogs. However, the latter ones are generally considered as the most promising and practically viable.Prussian Blue analogs form a big family of electrode materials with the general formula KxM1[M2(CN)6]∙nH2O, where x=0...2, and Mi are any possible 3d transition metals. The most well-known and commercially available is based on a hexacyanoferrate anion [Fe(CN)6]n-, while other transition metals can also form hexacyanometallate complexes, but are poorly studied or not known at all. The most part of published works shed light on the morphology and low content of water defects inside crystallites counting the lack of hexacyanoferrate, and their influence on realized capacity and capacity fades, while the fundamental principles and real water position presence which guide the electrochemical activity of high- and low-spin cations in these materials are totally missed.In our work, we also started with the intrinsic water defects and their impact on crystal and physicochemical properties in K2Mn[Fe(CN)6]∙nH2O but with sensitive to light atoms neutron diffraction technique. We observed that water content does not effect the whole crystal symmetry but slightly amend unit cell parameters. Besides the fact of decreasing a decomposition temperature in “watered” Prussian Blue analog, electrochemical properties were found close. Therefore, we concluded that intrinsic water does not notably influence material properties. Continuing with potassium manganese hexacyanoferrate and electronic structure impact to the compound properties, to reveal the best synergetic stabilizing agent during cycling, we synthesized and studied the system K2-γMn1-xCox[Fe(CN)6]∙nH2O with x=0, 0.05,...1. In addition to symmetry and composition transformations, magnetic and electrochemical properties also significantly differ, while higher cobalt content increases the redox potential of iron, but drastically decreases total capacity due to the inability of reaching iron oxidation. The fact of notable changing of redox potentials in K2-γMn1-xCox[Fe(CN)6]∙nH2O is inspiring, and we have been extending with other hexacyanometallates. Hexacyanomanganate-ion [Mn(CN)6]n- is one of the promising pathways to investigate such systems with a more fundamental point of view, and the obtained experimental results confirm the hypothesis. We discovered that cation position exchange totally alters the electrochemistry of the Prussian Blue cathode materials, and the interpretation still raises a lot of questions and assumptions, and together with computational chemistry, we will try to answer the fundamental questions about electronic, crystal structures and electrochemical properties. In this report we will present the new correlations of redox potential in Prussian Blue analogs depending on transition metal position and electronic structure, evaluate diffusion of potassium in these materials, and try to answer the possibility to use these compounds in potassium-ion batteries.

Synergistic regulation of low-defects manganese hexacyanoferrates with stable electrode/electrolyte interface for enhancing electrochemical potassium storage performance

Potassium manganese hexacyanoferrate (K-MnHCF) has been considered as a promising cathode material for potassium-ion batteries (PIBs) on account of their high potential and capacity, but suffers from the poor cyclic performance. Herein, a synergistic strategy combining the defects-reducing of K-MnHCF together with the electrode/electrolyte interface-stabilizing is developed to boost the electrochemical potassium-ions storage performance. By adjusting the co-precipitation process, K-MnHCF displays the regular cubic particle with the size of 100–150 nm, exhibiting the reversable capacity of 95 mAh g−1 at 50 mA g−1 in KPF6 electrolyte system. Stabilizing the interface between K-MnHCF cathode material and electrolyte is further applied to enhance the cyclic stability and coulombic efficiency (97.6 %) of K-MnHCF cathode, by reducing the side reactions at the interface in high concentration KFSI electrolyte system. This work provides a strategy to boost the electrochemical potassium-ions storage performance of PIB cathode by synergistic stabilizing of the cathode material and the electrode/electrolyte interface.

Potassium manganese hexacyanoferrate with improved lifespan in Zn(CF3SO3)2 electrolyte for aqueous zinc-ion batteries

Potassium manganese hexacyanoferrate undergoes severe dissolution and Zn displacement in aqueous Zn-ion batteries. Its electrochemical performance is improved by tuning the anions in the electrolyte.

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.

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

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

Reactant Concentration and Aging-Time-Regulated Potassium Manganese Hexacyanoferrate as a Superior Cathode for Sodium-Ion Batteries

Reactant Concentration and Aging-Time-Regulated Potassium Manganese Hexacyanoferrate as a Superior Cathode for Sodium-Ion Batteries

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.

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 High-Voltage Potassium-Ion Battery with a Potassium Manganese Hexacyanoferrate Cathode

Potassium-ion batteries (KIBs) are a promising complementary technology to lithium-ion batteries that have the potential to be more sustainable and lower in cost. In KIBs lithium is replaced with potassium, the copper current collector is replaced with aluminium, and cathodes can be cobalt and nickel free. KIBs have the crucial advantage over sodium-ion batteries that potassium can reversibly intercalate into graphite whilst sodium cannot. Graphite offers a low operating voltage, high capacity, and good rate performance as a potassium-ion anode material.There remain significant scientific challenges in the development of commercial potassium-ion batteries, notably developing high-capacity high-voltage cathode materials and electrolytes that are compatible with high-voltage cathodes and graphite anodes. In a recent perspective we identified potassium manganese hexacyanoferrate (KMF) as the most promising cathode material as it offers high energy density whilst also being low in cost [1]. However, the cyclability and coulombic efficiency of KMF are typically poor owing to parasitic reactions from water remaining in its structure and aluminium current collector corrosion among other mechanisms.Here we present recent work to improve the performance of the KMF//graphite electrochemical system. Firstly, the citrate-assisted synthesis of KMF is optimised using high throughput synthesis and characterisation techniques to reduce the defect and water content whilst tailoring the particle size [2]. Secondly, an ionic-liquid based electrolyte, 1 M KFSI in Pyr1,3FSI (potassium bis(fluorosulfonyl)imide in N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide)), is employed. This electrolyte successfully forms a passivating layer on the KMF cathode and surpresses aluminium corrosion which enables highly efficient cycling. Additionally, graphite operates exceptionally well with this same ionic-liquid based electrolyte.We will end by offering our perspective on the current state of this research area and promising research directions.

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.

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.

Effect of Particle Size and Anion Vacancies on Electrochemical Performances of Potassium Manganese Hexacyanoferrate for Potassium-Ion Batteries

K-ion battery (KIB) has recently attracted much attention as a potential high-voltage and high-power battery owing to low standard electrode potential of K/K+ and fast ionic diffusion of K+ ion in electrolyte solutions, respectively.1 Among positive electrode materials reported so far, K x Mn[Fe(CN)6] y (KMnHCF) is a promising material because of a high working potential (ca. 3.8 V vs. K/K+).2 Although the theoretical capacity of stoichiometric K2Mn[Fe(CN)6] is as high as 155 mAh g–1, the reported reversible capacities range from 40 to 140 mAh g–1.1 Moreover, the reason for the different capacities is still unclear. Previous studies on K x Fe[Fe(CN)6] y proposed the potential two factors which may affect the electrochemical performance. These are particle size 3 and number of [Fe(CN)6] n - anion vacancies.4 However, the difficulty of independent controlling the particle size and number of anion vacancies has hindered understanding the impact of each factor. In this study, we investigated the effect of the factors on electrochemical performance by controlling the particle size and number of the anion vacancies via chelate assisted and ionic exchange synthesis, respectively.KMnHCFs were synthesized by direct precipitation with chelating agents or by ionic exchange from the NaMnHCF. Samples with small (S-KMnHCF) and large particle size (L-KMnHCF) precipitated in 0.2 M and 1 M potassium citrate solutions, respectively. Samples with a large number of vacancies (IE-KMnHCF) were obtained by the ionic exchange method. All the samples were carefully dehydrated at 200 °C under vacuum to ignore the effect of interstitial water. The samples were characterized by powder X-ray diffraction (XRD), inductively coupled plasma emission spectroscopy (ICP-AES), scanning electron microscopy (SEM), and galvanostatic charge/discharge measurements.Figure 1a shows the XRD patterns and their fitted pattern by Rietveld refinement of S-, L-, and IE-KMnHCFs. The diffraction patterns of all the samples were successfully fitted with the monoclinic structures (P21/n), and no crystalline impurities were confirmed. Rietveld refinement and ICP-AES revealed that the composition of S-, L-, and IE-KMnHCF were K1.9Mn[Fe(CN)6]1.0, K1.8Mn[Fe(CN)6]0.99□0.01 (□ = anion vacancy), and Na0.10K1.6Mn[Fe(CN)6]0.85□0.15, respectively. Thus, S- and L-KMnHCF have negligible numbers of [Fe(CN)6]4- vacancies, while IE-KMnHCF has many anion vacancies. SEM image of Figs. 1b–1d display that the particle size of S-KMnHCF was 100–200 nm, whereas the particle size of L- and IE-KMnHCF was approximately 1–2 μm.Figure 2 shows the initial charge/discharge curves for S-, L-, and IE-KMnHCF in K cells. S-KMnHCF delivered a large reversible capacity of 137 mAh g-1, whereas L-KMnHCF showed a small reversible capacity of 49 mAh g-1. In contrast, IE-KMnHCF exhibited a reversible capacity of 117 mAh g-1 despite the large particle size. The reason behind the relatively large capacity of IE-KMnHCF would be enhanced potassium ion diffusion caused by the [Fe(CN)6]4- vacancies.4 In the presentation, we will also discuss the impact of the anion vacancies on K+ ion diffusion barrier and structural changes during charge and discharge process. References T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev.,DOI: 10.1021/acs.chemrev.9b00463, in press. X. Bie, K. Kubota, T. Hosaka, K. Chihara and S. Komaba, J. Mater. Chem. A, 5, 4325 (2017). G. He and L. F. Nazar, ACS Energy Lett., 2, 1122 (2017). M. Ishizaki, H. Ando, N. Yamada, K. Tsumoto, K. Ono, H. Sutoh, T. Nakamura, Y. Nakao and M. Kurihara, J. Mater. Chem. A, 7, 4777 (2019). Figure 1