Cathode Material For Potassium-ion Batteries Research Articles

Hexagonal δ-MnO2 nanoplates as efficient cathode material for potassium-ion batteries

Searching appropriate cathode electrode materials for potential energy storage applications with relatively big sized potassium (K) ions is particularly difficult. Herein, δ-MnO2 with hierarchical hexagonal nanoplate structure has been synthesized to combine together to form the material with vast space for large ions to transport and store. The battery performance as cathode material associated with K-ions, investigated by constant charge-discharge studies, reveals that the MnO2 electrode exhibits an initial discharge capacity of 154 mAh/g at current rate of C/20 (15.4 mA/g) versus potassium in 1 M KPF6 in EC:DEC electrolyte. As expected, the rate performances of MnO2 in 1 M KPF6 in EC:DEC electrolyte (108 mAh/g) are better than that of 1 M KPF6 in PC electrolyte (93 mAh/g) at same rate of C/20. Not only δ-MnO2 delivers stable capacity up to 200 cycles at C/10 rate (30.8 mA/g) but coulombic efficiency of nearly 100% is sustained for repetitive cycling in 1 M KPF6 in EC:DEC electrolyte. Overall, MnO2 exhibits improved electrochemical properties for K-ion batteries by improving charge storage mechanism associated with structural integrity, establishing MnO2 as a potential candidate for commercialization in future.

Suppressing Jahn-Teller distortion and phase transition of K0.5MnO2 by K-site Mg substitution for potassium-ion batteries

In the development of cathode materials for potassium-ion batteries, K0.5MnO2 cathode has shown great potential due to its high capacity and high energy density. However, it suffers from poor cycle stability due to the strong Jahn-Teller effect of Mn3+ and structure degradation. Here, by introducing Mg into the potassium layer, we design a new layered P3-type K0.5Mg0.15[Mn0.8Mg0.05]O2 cathode material. By using X-ray diffraction and absorption techniques, it is revealed that Mg ions in potassium layer can not only suppress the Jahn-Teller effect and prevent phase transition effectively, but also enhance the kinetics and thermodynamic properties of the cathode materials, thus greatly improve the cycle stability and rate capability. The strategy may open a new opportunity to design layered cathode materials for high performance potassium-ion batteries.

High-Quality Prussian Blue Analogues K2zn3[Fe(Cn)6]2 Crystals as a Stable and High Rate Cathode Material for Potassium-Ion Batteries

High-Quality Prussian Blue Analogues K2zn3[Fe(Cn)6]2 Crystals as a Stable and High Rate Cathode Material for Potassium-Ion Batteries

Facile synthesis of KVPO4F/reduced graphene oxide hybrid as a high-performance cathode material for potassium-ion batteries

Potassium-ion batteries (PIBs) as a substitute for lithium-ion batteries have aroused widespread attention and have been rapidly developed. In the positive electrode materials, polyanionic compound has a high working voltage and large reversible capacity on account of its distinct framework and the strong inducing effect of the anionic group. Herein, a KVPO4F/reduced graphene oxide (KVPF/rGO) hybrid was fabricated via a simple multi-step approach as the polyanionic cathode material for PIBs. Profiting from the small size of KVPF nanoparticles and their uniform distribution in the rGO framework, the as-synthesized KVPF/rGO hybrid manifests a large discharge capacity of 103.2 mAh g−1 with an outstanding energy density of 436.5 Wh kg−1. Through rGO decoration, the hybrid also demonstrates remarkable rate and cycling properties. By employing ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques, the potassium storage mechanism of KVPF was clearly revealed. The facile preparation procedure and superior properties endow it great application prospects in large-scale energy storage.

Coexistence of two coordinated states contributing to high-voltage and long-life Prussian blue cathode for potassium ion battery

Prussian blue analogues (PBAs) are promising cathode materials for potassium-ion batteries (PIBs) due to the large interstitial voids to accommodate K+ with large size. Mn-based PBA (MnHCF) shows high redox potential but considerable capacity degradation. By contrast, Fe-based PBA (FeHCF) exhibits good cyclic stability but relatively low redox potential. The different electrochemical performance is mainly due to different coordinated states, i.e., Fe-C≡N-Fe in FeHCF and Fe-C≡N-Mn in MnHCF. In this work, those two coordinated states are incorporated to construct Fe/Mn coexistent PBA (FeMnHCF), which shows not only high average discharge voltage (3.82 V vs. K+/K), but also excellent cyclic stability (the capacity can maintain 90 mAh g−1 after 600 cycles) in commonly used ester electrolyte. The results demonstrate that the coexistent coordinated structure not only improves the redox potential of N-coordinated Fe, but also increases the capacity contribution at high voltage (above 3.5 V). Furthermore, the coexistent structure can effectively inhibit the dissolution of transition metal (TM) elements and maintain structural integrity during K+ insertion/extraction, contributing to excellent cyclic stability. This work provides a promising strategy to design stable high-voltage PBA cathode materials.

Layered K0.54Mn0.78Mg0.22O2 as a high-performance cathode material for potassium-ion batteries

Layered K0.54Mn0.78Mg0.22O2 as a high-performance cathode material for potassium-ion batteries

Suppressing the interlayer-gliding of layered P3-type K0.5Mn0.7Co0.2Fe0.1O2 cathode materials on electrochemical potassium-ion storage

In recent years, potassium-ion batteries (KIBs) have emerged as a promising alternative candidate to replace lithium-ion batteries for large-scale energy storage devices owing to the natural abundance of potassium and similar mechanism as lithium-ion batteries. In particular, transition metal oxide cathode materials have attracted growing attention due to their high theoretical capacities and low cost compared with other cathode materials. Nevertheless, due to the larger ionic radius of K-ions, transition metal oxide cathode materials suffer from irreversible structural evolution and interlayer-gliding of transition metal layers in potassiation/depotassiation, which results in sluggish kinetics and structural instability. This limited capacity and unsatisfactory cycling properties inhibit the practical application of potassium-ion batteries. It still remains a challenge to develop the suitable cathode materials for potassium-ion batteries. In this work, the interlayer-gliding and irreversible P3–O3 structure transition were suppressed via the replacement of cobalt and iron, and the doping mechanism was investigated by in situ x-ray diffraction. The incorporation of Co ions and Fe ions enlarges the d-space between the transition metal layers, reduces the resistance of K+ migration, and provides the buffer spaces to suppress the interlayer-gliding and P3–O3 phase transformation in electrochemical potassium-ion storage, leading to an enhanced rate capability (58 mA h g−1 at 1 A g−1) and superior cycling stability (71% after 300 cycles at 200 mA g−1). This strategy provides a better understanding for the effect of Co–Fe substitution in suppressing interlayer-gliding and improving electrochemical properties for the development of a novel cathode material for potassium-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.

Spray drying derived wrinkled pea-shaped carbon-matrixed KVP2O7 as a cathode material for potassium-ion batteries

Searching for advanced cathode materials is indispensable for the eventual practical applications of potassium-ion batteries (KIBs). Herein, we design and synthesize a carbon-matrixed potassium vanadium pyrophosphate powder (KVP2O7@C) with a wrinkled pea-shaped particle morphology via a spray drying process. As a cathode material for KIBs, this KVP2O7@C exhibits a reversible capacity of 60 mA h g−1 with two discharge plateaus at 4.1 V and 4.5 V (vs. K+/K), leading to a high energy density of 250 W h kg−1 at 0.2 C in half cells. The potassium storage mechanism of KVP2O7 is revealed by ex-situ X-ray diffraction technology as an intercalation/de-intercalation reaction during electrochemical cycling, i.e. KVP2O7 (P21/c + P1¯) ↔ KxVP2O7 (x ≈ 0.4, P1¯). A full cell constructed by coupling KVP2O7@C cathode with a hard carbon anode can achieve a high operating voltage (3.55 V), high energy density (184 W h kg−1) and superior cycling stability (89% capacity retention after 400 cycles).

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

AbstractMost 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 K1.84Ni[Fe(CN)6]0.88⋅0.49 H2O (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.

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 K1.84 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.

A New Candidate in Polyanionic Compounds for a Potassium-Ion Battery Cathode: KTiOPO4

First-principles computations were performed to investigate the performance of KTiOPO4 (KTP) as a cathode material for potassium-ion batteries (PIBs), including the stability and electronic properties of depotassiated structures and mechanisms of K deintercalation and diffusion. As depotassiation proceeds, oxygen hole polarons are produced, and there are not peroxides or superoxides formed after deep depotassiation. The anionic oxygen redox in KTP provides a voltage vs K/K+ over 4 V by the PBE+U method and over 5 V with the more reliable HSE06 hybrid functional. When all K in KTP is removed, the calculated volume compression is only 1.528%. The AIMD simulations at 300 K for TiOPO4 verify its thermal stability. The PBE+U calculations predict a low ion diffusion barrier of 0.29 eV in bulk KTP, indicating a good charge-discharge rate for KTP as a cathode for PIBs. All of the calculated results indicate that KTP can be a promising cathode material for PIBs.

Yolk–Shell P3‐Type K0.5[Mn0.85Ni0.1Co0.05]O2: A Low‐Cost Cathode for Potassium‐Ion Batteries

Low‐cost preparation methods for cathodes with high capacity and long cycle life are crucial for commercializing potassium‐ion batteries (PIBs). Presently, the charging/discharging strain that develops in the active cathode material of PIBs causes cracks in the particles, leading to a sharp capacity fade. Here, to abate the strain release and the need for an industrially relevant process, a simple low‐cost co‐precipitation method for synthesizing yolk–shell P3‐type K0.5[Mn0.85Ni0.1Co0.05]O2 (YS‐KMNC) was reported. As cathode material for PIBs, the YS‐KMNC delivers a high reversible capacity (96 mAh g–1 at 20 mA g–1) and excellent cycle stability (80.5% retention over 400 cycles at a high current density of 200 mA g–1). More importantly, a full battery assembled with the YS‐KMNC cathode and a commercial graphite anode exhibits a high operating voltage (0.5‐3.4 V) and an excellent cycling performance (84.2% retention for 100 cycles at 100 mA g–1). Considering the low‐cost, simple production process and high performance of YS‐KMNC cathode, this work could pave the way for the commercial development of PIBs.

Synthesis and Characterization of a Novel Hydrated Layered Vanadium(III) Phosphate Phase K3V3(PO4)4·H2O: A Functional Cathode Material for Potassium-Ion Batteries.

Hydrated vanadium(III) phosphate, K3V3(PO4)4·H2O, has been synthesized by a facile aqueous hydrothermal reaction. The crystal structure of the compound is determined using X-ray diffraction (XRD) analysis aided by density functional theory (DFT) computational investigation. The structure contains layers of corner-sharing VO6 octahedra connected by corner and edge-sharing PO4 tetrahedra with a hydrated K+ ion interlayer. The unit cell is assigned to the orthorhombic system (space group Pnna) with a = 10.7161(4) Å, b = 20.8498(10) Å, and c = 6.5316(2) Å. Earlier studies of this material report a K3V2(PO4)3 stoichiometry with a NASICON structure (space group R3®c). Previously reported XRD and electrochemical data on K3V2(PO4)3 are critically evaluated and we suggest that they display mixed phase compositions of K3V3(PO4)4·H2O and known electrochemically active phases KVP2O7 and K3V(PO4)2. In the present study, the synthesis conditions, structural parameters, and electrochemical properties (vs K/K+) of K3V3(PO4)4·H2O are clarified along with further physical characterization by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray fluorescence (XRF), Raman spectroscopy, Fourier transform infrared (FT-IR), and thermogravimetric analysis (TGA).

Understanding the electrochemical properties and phase transformations of layered VOPO4⋅H2O as a potassium-ion battery cathode

Layered oxovanadium phosphate hydrate (VOPO4·xH2O) is considered to be a promising cathode material for potassium-ion batteries. However, its correlation between electrochemical properties and structural changes during electrochemical processes still need further exploration. Herein, monohydrate VOPO4·H2O is used as an alkali ion (Li+, Na+, K+) host material that gives superior electrochemical performances in potassium-ion batteries. It has a high discharge capacity of 90 mA h g−1 with a high average potential of 3.5 V and a lifespan of over 650 cycles with 61% retention at 1C rate. Three phase transitions during the potassiation/depotassiation process are observed: KxVOPO4 (0 < x < 0.12, d = 7.06 Å) → αI-KxVOPO4 (0.12 < x < 0.48, d = 6.73 Å) → αII-KxVOPO4 (0.30 < x < 0.55, d = 6.55 Å). The small structural change (≤6% on the c-axis) and the full reversibility of the structural transformation ensure the stable cycling; the large interlayer distance allows a fast K-ion diffusion (10−9–10−11 cm2 s−1). This study provides a comprehensive understanding on electrochemical performances and structural evolutions of VOPO4·H2O. Meanwhile, the small volume variation, high operating potential, and excellent cycling stability of VOPO4·H2O can attract more attention to develop other layered materials beyond transition metal oxides.

High-power rhombohedral-Fe2(SO4)3 with outstanding cycle-performance as Fe-based cathode for K-ion batteries

We report rhombohedral-Fe2(SO4)3 as a promising cathode material for potassium-ion batteries. Detailed structure information for rhombohedral-Fe2(SO4)3 was obtained using X-ray diffraction with Rietveld refinement. Based on the structural information, possible atomic sites for K ions and ion diffusion pathway in the structure were determined using bond-valence energy landscape analyses. At C/20 (1C = 112 mA g−1), rhombohedral-KxFe2(SO4)3 delivered a specific capacity of ~100 mAh g−1 an average operation voltage of ~3.3 V (vs. K+/K), corresponding to reversible de/intercalation of ~1.78 mol K+ ions. Moreover, even at 5C, rhombohedral-KxFe2(SO4)3 exhibited capacity retention of ~80% of the capacity measured at C/20, indicating the outstanding power-capability. After 300 cycles at 2C, rhombohedral-KxFe2(SO4)3 maintained ~83% of its initial specific capacity with high Coulombic efficiency of more than 99%. Operando XRD analyses revealed that the XRD peaks of rhombohedral KxFe2(SO4)3 monotonously shifted during K+ de/intercalation, indicating a single phase reaction. First-principles calculation confirmed the good agreement between the theoretical properties of rhombohedral-KxFe2(SO4)3 and the experimental results, including the average operation voltage, power-capability, and phase reaction.

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 K1.90Ni0.5Fe0.5[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 FeII/FeIII 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.

Designing advanced P3-type K0.45Ni0.1Co0.1Mn0.8O2 and improving electrochemical performance via Al/Mg doping as a new cathode Material for potassium-ion batteries

Developing stable and advanced cathode materials that accommodate larger K-ions becomes a serious challenge for potassium-ion batteries (KIBs). Herein, we propose a new layered oxide K0.45Ni0.1Co0.1Mn0.8O2 (pristine) without phase transition during the cycling process as a cathode material for KIBs. The pristine material is further doped with Mg2+ and Al3+ (K0.45Ni0.1Co0.1Mg/Al0.05Mn0.75O2) into Mn site to pursue better electrochemical performances. A detailed insight into the structural stability, charge compensation mechanism, and K+ kinetics study is clearly provided. The Mg/Al-doped electrodes exhibit outstanding cycling performance and rate capability because of the enhanced layered structure stability, enlarged K+ diffusion layer and optimized Jahn-Taller distortion of Mn3+. Furthermore, the Mg-doped electrode shows better effect in the suppression of Jahn-Taller distortion of Mn3+ by comparison with that of Al-doped electrode, but at the expanse of lower capacity as the decreased content of active redox of Mn3+/Mn4+.This research provides new insight into the future design of advanced cathode materials for KIBs.

Layered P3 type K0.48Ni0.2Co0.2Mn0.6O2 with microspherical and microcubic mixed morphology as a cathode material for potassium-ion batteries

Potassium-ion batteries (KIBs) have been recognized as one promising type of new energy storage technology. Exploring and developing novel and effective cathode materials has become the hotspot research area. Herein, a layered K0.48Ni0.2Co0.2Mn0.6O2 (KNCMO) with a mixed morphology of microspheres and microcubes was prepared. When evaluated for use in KIBs, the KNCMO exhibited outstanding electrochemical properties. In practical terms, KNCMO can deliver a reversible capacity of ~57 mAh g−1 at 40 mA g−1, and shows good rate performance. When the specific current was increased to 400 mA g−1, a considerable reversible capacity of ~35 mAh g−1 after 350 cycles can be obtained. This work can serve as a broader insight into the future development of cathode materials for potassium-ion batteries.

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.