High-Performance Cathode Materials for Potassium-Ion Batteries: Structural Design and Electrochemical Properties

Multiple Electron Transfers Enable High-Capacity Cathode Through Stable Anionic Redox

Single-electron transfer, low alkali metal contents, and large-molecular masses limit the capacity of cathodes. This study uses a cost-effective and light-molecular-mass orthosilicate material, K2FeSiO4, with a high initial potassium content, as a cathode for potassium-ion batteries to enable the transfer of more than one electron. Despite the limited valence change of Fe ions during cycling, K2FeSiO4 can undergo multiple electron transfers via successive oxygen anionic redox reactions to generate a high reversible capacity. Although the formation of O‒O dimers in K2FeSiO4 occur upon removing large amounts of potassium, the strong binding effect of Si on O mitigates irreversible oxygen release and voltage degradation during cycling. K2FeSiO4 achieves 236 mAh g-1 at 50 mA g-1, with an energy density of 520 Wh kg-1, which can be comparable with commercial LiFePO4 materials. Moreover, it also exhibits 1400 stable cycles under high-current conditions. These findings enhance the potential commercialization prospects for potassium-ion batteries.

10 Years Development of Potassium-Ion Batteries

Potassium-ion batteries (PIBs), with abundant resources and low cost, are considered as a promising alternative to commercial lithium-ion batteries for low-cost and large-scale applications. Over the past decade, significant academic progresses are made in the development of PIBs, including advancements in cathodes, anodes, and electrolytes. However, most improvements are achieved under laboratory conditions (e.g., K metal-based half-cells and low mass loading of active materials), and the performance of PIBs in full cells is still far from the requirements for commercial applications. A critical insight bridging the academic research and the commercialization of PIBs is urgently needed to guide future research in this field. This review will discuss the challenges and improvement strategies in the development of PIBs, focusing on the potential practical cathodes, anodes, and electrolytes, as well as their performance in full cells. It aims to give the readers a clear and logical understanding of the development of PIBs. The application analysis is also discussed to provide a comprehensive understanding of the commercialization potential of PIBs. Finally, perspectives are provided for the future development of PIBs.

Realizing an Energy-Dense Potassium Metal Battery at -40 °C via an Integrated Anode-Free and Dual-Ion Strategy

Potassium (K)-based batteries hold great promise for cryogenic applications owing to the small Stokes radius and weak Lewis acidity of K+. Nevertheless, energy-dense (>200 W h kg-1cathode+anode) K batteries under subzero conditions have seldom been reported. Here, an over 400 W h kg-1cathode+anode K battery is realized at -40 °C via an anode-free and dual-ion strategy, surpassing these state-of-the-art K batteries and even most Li/Na batteries at low temperatures (LTs). By introduction of a strongly associating salt as an additive to this anode-free K battery, an anion-derived solid electrolyte interphase can be established for a highly reversible, zero-excess K plating/stripping behavior on a bare current collector. Meanwhile, a binary solvent is rationally designed for lowering the cation desolvation energy barrier, which ensures comparably facile cation and desolvation-free anion kinetics in this dual-ion structure even at LTs. Consequently, the K||Al half-cell delivers a high Coulombic efficiency of 99.98% at -40 °C. By pairing with a high-energy cathode, a proof-of-concept anode-free K dual-ion battery (N/P = 0) is fabricated, delivering a record-high energy density of 407 W h kg-1cathode+anode with stable cycling of 183 cycles (80% capacity retention) at -40 °C. This work paves a way toward energy-dense batteries at extreme scenarios.

Boosting K+-Ionic Conductivity of Layered Oxides via Regulating P2/P3 Heterogeneity and Reciprocity for Room-Temperature Quasi-Solid-State Potassium Metal Batteries

Solid-state potassium metal batteries are promising candidates for grid-scale energy storage due to their low cost, high energy density and inherent safety. However, solid state K-ion conductors struggle with poor ionic conductivity due to the large ionic radius of K+-ions. Herein, we report precise regulation of phase heterogeneity and reciprocity of the P2/P3-symbiosis K0.62Mg0.54Sb0.46O2 solid electrolyte (SE) for boosting a high ionic conductivity of 1.6×10-4 S cm-1 at 25 °C. The bulk ionic conducting mechanism is explored by elucidating the effect of atomic stacking mode within the layered framework on K+-ion migration barriers. For ion diffusion at grain boundaries, the P2/P3 biphasic symbiosis property assists in tunning the SE microstructure, which crystallizes in rod-like particles with lengths of tens of micrometers facilitating long-distance ion transport and significantly decreasing grain boundary resistance. Potassium metal symmetric cells using the modified SE deliver excellent cycling life over 300 h at 0.1 mA cm-2 and a high critical current density of 0.68 mA cm-2. The quasi-solid-state potassium metal batteries (QSSKBs) coupled with two kinds of layered oxide cathodes demonstrate remarkable stability over 300 cycles, outperforming the liquid electrolyte counterparts. The QSSKB system provides a promising strategy for high-efficiency, safe, and durable large-scale energy storage.

Sodium Vanadates for Metal-Ion Batteries: Recent Advances and Perspectives

Rechargeable metal-ion batteries (MIBs) play a pivotal role in advancing the stable supply of renewable energy by efficiently storing and discharging electrical energy. In recent years, to propel the continuous advancement of MIB technology, numerous studies have concentrated on exploring and innovating electrode materials, aiming to engineer commercial batteries with high energy density, superior power density, and extended cycle life. Notably, sodium vanadates have garnered significant attention in the realm of MIBs owing to their distinctive crystal structures, abundant resource reservoirs, and exceptional electrochemical properties. This paper provides a prompt and comprehensive review of the research landscape for various sodium vanadates (such as NaxV2O5, Na1+ xV3O8, Na2V6O16·xH2O, etc.) in the domain of MIBs over the past five years. It delves into the structural characteristics, electrochemical performances, and energy storage mechanisms of these materials, while also proposing some effective strategies to augment their electrochemical capabilities. Building upon these insights and prevailing research outcomes, this review envisions the future developmental pathways of sodium vanadates for MIBs and aims to reveal the vast potential of sodium vanadates in the emerging energy storage field and provide researchers with clear insights and perspectives for developing optimal sodium vanadate electrodes.

K-Rich Spinel Interface of Air-Stable Layered Oxide Cathodes for Potassium-Ion Batteries

Potassium-containing transition metal layered oxides (KxTmO2), although possessing high energy density and suitable operating voltage, suffer from severe hygroscopic properties due to their two dimensional (2D) layered structure. Their air sensitivity compromises structural stability during prolonged air exposure, therefore increasing the cost. The common sense for designing air-stable layered cathode materials is to avoid contact with H2O molecules. In this study, it is surprisingly found that P3-type KxTmO2 forms an ultra-thin, potassium-rich spinel phase wrapping layer after simply water immersion, remarkedly reduces the reaction activity of the material's surface with air. Combined with Density Function Theory (DFT) calculations, this spinel phase is found to be able to effectively withstand air deterioration and preserving the crystal structure. Consequently, the water-treated material, when exposed to air, can largely maintain its good electrochemical performance, with capacity retention up to 99.15% compared to the fresh samples. Such an in situ surface phase transformation mechanism is also corroborated in other KxTmO2, underscoring its effectiveness in enhancing the air stability of P3-type layered oxides for K+ storage.

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

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

Heat-Resistant Carbon-Coated Potassium Magnesium Hexacyanoferrate Nanoplates for High-Performance Potassium-Ion Batteries

Metal hexacyanoferrates (HCFs) are regarded as promising cathode materials for potassium-ion batteries (PIBs) on account of their low cost and high energy density. However, the difficult-to-remove [Fe(CN)6] vacancies and crystal water lead to structural instability and capacity deterioration as well as the stereotype of poor thermostability of conventional HCFs. Herein, we report (100) face-oriented potassium magnesium hexacyanoferrate (KMgHCF) nanoplates with low [Fe(CN)6] vacancies and high crystallinity, enabling thermostability up to 550 °C, high-temperature carbon coating and crystal water elimination. The as-obtained KMgHCF/C nanoplates exhibit superior potassium storage properties, including a large reversible capacity of 84.6 mAh g-1, a high voltage plateau of 3.87 V, excellent long-term cycling performance over 15000 cycles and high rate capability at 5 A g-1. The unprecedented cycling stability of KMgHCF/C is attributed to the synergistic effect of a highly reversible two-phase reaction, low [Fe(CN)6] vacancies and no crystal water, a specially exposed steady (100) surface, and a protective carbon coating. This work provides a new material selection and modification strategy for the practical application of HCFs in PIBs.

Nonflammable Phosphate-Based Electrolyte for Safe and Stable Potassium Batteries Enabled by Optimized Solvation Effect

Current potassium-ion batteries (PIBs) are limited in safety and lifetime owing to the lack of suitable electrolyte solutions. To address these issues, herein, we report an innovative non-flammable electrolyte design strategy that leverages an optimal moderate solvation phosphate-based solvent which strikes a balance between solvation capability and salt dissociation ability, leading to superior electrochemical performance. The formulated electrolyte simultaneously exhibits the advantages of low salt concentration (only 0.6 M), low viscosity, high ionic conductivity, high oxidative stability, and safety. Our electrolyte also promotes the formation of self-limiting inorganic-rich interphases at the anode surface, alongside robust cathode-electrolyte interphase on iron-based Prussian blue analogues, mitigating electrode/electrolyte side reactions and preventing Fe dissolution. Notably, the PIBs employing our electrolyte exhibit exceptional durability, with 80 % capacity retention after 2,000 cycles at high-voltage of 4.2 V in a coin cell. Impressively, in a larger scale pouch cell, it maintains over 81 % of its initial capacity after 1,400 cycles at 1 C-rate with high average Coulombic efficiency of 99.6 %. This work represents a significant advancement toward the realization of safe, sustainable, and high-performance PIBs.

Calcium-Pillar Boosting Smooth Phase Transition in Potassium Vanadate Nanobelts toward Superior Cycling Performance in Potassium-Ion Batteries

The depletion of lithium resources has prompted exploration into alternative rechargeable energy storage systems, and potassium-ion batteries (PIBs) have emerged as promising candidates. As an active cathode material for PIBs, potassium vanadate (KxV2O5) usually suffers from structural damage during electrochemical K-ion insertion/extraction and hence leading to unsatisfactory cycling performance. Here, we introduce Ca2+ ions as pillars into the potassium vanadate to enhance its structural stability and smooth its phase transition behavior. The additional Ca2+ not only stabilizes the layered structure but also promotes the rearrangement of interlayer ions and leads to a smooth solid-solution phase transition. The optimal composition K0.36Ca0.05V2O5 (KCVO) exhibits outstanding cyclic stability, delivering a capacity of ∼90 mA h g-1 at 20 mA g-1 with negligible capacity decay even after 700 cycles at 500 mA g-1. Theoretical calculations indicate lower energy barriers for K+ diffusion, promoting rapid reaction kinetics. The excellent performances and detailed investigations offer insights into the structural regulation of layered vanadium cathodes.

Rational Regulation of High-Voltage Stability in Potassium Layered Oxide Cathodes

Layered oxide cathode materials may undergo irreversible oxygen loss and severe phase transitions during high voltage cycling and may be susceptible to transition metal dissolution, adversely affecting their electrochemical performance. Here, to address these challenges, we propose synergistic doping of nonmetallic elements and in situ electrochemical diffusion as potential solution strategies. Among them, the distribution of the nonmetallic element fluorine within the material can be regulated by doping boron, thereby suppressing manganese dissolution through surface enrichment of fluorine. Furthermore, in situ electrochemical diffusion of fluorine from the surface into the bulk of the materials after charging reduces the energy barrier of potassium ion diffusion while effectively inhibiting irreversible oxygen loss under high voltage. The modified K0.5Mn0.83Mg0.1Ti0.05B0.02F0.1O1.9 layered oxide cathode exhibits a high capacity of 147 mAh g-1 at 50 mA g-1 and a long cycle life of 2200 cycles at 500 mA g-1. This work demonstrates the efficacy of synergistic doping and in situ electrochemical diffusion of nonmetallic elements and provides valuable insights for optimizing rechargeable battery materials.

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

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

Architectural design of anode materials for superior alkali-ion (Li/Na/K) batteries storage

Developing high-performance anode materials remains a significant challenge for clean energy storage systems. Herein, we investigated the (MXene/MoSe2@C) heterostructure hybrid nanostructure as a superior anode material for application in lithium, sodium, and potassium ion batteries (LIBs, SIBs, and PIBs). Moreover, the anode structure's stability was examined via the open-source Large-scale atomic/molecular massively Parallel Simulator code. Our results indicated that the migration of SIBs toward the anode material is significantly greater than other ions during charge and discharge cycles. Therefore, SIBs systems can be competitive with PIBs and LIBs systems. In addition, the average values of the potential energies for the anode materials/ions complexes are about ~ - 713.65, ~  - 2030.41, and ~ - 912.36 kcal mol-1 in systems LIBs, SIBs, and PIBs, respectively. This study provides a rational design strategy to develop high-performance anode materials in SIBs/PIBs/LIBs systems, which can be developed for other transition metal chalcogenide-based composites as a superior anode of alkali metal ion battery storage systems.

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

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

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

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

A Conformal Protective Skin Producing Stable Cathode-Electrolyte Interface for Long-Life Potassium-Ion Batteries

The development of K-based layered oxide cathodes is essential for boosting the competitiveness of potassium-ion batteries (PIBs) in grid-scale energy storage. However, their service life is dramatically limited by interfacial instability issues, which is still poorly understood. In this work, amorphous FePO4 (a-FP) is built on K0.5 Ni0.1 Mn0.9 O2 (KNMO) as the protective skin, whose elasticity for strain relaxation and the K-conducting nature guarantee its integrity during fast and constant K-ion insertion/extraction. The conformal coating leads to a robust interphase on the cathode surface, which qualifies excellent K-transport ability and significantly suppresses the mechanical cracking and transition metal dissolution. Breakthrough in cycle life of the K-based layered cathodes is therefore achieved, which of the amorphous FePO4 coated K0.5 Ni0.1 Mn0.9 O2 (KNMO@a-FP) reaches 2500 cycles. The insights gained from the surface protection layer construction and the in-depth analysis of its working mechanism pave the way for further development of K-based layered cathodes with both bulk structural and interfacial stability.

Solvation Structure Modulation of High-Voltage Electrolyte for High-Performance K-Based Dual-Graphite Battery

Due to the advantages of dual-ion batteries (DIBs) and abundant resources, potassium-based dual-carbon batteries (K-DCBs) have wide application prospects. However, conventional carbonate ester-based electrolyte systems have obvious drawbacks such as poor oxidation resistance and difficulty in sustaining the anion intercalation process at high voltages, which seriously affect the capacity and cycle performance of K-DCBs. Therefore, a rational design of more efficient novel electrolyte systems is urgently required to realize high-performance K-DCBs. Herein, a solvation structure modulation strategy for the K-DCB electrolyte systems is reported. Consequently, substantial K⁺ ion storage improvement at the graphite anode and enhanced bis(fluorosulfonyl)imide anion (FSI^- ) intercalation capacity at the graphite cathode are successfully realized simultaneously. As a proof-of-concept, the assembled K-DCB exhibited a discharge capacity of 103.4 mAh g^-1 , and after 400 cycles, ≈90% capacity retention is observed. Moreover, the energy density of the K-DCB full cell reached 157.6 Wh kg^-1 , which is the best performance in reported K-DCBs till date. This study demonstrates the effectiveness of solvation modulation in improving the performance of K-DCBs.

Constructing Active BN Sites in Carbon Nanosheets for High-Capacity and Fast Charging Toward Potassium Ion Storage

Nitrogen doping is an effective strategy to improve potassium ion storage of carbon electrodes via the creation of adsorption sites. However, various undesired defects are often uncontrollably generated during the doping process, limiting doping effect on capacity enhancement and deteriorating the electric conductivity. Herein, boron element is additionally introduced to construct 3D interconnected B, N co-doped carbon nanosheets to remedy these adverse effects. This work demonstrates that boron incorporation preferentially converts pyrrolic N species into BN sites with lower adsorption energy barrier, further enhancing the capacity of B, N co-doped carbon. Meanwhile, the electric conductivity is modulated via the conjugation effect between the electron-rich N and electron-deficient B, accelerating the charge-transfer kinetics of potassium ions. The optimized samples deliver a high specific capacity, high rate capability, and long-term cyclic stability (532.1 mAh g^-1 at 0.05 A g^-1 , 162.6 mAh g^-1 at 2 A g^-1 over 8000 cycles). Furthermore, hybrid capacitors using the B, N co-doped carbon anode deliver a high energy and power density with excellent cycle life. This study demonstrates a promising approach using BN sites for adsorptive capacity and electric conductivity enhancement in carbon materials for electrochemical energy storage applications.

Highly Stable Sb/C Anode for K+ and Na+ Energy Storage Enabled by Pulsed Laser Ablation and Polydopamine Coating

Potassium- and sodium-ion batteries (PIBs and SIBs) have great potential as the next-generation energy application owing to the natural abundance of K and Na. Antimony (Sb) is a suitable alloying-type anode for PIBs and SIBs due to its high theoretical capacity and proper operation voltage; yet, the severe volume variation remains a challenge. Herein, a preparation of N-doped carbon-wrapped Sb nanoparticles (L-Sb/NC) using pulsed laser ablation and polydopamine coating techniques, is reported. As the anode for PIB and SIB, the L-Sb/NC delivers superior rate capabilities and excellent cycle stabilities (442.2 and 390.5 mA h g^-1 after 250 cycles with the capacity decay of 0.037% and 0.038% per cycle) at the current densities of 0.5 and 1.0 A g^-1 , respectively. Operando X-ray diffraction reveals the facilitated and stable potassiation and sodiation mechanisms of L-Sb/NC enabled by its optimal core-shell structure. Furthermore, the SIB full cell fabricated with L-Sb/NC and Na₃ V₂ (PO₄ )₂ F₃ shows outstanding electrochemical performances, demonstrating its practical energy storage application.

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

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

Synthesis of KVPO4F/Carbon Porous Single Crystalline Nanoplates for High-Rate Potassium-Ion Batteries

With high theoretical capacity and operating voltage, KVPO₄F is a potential high energy density cathode material for potassium-ion batteries. However, its performance is usually limited by F loss, poor electronic conductivity, and unsteady electrode/electrolyte interface. Herein, a simple one-step sintering process is developed, where vanadium-oxalate-phosphite/phosphate frameworks and fluorinated polymer are used to synthesize carbon-coated KVPO₄F nanoplates. It is found that the V-F-C bond generated by fluorinated-polymer-derived carbon at the interface of KVPO₄F/C nanoplates diminishes the F loss, as well as enhances K-ions migration ability and the electronic conductivity of KVPO₄F. The as-synthesized KVPO₄F/C cathode delivers a reversible capacity of 106.5 mAh g^-1 at 0.2 C, a high working voltage of 4.28 V, and a rate capability with capacity of 73.8 mAh g^-1 at the ultrahigh current density of 100 C. In addition, a KVPO₄F/C//soft carbon full cell exhibits a high energy density of 235.5 Wh kg^-1.

Lignin as Polymer Electrolyte Precursor for Stable and Sustainable Potassium Batteries

Potassium batteries show interesting peculiarities as large-scale energy storage systems and, in this scenario, the formulation of polymer electrolytes obtained from sustainable resources or waste-derived products represents a milestone activity. In this study, a lignin-based membrane is designed by crosslinking a pre-oxidized Kraft lignin matrix with an ethoxylated difunctional oligomer, leading to self-standing membranes that are able to incorporate solvated potassium salts. The in-depth electrochemical characterization highlights a wide stability window (up to 4 V) and an ionic conductivity exceeding 10^-3  S cm^-1 at ambient temperature. When potassium metal cell prototypes are assembled, the lignin-based electrolyte attains significant electrochemical performances, with an initial specific capacity of 168 mAh g^-1 at 0.05 A g^-1 and an excellent operation for more than 200 cycles, which is an unprecedented outcome for biosourced systems in potassium batteries.

Cardanol-Derived Epoxy Resins as Biobased Gel Polymer Electrolytes for Potassium-Ion Conduction

In this study, biobased gel polymer electrolyte (GPE) membranes were developed via the esterification reaction of a cardanol-based epoxy resin with glutaric anhydride, succinic anhydride, and hexahydro-4-methylphthalic anhydride. Nonisothermal differential scanning calorimetry was used to assess the optimal curing time and temperature of the formulations, evidencing a process activation energy of ∼65-70 kJ mol-1. A rubbery plateau modulus of 0.65-0.78 MPa and a crosslinking density of 2 × 10-4 mol cm-3 were found through dynamic mechanical analysis. Based on these characteristics, such biobased membranes were tested for applicability as GPEs for potassium-ion batteries (KIBs), showing an excellent electrochemical stability toward potassium metal in the -0.2-5 V voltage range and suitable ionic conductivity (10-3 S cm-1) at room temperature. This study demonstrates the practical viability of these biobased materials as efficient GPEs for the fabrication of KIBs, paving the path to increased sustainability in the field of next-generation battery technologies.

A Stabilized Polyacrylonitrile-Encapsulated Matrix on a Nanolayered Vanadium-Based Cathode Material Facilitating the K-Storage Performance

Layered vanadium-based metal oxides were regarded as promising cathode materials accounting for suitable K⁺ transport channels as well as high work potential in K-ion batteries. Nevertheless, because of the large radius of K⁺ and the rigid structure of inorganic materials, the typical K_0.486V₂O₅ suffers from volume expansion seriously in the repeated charging and discharging processes along with poor ionic and electronic conductivity, consequently determining inevitably poor electrochemical properties. Herein, we proposed a stabilized polymer (PAN) matrix on K_0.486V₂O₅ nanobelts by a liquid-assisted methodology and further electrospinning technology. As a result, a nanocomposite containing a 3D conductive and interconnected mesh structure was thus constructed. By avoiding the full carbonization of polyacrylonitrile (PAN) with appropriate thermal treatment, the elastic properties of the PAN precursor can be retained, effectively inhibiting the volume effect, and the stabilized PAN-encapsulated matrix can also greatly accelerate transport rates of K⁺ and electrons at a high rate as well as restrict the decomposition of organic electrolytes and side reactions. This work can supply significant basic scientific value of the polymer surface coating methodology for the far-reaching development of inorganic cathode materials in K-ion batteries.

Anion Doping for Layered Oxides with a Solid-Solution Reaction for Potassium-Ion Battery Cathodes

The development of potassium-ion batteries (PIBs) is challenged by the shortage of stable cathode materials capable of reversibly hosting the large-sized K⁺ (1.38 Å), which is prone to cause severe structural degradation and complex phase evolution during the potassiation/depotassiation process. Here, we identified that anionic doping of the layered oxides for PIBs is effective to combat their capacity fading at high voltage (>4.0 V). Taking P2-type K_2/3Mn_7/9Ni_1/9Ti_1/9O_17/9F_1/9 (KMNTOF) as an example, we showed that the partial substitution of O^2- by F^- enlarged the interlayer distance of the K_2/3Mn_7/9Ni_1/9Ti_1/9O₂ (KMNTO), which becomes more favorable for fast K⁺ transition without violent structural destruction. Meanwhile, based on the experimental data and theoretical results, we identified that the introduction of F^- anions effectively increased the redox-active Mn cationic concentration by lowering the average valence of the Mn element, accordingly providing more reversible capacity derived from the Mn^3+/4+ redox couple, rather than oxygen redox. This anionic doping strategy enables the KMNTOF cathode to deliver a high reversible capacity of 132.5 mAh g^-1 with 0.53 K⁺ reversible (de)intercalation in the structure. We expect that the discovery provides new insights into structural engineering for pursuing stable cathodes to facilitate the future applications of high-performance PIBs.

A Low-Strain Phosphate Cathode for High-Rate and Ultralong Cycle-Life Potassium-Ion Batteries

Most potassium-ion battery (PIB) cathode materials have deficient structural stability because of the huge radius of potassium ion, leading to inferior cycling performance. We report the controllable synthesis of a novel low-strain phosphate material K₃ (VO)(HV₂ O₃ )(PO₄ )₂ (HPO₄ ) (denoted KVP) nanorulers as an efficient cathode for PIBs. The as-synthesized KVP nanoruler cathode exhibits an initial reversible capacity of 80.6 mAh g^-1 under 20 mA g^-1 , with a large average working potential of 4.11 V. It also manifests an excellent rate property of 54.4 mAh g^-1 under 5 A g^-1 , with a high capacity preservation of 92.1 % over 2500 cycles. The outstanding potassium storage capability of KVP nanoruler cathode originates from a low-strain K⁺ uptake/removal mechanism, inherent semiconductor characteristic, and small K⁺ migration energy barrier. The high energy density and prolonged cyclic stability of KVP nanorulers//polyaniline-intercalated layered titanate full battery verifies the superiority of KVP nanoruler cathode in PIBs.