Electrolytes and Interphases in Potassium Ion Batteries

Construction of N-doped carbon encapsulated CoP hollow nanofibers as multifunctional electrode materials for potassium-ion and lithium-sulfur batteries

Herein, a composite of N-doped carbon coated phosphating cobalt hollow nanofibers (N/C@CoP-HNFs) was synthesized by electrospinning, phosphating, and carbon coating processes. When employed as multifunctional electrode materials for potassium-ion batteries (PIBs) and lithium-sulfur (Li-S) batteries, the N/C@CoP-HNFs demonstrated notable electrochemical properties. Specifically, it delivered an initial specific capacity of 420.4 mA h g-1 at a current density of 100 mA g-1, with a sustained capacity of 190.8 mA h g-1 after 200 cycles in PIBs, and a specific capacity of 1448 mA h g-1 at a current density of 0.5C in Li-S batteries, which is considered relatively high for these types of battery technology. This good performance may due to the combination of the carbon nitrogen layer and cobalt phosphide bilayer hollow tube structure, which is conducive to telescoping the diffusion length of ions and electrons and buffer volume variation, and effectively inhibits the shuttle effect. Density functional theory (DFT) calculations were also used to explore the energy storage mechanism of the material. The possible adsorption sites and corresponding adsorption energy of K+ were analyzed, and the advantages of the material were explored by calculating the diffusion barrier and state density. The theoretical simulations further validated the strong adsorption capability of CoP for polysulfides. This work is expected to provide new ideas for new energy storage materials.

Long-life potassium metal batteries enabled by anion-derived solid electrolyte interphase using concentrated ionic liquid electrolytes

Potassium metal batteries (PMBs) show great potential as next-generation energy storage systems yet face challenges such as the dendritic growth of the potassium anode, leading to issues with cycle life and safety. This study reports a potassium salt-concentrated ionic liquid electrolyte (PCIL) consisting of potassium bis(fluorosulfonyl)imide (KFSI) and 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (Pyr13FSI) to achieve long-life and, safe PMBs. PCIL presents several advantages including outstanding oxidation stability (≈5.2 V), decent ionic conductivity (4.0 mS cm-1 at 25 °C), and negligible flammability. Moreover, PCIL promotes the development of anion-derived solid-electrolyte interphase (SEI) with high inorganic content. This not only hinders the growth of potassium dendrites but also facilitates facile interfacial charge transfer kinetics. Benefiting from these advantages, PMBs (K||KVPO4F) employing PCIL exhibit remarkable cycle performances at both ambient and elevated temperatures (capacity retention after 300 cycles: 74.8% at 25 °C and 82.9% at 45 °C), surpassing the performance of conventional carbonate (1 M KPF6 EC/PC) and dilute potassium ionic liquid electrolyte (PIL). This work demonstrates the tangible capability of PCIL in realizing practical PMBs.

Operando Studies of Bismuth Nanoparticles Embedded in N, O-Doped Porous Carbon for High-Performance Potassium-Ion Hybrid Capacitor

A highly viable alternative to lithium-ion batteries for stationary electrochemical energy-storage systems is the potassium dual-ion hybrid capacitor (PIHC), especially toward fast-charging capability. However, the sluggish reaction kinetics of negative electrode materials seriously impedes their practical implementation. In this paper, a new negative electrode Bi@RPC (Nano-bismuth confined in nitrogen- and oxygen-doped carbon with rationally designed pores, evidenced by advanced characterization) is developed, leading to a remarkable electrochemical performance. PIHCs building with the active carbon YP50F positive electrode result in a high operation voltage (0.1-4 V), and remarkably well-retained energy density at a high-power density (11107 W kg-1 at 98 Wh kg-1). After 5000 cycles the proposed PHICs still show a superior capacity retention of 92.6%. Moreover, a reversible mechanism of "absorption-alloying" of the Bi@RPC nanocomposite is revealed by operando synchrotron X-ray diffraction and Raman spectroscopy. With the synergistic potassium ions storage mechanism arising from the presence of well-structured pores and nano-sized bismuth, the Bi@RPC electrode exhibits an astonishingly rapid kinetics and high energy density. The results demonstrate that PIHCs with Bi@RPC-based negative electrode is the promising option for simultaneously high-capacity and fast-charging energy storage devices.

Anion-Reinforced Solvation Structure Enables Stable Operation of Ether-Based Electrolyte in High-Voltage Potassium Metal Batteries

Electrolytes with anion-dominated solvation are promising candidates to achieve dendrite-free and high-voltage potassium metal batteries. However, it's challenging to form anion-reinforced solvates at low salt concentrations. Herein, we construct an anion-reinforced solvation structure at a moderate concentration of 1.5 M with weakly coordinated cosolvent ethylene glycol dibutyl ether. The unique solvation structure accelerates the desolvation of K+, strengthens the oxidative stability to 4.94 V and facilitates the formation of inorganic-rich and stable electrode-electrolyte interface. These enable stable plating/stripping of K metal anode over 2200 h, high capacity retention of 83.0 % after 150 cycles with a high cut-off voltage of 4.5 V in K0.67MnO2//K cells, and even 91.5 % after 30 cycles under 4.7 V. This work provides insight into weakly coordinated cosolvent and opens new avenues for designing ether-based high-voltage electrolytes.

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.

Homogeneous Adsorption of Multiple Potassiation Products of Red Phosphorus Anode toward Stable Potassium Storage

Potassium ion batteries (PIBs) are a viable alternative to lithium-ion batteries for energy storage. Red phosphorus (RP) has attracted a great deal of interest as an anode for PIBs owing to its cheapness, ideal electrode potential, and high theoretical specific capacity. However, the direct preparation of phosphorus-carbon composites usually results in exposure of the RP to the exterior of the carbon layer, which can lead to the deactivation of the active material and the production of "dead phosphorus". Here, the advantage of the π-π bond conjugated structure and high catalytic activity of metal phthalocyanine (MPc) is used to prepare MPc@RP/C composites as a highly stable anode for PIBs. It is shown that the introduction of MPc greatly improves the uneven distribution of the carbon layer on RP, and thus improves the initial Coulombic efficiency (ICE) of PIBs (the ICE of FePc@RP/C is 75.5% relative to 62.9% of RP/C). The addition of MPc promotes the growth of solid electrolyte interphase with high mechanical strength, improving the cycle stability of PIBs (the discharge-specific capacity of FePc@RP/C is 411.9 mAh g-1 after 100 cycles at 0.05 A g-1). Besides, density functional theory theoretical calculations show that MPc exhibits homogeneous adsorption energies for multiple potassiation products, thereby improving the electrochemical reactivity of RP. The use of organic molecules with high electrocatalytic activity provides a universal approach for designing superior high-capacity, large-volume expansion anodes for PIBs.

Flame-Retardant Nonaqueous Electrolytes for High-Safety Potassium-Ion Batteries

Potassium-ion batteries (PIBs) with conventional organic-based flammable electrolytes suffer from serious safety issues with a high risk of ignition and burning especially under harsh conditions, which significantly limits their widespread applications. Flame-retardant electrolytes (FREs) are considered as one of the most effective strategies to address these safety issues. Therefore, it's much necessary to summarize the challenges, recent progress, and design principles of flame-retardant nonaqueous electrolytes for PIBs to guide their development and future applications. In this review, an in-depth introduction and explanation of the origins of electrolyte flammability are first presented. Particularly, the state-of-the-art design principles of FREs for PIBs are extensively summarized and emphasized, including the electrolyte flame-retardant solvents/additives, highly concentrated electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), ionic liquids-based electrolytes and solid-state electrolytes. Moreover, the advantages and drawbacks of each approach are systematically presented and discussed, following by proposed perspectives to guide the rational development of next-generation high-safety PIBs for practical applications.

An Elastomeric Lithium-Conducting Interlayer for High-Performance LATP-Based Lithium Metal Batteries

In response to the critical challenges of interfacial impedance and volumetric changes in Li(1+x)AlxTi(2‑x)(PO4)3 (LATP)-based lithium metal batteries, an elastomeric lithium-conducting interlayer fabricates from fluorinated hydrogenated nitrile butadiene rubber (F-HNBR) matrix is introduced herein. Owing to the vulcanization, vapor-phase fluorination, and plasticization processes, the lithium-conducting interlayer exhibits a high elasticity of 423%, exceptional fatigue resistance (10 000 compression cycles), superior ionic conductivity of 6.3 × 10-4 S cm-1, and favorable lithiophilicity, rendering it an ideal buffer layer. By integrating the F-HNBR interlayer, the LATP-based lithium symmetric cells demonstrate an extended cycle life of up to 1600 h at 0.1 mA cm-2 and can also endure deep charge/discharge cycles (0.5 mAh cm-2) for the same duration. Furthermore, the corresponding lithium metal full cells achieve 500 cycles at 0.5 C with 98.3% capacity retention and enable a high-mass-loading cathode of 11.1 mg cm-2 to operate at room temperature.

Chloro-Functionalized Ether-Based Electrolyte for High-Voltage and Stable Potassium-Ion Batteries

Ether-based electrolyte is beneficial to obtaining good low-temperature performance and high ionic conductivity in potassium ion batteries. However, the dilute ether-based electrolytes usually result in ion-solvent co-intercalation of graphite, poor cycling stability, and hard to withstand high voltage cathodes above 4.0 V. To address the aforementioned issues, an electron-withdrawing group (chloro-substitution) was introduced to regulate the solid-electrolyte interphase (SEI) and enhance the oxidative stability of ether-based electrolytes. The dilute (~0.91 M) chloro-functionalized ether-based electrolyte not only facilitates the formation of homogeneous dual halides-based SEI, but also effectively suppress aluminum corrosion at high voltage. Using this functionalized electrolyte, the K||graphite cell exhibits a stability of 700 cycles, the K||Prussian blue (PB) cell (4.3 V) delivers a stability of 500 cycles, and the PB||graphite full-cell reveals a long stability of 6000 cycles with a high average Coulombic efficiency of 99.98 %. Additionally, the PB||graphite full-cell can operate under a wide temperature range from -5 °C to 45 °C. This work highlights the positive impact of electrolyte functionalization on the electrochemical performance, providing a bright future of ether-based electrolytes application for long-lasting, wide-temperature, and high Coulombic efficiency PIBs and beyond.

Restructuring Electrolyte Solvation by a Partially and Weakly Solvating Cosolvent toward High-Performance Potassium-Ion Batteries

Ether-based electrolytes are among the most important electrolytes for potassium-ion batteries (PIBs) due to their low polarization voltage and notable compatibility with potassium metal. However, their development is hindered by the strong binding between K+ and ether solvents, leading to [K+-solvent] cointercalation on graphite anodes. Herein, we propose a partially and weakly solvating electrolyte (PWSE) wherein the local solvation environment of the conventional 1,2-dimethoxyethane (DME)-based electrolyte is efficiently reconfigured by a partially and weakly solvating diethoxy methane (DEM) cosolvent. For the PWSE in particular, DEM partially participates in the solvation shell and weakens the chelation between K+ and DME, facilitating desolvation and suppressing cointercalation behavior. Notably, the solvation structure of the DME-based electrolyte is transformed into a more cation-anion-cluster-dominated structure, consequently promoting thin and stable solid-electrolyte interphase (SEI) generation. Benefiting from optimized solvation and SEI generation, the PWSE enables a graphite electrode with reversible K+ (de)intercalation (for over 1000 cycles) and K with reversible plating/stripping (the K||Cu cell with an average Coulombic efficiency of 98.72% over 400 cycles) and dendrite-free properties (the K||K cell operates over 1800 h). We demonstrate that rational PWSE design provides an approach to tailoring electrolytes toward stable PIBs.

Strategies for developing layered oxide cathodes, carbon-based anodes, and electrolytes for potassium ion batteries

Lithium-ion batteries (LIBs) have become the most popular portable secondary energy storage facilities. However, the limited lithium resource results in possible unsustainable development. Potassium-ion batteries (PIBs) are considered promising alternatives to LIBs because of their high resource availability, low cost, and environmentally friendly features. In this field, high energy density layered cathodes and carbon-based anodes are also the main research objectives. However, compared to the most appealing alternative sodium-ion batteries (SIBs), despite having various theoretical advantages, PIBs exhibit poorer electrochemical performance in practice. Their poor capacity retention and narrow working voltage range seriously limit their applications. The performance of the electrodes is usually considered an important factor for battery performance, life, and safety. To solve these problems, many significant research studies have been carried out in the last decade, achieving numerous breakthroughs. Nevertheless, there are still many drawbacks and unclear mechanisms. In this comprehensive review, we examine the current state of high-performance layered oxide cathodes, electrolytes, and carbon-based anodes, to identify potential candidates for PIBs. Our focus lies on their structural characteristics, interface properties, underlying mechanisms, and modification techniques. The viewpoints of these advanced strategies are integrated, and concise development suggestions and strategies are subsequently proposed.

Electrolyte-Induced Morphology Evolution to Boost Potassium Storage Performance of Perylene-3,4,9,10-tetracarboxylic Dianhydride

Organic materials have attracted extensive attention for potassium-ion batteries due to their flexible structure designability and environmental friendliness. However, organic materials generally suffer from unavoidable dissolution in aprotic electrolytes, causing an unsatisfactory electrochemical performance. Herein, we designed a weakly solvating electrolyte to boost the potassium storage performance of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). The electrolyte induces an in situ morphology evolution and achieves a nanowire structure. The weakly dissolving capability of ethylene glycol diethyl ether-based electrolyte and unique nanowire structure effectively avoid the dissolution of PTCDA. As a result, PTCDA shows excellent cycling stability (a capacity retention of 89.1% after 2000 cycles) and good rate performance (70.3 mAh g-1 at 50C). In addition, experimental detail discloses that the sulfonyl group plays a key role in inducing morphology evolution during the charge/discharge process. This work opens up new opportunities in electrolyte design for organic electrodes and illuminates further developments of potassium-ion batteries.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

Elastic MXene conductive layers and electrolyte engineering enable robust potassium storage

The precisely engineered structures of materials greatly influence the manifestation of their properties. For example, in the process of alkali metal ion storage, a carefully designed structure capable of accommodating inserted and extracted ions will improve the stability of material cycling. The present study explores the uniform distribution of self-grown carbon nanotubes to provide structural support for the conductive and elastic MXene layers of Ti3C2Tx-Co@NCNTs. Furthermore, a compatible electrolyte system has been optimized by analyzing the solvation structure and carefully regulating the component in the solid electrolyte interphase (SEI) layer. Mechanistic studies demonstrate that the decomposition predominantly controlled by FSI- leads to the formation of a robust inorganic SEI layer enriched with KF, thus effectively inhibiting irreversible side reactions and major structural deterioration. Confirming our expectations, Ti3C2Tx-Co@NCNTs exhibits an impressive reversible capacity of 260 mA h g-1, even after 2000 cycles at 500 mA g-1 in 1 M KFSI (DME), surpassing most MXene-based anodes reported for PIBs. Additionally, density functional theory (DFT) calculations verify the superior electronic conductivity and lower K+ diffusion energy barriers of the novel superstructure of Ti3C2Tx-Co@NCNTs, thereby affirming the improved electrochemical kinetics. This study presents systematic evaluation methodologies for future research on MXene-based anodes in PIBs.

Potassium Decahydrido-closo-Decaborane Urea Complex as a Potential Solid-State Electrolyte for Potassium Metal Batteries

All-solid-state potassium metal batteries have been considered promising candidates for large-scale energy storage because of abundance and wide availability of K resources, elimination of flammable liquid organic electrolytes, and incorporation of high-capacity K metal anode. However, unideal K-ion conductivities of most reported K-ion solid electrolytes have restricted the development of these batteries. Herein, a novel K2B10H10·CO(NH2)2 complex is reported, forming by incorporating urea into K2B10H10, to achieve an enhanced K-ion conductivity. The crystal structure of K2B10H10·CO(NH2)2 was determined as a monoclinic lattice with the space group of C2/c (No. 15). K2B10H10·CO(NH2)2 delivers an ionic conductivity of 2.7 × 10-8 S cm-1 at 25 °C, and reaching 1.3 × 10-4 S cm-1 at 80 °C, which is about 4 orders of magnitude higher than that of K2B10H10. One possible reason is the anion expansion in size due to the presence of dihydrogen bonds in K2B10H10·CO(NH2)2, resulting in an increase in the K-H bond distance and the electrostatic potential, thereby enhancing the mobility of K+. The K-ion conductivity is also higher than those of most hydridoborate-based K-ion conductors reported. Besides, K2B10H10·CO(NH2)2 reveals a wide electrochemical stability window and remarkable interface compatibility with K metal electrodes, suggesting a promising electrolyte for all-solid-state K metal batteries.

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

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

Recent advances in battery characterization using in situ XAFS, SAXS, XRD, and their combining techniques: From single scale to multiscale structure detection

Revealing and clarifying the chemical reaction processes and mechanisms inside the batteries will bring a great help to the controllable preparation and performance modulation of batteries. Advanced characterization techniques based on synchrotron radiation (SR) have accelerated the development of various batteries over the past decade. In situ SR techniques have been widely used in the study of electrochemical reactions and mechanisms due to their excellent characteristics. Herein, the three most wide and important synchrotron radiation techniques used in battery research were systematically reviewed, namely X-ray absorption fine structure (XAFS) spectroscopy, small-angle X-ray scattering (SAXS), and X-ray diffraction (XRD). Special attention is paid to how these characterization techniques are used to understand the reaction mechanism of batteries and improve the practical characteristics of batteries. Moreover, the in situ combining techniques advance the acquisition of single scale structure information to the simultaneous characterization of multiscale structures, which will bring a new perspective to the research of batteries. Finally, the challenges and future opportunities of SR techniques for battery research are featured based on their current development.

Nitrogen-Doped Cellulose-Derived Porous Carbon Fibers for High Mass-Loading Aqueous Supercapacitors

Biomass-based porous carbon with renewability and flexible structure tunability is a promising electrode material for supercapacitors. However, there is a huge gap between experimental research and practical applications. How to maintain good electrochemical performance of high mass-loading electrodes and suppress the self-discharge of supercapacitors is a key issue that urgently needs to be addressed. The structure regulation of electrode materials such as heteroatom doping is a promising optimization strategy for high mass-loading electrodes. In this work, nitrogen-doped cellulose-derived porous carbon fibers (N-CHPCs) were prepared by a facile bio-template method using cotton cellulose as raw material and urea as dopant. The prepared N-CHPCs have high specific surface area, excellent hierarchical porous structure, partial graphitization properties and suitable heteroatom content. The assembled high mass-loading (12.8 mg cm-2 ; 245 μm) aqueous supercapacitor has excellent electrochemical performance, i. e., low open-circuit voltage attenuation rate (21.39 mV h-1 ), high voltage retention rate (78.81 %), high specific capacitance (295.8 F g-1 at 0.1 A g-1 ), excellent area capacitance (3.79 F cm-2 at 0.1 A g-1 ), excellent cycling stability (97.28 % over 20,000 cycles at 1.0 A g-1 ). The excellent performance of high mass-loading N-CHPCs is of great significance for their practical applications in advanced aqueous supercapacitors.

Architecting carbon-coated Mo2CTx/MoSe2 heterostructures enables robust potassium storage

Herein, carbon-coated MoSe2 decorated Mo2CTx MXene heterostructures (MoSe2/Mo2CTx@C) have been fabricated. Mo2CTx works as a dual-function electron/ion conductor, which not only provides high conductivity and mechanical strength, but also prevents the severe self-aggregation of few layered MoSe2 nanosheets. The high reversible capacities of 405 mA h g-1 at 100 mA g-1 after 150 cycles and 258 mA h g-1 at 2000 mA g-1 after 400 cycles could be achieved for a potassium-ion battery.

Advancements and Prospects of Graphite Anode for Potassium-Ion Batteries

Potassium-ion batteries (KIBs) have recently attracted considerable attention owing to their resource abundance, low cost and environmental friendliness. Graphite as a mature commercial anode material for lithium-ion batteries, has been proved as a promising anode candidate for KIBs by reversible forming potassium-graphite intercalation compounds. However, large volume expansion and sluggish K+ kinetics caused by the incompatibility between large radius of K+ and the small interlayer spacing of graphite, result in the poor cycle stability and rate performances, hindering its practical application. Extensive research efforts have focused on improving the potassium storage performance of graphite anodes. This review provides an overview of recent advances in addressing these challenges and optimizing the electrochemical performance of graphite anodes for KIBs. Various strategies to improve the electrochemical performance of graphite and graphitic carbon anodes, such as microcrystalline regulation, heteroatom doping, morphological adjustment, and coating modification, are discussed, while the critical issues and challenges associated with graphite anodes and the prospects for their advancement in KIBs are highlighted. The review offers valuable guidelines for rational structural design and promotes the commercial development of high-performance graphite anode materials for KIBs.

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

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

Ultrastable Dendrite-Free Potassium Metal Batteries Enabled by Weakly-Solvated Electrolyte

Potassium (K) metal is considered one of the most promising anodes for potassium metal batteries (PMBs) because of its abundant and low-cost advantages but suffers from serious dendritic growth and parasitic reactions, resulting in poor cyclability, low Coulombic efficiency (CE), and safety concerns. In this work, we report a localized high-concentration electrolyte (LHCE) consisting of potassium bis(fluorosulfonyl)imide (KFSI) in a cosolvent of 1,2-dimethoxyethane (DME) and 1,1,2,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) to solve the problems of PMBs. TTE as a diluent not only endows LHCE with advantages of low viscosity, good wettability, and improved conductivity but also solves the dendrite problem pertaining to K metal anodes. Using the formulation of LHCE, a CE of 98% during 800 cycles in the K||Cu cell and extremely stable cycling of over 2000 h in the K||K symmetric cell are achieved at a current density of 0.1 mA cm-2. In addition, the LHCE shows good compatibility with a Prussian Blue cathode, allowing almost 99% CE for the K||KFeIIFeIII(CN)6 full cell during 100 cycles. This promising electrolyte design realizes high-safety and energy-dense PMBs.

Carboxylated Poly (p-Phenylene Terephthalamide) Reinforced Polyetherimide for High-Temperature Dielectric Energy Storage

Dielectric energy storage polymers play a vital role in advanced electronics and electrical systems, due to their high breakdown strength, excellent reliability, and easy fabrication. However, the low dielectric constant and poor thermal resistance of dielectric polymers limit their energy storage density and working temperatures, making them less versatile for broader applications. In this work, a novel carboxylated poly (p-phenylene terephthalamide) (c-PPTA) is synthesized and employed to simultaneously enhance the dielectric constant and thermal resistance of polyetherimide (PEI), leading to a discharged energy density of 6.4 J cm-3 at 150 °C. The introduction of c-PPTA molecules effectively reduces the ΠΠ stacking effect and increases the average chain spacing between polymer molecules, which is conducive to improving the dielectric constant. Additionally, c-PPTA molecules with stronger positive charges and high dipole moments can capture electrons, resulting in reduced conduction loss and enhanced breakdown strength at high temperatures. The coiled capacitor fabricated with the PEI/c-PPTA film exhibits superior capacitance performances and higher working temperatures compared to commercial metalized PP capacitors, demonstrating great potential for dielectric polymers in high-temperature electronic and electrical energy storage systems.

Low-temperature anode-free potassium metal batteries

In contrast to conventional batteries, anode-free configurations can extend cell-level energy densities closer to the theoretical limit. However, realizing alkali metal plating/stripping on a bare current collector with high reversibility is challenging, especially at low temperature, as an unstable solid-electrolyte interphase and uncontrolled dendrite growth occur more easily. Here, a low-temperature anode-free potassium (K) metal non-aqueous battery is reported. By introducing Si-O-based additives, namely polydimethylsiloxane, in a weak-solvation low-concentration electrolyte of 0.4 M potassium hexafluorophosphate in 1,2-dimethoxyethane, the in situ formed potassiophilic interface enables uniform K deposition, and offers K||Cu cells with an average K plating/stripping Coulombic efficiency of 99.80% at -40 °C. Consequently, anode-free Cu||prepotassiated 3,4,9,10-perylene-tetracarboxylicacid-dianhydride full batteries achieve stable cycling with a high specific energy of 152 Wh kg-1 based on the total mass of the negative and positive electrodes at 0.2 C (26 mA g-1) charge/discharge and -40 °C.

Unlocking Deep and Fast Potassium-Ion Storage through Phosphorus Heterostructure

Potassium-ion battery represents a promising alternative of conventional lithium-ion batteries in sustainable and grid-scale energy storage. Among various anode materials, elemental phosphorus (P) has been actively pursued owing to the ideal natural abundance, theoretical capacity, and electrode potential. However, the sluggish redox kinetics of elemental P has hindered fast and deep potassiation process toward the formation of final potassiation product (K3 P), which leads to inferior reversible capacity and rate performance. Here, it is shown that rational design on black/red P heterostructure can significantly improve K-ion adsorption, injection and immigration, thus for the first time unlocking K3 P as the reversible potassiation product for elemental P anodes. Density functional theory calculations reveal the fast adsorption and diffusion kinetics of K-ion at the heterostructure interface, which delivers a highly reversible specific capacity of 923 mAh g-1 at 0.05 A g-1 , excellent rate capability (335 mAh g-1 at 1 A g-1 ), and cycling performance (83.3% capacity retention at 0.8 A g-1 after 300 cycles). These results can unlock other sluggish and irreversible battery chemistries toward sustainable and high-performing energy storage.

In Situ Fabricated Non-Flammable Quasi-Solid Electrolytes for Li-Metal Batteries

Lithium metal batteries (LMBs) are viewed as one of the most promising high energy density battery systems, but their practical application is hindered by significant fire hazards and fast performance degradation due to the lack of a safe and compatible configuration. Herein, nonflammable quasi-solid electrolytes (NQSEs) are designed and fabricated by using the in situ polymerization method, in which 1,3,2-dioxathiolan-2,2-oxide is used as both initiator to trigger the in situ polymerization of solvents and interphase formation agent to construct robust interface layers to protect the electrodes, and triethyl phosphate as a fire-retardant agent. The NQSEs show a high ionic conductivity of 0.38 mS cm-1 at room temperature and enable intimate solid-electrolyte interphases, and demonstrate excellent performance with stable plating/striping of Li metal anode, and high voltage (4.5 V) and high temperature (>60 °C) survivability. The findings provide an effective strategy to build high-temperature, high-energy density, and safe quasi-solid LMBs.

A host potassiophilicity strategy for unprecedentedly stable and safe K metal batteries

Creating high-performance host materials for potassium (K) metal anodes remains a significant challenge due to the complex preparation process and poor K reversibility. In our work, we developed a potassiophilicity strategy using an oxygen-modified carbon cloth (O-CC) network as a host for K metal anodes. The O-CC network exhibited superior potassiophilic ability, and this improvement was also observed in other carbon hosts using the same process. The oxygen-induced epoxy group in the carbon network regulates interface electrons and enables strong binding of K adatoms through orbital hybridization, resulting in fewer side reactions with the electrolyte and promoting K-ion desolvation and uniform deposition. These factors result in unprecedented stability of the carbon network host, with a long lifespan of over 5500 hours at 0.5 mA cm-2/0.5 mA h cm-2 and 3500 h at 1 mA cm-2/0.5 mA h cm-2 in symmetric cells for K metal anodes, surpassing the cycle life of all previously reported K metal anodes. Furthermore, a high average coulombic efficiency of over 99.3% is demonstrated in O-CC//K cells during 210 cycles. The O-CC also exhibited a stable electrochemical performance, with a capacity retention of 73.3% in full cells coupled with a perylene-3,4,9,10-tetracarboxylic dianhydride cathode. We believe that this new strategy holds great promise for metal anodes in battery applications.

Pathways to Next-Generation Fire-Safe Alkali-Ion Batteries

High energy and power density alkali-ion (i.e., Li+ , Na+ , and K+ ) batteries (AIBs), especially lithium-ion batteries (LIBs), are being ubiquitously used for both large- and small-scale energy storage, and powering electric vehicles and electronics. However, the increasing LIB-triggered fires due to thermal runaways have continued to cause significant injuries and casualties as well as enormous economic losses. For this reason, to date, great efforts have been made to create reliable fire-safe AIBs through advanced materials design, thermal management, and fire safety characterization. In this review, the recent progress is highlighted in the battery design for better thermal stability and electrochemical performance, and state-of-the-art fire safety evaluation methods. The key challenges are also presented associated with the existing materials design, thermal management, and fire safety evaluation of AIBs. Future research opportunities are also proposed for the creation of next-generation fire-safe batteries to ensure their reliability in practical applications.

Self-Healing Binder for High-Voltage Batteries

Lithium-ion batteries are core components of flexible electronic devices. However, deformation types, such as impinging, bending, stretching, folding, and twisting, can cause internal cracks and, eventually, damage these batteries. The cracks separate the active particles from the conductive particles and the binder, as well as the electrode from the collector. Self-healing binders can alleviate this mechanical damage and improve the stress response of active material particles during high rates of charging and discharging of these batteries and the operation at a high voltage, thereby enhancing their cycle performance. In the present study, a thermoplastic intrinsic self-healing polymer (TISP) binder is proposed. The TISP is obtained by polymerization of butanediol (2,3-BDO), propylene glycol (1,3-PDO), succinic acid (SuA), sebacic acid (SeA), and iconic acid (IA). The hydroxyl and ester groups in its structure can form diverse bonds including the hydrogen and ion-dipole with active particles and the current collector, thereby producing elevated adhesion. Its properties, including a low glass transition temperature (-60 °C), amorphous structure, and low cross-link density, improve the mobility of polymer chains at 40 °C, and this facilitates structural recovery and the maintenance of strong adhesions. Owing to its higher occupied molecular orbital (HOMO) level than the electrolyte solvent, the TISP is likely oxidized before the main component of the electrolyte during charging. This decomposition produces a chemical passivation interphase on the cathode which reduces side reactions of LiCoO₂ and the electrolyte under high-voltage conditions. Tests reveal that a LiCoO₂ electrode battery using the TISP as a binder retains 162.4 mAh g^-1 after 349 cycles at 4.5 V, and this represents an 86.5% capacity retention. In addition, heating (40 °C, 1 h) of a scratch-damaged electrode can recover a specific capacity of 156.6 mAh g^-1 after 349 cycles at 4.5 V. Relative to a battery without any mechanical scratch, this capacity recovery represents approximately 96%, and this demonstrates the importance of the TISP to the high-voltage damaged electrode.

Investigation of SnS2 -rGO Sandwich Structures as Negative Electrode for Sodium-Ion and Potassium-Ion Batteries

Sodium-ion and potassium-ion batteries (NIBs and KIBs) are considered promising alternatives to replace lithium-ion batteries (LIBs) in energy storage applications due to the natural abundance and low cost of Na and K. Nevertheless, a critical challenge is that the large size of Na⁺ /K⁺ leads to a huge volume change of the hosting material during electrochemical cycling, resulting in rapid capacity decay. Among negative candidates for alkali-metal-ion batteries, SnS₂ is attractive due to the competitively high specific capacity, low redox potential and high abundance. Porous few-layer SnS₂ nanosheets are in situ grown on reduced graphene oxide, forming a SnS₂ -rGO sandwich structure via strong C-O-Sn bonds. This nano-scaled sandwich structure not only shortens Na⁺ /K⁺ and electron transport pathways but also accommodates volume expansion, thereby enabling high and stable electrochemical cycling performance of SnS₂ -rGO. This work explores the influence of different conductive carbons (Super P and C65) on the SnS₂ -rGO electrode. In addition, the effects of the electrolyte additive fluoroethylene carbonate (FEC) on the electrochemical performance in NIBs and KIBs is evaluated. This work provides guidelines for optimized electrode structure design, electrolyte additives and carbon additives for the realization of better NIBs and KIBs.

A Review on Regulating Li+ Solvation Structures in Carbonate Electrolytes for Lithium Metal Batteries

Lithium metal batteries (LMBs) are considered promising candidates for next-generation battery systems due to their high energy density. However, commercialized carbonate electrolytes cannot be used in LMBs due to their poor compatibility with lithium metal anodes. While increasing cut-off voltage is an effective way to boost the energy density of LMBs, conventional ethylene carbonate-based electrolytes undergo a number of side reactions at high voltages. It is therefore critical to upgrade conventional carbonate electrolytes, the performance of which is highly influenced by the solvation structure of lithium ions (Li⁺ ). This review provides a comprehensive overview of the strategies to regulate the solvation structure of Li⁺ in carbonate electrolytes for LMBs by better understanding the science behind the Li⁺ solvation structure and Li⁺ behavior. Different strategies are systematically compared to help select better electrolytes for specific applications. The remaining scientific and technical problems are pointed out, and directions for future research on carbonate electrolytes for LMBs are proposed.

Electrospun carbon-based nanomaterials for next-generation potassium batteries

Rechargeable potassium (K) batteries that are of low cost, with high energy densities and long cycle lives have attracted tremendous interest in affordable and large-scale energy storage. However, the large size of the K-ion leads to sluggish reaction kinetics and causes a large volume variation during the ion insertion/extraction processes, thus hindering the utilization of active electrode materials, triggering a serious structural collapse, and deteriorating the cycling performance. Therefore, the exploration of suitable materials/hosts that can reversibly and sustainably accommodate K-ions and host K metals are urgently needed. Electrospun carbon-based materials have been extensively studied as electrode/host materials for rechargeable K batteries owing to their designable structures, tunable composition, hierarchical pores, high conductivity, large surface areas, and good flexibility. Here, we present the recent developments in electrospun CNF-based nanomaterials for various K batteries (e.g., K-ion batteries, K metal batteries, K-chalcogen batteries), including their fabrication methods, structural modulation, and electrochemical performance. This Feature Article is expected to offer guidelines for the rational design of novel electrospun electrodes for the next-generation K batteries.

Electrolytes in Organic Batteries

Organic batteries using redox-active polymers and small organic compounds have become promising candidates for next-generation energy storage devices due to the abundance, environmental benignity, and diverse nature of organic resources. To date, tremendous research efforts have been devoted to developing advanced organic electrode materials and understanding the material structure-performance correlation in organic batteries. In contrast, less attention was paid to the correlation between electrolyte structure and battery performance, despite the critical roles of electrolytes for the dissolution of organic electrode materials, the formation of the electrode-electrolyte interphase, and the solvation/desolvation of charge carriers. In this review, we discuss the prospects and challenges of organic batteries with an emphasis on electrolytes. The differences between organic and inorganic batteries in terms of electrolyte property requirements and charge storage mechanisms are elucidated. To provide a comprehensive and thorough overview of the electrolyte development in organic batteries, the electrolytes are divided into four categories including organic liquid electrolytes, aqueous electrolytes, inorganic solid electrolytes, and polymer-based electrolytes, to introduce different components, concentrations, additives, and applications in various organic batteries with different charge carriers, interphases, and separators. The perspectives and outlook for the future development of advanced electrolytes are also discussed to provide a guidance for the electrolyte design and optimization in organic batteries. We believe that this review will stimulate an in-depth study of electrolytes and accelerate the commercialization of organic batteries.

Ether-based electrolytes enable the application of nitrogen and sulfur co-doped 3D graphene frameworks as anodes in high-performance sodium-ion batteries

The development of graphitic carbon materials as anodes of sodium-ion batteries (SIBs) is greatly restricted by their inherent low specific capacity. Herein, nitrogen and sulfur co-doped 3D graphene frameworks (NSGFs) were successfully synthesized via a simple and facile one-step hydrothermal method and exhibited high Na storage capacity in ether-based electrolytes. A systematic comparison was made between NSGFs, undoped graphene frameworks (GFs) and nitrogen-doped graphene frameworks (NGFs). It is demonstrated that the high specific capacity of NSGFs can be attributed to the free diffusion of Na ions within the graphene layer and reversible reaction between -C-S_x-C- covalent chains and Na ions thanks to the large interplanar distance and the dominant -C-S_x-C- covalent chains in NSGFs. NSGF anodes, therefore, exhibit a high initial coulombic efficiency (ICE) (92.8%) and a remarkable specific capacity of 834.0 mA h g^-1 at 0.1 A g^-1. Kinetic analysis verified that the synergetic effect of N/S co-doping not only largely enhanced the Na ion diffusion rate but also reduced the electrochemical impedance of NSGFs. Postmortem techniques, such as SEM, ex situ XPS, HTEM and ex situ Raman spectroscopy, all demonstrated the extremely physicochemically stable structure of the 3D graphene matrix and ultrathin inorganic-rich solid electrolyte interphase (SEI) films formed on the surface of NSGFs. Yet it is worth noting that the Na storage performance and mechanism are exclusive to ether-based electrolytes and would be inhibited in their carbonate ester-based counterparts. In addition, the corrosion of copper foils under the synergetic effect of S atoms and ether-based electrolytes was reported for the first time. Interestingly, by-products derived from this corrosion could provide additional Na storage capacity. This work sheds light on the mechanism of improving the electrochemical performance of carbon-based anodes by heteroatom doping in SIBs and provides a new insight for designing high-performance anodes of SIBs.

Electrolyte Design Strategies for Non-Aqueous High-Voltage Potassium-Based Batteries

High-voltage potassium-based batteries are promising alternatives for lithium-ion batteries as next-generation energy storage devices. The stability and reversibility of such systems depend largely on the properties of the corresponding electrolytes. This review first presents major challenges for high-voltage electrolytes, such as electrolyte decomposition, parasitic side reactions, and current collector corrosion. Then, the state-of-the-art modification strategies for traditional ester and ether-based organic electrolytes are scrutinized and discussed, including high concentration, localized high concentration/weakly solvating strategy, multi-ion strategy, and addition of high-voltage additives. Besides, research advances of other promising electrolyte systems, such as potassium-based ionic liquids and solid-state-electrolytes are also summarized. Finally, prospective future research directions are proposed to further enhance the oxidative stability and non-corrosiveness of electrolytes for high-voltage potassium batteries.

Low-Grade Thermal Energy Harvesting and Self-Powered Sensing Based on Thermogalvanic Hydrogels

Thermoelectric cells (TEC) directly convert heat into electricity via the Seebeck effect. Known as one TEC, thermogalvanic hydrogels are promising for harvesting low-grade thermal energy for sustainable energy production. In recent years, research on thermogalvanic hydrogels has increased dramatically due to their capacity to continuously convert heat into electricity with or without consuming the material. Until recently, the commercial viability of thermogalvanic hydrogels was limited by their low power output and the difficulty of packaging. In this review, we summarize the advances in electrode materials, redox pairs, polymer network integration approaches, and applications of thermogalvanic hydrogels. Then, we highlight the key challenges, that is, low-cost preparation, high thermoelectric power, long-time stable operation of thermogalvanic hydrogels, and broader applications in heat harvesting and thermoelectric sensing.

Research progress of biomass carbon materials as anode materials for potassium-ion batteries

Biochar materials have attracted people's attention because of their environmental friendliness, abundant resources, and the use of waste resources for reuse. As a potassium-ion anode material, biomass char materials synthesized by different methods have broad application prospects. However, due to the problems of low initial magnification and limited potassium-storage capacity, it is necessary to improve the electrochemical performance through modifications, such as atomic doping. Atomic doping is an effective way to improve battery conductivity and potassium storage. In this paper, the synthesis method of biochar as an anode material for potassium-ion batteries and the influence of atomic doping on its modification in recent years are reviewed.

Understanding the Predominant Potassium-Ion Intercalation Mechanism of Single-Phased Bimetal Oxides by in Situ Magnetometry

The electrochemical performance of electrode materials is largely dependent on the structural and chemical evolutions during the charge-discharge processes. Hence, revealing ion storage chemistry could enlighten mechanistic understanding and offer guidance for rational design for energy storage materials. Here, we investigate the mechanisms of potassium (K)-ion storage in the promising bimetal oxide materials by in situ magnetometry. We focus on a single-phased hollow FeTiO₃ (SPH-FTO) hexagonal prism synthesized through a complexing-reagent assisted approach and find that the K-ion storage in this compound occurs predominantly with an intercalation mechanism and fractionally a conversion mechanism. We also demonstrate a K-ion hybrid capacitor assembled with the prepared SPH-FTO hexagonal prism anode and activated carbon cathode, delivering a high energy density and high power density as well as extraordinary cycling stability. This new understanding is used to showcase the inherently high K-ion storage properties from the earth-abundant FeTiO₃.

Manipulating K-Storage Mechanism of Soft Carbon via Molecular Design-Driven Structure Transformation

The emerging potassium-ion batteries (PIBs) have been placing stratospheric expectations for realizing grid-scale electrochemical storage of renewable energy. However, the unsatisfactory K-storage of PIB anode materials, especially promising carbonaceous materials, significantly limited the development of PIBs. Here, a molecular design strategy was proposed to realize controllable structure transformation of soft carbon (SC) materials for enhanced K-storage performance. The optimized SC-PCN material delivered a high reversible K-storage capacity of 838 mAh/g at 50 mA/g, outstanding rate capability (213 mAh/g at 1000 mA/g), and excellent long-term cycling performance (301 mAh/g maintained after 300 cycles at 500 mA/g), superior to most previously reported carbon-based PIB anodes materials. Reaction kinetic analysis revealed that the proposed molecular design strategy can achieve the transformation from a surface capacitive-dominated mechanism to a capacitive-diffusion hybrid mechanism for SC-PCN, benefiting from its unique microstructures with highly defective surface generated via the synergistic effect from template removal, N doping, and surface reconstruction. The optimal hybrid K-storage mechanism should be responsible for the excellent K-storage properties of the prepared SC-PCN.

A Fast-Charging and High-Temperature All-Organic Rechargeable Potassium Battery

Developing fast-charging, high-temperature, and sustainable batteries is critical for the large-scale deployment of energy storage devices in electric vehicles, grid-scale electrical energy storage, and high temperature regions. Here, a transition metal-free all-organic rechargeable potassium battery (RPB) based on abundant and sustainable organic electrode materials (OEMs) and potassium resources for fast-charging and high-temperature applications is demonstrated. N-doped graphene and a 2.8 m potassium hexafluorophosphate (KPF₆ ) in diethylene glycol dimethyl ether (DEGDME) electrolyte are employed to mitigate the dissolution of OEMs, enhance the electrode conductivity, accommodate large volume change, and form stable solid electrolyte interphase in the all-organic RPB. At room temperature, the RPB delivers a high specific capacity of 188.1 mAh g^-1 at 50 mA g^-1 and superior cycle life of 6000 and 50000 cycles at 1 and 5 A g^-1 , respectively, demonstrating an ultra-stable and fast-charging all-organic battery. The impressive performance at room temperature is extended to high temperatures, where the high-mass-loading (6.5 mg cm^-2 ) all-organic RPB exhibits high-rate capability up to 2 A g^-1 and a long lifetime of 500 cycles at 70-100 °C, demonstrating a superb fast-charging and high-temperature battery. The cell configuration demonstrated in this work shows great promise for practical applications of sustainable batteries at extreme conditions.

Ionothermal Synthesis of Fully Conjugated Covalent Organic Frameworks for High-Capacity and Ultrastable Potassium-Ion Batteries

Fully aromatic conjugated covalent organic frameworks (FAC-COFs) with excellent physicochemical stability have been emerging as active semiconductors for diverse potential applications. Developing efficient synthesis methods for fabricating FAC-COFs will significantly facilitate the exploration over their material and photonic/electronic functionalities. Herein, a facile solvent-free strategy is developed for the synthesis of 2D phthalocyanine-based FAC-COFs (FAC-Pc-COFs). Cyclopolymerization of benzo[1,2-b:4,5-b']bis[1,4]benzodioxin-2,3,9,10-tetracarbonitrile (BBTC) and quinoxalino[2',3':9,10]phenanthro[4,5-abc]phenazine-6,7,15,16-tetracarbonitrile (QPPTC) in ZnCl₂  leads to the fast formation and isolation of BB-FAC-Pc-COF and QPP-FAC-Pc-COF, respectively. Powder X-ray diffraction and electron microscopy analysis reveal their crystalline nature with sql topology and AA stacking configuration. Thermogravimetric analysis and immersion experiment indicate their excellent stability. The conductivity test demonstrates their high conductivity of 0.93-1.94 × 10^-4  S cm^-1  owing to the fully π-conjugated electronic structural nature. In particular, the as-prepared FAC-Pc-COFs show high-performance K⁺ storage in potassium-ion batteries due to their excellent conductivity, highly ordered and robust structure, and N/O-rich framework nature. Impressively, QPP-FAC-Pc-COF shows a large reversible capacity of 424 mA h g^-1  after 100 cycles at 50 mA g^-1  and a capacity retention of nearly 100% at 2000 mA g^-1  for over 10 000 cycles.

Green and Scalable Template-Free Strategy to Fabricate Honeycomb-Like Interconnected Porous Micro-Sized Layered Sb for High-Performance Potassium Storage

The tremendous volume change and severe pulverization of micro-sized Sb anode generate no stable capacity in potassium-ion batteries (PIBs). The honeycomb-like porous structure provides free spaces to accommodate its volume expansion and offers efficient ion transport, yet complex synthesis and low yield limits its large-scale application. Here, a green, scalable template-free method for designing a 3D honeycomb-like interconnected porous micro-sized Sb (porous-Sb) is proposed. Its honeycomb-like porous formation mechanism is also verified. Under hydrothermal conditions, Sb reacts with water and dissolved oxygen in water, undergoing non-homogeneous and continuous corrosion at grain boundaries, and producing soluble H₂ Sb₂ O₆ (H₂ O), which regulates the porous structure of Sb by controlling reaction time. Benefiting from its porous structure and micron size, porous-Sb anode displays large gravimetric and volumetric capacities with 655.5 mAh g^-1 and 2,001.9 mAh cm^-3 at 0.05 A g^-1 and superior rate performance of 441.9 mAh g^-1 at 2.0 A g^-1 in PIBs. Furthermore, ex situ characterization and kinetic analysis uncover the small volume expansion and fast K⁺ reaction kinetics of porous Sb during potassiation/depotassiation, originating from its large electrolyte contact area and internal expansion mechanism. It verifies a green, scalable template-free strategy to construct honeycomb-like porous metals for energy storage and conversion.

Less is More: Trace Amount of a Cyclic Sulfate Electrolyte Additive Enable Ultra-Stable Graphite Anode for High-Performance Potassium-Ion Batteries

Graphite can be successfully used as an anode for potassium-ion batteries (PIBs), while its conversion to KC₈ leads to huge volume expansion, destruction of solid electrolyte interphase (SEI), and thus poor cycling stability. Incorporating additives into electrolytes is an economical and effective way to construct robust SEI for high-performance PIBs. Herein, we developed a series of sulfur-containing additives for PIB graphite anodes, and the impacts of their molecular structure and contents on the SEI are also systematically investigated. Compared with butylene sulfites and 1,3-propane sultone, the 1,3,2-dioxathiolane 2,2-dioxide (DTD) additive endows the graphite electrode (GE) with a higher reversible capacity, and better cycling stability in both the dilute potassium bis(fluorosulfonyl)imide (KFSI)- and potassium hexafluorophosphate (KPF₆)-based carbonate electrolyte, as a result of a thinner and sulfate-enriched SEI. Moreover, the addition of a trace amount (0.2 wt %) DTD to the electrolyte can effectively protect the GE running over 800 cycles at 1 C. Excessive additives in the electrolyte will induce continuous SEI growth and render a rapid capacity fading of the GE. This strategy using the electrolyte additive paves the way for the design of novel PIB electrolytes and thus provides a great opportunity for commercial PIBs.

Electrolyte Regulation for Non-Graphitic Carbon to Achieve Stable Long-Cycling K-Storage

Potassium-ion batteries have been considered as a promising next-generation energy storage system due to low cost but comparable energy density to lithium-ion batteries. However, carbon-based anode materials usually delivered unsatisfactory K-storage capacity as well as long-cycling performance due to poor matching with common electrolytes, thus forming an unstable solid electrolyte interphase (SEI). Herein, a robust KF-rich SEI can be achieved on the as-prepared non-graphitic carbon surface by regulating the electrolyte solvation structures, which can significantly suppress redox reaction of solvents and ensure highly reversible K⁺ intercalation/deintercalation. As a result, the as-synthesized non-graphitic carbon anode predictably exhibits super long-cycling performance with about 200 mA h/g at 100 mA/g for 1000 cycles and a stable capacity of 135 mA h/g at 500 mA/g for 2000 cycles with negligible capacity decay in the optimized 3 M KFSI/DME electrolyte. This work provides deep insights into further development and improvement of advanced electrolyte systems for next generation energy storage devices.

Insights into Cationic Transference Number Values and Solid Electrolyte Interphase Growth in Liquid/Solid Electrolytes for Potassium Metal Batteries

Liquid/solid battery electrolytes make separators dispensable and enable a high cationic transference number with liquid-like room temperature ionic conductivity. This work gives insights into electrochemical behavior (galvanostatic polarization and time-dependent impedance spectroscopy) of liquid/solid electrolytes containing potassium salts in battery cells enclosing potassium metal anodes. Very high potassium transference numbers (t _K = 0.88) are observed in carbonate-based electrolytes, linked with long-term mechanical instability of the solid electrolyte interphase on the potassium anode. In the case of glyme-based electrolytes, electrochemical behavior indicates the existence of the highly porous solid electrolyte interphase and additional surface porosity of the potassium electrode.

Phase Transformations and Phase Segregation during Potassiation of Sn x P y Anodes

K-ion batteries (KIBs) have the potential to offer a cheaper alternative to Li-ion batteries (LIBs) using widely abundant materials. Conversion/alloying anodes have high theoretical capacities in KIBs, but it is believed that electrode damage from volume expansion and phase segregation by the accommodation of large K-ions leads to capacity loss during electrochemical cycling. To date, the exact phase transformations that occur during potassiation and depotassiation of conversion/alloying anodes are relatively unexplored. In this work, we synthesize two distinct compositions of tin phosphides, Sn₄P₃ and SnP₃, and compare their conversion/alloying mechanisms with solid-state nuclear magnetic resonance (SSNMR) spectroscopy, powder X-ray diffraction (XRD), and density functional theory (DFT) calculations. Ex situ ³¹P and ¹¹⁹Sn SSNMR analyses reveal that while both Sn₄P₃ and SnP₃ exhibit phase separation of elemental P and the formation of KSnP-type environments (which are predicted to be stable based on DFT calculations) during potassiation, only Sn₄P₃ produces metallic Sn as a byproduct. In both anode materials, K reacts with elemental P to form K-rich compounds containing isolated P sites that resemble K₃P but K does not alloy with Sn during potassiation of Sn₄P₃. During charge, K is only fully removed from the K₃P-type structures, suggesting that the formation of ternary regions in the anode and phase separation contribute to capacity loss upon reaction of K with tin phosphides.