Flexible Carbon Dots-Intercalated MXene Film Electrode with Outstanding Volumetric Performance for Supercapacitors

2D MXenes have emerged as promising supercapacitor electrode materials due to their metallic conductivity, pseudo-capacitive mechanism, and high density. However, layer-restacking is a bottleneck that restrains their ionic kinetics and active site exposure. Herein, a carbon dots-intercalated strategy is proposed to fabricate flexible MXene film electrodes with both large ion-accessible active surfaces and high density through gelation of calcium alginate (CA) within the MXene nanosheets followed by carbonization. The formation of CA hydrogel within the MXene nanosheets accompanied by evaporative drying endow the MXene/CA film with high density. In the carbonization process, the CA-derived carbon dots can intercalate into the MXene nanosheets, increasing the interlayer spacing and promoting the electrolytic diffusion inside the MXene film. Consequently, the carbon dots-intercalated MXene films exhibit high volumetric capacitance (1244.6 F cm⁻³ at 1 A g⁻¹), superior rate capability (662.5 F cm⁻³ at 1000 A g⁻¹), and excellent cycling stability (93.5% capacitance retention after 30 000 cycles) in 3 m H₂SO₄. Additionally, an all-solid-state symmetric supercapacitor based on the carbon dots-intercalated MXene film achieves a high volumetric energy density of 27.2 Wh L⁻¹. This study provides a simple yet efficient strategy to construct high-volumetric performance MXene film electrodes for advanced supercapacitors.     

Flexible Transparent Bifunctional Capacitive Sensors with Superior Areal Capacitance and Sensing Capability based on PEDOT:PSS/MXene/Ag Grid Hybrid Electrodes

Flexible transparent supercapacitors (FTSs) have aroused considerable attention. Nonetheless, balancing energy storage capability and transparency remains challenging. Herein, a new type of FTSs with both excellent energy storage and superior transparency is developed based on PEDOT:PSS/MXene/Ag grid ternary hybrid electrodes. The hybrid electrodes can synergistically utilize the high optoelectronic properties of Ag grids, the excellent capacitive performance of MXenes, and the superior chemical stability of PEDOT:PSS, thus, simultaneously demonstrating excellent optoelectronic properties (T: ≈89%, R_s: ≈39 Ω sq⁻¹), high areal specific capacitance, superior mechanical softness, and excellent anti-oxidation capability. Due to the excellent comprehensive performances of the hybrid electrodes, the resulting FTSs exhibit both high optical transparency (≈71% and ≈60%) and large areal specific capacitance (≈3.7 and ≈12 mF cm⁻²) besides superior energy storage capacity (P: 200.93, E: 0.24 µWh cm⁻²). Notably, the FTSs show not only excellent energy storage but also exceptional sensing capability, viable for human activity recognition. This is the first time to achieve FTSs that combine high transparency, excellent energy storage and good sensing all-in-one, which make them stand out from conventional flexible supercapacitors and promising for next-generation smart flexible energy storage devices.     

Reassembly of MXene Hydrogels into Flexible Films towards Compact and Ultrafast Supercapacitors

The freestanding MXene films are promising for compact energy storage ascribing to their high pseudocapacitance and density, yet the sluggish ion transport caused by the most densely packed structure severely hinders their rate capability. Here, a reassembly strategy for constructing freestanding and flexible MXene-based film electrodes with a tunable porous structure is proposed, where the Ti₃C₂T_x microgels disassembled from 3D structured hydrogel are reassembled together with individual Ti₃C₂T_x nanosheets in different mass ratios to form a densely packed 3D network in microscale and a film morphology in macroscale. The space utilization of produced film can be maximized by a good balance of the density and porosity, resulting in a high volumetric capacitance of 736 F cm⁻³ at an ultrahigh scan rate of 2000 mV s⁻¹. The fabricated supercapacitor yields a superior energy density of 40 Wh L⁻¹ at a power density of 0.83 kW L⁻¹, and an energy density of 21 Wh L⁻¹ can be still maintained even when the power density reaches 41.5 kW L⁻¹, which are the highest values reported to date for symmetric supercapacitors in aqueous electrolytes. More promisingly, the reassembled films can be used as electrodes of flexible supercapacitors, showing excellent flexibility and integrability.     

Nature-Inspired Interconnected Macro/Meso/Micro-Porous MXene Electrode

The geometric multiplication development of MXene has promoted it to become a star material in numerous applications including, but not limited to, energy storage. It is found that pore structure modulation engineering can improve the inherent properties of MXene, in turn significantly enhancing its electrochemical performance. However, most of the current works have focused on exploring the structure-effective relationships of the single-scale pore structure regulation of MXene. Inspired by Murray's law from nature where a highly graded structure of the organisms is discovered and used to achieve effective diffusion and maximize mass transfer, a hierarchically interconnected porous MXene electrode across micro-meso-macroporous is constructed. This MXene-based electrode provides large amounts of active sites while greatly shortening the ion diffusion channel. Finally, the zinc ion microcapacitor based on this MXene electrode exhibits an ultrahigh area-specific capacitance up to 410 mF cm⁻² and an energy density up to 103 µWh cm⁻² at a power density of 2100 µW cm⁻². The areal energy density outperforms the currently reported zinc ion microcapacitors. This study supports an effective strategy for electrode materials (including but not limited to MXene) to achieve ultra-short ion diffusion channels and maximum transport efficiency for next-generation high-performance energy storage.     

Controllable Patterning of Porous MXene (Ti3C2) by Metal-Assisted Electro-Gelation Method

Since discovered in 2011, transition metal carbides or nitrides (MXenes) have attracted enormous attention due to their unique properties. Morphology regulation strategies assembling 2D MXene sheets into 3D architecture have endowed the as-formed porous MXene with a better performance in various fields. However, the direct patterning strategy for the porous MXene into integration with multifunctional and multichannel electronic devices still needs to be investigated. The metal-assisted electro-gelation method the authors propose can directly generate porous-structured MXene hydrogel with a tunable feature. By electrolyzing the sacrificial metal, the released metal cations initiate the electro-gelation process during which electrostatic interactions occur between cations and the MXene sheets. A high spatial resolution down to micro-meter level is achieved utilizing the method, enabling high-performance hydrogels with more complex architectures. Electronics prepared through this metal-assisted electro-gelation process have shown promising applications of the porous MXene in energy and biochemical sensing fields. Energy storage devices with a capacitance at 33.3 mF cm⁻² and biochemical sensors show prominent current responses towards metabolites (sensitivity of H₂O₂: 165.6  µ A mm ⁻¹ cm⁻²; sensitivity of DA: 212 nA  µ m ⁻¹ cm⁻²), suggesting that the metal-assisted electro-gelation method will become a prospective technique for advanced fabrication of MXene-based devices.     

Self-Adhesive Polydimethylsiloxane Foam Materials Decorated with MXene/Cellulose Nanofiber Interconnected Network for Versatile Functionalities

Polydimethylsiloxanes (PDMS) foam as one of next-generation polymer foam materials shows poor surface adhesion and limited functionality, which greatly restricts its potential applications. Fabrication of advanced PDMS foam materials with multiple functionalities remains a critical challenge. In this study, unprecedented self-adhesive PDMS foam materials are reported with worm-like rough structure and reactive groups for fabricating multifunctional PDMS foam nanocomposites decorated with MXene/cellulose nanofiber (MXene/CNF) interconnected network by a facile silicone foaming and dip-coating strategy followed by silane surface modification. Interestingly, such self-adhesive PDMS foam produces strong interfacial adhesion with the hybrid MXene/CNF nano-coatings. Consequently, the optimized PDMS foam nanocomposites have excellent surface super-hydrophobicity (water contact angle of ≈159^o), tunable electrical conductivity (from 10⁻⁸ to 10 S m⁻¹), stable compressive cyclic reliability in both wide-temperature range (from −20 to 200 ^oC) and complex environments (acid, sodium, and alkali conditions), outstanding flame resistance (LOI value of >27% and low smoke production rate), good thermal insulating performance and reliable strain sensing in various stress modes and complex environmental conditions. It provides a new route for the rational design and development of advanced PDMS foam nanocomposites with versatile multifunctionalities for various promising applications such as intelligent healthcare monitoring and fire-safe thermal insulation.     

Redox-Active Polymer Integrated with MXene for Ultra-Stable and Fast Aqueous Proton Storage

Proton is a charge carrier with the smallest ionic size and quickest kinetics, making aqueous proton batteries (APBs), a promising technology for safe and profitable energy storage systems. Despite being potential electrode materials, organic compounds have not yet been fully investigated in terms of proton storage properties and APB applications due to their low capacity and unstable cycle life in aqueous electrolytes. Herein, a novel redox-active polymer (PDPZ) with diquinoxalino-phenazine as the structural unit has been designed, which is further integrated with MXene nanosheets to construct a flexible PDPZ@MXene electrode material with a rapid and ultra-stable proton storage behavior. In-operando monitoring techniques, i.e., in situ Raman and in situ FTIR, demonstrate the highly reversible redox reaction between CN and C N/N H bonds in electro-active PDPZ molecule with the strong proton absorption ability. Theoretical calculation further proves the electron transfer from MXene to PDPZ promotes the redox reaction of the PDPZ@MXene electrode. As a result, a flexible APB device is developed with a considerable energy density (64.3 mWh cm⁻³), a supercapacitor-level power density (6000 mW cm⁻³), and a record lifespan with ≈98.2% capacity retention over 10 000 cycles, revealing its potential applications in satisfying the various requirements of energy storage systems.     

Uniformly MXene-Grafted Eutectic Aluminum-Cerium Alloys as Flexible and Reversible Anode Materials for Rechargeable Aluminum-Ion Battery

Aluminum is an attractive anode material in aqueous multivalent-metal batteries for large-scale energy storage because of its high Earth abundance, low cost, high theoretic capacity, and safety. However, state-of-the-art aqueous aluminum-ion batteries based on aluminum anode persistently suffer from poor rechargeability and low coulombic efficiency due to irreversibility of aluminum stripping/plating and dendrite growth. Here eutectic aluminum-cerium alloys in situ grafted with uniform ultrathin MXene (MXene/E-Al₉₇Ce₃) as flexible, reversible, and dendrite-free anode materials for rechargeable aqueous aluminum-ion batteries is reported. As a result of the MXene serving as stable solid electrolyte interphase to inhibit side reactions and the lamella-nanostructured E-Al₉₇Ce₃ enabling directional Al stripping and deposition by making use of symbiotic α-Al metal and intermetallic Al₁₁Ce₃ lamellas, the MXene/E-Al₉₇Ce₃ hybrid electrodes exhibit reversible and dendrite-free Al stripping/plating with low voltage polarization of ± 54 mV for ≥1000 h in a low-oxygen-concentration aqueous aluminum trifluoromethanesulfonate (Al(OTF)₃) electrolyte. These superior electrochemical properties endow soft-package aluminum-ion batteries assembled with MXene/E-Al₉₇Ce₃ anode and Al_xMnO₂ cathode to have high initial discharge capacity of ≈360 mAh g⁻¹ at 1 A g⁻¹, and retain ≈85% after 500 cycles, along with the coulombic efficiency of as high as 99.5%.     

Overcoming the Challenges of Freestanding Tin Oxide-Based Composite Anodes to Achieve High Capacity and Increased Cycling Stability

Freestanding electrodes are a promising way to increase the energy density of the batteries by decreasing the overall amount of electrochemically inactive materials. Freestanding antimony doped tin oxide (ATO)-based hybrid materials have not been reported so far, although this material has demonstrated excellent performance in conventionally designed electrodes. Two different strategies, namely electrospinning and freeze-casting, are explored for the fabrication of ATO-based hybrid materials. It is shown that the electrospinning of ATO/carbon based electrodes from polyvinyl pyrrolidone polymer (PVP) solutions was not successful, as the resulting electrode material suffers from rapid degradation. However, freestanding reduced graphene oxide (rGO) containing ATO/C/rGO nanocomposites prepared via a freeze-casting route demonstrates an impressive rate and cycling performance reaching 697 mAh g⁻¹ at a high current density of 4 A g⁻¹, which is 40 times higher as compared to SnO₂/rGO and also exceeds the freestanding SnO₂-based composites reported so far. Antimony doping of the nanosized tin oxide phase and carbon coating are thereby shown to be essential factors for appealing electrochemical performance. Finally, the freestanding ATO/C/rGO anodes are combined with freestanding LiFe_0.2Mn_0.8PO₄/rGO cathodes to obtain a full freestanding cell operating without metal current collector foils showing nonetheless an excellent cycling stability.     

3D Printed Nanocarbon Frameworks for Li-Ion Battery Cathodes

The use of conductive carbon materials in 3D-printing is attracting growing academic and industrial attention in electrochemical energy storage due to the high customization and on-demand capabilities of the additive manufacturing. However, typical polymers used in conductive filaments for 3D printing show high resistivity and low compatibility with electrochemical energy applications. Removal of insulating thermoplastics in as-printed materials is a common post-printing strategy, however, excessive loss of thermoplastics can weaken the structural integrity. This work reports a two-step surface engineering methodology for fabrication of 3D-printed carbon materials for electrochemical applications, incorporating conductive poly(ortho-phenylenediamine) (PoPD) via electrodeposition. A conductive PoPD effectively enhances the electrochemical activities of 3D-printed frameworks. When PoPD-refilled frameworks casted with LiMn₂O₄ (LMO) composite materials used as battery cathode, it delivers a capacity of 69.1 mAh g⁻¹ at a current density of 0.036 mA cm⁻² ( ≈ 1.2 C discharge rate) and good cyclability with a retained capacity of 84.4% after 200 cycles at 0.36 mA cm⁻². This work provides a pathway for developing electroactive 3D-printed electrodes particularly with cost-efficient low-dimensional carbon materials for aqueous rechargeable Li-ion batteries.     

Paper-Mill Waste Reinforced Nanofluidic Membrane as High-Performance Osmotic Energy Generators

Nanofluidic membranes consisting of 2D materials and polymers are considered promising candidates for harvesting osmotic energy from river estuaries owing to their unique ion channels. However, micron-scale polymer chains agglomerate in the nanochannels, resulting in steric hindrance and affection ion transport. Herein, a nanofluidic membrane is designed from MXene and xylan nanoparticles that are derived from paper-mill waste. The demonstrated membrane reinforced by paper-mill waste has the characteristics of green, low-cost, and outstanding performance in mechanical properties and surface-charge-governed ionic transport. The MXene/carboxmethyl xylan (CMX) membrane demonstrates a high surface charge (ζ-potential of −44.3 mV) and 12 times higher strength (284.96 MPa) than the pristine MXene membrane. The resulting membrane shows intriguing features of high surface charge, high ion selectivity, and reduced steric hindrance, enabling it high osmotic energy generation performance. A potential of the nanofluidic membrane is ≈109 mV, the corresponding current of up to 2.73 µA, and the output power density of 14.52 mW m⁻² are obtained under a 1000-fold salt concentration gradient. As the electrolyte pH increases, the power density reaches 56.54 mW m⁻². This works demonstrate that CMX nanoparticles can effectively enhance the properties of the nanofluidic membrane and provide a promising strategy to design high-performance nanofluidic devices.     

Metal Single-Site Molecular Complex–MXene Heteroelectrocatalysts Interspersed Graphene Nanonetwork for Efficient Dual-Task of Water Splitting and Metal–Air Batteries

Development of multifunctional electrocatalysts with high efficiency and stability is of great interest in recent energy conversion technologies. Herein, a novel heteroelectrocatalyst of molecular iron complex (Fe_MC)-carbide MXene (Mo₂TiC₂T_x) uniformly embedded in a 3D graphene-based hierarchical network (GrH) is rationally designed. The coexistence of Fe_MC and MXene with their unique interactions triggers optimum electronic properties, rich multiple active sites, and favorite free adsorption energy for excellent trifunctional catalytic activities. Meanwhile, the highly porous GrH effectively promotes a multichannel architecture for charge transfer and gas/ion diffusion to improve stability. Therefore, the Fe_MC–MXene/GrH results in superb performances towards oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in alkaline medium. The practical tests indicate that Zn/Al–air batteries derived from Fe_MC–MXene/GrH cathodic electrodes produce high power densities of 165.6 and 172.7 mW cm⁻², respectively. Impressively, the liquid-state Zn–air battery delivers excellent cycling stability of over 1100 h. In addition, the alkaline water electrolyzer induces a low cell voltage of 1.55 V at 10 mA cm⁻² and 1.86 V at 0.4 A cm⁻² in 30 wt.% KOH at 80 °C, surpassing recent reports. The achievements suggest an exciting multifunctional electrocatalyst for electrochemical energy applications.     

Si/SiO2@Graphene Superstructures for High-Performance Lithium-Ion Batteries

The superstructure composed of various functional building units is promising nanostructure for lithium-ion batteries (LIBs) anodes with extreme volume change and structure instability, such as silicon-based materials. Here, a top-down route to fabricate Si/SiO₂@graphene superstructure is demonstrated through reducing silicalite-1 with magnesium reduction and depositing carbon layers. The successful formation of superstructure lies on the strong 3D network formed by the bridged-SiO₂ matrix coated around silicon nanoparticles. Furthermore, the mesoporous Si/SiO₂ with amorphous bridged SiO₂ facilitates the deposition of graphene layers, resulting in excellent structural stability and high ion/electron transport rate. The optimized Si/SiO₂@graphene superstructure anode delivers an outstanding cycling life for ≈1180 mAh g⁻¹ at 2 A g⁻¹ over 500 cycles, excellent rate capability for ≈908 mAh g⁻¹ at 12 A g⁻¹, great areal capacity for ≈7 mAh cm⁻² at 0.5 mA cm⁻², and extraordinary mechanical stability. A full cell test using LiFePO₄ as the cathode manifests a high capacity of 134 mAh g⁻¹ after 290 loops. More notably, a series of technologies disclose that the Si/SiO₂@graphene superstructure electrode can effectively maintain the film between electrode and electrolyte in LIBs. This design of Si/SiO₂@graphene superstructure elucidates a promising potential for commercial application in high-performance LIBs.     

Controlled Assembly of MXene Nanosheets as an Electrode and Active Layer for High-Performance Electronic Skin

MXenes are an emerging class of 2D transition metal carbides and nitrides. They have been widely used in flexible electronics owing to their excellent conductivity, mechanical flexibility, and water dispersibility. In this study, the electrode and active layer applications of MXene materials in electronic skins are realized. By utilizing vacuum filtration technology, few-layer MXene electrodes are integrated onto the top and bottom surfaces of the 3D polyacrylonitrile (PAN) network to form a stable electronic skin. The fabricated flexible device with Ti₃C₂T_x MXene electrodes outperforms those with other electrodes and exhibits excellent device performance, with a high sensitivity of 104.0 kPa⁻¹, fast response/recovery time of 30/20 ms, and a low detection limit of 1.5 Pa. Furthermore, the electrode and the constructed MXene/PAN-based flexible pressure sensor exhibit robust mechanical stability and can survive 240 bending cycles. Such a robust, flexible device can be enlarged or folded like a jigsaw puzzle or origami and transformed from 2D to 3D structures; moreover, it can detect tiny movements of human muscles, such as movements corresponding to sound production and intense movements during bending of fingers.     

Interlayer Injection of Low-Valence Zn Atoms to Activate MXene-Based Micro-Redox Capacitors With Battery-Type Voltage Plateaus

Insufficient and unstable energy output is the bottleneck issue radically restricting the application of micro-supercapacitors (MSCs). Herein, an interlayer atom injection strategy that can anchor low-valence Zn atoms (Zn^δ+, 0 < δ <2) on O-terminals of Ti₃C₂T_x (MXene) flakes within the MXene/silver-nanowires hybrid cathode of symmetric MSCs is first presented. Combining the polyacrylamide/ZnCl₂ hydrogel electrolyte rich in Cl⁻ and Zn²⁺ ions, the matched Zn^δ+/Zn²⁺ (−0.76 V vs SHE) and Ag/AgCl (0.23 V vs SHE), redox couples between the symmetrical electrodes are activated to offer faradaic charge storage beside ions-intercalation involved pseudocapacitance. Thus, a battery-type voltage plateau (≈0.9 V) appears in the discharge curve of a fabricated pseudo-symmetric micro-redox capacitor, simultaneously achieving energy density enhancement (117 µWh cm⁻² at 0.5 mA cm⁻²) and substantially improved power output stability (46% of the energy from the plateau region) relative to that before activation (98 µWh cm⁻² without voltage platform). The work provides a fire-new strategy to overcome the performance bottlenecks confronting conventional MSCs.     

3D HfO2 Thin Film MEMS Capacitor with Superior Energy Storage Properties

Capacitors are ubiquitous and crucial components in modern technologies. Future microelectronic devices require novel dielectric capacitors with higher energy storage density, higher efficiency, better frequency and temperature stabilities, and compatibility with integrated circuit (IC) processes. Here, in order to overcome these challenges, a novel 3D HfO₂ thin film capacitor is designed and fabricated by an integrated microelectromechanical system (MEMS) process. The energy storage density (ESD) of the capacitor reaches 28.94 J cm⁻³, and the energy storage efficiency of the capacitor is up to 91.3% under an applied electric field of 3.5 MV cm⁻¹. The ESD can be further improved by reducing the minimum period structure size of the 3D capacitor. Moreover, the 3D capacitor exhibits excellent temperature stability (up to 150 °C) and charge-discharge endurance (10⁷ cycles). The results indicate that the 3D HfO₂ thin film MEMS capacitor has enormous potential in energy storage applications in harsh environments, such as pulsed discharge and power conditioning electronics.     

Hyperelastic,Robust, Fire-Safe Multifunctional MXene Aerogels with Unprecedented Electromagnetic Interference Shielding Efficiency

MXene aerogels have shown great potential for many important functional applications, in particular electromagnetic interference (EMI) shielding. However, it has been a grand challenge to create mechanically hyperelastic, air-stable, and durable MXene aerogels for enabling effective EMI protection at low concentrations due to the difficulties in achieving tailorable porous structures, excellent mechanical elasticity, and desired antioxidation capabilities of MXene in air. Here, a facile strategy for fabricating MXene composite aerogels by co-assembling MXene and cellulose nanofibers during freeze-drying followed by surface encapsulation with fire-retardant thermoplastic polyurethane (TPU) is reported. Because of the maximum utilization of pore structures of MXene, and conductive loss enhanced by multiple internal reflections, as-prepared aerogel with 3.14 wt% of MXene exhibits an exceptionally high EMI shielding effectiveness of 93.5 dB, and an ultra-high MXene utilization efficiency of 2977.71 dB g g⁻¹, tripling the values in previous works. Owing to the presence of multiple hydrogen bonding and the TPU elastomer, the aerogel exhibits a hyperelastic feature with additional strength, excellent stability, superior durability, and high fire safety. This study provides a facile strategy for creating multifunctional aerogels with great potential for applications in EMI protection, wearable devices, thermal management, pressure sensing, and intelligent fire monitoring.     

Bilayered VOPO4⋅2H2O Nanosheets with High-Concentration Oxygen Vacancies for High-Performance Aqueous Zinc-Ion Batteries

2D materials with atomically precise thickness and tunable chemical composition hold promise for potential applications in nanoenergy. Herein, a bilayer-structured VOPO₄⋅2H₂O (bilayer-VOP) nanosheet is developed with high-concentration oxygen vacancies ([Vo˙˙]) via a facile liquid-exfoliation strategy. Galvanostatic intermittent titration technique study indicates a 6 orders of magnitude higher zinc-ion coefficient in bilayer-VOP nanosheets (4.6 × 10⁻⁷ cm⁻² s⁻¹) compared to the bulk counterpart. Assistant density functional theory (DFT) simulation indicates a remarkably enhanced electron conductivity with a reduced bandgap of ≈ 0.2 eV (bulk sample: 1.5 eV) along with an ultralow diffusion barrier of ≈ 0.08 eV (bulk sample: 0.13 eV) in bilayer-VOP nanosheets, thus leading to superior diffusion kinetics and electrochemical performance. Mott–Schottky (impedance potential) measurement also demonstrates a great increase in electronic conductivity with ≈ 57-fold increased carrier concentration owing to its high concentration [Vo˙˙]. Benefited by these unique features, the rechargeable zinc-ion battery composed of bilayer-VOP nanosheets cathode exhibits a remarkable capacity of 313.6 mAh g⁻¹ (0.1 A g⁻¹), an energy density of 301.4 Wh kg⁻¹, and a prominent rate capability (168.7 mAh g⁻¹ at 10 A g⁻¹).     

Mg-Doped Na4Fe3(PO4)2(P2O7)/C Composite with Enhanced Intercalation Pseudocapacitance for Ultra-Stable and High-Rate Sodium-Ion Storage

Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is considered as a promising cathode material for sodium-ion batteries (SIBs) due to its low cost, non-toxicity, and high structural stability, but its electrochemical performance is limited by the poor electronic conductivity. In this study, Mg-doped NFPP/C composites are presented as cathode materials for SIBs. Benefiting from the enhanced electrochemical kinetics and intercalation pseudocapacitance resulted from the Mg doping, the optimal Mg-doped NFPP/C composite (NFPP-Mg5%) delivers high rate performance (capacity of ≈40 mAh g⁻¹ at 20 A g⁻¹) and ultra-long cycling life (14 000 cycles at 5 A g⁻¹ with capacity retention of 80.8%). Moreover, the in situ X-ray diffraction and other characterizations reveal that the sodium storage process of NFPP-Mg5% is dominated by the intercalation pseudocapacitive mechanism. In addition, the full SIB based on NFPP-Mg5% cathode and hard carbon anode exhibits the discharge capacity of ≈50 mAh g⁻¹ after 200 cycles at 500 mA g⁻¹. This study demonstrates the feasibility of improving the electrochemical performance of NFPP by doping strategy and presents a low-cost, ultra-stable, and high-rate cathode material for SIBs.     

Ultralow Loss and High Tunability in a Non-perovskite Relaxor Ferroelectric

Dielectric ceramics are fundamental for electronic systems, including energy storages, microwave applications, ultrasonics, and sensors. Relaxor ferroelectrics show superb performance among dielectrics due to their high efficiency and energy density by the nature of nanodomains. Here, a novel non-perovskite relaxor ferroelectric, Bi₆Ti₅WO₂₂, with ultralow loss, ≈10⁻³, highly tunable permittivity, ≈2200 at room temperature with 40% tunability and the superparaelectric region at room temperature is presented. The actual crystal structure and the nanodomains of Bi₆Ti₅WO₂₂ are demonstrat Various-temperature neutron powder diffraction and in situ high-resolution transmission-electron-microscopy illustrate the twinning effect, subtle structure change and micro-strain in the material influenced by temperature, manifesting the actual crystal structure of Bi₆Ti₅WO₂₂. Compared with dielectric loss of BaTiO₃-based dielectric tunable materials, the loss of Bi₆Ti₅WO₂₂ is more than an order of magnitude lower, which makes it exhibit a figure of merit (≈240), much higher than that of conventional dielectric tunable materials (< 100), endorse the material great potential for direct applications. The present research offers a strategy for discovering novel relaxor ferroelectrics and a highly desirable material for fabricating energy storage capacitors, microwave dielectrics, and ultrasonics.