Strong Surface‐Bound Sulfur in Carbon Nanotube Bridged Hierarchical Mo2C‐Based MXene Nanosheets for Lithium–Sulfur Batteries

In this work, hydroxyl‐functionalized Mo₂C‐based MXene nanosheets are synthesized by facilely removing the Sn layer of Mo₂SnC. The hydroxyl‐functionalized surface of Mo₂C suppresses the shuttle effect of lithium polysulfides (LiPSs) through strong interaction between Mo atoms on the MXenes surface and LiPSs. Carbon nanotubes (CNTs) are further introduced into Mo₂C phase to enlarge the specific surface area of the composite, improve its electronic conductivity, and alleviate the volume change during discharging/charging. The strong surface‐bound sulfur in the hierarchical Mo₂C‐CNTs host can lead to a superior electrochemical performance in lithium–sulfur batteries. A large reversible capacity of ≈925 mAh g ^{? 1} is observed after 250 cycles at a current density of 0.1 C (1 C = 1675 mAh g^?1) with good rate capability. Notably, the electrodes with high loading amounts of sulfur can also deliver good electrochemical performances, i.e., initial reversible capacities of ≈1314 mAh g^?1 (2.4 mAh cm^?2), ≈1068 mAh g^?1 (3.7 mAh cm^?2), and ≈959 mAh g^?1 (5.3 mAh cm^?2) at various areal loading amounts of sulfur (1.8, 3.5, and 5.6 mg cm^?2) are also observed, respectively.     

Interfacial Engineering Coupled Valence Tuning of MoO3 Cathode for High‐Capacity and High‐Rate Fiber‐Shaped Zinc‐Ion Batteries

Aqueous Zn‐ion batteries (ZIBs) have garnered the researchers' spotlight owing to its high safety, cost effectiveness, and high theoretical capacity of Zn anode. However, the availability of cathode materials for Zn ions storage is limited. With unique layered structure along the [010] direction, α‐MoO₃ holds great promise as a cathode material for ZIBs, but its intrinsically poor conductivity severely restricts the capacity and rate capability. To circumvent this issue, an efficient surface engineering strategy is proposed to significantly improve the electric conductivity, Zn ion diffusion rate, and cycling stability of the MoO₃ cathode for ZIBs, thus drastically promoting its electrochemical properties. With the synergetic effect of Al₂O₃ coating and phosphating process, the constructed Zn//P‐MoO_3?x@Al₂O₃ battery delivers impressive capacity of 257.7 mAh g^?1 at 1 A g^?1 and superior rate capability (57% capacity retention at 20 A g^?1), dramatically surpassing the pristine Zn//MoO₃ battery (115.8 mAh g^?1; 19.7%). More importantly, capitalized on polyvinyl alcohol gel electrolyte, an admirable capacity (19.2 mAh cm^?3) as well as favorable energy density (14.4 mWh cm^?3; 240 Wh kg^?1) are both achieved by the fiber‐shaped quasi‐solid‐state ZIB. This work may be a great motivation for further research on molybdenum or other layered structure materials for high‐performance ZIBs.     

Formation of Mo–Polydopamine Hollow Spheres and Their Conversions to MoO2/C and Mo2C/C for Efficient Electrochemical Energy Storage and Catalyst

Highly uniform hierarchical Mo–polydopamine hollow spheres are synthesized for the first time through a liquid‐phase reaction under ambient temperature. A self‐assembly mechanism of the hollow structure of Mo–polydopamine precursor is discussed in detail, and a determined theory is proposed in a water‐in‐oil system. Via different annealing process, these precursors can be converted into hierarchical hollow MoO₂/C and Mo₂C/C composites without any distortion in shape. Owing to the well‐organized structure and nanosize particle embedding, the as‐prepared hollow spheres exhibit appealing performance both as the anode material for lithium‐ion batteries and as the catalyst for hydrogen evolution reaction (HER). Accordingly, MoO₂/C delivers a high reversible capacity of 940 mAh g^?1 at 0.1 A g^?1 and 775 mAh g^?1 at 1 A g^?1 with good rate capability and long cycle performance. Moreover, Mo₂C/C also exhibits an enhanced electrocatalytic performance with a low overpotential for HER in both acidic and alkaline conditions, as well as remarkable stability.     

Golden Bristlegrass‐Like Hierarchical Graphene Nanofibers Entangled with N‐Doped CNTs Containing CoSe2 Nanocrystals at Each Node as Anodes for High‐Rate Sodium‐Ion Batteries

Golden bristlegrass‐like unique nanostructures comprising reduced graphene oxide (rGO) matrixed nanofibers entangled with bamboo‐like N‐doped carbon nanotubes (CNTs) containing CoSe₂ nanocrystals at each node (denoted as N‐CNT/rGO/CoSe₂ NF) are designed as anodes for high‐rate sodium‐ion batteries (SIBs). Bamboo‐like N‐doped CNTs (N‐CNTs) are successfully generated on the rGO matrixed nanofiber surface, between rGO sheets and mesopores, and interconnected chemically with homogeneously distributed rGO sheets. The defects in the N‐CNTs formed by a simple etching process allow the complete phase conversion of Co into CoSe₂ through the efficient penetration of H₂Se gas inside the CNT walls. The N‐CNTs bridge the vertical defects for electron transfer in the rGO sheet layers and increase the distance between the rGO sheets during cycles. The discharge capacity of N‐CNT/rGO/CoSe₂ NF after the 10 000th cycle at an extremely high current density of 10 A g^?1 is 264 mA h g^?1, and the capacity retention measured at the 100th cycle is 89%. N‐CNT/rGO/CoSe₂ NF has final discharge capacities of 395, 363, 328, 304, 283, 263, 246, 223, 197, 171, and 151 mA h g^?1 at current densities of 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 A g^?1, respectively.     

Nitrogen-doped porous carbon as cathodes for aluminum-ion batteries

Abstract

Aluminum-ion batteries are favorable for the next generation of rechargeable batteries by virtue of their high theoretical energy density, high safety and low cost. Graphite with excellent electronic conductivity and large surface area is likely to be the most pleasurable cathode material for aluminum-ion batteries. However, due to the lack of high-performance cathodes and cost-effective electrolytes, aluminum-ion batteries are greatly hindered. Herein, a nitrogen doped three-dimensional porous carbon material cathode (N-3PC) with controlled porous and disordered structures is synthesized via a facile heating reaction in oil bath combined with sintering method. Moreover, the controlled porous structures by the Zn(NO₃)₂ pore-forming agent method support a very large specific surface area, which is beneficial to a relatively high initial Coulombic efficiency. In terms of open porous structures of N-3PC, the increased disordered structures can not only benefit the diffusion of Al³⁺ ions but also enlarge the reversible capacity of Al storage. When applied as cathode materials for aluminum-ion batteries, N-3PC cathode showcases rosy rate capability (33?mA h g^?1 at 500?mA g^?1) and perfect reversible capacity (13?mA h g^?1 at a high current density of 2000?mA g^?1). Furthermore, the high-performance N-3PC cathode material is prepared via a green approach stemming from low-cost and Zn(NO₃)₂ template, demonstrating a feasible development of carbon cathode materials for aluminum-ion batteries.     

Hierarchical Porous Nanosheets Constructed by Graphene‐Coated,Interconnected TiO2 Nanoparticles for Ultrafast Sodium Storage

Sodium‐ion batteries (SIBs) are considered promising next‐generation energy storage devices. However, a lack of appropriate high‐performance anode materials has prevented further improvements. Here, a hierarchical porous hybrid nanosheet composed of interconnected uniform TiO₂ nanoparticles and nitrogen‐doped graphene layer networks (TiO₂@NFG HPHNSs) that are synthesized using dual‐functional C₃N₄ nanosheets as both the self‐sacrificing template and hybrid carbon source is reported. These HPHNSs deliver high reversible capacities of 146 mA h g^?1 at 5 C for 8000 cycles, 129 mA h g^?1 at 10 C for 20 000 cycles, and 116 mA h g^?1 at 20 C for 10 000 cycles, as well as an ultrahigh rate capability up to 60 C with a capacity of 101 mA h g^?1. These results demonstrate the longest cyclabilities and best rate capability ever reported for TiO₂‐based anode materials for SIBs. The unprecedented sodium storage performance of the TiO₂@NFG HPHNSs is due to their unique composition and hierarchical porous 2D structure.     

Ultrahigh Capacity Due to Multi‐Electron Conversion Reaction in Reduced Graphene Oxide‐Wrapped MoO2 Porous Nanobelts

Multivalent transition metal oxides (MO_x) containing redox centers which can theoretically accept more than one electron have been suggested as promising anode materials for high‐performance lithium ion batteries (LIBs). The Li‐storage mechanism of these oxides is suggested to involve an unusual conversion reaction leading to the formation of metallic nanograins and Li₂O; however, a full‐scale conversion reaction is seldom observed in molybdenum dioxide (MoO₂) at room temperature due to slow kinetics. Herein, a full‐scale multi‐electron conversion reaction, leading to a high reversible capacity (974 mA h g^?1 charging capacity at 60 mA g^?1) in LIBs, is realized in a hybrid consisting of reduced graphene oxide (rGO) sheet‐wrapped MoO₂ porous nanobelts (rGO/MoO₂ NBs). The rGO wrapping layers stabilize the nanophase transition in MoO₂ and alleviate volume swing effects during lithiation/delithiation processes. This enables the hybrid to exhibit great cycle stability (tested to around 1900 cycles) and ultrafast rate capability (tested up to 50 A g^?1).     

Stabilizing Lithium–Sulfur Batteries through Control of Sulfur Aggregation and Polysulfide Dissolution

Lithium–sulfur (Li–S) batteries are investigated intensively as a promising large‐scale energy storage system owing to their high theoretical energy density. However, the application of Li–S batteries is prevented by a series of primary problems, including low electronic conductivity, volumetric fluctuation, poor loading of sulfur, and shuttle effect caused by soluble lithium polysulfides. Here, a novel composite structure of sulfur nanoparticles attached to porous‐carbon nanotube (p‐CNT) encapsulated by hollow MnO₂ nanoflakes film to form p‐CNT@Void@MnO₂/S composite structures is reported. Benefiting from p‐CNTs and sponge‐like MnO₂ nanoflake film, p‐CNT@Void@MnO₂/S provides highly efficient pathways for the fast electron/ion transfer, fixes sulfur and Li₂S aggregation efficiently, and prevents polysulfide dissolution during cycling. Besides, the additional void inside p‐CNT@Void@MnO₂/S composite structure provides sufficient free space for the expansion of encapsulated sulfur nanoparticles. The special material composition and structural design of p‐CNT@Void@MnO₂/S composite structure with a high sulfur content endow the composite high capacity, high Coulombic efficiency, and an excellent cycling stability. The capacity of p‐CNT@Void@MnO₂/S electrode is ≈599.1 mA h g^?1 for the fourth cycle and ≈526.1 mA h g^?1 after 100 cycles, corresponding to a capacity retention of ≈87.8% at a high current density of 1.0 C.     

Core–Shell Structure of Hierarchical Quasi‐Hollow MoS2 Microspheres Encapsulated Porous Carbon as Stable Anode for Li‐Ion Batteries

Monodisperse sulfonated polystyrene (SPS) microspheres are employed as both the template and carbon source to prepare MoS₂ quasi‐hollow microspheres‐encapsulated porous carbon. The synthesis procedure involves the hydrothermal growth of MoS₂ ultrathin nanosheets on the surface of SPS microspheres and subsequent annealing to remove SPS core. Incomplete decomposition of SPS during annealing due to the confining effect of MoS₂ shells leaves residual porous carbon in the interior. When being evaluated as the anode materials of Li‐ion batteries, the as‐prepared C@MoS₂ microspheres exhibit excellent cycling stability (95% of capacity retained after 100 cycles) and high rate behavior (560 mAh g^?1 at 5 A g^?1).     

Carbon–Heteroatom Bond Formation by an Ultrasonic Chemical Reaction for Energy Storage Systems

The direct formation of C? N and C? O bonds from inert gases is essential for chemical/biological processes and energy storage systems. However, its application to carbon nanomaterials for improved energy storage remains technologically challenging. A simple and very fast method to form C? N and C? O bonds in reduced graphene oxide (RGO) and carbon nanotubes (CNTs) by an ultrasonic chemical reaction is described. Electrodes of nitrogen‐ or oxygen‐doped RGO (N‐RGO or O‐RGO, respectively) are fabricated via the fixation between N₂ or O₂ carrier gas molecules and ultrasonically activated RGO. The materials exhibit much higher capacitance after doping (133, 284, and 74 F g^?1 for O‐RGO, N‐RGO, and RGO, respectively). Furthermore, the doped 2D RGO and 1D CNT materials are prepared by layer‐by‐layer deposition using ultrasonic spray to form 3D porous electrodes. These electrodes demonstrate very high specific capacitances (62.8 mF cm^?2 and 621 F g^?1 at 10 mV s^?1 for N‐RGO/N‐CNT at 1:1, v/v), high cycling stability, and structural flexibility.     

Fe2VO4 Hierarchical Porous Microparticles Prepared via a Facile Surface Solvation Treatment for High‐Performance Lithium and Sodium Storage

Preventing the aggregation of nanosized electrode materials is a key point to fully utilize the advantage of the high capacity. In this work, a facile and low‐cost surface solvation treatment is developed to synthesize Fe₂VO₄ hierarchical porous microparticles, which efficiently prevents the aggregation of the Fe₂VO₄ primary nanoparticles. The reaction between alcohol molecules and surface hydroxy groups is confirmed by density functional theory calculations and Fourier transform infrared spectroscopy. The electrochemical mechanism of Fe₂VO₄ as lithium‐ion battery anode is characterized by in situ X‐ray diffraction for the first time. This electrode material is capable of delivering a high reversible discharge capacity of 799 mA h g^?1 at 0.5 A g^?1 with a high initial coulombic efficiency of 79%, and the capacity retention is 78% after 500 cycles. Moreover, a remarkable reversible discharge capacity of 679 mA h g^?1 is achieved at 5 A g^?1. Furthermore, when tested as sodium‐ion battery anode, a high reversible capacity of 382 mA h g^?1 can be delivered at the current density of 1 A g^?1, which still retains at 229 mA h g^?1 after 1000 cycles. The superior electrochemical performance makes it a potential anode material for high‐rate and long‐life lithium/sodium‐ion batteries.     

In Situ Fabrication of Branched TiO2/C Nanofibers as Binder‐Free and Free‐Standing Anodes for High‐Performance Sodium‐Ion Batteries

Herein, 1D free‐standing and binder‐free hierarchically branched TiO₂/C nanofibers (denoted as BT/C NFs) based on an in situ fabrication method as an anode for sodium‐ion batteries are reported. The in situ fabrication endows this material with large surface area and strong structural stability, providing this material with abundant active sites and smooth channels for fast ion transportation. As a result, BT/C NFs with the character of free‐standing membranes are directly used as binder‐free anode for sodium‐ion batteries, delivering a capacity of 284 mA h g^?1 at a current density of 200 mA g^?1 after 1000 cycles. Even at a high current density of 2000 mA g^?1, the reversible capacity can still achieve as high as 204 mA h g^?1. By means of kinetic analysis, it is demonstrated that the remarkable surface pseudocapacitive behavior is also a major factor to achieve excellent performance. The rationally designed structure coupled with the inherent pseudocapacitive behavior gives this material potential for sodium‐ion batteries.     

Surface‐Engineered Black Niobium Oxide@Graphene Nanosheets for High‐Performance Sodium‐/Potassium‐Ion Full Batteries

Nanoscale surface‐engineering plays an important role in improving the performance of battery electrodes. Nb₂O₅ is one typical model anode material with promising high‐rate lithium storage. However, its modest reaction kinetics and low electrical conductivity obstruct the efficient storage of larger ions of sodium or potassium. In this work, partially surface‐amorphized and defect‐rich black niobium oxide@graphene (black Nb₂O_5?x@rGO) nanosheets are designed to overcome the above Na/K storage problems. The black Nb₂O_5?x@rGO nanosheets electrodes deliver a high‐rate Na and K storage capacity (123 and 73 mAh g^?1, respectively at 3 A g^?1) with long‐term cycling stability. Besides, both Na‐ion and K‐ion full batteries based on black Nb₂O_5?x@rGO nanosheets anodes and vanadate‐based cathodes (Na_0.33V₂O₅ and K_0.5V₂O₅ for Na‐ion and K‐ion full batteries, respectively) demonstrate promising rate and cycling performance. Notably, the K‐ion full battery delivers higher energy and power densities (172 Wh Kg^?1 and 430 W Kg^?1), comparable to those reported in state‐of‐the‐art K‐ion full batteries, accompanying with a capacity retention of ≈81.3% over 270 cycles. This result on Na‐/K‐ion batteries may pave the way to next‐generation post‐lithium batteries.     

Porous Carbon Nanofibers Encapsulated with Peapod‐Like Hematite Nanoparticles for High‐Rate and Long‐Life Battery Anodes

Fe₂O₃ is regarded as a promising anode material for lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs) due to its high specific capacity. The large volume change during discharge and charge processes, however, induces significant cracking of the Fe₂O₃ anodes, leading to rapid fading of the capacity. Herein, a novel peapod‐like nanostructured material, consisting of Fe₂O₃ nanoparticles homogeneously encapsulated in the hollow interior of N‐doped porous carbon nanofibers, as a high‐performance anode material is reported. The distinctive structure not only provides enough voids to accommodate the volume expansion of the pea‐like Fe₂O₃ nanoparticles but also offers a continuous conducting framework for electron transport and accessible nanoporous channels for fast diffusion and transport of Li/Na‐ions. As a consequence, this peapod‐like structure exhibits a stable discharge capacity of 1434 mAh g^?1 (at 100 mA g^?1) and 806 mAh g^?1 (at 200 mA g^?1) over 100 cycles as anode materials for LIBs and SIBs, respectively. More importantly, a stable capacity of 958 mAh g^?1 after 1000 cycles and 396 mAh g^?1 after 1500 cycles can be achieved for LIBs and SIBs, respectively, at a large current density of 2000 mA g^?1. This study provides a promising strategy for developing long‐cycle‐life LIBs and SIBs.     

Graphene‐Encapsulated FeS2 in Carbon Fibers as High Reversible Anodes for Na+/K+ Batteries in a Wide Temperature Range

Developing low cost, long life, and high capacity rechargeable batteries is a critical factor towards developing next‐generation energy storage devices for practical applications. Therefore, a simple method to prepare graphene‐coated FeS₂ embedded in carbon nanofibers is employed; the double protection from graphene coating and carbon fibers ensures high reversibility of FeS₂ during sodiation/desodiation and improved conductivity, resulting in high rate capacity and long‐term life for Na⁺ (305.5 mAh g^?1 at 3 A g^?1 after 2450 cycles) and K⁺ (120 mAh g^?1 at 1 A g^?1 after 680 cycles) storage at room temperature. Benefitting from the enhanced conductivity and protection on graphene‐encapsulated FeS₂ nanoparticles, the composites exhibit excellent electrochemical performance under low temperature (0 and ?20 °C), and temperature tolerance with stable capacity as sodium‐ion half‐cells. The Na‐ion full‐cells based on the above composites and Na₃V₂(PO₄)₃ can afford reversible capacity of 95 mAh g^?1 at room temperature. Furthermore, the full‐cells deliver promising discharge capacity (50 mAh g^?1 at 0 °C, 43 mAh g^?1 at ?20 °C) and high energy density at low temperatures. Density functional theory calculations imply that graphene coating can effectively decrease the Na⁺ diffusion barrier between FeS₂ and graphene heterointerface and promote the reversibility of Na⁺ storage in FeS₂, resulting in advanced Na⁺ storage properties.     

Ionic Liquid‐Controlled Growth of NiCo2S4 3D Hierarchical Hollow Nanoarrow Arrays on Ni Foam for Superior Performance Binder Free Hybrid Supercapacitors

A significant development in the design of a NiCo₂S₄ 3D hierarchical hollow nanoarrow arrays (HNA)‐based supercapacitor binder free electrode assembled by 1D hollow nanoneedles and 2D nanosheets on a Ni foam collector through controlling ionic liquid 1‐octyl‐3‐methylimidazolium chloride ([OMIm]Cl) concentration is reported. The unique NiCo₂S₄‐HNA electrode acquires high specific capacity (1297 C g^?1 at 1 A g^?1, 2.59 C cm^?2 at 2 mA cm^?2), excellent rate capability (maintaining 73.0% at 20 A g^?1), and long operational life (maintaining 92.4% after 10 000 cycles at 5 A g^?1), which are superior to those for 1D hollow nanoneedle arrays (HNN) and 2D porous nanoflake arrays (PNF). The outstanding electrochemical performance is attributed to the novel 3D structure with large specific surface, hollow cores, high porosity as well as stable architecture. In addition, a hybrid supercapacitor applying 3D NiCo₂S₄‐HNA as the positive electrode and active carbon as the negative electrode exhibits a high energy density of 42.5 Wh kg^?1 at a power density of 2684.2 W kg^?1 in an operating voltage of 1.6 V. Robust cycling stability is also expressed with 84.9% retention after repeating 10 000 cycles at 5 A g^?1, implying their great potential in superior‐performance supercapacitors.     

Engineering 2D Nanofluidic Li‐Ion Transport Channels for Superior Electrochemical Energy Storage

Rational surface engineering of 2D nanoarchitectures‐based electrode materials is crucial as it may enable fast ion transport, abundant‐surface‐controlled energy storage, long‐term structural integrity, and high‐rate cycling performance. Here we developed the stacked ultrathin Co₃O₄ nanosheets with surface functionalization (SUCNs‐SF) converted from layered hydroxides with inheritance of included anion groups (OH^?, NO₃^?, CO₃^2?). Such stacked structure establishes 2D nanofluidic channels offering extra lithium storage sites, accelerated Li‐ion transport, and sufficient buffering space for volume change during electrochemical processes. Tested as an anode material, this unique nanoarchitecture delivers high specific capacity (1230 and 1011 mAh g^?1 at 0.2 and 1 A g^?1, respectively), excellent rate performance, and long cycle capability (1500 cycles at 5 A g^?1). The demonstrated advantageous features by constructing 2D nanochannels in nonlayered materials may open up possibilities for designing high‐power lithium ion batteries.     

Oxygen‐Vacancy and Surface Modulation of Ultrathin Nickel Cobaltite Nanosheets as a High‐Energy Cathode for Advanced Zn‐Ion Batteries

The development of high‐capacity, Earth‐abundant, and stable cathode materials for robust aqueous Zn‐ion batteries is an ongoing challenge. Herein, ultrathin nickel cobaltite (NiCo₂O₄) nanosheets with enriched oxygen vacancies and surface phosphate ions (P–NiCo₂O_4‐x) are reported as a new high‐energy‐density cathode material for rechargeable Zn‐ion batteries. The oxygen‐vacancy and surface phosphate‐ion modulation are achieved by annealing the pristine NiCo₂O₄ nanosheets using a simple phosphating process. Benefiting from the merits of substantially improved electrical conductivity and increased concentration of active sites, the optimized P–NiCo₂O_4‐x nanosheet electrode delivers remarkable capacity (309.2 mAh g^?1 at 6.0 A g^?1) and extraordinary rate performance (64% capacity retention at 60.4 A g^?1). Moreover, based on the P–NiCo₂O_4‐x cathode, our fabricated P–NiCo₂O_4‐x//Zn battery presents an impressive specific capacity of 361.3 mAh g^?1 at the high current density of 3.0 A g^?1 in an alkaline electrolyte. Furthermore, extremely high energy density (616.5 Wh kg^?1) and power density (30.2 kW kg^?1) are also achieved, which outperforms most of the previously reported aqueous Zn‐ion batteries. This ultrafast and high‐energy aqueous Zn‐ion battery is promising for widespread application to electric vehicles and intelligent devices.     

Enhanced Capacity and Rate Capability of Nitrogen/Oxygen Dual‐Doped Hard Carbon in Capacitive Potassium‐Ion Storage

The intercalation of potassium ions into graphite is demonstrated to be feasible, while the electrochemical performance of potassium‐ion batteries (KIBs) remains unsatisfying. More effort is needed to improve the specific capacity while maintaining a superior rate capability. As an attempt, nitrogen/oxygen dual‐doped hierarchical porous hard carbon (NOHPHC) is introduced as the anode in KIBs by carbonizing and acidizing the NH₂‐MIL‐101(Al) precursor. Specifically, the NOHPHC electrode delivers high reversible capacities of 365 and 118 mA h g^?1 at 25 and 3000 mA g^?1, respectively. The capacity retention reaches 69.5% at 1050 mA g^?1 for 1100 cycles. The reasons for the enhanced electrochemical performance, such as the high capacity, good cycling stability, and superior rate capability, are analyzed qualitatively and quantitatively. Quantitative analysis reveals that mixed mechanisms, including capacitance and diffusion, account for the K‐ion storage, in which the capacitance plays a more important role. Specifically, the enhanced interlayer spacing (0.39 nm) enables the intercalation of large K ions, while the high specific surface area of ≈1030 m² g^?1 and the dual‐heteroatom doping (N and O) are conducive to the reversible adsorption of K ions.     

Multichannel Porous TiO2 Hollow Nanofibers with Rich Oxygen Vacancies and High Grain Boundary Density Enabling Superior Sodium Storage Performance

TiO₂ as an anode for sodium‐ion batteries (NIBs) has attracted much recent attention, but poor cyclability and rate performance remain problematic owing to the intrinsic electronic conductivity and the sluggish diffusivity of Na ions in the TiO₂ matrix. Herein, a simple process is demonstrated to improve the sodium storage performance of TiO₂ by fabricating a 1D, multichannel, porous binary‐phase anatase‐TiO₂–rutile‐TiO₂ composite with oxygen‐deficient and high grain‐boundary density (denoted as a‐TiO_2?x /r‐TiO_2?x ) via electrospinning and subsequent vacuum treatment. The introduction of oxygen vacancies in the TiO₂ matrix enables enhanced intrinsic electronic conductivity and fast sodium‐ion diffusion kinetics. The porous structure offers easy access of the liquid electrolyte and a short transport path of Na⁺ through the pores toward the TiO₂ nanoparticle. Furthermore, the high density of grain boundaries between the anatase TiO₂ and rutile TiO₂ offer more interfaces for a novel interfacial storage. The a‐TiO_2?x /r‐TiO_2?x shows excellent long cycling stability (134 mAh g^?1 at 10 C after 4500 cycles) and superior rate performance (93 mAh g^?1 after 4500 cycles at 20 C) for sodium‐ion batteries. This simple and effective process could serve as a model for the modification of other materials applied in energy storage systems and other fields.