Free‐Standing 3D Nanoporous Duct‐Like and Hierarchical Nanoporous Graphene Films for Micron‐Level Flexible Solid‐State Asymmetric Supercapacitors

High energy density and power density within a limited volume of flexible solid‐state supercapacitors are highly desirable for practical applications. Here, free‐standing high‐quality 3D nanoporous duct‐like graphene (3D‐DG) films are fabricated with high flexibility and robustness as the backbones to deposit flower‐like MnO₂ nanosheets (3D‐DG@MnO₂). The 3D‐DG is the ideal support for the deposition of large amount of active materials because of its large surface area, appropriate pore structure, and negligible volume compared with other kinds of carbon backbones. Moreover, the 3D‐DG preserve the distinctive 2D coherent electronic properties of graphene, in which charge carriers move rapidly with a small resistance through the high‐quality and continuous chemical vapor deposition‐grown graphene building blocks, which results in a high rate performance. Marvelously, ultrathin (≈50 μm) flexible solid‐state asymmetric supercapacitors (ASCs) using 3D‐DG@MnO₂ as the positive electrode and 3D hierarchical nanoporous graphene films as the negative electrode display ultrahigh volumetric energy density (28.2 mW h cm^?3) and power density (55.7 W cm^?3) at 2.0 V. Furthermore, as‐prepared ASCs show high cycle stability clearly demonstrating their broad applications as power supplies in wearable electronic devices.     

Superior Oxygen Reduction Reaction on Phosphorus‐Doped Carbon Dot/Graphene Aerogel for All‐Solid‐State Flexible Al–Air Batteries

Carbon dots have been recognized as one of the most promising candidates for the oxygen reduction reaction (ORR) in alkaline media. However, the desired ORR performance in metal–air batteries is often limited by the moderate electrocatalytic activity and the lack of a method to realize good dispersion. To address these issues, herein a biomass‐deriving method is reported to achieve the in situ phosphorus doping (P‐doping) of carbon dots and their simultaneous decoration onto graphene matrix. The resultant product, namely P‐doped carbon dot/graphene (P‐CD/G) nanocomposites, can reach an ultrahigh P‐doping level for carbon nanomaterials. The P‐CD/G nanocomposites are found to exhibit excellent ORR activity, which is highly comparable to the commercial Pt/C catalysts. When used as the cathode materials for a primary liquid Al–air battery, the device shows an impressive power density of 157.3 mW cm^?2 (comparing to 151.5 mW cm^?2 of a similar Pt/C battery). Finally, an all‐solid‐state flexible Al–air battery is designed and fabricated based on our new nanocomposites. The device exhibits a stable discharge voltage of ≈1.2 V upon different bending states. This study introduces a unique biomass‐derived material system to replace the noble metal catalysts for future portable and wearable electronic devices.     

Ultrafast All‐Solid‐State Coaxial Asymmetric Fiber Supercapacitors with a High Volumetric Energy Density

Fiber‐supercapacitors (FSCs) are promising energy storage devices that can complement or even replace microbatteries in miniaturized portable and wearable electronics. Currently, a major challenge for FSCs is achieving ultrahigh volumetric energy and power densities simultaneously, especially when the charge/discharge rates exceed 1 V s^?1. Herein, an Au‐nanoparticle‐doped‐MnO_x@CoNi‐alloy@carbon‐nanotube (Au–MnO_x@CoNi@CNT) core/shell nanocomposite fiber electrode is designed, aiming to boost its charge/discharge rate by taking advantage of the superconductive CoNi alloy network and the greatly enhanced conductivity of the Au doped MnO_x active materials. An all‐solid‐state coaxial asymmetric FSC (CAFSC) prototype device made by wrapping this fiber with a holey graphene paper (HGP) exhibits excellent performance at rates up to 10 V s^?1, which is the highest charge rate demonstrated so far for FSCs based on pseudocapacitive materials. Furthermore, our fully packaged CAFSC delivers a volumetric energy density of ≈15.1 mW h cm^?3, while simultaneously maintaining a high power density of 7.28 W cm^?3 as well as a long cycle life (90% retention after 10 000 cycles). This value is the highest among all reported FSCs, even better than that of a typical 4 V/500 µA h thin‐film lithium battery.     

Graphitic Petal Electrodes for All‐Solid‐State Flexible Supercapacitors

The charge storage characteristics of a composite nanoarchitecture with a highly functional 3D morphology are reported. The electrodes are formed by the electropolymerization of aniline monomers into a nanometer‐thick polyaniline (PANI) film that conformally coats graphitic petals (GPs) grown by microwave plasma chemical vapor deposition (MPCVD) on conductive carbon cloth (CC). The hybrid CC/GPs/PANI electrodes yield results near the theoretical maximum capacitance for PANI of 2000 F g^?1 (based on PANI mass) and a large area‐normalized specific capacitance of ≈2.6 F cm^?2 (equivalent to a volumetric capacitance of ≈230 F cm^?3) at a low current density of 1 A g^?1 (based on PANI mass). The specific capacitances remain above 1200 F g^?1 (based on PANI mass) for currents up to 100 A g^?1 with correspondingly high area‐normalized values. The hybrid electrodes also exhibit a high rate capability with an energy density of 110 Wh kg^?1 and a maximum power density of 265 kW kg^?1 at a current density of 100 A g^?1. Long‐term cyclic stability is good (≈7% loss of initial capacitance after 2000 cycles), with coulombic efficiencies >99%. Moreover, prototype all‐solid‐state flexible supercapacitors fabricated from these hybrid electrodes exhibit excellent energy storage performance.     

3D Printing Quasi‐Solid‐State Asymmetric Micro‐Supercapacitors with Ultrahigh Areal Energy Density

A 3D printing approach is first developed to fabricate quasi‐solid‐state asymmetric micro‐supercapacitors to simultaneously realize the efficient patterning and ultrahigh areal energy density. Typically, cathode, anode, and electrolyte inks with high viscosities and shear‐thinning rheological behaviors are first prepared and 3D printed individually on the substrates. The 3D printed asymmetric micro‐supercapacitor with interdigitated electrodes exhibits excellent structural integrity, a large areal mass loading of 3.1 mg cm^?2, and a wide electrochemical potential window of 1.6 V. Consequently, this 3D printed asymmetric micro‐supercapacitor displays an ultrahigh areal capacitance of 207.9 mF cm^?2. More importantly, an areal energy density of 73.9 µWh cm^?2 is obtained, superior to most reported interdigitated micro‐supercapacitors. It is believed that the efficient 3D printing strategy can be used to construct various asymmetric micro‐supercapacitors to promote the integration in on‐chip energy storage systems.     

Nanostructured Ternary Electrodes for Energy‐Storage Applications

A three‐component, flexible electrode is developed for supercapacitors over graphitized carbon fabric, utilizing γ‐MnO₂ nanoflowers anchored onto carbon nanotubes (γ‐MnO₂/CNT) as spacers for graphene nanosheets (GNs). The three‐component, composite electrode doubles the specific capacitance with respect to GN‐only electrodes, giving the highest‐reported specific capacitance (308 F g^?1) for symmetric supercapacitors containing MnO₂ and GNs using a two‐electrode configuration, at a scan rate of 20 mV s^?1. A maximum energy density of 43 W h kg^?1 is obtained for our symmetric supercapacitors at a constant discharge‐current density of 2.5 A g^?1 using GN–(γ‐MnO₂/CNT)‐nanocomposite electrodes. The fabricated supercapacitor device exhibits an excellent cycle life by retaining ≈90% of the initial specific capacitance after 5000 cycles.     

All‐Solid‐State On‐Chip Supercapacitors Based on Free‐Standing 4H‐SiC Nanowire Arrays

All‐solid‐state on‐chip SiC supercapacitors (SCs) based on free‐standing SiC nanowire arrays (NWAs) are reported. In comparison to the widely used technique based on the interdigitated fingers, the present strategy can be much more facile for constructing on‐chip SCs devices, which is directly sandwiched with a solid electrolyte layer between two pieces of SiC NWAs film without any substrate. The mass loading of active materials of on‐chip SiC SCs can be up to ≈5.6 mg cm^?2, and the total device thickness is limited in ≈40 µm. The specific area energy and power densities of the SCs device reach 5.24 µWh cm^?2 and 11.2 mW cm^?2, and their specific volume energy and power densities run up to 1.31 mWh cm^–3 and 2.8 W cm^?3, respectively, which are two orders of magnitude higher than those of state‐of‐the‐art SiC‐based SCs, and also much higher than those of other solid‐state carbon‐based SCs ever reported. Furthermore, such on‐chip SCs exhibit superior rate capability and robust stability with over 94% capacitance retention after 10 000 cycles at a scan rate of 100 mV s^?1, representing their high performance in all merits.     

A High Performance Stretchable Asymmetric Fiber‐Shaped Supercapacitor with a Core‐Sheath Helical Structure

The emerging fiber‐shaped supercapacitors (FSSs) have motivated tremendous research interest in energy storage devices. However, challenges still exist in the pursuit of combination of excellent electrochemical performance and mechanical stretchability. Here, a core‐sheath asymmetric FSS is first made by wrapping gel electrolyte coated carbon nanotube (CNT)@MnO₂ core fiber with CNT@PPy composite film. Then a stretchable helical structure is formed by over‐twisting the FSS. The resulted stretchable asymmetric FSS exhibits a specific capacitance of 60.435 mF cm^?2 at the scan rate of 10 mV s^?1 and the capacitance performance is well maintained during repeated stretching to 20% strain. Furthermore, a high energy density of 18.88 μW h cm^?2 is achieved for the stretchable FSS due to its high specific capacitance and extended potential window of 1.5 V.     

High‐Performance Fiber‐Shaped All‐Solid‐State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics

Flexible fiber‐shaped supercapacitors have shown great potential in portable and wearable electronics. However, small specific capacitance and low operating voltage limit the practical application of fiber‐shaped supercapacitors in high energy density devices. Herein, direct growth of ultrathin MnO₂ nanosheet arrays on conductive carbon fibers with robust adhesion is exhibited, which exhibit a high specific capacitance of 634.5 F g^?1 at a current density of 2.5 A g^?1 and possess superior cycle stability. When MnO₂ nanosheet arrays on carbon fibers and graphene on carbon fibers are used as a positive electrode and a negative electrode, respectively, in an all‐solid‐state asymmetric supercapacitor (ASC), the ASC displays a high specific capacitance of 87.1 F g^?1 and an exceptional energy density of 27.2 Wh kg^?1. In addition, its capacitance retention reaches 95.2% over 3000 cycles, representing the excellent cyclic ability. The flexibility and mechanical stability of these ASCs are highlighted by the negligible degradation of their electrochemical performance even under severely bending states. Impressively, as‐prepared fiber‐shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage. The excellent performance of all‐solid‐state ASCs opens up new opportunity for development of wearable and self‐powered nanodevices in near future.     

All‐Solid‐State,Foldable, and Rechargeable Zn‐Air Batteries Based on Manganese Oxide Grown on Graphene‐Coated Carbon Cloth Air Cathode

Pliable, safe, and inexpensive energy storage devices are in demand to power modern flexible electronics. In this work, a foldable battery based on a solid‐state and rechargeable Zn‐air battery is introduced. The air cathode is prepared by coating graphene flakes on pretreated carbon cloth to form a dense, interconnected, and conducting carbon network. Manganese oxide hierarchical nanostructures are subsequently grown on the large surface area carbon network, leading to high loading of active catalyst per unit volume while maintaining the mechanical and electrical integrity of the air cathode. Solid‐state and rechargeable Zn‐air battery with such air cathode exhibits similar polarization curve and resistance at its flat and folded states. The folded battery is able to deliver a power density as high as ≈32 mW cm^?2 and good cycling stability of up to 110 cycles. In addition, the flat battery shows similar discharge/charge curve and stable cycling performance after 100 times of repeated folding and unfolding, indicating its high mechanical robustness.     

Hierarchical Zn–Co–S Nanowires as Advanced Electrodes for All Solid State Asymmetric Supercapacitors

A facile two‐step strategy is developed to design the large‐scale synthesis of hierarchical, unique porous architecture of ternary metal hydroxide nanowires grown on porous 3D Ni foam and subsequent effective sulfurization. The hierarchical Zn–Co–S nanowires (NWs) arrays are directly employed as an electrode for supercapacitors application. The as‐synthesized Zn–Co–S NWs deliver an ultrahigh areal capacity of 0.9 mA h cm^?2 (specific capacity of 366.7 mA h g^?1) at a current density of 3 mA cm^?2, with an exceptional rate capability (≈227.6 mA h g^?1 at a very high current density of 40 mA cm^?2) and outstanding cycling stability (≈93.2% of capacity retention after 10 000 cycles). Most significantly, the assembled Zn–Co–S NWs//Fe₂O₃@reduced graphene oxide asymmetric supercapacitors with a wide operating potential window of ≈1.6 V yield an ultrahigh volumetric capacity of ≈1.98 mA h cm^?3 at a current density of 3 mA cm^?2, excellent energy density of ≈81.6 W h kg^?1 at a power density of ≈559.2 W kg^?1, and exceptional cycling performance (≈92.1% of capacity retention after 10 000 cycles). This general strategy provides an alternative to design the other ternary metal sulfides, making it facile, free‐standing, binder‐free, and cost‐effective ternary metal sulfide‐based electrodes for large‐scale applications in modern electronics.     

All‐Solid‐State Fiber Supercapacitors with Ultrahigh Volumetric Energy Density and Outstanding Flexibility

Fiber supercapacitors (FSCs) represent a promising class of energy storage devices that can complement or even replace microbatteries in miniaturized portable and wearable electronics. One of their main limitations, however, is the low volumetric energy density when compared with those of rechargeable batteries. Considering the energy density of FSC is proportional to CV² (E = 1/2 CV², where C is the capacitance and V is the operating voltage), one would explore high operating voltage as an effective strategy to promote the volumetric energy density. In the present work, an all‐solid‐state asymmetric FSC (AFSC) with a maximum operating voltage of 3.5 V is successfully achieved, by employing an ionic liquid (IL) incorporated gel‐polymer as the electrolyte (EMIMTFSI/PVDF‐HFP). The optimized AFSC is based on MnO_x@TiN nanowires@carbon nanotube (NWs@CNT) fiber as the positive electrode and C@TiN NWs@CNT fiber as the negative electrode, which gives rise to an ultrahigh stack volumetric energy density of 61.2 mW h cm^?3, being even comparable to those of commercially planar lead‐acid batteries (50–90 mW h cm^?3), and an excellent flexibility of 92.7% retention after 1000 blending cycles at 90°. The demonstration of employing the ILs‐based electrolyte opens up new opportunities to fabricate high‐performance flexible AFSC for future portable and wearable electronic devices.     

MoS2‐Based All‐Purpose Fibrous Electrode and Self‐Powering Energy Fiber for Efficient Energy Harvesting and Storage

Here an all‐purpose fibrous electrode based on MoS₂ is demonstrated, which can be employed for versatile energy harvesting and storage applications. In this coaxial electrode, ultrathin MoS₂ nanofilms are grown on TiO₂ nanoparticles coated carbon fiber. The high electrochemical activity of MoS₂ and good conductivity of carbon fiber synergistically lead to the remarkable performances of this novel composite electrode in fibrous dye‐sensitized solar cells (showing a record‐breaking conversion efficiency of 9.5%) and high‐capacity fibrous supercapacitors. Furthermore, a self‐powering energy fiber is fabricated by combining a fibrous dye‐sensitized solar cell and a fibrous supercapacitor into a single device, showing very fast charging capability (charging in 7 s under AM1.5G solar illumination) and an overall photochemical‐electricity energy conversion efficiency as high as 1.8%. In addition, this wire‐shaped electrode can also be used for fibrous Li‐ion batteries and electrocatalytic hydrogen evolution reactions. These applications indicate that the MoS₂‐based all‐purpose fibrous electrode has great potential for the construction of high‐performance flexible and wearable energy devices.     

All‐Printed MnHCF‐MnOx‐Based High‐Performance Flexible Supercapacitors

Here, a simple active materials synthesis method is presented that boosts electrode performance and utilizes a facile screen‐printing technique to prepare scalable patterned flexible supercapacitors based on manganese hexacyanoferrate‐manganese oxide and electrochemically reduced graphene oxide electrode materials (MnHCF‐MnO_x/ErGO). A very simple in situ self‐reaction method is developed to introduce MnO_x pseudocapacitor material into the MnHCF system by using NH₄F. This MnHCF‐MnO_x electrode materials can deliver excellent capacitance of 467 F g^?1 at a current density of 1 A g^?1, which is a 2.4 times capacitance increase compared to MnHCF. In addition a printed, patterned, flexible MnHCF‐MnO_x/ErGO supercapacitor is fabricated, showing a remarkable areal capacitance of 16.8 mF cm^?2 and considerable energy and power density of 0.5 mWh cm^?2 and 0.0023 mW cm^?2, respectively. Furthermore, the printed patterned flexible supercapacitors also exhibit exceptional flexibility, and the capacitance remains stable, even while bending to various angles (60°, 90°, and 180°) and for 100 cycles. The flexible supercapacitor arrays integrated by multiple prepared single supercapacitors can power various LEDs even in the bent states. This approach offers promising opportunities for the development of printable energy storage materials and devices with high energy density, large scalability, and excellent flexibility.     

Monolithic Integration of All‐in‐One Supercapacitor for 3D Electronics

A supercapacitor is usually stacked in the configuration of a layered sandwiched architecture, and has been adopted as discrete energy storage device or circuit component. However, this stacked structure decreases mechanical integrity, leads to low specific capacity, and prevents high‐density monolithic integration. Here all‐in‐one supercapacitors are fabricated by integrating cathode, anode, current collector, and separator into one monolithic glass fiber (GF) substrate together with other circuit components through matured and scalable fabrication techniques, the all‐in‐one supercapacitor is embedded as a component for 3D electronics. This all‐in‐one architecture demonstrates its effectiveness in the prevention of the delamination of the sandwiched supercapacitor and the minimization of the proportion of inactive materials. The supercapacitor delivers high power density (320 mW cm^?3) and energy density (2.12 mWh cm^?3), and exhibits a capacitance retention of 100% even after a continuous cycling of 431 h. Furthermore, a 3D polydimethylsiloxane/GF architecture is constructed for driving a flash light emitting diode system, where the all‐in‐one supercapacitor is monolithically integrated in the 3D system, and each layer is connected via vertical through‐holes. This all‐in‐one device can be constructed with a macroscopically available membrane and readily integrated into 3D systems without secondary packaging, providing the potential for high‐density heterogeneous 3D electronics.     

All Pseudocapacitive MXene‐RuO2 Asymmetric Supercapacitors

2D transition metal carbides and nitrides, known as MXenes, are an emerging class of 2D materials with a wide spectrum of potential applications, in particular in electrochemical energy storage. The hydrophilicity of MXenes combined with their metallic conductivity and surface redox reactions is the key for high‐rate pseudocapacitive energy storage in MXene electrodes. However, symmetric MXene supercapacitors have a limited voltage window of around 0.6 V due to possible oxidation at high anodic potentials. In this study, the fact that titanium carbide MXene (Ti₃C₂T_x) can operate at negative potentials in acidic electrolyte is exploited, to design an all‐pseudocapacitive asymmetric device by combining it with a ruthenium oxide (RuO₂) positive electrode. This asymmetric device operates at a voltage window of 1.5 V, which is about two times wider than the operating voltage window of symmetric MXene supercapacitors, and is the widest voltage window reported to date for MXene‐based supercapacitors. The complementary working potential windows of MXene and RuO₂, along with proton‐induced pseudocapacitance, significantly enhance the device performance. As a result, the asymmetric devices can deliver an energy density of 37 µW h cm^?2 at a power density of 40 mW cm^?2, with 86% capacitance retention after 20 000 charge–discharge cycles. These results show that pseudocapacitive negative MXene electrodes can potentially replace carbon‐based materials in asymmetric electrochemical capacitors, leading to an increased energy density.     

Artificial Solid‐Electrolyte Interphase Enabled High‐Capacity and Stable Cycling Potassium Metal Batteries

Secondary batteries based on earth‐abundant potassium metal anodes are attractive for stationary energy storage. However, suppressing the formation of potassium metal dendrites during cycling is pivotal in the development of future potassium metal‐based battery technology. Herein, a promising artificial solid‐electrolyte interphase (ASEI) design, simply covering a carbon nanotube (CNT) film on the surface of a potassium metal anode, is demonstrated. The results show that the spontaneously potassiated CNT framework with a stable self‐formed solid‐electrolyte interphase layer integrates a quasi‐hosting feature with fast interfacial ion transport, which enables dendrite‐free deposition of potassium at an ultrahigh capacity (20 mAh cm^?2). Remarkably, the potassium metal anode exhibits an unprecedented cycle life (over 1000 cycles, over 2000 h) at a high current density of 5 mA cm^?2 and a desirable areal capacity of 4 mAh cm^?2. Dendrite‐free morphology in carbon‐fiber and carbon‐black‐based ASEI for potassium metal anodes, which indicates a broader promise of this approach, is also observed.     

Metal–Organic Framework Derived Honeycomb Co9S8@C Composites for High‐Performance Supercapacitors

Unique nanostructures always lead to extraordinary electrochemical energy storage performance. Here, the authors report a new strategy for using Metal‐organic frameworks (MOFs) derived cobalt sulfide in a carbon matrix with a 3D honeycombed porous structure, resulting in a high‐performance supercapacitor with unrivalled capacity of ≈1887 F g^‐1 at the current density of 1 A g^‐1. The honeycomb‐like structure of Co₉S₈@C composite is loosely adsorbed, with plentiful surface area and high conductivity, leading to improved Faradaic processes across the interface and enhanced redox reactions at active Co₉S₈ sites. Therefore, the heterostructure‐fabricated hybrid supercapacitor, using activated carbon as the counter electrode, demonstrates a high energy density of 58 Wh kg^‐1 at the power density of 1000 W kg^‐1. Even under an ultrahigh power density of 17 200 W kg^‐1, its energy density maintains ≈38 Wh kg^‐1. The hybrid supercapacitor also exhibits suitable cycling stability, with ≈90% capacity retention after 10 000 continuous cycles at the current density of 5 A g^‐1. This work presents a practical method for using MOFs as sacrificial templates to synthesize metal‐sulfides for highly efficient electrochemical energy storage.     

In Situ Formation of Co9S8 Quantum Dots in MOF‐Derived Ternary Metal Layered Double Hydroxide Nanoarrays for High‐Performance Hybrid Supercapacitors

Layered double hydroxides (LDHs) are promising cathode materials for supercapacitors because of the enhanced flow efficiency of ions in the interlayers. However, the limited active sites and monotonous metal species further hinder the improvement of the capacity performance. Herein, cobalt sulfide quantum dots (Co₉S₈‐QDs) are effectively created and embedded within the interlayer of metal‐organic‐frameworks‐derived ternary metal LDH nanosheets based on in situ selective vulcanization of Co on carbon fibers. The hybrid CF@NiCoZn‐LDH/Co₉S₈‐QD retains the lamellar structure of the ternary metal LDH very well, inheriting low transfer impedance of interlayer ions. Significantly, the selectively generated Co₉S₈‐QDs expose more abundant active sites, effectively improving the electrochemical properties, such as capacitive performance, electronic conductivity, and cycling stability. Due to the synergistic relationship, the hybrid material delivers an ultrahigh electrochemical capacity of 350.6 mAh g^?1 (2504 F g^?1) at 1 A g^?1. Furthermore, hybrid supercapacitors fabricated with CF@NiCoZn‐LDH/Co₉S₈‐QD and carbon nanosheets modified by single‐walled carbon nanotubes display an outstanding energy density of 56.4 Wh kg^?1 at a power density of 875 W kg^?1, with an excellent capacity retention of 95.3% after 8000 charge–discharge cycles. Therefore, constructing hybrid electrode materials by in situ‐created QDs in multimetallic LDHs is promising.     

Yolk–Shell NiS2 Nanoparticle‐Embedded Carbon Fibers for Flexible Fiber‐Shaped Sodium Battery

Fiber‐shaped rechargeable batteries hold promise as the next‐generation energy storage devices for wearable electronics. However, their application is severely hindered by the difficulty in fabrication of robust fiber‐like electrodes with promising electrochemical performance. Herein, yolk–shell NiS₂ nanoparticles embedded in porous carbon fibers (NiS₂?PCF) are successfully fabricated and developed as high‐performance fiber electrodes for sodium storage. Benefiting from the robust embedded structure, 3D porous and conductive carbon network, and yolk–shell NiS₂ nanoparticles, the as‐prepared NiS₂?PCF fiber electrode achieves a high reversible capacity of about 679 mA h g^?1 at 0.1 C, outstanding rate capability (245 mA h g^?1 at 10 C), and ultrastable cycle performance with 76% capacity retention over 5000 cycles at 5 C. Notably, a flexible fiber‐shaped sodium battery is assembled, and high reversible capacity is kept at different bending states. This work offers a new electrode‐design paradigm toward novel carbon fiber electrodes embedded with transition metal oxides/sulfides/phosphides for application in flexible energy storage devices.