Winding aligned carbon nanotube composite yarns into coaxial fiber full batteries with high performances

Recent progress of flexible rechargeable batteries

The rapid popularization of wearable electronics, soft robots and implanted medical devices has stimulated extensive research in flexible batteries, which are bendable, foldable, knittable, wearable, and/or stretchable. Benefiting from these distinct characteristics, flexible batteries can be seamlessly integrated into various wearable/implantable devices, such as smart home systems, flexible displays, and implantable sensors. In contrast to conventional lithium-ion batteries necessitating the incorporation of stringent current collectors and packaging layers that are typically rigid, flexible batteries require the flexibility of each component to accommodate diverse shapes or sizes. Accordingly, significant advancements have been achieved in the development of flexible electrodes, current collectors, electrolytes, and flexible structures to uphold superior electrochemical performance and exceptional flexibility. In this review, typical structures of flexible batteries are firstly introduced and classified into mono-dimensional, two-dimensional, and three-dimensional structures according to their configurations. Subsequently, five distinct types of flexible batteries, including flexible lithium-ion batteries, flexible sodium-ion batteries, flexible zinc-ion batteries, flexible lithium/sodium-air batteries, and flexible zinc/magnesium-air batteries, are discussed in detail according to their configurations, respectively. Meanwhile, related comprehensive analysis is introduced to delve into the fundamental design principles pertaining to electrodes, electrolytes, current collectors, and integrated structures for various flexible batteries. Finally, the developments and challenges of flexible batteries are summarized, offering viable guidelines to promote the practical applications in the future.

Multilayer Core-Shell Fiber Device for Improved Strain Sensing and Supercapacitor Applications

1D fiber devices, known for their exceptional flexibility and seamless integration capabilities, often face trade-offs between desired wearable application characteristics and actual performance. In this study, a multilayer device composed of carbon nanotube (CNT), transition metal carbides/nitrides (MXenes), and cotton fibers, fabricated using a dry spinning method is presented, which significantly enhances both strain sensing and supercapacitor functionality. This core-shell fiber design achieves a record-high sensitivity (GF ≈ 4500) and maintains robust durability under various environmental conditions. Furthermore, the design approach markedly influences capacitance, correlating with the percentage of active material used. Through systematic optimization, the fiber device exhibited a capacitance 26-fold greater than that of a standard neat CNT fiber, emphasizing the crucial role of innovative design and high active material loading in improving device performance.

Silicon Nanowire Array Weaved by Carbon Chains for Stretchable Lithium-Ion Battery Anode

To manufacture flexible batteries, it can be a challenge for silicon base anode materials to maintain structural integrity and electrical connectivity under bending and torsion conditions. In this work, 1D silicon nanowire array structures combined with flexible carbon chains consisting of short carbon nanofibers (CNFs) and long carbon nanotubes (CNTs) are proposed. The CNFs and CNTs serve as chain joints and separate chain units, respectively, weaving the well-ordered Si nanowire array into a robust and integrated configuration. The prepared flexible and stretchable silicon array anode exhibits excellent electrochemical performance during dynamic operation. A high initial specific capacity of 2856 mAh g-1 is achieved. After 1000 cycles, a capacity retention of 60% (1602 mAh g-1) is maintained. Additionally, the capacity attenuation is less than 1% after 100 bending cycles. This excellent cycling stability is obtained with a high Si loading of 6.92 mg cm-2. This novel approach offers great promise for the development of high-loading flexible energy-storage devices.

High-performance fibre battery with polymer gel electrolyte

Replacement of liquid electrolytes with polymer gel electrolytes is recognized as a general and effective way of solving safety problems and achieving high flexibility in wearable batteries1-6. However, the poor interface between polymer gel electrolyte and electrode, caused by insufficient wetting, produces much poorer electrochemical properties, especially during the deformation of the battery7-9. Here we report a strategy for designing channel structures in electrodes to incorporate polymer gel electrolytes and to form intimate and stable interfaces for high-performance wearable batteries. As a demonstration, multiple electrode fibres were rotated together to form aligned channels, while the surface of each electrode fibre was designed with networked channels. The monomer solution was effectively infiltrated first along the aligned channels and then into the networked channels. The monomers were then polymerized to produce a gel electrolyte and form intimate and stable interfaces with the electrodes. The resulting fibre lithium-ion battery (FLB) showed high electrochemical performances (for example, an energy density of about 128 Wh kg-1). This strategy also enabled the production of FLBs with a high rate of 3,600 m h-1 per winding unit. The continuous FLBs were woven into a 50 cm × 30 cm textile to provide an output capacity of 2,975 mAh. The FLB textiles worked safely under extreme conditions, such as temperatures of -40 °C and 80 °C and a vacuum of -0.08 MPa. The FLBs show promise for applications in firefighting and space exploration.

Fiber Crossbars: An Emerging Architecture of Smart Electronic Textiles

Smart wearables have a significant impact on people's daily lives, enabling personalized motion monitoring, realizing the Internet of Things, and even reshaping the next generation of telemedicine systems. Fiber crossbars (FCs), constructed by crossing two fibers, have become an emerging architecture among the accessible structures of state-of-the-art smart electronic textiles. The mechanical, chemical, and electrical interactions between crossing fibers result in extensive functionalities, leading to the significant development of innovative electronic textiles employing FCs as their basic units. This review provides a timely and comprehensive overview of the structure designs, material selections, and assembly techniques of FC-based devices. The recent advances in FC-based devices are summarized, including multipurpose sensing, multiple-mode computing, high-resolution display, high-efficient power supply, and large-scale textile systems. Finally, current challenges, potential solutions, and future perspectives for FC-based systems are discussed for their further development in scale-up production and commercial applications.

Modern Developments for Textile-Based Supercapacitors

Smart textiles are transforming the future of wearable technology, and due to that, there has been a great deal of new research looking for alternative energy storage. Supercapacitors offer high discharge rates, flexibility, and long life cycles and can be integrated fully into a textile. Optimization of these new systems includes utilizing electrically conductive materials, employing successful electrostatic charge and/or faradaic responses, and fabricating a textile-based energy storage system without disrupting comfort, washability, and life cycle. This paper examines recent developments in fabrication methods and materials used to create textile supercapacitors and what challenges still remain.

Densifiable Ink Extrusion for Roll-To-Roll Fiber Lithium-Ion Batteries with Ultra-High Linear and Volumetric Energy Densities

Conventional bulky and rigid planar architecting power systems are difficult to satisfy the growing demand for wearable applications. 1D fiber batteries bearing appealing features of miniaturization, adaptability, and weavability represent a promising solution, yet challenges remain pertaining to energy density and scalability. Herein, an ingenious densifiable functional ink is invented to fabricate scalable, flexible, and high-mass-loading fiber lithium-ion batteries (LIBs) by adopting a fast ink-extrusion technology. In the formulated ink, pyrrole-modified reduced graphene oxide is elaborately introduced and exerts multiple influences; it not only assembles carbon nanotubes and poly(vinylidene fluoride-co-hexafluoropropylene) to compose a sturdy, conductive, and agglomeration-free 3D network that realizes an ultra-high content (75 wt%) of the active materials and endows the electrode excellent flexibility but also serves as a capillary densification inducer, encouraging an extremely large linear mass loading (1.01 mg cm^-1 per fiber) and packing density (782.1 mg cm^-3 ). As a result, the assembled fiber LIBs deliver impressive linear and volumetric energy densities with superb mechanical compliance, demonstrating the best performance among all the reported extruded fiber batteries. This work highlights a highly effective and facile approach to fabricate high-performance fiber energy storage devices for future practical wearable applications.

Functional Fiber Materials to Smart Fiber Devices

The development of fiber materials has accompanied the evolution of human civilization for centuries. Recent advances in materials science and chemistry offered fibers new applications with various functions, including energy harvesting, energy storing, displaying, health monitoring and treating, and computing. The unique one-dimensional shape of fiber devices endows them advantages to work as human-interfaced electronics due to the small size, lightweight, flexibility, and feasibility for integration into large-scale textile systems. In this review, we first present a discussion of the basics of fiber materials and the design principles of fiber devices, followed by a comprehensive analysis on recently developed fiber devices. Finally, we provide the current challenges facing this field and give an outlook on future research directions. With novel fiber devices and new applications continuing to be discovered after two decades of research, we envision that new fiber devices could have an important impact on our life in the near future.

Nb and Ni Nanoparticles Anchored on N-Doped Carbon Nanofiber Membrane as Self-Supporting Anode for High-Rate Lithium-Ion Batteries

A flexible N-doped carbon nanofiber membrane loaded with Nb and Ni nanoparticles (Nb/Ni@NC) was prepared using electrospinning technology and a subsequent thermal annealing method and used as a self-supporting anode material for lithium-ion batteries. The Nb/Ni@NC nanofiber membrane had excellent flexibility and could be folded and bent at will without fragmentation and wrinkling; the nanofibers also had a uniform and controllable morphology with a diameter of 300-400 nm. The electrochemical results showed that the flexible Nb/Ni@NC electrode could deliver a high discharge capacity of 378.7 mAh g^-1 after 200 cycles at 0.2 A g^-1 and an initial coulombic efficiency of 67.7%, which was higher than that of the pure flexible NC anode in contrast. Moreover, a reversible discharge capacity of 203.6 mAh g^-1 after 480 cycles at 1.0 A g^-1 was achieved by the flexible Nb/Ni@NC electrode with a capacity decay for each cycle of only 0.075%, which showed an excellent rate capability and cycling stability.

Plasma-Enhanced Carbon Nanotube Fiber Cathode for Li-S Batteries

Fiber-shaped batteries have attracted much interest in the last few years. However, a major challenge for this type of battery is their relatively low energy density. Here, we present a freestanding, flexible CNT fiber with high electrical conductivity and applied oxygen plasma-functionalization, which was successfully employed to serve as an effective cathode for Li-S batteries. The electrochemical results obtained from the conducted battery tests showed a decent rate capability and cyclic stability. The cathode delivered a capacity of 1019 mAh g−1 at 0.1 C. It accommodated a high sulfur loading of 73% and maintained 47% of the initial capacity after 300 cycles. The demonstrated performance of the fiber cathode provides new insights for the designing and fabrication of high energy density fiber-shaped batteries.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

Advanced materials for personal thermal and moisture management of health care workers wearing PPE

In recent years, the development of personal protective equipment (PPE) for health care workers (HCWs) attracted enormous attention, especially during the pandemic of COVID-19. The semi-permeable protective clothing and the prolonged working hours make the thermal comfort a critical issue for HCWs. Although there are many commercially available personal cooling products for PPE systems, they are either heavy in weight or have limited durability. Besides, most of the existing solutions cannot relieve the perspiration efficiently within the insolation gowns. To avoid heat strain and ensure a longtime thermal comfort, new strategies that provide efficient personal thermal and moisture management without compromising health protection are required. This paper reviews the emerging materials for protective gown layers and advanced technologies for personal thermal and moisture management of PPE systems. These materials and strategies are examined in detail with respect to their fundamental working principles, thermal and mechanical properties, fabrication methods as well as advantages and limitations in their prospective applications, aiming at stimulating creative thinking and multidisciplinary collaboration to improve the thermal comfort of PPEs.

Fiber‐Shaped Electronic Devices

Textile electronics embedded in clothing represent an exciting new frontier for modern healthcare and communication systems. Fundamental to the development of these textile electronics is the development of the fibers forming the cloths into electronic devices. An electronic fiber must undergo diverse scrutiny for its selection for a multifunctional textile, viz., from the material selection to the device architecture, from the wearability to mechanical stresses, and from the environmental compatibility to the end‐use management. Herein, the performance requirements of fiber‐shaped electronics are reviewed considering the characteristics of single electronic fibers and their assemblies in smart clothing. Broadly, this article includes i) processing strategies of electronic fibers with required properties from precursor to material, ii) the state‐of‐art of current fiber‐shaped electronics emphasizing light‐emitting devices, solar cells, sensors, nanogenerators, supercapacitors storage, and chromatic devices, iii) mechanisms involved in the operation of the above devices, iv) limitations of the current materials and device manufacturing techniques to achieve the target performance, and v) the knowledge gap that must be minimized prior to their deployment. Lessons learned from this review with regard to the challenges and prospects for developing fiber‐shaped electronic components are presented as directions for future research on wearable electronics.

Directed Assembly of Particles for Additive Manufacturing of Particle-Polymer Composites

Particle-polymer dispersions are ubiquitous in additive manufacturing (AM), where they are used as inks to create composite materials with applications to wearable sensors, energy storage materials, and actuation elements. It has been observed that directional alignment of the particle phase in the polymer dispersion can imbue the resulting composite material with enhanced mechanical, electrical, thermal or optical properties. Thus, external field-driven particle alignment during the AM process is one approach to tailoring the properties of composites for end-use applications. This review article provides an overview of externally directed field mechanisms (e.g., electric, magnetic, and acoustic) that are used for particle alignment. Illustrative examples from the AM literature show how these mechanisms are used to create structured composites with unique properties that can only be achieved through alignment. This article closes with a discussion of how particle distribution (i.e., microstructure) affects mechanical properties. A fundamental description of particle phase transport in polymers could lead to the development of AM process control for particle-polymer composite fabrication. This would ultimately create opportunities to explore the fundamental impact that alignment has on particle-polymer composite properties, which opens up the possibility of tailoring these materials for specific applications.

Smart fibers for energy conversion and storage

Fibers have played a critical role in the long history of human development. They are the basic building blocks of textiles. Synthetic fibers not only make clothes stronger and more durable, but are also customizable and cheaper. The growth of miniature and wearable electronics has promoted the development of smart and multifunctional fibers. Particularly, the incorporation of functional semiconductors and electroactive materials in fibers has opened up the field of fiber electronics. The energy supply system is the key branch for fiber electronics. Herein, after a brief introduction on the history of smart and functional fibers, we review the current state of advanced functional fibers for their application in energy conversion and storage, focusing on nanogenerators, solar cells, supercapacitors and batteries. Subsequently, the importance of the integration of fiber-shaped energy conversion and storage devices via smart structure design is discussed. Finally, the challenges and future direction in this field are highlighted. Through this review, we hope to inspire scientists with different research backgrounds to enter this multi-disciplinary field to promote its prosperity and development and usher in a truly new era of smart fibers.

Application-Driven Carbon Nanotube Functional Materials

Carbon nanotube functional materials (CNTFMs) represent an important research field in transforming nanoscience and nanotechnology into practical applications, with potential impact in a wide realm of science, technology, and engineering. In this review, we combine the state-of-the-art research activities of CNTFMs with the application prospect, to highlight critical issues and identify future challenges. We focus on macroscopic long fibers, thin films, and bulk sponges which are typical CNTFMs in different dimensions with distinct characteristics, and also cover a variety of derived composite/hierarchical materials. Critical issues related to their structures, properties, and applications as robust conductive skeletons or high-performance flexible electrodes in mechanical and electronic devices, advanced energy conversion and storage systems, and environmental areas have been discussed specifically. Finally, possible solutions and directions are proposed for overcoming current obstacles and promoting future efforts in the field.

Carbon nanotubes for flexible batteries: recent progress and future perspective

Flexible batteries, which maintain their functions potently under various mechanical deformations, attract increasing interest due to potential applications in emerging portable and wearable electronics. Significant efforts have been devoted to material synthesis and structural designs to realize the mechanical flexibility of various batteries. Carbon nanotubes (CNTs) have a unique one-dimensional (1D) nanostructure and are convenient to further assemble into diverse macroscopic structures, such as 1D fibers, 2D films and 3D sponges/aerogels. Due to their outstanding mechanical and electrical properties, CNTs and CNT-based hybrid materials are superior building blocks for different components in flexible batteries. This review summarizes recent progress on the application of CNTs in developing flexible batteries, from closed-system to open-system batteries, with a focus on different structural designs of CNT-based material systems and their roles in various batteries. We also provide perspectives on the challenges and future research directions for realizing practical applications of CNT-based flexible batteries.

Electroluminescent Fabric Woven by Ultrastretchable Fibers for Arbitrarily Controllable Pattern Display

Flexible textile displays can be revolutionary for information transmission at any place and any time. Typically, textile displays are fabricated by traditional rigid electronics that sacrifice mechanical flexibility of devices or by flexible electronics that do not have an appropriate choice to arbitrarily control single pixels. This work reports on an electroluminescent fabric woven by ultrastretchable fibers (electroluminescent fibers up to 400% stretch, electrode fibers up to 250% stretch), which can exhibit the pixel-based arbitrarily controllable pattern display by a mobile phone application. To realize ultrastretchability, we made these fibers by encapsulating liquid metals on a polyurethane core (high elasticity). To realize arbitrary control, the design shows a plain-woven structure comprising ZnS-based electroluminescent fibers and perpendicular electrode fibers. The cross-points between the electroluminescent fiber and the electrode fiber form pixels that can be switched on or off independently and can further form the pixel-based arbitrarily controllable pattern display. By doping with different elements, ZnS-based electroluminescent fibers can emit green, blue, or yellow lights. Meanwhile, the fabrication of these fibers employs dip-coating, a scalable manufacturing method without high temperature or vacuum atmosphere. These fabrics show great potential in a wide range of applications such as wearable electronic devices, healthcare, and fashion design.

Aligned carbon nanotube fibers for fiber-shaped solar cells, supercapacitors and batteries

Aligned carbon nanotube (CNT) fibers have been considered as one of the ideal candidate electrodes for fiber-shaped energy harvesting and storage devices, due to their merits of flexibility, lightweight, desirable mechanical property, outstanding electrical conductivity as well as high specific surface area. Herein, the recent advancements on the aligned CNT fibers for energy harvesting and storage devices are reviewed. The synthesis, structure, and properties of aligned carbon nanotube fibers are briefly summarized. Then, their applications in fiber-shaped energy harvesting and storage devices (i.e., solar cells, supercapacitors, and batteries) are demonstrated. The remaining challenges are finally discussed to highlight the future research direction in the development of aligned CNT fibers for fiber-shaped energy devices.

Fiber-Based Sensors and Energy Systems for Wearable Electronics

Wearable electronics have been receiving increasing attention for the past few decades. Particularly, fiber-based electronics are considered to be ideal for many applications for their flexibility, lightweight, breathability, and comfortability. Furthermore, fibers and fiber-based textiles can be 3D-molded with ease and potentially integrated with everyday clothes or accessories. These properties are especially desired in the fields of bio-related sensors and energy-storage systems. Wearable sensors utilize a tight interface with human skin and clothes for continuous environmental scanning and non-invasive health monitoring. At the same time, their flexible and lightweight properties allow more convenient and user-friendly experiences to the wearers. Similarly, for the wearable devices to be more accessible, it is crucial to incorporate energy harvesting and storage systems into the device themselves, removing the need to attach an external power source. This review summarizes the recent applications of fibers and fiber-based textiles in mechanical, photonic, and biomedical sensors. Pressure and strain sensors and their implementation as electronic skins will be explored, along with other various fiber sensors capable of imaging objects or monitoring safety and health markers. In addition, we attempt to elucidate recent studies in energy-storing fibers and their implication in self-powered and fully wireless wearable devices.

Spirally Wrapped Carbon Nanotube Microelectrodes for Fiber Optoelectronic Devices beyond Geometrical Limitations toward Smart Wearable E-Textile Applications

Fiber optoelectronics technology has recently attracted attention as enabling various form factors of wearable electronics, and the issue of how to control and optimize the configuration and physical properties of the electrode micropatterns in the microfiber devices has become important. Here, spirally wrapped carbon nanotube (CNT) microelectrodes with a controlled dimension are demonstrated for high-performance fiber optoelectronic devices. Inkjet-printed CNT microelectrodes with the desired dimension on an agarose hydrogel template are rolling-transferred onto a microfiber surface with an efficient electrical interface. A fiber organic field-effect transistor with spirally wrapped CNT microelectrodes verifies the feasibility of this strategy, where the transferred microelectrodes intimately contact the organic semiconductor active layer and the output current characteristics are simply controlled, resulting in characteristics that exceed the previous structural limitations. Furthermore, a fiber organic photodiode with spirally wrapped CNT microelectrodes, when used as a transparent electrode, exhibits a high I_light/I_dark ratio and good durability of bending. This fiber photodiode can be successfully incorporated into a textile photoplethysmography bandage for the real-time monitoring of human vital signals. This work offers a promising and efficient strategy to overcome the geometric factors limiting the performance of fiber-optic optoelectronic devices.

Autonomous Energy Harvester Based on Textile-Based Enzymatic Biofuel Cell for On-Demand Usage

This paper presents an autonomous energy harvester based on a textile-based enzymatic biofuel cell, enabling an efficient power management and on-demand usage. The proposed biofuel cell works by an enzymatic reaction with glucose in sweat absorbed by the specially designed textile for sustainable and efficient energy harvesting. The output power of the textile-based biofuel cell has been optimized by changing electrode size and stacking electrodes and corresponding fluidic channels suitable for following power management circuit. The output power level of single electrode is estimated less than 0.5 μW and thus a two-staged power management circuit using intermediate supercapacitor has been presented. As a solution to produce a higher power level, multiple stacks of biofuel cell electrodes have been proposed and thus the textile-based biofuel cell employing serially connected 5 stacks produces a maximal power of 13 μW with an output voltage of 0.88 V when load resistance is 40 kΩ. A buck-boost converter employing a crystal oscillator directly triggered by DC output voltage of the biofuel cell makes it possible to obtain output voltage of the DC-DC converter is 6.75 V. The efficiency of the DC-DC converter is estimated as approximately 50% when the output power of the biofuel cell is tens microwatts. In addition, LT-spice modeling and simulation has been presented to estimate power consumption of each element of the proposed DC-DC converter circuit and the predicted output voltage has good agreement with measurement result.

A zinc ion yarn battery with high capacity and fire retardancy based on a SiO2 nanoparticle doped ionogel electrolyte

Ionogel electrolytes are proposed to be an excellent substitute for liquid electrolytes due to their superior portability, flexibility and safety without any leakage that may lead to the failure of batteries. Given these natural characteristics, ionogels can provide batteries with good flexibility and wearability. However, the instability under different extreme working conditions and low ionic conductivity limit the further applications of ionogels. In this research, an ionogel electrolyte is synthesized by polymerizing 1-vinyl-3-ethylimidazolium dicyanamide ([Veim][DCA]) and N,N-methylenebisacrylamide (NNMBA) in zinc acetate (Zn(CH3COO)2) and manganese sulfate (MnSO4) dissolved 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCA]). The fire retardancy of the ionogel electrolyte is improved by adding SiO2 nanoparticles into the electrolyte. The ionogel electrolyte with 40 wt% silicon dioxide (SiO2) nanoparticles possesses a high ionic conductivity of 0.013 S cm-1 and the zinc//manganese dioxide (Zn//MnO2) battery based on the ionogel electrolyte is able to deliver a high specific capacity of 277 mA h g-1 and a high energy density of 283 W h kg-1. Moreover, the SiO2 nanoparticles contained in the ionogel endow the battery with good fire retardancy. Put together, the yarn battery successfully meets the requirements of modern flexible and wearable electronics.

Soft Materials for Wearable/Flexible Electrochemical Energy Conversion, Storage, and Biosensor Devices

Next-generation wearable technology needs portable flexible energy storage, conversion, and biosensor devices that can be worn on soft and curved surfaces. The conformal integration of these devices requires the use of soft, flexible, light materials, and substrates with similar mechanical properties as well as high performances. In this review, we have collected and discussed the remarkable research contributions of recent years, focusing the attention on the development and arrangement of soft and flexible materials (electrodes, electrolytes, substrates) that allowed traditional power sources and sensors to become viable and compatible with wearable electronics, preserving or improving their conventional performances.

High-performance fiber-shaped lithium-ion batteries

This short review summarizes our recent progress in fiber-shaped lithium-ion batteries and lithium-air batteries based on carbon nanotube hybrid fiber electrodes. The fiber architecture allows batteries to be deformable in all dimensions and bear various deformations such as bending, tying, twisting and even stretching. They are scaled up and further woven into breathable, flexible, stretchable and shape-memory textiles to effectively meet the requirements of modern electronics such as wearable products.

Cost-Effective Yarn-Shaped Lithium-Ion Battery with High Wearability

Because of flexibility, compactness, weavability, and ergonomic design, yarn-shaped lithium-ion batteries (LIBs) have enormous potential applications in wearable electronics. Still, the yarn-shaped LIB with the ability to meet commercialization requirements has never been reported, owing to the current challenge in complex material synthesis technologies, expensive raw material costs, poor safety performance, and nonstandard manufacturing equipment. Herein, we propose a yarn-shaped LIB that meets the aforementioned requirements. With a highly conductive and flexible stainless-steel yarn acting as the current collector, the electrode active materials and the gel electrolyte, which are commercially available at low cost, are uniformly coated onto the stainless-steel yarn by a simple and facile dipping-drying method. Even at different deformation conditions (i.e., bending or knotting), the specific capacity of the yarn-shaped LIB (7 cm long, <2 mm in diameter) assembled from graphite and lithium iron phosphate electrodes is maintained >85%. After charged treatment, it can successfully power up an electronic watch and an electronic thermo-hygrometer. Thanks to the simple preparation process, low cost of raw materials, and good safety performance, this work can promote the commercialization of wearable energy storage devices.

Recent Advances in Fiber-Shaped Supercapacitors and Lithium-Ion Batteries

The rapid development in wearable electronics has spurred a great deal of interest in flexible energy storage devices, particularly fiber-shaped energy storage devices (FSESDs), such as fiber-shaped supercapacitors (FSSCs) and fiber-shaped batteries (FSBs). Depending on their electrode configurations, FSESDs can contain five differently structured electrodes, including parallel fiber electrodes (PFEs), twisted fiber electrodes (TFDs), wrapped fiber electrodes (WFEs), coaxial fiber devices (CFEs), and rolled electrodes (REs). Various rational methods have been devised to incorporate these fiber-shaped electrodes into multifunctional FSESDs, including fiber-shaped supercapacitors, lithium-ion batteries, lithium-sulfur batteries, lithium-air batteries, zinc-air batteries, and aluminum-air batteries. Although significant progress has been made in FSESDs, it remains a major challenge to make high-performance fiber-shaped devices at low cost. A focused and critical review of the recent advancements in fiber-shaped supercapacitors and lithium-ion batteries is provided here. The pros and cons for each of the aforementioned electrode configurations and FSESDs are discussed, along with current challenges and future opportunities for FSESDs.

Understanding the Mechanical and Conductive Properties of Carbon Nanotube Fibers for Smart Electronics

The development of fiber-based smart electronics has provoked increasing demand for high-performance and multifunctional fiber materials. Carbon nanotube (CNT) fibers, the 1D macroassembly of CNTs, have extensively been utilized to construct wearable electronics due to their unique integration of high porosity/surface area, desirable mechanical/physical properties, and extraordinary structural flexibility, as well as their novel corrosion/oxidation resistivity. To take full advantage of CNT fibers, it is essential to understand their mechanical and conductive properties. Herein, the recent progress regarding the intrinsic structure-property relationship of CNT fibers, as well as the strategies of enhancing their mechanical and conductive properties are briefly summarized, providing helpful guidance for scouting ideally structured CNT fibers for specific flexible electronic applications.

Flexible 1D Batteries: Recent Progress and Prospects

With the rapid development of wearable and portable electronics, flexible and stretchable energy storage devices to power them are rapidly emerging. Among numerous flexible energy storage technologies, flexible batteries are considered as the most favorable candidate due to their high energy density and long cycle life. In particular, flexible 1D batteries with the unique advantages of miniaturization, adaptability, and weavability are expected to be a part of such applications. The development of 1D batteries, including lithium-ion batteries, zinc-ion batteries, zinc-air batteries, and lithium-air batteries, is comprehensively summarized, with particular emphasis on electrode preparation, battery design, and battery properties. In addition, the remaining challenges to the commercialization of current 1D batteries and prospective opportunities in the field are discussed.

A Route Toward Smart System Integration: From Fiber Design to Device Construction

Fiber is a symbol of human civilization, being ubiquitous but obscure in society over most of history. Fiber has been revived upon the advent of fiber-based electronic devices in the past two decades. This is due to its desirable lightweight, flexible, and conformable characteristics, which enable it to play a fundamental role in the electronic and information era. Numerous fiber-based electronic devices have sprung up in energy conversion, energy storage, sensing, actuation, etc. A possibility is thereby conceived that they can be integrated into smart systems compatible with the human body, consisting of biotic fiber-based organs and tissues, which possess similar but more advanced functions. However, the design of mono-/multifibers, the construction of fiber-based devices, and the integration of these smart systems represent great challenges in fundamental understanding and practical implementation. A systematic review of the current state of the art with respect to the design and fabrication of electronic fiber materials, construction of fiber-based devices, and integration of smart systems is presented. In addition, limitations of current fiber-based devices and perspectives are explored toward potential and promising smart integration.

An Overview of Fiber-Shaped Batteries with a Focus on Multifunctionality, Scalability, and Technical Difficulties

Flexible and wearable energy storage devices are receiving increasing attention with the ever-growing market of wearable electronics. Fiber-shaped batteries display a unique 1D architecture with the merits of superior flexibility, miniaturization potential, adaptability to deformation, and compatibility with the traditional textile industry, which are especially advantageous for wearable applications. In the recent research frontier in the field of fiber-shaped batteries, in addition to higher performance, advances in multifunctional, scalable, and integrable systems are also the main themes. However, many difficulties exist, including difficult encapsulation and installation of separators, high internal resistance, and poor durability. Herein, the design principles (e.g., electrode preparation and battery assembly) and device performance (e.g., electrochemical and mechanical properties) of fiber-shaped batteries, including lithium-based batteries, zinc-based batteries, and some other representative systems, are summarized, with a focus on multifunctional devices with environmental adaptability, stimuli-responsive properties, and scalability up to energy textiles, with the hope of enlightening future research directions. Finally, technical challenges in the realistic wearable application of these batteries are also discussed with the aim of providing possible solutions and new insights for further improvement.

Recent Advances in 1D Stretchable Electrodes and Devices for Textile and Wearable Electronics: Materials, Fabrications, and Applications

Research on wearable electronic devices that can be directly integrated into daily textiles or clothes has been explosively grown holding great potential for various practical wearable applications. These wearable electronic devices strongly demand 1D electronic devices that are light-weight, weavable, highly flexible, stretchable, and adaptable to comport to frequent deformations during usage in daily life. To this end, the development of 1D electrodes with high stretchability and electrical performance is fundamentally essential. Herein, the recent process of 1D stretchable electrodes for wearable and textile electronics is described, focusing on representative conductive materials, fabrication techniques for 1D stretchable electrodes with high performance, and designs and applications of various 1D stretchable electronic devices. To conclude, discussions are presented regarding limitations and perspectives of current materials and devices in terms of performance and scientific understanding that should be considered for further advances.

Application Challenges in Fiber and Textile Electronics

Modern electronic devices are moving toward miniaturization and integration with an emerging focus on wearable electronics. Due to their close contact with the human body, wearable electronics have new requirements including low weight, small size, and flexibility. Conventional 3D and 2D electronic devices fail to efficiently meet these requirements due to their rigidity and bulkiness. Hence, a new family of 1D fiber-shaped electronic devices including energy-harvesting devices, energy-storage devices, light-emitting devices, and sensing devices has risen to the challenge due to their small diameter, lightweight, flexibility, and weavability into soft textile electronics. The application challenges faced by fiber and textile electronics from single fiber-shaped devices to continuously scalable fabrication, to encapsulation and testing, and to application mode exploration, are discussed. The evolutionary trends of fiber and textile electronics are then summarized. Finally, future directions required to boost their commercialization are highlighted.

Aerosol-Jet-Printed Preferentially Aligned Carbon Nanotube Twin-Lines for Printed Electronics

The alignment of carbon nanotubes (CNTs) is of great importance for the fabrication of high-speed electronic devices such as a transistor as the electron mobilities can be greatly enhanced with aligned CNT architectures. Here, we report, for the first time, a methodology to obtain preferentially aligned CNT traces on a flexible polyimide substrate utilizing the high-resolution aerosol jet printing technique and evaporation-driven self-assembly process. A self-assembled twin-line of CNT ("coffee-ring" effect) is observed in the deposit patterns, and the field-emission scanning electron microscopy (FESEM) images reveal highly self-ordered CNT in the resulting CNT twin-line. Various aerosol jet parameters have been investigated to obtain printed tracks in the range of 30-80 μm and conductive tracks (single CNT twin-line width) in the range of 600-1500 nm. The smallest CNT twin-line obtained in this experiment is found to be approximately 16 μm using a suitable sheath-to-atomizer flow ratio. Image analysis of FESEM images confirms the formation of aligned CNT traces at the ink periphery. The effect of the line width on the degree of alignment of the CNT is studied and evaluated. The electrical resistance of the CNT trace is adjustable by controlling the number of print passes and print speed.

Flexible and High-Voltage Coaxial-Fiber Aqueous Rechargeable Zinc-Ion Battery

Extensive efforts have been devoted to construct a fiber-shaped energy-storage device to fulfill the increasing demand for power consumption of textile-based wearable electronics. Despite the myriad of available material selections and device architectures, it is still fundamentally challenging to develop eco-friendly fiber-shaped aqueous rechargeable batteries (FARBs) on a single-fiber architecture with high energy density and long-term stability. Here, we demonstrate flexible and high-voltage coaxial-fiber aqueous rechargeable zinc-ion batteries (CARZIBs). By utilizing a novel spherical zinc hexacyanoferrate with prominent electrochemical performance as cathode material, the assembled CARZIB offers a large capacity of 100.2 mAh cm^-3 and a high energy density of 195.39 mWh cm^-3, outperforming the state-of-the-art FARBs. Moreover, the resulting CARZIB delivers outstanding flexibility with the capacity retention of 93.2% after bending 3000 times. Last, high operating voltage and output current are achieved by the serial and parallel connection of CARZIBs woven into the flexible textile to power high-energy-consuming devices. Thus, this work provides proof-of-concept design for next-generation wearable energy-storage devices.

Electrochemical energy storage devices for wearable technology: a rationale for materials selection and cell design

Compatible energy storage devices that are able to withstand various mechanical deformations, while delivering their intended functions, are required in wearable technologies. This imposes constraints on the structural designs, materials selection, and miniaturization of the cells. To date, extensive efforts have been dedicated towards developing electrochemical energy storage devices for wearables, with a focus on incorporation of shape-conformable materials into mechanically robust designs that can be worn on the human body. In this review, we highlight the quantified performances of reported wearable electrochemical energy storage devices, as well as their micro-sized counterparts under specific mechanical deformations, which can be used as the benchmark for future studies in this field. A general introduction to the wearable technology, the development of the selection and synthesis of active materials, cell design approaches and device fabrications are discussed. It is followed by challenges and outlook toward the practical use of electrochemical energy storage devices for wearable applications.

Mechanical properties of carbon nanotube fibers at extreme temperatures

Carbon nanotube (CNT) fibers are strong, flexible, and multifunctional, which makes them promising candidates for use at extreme temperatures. However, the current reported mechanical properties of CNT fibers were commonly obtained at room temperature. Here, we report the measurement of the mechanical properties of CNT fibers at temperatures ranging from -196 °C to 2400 °C. Compared with the room temperature strength and modulus, CNT fibers tested at 1000 °C and 2400 °C retained 82% and 54% of the strength, and 71% and 50% of the modulus, respectively, while 68% and 220% increases in the strength and modulus, respectively, were observed for CNT fibers tested at -196 °C. We attributed the decay in the mechanical properties at high temperatures to the weakening of individual nanotubes and intertube interactions, and the strength enhancement at low temperature to the increased activation energy to break the nanotubes. The present study provides the fundamental mechanical properties of CNT fibers at extreme temperatures, which could facilitate the applications of CNT fibers in aeronautics and astronautics where extreme temperature conditions commonly exist.

Carbon-Nanomaterial-Based Flexible Batteries for Wearable Electronics

Wearable electronics have received considerable attention in recent years. These devices have penetrated every aspect of our daily lives and stimulated interest in futuristic electronics. Thus, flexible batteries that can be bent or folded are desperately needed, and their electrochemical functions should be maintained stably under the deformation states, given the increasing demands for wearable electronics. Carbon nanomaterials, such as carbon nanotubes, graphene, and/or their composites, as flexible materials exhibit excellent properties that make them suitable for use in flexible batteries. Herein, the most recent progress on flexible batteries using carbon nanomaterials is discussed from the viewpoint of materials fabrication, structure design, and property optimization. Based on the current progress, the existing advantages, challenges, and prospects are outlined and highlighted.

Facile Development Strategy of a Single Carbon-Fiber-Based All-Solid-State Flexible Lithium-Ion Battery for Wearable Electronics

Microsized and shape-versatile flexible and wearable lithium-ion batteries (LIBs) are promising and smart energy storage devices for next-generation electronics. In the present work, we design and fabricate the first prototype of microsized fibrous LIBs (thickness ≈ 22 μm) based on multilayered coaxial structure of solid-state battery components over flexible and electrically conductive carbon fibers (CFs). The micro coaxial batteries over the CF surface were fabricated via electrophoretic deposition and dip-coating methods. The microfiber battery showed a stable potential window of 2.5 V with an areal discharge capacity of ∼4.2 μA h cm^-2 at 13 μA cm^-2 of the current density. The as-assembled battery fiber delivered a comparable energy density (∼0.006 W h cm^-3) with solid-state lithium thin-film batteries at higher power densities (∼0.0312 W cm^-3). The fibrous batteries were also connected in parallel and in series to deliver large current and high voltage, respectively. The fibrous battery also retains up to 85% discharge capacity even after 100 charge-discharge cycles. Furthermore, these battery fibers performed well under both static and bending conditions, which shows the robustness of the battery fiber. Therefore, this type of fibrous microbattery can be used in advanced flexible and wearable microelectronics, bioelectronics, robotics, and textile applications.

Function-driven engineering of 1D carbon nanotubes and 0D carbon dots: mechanism, properties and applications

Metal-free carbonaceous nanomaterials have witnessed a renaissance of interest due to the surge in the realm of nanotechnology. Among myriads of carbon-based nanostructures with versatile dimensionality, one-dimensional (1D) carbon nanotubes (CNTs) and zero-dimensional (0D) carbon dots (CDs) have grown into a research frontier in the past few decades. With extraordinary mechanical, thermal, electrical and optical properties, CNTs are utilized in transparent displays, quantum wires, field emission transistors, aerospace materials, etc. Although CNTs possess diverse characteristics, their most attractive property is their unique photoluminescence. On the other hand, another growing family of carbonaceous nanomaterials, which is CDs, has drawn much research attention due to its cost-effectiveness, low toxicity, environmental friendliness, fluorescence, luminescence and simplicity to be synthesized and functionalized with surface passivation. Benefiting from these unprecedented properties, CDs have been widely employed in biosensing, bioimaging, nanomedicine, and catalysis. Herein, we have systematically presented the fascinating properties, preparation methods and multitudinous applications of CNTs and CDs (including graphene quantum dots). We will discuss how CNTs and CDs have emerged as auspicious nanomaterials for potential applications, especially in electronics, sensors, bioimaging, wearable devices, batteries, supercapacitors, catalysis and light-emitting diodes (LEDs). Last but not least, this review is concluded with a summary, outlook and invigorating perspectives for future research horizons in this emerging platform of carbonaceous nanomaterials.

Highly conductive porous graphene/sulfur composite ribbon electrodes for flexible lithium-sulfur batteries

Flexible batteries have become an indispensable component of emerging devices, such as wearable, foldable electronics and sensors. Although various flexible batteries have been explored based on one-dimensional and two-dimensional platforms, developing a high energy density electrode with high structural integrity remains challenging. Herein, a scalable, one-pot wet spinning strategy is used to synthesize a flexible porous cathode for lithium-sulfur batteries (LSBs) for the first time, which consists of reduced graphene oxide (rGO), graphene crumples (GCs) and sulfur powders. The electrode structures are tailored using GCs with different dimensions and functional features that are critical to its robustness under mechanical deformation and electrolyte penetration into the battery components. The optimized rGO/GC/S composite ribbon cathodes deliver a high capacity of 524 mA h g^-1 after 100 cycles at a current rate of 0.2 C. A shape-conformable battery prototype comprising an rGO/GC/S cathode and a lithium anode demonstrates a stable discharge characteristic under repeated bending/flattening cycles. The LSB prototype supported by an elastomer presents stable discharge behavior with high mechanical robustness against an extension of up to 50%. The above-mentioned findings shed new light on developing sulfur cathodes for flexible, high performance LSBs based on the rational design of graphene structures.

Issues and Challenges Facing Flexible Lithium-Ion Batteries for Practical Application

With the advent of flexible electronics, lithium-ion batteries have become a key component of high performance energy storage systems. Thus, considerable effort is made to keep up with the development of flexible lithium-ion batteries. To date, many researchers have studied newly designed batteries with flexibility, however, there are several significant challenges that need to be overcome, such as degradation of electrodes under external load, poor battery performance, and complicated cell preparation procedures. In addition, an in-depth understanding of the current challenges for flexible batteries is rarely addressed in a systematical and practical way. Herein, recent progress and current issues of flexible lithium-ion batteries in terms of battery materials and cell designs are reviewed. A critical overview of important issues and challenges for the practical application of flexible lithium-ion batteries is also provided. Finally, the strategies are discussed to overcome current limitations of the practical use of flexible lithium-based batteries, providing a direction for future research.

Thermally Responsive Torsional and Tensile Fiber Actuator Based on Graphene Oxide

Graphene-based actuators are of practical interest because of their relatively low cost compared with other nanocarbon materials, such as carbon nanotubes. We demonstrate the simple fabrication of graphene oxide (GO)-based fibers with an infiltrated nylon-6,6 polymer by wet spinning. These fibers could be twisted to form torsional actuators and further coiled to form tensile actuators. By controlling the relative twisting and coiling direction of the GO/nylon fiber, we were able to realize reversible contraction or elongation actuation with strokes as high as -80 and 75%, respectively, when the samples were heated to 200 °C. The tensile actuation showed a remarkably little hysteresis. Moreover, this GO/nylon actuator could lift loads over 100 times heavier than itself and generate a stable actuation at high temperatures over the melting point of the polymer. This novel kind of GO-based actuator, which has a multidirectional actuation, has potential for a wide range of applications such as artificial muscles, robotics, and temperature sensing.

Fiber-Type Solar Cells, Nanogenerators, Batteries, and Supercapacitors for Wearable Applications

Wearable electronic devices represent a paradigm change in consumer electronics, on-body sensing, artificial skins, and wearable communication and entertainment. Because all these electronic devices require energy to operate, wearable energy systems are an integral part of wearable devices. Essentially, the electrodes and other components present in these energy devices should be mechanically strong, flexible, lightweight, and comfortable to the user. Presented here is a critical review of those materials and devices developed for energy conversion and storage applications with an objective to be used in wearable devices. The focus is mainly on the advances made in the field of solar cells, triboelectric generators, Li-ion batteries, and supercapacitors for wearable device development. As these devices need to be attached/integrated with the fabric, the discussion is limited to devices made in the form of ribbons, filaments, and fibers. Some of the important challenges and future directions to be pursued are also highlighted.