Fiber/Fabric-Based Piezoelectric and Triboelectric Nanogenerators for Flexible/Stretchable and Wearable Electronics and Artificial Intelligence

Integration of advanced nanogenerator technology with conventional textile processes fosters the emergence of textile-based nanogenerators (NGs), which will inevitably promote the rapid development and widespread applications of next-generation wearable electronics and multifaceted artificial intelligence systems. NGs endow smart textiles with mechanical energy harvesting and multifunctional self-powered sensing capabilities, while textiles provide a versatile flexible design carrier and extensive wearable application platform for their development. However, due to the lack of an effective interactive platform and communication channel between researchers specializing in NGs and those good at textiles, it is rather difficult to achieve fiber/fabric-based NGs with both excellent electrical output properties and outstanding textile-related performances. To this end, a critical review is presented on the current state of the arts of wearable fiber/fabric-based piezoelectric nanogenerators and triboelectric nanogenerators with respect to basic classifications, material selections, fabrication techniques, structural designs, and working principles, as well as potential applications. Furthermore, the potential difficulties and tough challenges that can impede their large-scale commercial applications are summarized and discussed. It is hoped that this review will not only deepen the ties between smart textiles and wearable NGs, but also push forward further research and applications of future wearable fiber/fabric-based NGs.     

An Ultrastable and High‐Performance Flexible Fiber‐Shaped Ni–Zn Battery based on a Ni–NiO Heterostructured Nanosheet Cathode

Currently, the main bottleneck for the widespread application of Ni–Zn batteries is their poor cycling stability as a result of the irreversibility of the Ni‐based cathode and dendrite formation of the Zn anode during the charging–discharging processes. Herein, a highly rechargeable, flexible, fiber‐shaped Ni–Zn battery with impressive electrochemical performance is rationally demonstrated by employing Ni–NiO heterostructured nanosheets as the cathode. Benefiting from the improved conductivity and enhanced electroactivity of the Ni–NiO heterojunction nanosheet cathode, the as‐fabricated fiber‐shaped Ni–NiO//Zn battery displays high capacity and admirable rate capability. More importantly, this Ni–NiO//Zn battery shows unprecedented cyclic durability both in aqueous (96.6% capacity retention after 10 000 cycles) and polymer (almost no capacity attenuation after 10 000 cycles at 22.2 A g^?1) electrolytes. Moreover, a peak energy density of 6.6 µWh cm^?2, together with a remarkable power density of 20.2 mW cm^?2, is achieved by the flexible quasi‐solid‐state fiber‐shaped Ni–NiO//Zn battery, outperforming most reported fiber‐shaped energy‐storage devices. Such a novel concept of a fiber‐shaped Ni–Zn battery with impressive stability will greatly enrich the flexible energy‐storage technologies for future portable/wearable electronic applications.     

Ultrastretchable and Wireless Bioelectronics Based on All‐Hydrogel Microfluidics

Hydrogel bioelectronics that can interface biological tissues and flexible electronics is at the core of the growing field of healthcare monitoring, smart drug systems, and wearable and implantable devices. Here, a simple strategy is demonstrated to prototype all‐hydrogel bioelectronics with embedded arbitrary conductive networks using tough hydrogels and liquid metal. Due to their excellent stretchability, the resultant all‐hydrogel bioelectronics exhibits stable electrochemical properties at large tensile stretch and various modes of deformation. The potential of fabricated all‐hydrogel bioelectronics is demonstrated as wearable strain sensors, cardiac patches, and near‐field communication (NFC) devices for monitoring various physiological conditions wirelessly. The presented simple platform paves the way of implantable hydrogel electronics for Internet‐of‐Things and tissue–machine interfacing applications.     

Degradable piezoelectric biomaterials for wearable and implantable bioelectronics

Current bioelectronics are facing a paradigm shift from old-fashioned unrecyclable materials to green and degradable functional materials with desired biocompatibility. As an essential electromechanical coupling component in many bioelectronics, new piezoelectric materials are being developed with biodegradability, as well as desired mechanical and electromechanical properties for the next generation implantable and wearable bioelectronics. In this review, we provide an overview of the major advancements in biodegradable piezoelectric materials. Different natural (such as peptide, amino acids, proteins, cellulose, chitin, silk, collagen, and M13 phage) and synthetic piezoelectric materials (such as polylactic acid) are discussed to reveal the underlying electromechanical coupling mechanism at the molecular level, together with typical approaches to the alignment of orientation and polarization to boost their electromechanical performance. Meanwhile, in vivo and in vitro degradation manners of those piezoelectric materials are summarized and compared. Representative developments of typical electronic prototypes leveraging these materials are also discussed. At last, challenges toward practical applications are pointed out together with potential research opportunities that might be critical in this new materials research area.     

Zn Microbatteries Explore Ways for Integrations in Intelligent Systems

As intelligent microsystems develop, many revolutionary applications, such as the swallowing surgeon proposed by Richard Feynman, are about to evolve. Nonetheless, integrable energy storage satisfying the demand for autonomous operations has emerged as a major obstacle to the deployment of intelligent microsystems. A reason for the lagging development of integrable batteries is the challenge of miniaturization through microfabrication procedures. Lithium batteries, generated by the most successful battery chemistry, are not stable in the air, thus creating major manufacturing challenges. Other cations (Na⁺, Mg²⁺, Al³⁺, K⁺) are still in the early stages of development. In contrast, the superior stability of zinc batteries in the air brings high compatibility to microfabrication protocols and has already demonstrated excellent practicability in full-sized devices. To obtain energy-dense and high-power zinc microbatteries within square-millimeter or smaller footprints, sandwich, pillar, and Swiss-roll configurations are developed. Thin interdigital and fiber microbatteries find their applications being integrated into wearable devices and electronic skin. It is foreseeable that zinc microbatteries will find their way into highly integrated microsystems unlocking their full potential for autonomous operation. This review summarizes the material development, configuration innovation, and application-oriented integration of zinc microbatteries.     

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.     

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.     

Toward Wearable Self‐Charging Power Systems: The Integration of Energy‐Harvesting and Storage Devices

One major challenge for wearable electronics is that the state‐of‐the‐art batteries are inadequate to provide sufficient energy for long‐term operations, leading to inconvenient battery replacement or frequent recharging. Other than the pursuit of high energy density of secondary batteries, an alternative approach recently drawing intensive attention from the research community, is to integrate energy‐generation and energy‐storage devices into self‐charging power systems (SCPSs), so that the scavenged energy can be simultaneously stored for sustainable power supply. This paper reviews recent developments in SCPSs with the integration of various energy‐harvesting devices (including piezoelectric nanogenerators, triboelectric nanogenerators, solar cells, and thermoelectric nanogenerators) and energy‐storage devices, such as batteries and supercapacitors. SCPSs with multiple energy‐harvesting devices are also included. Emphasis is placed on integrated flexible or wearable SCPSs. Remaining challenges and perspectives are also examined to suggest how to bring the appealing SCPSs into practical applications in the near future.     

Crossroads in the renaissance of rechargeable aqueous zinc batteries

Aqueous zinc batteries dominate the primary battery market with alkaline chemistries and recently have been rejuvenated as rechargeable devices to compete for grid-scale energy storage applications. Tremendous effort has been made in the past few years and improved cyclability has been demonstrated in both alkaline, neutral, and mild acidic systems. In this review/perspective, we will elucidate the merits of rechargeable aqueous zinc batteries through side-by-side comparison to Li-ion batteries, examine the challenges and progress made in the pursuit of highly rechargeable alkaline and mild acidic batteries, and finally provide a holistic forward look at the technology. The focus is placed on static closed cell designs, while flow batteries and open systems like zinc-air batteries will not be included due to space constraint.     

Commencing an Acidic Battery Based on a Copper Anode with Ultrafast Proton‐Regulated Kinetics and Superior Dendrite‐Free Property

Building aqueous acidic batteries is in its infancy. There are several sporadic attempts that show desirable electrochemical performance, especially rate stability and high power density. The direct use of a metal anode is regarded as the best protocol for fabricating metal‐based batteries. However, introducing an acid‐tolerant and electrochemically reversible metal anode into an acidic aqueous battery system remains a considerable challenge. In this work, copper (Cu) metal is used as a reversible metal anode to match acidic regimes with a nearly 100% deposition–dissolution efficiency. The reaction kinetics and mechanism of the Cu anode can be regulated by protons with 400% kinetic acceleration compared with a mild electrolyte. In addition, the anode exhibits a dendrite‐free morphology after cycling due to the surface roughening effect, which is different from the morphologies of widely used Zn‐ and Li‐metal anodes. When coupled with the Prussian blue analog as cathodes, the battery delivers ultrafast kinetics of 1830 W kg^?1 at 75 C, which is comparable to the power performance of supercapacitors. Long‐term cyclic stability is evaluated, where the capacity retention is 85.6% after 5000 cycles. Finally, flexible fiber‐shaped acidic Cu‐based batteries are demonstrated for potential wearable applications.     

High‐Performance Integrated Self‐Package Flexible Li–O2 Battery Based on Stable Composite Anode and Flexible Gas Diffusion Layer

With the rising development of flexible and wearable electronics, corresponding flexible energy storage devices with high energy density are required to provide a sustainable energy supply. Theoretically, rechargeable flexible Li–O₂ batteries can provide high specific energy density; however, there are only a few reports on the construction of flexible Li–O₂ batteries. Conventional flexible Li–O₂ batteries possess a loose battery structure, which prevents flexibility and stability. The low mechanical strength of the gas diffusion layer and anode also lead to a flexible Li–O₂ battery with poor mechanical properties. All these attributes limit their practical applications. Herein, the authors develop an integrated flexible Li–O₂ battery based on a high‐fatigue‐resistance anode and a novel flexible stretchable gas diffusion layer. Owing to the synergistic effect of the stable electrocatalytic activity and hierarchical 3D interconnected network structure of the free‐standing cathode, the obtained flexible Li–O₂ batteries exhibit superior electrochemical performance, including a high specific capacity, an excellent rate capability, and exceptional cycle stability. Furthermore, benefitting from the above advantages, the as‐fabricated flexible batteries can realize excellent mechanical and electrochemical stability. Even after a thousand cycles of the bending process, the flexible Li–O₂ battery can still possess a stable open‐circuit voltage, a high specific capacity, and a durable cycle performance.     

Atomically Thin Mesoporous Co3O4 Layers Strongly Coupled with N‐rGO Nanosheets as High‐Performance Bifunctional Catalysts for 1D Knittable Zinc–Air Batteries

Under development for next‐generation wearable electronics are flexible, knittable, and wearable energy‐storage devices with high energy density that can be integrated into textiles. Herein, knittable fiber‐shaped zinc–air batteries with high volumetric energy density (36.1 mWh cm^?3) are fabricated via a facile and continuous method with low‐cost materials. Furthermore, a high‐yield method is developed to prepare the key component of the fiber‐shaped zinc–air battery, i.e., a bifunctional catalyst composed of atomically thin layer‐by‐layer mesoporous Co₃O₄/nitrogen‐doped reduced graphene oxide (N‐rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co₃O₄ and N‐rGO nanosheets, the bifunctional catalyst exhibits high activity and superior durability for oxygen reduction and evolution reactions. Compared to a fiber‐shaped zinc–air battery using state‐of‐the‐art Pt/C + RuO₂ catalysts, the battery based on these Co₃O₄/N‐rGO nanosheets demonstrates enhanced and stable electrochemical performance, even under severe deformation. Such batteries, for the first time, can be successfully knitted into clothes without short circuits under external forces and can power various electronic devices and even charge a cellphone.     

Lithium-Ion Battery Screening by K-Means with DBSCAN for Denoising

Batteries are often packed together to meet voltage and capability needs. However, due to variations in raw materials, different ages of equipment, and manual operation, there is inconsistency between batteries, which leads to reduced available capacity, variability of resistance, and premature failure. Therefore, it is crucial to pack similar batteries together. The conventional approach to screening batteries is based on their capacity, voltage and internal resistance, which disregards how batteries perform during manufacturing. In the battery discharge process, real time discharge voltage curves (DVCs) are collected as a set of unlabeled time series, which reflect how the battery voltage changes. However, few studies have focused on DVC based battery screening. In this paper, we provide an effective approach for battery screening. First, we apply interpolation on DVCs and give a method to transform them into slope sequences. Then, we use density-based spatial clustering of applications with noise (DBSCAN) for denoising and treat the remaining data as input to the K-means algorithm for screening. Finally, we provide the experimental results and give our evaluation. It is proved that our method is effective.     

A Quasi‐Solid‐State Flexible Fiber‐Shaped Li–CO2 Battery with Low Overpotential and High Energy Efficiency

The rapid development of wearable electronics requires a revolution of power accessories regarding flexibility and energy density. The Li–CO₂ battery was recently proposed as a novel and promising candidate for next‐generation energy‐storage systems. However, the current Li–CO₂ batteries usually suffer from the difficulties of poor stability, low energy efficiency, and leakage of liquid electrolyte, and few flexible Li–CO₂ batteries for wearable electronics have been reported so far. Herein, a quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery with low overpotential and high energy efficiency, by employing ultrafine Mo₂C nanoparticles anchored on a carbon nanotube (CNT) cloth freestanding hybrid film as the cathode, is demonstrated. Due to the synergistic effects of the CNT substrate and Mo₂C catalyst, it achieves a low charge potential below 3.4 V, a high energy efficiency of ≈80%, and can be reversibly discharged and charged for 40 cycles. Experimental results and theoretical simulation show that the intermediate discharge product Li₂C₂O₄ stabilized by Mo₂C via coordinative electrons transfer should be responsible for the reduction of overpotential. The as‐fabricated quasi‐solid‐state flexible fiber‐shaped Li–CO₂ battery can also keep working normally even under various deformation conditions, giving it great potential of becoming an advanced energy accessory for wearable electronics.     

Bamboo‐Like Nitrogen‐Doped Carbon Nanotube Forests as Durable Metal‐Free Catalysts for Self‐Powered Flexible Li–CO2 Batteries

The Li–CO₂ battery is a promising energy storage device for wearable electronics due to its long discharge plateau, high energy density, and environmental friendliness. However, its utilization is largely hindered by poor cyclability and mechanical rigidity due to the lack of a flexible and durable catalyst electrode. Herein, flexible fiber‐shaped Li–CO₂ batteries with ultralong cycle‐life, high rate capability, and large specific capacity are fabricated, employing bamboo‐like N‐doped carbon nanotube fiber (B‐NCNT) as flexible, durable metal‐free catalysts for both CO₂ reduction and evolution reactions. Benefiting from high N‐doping with abundant pyridinic groups, rich defects, and active sites of the periodic bamboo‐like nodes, the fabricated Li–CO₂ battery shows outstanding electrochemical performance with high full‐discharge capacity of 23 328 mAh g^?1, high rate capability with a low potential gap up to 1.96 V at a current density of 1000 mA g^?1, stability over 360 cycles, and good flexibility. Meanwhile, the bifunctional B‐NCNT is used as the counter electrode for a fiber‐shaped dye‐sensitized solar cell to fabricate a self‐powered fiber‐shaped Li–CO₂ battery with overall photochemical–electric energy conversion efficiency of up to 4.6%. Along with a stable voltage output, this design demonstrates great adaptability and application potentiality in wearable electronics with a breath monitor as an example.     

Advanced Multimaterial Electronic and Optoelectronic Fibers and Textiles

The ability to integrate complex electronic and optoelectronic functionalities within soft and thin fibers is one of today's key advanced manufacturing challenges. Multifunctional and connected fiber devices will be at the heart of the development of smart textiles and wearable devices. These devices also offer novel opportunities for surgical probes and tools, robotics and prostheses, communication systems, and portable energy harvesters. Among the various fiber‐processing methods, the preform‐to‐fiber thermal drawing technique is a very promising process that is used to fabricate multimaterial fibers with complex architectures at micro‐ and nanoscale feature sizes. Recently, a series of scientific and technological breakthroughs have significantly advanced the field of multimaterial fibers, allowing a wider range of functionalities, better performance, and novel applications. Here, these breakthroughs, in the fundamental understanding of the fluid dynamics, rheology, and tailoring of materials microstructures at play in the thermal drawing process, are presented and critically discussed. The impact of these advances on the research landscape in this field and how they offer significant new opportunities for this rapidly growing scientific and technological platform are also discussed.     

碳纤维复合材料结构锂离子电池研究综述

碳纤维复合材料结构锂离子电池是将结构件和储能系统相结合，在保持碳纤维力学性能的同时，赋予其优异的储能性能，使动力电池组在减重的同时简化结构设计，提高能量效率和结构效率。在低碳经济的大环境下，碳纤维复合材料结构锂离子电池作为一种新型储能器件引起了国内外学者的极大关注。本文综述了嵌入集成式结构电池和多功能复合材料结构电池的工作原理、制备工艺及储能性能等基础问题的研究现状，提出了全碳纤维固态结构电池的概念及其设计原型。同时简要介绍了现阶段碳纤维复合材料结构锂离子电池最具代表性的应用，并展望了其在航空航天和交通运输等领域的应用价值。     

Constructing Carbon Nanotube Hybrid Fiber Electrodes with Unique Hierarchical Microcrack Structure for High-Voltage,Ultrahigh-Rate,and Ultralong-Life Flexible Aqueous Zinc Batteries

Flexible aqueous zinc batteries are promising candidates as safe power sources for fast-growing portable and wearable electronics. However, the low working voltage, poor rate capability, and cycling stability have greatly restricted their development and applications. Here, a new family of flexible bimetallic phosphide/carbon nanotube hybrid fiber electrodes with unique macroscopic microcrack structure and microscopic porous nanoflower structure is reported. The hierarchical microcrack structure not only facilitates the penetration of electrolyte for effective exposure of active sites, but also can serve as buffers to relieve the stress concentrations of the fiber electrode under deformations, enabling impressive electrochemical performance and mechanical flexibility. Particularly, the fabricated flexible aqueous zinc batteries demonstrate high working voltage plateau and specific capacity (≈1.7 V, 258.9 mAh g⁻¹ at 2 A g⁻¹), ultrahigh rate capability (135.8 mAh g⁻¹ at 50 A g⁻¹, fully charged in only 9.8 s) and impressive power density of 79 000 W kg⁻¹. Moreover, the flexible batteries show ultralong cycling life with 74.6% capacity retention after 20 000 cycles. The fiber batteries are also highly flexible and can be easily knitted into soft electronic textiles to power a smartphone, which are particularly promising for the next-generation of flexible and wearable electronics.     

Toward stretchable batteries: 3D-printed deformable electrodes and separator enabled by nanocellulose

The needs for stretchable batteries surge as wearable and epidermal electronics emerge. The development of stretchable batteries, however, remains a grand challenge, as the battery components are intrinsically brittle and fracture easily under mechanical loading. Existing efforts to increase the stretchability of battery components often involve complex fabrication processes and thus are not viable for scalable and cost-effective manufacturing. To address this challenge, herein a facile yet effective strategy is developed to fabricate stretchable electrodes and separator for Li-ion batteries using extrusion-based 3D printing of active materials mixed with nanofibrillated cellulose. The resulting electrodes and separator can achieve reversible stretchability of 50%. After 50 stretching cycles, the resistance of the electrodes under 50% stretch only increases by 3%. The origin of the exceptional mechanical and electrical performances of the 3D-printed battery components is twofold: (i) excellent deformability enabled by the 3D-printed serpentine structure at the component level; (ii) the robust nanoscale structure due to the high aspect ratios of nanofibrillated cellulose and carbon nanotubes and the strong interactions between nanofibrillated cellulose and carbon nanotubes or among the individual cellulose fibers at the material structure level. The facile 3D printing of the patterned electrodes/separator leads to low-cost manufacturing of high-performance stretchable Li-ion batteries, demonstrating its promising potential to enable stretchable energy storage devices for wearable and epidermal electronics.     

Smart Textile-Integrated Microelectronic Systems for Wearable Applications

The programmable nature of smart textiles makes them an indispensable part of an emerging new technology field. Smart textile-integrated microelectronic systems (STIMES), which combine microelectronics and technology such as artificial intelligence and augmented or virtual reality, have been intensively explored. A vast range of research activities have been reported. Many promising applications in healthcare, the internet of things (IoT), smart city management, robotics, etc., have been demonstrated around the world. A timely overview and comprehensive review of progress of this field in the last five years are provided. Several main aspects are covered: functional materials, major fabrication processes of smart textile components, functional devices, system architectures and heterogeneous integration, wearable applications in human and nonhuman-related areas, and the safety and security of STIMES. The major types of textile-integrated nonconventional functional devices are discussed in detail: sensors, actuators, displays, antennas, energy harvesters and their hybrids, batteries and supercapacitors, circuit boards, and memory devices.