Fabric-Type Flexible Energy-Storage Devices for Wearable Electronics

Abstract

With the rapid advancements in flexible wearable electronics, there is increasing interest in integrated electronic fabric innovations in both academia and industry. However, currently developed plastic board-based batteries remain too rigid and bulky to comfortably accommodate soft wearing surfaces. The integration of fabrics with energy-storage devices offers a sustainable, eco-friendly, and pervasive energy solution for wearable distributed electronics. Fabric-type flexible energy-storage devices are particularly advantageous as they conform well to the curved body surface and the various movements associated with wearing habits such as running. This review presents a comprehensive overview of the advances in flexible fabric-type energy-storage devices for wearable electronics, including their significance, construction methods, structure design, hybrid forms with other energy sources, and the existing challenges and future directions. With worldwide efforts on materials and technologies, we hope that progress in this review will revolutionize our way of life.

1. Introduction

As human civilization progresses, the desire for communication, self-awareness, and cooperation continues to grow. Wearable electronics have emerged as a means of meeting these needs, offering more convenient communication than ever before. As a result, the development of wearable electronics has accelerated in recent years. According to Verified Market Research, the global wearable electronic devices market was valued at USD 855.46 million in 2021, and it is expected to reach USD 5561.17 million by 2030, growing at a CAGR of 23.12% from 2023 to 2030. Today, stylish, lightweight, and flexible wearable electronics have become increasingly common in our daily lives [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. Given their durability, high demand is placed on the devices, including tensile, bending, and wear resistance. However, the traditional rigid plate-structured power supply system has impeded the light weight, miniaturization, and integration of these devices. Additionally, frequent charging remains a problem that must be addressed [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39].

Clothing has been a fundamental aspect of human society since the earliest days of human existence 34,000 years ago, if not earlier, when fabric-like materials were first discovered. Traditionally, fabrics were primarily used for skin protection, regulating body temperature, adornment, and aesthetics. In recent years, the integration of electronics and fabrics has gained popularity, offering a platform for electronic functionality while maintaining the characteristics of fabrics [40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61]. Among all the types of wearable electronic fabrics, fabric-type flexible energy-storage devices are of particular interest due to the increasing power supply requirements of portable electronics. This has stimulated the development of flexible energy-storage devices with higher energy density. Since 2013, energy-storage devices with fiber and woven fabric structures have been rapidly developed and successfully applied to supercapacitors and batteries [62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81].

Energy-storage devices with different constructions require a fabrication process that is similar to that of electronic devices. Functional materials with specific nanostructures are assembled layer-by-layer on fibers and fabrics, which are then used as electrodes for smart devices in wearable and portable electronic products [82,83,84,85,86,87]. Moreover, weaving structures provide fiber and fabric device units with more structural freedom, enabling them to be woven into self-designed shapes that can adapt to any surface. This creates fabric-type flexible energy-storage devices that pave the way for a new era in energy storage [88,89,90,91,92,93,94,95,96,97,98,99,100,101,102].

In addition, emerging wearable smart fabrics must have integrated electronic systems to achieve a sustainable power supply [103,104]. Hybrid energy harvesting and storage is a productive approach for providing uninterrupted power supply by harvesting various energies in the environment as energy replenishment. For example, hybrid energy fabrics can be created using solar cells and batteries, triboelectric nanogenerators (TENGs) and supercapacitors (SCs), solar cells, and SCs, among others [105,106,107,108,109,110,111].

This review provides an overview of the advancements made in fabric-type flexible energy-storage devices for wearable electronics. It mainly covers the preparation method of fiber/fabric devices, fiber-structured energy-storage devices, energy-storage fabrics, and harvesting–storage integrated fabrics.

2. From Fiber Materials to Fiber Devices

2.1. From Fiber Materials to Fiber Electrode

Fiber is a slender, soft material with a length/width ratio greater than 10³ and a thickness ranging from several to hundreds of microns. It is commonly known as fabric fiber because it is primarily used in fabric production. Based on their raw materials, fabric fibers can be categorized into natural and chemical fibers. Recently, the need for functional fibers exhibiting distinctive traits such as flame retardancy, hydrophilicity, and antistaticity has grown substantially. These fibers serve as the foundation for developing intelligent fibers and fabrics, which are often created by modifying traditional fibers through surface modification or layer-by-layer function coating. Functional fiber materials can be produced in two ways: by incorporating additives into the raw materials during the extension process or by modifying the surface of classical fibers. These techniques have been widely adopted and have become a part of our daily lives.

As cutting-edge advances, functional fibers are further being coated with conductors and semiconductors to serve as electrodes for electronic devices. This expansion of the concept of fiber has garnered significant attention in the field of wearable electronics due to their low cost, light weight, weave-ability, and flexibility. After the fiber electrodes are prepared, fiber-type devices can be further prepared by twisting the fiber electrodes, and fabric-type devices can be obtained by directly weaving the fiber electrodes. In addition to directly weaving the fiber electrodes, with fibers as the basic units of fabrics, a series of assembly methods have been developed to prepare fiber-based devices.

2.2. From Fiber Electrode to Fiber-Type Device

Fiber electrodes are the basis of smart fiber-type devices. Before preparing smart fiber-type devices, one or multiple ordered layers of nano/micro-functional structures are usually assembled on fibers to make fiber electrodes. The fiber electrodes are then twisted to form smart fiber-type devices.

Based on fiber electrodes, various fiber-type devices, such as solar cells, batteries, and supercapacitors, have been developed, which have largely expanded the range of fiber-type electronics. For example, Fan and colleagues devised flexible fiber-type dye-sensitized solar cells (DSSCs) based on a twisted structure, which is similar to the DNA double-helix structure [112]. Its open circuit voltage (Voc), short circuit current density (Isc), and fill factor (FF) are 0.61 V, 1.2 mA cm⁻², and 0.38, respectively. Subsequently, research reported fiber-type solar devices made of manganese-based and other composite fiber electrode materials [113,114,115]. The encapsulated fiber-type devices achieved a power conversion efficiency of up to 7.02%. Zhang et al. reported a stretchable fiber-type lithium-ion battery. For this, multi-walled carbon-nanotube (MWCNT) fiber-based cathodes and anodes were coated with LiMn₂O₄ and Li₄Ti₅O₁₂, respectively. The two fiber electrodes were then twisted onto a polydimethylsiloxane fiber at a specific helical angle. The space between the two twisted fiber electrodes served as the separator to prevent short circuits [116].

2.3. Directly Weaving Fiber Electrodes into Fabric-Type Devices

In addition to twisting fiber electrodes to assemble fiber-type devices, fiber electrodes can likewise be straightaway woven into fabric-type devices. For this approach, distinct fiber electrodes were initially constructed, mounted on a loom as either weft or warp threads, and subsequently interlaced through a shuttle flight progression to create fabric-type devices.

In 2016, Zhang et al. [117] for the first time demonstrated an all-solid photovoltaic cloth that can power portable wearable electronics. The lightweight and inexpensive Mn-plated polymer wire with ZnO nano-arrays served as the substrate for fabricating the fiber photoanode. A lamina of CuI functioned as the hole-transfer material, and Cu-coated polymer fibers were employed to assemble the counter electrodes. Through a shuttle flight progression, fiber photoanodes and counter electrodes were interlaced in a staggered fashion on an industrial knitting contraption to create the photovoltaic fabric. The resultant fabric exhibited an efficiency of 1.3% when subjected to a benchmark luminosity intensity of 100 mW/cm² (Figure 1a,b). This effort validated the convenient mass production of fabric DSSCs, exploiting conventional weaving technologies, and the consequent all-solid fabric-type DSSCs displayed excellent abrasion resilience, rendering them a promising prospect for wearable energy in the future.

Subsequently, Liu et al. [118] described an organic photovoltaic (OPV) fabric by interlacing P3HT:PCBM-based cathode fibers and Ag-coated nylon-based anode fibers in a loom. This flexible fabric-type OPV material attained a Voc of 0.48 V, Jsc of 7.39 mA/cm², and a PCE of up to 1.62%, which was capable of powering an electronic watch (Figure 1c). Furthermore, it is well-suited for diverse nascent applications such as wearable electronics, biomedical electronics, and artificial intelligence. Gao et al. [119] likewise constructed a flexible fabric-type DSSC on a vast scale. By harnessing analogous interweaving technologies, TiO₂-based photoanode and Ag-coated nylon filaments were crossed together (Figure 1d). The solid fabric-type DSSCs achieved a Voc of 0.4 V, Jsc of 9.42 mA/cm², and an enhanced PCE of up to 1.92%.

2.4. Other Strategy of Assembling Fiber-Based Devices

As the most suitable device type for wearable electronics, fabric-type devices have recently developed several other assembly methods, such as direct coating based on the fabric [120,121,122,123,124] and layer stacking of fabric electrodes [125,126,127,128]. Furthermore, fiber-type devices, as the core basic units, can also be woven into fabric-type devices through assembly strategies such as weaving of fiber devices [111].

Direct coating based on the fabric typically encompasses three stages. Initially, semiconductor materials are deposited on a conductive fabric-type substrate. Secondly, a conductor layer is added on top of the semiconductor layer as the counter electrode. Finally, after the encapsulation process, an intelligent fabric can be manufactured.

For example, Wu et al. [121] demonstrated a fabric-type solar device by layering P3HT:PCBM and electrodes on a fiber-type conductive fabric (Figure 2a). Later, Liu et al. [129] introduced a novel fabrication process for a monolithic-structured solid-state dye-sensitized solar cell (DSSC) on fabric using whole solution-based methods (Figure 2b).

The procedure involved five main steps: Initially, the silver electrode was coated on the fabric. Secondly, the TiO₂ compact lamina was employed by a predesigned shadow mask. Thirdly, the mesoporous TiO₂ stratum was coated on the TiO₂ compact lamina and sintered to form photoanodes following the dye staining progression. Fourthly, electrolyte solution was deposited on it. After 16 h, the electrolyte could fully permeate the TiO₂ mesoporous film. Finally, the fabric was coated with PEDOT:PSS and AgNW as additional electrodes. The fabricated fabric-type device displayed a maximum PCE of 0.4%. Furthermore, Cho et al. [124] described a high-performance fabric-type polymer solar cell by implementing plasmonic nanostructures on a commercially woven fabric, achieving a PCE of approximately 8.71% (Figure 2c). The proposed wearable polymer solar cells were found to be stable after bending for 100 cycles (Figure 2d).

Layer stacking of fabric electrodes involves depositing conductor and semiconductor materials directly onto a fabric substrate. Two or more fabric-type electrodes can be stacked on top of one another to resemble a sandwich. Dip deposition, spray deposition, and in situ chemical synthesis are all practicable coating techniques. A double-layered fabric can be constructed by stacking the working and counter fabric electrodes, which can be readily incorporated into diverse fabrics or other architectures.

In 2014, Pan et al. [126] reported a novel technique for creating flexible and wearable DSSC fabrics through stacking two fabric electrodes (Figure 3a). They initially wove Ti wires into fabrics for working electrodes. Following electrochemical anodizing, aligned TiO₂ nanotubes were produced perpendicularly on the Ti wire surface. After absorbing dye N719, the working electrode based on Ti wire fabric was obtained. Next, the counter electrodes were assembled by weaving CNT fiber. After consolidating the electrolyte, the DSSC fabric was created by stacking the CNT fabric and Ti wire fabric. Subsequently, Zhang et al. [127] coated a stratum of TiO₂ on Ti wire fabric through anodization. After coating a lamina of P3HT:PCBM and PEDOT:PSS on the fabric, they finally covered a pliant carbon nanotube on the fabric to generate a “sandwich” architecture (Figure 3b). The fabric-type OPV fabric has nearly the same PCE on both sides, and after 200 instances of bending, the PCE loss did not surpass 3% (Figure 3c). To enhance the PCE, Zhen et al. [128] likewise demonstrated a flexible OPV material by placing a PEDOT:PSS/P3HT:PCBM electrode on an Ag-coated fabric. This OPV fabric achieves nearly the same PCE as that of conventional counterparts.

Although layer stacking is a convenient fabrication method for assembling intelligent fabrics, it has drawbacks. When functional layers are stacked with spacers in between, the resulting fabric becomes thicker, which reduces the wearing comfort as well as the fabric’s breathability, pliability, and sensitivity to deformation caused by movement. However, these issues can be circumvented by constructing intelligent fabrics based on the weaving of fiber devices.

The procedure of weaving the fiber devices encompasses three stages. Firstly, the fiber electrodes are created to assemble the fiber-type devices. Next, the fiber-type devices are fed into the weaving contraption as either the weft or warp threads. Finally, the fiber-type devices are interlaced with cotton or other fibers to assemble the fabrics.

Zhang et al. [111] reported a high-performance and sustainable photo-rechargeable fabric in 2020, which serves as a reliable power source for wearable electronics. This fabric integrates a photovoltaic fabric and a rechargeable battery fabric, enabling it to self-charge. The energy-storage devices were woven using conductive multi-walled carbon nanotube/polyethylene terephthalate (MWCNT/PET) wires, which were assembled with MnO₂ and Zn electrodes. These wires were created by enclosing a PET wire strand into the MWCNTs. The Zn electrodes were electro-deposited, while the MnO₂ electrodes were brush-coated with MnO₂ nanofibers. To prevent any contact between the electrolyte and human skin, a pair of MnO₂ and Zn electrodes were aligned in parallel and embedded within a gel electrolyte. They were then encapsulated in a lamina of polymethyl methacrylate (PMMA), and the battery was wrapped with thin colored cotton wires, instead of the usual plastic pipe, to give it a cotton-like softness while protecting the battery center. Finally, a scalable shuttle-flying method was used to directly weave the energy-storage fabric (Figure 4).

As a supplement to the four device assembly techniques mentioned, other techniques such as embroidery have likewise been reported for assembling fiber electrodes into devices to create lightweight and breathable intelligent fabrics that can adapt to various body movements. For example, Yu et al. [130] demonstrated a foldable photovoltaic fabric with colorful patterns, which was assembled from abbreviated photoanode filaments via a scalable couching embroidery method (Figure 5). Due to its dependable electric nexus, this fabric can be comfortably worn and can provide any desired output for wearable electronics. Furthermore, embroidery technologies can likewise accommodate other functional fiber devices, giving them nearly unlimited applications.

In contradistinction to the device assembling method of electrode stacking or interweaving, the embroidery method facilitates convenient and dependable stitching of both lengthy and abbreviated fiber electrodes together with the requisite output. This technology is more suitable for wearable electronics. However, a key issue for flexible electronics is sustainable energy supply, which requires highly capacitive and wearable energy-storage devices as power sources. Therefore, it is essential to develop flexible fiber-type or fabric-type energy-storage devices.

3. Fiber-Type Energy-Storage Device

The need for high-performance energy-storage devices has become increasingly urgent in modern society, as ecological problems continue to emerge. In response, the electronics and electric vehicle markets have stimulated the progression of wearable energy-storage devices. To fulfill the necessities of wearable technology, energy-storage devices with diverse architectures have been developed, including two-dimensional (2D) planes, one-dimensional (1D) fibers, and interwoven fabrics. Fiber-type and fabric-type energy-storage devices are particularly suitable for people’s wearing habits.

Among these various structures, 1D energy-storage fiber has received considerable attention owing to its excellent structural flexibility for wearable applications and can be readily interwoven into any desired contour or knitted into fabrics, rendering it highly versatile. Thus, the evolution of fiber-type energy-storage devices is of substantial interest and is anticipated to unfurl novel avenues in the domain of energy storage.

There are two primary categories of fiber-type energy-storage devices: fiber-type supercapacitors and batteries. The power density of SCs is greater than that of conventional capacitors, while batteries possess a substantially superior energy density, although with an inferior power density to SCs.

3.1. Fiber-Type Supercapacitors

Fiber-type SCs typically consist of two electrodes connected by an electrolyte and then separated by a separator, as shown in Figure 6 [82]. Supercapacitor fibers can be bent repeatedly without significantly affecting their ability to store energy, making them reasonably flexible. Furthermore, they can be woven or knitted into wearable fabrics, making them ideal for compact or curved spaces due to their fibrous structure. Supercapacitor fibers also have two electrodes that sandwich the electrolyte and separator. The capacitive materials used for charge storage and transportation have a significant impact on the device’s performance. Carbon-based materials, transition metal oxides, and conducting polymers are some examples of capacitive materials. However, capacitive materials are more challenging to assemble for supercapacitor fibers, as typical supercapacitors can easily break under deformation.

In accordance with the category of capacitor materials, fiber-type supercapacitors usually apply carbon-based capacitive materials and compounded capacitive materials.

Carbon-based supercapacitor fibers (or carbon-based SCs) are those in which both electrodes are coated with carbon materials, such as carbon nanotubes or graphene. Electric double-layer capacitors (EDLCs) are a common type of carbon-based SC. Carbon materials possess manifold advantages as capacitive materials, encompassing a vast specific surface area, optimal conductivity, elevated rate capacity, and favorable cycle stability. Among fiber-type SCs, carbon-carbon composite fibers are the most popular due to their one-dimensional, flexible, and conductive properties. For instance, Yang et al. [65] developed a series of fiber-type supercapacitors with high performance (Figure 7a–c). The supercapacitor electrodes were made of aligned carbon nanotube sheets, providing remarkable properties such as high flexibility, tensile strength, electrical conductivity, and mechanical and thermal stability. Even after being stretched for 100 cycles, Functional supercapacitor fibers can be designed to possess additional properties. For instance, Meng et al. [66] developed a fiber-type supercapacitor that exhibits remarkable stability even under torsional deformation (Figure 7d–f). The torsional supercapacitors were constructed using CNT films as an electrode. The consequent supercapacitor can endure torsional degrees of up to 20,000 rad/m and uphold relatively invariable electrical conductivity.

Hybrid carbon-based supercapacitors employ two or more carbon materials to achieve high performance. These materials are gaining increasing attention as they combine the strengths of each individual component. Among hybrid carbon-based supercapacitors, CNTs are the most commonly used material. The most widely investigated hybrid carbon-based supercapacitor fibers include CNT/graphene and CNT/carbon microfiber supercapacitor fibers. Sun et al. [69] synthesized graphene/carbon composite fibers (Figure 8a–c). The composite fibers achieved high electrical conductivity, leading to specific capacitances of up to 31.50 F/g (4.97 mF/cm² or 27.1 F/cm). In another study, Le et al. [70] developed a coaxial fiber-type supercapacitor composed of carbon bundles coated with MWCNTs as the core electrodes. Carbon nanofibers were used as the external electrodes (Figure 8d–f). The cyclic voltammetry characteristic changes of the supercapacitor fiber were negligible even when it was bent 180 degrees.

For supercapacitors with composite capacitive materials, the duo electrodes are customarily coated with two categories of composite materials, encompassing carbon/conducting polymers and carbon/metal oxides. Wang et al. [71] reported a supercapacitor composed of CNT and PANI nanowires (Figure 9a). In the supercapacitor, the CNT nanowires were utilized as an active material and current collector. PANI was employed as electrode due to its elevated energy-storage capacity. The supercapacitors have excellent electrochemical capacity. When interwoven into wearable electronic devices, they can be stretched without a substantial loss of capacitance. Shang et al. [72] reported a fiber-type supercapacitor based on helical CNT wires (Figure 9b). For a fiber-type supercapacitor with CNT/PEDOT:PSS composite, a high-performance supercapacitor has been fabricated from CNT wire using an undemanding process [73] (Figure 9c). The coating of PEDOT:PSS further ameliorates the capacity of the supercapacitor. Lu et al. [75] reported a technique of fabricating an asymmetric fiber-type supercapacitor by electrodeposition, in which carbon fiber bundle@CNT-NiCo(OH)x (CF-@CNC) was the positive electrode, and activated carbon (AC) coated CFs (CF-@AC) were the negative electrode (Figure 9d). To surmount the inherent disadvantages of CFs, such as hydrophobicity, suboptimal electrical conductivity, and low surface area, CNT modification of CF@CNC and air plasma treatment were adopted. This technique not only enhances ion and electron transport but also prevents detachment of NiCo(OH)x deposition.

3.2. Fiber-Type Batteries

A fiber-type battery is a device that uses metal electrodes and an electrolyte solution to convert chemical energy into electrical energy. This type of battery can be classified into primary and secondary batteries. Primary batteries cannot be recharged, whereas secondary batteries, such as lithium-ion batteries, lithium–sulfur batteries, and metal–air batteries, can be recharged.

In a fiber-type primary battery, such as an alkaline zinc–manganese battery, the negative electrode mainly consists of zinc, while the positive electrode is typically composed of manganese dioxide (MnO₂) and graphite powder, and the electrolyte is a potassium hydroxide solution (Figure 10a). Yu et al. have demonstrated a fiber-type zinc–manganese battery that uses lightweight and low-cost commercial carbon fibers as collectors, coated with anode or cathode materials. The anode and cathode are enclosed in a transparent flexible polymer plastic tube filled with electrolyte, resulting in an open circuit voltage of 1.5 V, which is close to the theoretical voltage. At a discharge density of 70 mA/g or 1.3 mA/cm², the discharge capacity is 158 mAh/g or 1.50 mA/cm² (calculated by the mass of positive MnO₂). The fiber battery also exhibits good flexibility, with no loss of capacity when bent to a radius between 3.0 cm and 0.7 cm. Moreover, increasing the length of the battery fiber from 2.0 cm to 8.0 cm has no significant effect on the discharge capacity, indicating that fiber-type batteries can be easily assembled without compromising their performance (Figure 10b–d) [34].

For fiber-type secondary batteries, the most commonly used type are lithium-ion batteries (LIBs), which were first introduced by Sony in 1991. Over the past two decades, researchers worldwide have focused on improving the energy density of LIBs owing to the increasing demand for vehicle electrification and grid energy storage. With a theoretical specific energy of 400 Wh/kg, lithium-ion batteries are extensively utilized in wearable electronics, owing to their elevated energy density and prolonged cycle life. However, traditional LIBs are deficient in fulfilling the prerequisites of wearable electronics in terms of superior flexibility and breathability. As a solution, fiber-type electrochemical storage devices that are flexible and can be woven into fabrics have emerged and gained traction.

For example, Kwon et al. [131] introduced a cable-type lithium-ion battery that achieves unparalleled mechanical flexibility, offering a potential breakthrough in wearable electronics (Figure 11a). This battery can also be worn on the neck, or other body parts, allowing maximum liberty in design and significantly promoting the application in wearable electronics. Similarly, Ren et al. [88] reported a MWCNT fiber-based battery with outstanding performance (Figure 11b,c). The fiber-type battery was assembled by twisting MWCNT fiber and lithium fiber, which were used as the positive and negative electrodes, respectively. The specific capacity was computed at 94.37 mAh/cm³. Later, a stretchable fiber-type lithium-ion battery was produced based on two MWCNT/lithium oxide composite yarns as the two electrodes without additional current collectors and binders [89] (Figure 11d).

The combination of two composite yarns results in a safe battery with superior electrochemical properties, including energy densities of 27 Wh/kg or 17.7 mWh/cm³ and power densities of 880 W/kg or 0.56 W/cm³. In 2015, a CNT/MoS₂-based hybrid fiber was assembled to demonstrate exceptional electronic and electrochemical properties (Figure 11e,f) [90]. This hybrid fiber was utilized to fabricate lithium-ion batteries with a high specific capacitance of 135 F/cm³ and a high specific capacity of 1298 mAh/g. Furthermore, Zhang et al. [84] reported a fiber-type lithium-ion battery (FAL) in 2016, using polyimide/CNT composite fiber and LiMn₂O₄/CNT composite fiber as the two electrodes (Figure 11g,h). This battery boasts an output power density of 10,217.74 W/kg, surpassing that of most supercapacitors, as well as an energy density of 48.93 Wh/kg, equivalent to that of a thin-film lithium-ion battery. Moreover, the fiber’s 3D deformability provides unique advantages over traditional planar structures.

Another popular battery type is the lithium–sulfur (Li-S) battery. The fiber-type Li-S battery was developed firstly by Fang et al., utilizing carbon fibers as both the sulfur support and current collector (Figure 12a,b) [92]. When combined with a lithium fiber anode, the resulting fiber-type Li-S battery showed a specific capacity of 400–700 mAh/g, which is higher than that of previously reported counterparts. Chong et al. then reported graphene-based rGO/CNT/S hybrid fiber-type cathodes and a fiber-type lithium–sulfur battery [93] (Figure 12c,d). Later, a fiber-type cathode was created based on rGO/sulfur composites, as shown in Figure 12e,f [94]. The resulting fiber-type lithium–sulfur battery demonstrated excellent flexibility and elevated electrochemical performance.

Metal–air batteries are considered promising candidates for meeting the increasing demand for high-energy-density, low-cost, and environmentally friendly power sources for mobile electronics and electric vehicles, as an alternative to lithium-ion batteries. Typically, metal–air batteries consist of a porous cathode, electrolyte, and metal anode. Anodes made of metals such as Li, Na, and Zn are suitable for metal–air batteries. Metal–air batteries function based on the dissolution of metals on negative electrodes and oxygen reduction or oxygen evolution reactions on positive electrodes. Common metal–air battery fibers include fiber-type lithium–air battery, fiber-type zinc–air battery, fiber-type aluminum–air battery, and fiber-type silicon–air battery.

For instance, Yin et al. [95] reported a fiber-type Li-O₂ (SFLO) battery (Figure 13a). Later, Peng’s group reported a new fiber-type aluminum–air battery based on CNT/silver nanoparticles as an air cathode on a gel electrolyte (Figure 13b) [132]. These fibers are highly versatile and can be woven into various fabrics for use in a range of applications on a large scale. The same group has also demonstrated a flexible fiber-type zinc–air battery based on CNT sheets (Figure 13c) [96]. The CNT-based zinc–air battery possessed a stable electrochemical performance, which can stably discharge at 10 A/g. In the case of silicon–air battery fibers, Peng’s research group has developed a fiber-type silicon–air battery based on silicon/CNT (as shown in Figure 13d). This design has resulted in a silicon–oxygen battery (SOB) fiber with exceptional flexibility and a high energy density of 512 Wh/kg. Furthermore, the hybrid fiber produced in this study has the advantage of avoiding dendrite formation and lithium metal safety issues while also maintaining its ultra-high flexibility. The SOB fiber has been shown to function effectively even after 20,000 bending cycles [97].

4. Fabric-Type Energy-Storage Device

Fabric is an ideal medium for fiber-type devices in wearable applications. To create an uninterrupted power supply for wearables, energy-storage fabrics that are both versatile and efficient are essential. Traditional energy-storage technologies, which are often flat or cylindrical, do not conform well to the curved surfaces of the human body. As a result, fabric-type energy-storage devices, such as supercapacitors and batteries, have attracted significant interest due to their ability to conform to the body and accommodate long-term wear.

4.1. Fabric-Type Supercapacitors

Supercapacitor fabrics typically rely on liquid electrolytes such as sulfuric acid, ionic liquid, or aqueous solutions of potassium hydroxide (BMIPF6, EMIPF6, etc.). Such fabrics are usually created by sewing supercapacitor fibers or threads enclosed in polymer tubes onto pre-existing pieces of cloth. To address this limitation, Jost et al. [76] reported a fiber coating technology capable of producing hundreds of yarns at once, enabling uniform mass loading of active ingredients (as shown in Figure 14a,b). Although the resulting fabric-type supercapacitor can be stretched like regular cloth, the electrolyte must be deposited into the cloth after knitting, which demands additional encapsulation and reduces flexibility. To solve this issue, Cheng et al. stitched three fiber-type supercapacitors into a fabric, using expandable non-liquid crystal spinning technology to create a Redox graphene (rGO) fiber electrode (as shown in Figure 14c,d) [77]. The as-assembled fiber-type supercapacitor showed a specific capacitance of 226 F/cm³. Using a non-liquid-crystal spinning technique, the sewn fabric supercapacitor was able to achieve a capacitance of 1.12 mF at a current of 7.45 A. Additionally, it could be rapidly charged to 3 V, providing enough power to light an LED. This method of assembly could also be employed with other two-dimensional nanomaterials to create macroscopic fibers for various applications, including micro-devices, wearable electronics, and smart fabrics.

Supercapacitor fabrics that utilize solid-type electrolytes typically rely on gel electrolytes, such as potassium hydroxide solution of potassium polyacrylate or sulfuric acid solution of ammonium polyacrylate. These fabrics have a similar structure to those based on liquid-type electrolytes but are more stable and easier to encapsulate. Zhai et al. [78] demonstrated all-carbon solid-state yarn supercapacitors for use in smart fabrics (Figure 15a). The supercapacitors were composed of two hybrid carbon yarn electrodes twisted together in a polyvinyl alcohol/H₃PO₄ polymer gel, which acted as both an electrolyte and a separator. A power density of 27.5 μw/cm was achieved. Similarly, Yang et al. [79] reported on fabric-type all-solid SCs with a high-performance. The all-solid SCs device achieved discharge currents of 0.2–1 A and a specific capacitance increase to 3.2 F/cm² after 10,000 charge and discharge cycles (Figure 15b). Fan et al. [80] demonstrated an all-solid fabric-type supercapacitor by weaving fiber-type electrodes, which achieved excellent mechanical and charge–discharge stability. It outputs an energy density of 0.1408 mWh/cm² and power density of 3.01 mW/cm² when charged up to 1.8 V. The idea of using a micro-dendritic electrode structure provides an inspiring strategy to improve the performance of nanomaterials in final devices (Figure 15c,d). This approach is suitable for future mass production and could be extensively applied to other types of energy harvesting and storage devices.

4.2. Fabric-Type Batteries

Furthermore, while energy-storage fabrics have been extensively researched with regards to supercapacitor fabrics, secondary batteries have received less attention. Nevertheless, the incorporation of fibers from secondary batteries into fabrics can significantly enhance the overall power output. For instance, Zhang et al. [81] developed a fiber-type aqueous lithium by utilizing a polyimide/carbon nanotube hybrid fiber as the anode and LiMn₂O₄/carbon nanotube hybrid fiber as the cathode (Figure 16a). These fiber-type LIBs can be woven into fabrics that are flexible enough to be bent and twisted into various shapes and architectures, and connected in series, while maintaining their electrochemical performance under different deformations. As batteries using aqueous electrolytes tend to suffer from electrolyte leakage and low flexibility, batteries using solid electrolytes are gaining increasing attention. Wang et al. [133] reported fiber-type lithium-ion batteries (Figure 16b,c). They can be weaved into fabrics to serve as a power source for wearable electronics in the future. This method offers a new approach to large-scale assembly of flexible fabric-type lithium-ion batteries.

The working principle of zinc-ion batteries relies on the movement of zinc ions between the positive and negative electrodes. Due to their high energy density and high safety, they have been widely used. To make them more suitable for wearable devices, flexible fabric-type zinc-ion batteries are in development. Li et al. [102] reported a fiber-type zinc-ion battery (ZIB) based on a cross-linked polyacrylamide (PAM) electrolyte (Figure 16d,e). The prepared fabric-type ZIB showed a high specific capacity of 302.1 mAh/g and volumetric energy density of 53.8 mWh/cm³, as well as excellent cycling stability. Moreover, the quasi-solid-state yarn ZIB also exhibited superior knittability, good stretchability (up to 300% strain), and excellent waterproof capability (high capacity retention of 96.5% after 12 h of underwater operation). Recently, Xiao et al. [134] reported a rechargeable solid-state Zn/MnO₂ fiber battery with stable cyclic performance exceeding 500 h, while maintaining 98.0% capacity after more than 1000 charging/recharging cycles. The Zn/MnO₂ fiber battery boasts several essential features, such as its light weight, slim design, affordability, biocompatibility, and high energy density. Due to these features, it can be seamlessly integrated into a multifunctional on-body e-fabric, providing a stable power source for continuous and simultaneous monitoring of heart rate, temperature, humidity, and altitude.

For metal–air battery fabrics, Zhang et al. [135] reported a fiber-type Li–air battery for metal–air battery fabrics. The prepared fabric-type battery produced a discharging voltage of 8 V. This fabric was used to power electronic devices (as shown in Figure 17a,b). Later, Wang et al. [136] reported a fiber-type Li–air battery. The low-density polyethylene film can suppress the side reactions of the discharge product of Li₂O₂ to Li₂CO₃ in ambient air, while the LiI facilitates the electrochemical decomposition of Li₂O₂ during charging. The resulting Li–air battery demonstrated an impressive life of over 600 cycles. The prepared fabric-type battery can effectively charge a smartphone (Figure 17c). Xu et al. [136] developed a new family of all-solid-state fiber-shaped aluminum–air batteries, offering a specific capacity of 935 mAh/g and an energy density of 1168 wh/kg. The electrochemical performance was enhanced by synthesizing an electrode composed of cross-stacked aligned carbon-nanotube/silver-nanoparticle sheets. To demonstrate wearable applications, The prepared fabric-type batteries can power an LED watch (Figure 17d). Moreover, Li et al. [137] demonstrated a fiber-type Zn–air battery (ZAB) utilizing a continuous procedure (Figure 17e,f). It showed excellent charge–discharge performance, achieving an energy efficiency of 60%. The flexible fabric-type zinc–air batteries can power various wearable electronics.

5. Harvesting–Storage Integrated Fabrics

Hybrid energy devices, which combine energy harvesting and energy storage, are an effective strategy for sustainable energy supply. While a single energy-harvesting device cannot capture solar, wind, and mechanical energy simultaneously, weaving together diverse types of fiber-shaped or fabric-shaped energy harvesting devices allows for energy harvesting from multiple sources. Moreover, integrating energy harvesting and energy-storage devices is imperative to achieve continuous power supply, given that sunlight, wind, and human movement are inconstant in time and space.

Compared to conventional, bulky hybrid energy devices, fabric-type hybrids, such as hybrid energy fabric, have become popular due to their light weight, portability, and durability. As a result, researchers have extensively studied combinations of diverse energy harvesting and storage devices, for example hybrid energy fabrics including solar cells with supercapacitors, TENGs with supercapacitors, and solar cells and triboelectric NGs with supercapacitors.

For integrated energy fabrics with solar cells and supercapacitors, Chai et al. [105] developed an all-solid, customizable energy fabric that integrates solar energy harvesting and storage. Fiber-shaped supercapacitors with ultra-fast charging and ultra-high bending resistance are used as energy-storage modules, while a DSSC fabric is used for solar energy harvesting (Figure 18a). Using self-collected solar energy, the fabric can be charged to 1.2 V within 17 s and discharged completely within 78 s at a discharge current density of 0.1 mA.

For integrated energy fabrics with TENGs and supercapacitors, Chen et al. [106] proposed a self-charging power fabric (SCPT) that combines a fabric triboelectric nanogenerator (FTENG) with a woven supercapacitor (W-SC) (Figure 18b). This single-piece self-powered/self-charged fabric can be easily woven using traditional methods and can harvest energy while powering wearable electronics.

For integrated energy fabrics with solar cells and batteries, Zhang et al. [111] presented a photo-rechargeable fabric with economically viable materials and scalable fabrication technologies, integrating solar cells and batteries. This fabric can constantly deliver electric power for 10 min at 0.1 mA after being charged for 1 min under standard 1 sun conditions. It can also function normally under twisted and watery circumstances and store energy for over 60 days without significant voltage loss. The photo-rechargeable fabric was demonstrated to power a body area sensor network for personalized healthcare (Figure 18c,d). Huang et al. [108] proposed an industrially scalable couch embroidery electrode assembly strategy for sustainable energy devices that can create fiber-type solar cells, Zn-MnO₂ batteries, and integrated power sources by simultaneously constructing several cable electrode types with extreme length differences into a single fabric-type device. These methods make the manufacturing of integrated fabric-type energy devices easier and make the structure and function of electronic power supplies more flexible on the design side (Figure 18e–g).

For integrated fabrics with solar cells, TENGs, and supercapacitors, Wang et al. [107] presented a hybrid self-charging energy fabric system that integrates solar cells, triboelectric nanogenerators, and supercapacitors. All the harvested energies from solar and mechanical sources can be converted into electricity using fiber-shaped dye-sensitized solar cells and TENGs and then further stored in fabric-type supercapacitors. A single solar cell yields an open circuit voltage, short circuit current density, and power conversion efficiency of 0.74 V, 11.92 mA/cm², and 5.64%, respectively. The flexible TENG can use mechanical energy from the environment, such as human motions, to produce an output of 0.91 mA. The SC unit provides a specific capacitance of 1.9 mF/cm, making it a flexible and effective energy-storage device.

These novel hybrid energy fabrics are advantageous for fulfilling the power and mechanical resilience prerequisites of wearable electronics, attributable to their appealing performance, successful validation in on-body testing, as well as their unique structure and inexpensive fabrication.

6. Conclusions and Prospects

In this review, we have systematically summarized the state-of-the-art developments in flexible fabric-type energy-storage devices, as well as their hybrid fabrics for energy harvesting and storage in wearable electronics. We have introduced four assembly strategies for forming intelligent fibers into intelligent fabrics, including direct coating based on the fabric, layer stacking of fabric electrodes, interweaving of fiber electrodes, and weaving of fiber devices. We have also discussed fiber-type and fabric-type energy-storage devices based on supercapacitors and batteries. Notably, energy-storage fabrics can be integrated with other energy harvesting fabrics such as solar energy fabrics and mechanical energy fabrics to form hybrid energy fabrics for simultaneous energy harvesting and storage. These can be used as a sustainable energy source to power portable and wearable electronic devices (Figure 19).

Despite many advancements and ongoing research, challenges remain in commercial applications, such as enhancing energy and power density, improving cycling lifetime, developing scalable fabrication techniques, improving safety, and ensuring comfortable wearing experiences.

For enhancing energy and power density, although significant progress has been made in developing energy-storage materials with high energy densities, it is important to recognize that the performance of such devices can be significantly impacted by the unique structure of the fiber and fabric substrate used. In order to address this challenge, one key focus is on the assembly of functional materials with nanostructures on curved surfaces. As such, it is crucial to identify suitable materials and structures that are well-matched to the fiber and fabric substrates.

For improving cycling lifetime, encapsulation is crucial. To reduce device sensitivity to water and oxygen, efficient materials and technologies must be developed that are suitable for curved structures, such as fibers or fabrics. However, current packaging technologies cannot suit both planar and curved structures, requiring the joint efforts of academia and industry to develop new encapsulation materials and technologies.

For developing scalable fabrication techniques, large-scale fabrication technology is necessary for practical commercial applications. However, developing processing equipment that can achieve continuous and stable coating on high-curvature interfaces is difficult, particularly while possessing negligible defects. Therefore, it is necessary to develop technology suitable for the processing of long-dimension fiber electrode materials and then realize scalable fabrication of devices.

For improving safety, biocompatibility and safety are top priorities for wearable devices. Increasing the energy density of the energy device to very high levels usually leads to safety concerns, particularly in wearable scenarios. The potential for unintentional friction could further exacerbate these safety hazards. Once the energy density is increased beyond a certain threshold, it becomes challenging to develop effective safety technologies such as aqueous batteries. Therefore, the development of reliable encapsulation technology is still required to ensure the safety of device applications.

For satisfying comfortable wearing experiences, long-term comfort of the human body should be considered, such as breathability, light weight, and softness. An ideal wearing experience can be achieved by developing lightweight polymer fibers as electrode substrates that can replace traditional metal conductive substrates, as well as combining smart devices with conventional cotton thread or silk.

In summary, fabric-type flexible energy-storage devices unlock an attractive power source for emerging wearable electronics, leading to great advancements in future technology.