Recently, Professor Zhi Chunyi from City University of Hong Kong reviewed the design principles (such as electrode preparation and battery assembly) and device performance (such as electrochemical and mechanical properties) of fiber-shaped batteries, including lithium-based batteries, zinc-based batteries and other representative ones. system, and focuses on the multi-functional devices of energy storage textiles with environmental adaptability, stimulus responsiveness and scalability, hoping to inspire future research directions. Finally, the technical difficulties of these batteries in practical wearable applications are also discussed, aiming to provide possible solutions and new insights for further improvements. The review was published in Adv. Mater., titled "An Overview of Fiber-Shaped Batteries with a Focus on Multifunctionality, Scalability, and Technical Difficulties".

1. Fiber battery design principle - schematic diagram of three fiber battery configurations

2. Research progress of fibrous batteries

2.1 Fibrous battery development timetable

2.2 Fiber-shaped lithium-sulfur ion battery

2.3 Fiber-shaped sodium-based battery

2.4 Fiber-shaped zinc-based batteries

2.5 fiber lithium air battery

2.6 Fiber-shaped zinc-air battery

3. Multifunctional and integrated systems

3.1 Waterproof/fire type fiber battery

3.2 Shape memory and self-healing fibrous batteries

3.3. Printable fiber-shaped batteries

3.4 Fiber battery integrated system

4. From fiber batteries to fabric batteries

4.2 Woven/knitted fabric battery

The above content mainly introduces the design basis of fiber-shaped batteries, including typical electrodes, electrolytes, device structures and corresponding manufacturing methods. Then, cutting-edge research on electrode preparation, battery assembly, electrochemical performance, and flexibility evaluation of different fiber-shaped battery systems are discussed in detail, and the key results of each step are briefly introduced. Versatility, scalability, and forward-looking directions for integration with other energy sources.

In addition, the authors of the article also discuss the main challenges faced by fiber-shaped batteries in future wearable applications and provide possible solutions and some insights for the required improvements. The main insights are as follows: The authors believe that a promising direction for future applications of fiber-shaped batteries is to carry a rechargeable (micro) battery cluster, such as a structural material battery system that can be placed in a pocket to power smart textiles due to their small size. , portability and adaptability. Another direction is wearable energy storage textiles. The choice of future smart fabric batteries among energy storage textiles, structural material batteries or other alternative systems depends largely on the specific situation and requirements. Despite significant efforts in the development of fiber-shaped batteries, much more needs to be done before these basic wearable devices are actually available. With the increasing popularity of energy storage textiles, in addition to electrochemical performance, safety, comfort, convenience, durability, etc. will become other major considerations. From our perspective, solving the technical issues involved in the following aspects is crucial to convert existing academic research results into practical applications in the future.

1. High internal resistance caused by slender structure

The first problem with this slender structure of fiber-shaped batteries is their internal resistance. Most of the fiber-shaped devices have been reported to be on the centimeter scale, and few reports have mentioned their resistance. However, as the length of the battery fiber increases, especially when the amplification fiber is operated in a deformed state, the higher resistance has a more obvious adverse effect on the electrochemical performance. The internal resistance of the battery includes the following aspects: the conductivity of the fiber electrode, the interface resistance between the electrolyte and the electrode, and the ionic conductivity of the electrolyte.

2. Create difficulties

Compared with planar batteries, the manufacturing process requirements for fibrous batteries are more stringent and complex. The main problem lies in the complex interdependence between the one-dimensional geometry and the limited free motion of discrete components, especially when considering the coordination of flexibility, adaptability and performance of the battery. In addition, other challenges such as active material dissolution, inter-component adhesion, electrolyte leakage and sedimentation should be carefully considered.

3. Difficulty setting up the diaphragm

Flexible energy storage devices usually require a diaphragm because in practical applications, there is a high risk of short circuit caused by contact between two electrodes. Using a diaphragm is an effective strategy to prevent this from happening. However, unlike planar batteries with sandwich structures, how to place separators in battery devices with fiber structures remains a huge challenge. In laboratory studies, most reported fiber-shaped batteries use polymer gel electrolytes as separators, but the limited mechanical strength of polymer electrolytes is still insufficient to meet practical requirements, especially when large deformations are imposed. Electrolytes are often overwhelming. For future wearable applications, placing an effective separator can effectively ensure the long-term stability of the battery. However, considering the slender structure of the fiber battery, how to optimize the adaptability between the separator and other inner components to adapt to various complex Deformation is still a difficult problem that needs to be solved urgently.

4. Difficulty in packaging

In the practical application of fiber-shaped batteries, effective packaging is essential. Compared with planar devices, the encapsulation process is more challenging for one-dimensional fiber-shaped devices with high curvature interfaces, especially those for air batteries where the air electrode must be exposed to the air for gas diffusion. In terms of packaging materials, polymer materials such as heat shrink tubing, polyester film, and silicone rubber have been widely used in flexible packaging layers, but their water vapor and oxygen barrier functions are far from satisfactory. In addition, after fiber-shaped batteries are integrated into wearable fabrics or textiles, they must be able to withstand long-term water washing and sweat penetration. However, metal anodes (such as lithium and sodium metal wires) and organic electrolytes usually require strict hydrophobic encapsulation to prevent water intrusion. Some literature reports have tried some encapsulation technologies, but did not implement strict and comprehensive waterproof performance testing. In addition, encapsulation materials greatly increase the diameter of fiber-shaped batteries, which adversely affects flexibility. Therefore, more effective packaging strategies are currently needed to realize ideal fiber-shaped batteries that can be washed like ordinary clothes.

5. Difficulty minimizing size

The flexibility and softness of fiber-shaped batteries depends largely on the diameter of the entire device. However, the dimensions of currently reported fiber-shaped batteries are far from satisfactory because their complex structures consist of several essential components, including electrodes, electrolytes, and separators/encapsulation layers, and their superimposed effects inevitably increase Battery size and thickness. In addition, device configuration is another important factor. For example, due to the multi-layer structure of coaxial fiber-shaped batteries, the cell diameter is often larger than 2 mm. In comparison, fiber-shaped batteries with parallel and twisted structures usually have relatively thin diameters (about 1-2 mm). Nonetheless, compared with natural/synthetic yarns in fabrics with a typical diameter of only 200-300 μm, the diameter of existing fiber-shaped batteries is simply too large and far from being able to meet the needs of large-scale weaving/knitting. Currently Technology is still elusive in achieving energy storage textiles that are as dense and soft as pure natural/synthetic fabrics.

6. Low mechanical strength

Currently, there is little research into manufacturing processes using machines, and most fibrous devices are prepared and woven into textiles by hand. Based on existing technology, how to achieve large-scale production of fibrous batteries and how to easily weave fibrous batteries into breathable energy storage textiles using machine weaving/knitting methods is still a difficult problem to be solved. In terms of woven/knitting technology, CCI rapier looms and Stoll knitting machines usually exert a tensile stress of 400-800 MPa on the yarn. Although the stress is not very high, the friction generated cannot be ignored. In addition, in the study of fiber-shaped batteries, attention should be paid to the mechanical properties of each fiber-shaped component. For most reported fiber-shaped batteries made of graphene or carbon nanotube-based fiber substrates, their mechanical strength (10-2-10-3MPa) is far insufficient to enable industrial woven/knitting requirements. If high-strength conductive metal wires are used as fiber current collectors, their extremely poor flexibility will bring great difficulties to the weaving process, and the smooth surface of the metal wires will also cause the internal components of the energy storage textile to slip. In addition, when the original shape of the fiber-shaped battery cell inevitably undergoes severe deformation during the weaving/knitting process, how to maintain its electrochemical performance is a major challenge.

7. Yarn texture is difficult to achieve

Fiber is the basic unit for manufacturing textiles. The threads formed by bundles of filaments or stable fibers coaxially rotated together are called "yarns", and the yarns can be further woven/knitted into textile products. When the diameter of the yarn is no more than 10 μm, it will have a soft texture, giving the wearer a comfortable feeling. However, based on current manufacturing technology, it is impossible to make fiber-shaped batteries as soft as natural/synthetic fibers because fiber-shaped batteries are usually covered with gel electrolytes and/or elastomer coatings for encapsulation purposes. Existing fiber-shaped batteries have a texture more like plastic threads than yarn.

8. Lack of testing standards to evaluate mechanical properties

Although great progress has been made in the research of battery performance and device configuration, how to fairly compare the "flexibility" and "wear resistance" of different fiber-shaped batteries is still a problem. Unlike planar energy storage devices, this long and thin structure can easily cause slippage between the fiber-shaped battery and the test fixture during mechanical performance testing. Furthermore, for conventional electronic stretching machines, a more sensitive stress sensing probe is required due to the lower mechanical strength of fiber-shaped cells. Due to these limitations, most reported works usually adopt simple bending, torsion, and tensile tests to determine the mechanical flexibility and durability of fiber-shaped batteries without obtaining quantitative data related to modulus, tensile strength, etc. , this simple assessment seems somewhat arbitrary. Here, the authors call for the establishment of a series of systematic criteria to evaluate the mechanical flexibility and durability of fiber-shaped batteries.

9. Security issues

In actual wearable applications, direct contact between fiber-shaped batteries and the human body is unavoidable, so it is crucial to ensure that these batteries are absolutely safe. The first major safety concern is hazardous material leakage and toxicity. For example, some corrosive or flammable electrolytes may leak from battery devices, and some heavy metals, such as cobalt-containing electrode materials or catalysts, can also be harmful to humans. Therefore, non-toxic electrode materials and mild aqueous electrolytes have been developed to solve such problems. In addition, using polymer gels or solid electrolytes instead of traditional organic electrolytes is also considered an effective solution to avoid electrolyte leakage. At the same time, researchers should also pay attention to developing some anti-leakage packaging technologies. As we all know, thermal runaway has always been a common problem for high-power aprotic batteries, especially during ultra-fast charging and discharging processes or under dangerous conditions (such as short circuit and overcharging). Therefore, it is important to develop effective heat dissipation mechanisms to avoid overheating while also avoiding short circuit issues.

10. Multifunctional and integrated

Multifunctional and integrated systems are crucial to expanding the application range of fiber-shaped batteries. For example, in terms of multifunctionality, electrochromism, photoresponse, thermal response and low-temperature antifreeze properties have all been successfully implemented and optimized in planar batteries or fiber-shaped supercapacitors, but fiber-shaped batteries with these special functions are lacking. Related reports. One possible explanation is that due to the complex electrochemical reactions of batteries, the selection criteria of materials related to conductivity, transparency, mechanical strength, etc. are more stringent for the preparation of batteries than for supercapacitors. In addition, during the manufacturing process, it is much more difficult to assemble one-dimensional fiber-shaped devices than to assemble two-dimensional sandwich structure devices, so most attempts at improvement were first performed on planar devices. In future research, we expect people to introduce accumulated experience and technology into the field of multifunctional fiber batteries. In addition, the integration of fiber-shaped batteries with other systems such as energy conversion devices, self-powered generators and sensors is a very promising research direction in the future. Compared with integrated devices that realize the switching functions of energy storage and energy collection through simple connections, integrating one-dimensional energy storage components and energy collection components into one device can achieve dual functions at the same time. However, building such an integrated device is very complex, requiring researchers to have a deep understanding of the intersecting working mechanisms of different fields of science and technology, as well as careful design of device assembly, component compatibility, and energy management.

【Summary】

In summary, the field of fiber-shaped energy storage batteries has developed rapidly in recent years, achieved great achievements, and shows great prospects in actual wearable applications. Therefore, this review paper reviews the key progress made so far in fibrous battery systems from aspects such as electrode preparation, novel structural design, electrochemical performance and flexibility evaluation. Researchers will continue to pursue higher electrochemical performance, explore new materials, effective preparation strategies and reduce their production costs. In addition, the large-scale production of energy storage textiles with high performance, biocompatibility, and wearing comfort from ordinary fiber-shaped batteries is of great significance for wearable applications. More importantly, by optimizing new smart functional materials and cleverly combining them with basic fibrous structures, devices can be multifunctional, effectively broadening the application range of these one-dimensional fibrous batteries. Of course, special functions and electrochemical performance may sometimes not be optimal in a device at the same time, in which case a balance needs to be reached between the two. The integration of fiber-shaped batteries with other systems (such as photoelectric conversion systems, nanogenerators, and medical sensors) can bring higher application value to consumers and may bring about technological revolutions in future research fields.

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