The flexible lithium battery prepared based on the flexible fabric composite electrode can still supply power stably under the bending state. Photo courtesy of Zheng Zijian's research group, The Hong Kong Polytechnic University
As far as the development of the flexible electronics industry is concerned, flexible batteries are an indispensable part. At present, many electronic components such as display screens and sensors can already be flexibly bent or even folded and stretched, but the battery that provides power for them still stays in the rigid design stage.
The emergence of flexible batteries can get rid of the limitations of the existing metal foil rigid batteries on the design of product shapes and structures. After integrating with flexible wearable electronics, they can take advantage of their arbitrary deformation.
In most literatures, battery flexibility is mainly judged by the radius of curvature (r) when the material yields. In this regard, a research group led by Professor Zheng Zijian of the Department of Textiles and Clothing of the Hong Kong Polytechnic University recently gave a constructive comment in Joule, highlighting the importance of battery strain in evaluating flexibility, and first proposed the area energy density and curvature The ratio of radii (Ea/r) serves as a better evaluation of the performance factor of flexible batteries.
The flexibility of lithium batteries depends on this index
At present, the commonly used lithium ion batteries cannot be bent, because the layered metal foil electrode in the battery is easily broken after bending. However, to supply power to flexible electronic fabrics, wearable devices, etc., lithium-ion batteries must have some flexibility.
Zheng Zijian told China Science Daily that he found that evaluating the flexibility with the commonly used radius of curvature is prone to misunderstanding, because the yield when the material is bent is not only related to the radius of curvature, but also to the thickness of the material (h). Even with the same material, thicker foils yield larger radii of curvature than thinner foils.
If the tensile strain of the outer layer of the material (ε=h/2r) is used as the basis for evaluation, the thickness can be introduced to make the evaluation more fair. Moreover, Zheng Zijian believes that the flexibility of the battery is related to the battery structure, which determines the energy density of the battery.
"The previous literature separates the energy density from the flexibility independently, which is unfair." Zheng Zijian said, "The performance of a flexible battery with a high energy density and a large radius of curvature is not necessarily smaller than that of a low energy density (better flexibility). Poor and vice versa."
In some studies, in order to increase flexibility, carbon-based materials are used instead of metal foil, or the metal foil is made into a hollow mesh. Although the flexibility is increased, the volumetric energy density is reduced. Therefore, Zheng Zijian multiplies ε by the volume energy density to obtain 1/2 (Ea/r).
"The area energy density Ea and r are the data available in the paper, and there is not much calculation of "moisture", which is lowly misleading." Zheng Zijian said, "Furthermore, for the calculation of volume energy density, because everyone uses The standard is different, the difference can be ten times to hundreds of times. And Ea and r are unchanged."
Therefore, Zheng Zijian proposed to use Ea/r as the performance factor of flexible batteries in the paper. Generally, a certain research field will use a digital index as a performance factor for performance comparison and discussion, but there is no performance factor in the field of flexible batteries.
"Performance factor is more an academic standard in the field of research, but it can be directly compared with commercial batteries, so it is very meaningful." Zheng Zijian said.
Carbon-based flexible batteries will replace metal foils in the future
So, using the performance factor proposed this time, what kind of flexible battery performance is more prominent at present?
"Although we did not deliberately discuss which would be better, we found that, compared to metal mesh or carbon-based paper-based flexible battery materials, carbon-based fabric-based flexible batteries are more flexible and use their three-dimensional structure as the base material. Fluid assembled batteries can get a higher performance factor. "Zheng Zijian said, "We think that in the future flexible batteries will slowly change the current status based on the preparation of metal foil as a current collector."
According to the Printed Circuit Association (IPC) standard, flexible electronic equipment should be able to withstand at least 5% strain.
Chang Jian, the first author of the article and an assistant professor in the Research Institute of Frontier and Interdisciplinary Sciences of the Southern University of Science and Technology, told the China Science Journal that judging from the two key parameters of energy density and strain, some of the flexible batteries currently developed in academia have reached 350. Watt-hour/liter and 5%, which meets the matching needs of flexible and wearable electronic devices.
But generally speaking, the yield strain of metal materials is less than 1%, so 5% strain will cause fatigue on the metal foil.
"Metal foil is very thin. This is the result of decades of hard work in the entire battery industry. The volume has been maximized. At present, the carbon material is still thick, resulting in an invalid volume. It should be gradually improved in the future." Zheng Zijian said.
The second author of the article, Huang Qiyao, a postdoctoral fellow in the Department of Textiles and Clothing at the Hong Kong Polytechnic University, told the China Science Journal that the current research of Zheng Zijian's research group on flexible batteries is mainly focused on the development of flexible current collectors and the research of high energy density flexible batteries. Based on the first patented technology, polymer-assisted metal deposition (PAMD), they developed a series of conductive fabrics with high conductivity and flexibility. By replacing traditional metal foils as battery collectors, they combined with high-performance active materials to construct three-dimensional flexible fabrics Composite electrodes to prepare flexible wearable battery devices. This type of fabric battery is extremely soft and can be folded to a radius of less than 1 mm. Even if it is folded more than 1000 times, its performance remains unchanged.
"But the current commercial carbon-based fabrics are still relatively thick, which leads to an unsatisfactory volumetric energy density of existing fabric flexible batteries." Huang Qiyao said, "With the future development of textile industry technology, the large-scale production of ultra-thin carbon fabrics is bound to Bring a substantial increase in the energy density of the fabric battery."
Flexible battery can be expected in the future in multiple scenarios
Huang Qiyao believes that flexible batteries have broad application prospects in the future in wearable electronics, biosensors, human skin, software robots, etc., which can not only provide energy storage and power supply functions similar to conventional batteries, but also meet the flexibility of future flexible electronic devices. , Design flexibility and wearable comfort requirements.
At the same time, flexible batteries can be combined with other hot technologies, such as the Internet of Things and 5G. As an energy storage device, the flexible battery can also be used as a power source for electronic equipment. Its flexible and bendable performance can make full use of the effective space in the equipment, which is conducive to improving the overall battery capacity of electronic products and is also an important development for improving the endurance of electronic equipment. way.
However, flexible batteries are still in the research-small trial stage. Although there are some samples of flexible batteries on the market, they have not yet entered the stage of mass production and product application.
Zheng Zijian pointed out that the flexible battery is still a new research field, and there are still many problems that need to be solved, including how to improve the energy density and capacity, cycle stability and safety of the flexible battery while maintaining the battery's flexibility. The strain generated by a flexible battery under an applied force tends to exceed the yield strain of each component of the battery, which in turn leads to destruction of the battery structure and deterioration of stability.
To solve these bottleneck problems, it is necessary to design and change the internal structure of the battery, but also need to develop some new active materials, separators, electrolytes, and battery packaging materials.
Zheng Zijian believes that the promotion of flexible batteries and flexible electronics is of great significance. Based on the needs of electronic equipment for batteries, the promotion of flexible battery technology and flexible display screens, biosensors and flexible circuits and other technologies are conducive to the development of more flexible electronic devices, which are used in medical health monitoring, smart textiles, smart phones, global It is applied in multiple scenarios such as positioning system tracking, Internet of Things, and human-computer interaction. (â– Chi Han, a trainee reporter of this newspaper)
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