Comprehensive analysis of flexible lithium ion battery energy storage equipment

With the rapid development of electronic technology, more and more electronic devices are developing in the direction of thinness and flexibility. For example, Samsung and LG have launched their own flexible and foldable screens, and are planning to launch foldable mobile phones. Such products, currently display components and circuits can be flexible and foldable, the biggest challenge is the foldable energy storage power products, traditional lithium-ion batteries, super capacitors and other products, not only bulky, but also can not be folded, in When the volume changes too much, it may even cause a short circuit between the positive and negative electrodes, causing thermal runaway, leading to serious safety problems. Therefore, in order to adapt to the development of the next generation of flexible electronic devices, the development direction of lithium-ion batteries should also be developed in a flexible and foldable direction.

For chemical power sources such as lithium-ion batteries and supercapacitors, the biggest obstacle to flexibility is the flexible design of the current collector, which not only ensures good mechanical properties of the flexible electrode, but also has good electrochemical performance. Starting today, we will introduce the latest developments in flexible energy storage power supply design in two parts, including flexible lithium-ion batteries and flexible supercapacitors.

In order to realize the flexible design of lithium-ion batteries, various routes have been tried, such as polymer batteries, cellulose-based batteries and paper-based batteries. Research shows that the most important influence on the folding performance of lithium-ion batteries is the electrodes and current collectors. Design, below we will introduce the current development status of flexible lithium-ion battery design according to the technical direction.

Flexible paper battery

Speaking of paper, everyone is no stranger. Papermaking is one of the four inventions in China. Although it is not clear why we like to make up four, like the four famous books, the four great talents, the four big families, but the invention of papermaking It has indeed had a major impact on the progress of human civilization. The paper has a rough surface and high porosity, which is very suitable for the diffusion of ions, and the paper has the characteristics of flexible folding, so it is very suitable for the production of flexible lithium-ion batteries. In lithium-ion batteries, paper can be used both as a current collector and as a separator. Of course, this requires modification of traditional paper, such as coating a multi-walled carbon nanotube SWCNTs on the surface of the paper to enhance the paper. Conductivity, a carbon nanotube CNT and cellulose composite to make a flexible electrode with good conductivity, of course, paper can also be used as a separator. In a recent report, a multi-walled carbon nanotube film is used as a current collector, and The positive and negative active materials are coated separately, and the paper is used as the separator and the support structure. The battery not only has good foldability, but also has excellent electrochemical performance, especially the self-discharge performance of the battery. When stored for 350 hours, the voltage drops only 5.4. mV.

3D electrode flexible lithium ion battery

The capacity and rate performance of lithium-ion batteries are closely related to the active area of ​​the electrode. By improving the electrode structure of the lithium-ion battery, increasing the active area of ​​the electrode and increasing the diffusion kinetics of Li+ are important for improving the performance of the lithium-ion battery. way. Recent studies have found that carbon textiles with 3D structure have good electrical conductivity and excellent mechanical properties, so they are very suitable for replacing traditional metal current collectors. By growing a layer of ZnCo2O4 nanowire material on the surface of carbon fiber, the specific capacity of the negative electrode can reach 1300 mAh/g. With this material as the negative electrode, the lithium ion battery made of lithium cobaltate as the positive electrode can work normally even in the case of bending. In the durability test, the battery can still work normally after being bent for hundreds of times, but the main problem of the current carbon textile material is that the areal density is too high, which affects the energy density of the battery. If an ultra-thin titanium foil is used as the current collector, coating a layer of active material with a 3D structure can significantly improve the rate performance of the battery while ensuring good flexibility. Recently, the emerging graphene foam materials have attracted a lot of attention due to their light weight, good electrical conductivity and excellent foldability. For example, research shows that Li4TI5O12 and graphene foam form a composite material, and the ultra-high magnification at 200C. Underneath, the specific capacity of 86 mAh/g can still be obtained, and good foldability is maintained, and the capacity is only slightly reduced by 1% when the bending radius reaches 5 mm.

All solid electrolyte

Conventional liquid electrolytes have been greatly limited in flexibility due to thermal stability and poor mechanical stability. The plastic crystal electrolytes developed in recent years have solved the problem of poor stability of conventional liquid electrolytes. The plastic crystal electrolyte is mainly composed of lithium salt and plastic crystal, which has good thermal stability and good ionic conductivity, but the traditional plastic crystal electrolyte exhibits more liquid behavior at room temperature, thus causing its mechanical properties. Problems such as poor need to be modified by corresponding transformation means. By adding PET fiber to the plastic crystal electrolyte, the mechanical properties of the plastic crystal electrolyte can be significantly improved, with LiCoO2 as the positive electrode and Li4TI5O12 as the negative electrode. The crystal electrolyte is an electrolyte and a separator, and the battery exhibits good bendability and can work normally even if it is wound several times. However, the thickness of the above electrolyte is generally about 25 um, which cannot meet the design requirements of the thin and light lithium ion battery. Therefore, people have studied lithium-phosphorus oxide LPON materials, and the thickness of the solid electrolyte can be 2 um, and still maintain good electrochemical performance. performance.

Flexible ion battery structure design

For the design of flexible electronic components, the biggest limitation of the design is the shape of the battery. Compared with the traditional chip battery, the linear lithium-ion battery has a natural advantage in this respect. For example, LG Chemical recently introduced a new one. A linear lithium-ion battery having a hollow spiral type negative electrode structure, a modified non-woven diaphragm, and an external positive electrode structure, the battery voltage platform of 3.5V, and a capacity linear density of 1.0 mAh/cm, the battery having Good bendability, and more importantly, the battery does not have to be placed inside the electronic device like a conventional lithium-ion battery, and can be placed anywhere, thus greatly facilitating the use of the wearable device.

With the development of extendable electronic devices, flexible electronic devices not only have to withstand bending deformation, but also can be bent, stretched and compressed. Therefore, it is a huge challenge to provide energy storage batteries for extendable electronic devices. A feasible technical solution is to carry out the unitized design, that is, to divide the traditional lithium ion battery into a single small unit, generally using an elastic silicone film as a base, and different small units are connected to each other. Through testing, the battery prepared by this method can be extended to more than 300%, and still can maintain good electrochemical performance.

At present, the design of flexible lithium-ion batteries is only in its infancy, and there are still many problems to be solved. For example, how to improve the specific energy and safety performance of lithium-ion batteries under the premise of ensuring good mechanical properties of batteries, which requires electrolysis design. It can be done together with the electrolyte design and the design of the battery structure.

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