Picture of the world's first extensible thin film lithium-ion battery prototype (source: Chen Xi)
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According to the university's website, the battery follows the design of a commercial battery. Professor Max Nedberg, who leads the team, says research on foldable batteries has gone beyond applications. Mobile phoneThe foldable battery will be of great value for electronic products such as computers, smart watches and tablet computers that use a folding display.
Once the research results can be improved in the future and put into commercial applications, or will have a revolutionary impact on the design of future electronic products.
DeepTech exclusive interview with the lead author of this article, the main inventor of stretch lithium-ion battery, Dr. Chen Xi of Zurich Federal Institute of Technology, brings you an in-depth interpretation of this research and the future development of folding batteries.
Photo: Dr. Chen Xi, Federal Institute of Technology, Zurich, inventor of extendable thin film lithium-ion batteries (source: Chen Xi)
In the first half of this year, mobile phone giantsthree stars andHuawei One after another released their own collapsible mobile phones, amazing the world, hanging the appetite of consumers all over the world,milletXiaomi MIX alpha, its first folding concept phone, was also released not long ago, so this year is also considered the first year of folding.
Following multi-cameras, artificial intelligence and full screen, foldable mobile phones are likely to become an inevitable trend in the future, and related technology and product development has attracted the continuous attention of technology and business.
Tuhua's first collapsible mobile phone (source: MIT Technology Review)
Picture? Xiaomi's first 5D surround screen concept mobile phone (source: Xiaomi)
The reason why existing collapsible mobile phones can be folded depends on flexible screens. From Samsung toJD.COMSquare, foldableDisplay screen is one of the most proud technological breakthroughs in the electronic industry in recent years. But apart from the screen, the other parts of the foldable mobile phone are not very different from those of the ordinary mobile phone. This not only means that there are no more components in the connection between the two parts of the mobile phone, but also means that besides the flexure hinge, the other parts of the foldable mobile phone are still "bent rather than bent", slightly inadvertent and still have the risk of damage or even self-explosion.
And the reason for the self-explosion, is precisely from the battery.
The structure of a lithium-ion battery can be understood as a "sandwich". In the middle is the electrolyte / diaphragm which is responsible for the transfer of lithium ions, while at the upper and lower ends are the collector fluid which is loaded with positive and negative electrode materials and collects and conducts current.
At present, the electrolyte of mainstream commercial lithium-ion battery is a kind of liquid, which is composed of flammable and explosive organic solvent and lithium salt. Although the electrolyte has excellent performance, once the air tightness is not good, resulting in the battery being invaded by oxygen or water in the air, there may be dangerous side reactions, resulting in battery bulging, spontaneous combustion, and even the production of highly toxic hydrofluoric acid (HF). Therefore, mainstream commercial batteries must be tightly wrapped in steel or aluminum hard shells to prevent electrolyte contact with the outside world. With such a hard shell, folding is naturally impossible to talk about.
On the other hand, the positive and negative electrodes used for electrochemical reactions are made of powdered electrode materials deposited on copper and aluminium foils, i.e., collectors. As the name implies, the so-called collector is the component that stores the electrode material, collects and conducts the current in the battery.
In conventional lithium-ion batteries, the positive and negative film is tightly bonded to the diaphragm and wound, so it is difficult to bend. Even if the thin film battery is made of single layer electrode, once the collecting fluid made of metal material is bent, it will lead to the shedding of electrode powder, and the sharp fold will destroy the battery structure and even pierce the diaphragm between positive and negative electrodes, which will affect the performance of the battery, and the self-explosion of the battery will also be caused by the heavy one.
Don't test your mobile phone's bending resistance easily. Violence-induced deformation is one of the main causes of cell phone self-explosion (source: rankbank)
Therefore, it is impossible to rely on existing materials to achieve foldable batteries.
Researchers have to find new materials for every component in the battery to ensure deformation, while also taking into account the safety of the battery. Moreover, the electrochemical functions of different materials that make up different parts should be compatible with each other. Otherwise, the voltage that can make one part work normally may cause chemical reaction or even failure of another part.
It has always been a subject for scientists and engineers in the field of battery research to find suitable deformable materials for all the components in the battery. Chen Xi said that the first two years of their research on the subject were basically trial and error. After finding the right research direction, it took another two years to finally find a suitable solution for each component of the lithium-ion battery and produce the world's first collapsible, deformable lithium-ion battery.
Their final solution is a foldable solid-state lithium-ion battery, which is also made from a sandwich structure.
The so-called solid-state lithium-ion battery refers to a solid which is not a solution of an organic solvent and a lithium salt mentioned above in a battery, and is not a solution of the organic solvent and the lithium salt mentioned above. It is worth mentioning that John B. Goodwill, who has just won the Nobel Prize in chemistry on October 9, is the latest research result published in 2017 and a solid-state lithium-ion battery.
Chen Xi's electrolyte is a special substance between liquid and solid - hydrogel.
Hydrogel is a kind of hydrophilic three-dimensional polymer network structure gel. On the one hand, it has a rubber-like property, and has a large number of cross-linked organic polymer chains, which are often linked to each other by a covalent bond, a hydrogen bond, or an electrostatic force. Chen Xi, their chosen hydrogel, is cross-linked by the most powerful covalent bond, while being stretched, while each polymer chain is stretched, there is no relative slip between the molecular chains due to the presence of a strong covalent bond. When the tension disappears, the polymer chain contracts and the object will restore the original shape. Without such a covalent bond, the slippage between the chain and the chain will eventually lead to a fracture of the material and no recovery.
On the other hand, hydrophilic properties allow hydrogels to carry large amounts of moisture. The researchers dissolved the highly soluble water-soluble lithium salt into the water of the hydrogel.
Previous studies have shown that such an aqueous solution of a very high concentration of a lithium salt can be used to make the electrolyte of a lithium ion battery. The aqueous electrolyte has a distinct advantage in comparison with a flammable, explosive, highly toxic commercial electrolyte. It does not react with water and oxygen in the air and does not produce hydrofluoric acid.
This means that the batteries made of this electrolyte are not only non-flammable and non-toxic in the process of use, but also can be assembled directly in the air. Traditional lithium batteries must be filled with argon in a special environment to ensure safe production. Coupled with the excellent mechanical properties of hydrogels, they developed a new type of electrolyte that can be easily stretched, twisted and safely.
Fig. Schematic diagram of collapsible battery structure (source: ETH Zurich)
With electrolytes, fluid harvesting for both positive and negative materials needs to be addressed. As mentioned earlier, commercial electrodes using copper and aluminium foils can not be bent, but if the powders of electrodes are combined with materials that can be deformed and have good conductivity, can the electrodes also be deformed?
The difficulty of this matter is that for materials, elastic deformation and good conductivity are often a pair of contradictory needs. Elastic rubber is a good insulator, conductive metal, and even conductive plastic, are not elastic. Through continuous experiments, the researchers designed a special structure composed of four kinds of materials, and achieved the above two goals at the same time.
The first material is a thin film made of elastic polymers, which serves as a base for collecting fluids. The second and third materials are carbon nanotubes and carbon black particles dispersed in the base. These conductive fillers give the substrate a certain degree of conductivity. To solve the problem of conductivity thoroughly, the key is the fourth material - a layer of silver glue deposited on the base surface.
From a microscopic point of view, the metal silver in this structure is a stack of hexagonal two-dimensional sheet structure. Some of them are fixed on the substrate, others are mounted on other silver chips like tiles, and they come together to form a good electronic path. When the collecting fluid is stretched, there will be a relative slip between the silver sheets, but it can still ensure that the "hands and feet" of this piece can continue to rest on the "body" of the other piece, so that the passage of the current is not affected. Even in a few local places, there is a complete separation between silver sheets, and carbon nanotubes and carbon black particles dispersed in the substrate can play the role of conduction current. The researchers found that even if stretched to double the length, the unit surface conductivity of the collector decreased only slightly and performed very well.
The silver glue layer responsible for conducting current on the collector fluid (source: doi: 10.1002/adma. 201904648)
Finally, they combined the positive and negative electrode materials with the collecting fluid by spraying deposition, and used a structure similar to the photo frame, which sealed the collecting fluid and electrolyte together and made the finished product of the collapsible battery.
After 50 cycles of charging and discharging, the battery still has 28 mAh g_1 reusable power and 20 Wh kg_1 energy density, which indicates that the battery still has reliable repeatable charging and discharging performance under extreme mechanical pressure.
For electronics designers, this means that for the first time, they will be able to have the idea that, in addition to screens and circuits, batteries can bend along with them! And this is likely to lead to a new series of electronic products.
Needless to say, the foldable mobile phone mentioned above will probably change its shape in the future.
The battery can also be installed on tablets and smartwatches. You can imagine that in the future, the smartwatch will completely become a flexible "smart wristband", and the battery does not have to be installed under the dial, but can be hidden in the strap.
All kinds of wearable equipment for medical and sports training can finally be perfect "close to the body". Even your jacket and schoolbag itself may be a elegant and safe battery fabric.
In the unsold convenience store, the deformable battery can also become a part of the active electronic label affixed to the commodity, in the whole process of the product from production, processing to sales, and even express delivery to the door, it provides continuous power supply for traceability, price marking and positioning chip.
After extreme stretching, the battery can still maintain excellent performance (source: Chen Xi)
However, Chen Xi said he was only the first step in achieving a collapsible battery. The stretch battery technology still has a long way to go before it can be commercialized on a large scale.
Firstly, the performance of this battery is not comparable to that of mature commercial batteries, and the energy density needs to be significantly increased.
Secondly, it is necessary to optimize the charge-discharge control and improve the packaging process to prolong its cycle life.
Third, reduce the cost of batteries. As the world's first bent full-cell, some of the materials they use are still expensive and difficult to compete commercially with mainstream batteries.
Chen Xi's research team at the Federal Institute of Technology in Zurich has begun further research on these issues. It may not be long before you accidentally sit on the tablet you put on the sofa and never feel heartache again.
Chen X, Huang H, Pan L, Liu T, Niederberger M. Fully Integrated Design of a Stretchable Solid‐State Lithium‐Ion Full Battery. Adv.Mater.2019, first published: 06 September 2019 doi: 10.1002/adma.201904648