Wearing a piece of everyday clothes to go out, it can wirelessly charge our mobile phones, and it can also power smart medical fabrics in the future…The wearable devices that are hot in the frontier of science and technology have the possibility of further landing-research from China The team provides a new and effective path for the “heart” of wearable devices, that is, energy supply.
At 23:00 on September 1st, Beijing time, the top academic journal “Nature” published online a latest study by the Fudan University team, entitled “Scalable production of high-performance fiber lithium-ion batteries” (“Scalable production of high -performing woven lithium-ion fibre batteries”), the research has discovered the correlation between the internal resistance and length of fiber lithium-ion batteries (FLIBs), effectively solving the problem of active material and fiber electrode interface stability, and continuously constructing both A new type of fiber polymer lithium ion battery with high safety and high performance.
Their research results show that when the length is 1 meter, the fiber lithium-ion battery has a capacity of 25mAh, which can provide more than 2 days of power for commercial wearable devices such as heart rate monitors and oximeters. The energy density based on the overall mass exceeds 85 Wh/kg. At the same time, the fiber lithium ion battery has good cycle stability. After 500 cycles, the battery capacity retention rate still reaches 90.5%, and the coulombic efficiency is 99.8%. Even with a radius of curvature of 1 cm, the capacity retention rate of the fiber lithium-ion battery is still greater than 80% after bending 100,000 times.
The research team can further obtain high-performance and high-safety large-area battery fabrics through textile methods. If the battery fabric and the wireless charging transmitter are integrated, the smartphone can be wirelessly charged safely and stably; by integrating the fiber lithium ion battery and fiber sensor with the display fabric, the concentration of sodium and calcium ions in human sweat can also be achieved. Real-time monitoring and signal transmission and display.
The reviewers commented that this work is “a milestone research in the field of energy storage and wearable technology” (“landmark research not only in energy storage but also in wearable technology”) and “a milestone in the field of flexible electronics” (“a milestone research in the field of flexible electronics”). towards the prevalence of flexible electronics”).
Doctoral students He Jiqing and Lu Chenhao from the Department of Polymer Science of Fudan University are the co-first authors of the paper. The research was supported by projects such as the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Shanghai Municipal Science and Technology Commission.
The “heart” of wearable devices, began to explore more than a decade ago
“In the past ten years or so, people hope that fabrics are no longer simple and have certain single functions, but they are still smart.” In March this year, Peng Huisheng said when introducing the smart fabrics he has studied for many years to The Paper.
At that time, the team led by him published another blockbuster study in Nature. They used a 1.5-square-meter “cloth” to show the outside world. They could effectively integrate the preparation of the display device and the fabric weaving process. The interweaving points of polymer composite fibers integrate multifunctional micro light-emitting devices to realize large-area flexible display fabrics and intelligent integrated systems.
Such smart fabrics have the advantages of being smart, soft, adaptable to complex deformations, breathable and moisture conductive, and are an important development direction in the future wearable and other fields. However, in order to be widely used, one of the prerequisites also includes obtaining a high-performance and safe flexible battery.
Fiber lithium-ion batteries with diameters ranging from tens of microns to hundreds of microns are currently one of the mainstream directions. “As early as 2006, I began to hear that some people made lithium-ion batteries into thin films to obtain flexible energy systems. This direction is very important for the future development of human society and I am very interested.”
However, Peng Huisheng is a person who is willing to bite the “hard bones”, and he doesn’t really want to do what others have already done. “At that time, I kept thinking about this field, whether I could be something that no one has ever done or even thought about.” After returning to Fudan University in 2008, the previous hazy pursuit suddenly became concrete. Peng Huisheng thought: If lithium-ion batteries are made into fibers, it will be fun. More than a decade ago, Peng Huisheng didn’t even consider practicality at all.
The exploration process is long. Peng Huisheng mentioned that from 2008 to 2013, the world’s first fiber lithium ion battery was realized, and it was further expanded to fiber lithium sulfur battery, fiber zinc ion battery, fiber metal air battery and so on. However, after several generations of doctoral students and post-doctoral students in engineering research, progress has not been much.
He Jiqing said in an interview with The Paper (www.thepaper.cn) that the preparation of fiber lithium-ion batteries mainly faces two difficulties. “First, the internal resistance of a fiber lithium battery has an important impact on its electrochemical performance, but the relationship between its internal resistance and fiber length is still not very clear; second, because of the completely different device structure, it faces the bulk The electrode preparation and device construction methods of lithium-ion batteries are difficult to apply to fiber lithium batteries, and the continuous preparation of fiber lithium batteries is almost blank in the world.”
So far, the length of fiber lithium-ion batteries publicly reported is usually on the centimeter level, and the energy density based on the overall battery mass is also relatively low. In addition, the research team also mentioned in the paper that this short-fiber lithium-ion battery is difficult to use on a large scale in reality. This is because a large number of wire connection points can easily cause water and oxygen intrusion, electrolyte leakage and external force damage, resulting in battery performance. Performance degradation or even failure.
Large-scale production of long-fiber lithium-ion batteries while maintaining high performance remains an unfulfilled demand.
How does the internal resistance of the fiber battery affect the performance? “Outsiders” want to give it a try
The breakthrough contribution made by Peng Huisheng’s team to the field this time firstly benefited from an unexpected discovery.
He Jiqing was the first person to complete this research, but when he first joined Peng Huisheng’s team, he was more like an “outside man” and had not been exposed to battery-related research before.
After graduating with a master’s degree in chemistry in 2012, He Jiqing did not directly continue her Ph.D. In the next 5 years, he worked in two well-known multinational chemical giants one after another. This was considered by his mentor Peng Huisheng after returning to campus as one of his advantages. “He was very able to focus on problem-solving, collected information extensively and thought seriously and independently, and made this discovery. He used to work in industry and was familiar with R&D. Of course, we have also accumulated for a long time, so the design and development of the production line are very efficient.”
At the end of 2018, after He Jiqing gradually integrated into the research team, based on the research work of the previous graduated members, he changed the research ideas in time, faced practical applications, and worked closely with team members, and carried out a large number of materials screening, design and device construction methods. Experimental exploration.
In a preliminary attempt, He Jiqing and others manually twisted the positive electrode and the separator-wrapped negative electrode designed in the early stage to prepare fiber lithium-ion batteries with lengths of 0.1 m, 0.2 m, 0.5 m and 1 m respectively, and measured them. The electrochemical performance after adding electrolyte.
Surprisingly, the research team observed that the internal resistance of the longer fibers did not increase but decreased. This discovery exceeded their expectations. “Mr. Peng saw the result at the time, but at the same time he was very rigorous. He asked us to repeat it first. If there is no problem with repeatability, we will continue to see what to do next.” He Jiqing mentioned arrive.
Finally, they extended the length to 10 meters. The paper shows that when the fiber length is wider, from 0.01 meters to 10 meters, the internal resistance will also decrease. These results mean that it is possible to obtain high-performance long-fiber lithium-ion batteries.
Peng Huisheng has high requirements for scientific research. He hopes that his years of research results can be industrialized in the future, so he is extremely rigorous on the repeatability of his work. At the same time, he still hopes that there will be sufficient theoretical support behind the feasibility of the matter, and he knows the reason.
In the following nearly one year, the research team conducted a large number of comparative experiments, and finally concluded the law of the internal resistance of the fiber battery with the length, which provided theoretical support for the continuous construction of long-fiber lithium-ion batteries.
The internal resistance decreases with the increase of fiber length.
High-efficiency load of active materials and continuous construction of batteries
After the high-performance long-fiber lithium-ion battery is theoretically feasible, the next step of the research team is to achieve large-scale preparation at the method level.
One of the first challenges is to efficiently load a uniform active material coating on a long-fiber current collector with a micron diameter. “The first core point in the production of batteries is how to coat the positive and negative electrode slurry on the battery current collector.” He Jiqing further explained, “Generally speaking, most of the current commercial batteries are on flat substrates. Paint, this process is relatively simple, it can be applied very uniformly, and its thickness is also easy to control.”
It is not easy to do similar work on fibers. The paper mentioned that the active material coating on the curved fiber surface is often uneven. The reason for this phenomenon is that during the loading process of the active layer, the curved structure makes the active material bear greater surface tension. He Jiqing vividly stated, “This makes it easy to produce uneven beaded structures,” and this uneven active layer is very detrimental to battery performance and stability.
He Jiqing introduced that the team achieved uniform, stable and high-content active material loading of the fiber electrode mainly through the targeted design of the load device and the systematic regulation of the chemical composition of the active layer.
The research results show that on the electrode with a length of 100 meters, the load weight of the active material hardly changes, and there is no obvious shedding after tens of thousands of bends. Subsequently, the research team successively carried out methodological research such as continuous battery assembly and packaging, and finally realized the continuous and stable preparation of high-performance fiber lithium-ion batteries.
Continuous preparation and structural characterization of long-fiber lithium-ion batteries.
What is missing to realize the production of fiber lithium-ion batteries?
The final results of this study show that the capacity of fiber lithium-ion batteries increases linearly with length. Based on the total mass, the energy density of a 1-meter fiber lithium-ion battery exceeds 80 Wh/kg, which can provide more than 2 days of power for commercial wearable devices such as heart rate monitors and oximeters.
At the same time, the fiber lithium ion battery has good cycle stability. After 500 cycles, the battery capacity retention rate still reaches 90.5%, and the coulombic efficiency is 99.8%. Its comprehensive electrochemical performance is comparable to some small commercial batteries.
Electrochemical performance of fiber lithium ion battery.
This fiber lithium ion battery also has good mechanical properties and can be easily woven into a flexible and breathable fabric.
However, before being further applied as a fabric to wearable devices, we also need to confirm its safety, which is particularly important for lithium-ion batteries. The research of the paper shows that the fiber battery fabric can work normally under severe conditions such as bending, dynamic deformation, high and low temperature (-20℃ to 60℃), puncture and washing, showing good application potential.
The safety verification test showed that, for example, even if the fiber lithium-ion battery textiles were folded in various ways or crushed by cars, they did not burn or explode. In addition, even after the machine is washed or pierced, it can continue to charge the tablet. The research team also used an infrared imager to monitor the temperature around the puncture area and found no signs of temperature rise.
a-e) Obtain high-performance and high-safety fiber lithium-ion battery fabric through textile methods; f-h) Fiber lithium-ion battery fabric for wireless charging of smart phones.
The research team further obtained high-performance and high-safety large-area battery fabrics through textile methods, and performed some functional demonstrations in real application scenarios. They integrated the battery fabric and the wireless charging transmitter on a piece of clothing, which can safely and stably wirelessly charge the smartphone placed in the wearer’s pocket. The whole process lasted 40 minutes, no obvious heating phenomenon occurred, showing good safety.
In addition, the research team integrated fiber lithium-ion batteries and fiber sensors with the display fabric. When the user exercises, the fiber sensor detects the concentration of sodium and calcium ions in the sweat and sends the data to the signal processing chip, which can transmit the information to the textile display. This provides the possibility for future smart medical applications.
In the view of the research team, there is still work to be improved in its follow-up. “As a battery, what everyone cares most about is energy density. Although we can now manufacture fiber batteries on a large scale and continuously, and the energy density is also very high, compared with the existing mobile phone batteries and electric vehicle batteries, its There is still a certain gap in energy density.” Their main next step is to further increase the energy density of fiber lithium-ion batteries.
In addition, the fiber lithium-ion battery is an energy storage system, and its ultimate mission is to supply power in practical applications. “Another key task we will do next is to narrow the distance between it and the real product, or even eliminate it completely.”
Peng Huisheng also told The Paper that the current scientific community is committed to achieving higher energy density and high safety, and is also developing continuous preparation methods. The industry is committed to how to use various fiber batteries, including developing low-cost and high-efficiency production lines, establishing industry standards, developing integrated application methods, and so on.
“Judging from the current performance and engineering level of fiber lithium-ion batteries, it is expected to achieve large-scale production and application in 3-5 years. If resources are concentrated and efficiently used, it may be achieved in 2-3 years.” Peng Huisheng emphasized, To realize the production of fiber lithium-ion batteries, several problems still need to be solved urgently.
First of all, in terms of resources, sufficient funds, technical experts with rich battery research and development experience, and a certain amount of space support are needed. Secondly, 3-5 main application directions need to be determined, and then mainstream companies in these directions put forward application requirements, and conduct subsequent R&D and production around real application requirements. Finally, in terms of specific technologies, it is necessary to further develop standard continuous production lines and production processes, further provide performance, develop flexible packaging technology, and obtain high-efficiency integration methods to meet actual application requirements.