The emergence of flexible electronic devices has redefined the landscape of the global portable electronics industry. From healthcare to solar power to wearable consumer electronics, flexible electronic devices are can truly change how electronics are both manufactured and deployed..
Yet to maximize the defining characteristic of flexible circuits, it needs an equally flexible power source. In order to be incorporated in a flexible electronic device, the power source has to maintain stability and exhibit high performance under mechanical deformation. Flexible lithium-ion batteries (FLIBs) are designed to withstand folding, bending, stretching and other mechanical deformations while performing the same function as a traditional battery.
Flexible components for FLIBs
How do engineers design a flexible battery? The simplest and the most obvious way is to make all of its constituent components flexible. In this particular case, the electrodes and the electrolyte are the two key components of a FLIB that need flexible design.
Electrodes
The material of the current collectors and the binders play a critical role in the design of flexible electrodes. Current collectors act as a conduction bridge and collect the electrons generated as a result of the electrochemical reaction within the battery and deliver them to the external circuit. Traditionally, metals like aluminium and copper are used as current collectors and they are coated with active materials mixed with conductive agents and binders. These materials, however, are not suitable for a flexible design. In the case of FLIBs, carbon-based or other flexible materials are used as current collector substrates onto which the active material is loaded. Carbon materials not only exhibit excellent mechanical characteristics but also have strong electrical conductivity and high thermal and chemical stability. As a result, carbon materials such as graphene, carbon nanotubes and carbon nanofibers are frequently used in FLIBs as self-supporting flexible electrodes, obviating the necessity for metal current collectors.
Furthermore, traditional conductive materials and binders are also replaced with flexible polymers. Polymers are used as binders in traditional batteries as well but, they do not add any significant flexibility to the battery because their amount is usually very low. Moreover, traditionally, binders are insulating material and increasing their amount will have a significant impact on the conductivity of the electrode. An effective solution to this problem is the addition of conductive polymer in such a quantity that flexibility is achieved without compromising the electrical conductivity.
Construction materials for flexible electrodes are not limited to polymers and carbon materials. There are various other materials that can be used as well, depending on the requirements. MXene, for example, is another great choice for designing flexible electrodes. It is a two-dimensional material with excellent electrical and mechanical properties, a large surface area and a low lithium-ion diffusion barrier.
Electrolytes
In addition to the electrodes, flexible electrolyte also plays a critical role in the FLIB design. Liquid electrolytes used in traditional lithium-ion batteries are usually organic and have the desired properties for the good performance of the battery. There are, however, certain safety risks associated with these liquid electrolytes. Moreover, in the case of FLIBs, the use of liquid electrolytes can be even more dangerous as they can leak or flow when the batteries undergo mechanical deformation. Consequently, solid and gel electrolytes are a better choice for FLIBs.
Flexible polymer electrolytes are a good replacement for traditional liquid electrolytes but their use in FLIBs is limited by certain factors including narrow electrochemical windows and low ionic conductivity. It is possible to solve this problem and improve electrochemical stability as well as ionic conductivity by using composite polymers based on the combination of inorganic and organic materials as solid electrolytes or by using gel electrolytes. Gel electrolytes usually comprise a polymer matrix along with a plasticizer and a lithium salt. The polymer matrix provides mechanical strength while the plasticizer improves the ionic conductivity.
FLIB structures
For effective design and practical realization of a high-performance FLIB, it is important to develop flexible battery structures in addition to flexible internal components. Traditional batteries usually have a hard cell structure that is not capable of handling mechanical deformation.
The most common FLIB structures are the fiber type and thin-film structures. Fiber type structure is based on either a co-axial cable arrangement or is composed of two twisted fiber electrodes. FLIBs based on this structure can be easily integrated into wearable electronic devices. The thin-film structure is realized by stacking layers of components on top of each other. The external packaging is made of flexible polymeric material or plastic films. Mass production of this type of structure is very easy and the preparation process is quite simple. This is perhaps the most widely adopted cell structure type for commercially available FLIBs. Paper folding, island connection, wavy and bamboo slip are some other possible structures for FLIBs. Each fabrication method has its advantages and drawbacks but the main overall challenge is the reliability and scalability of the fabrication process. Cell capacity, degree of flexibility, process complexity, durability and versatility are some of the key features that need to be considered when evaluating different fabrication methods.
Challenges
A lot of research effort is being put into the design and development of high-performance reliable FLIBs. There is still a need to develop intrinsically flexible active materials for flexible electrodes. Large-scale adoption of various fabrication methods is limited by certain factors and researchers are continuously trying to develop methods that are both scalable and reliable. The lack of reliable testing methods that examine the performance of FLIBs under real-world mechanical deformations is also hindering progress in this arena. Consequently, FLIB technology is still not mature enough for large scale commercialization for high-capacity applications but it has taken its first steps and the future seems promising.
