The Key to Fully Flexible Electronics: Flexible Lithium Batteries

flexible battery

As Huawei and Samsung released the folding screen phones Mate X and Galaxy Fold respectively, they have a technologically-sense design and excellent hardware configuration. The unfolded area of the new folding screen mobile phone screen can reach more than 7.3 inches, which enhances the user experience. But in fact, a more disruptive, practical, and cost-effective mobile phone design is already on the way to development. It is a fully flexible wearable mobile phone.

Fully flexible mobile phones are dubbed “wrist phones” by the media. They are not limited to the folding of the middle part. They can bend the whole mobile phone freely, which is convenient to wear on the wrist and other parts to achieve better integration with the human body. The current folding screen mobile phones still use ordinary rigid batteries, avoiding the problem of using flexible batteries. If you want to introduce revolutionary fully flexible electronic devices, you must develop corresponding flexible power supplies and implant them. Therefore, the development of flexible lithium batteries with high energy density will be of great significance to promote the development of wearable flexible electronic devices.

The ideal flexible battery should have high flexibility, energy density, and power density at the same time. However, these factors often hinder each other in the flexible battery. In this review, this article made a detailed analysis based on the structural design of the battery components and the overall device level, and reviewed the latest developments in flexible lithium batteries, and summarized the current academic development ideas into the following four strategies:

1) Development of a deformable battery module with a porous structure

Such as porous current collectors, porous electrodes, flexible solid electrolytes, etc. Flexible porous structures are currently widely used in battery modules to cushion the strain generated when the battery device is subjected to bending and twisting.

1) Development of a deformable battery module with a porous structure

a) A conductive porous film of graphene oxide, which has a conductivity as high as 3112 S/cm. The flexible lithium battery assembled with this film as the current collector did not find a significant decrease in capacity after 100 cycles of high charge-discharge rate (5C).

b) A composite cathode material of single-walled carbon nanotubes and polymer (2,5-dihydroxy-1,1-benzoquinone sulfide) is used to assemble flexible lithium batteries. The flexible battery exhibits a specific discharge capacity of 182 mAh/g at low current (50 mA/g), and can still reach a specific capacity of 75 mAh/g when discharged at a large current (5000 mA/g).

c) Using bacterial cellulose as a template, develop a solid electrolyte composite of Li7La3Zr2O12 (LLZO) and polyethylene oxide. The porous interconnected polymer matrix is ​​used as a mechanical carrier that is soft and strong, and the LLZO particles used to transport Li-ions are embedded in it. The overall display shows a high ionic conductivity of 1.12×10-4 S/cm and excellent mechanical flexibility.

2) Ultra-thin battery design

Such as single-pair (or double-pair) positive/separator/negative structure. Compared with strategy 1, strategy 2 (ultra-thin battery design) requires battery design from the overall device level.

Ultra-thin battery design

a) The ultra-thin flexible lithium battery with a thickness of only 0.4mm released by GREPOW Battery Company can be applied to various wearable devices. Even after bending with a bending radius of 25 mm, or twisting to an angle of ±25 degrees for more than 1,000 times, this flexible battery can still maintain 99% of the capacity.

GREPOW ultra-thin flexible lithium battery

b) A Li4Ti5O12/LiPON/Li thin-film solid-state battery prepared based on the flame spray pyrolysis method with flexible polyimide as the supporting substrate. After charging and discharging at a rate of 1C and cycling in a flat, bent, and flatform for 90 times, the battery’s discharge capacity retention rate is still as high as 98%, showing excellent cycle performance.

3) Geometric topology battery design

Such as linear structure, Origami, Kirigami structure, etc. In addition to improving the flexibility of the battery component material itself, the battery structure designed by the principle of geometric topology can reduce the stress change generated in the battery during the deformation process.

Geometric topology battery design

a. This strategy was first reported in the work of its linear battery. The flexible battery can not only adapt to bending deformation, but also more complex shape changes, such as folding and twisting.

b. Using spring-like LiCoO2/reduced graphene oxide as the positive electrode material, combined with gel electrolyte, a self-healing flexible lithium battery was designed and assembled. Under complex deformation (bending and torsion), charging and discharging at a current density of 1 A/g, the battery can still maintain a discharge specific capacity of 82.6 mAh/g; even if the battery is cut and healed five times, the battery can still be The specific discharge capacity is 50.1 mAh/g.

c. In addition to linear structures, paper folding technology is also widely used in flexible batteries. With the Origami origami solution, a two-dimensional sheet material can be folded along a predetermined crease to create a compact and deformable three-dimensional structure that can withstand high-strength deformation.

d. Soon after, a Kirigami scheme was developed combining folding and cutting technology. After 100 charge-discharge cycles, the battery can achieve a capacity retention rate of more than 85% and a coulombic efficiency of 8%. After 3,000 battery deformations, the maximum output power of the battery has not been significantly reduced.

4) Decouple the flexible and energy storage parts of the battery

Such as spine battery, Zigzag battery, etc. For the above-mentioned flexible battery design, dislocation, peeling, and shedding between the active material and the current collector still occur during the complex deformation process. The increased overpotential and internal resistance of the battery due to poor contact will reduce the capacity retention rate and coulomb efficiency of the full battery, which is not conducive to the cycle performance of the battery. The potential solution is to redesign the battery architecture to separate energy storage and provide flexibility.

Decouple the flexible and energy storage parts of the battery

a. De Volder et al. demonstrated a layered tapered carbon nanotube structure, similar to the morning glory of a plant. The wide corolla is used to carry the positive and negative active material particles, and the slender stem part and the current collector are below. The parts are tightly combined. During the deformation of the battery, most of the stress is applied to the current collector itself, and the tapered structure hardly produces strain during this period, thus exhibiting extremely high flexibility. The Fe2O3/LiNi8Co0.2O2 full battery assembled with this tapered structure can be charged and discharged 500 times at a rate of 1C and still has a capacity retention rate of 88%.

b. Inspired by the good mechanical strength and flexibility of the animal spine, a method for large-scale production of high-energy-density flexible lithium-ion batteries: store energy by surrounding thick, rigid parts in the axial direction (corresponding to the spine), And the thin, non-circular flexible part (corresponding to the bone marrow and intervertebral disc) is used to connect the “spine”, thereby achieving good flexibility and high energy density of the entire device. Since the volume of the rigid electrode part is much larger than the flexible connection part, occupying more than 95% of the volume of the battery cell, the energy density of the whole battery can reach 242Wh/L. The reasonable bionic design makes it pass the strong dynamic mechanical load test.

Questions and answers from flexible lithium battery experts

Xie Ming, a well-known flexible lithium battery expert in the industry, accepted an exclusive interview from the material man. The following is the interview content.

Q: Based on the actual situation of current industrial production, please comment on the four flexible lithium battery development strategies summarized in the review.

A:  1) Development of flexible battery elements with porous structure carbon (nanotube) paper is used as a flexible current collector, the cost is relatively high, which is not easily accepted by manufacturers; secondly, when carbon paper is used as a negative current collector, its side reactions are very obvious. If a porous metal current collector (such as copper mesh, aluminum mesh, etc.) is used, its flexibility can meet actual needs, but during the coating process, the slurry easily penetrates from the mesh pores. The current development of related coating processes is still An important challenge facing the corporate world.

2) Ultra-thin battery design

In order to achieve stable mechanical flexibility and electrochemical performance, ultra-thin batteries mostly use single-pair (or double-pair) low-capacity (usually less than 60 mAh) design, so their application scenarios and markets are very limited. In order to make the battery thinner, GREPOW has introduced a mature technology ultra-thin battery. While meeting the actual requirements, the overall battery thickness can be less than 8mm, and the thinnest can reach 0.4mm. You can imagine this is like A battery as thin as paper.

3) Geometric topology battery design

The concept proposed by this strategy is very good, represented by the linear battery, its mechanical flexibility, and electrochemical performance can be guaranteed. At present, many research teams are committed to developing new types of fibrous and linear batteries to solve traditional bottlenecks. The fly in the ointment is that the positive electrode, negative electrode, and separator of this linear battery rely on self-synthesis, which is different from commercial battery components, which will increase production costs. In addition, most linear batteries use heat-shrinkable tubes instead of aluminum-plastic film packaging. The heat-shrinkable tube materials have a limited barrier to water vapor and oxygen, and it is difficult to meet actual needs in long-term use.

4) Decouple the flexible and energy storage parts of the battery

This strategy uses improved commercial battery components, which is of great significance to promote the development and production of flexible lithium batteries. As early as 14 years or so, some Chinese companies have begun to develop “bamboo-shaped” batteries that look similar to “spine” batteries. According to the latest literature report on “Zigzag” batteries, the energy density of flexible lithium batteries assembled with the “decoupling” strategy can reach 275 Wh/L. After the process optimization of industrial standards, the energy density can still be achieved. Room for improvement. At present, an MIT research group has developed a series of fully automatic and personalized battery pole piece winding equipment. It is believed that with the intervention of an intelligent manufacturing system (IMS) service providers, the shortcomings of this type of battery assembly process can be gradually overcome.

Q: Could you please introduce the application scenarios of flexible batteries in the future?

A: At present, flexible wearable devices are the largest application market for flexible batteries. Taking smartwatches as an example, many large consumer electronics manufacturers have proposed the idea of ​​implanting flexible batteries into smartwatch straps, removing the battery from the panel, and realizing an ultra-light and ultra-thin dial design. They hope that the capacity of the flexible battery can be close to 500 mAh, but the volume energy density of the more mature flexible lithium battery samples in the industry is about 300-400 Wh/L, which is temporarily difficult to achieve the above goal. In addition, in order to introduce the current in the watchband to the dial, it is necessary to design the circuit in the watchband, which will be a considerable investment in development.

In addition, fully flexible mobile phones will be an important application scenario for flexible lithium batteries in the future. A few days ago, Samsung released a new foldable mobile phone Galaxy Fold, but this phone still uses ordinary rigid batteries, avoiding the problem of using flexible batteries. If you want to launch a revolutionary fully flexible mobile phone, you must develop a high-capacity (more than 2000 mAh) flexible battery implanted in it. As mentioned in this review, energy density and mechanical flexibility are usually in a flexible battery. The combination of scientific experiments and theoretical simulation is used to deeply study the basic mechanical problems in flexible batteries. Development work is very helpful.

Q: Please talk about the core competitiveness of flexible batteries from the perspective of industrial applications.

A: Compared with traditional non-bendable batteries, the volumetric energy density of flexible batteries will definitely be affected. Therefore, flexible batteries must find their own application positioning, that is, place batteries in places where batteries could not be placed before, make full use of every effective space of electronic devices, and increase the standby time by increasing the overall battery capacity of electronic devices. This is another strategy to solve consumers’ anxiety about mobile phone standby when the energy density of lithium-ion battery materials has not been greatly developed.

In short, flexible batteries do have a broad potential application market. In recent years, well-known international companies such as Apple and Samsung have launched patent arrangements in related fields. However, most downstream electronic equipment manufacturers still expect to start the development of a fully flexible series of products after the flexible battery production technology has matured.

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