What is a Flexible Battery?

 

Flexible batteries refer to batteries that can be folded and twisted at will, including primary and secondary batteries. Unlike traditional rigid batteries, their design is conformal and flexible. They can maintain their characteristic shape even when continuously bent or twisted. It is to turn the traditional liquid electrolyte into a solid-state, and “print” the internal structure of the traditional lithium-ion battery on the flexible substrate so that the battery will not be “powered off” even if it is bent or folded, thereby ensuring the battery It can work normally even after bending or folding.

Demand for flexible batteries

We are used to thinking of batteries as bulky tools that can store energy and power electronic devices. For a long time, disposable carbon-zinc batteries, rechargeable lead-acid batteries, and nickel-cadmium batteries have been dominant.

Figure 1: Market descriptions by territory      Source: IDTechEx

With the emergence of portable devices such as netbooks, ultrabooks, and other handheld devices, the battery market has seen explosive growth of various types, among which the most popular is lithium-ion rechargeable batteries. However, as electronic products become thinner and more flexible, batteries must now get rid of their rigid form and adapt to their bending. Therefore, the thin-film flexible battery market has also followed.

Market observer IDTechEx predicts in their new report that by 2026, the current small thin-film battery market will reach US$470 million. According to He Xiaoxi, a technical analyst at IDTechEx, this is the reason why companies such as TDK, STMicroelectronics, LG, Samsung, and Apple are increasingly involved. Considering the Internet of Things, the deployment of wearable devices and other environmental sensors is getting faster and faster, and it is imperative to replace traditional battery technology. New dimensions and designs are urgently needed. For example, Samsung has a curved battery in the Gear Fit wristband.

Flexible battery manufacturers

The GREPOW battery manufacturer has more than 20 years of battery manufacturing experience. Special-shaped battery technology is mature. The advantages of the special-shaped battery are their adaptability, lightweight and portability, which makes them easy to be used in products such as small and wearable electronic devices. achieve. Therefore, GREPOW is working hard to manufacture different shaped batteries, including rechargeable batteries with high energy density and good shape, and is in a leading position in the industry. Here are two types of batteries related to flexible batteries: curved batteries, Thickness: 1.6 mm ~ 4.5 mm; Width: 6.0 mm ~ 50 mm; Inner arc length: 20 mm ~ 55 mm; Inner arc radius: ≥8.5 mm.

Figure 2: curved battery      Source: GREPOW battery

Another GREPOW special-shaped battery: ultra-thin battery, Charge the battery to 3.83v and fix the battery to the surface of the white PVC card. Fix the cell pole card to the bending and torsion tester, 15 degrees forward and backward, and 30 degrees total distortion, for bending and torsion test. After the bending and torsion test of the 0.45mm ultra-thin cell for 9000 times, the surface of the cell was folded and the internal pole sheet had creases. The internal resistance increased by about 45%. The voltage before and after the bending and torsion basically remained unchanged.

Figure 3: ultra-thin battery      Source: GREPOW battery

STMicroelectronics (ST) is producing thin-film solid-state lithium batteries in small quantities. The report said that two other companies are producing printed batteries. Therefore, there are now various flexible batteries on the market competing for power to power several devices.

Other companies are also trying other strategies. For example, TDK is developing battery-less energy harvesters. The idea is because IoT nodes and wearable devices require extremely low power to operate, so they can be operated by energy harvesters instead of batteries. Other companies such as Oakridge Global Energy Solutions Inc. plan to increase production capacity at their Brevard County, Florida plant. They will manufacture electrodes and batteries for thin-film solid-state lithium batteries. They acquired this technology from Oak Ridge Micro-Energy Inc. in 2002 and plan to start mass production in early 2017.

Types and applications of flexible batteries

Various flexible batteries will soon be on the market. These will include thin-film batteries, printed batteries, layered lithium polymer batteries, micro-batteries, advanced lithium-ion batteries, thin flexible supercapacitors, and stretchable batteries.

Understandably, they will have multiple uses. For example, wearable devices are expected to become the greatest potential for flexible batteries. Printed batteries in the form of skin patches have been used in the healthcare industry, and the market is growing steadily. At present, although the high cost of printed zinc batteries hinders its widespread application, this application has the greatest potential. According to the IDTechEx report, the market for micro-power batteries that power disposable medical devices will expand rapidly.

Figure 4: Applications of batteries with new form and structural factors     Source: IDTechEx

There are other requirements for batteries that power various types of power sources, displays, and flexible sensors. The U.S. Department of Defense has invested $75 million to create the Flexible Hybrid Electronics Manufacturing Institute in San Jose.

The promotion of flexible batteries and flexible electronics is of great significance. Based on the demand for electronic equipment for batteries, the promotion of flexible battery technology and the cooperation of flexible display, biosensor, and flexible circuit technologies will help to develop more flexible electronic devices for medical health monitoring, smart textiles, smartphones, and global It is applied in multiple scenarios such as positioning system tracking, Internet of Things, and human-computer interaction.

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Flexible Metal-air Battery’s Research Progress

 

With the development of flexible and wearable electronic devices, flexible energy storage devices have attracted extensive attention from the scientific and industrial communities. In recent years, a series of works have been carried out around flexible lithium-ion batteries and supercapacitors, and important progress has been made.

However, in order to extend the life of electronic devices, energy storage systems need to have a higher energy density. Flexible metal-air batteries that can be cyclically charged and discharged will greatly improve the endurance of flexible and wearable electronic devices, but more challenges need to be addressed.

On the one hand, the energy density, energy efficiency, and cycle life of the battery need to be improved; on the other hand, the electrode structure, electrolyte materials, and battery structure need to be optimized to maintain stable electrochemical performance under deformation conditions.

This article will introduce the recent advances in the structural design, electrode and electrolyte material development, and operating condition management of flexible metal-air batteries, mainly zinc-air and lithium-air batteries, and discusses future research directions and prospects.

Metal-air batteries have attracted extensive research attention due to their high theoretical capacity and energy density as shown in Figure 1. Among them, rechargeable zinc-air batteries in alkaline systems and lithium-air batteries in organic systems as typical representatives of aqueous and non-aqueous systems are the hotspots of recent research.

The working principle of the battery is shown in Figure 2. When the battery is prepared to be flexible, it is necessary to design new flexible structures, prepare flexible electrode materials, and solid electrolyte membranes to face more challenges.

Fig. 1 Comparison of capacity, energy density, and voltage of different metal-air batteries

 

Schematic diagram of the working principle of zinc-air battery in an alkaline system and lithium-air battery in a non-aqueous system.

Flexible battery structure and testing

The current cell structure widely used in flexible zinc-air and lithium-air batteries is a sandwich structure consisting of a flexible positive electrode, an electrolyte membrane, and a negative electrode stacked on top of each other. Another structure is a tubular one that uses a wire-shaped metal electrode with an electrolyte layer and an air electrode layer wrapped around the surface in turn. In addition, there are some new structures, such as a foldable battery structure and a flexible, ultra-light lithium-air battery inspired by bamboo sticks.

In addition to the charge/discharge and cycle life tests in conventional batteries, stability under external forces is of paramount importance for testing flexible batteries. These include electrochemical stability under bending, twisting at different angles and stretching at different lengths, as well as retention of performance under long-term fatigue.

Metal electrode

In flexible air batteries, metal sheets are often used directly as electrodes. However, the metal sheet may suffer from fatigue failure in long-term bending. In flexible zinc-air batteries, there is a combination of metal powder and binder and conductive carbon powder to form a composite electrode, which improves the flexibility and stability of the electrode. In flexible lithium-empty batteries, lithium metal and stainless steel mesh are rolled together to improve the fatigue resistance of the metal electrode.

In addition, in order to achieve a certain stretchability in a flexible battery, the metal electrodes can be made into a spring-like material or a combination of small pieces and a complete electrode to meet the need for stretching through the “integer into zero”.

Electrolyte film

In flexible zinc-air batteries, anion exchange membranes and alkaline gel electrolytes are mainly used as the electrolyte membranes of the batteries. In flexible lithium-air batteries, the electrolyte membranes mainly include gel, solid and composite polymer electrolyte membranes. In order to achieve the good electrochemical performance of flexible batteries, the electrolyte membrane is required to have good conductivity, chemical, and electrochemical stability and other properties of traditional liquid electrolytes, in addition to the interfacial problem with metal and air electrodes is a challenge that needs to be solved.

For the electrolyte membrane-metal electrode interface, the problems of dendrite and surface passivation need to be overcome. For the electrolyte membrane-air electrode interface, the solid electrolyte greatly reduces the effective reaction interface. For lithium-air batteries, the degradation of the reaction area is further exacerbated by the fact that the product is solid lithium peroxide. Thus, effective methods are needed to increase the reaction interface.

In addition, during the bending or twisting of the battery, the electrode and electrolyte membrane may separate due to differences in the mechanical properties of the electrode and electrolyte membranes. How to maintain the stability of the interface is the key to ensure the long-term stable operation of the battery.

Air electrode

Air electrodes, as an important component of metal-air batteries, have been the focus of research. On the one hand, an effective catalyst is needed to achieve rapid charging and discharging of the battery; on the other hand, a suitable structure is needed to ensure oxygen transport. In flexible batteries, it is even more necessary for the electrode to have good flexibility to meet the needs of deformation.

Currently, the main flexible electrodes include：

1. Electrodes consisting of a network of carbon cloth or carbon fiber.
2. e.g. carbon nanotube paper, graphite paper electrodes made of carbon nanotube materials (e.g. carbon nanotubes, graphene).
3. Electrodes formed from metal substrates such as stainless steel mesh, nickel mesh.
4. Some other new flexible electrodes.

operation management

Typically, zinc-air batteries operate directly in the air, while lithium-air batteries operate in oxygen. The operating conditions can seriously affect the performance of the battery. First of all, moisture in the air will affect the stability of the electrolyte film, while carbon dioxide in the air has a greater impact: carbonate will be formed in zinc-air batteries, affecting the conductivity of the electrolyte; and lithium carbonate will be formed as a solid by-product in lithium-air batteries, affecting the charging performance of the battery. Secondly, battery performance is usually tested at room temperature, while the actual use of the temperature has a large variation. For example, in wearable devices, the operating temperature of the battery may rise to thirty degrees or more due to contact with the human body. In different seasons and regions, the temperature will vary even more. Therefore, future battery testing will need to look in more detail at stability under different gas atmospheres and temperatures, and adopt appropriate management measures.

Future developments of flexible batteries

In recent years, a number of advances have been made in flexible metal-air batteries, with substantial hints of the energy density, efficiency, and cycle life of the batteries. Future research needs to further address the following issues.

1. New structural design of the battery, which meets the requirement of maintaining stable electrochemical performance under various deformation conditions.
2. The establishment of evaluation criteria to standardize the assessment of battery performance, such as the specification of accepted flexibility test standards (e.g., bending and twisting angles, tensile lengths, fatigue tests, etc.) based on uniform mass or volume.
3. development of flexible components, including metal and air electrodes, electrolyte membranes, collectors, and encapsulation materials.

Fourth, the management of the operating conditions will ensure that stable electrochemical performance is provided under different conditions.

In conclusion, future research needs to use a combination of experimental online monitoring and numerical simulation and other technical means to clearly elucidate the relationship between material transport, structural changes, and electrochemical reactions during battery operation to provide important guidance for rational battery design.

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1. Flexible Battery for Wearable Electronics
2. A Review of Flexible Battery Manufacturers
3. Flexible Batteries will Change the Future of Smart Devices
4. A Flexible Battery With a Thickness of Less Than 1 mm has been Developed in Japan
5. Flexible Paper Battery Offers Future Power
6. Highly Flexible High-energy Textile Lithium Battery for Wearable Electronics
7. Flexible Batteries Enable More Space in Foldable Phones

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