No extra steps required, just stick it on and it will charge

Produced by: Science Popularization China

Author: Yiyan Science Team

Producer: China Science Expo

Wireless charging is gradually becoming a part of daily electronic device charging, from electric toothbrushes to smart bracelets and mobile phones, the application scenarios of wireless charging are increasing. It not only simplifies the charging process, but also frees us from the constraints of charging cables. How can it be charged by "sticking"?

Wireless charging, also known as inductive charging or wireless energy transfer, is actually a technology that can charge devices without connecting a charging cable between the electronic device and the charging platform.

A mobile phone being wirelessly charged

(Photo source: veer photo gallery)

The most common application of wireless charging for electronic devices is the principle of electromagnetic induction, that is, in a closed circuit, when the magnetic flux passing through the circuit changes, an induced current will be generated in the circuit.

Wireless charging usually requires two coils, one is a transmitting coil placed in the charging platform, and the other is a receiving coil placed in the electronic device. When a changing current passes through the transmitting coil of the charging platform, a changing magnetic field is generated in the coil. When the electronic device is close to the charging platform, the receiving coil generates an induced current, which is converted into direct current through the circuit system to charge the device.

Mobile phone wireless charging diagram

(Image source: Reference 1)

Although wireless charging can replenish power without plugging in cables, the transmitter and receiver need to meet the requirements of position and distance. The transmission distance of electromagnetic induction is usually between a few millimeters and a few centimeters, and is also affected by the alignment of the coils. Therefore, when performing wireless charging, the closer the coils are and the better they are aligned, the faster the charging speed.

Some people may think that charging by "sticking a patch" is still not very convenient. Is it possible to charge without sticking a patch?

Wireless charging at a distance

Electromagnetic resonance is another principle of wireless charging application, which allows energy transmission at a longer distance. Electromagnetic resonance also requires two sets of coils. The transmitter generates a changing magnetic field through an oscillator and a transmitting coil. Since the receiving coil at the receiving end has the same resonant frequency as the transmitting coil, the receiving coil and the transmitting coil will have a strong coupling, converting the magnetic field energy into electrical energy, thus realizing wireless transmission of electrical energy.

Electromagnetic resonance principle diagram

(Image source: Reference 2)

In 2007, a research team from the Massachusetts Institute of Technology published a research result in the journal Science, showing that under the strong coupling state of the self-resonant coil, a copper coil with a diameter of 25 cm can achieve energy transmission over 2 meters, with a transmission power of 60 watts, enough to light a light bulb.

Magic coils - a necessary condition for wireless charging

Whether using the principle of electromagnetic induction or the principle of electromagnetic resonance, the coil is an important component of wireless charging technology, and the energy loss caused by the coil has a great impact on the efficiency of wireless charging.

So what materials are wireless charging coils made of?

Copper coils are widely used materials in wireless charging technology and are mostly used at the receiving end of wireless charging. Copper is a metal material with extremely low resistivity, and its conductivity is second only to silver. It has the advantages of low cost and good ductility. In addition, copper has a good heat dissipation effect and can effectively disperse the heat generated during the charging process to avoid overheating.

Copper coil

(Photo source: veer photo gallery)

Litz wire is a commonly used coil material for wireless charging transmitters. This type of wire is made of multiple strands of fine copper wires twisted together. The purpose is to reduce the skin effect (the uneven distribution of alternating current in the conductor), thereby reducing resistance and improving energy transmission efficiency.

Wireless charging coil

(Photo source: veer photo gallery)

Although coils made of metal materials such as copper have excellent conductivity and have been widely used in wireless charging coils, for flexible electronic devices that come into contact with the skin, it is difficult for rigid coils such as copper to change their shapes according to different application scenarios. In addition, in addition to the coil, wireless charging electronic devices also need a battery to store electrical energy, which not only increases the size of the device, but also increases the weight of the device. So, is it possible to store electrical energy directly in the coil and eliminate the battery?

Carbon fiber coil can both charge wirelessly and store electricity

On October 17, 2024, Chinese scientists published a research work on "Carbon Fiber Flexible Devices" in the international high-level journal "Advanced Materials", which used the conductivity of carbon fiber and the properties of being able to serve as supercapacitor electrodes to achieve a combination of wireless charging and energy storage.

The research results were published in Advanced Materials

(Image source: Advanced Materials magazine)

Double-layer supercapacitors are devices that store electrical energy by adsorbing electrostatic charges on electrodes. They have the advantage of fast charging and usually use carbon materials as electrodes. Based on this, the researchers prepared a 64-cm long flexible carbon fiber and immersed one end of the carbon fiber in graphene oxide and activated carbon slurry. After drying and high-temperature treatment, they obtained a carbon fiber/reduced graphene oxide/activated carbon (CF@rGO/AC) electrode.

Schematic diagram of the integration of carbon fiber wireless charging coil and supercapacitor; b. Schematic diagram of the preparation of carbon fiber coil supercapacitor

(Image source: Reference 3)

The researchers placed the separator between two identical CF@rGO/AC electrodes, dripped in ionic liquid electrolyte, and wrapped it with heat shrink tubing to form a supercapacitor at one end of the flexible carbon fiber.

This supercapacitor has the characteristics of adjustable electrode length and adjustable storage energy. When the electrode length reaches 8 cm, the capacity of the supercapacitor can reach 1490 μWh and the power can reach 35.8 mW.

The integrated carbon fiber device consists of a 62.5 cm long wireless coil and a 1.5 cm long supercapacitor. When the outer diameter of the carbon fiber coil is 6.8 cm, the supercapacitor can be fully charged in 2 seconds at a charging voltage of 3 V. The carbon fiber coil is folded into a three-dimensional spring shape, and the charging power can reach 313 mW.

Carbon fiber wireless charging coil and supercapacitor circuit diagram;

bd. Actual picture of carbon fiber integrated device;

e. Graph of different carbon fiber coil outer diameters and wireless charging time and initial discharge current;

f. Initial charging current diagram of different carbon fiber coil outer diameters;

g, h. Application pictures of carbon fiber integrated devices in wristbands and pet GPS;

ik. Carbon fiber integrated devices drive electric fans, light strips and toy cars

(Image source: Reference 3)

Based on the good flexibility of carbon fiber, the researchers converted the carbon fiber device into different shapes. It can not only be placed in a smart bracelet, but also in the form of a disc in a pet GPS, which has better convenience and adaptability.

Wireless charging will shine on a bigger stage in the future

The application of wireless charging technology for smart bracelets, mobile phones, etc. is already very common. So can new energy vehicles that also require electricity to drive also use wireless charging technology?

The answer is yes.

Compared with the plug-in charging method, wireless charging can greatly alleviate the problem of insufficient charging piles. Car owners only need to park their cars in a fixed location to charge, which greatly improves convenience. With the development of technology, it is possible to transform roads in the future to realize dynamic charging of new energy vehicles. Imagine that when you are driving a new energy vehicle on a specific road, charging while driving, it not only solves the range anxiety, but also gives you a cool feeling.

Concept diagram of dynamic wireless charging for cars

(Photo source: veer photo gallery)

Currently, many car companies are exploring the use of wireless charging to replenish power for new energy vehicles. However, problems such as electromagnetic wave energy loss and short transmission distance caused by the air medium have yet to be solved, and there is still a long way to go before large-scale application.

The development of science and technology is often not achieved overnight. The unremitting efforts of scientists are turning the beautiful scenes we imagine into reality. We have reason to believe that the progress of science and technology will bring a qualitative leap to our lives. Let us look forward to a more intelligent future together.

References:

1. Ye Yuhao. Principles and applications of wireless charging technology[J]. Science and Technology Innovation and Productivity, 2023.

2. Chen Yao. Current status of development and standardization of wireless power transmission technology[J]. Battery, 2021.

3. C. Gao, J. Liu, Y. Han, R. Chen, J. Huang, Y. Gu, Y. Zhao, L. Qu, An Energy-Adjustable, Deformable, and Packable Wireless Charging Fiber Supercapacitor[J]. Adv. Mater., 2024.

4.LL Zhang, XS Zhao, Carbon-based materials as supercapacitor electrodes[J]. Chemical Society Reviews, 2009.

5.LL Zhang, R. Zhou, XS Zhao, Graphene-based materials as supercapacitor electrodes[J]. Journal of Materials Chemistry, 2010.

6. Sun Kunya. Systematic research on wireless charging technology for electric vehicles[J], 2024.