Defects Deliver the Best of Both Worlds: Highly Efficient Ultrahigh Energy Density Capacitor

In order to manufacture new materials, a thin film is first deposited in this chamber by a pulsed laser deposition process. The bright "bubble" you see is the laser hitting the target and depositing material. Image source: Martin/University of California, Berkeley

Capacitors that quickly store and release electrical energy are key components in modern electronic and power systems. However, compared with other storage systems (such as batteries or fuel cells), the most commonly used energy density is lower, and these storage systems cannot quickly discharge and charge without being damaged.

Now, as reported in the magazine

, The researchers found a way to get the best of both worlds. A team led by researchers from the Department of Energy (DOE) Lawrence Berkeley National Laboratory (Berkeley Lab) proved that it can be processed universally by introducing isolated defects into a certain type of commercially available film in a simple post-processing step The material becomes the highest performance energy storage material.

This research was supported by the "Materials Project", an open-access online database that actually provides the largest collection of material properties for scientists worldwide. Today, the materials project combines calculation and experimental work to speed up the design of new functional materials and other goals. This includes knowing how to manipulate known materials in ways that improve them.

The increasing demand for cost reduction and device size reduction has driven the development of high energy density capacitors. Capacitors are commonly used in electronic devices to maintain power while charging the battery. The new materials developed by Berkeley Lab can finally combine the efficiency, reliability and robustness of capacitors with the energy storage capacity of large batteries. Applications include personal electronic devices, wearable technology and car audio systems.

The material is based on the so-called "relaxed ferroelectric", which is a ceramic material that has a rapid mechanical or electronic response to external electric fields and is usually used as a capacitor in applications such as ultrasonics, pressure sensors, and voltage generators. .

The applied field drives changes in the orientation of electrons in the material. At the same time, the energy stored in the magnetic field drive material changes, making it a good candidate for the use of small capacitors. The problem to be solved is how to optimize the ferroelectric so that it can be charged to high voltage and discharged quickly (billions of times or more) without being damaged, so it is not suitable for long-term use in applications such as computers and vehicles.

The researcher in Ryan Martin’s laboratory is a professor in the Department of Materials Science (MSD) at Berkeley Lab and a professor of materials science and engineering at the University of California, Berkeley. He achieved this by introducing local defects to enable them to withstand higher voltages. One goal.

"You may have experienced relaxation ferroelectrics on a gas grill. The button that lights the grill operates a spring hammer, which strikes a piezoelectric crystal, which is a relaxor, and generates a voltage that ignites the gas. ." Martin explained. "We have proven that they can also be made into some of the best materials for energy storage applications."

Placing a ferroelectric material between the two electrodes and increasing the electric field will cause the charge to accumulate. During the discharge process, the available energy depends on the strength of the orientation or polarization of the material electrons in response to the electric field. However, most of these materials generally cannot withstand large electric fields before the material fails. Therefore, the fundamental challenge is to find a way to increase the maximum possible electric field without sacrificing polarization.

The researchers turned to a method they had previously developed to "turn off" the conductivity of materials. By bombarding the film with high-energy charged particles called ions, they can introduce isolated defects. Defects trap electrons in the material, prevent their movement, and reduce the conductivity of the film by orders of magnitude.

"In ferroelectrics that should be insulators, leakage of charge through it is a major problem. By bombarding the ferroelectrics with high-energy ion beams, we know we can make them better insulators," a PhD researcher in Martin's research group , Said Jayne King, the lead author of the paper. "Then we asked, can we use the same method to make relaxor ferroelectrics able to withstand higher voltages and electric fields before catastrophic failure?"

The answer is "yes". Kim first produced a typical relaxor ferroelectric film called niobium magnesium lead-lead titanate. He then targeted thin films with high-energy helium ions in the ion beam analysis facility operated by the Accelerator Technology and Applied Physics (ATAP) department of Berkeley Lab. The helium ion knocks the target ion from its site, forming a point defect. Measurements show that the energy storage density of the ion bombardment film is more than twice the previously reported value, and the efficiency is increased by 50%.

"Our initial desired effect is mainly to reduce the leakage of isolation point defects. However, we realize that the changes in the polarization electric field relationship due to certain defects are also important." Martin said. "This transition means that more and more applied voltages are needed to produce the maximum polarization change." The results show that ion bombardment can help overcome the trade-off between high polarization and fragility.

The same ion beam method can also improve other dielectric materials to improve energy storage and provide researchers with a tool to repair problems in materials that have already been synthesized. Jin said: "It is great to see people use these ion beam methods to'repair' the materials in the equipment after their synthesis or production process is imperfect."

Reference: "Ion Bombardment Relaxation of Ultra-high Capacitance Energy Density in Ferroelectric Thin Films", Jieun Kim, Sahar Saremi, Megha Acharya, Gabriel Velarde, Eric Parsonnet, Patrick Donahue, Alexander Qualls, David Garcia and Lane W. Martin, 2020 July 3,

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This research was supported by the Office of Science of the US Department of Energy and was funded by the National Science Foundation.

The applied field drives changes in the orientation of electrons in the material.

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