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Highly efficient capacitor with ultra high energy density

Pulse laser deposition chamber

To manufacture the new material, the thin film is first deposited in this chamber using a pulsed laser deposition process. The bright “cloud”

; you see is the laser that hits the target and deposits the material. Photo credit: Martin / UC Berkeley

By introducing defects into a common material, the researchers at the Berkeley Lab create a highly efficient capacitor with a dramatically increased energy density.

Capacitors, which quickly store and release electrical energy, are key components in modern electronics and power supply systems. However, the most commonly used ones have low energy densities compared to other storage systems such as batteries or fuel cells, which in turn cannot discharge and recharge quickly without being damaged.

Well, as reported in the diary scienceResearchers have found the best of both worlds. A team of researchers from the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has shown that a common material can be processed into a high-performance energy storage material by introducing isolated defects in a commercially available thin film in a simple post-processing step.

The research is supported by the Materials Project, an open access online database that provides scientists around the world with virtually the largest collection of material properties. Today, the materials project combines both computational and experimental efforts to, among other things, accelerate the design of new functional materials. This includes understanding ways to manipulate known materials to improve their performance.

Growing demands on cost reduction and the miniaturization of components have driven the development of capacitors with high energy density. Capacitors are commonly used in electronic devices to maintain power while a battery is being charged. The new material developed at the Berkeley Lab could ultimately combine the efficiency, reliability, and robustness of capacitors with the energy storage capabilities of larger batteries. Applications include personal electronic devices, wearable technology, and car audio systems.

The material is based on a so-called “relaxor ferroelectric”, a ceramic material that experiences a rapid mechanical or electronic response to an external electric field and is commonly used as a capacitor in applications such as ultrasound, pressure sensors and voltage generators.

The applied field drives changes in the orientation of the electrons in the material. At the same time, the field causes a change in the energy stored in the materials, making them a good candidate for use beyond a small capacitor. The problem to be solved is to optimize the ferroelectric so that it can be charged to high voltages and discharged very quickly – billions of times or more – without suffering damage that would make it unsuitable for long-term use in applications such as computers and vehicles .

Researchers in the laboratory of Lane Martin, a faculty scientist in the Materials Science Department (MSD) at Berkeley Lab and a professor of materials science and engineering at UC Berkeley, did this by introducing local defects that enabled him to withstand greater stresses.

“You’ve probably seen Relaxor ferroelectrics on a gas grill. The button that lights the grill activates a spring-loaded hammer that strikes a piezoelectric crystal, which is a kind of relaxor, and creates a voltage that ignites the gas, ”explained Martin. “We have shown that they can also be made into some of the best materials for energy storage applications.”

By placing a ferroelectric material between two electrodes and increasing the electric field, a charge builds up. During the discharge, the amount of energy available depends on how strongly the electrons in the material orient or polarize in response to the electric field. However, most of these materials typically cannot withstand a large electric field before the material fails. The fundamental challenge is therefore to find a way to increase the maximum possible electric field without affecting the polarization.

The researchers turned to an approach they had previously developed to turn off conductivity in a material. By bombarding a thin film with high-energy charged particles known as ions, they were able to introduce isolated defects. The defects trap the electrons in the material, prevent their movement and reduce the conductivity of the film by orders of magnitude.

“With ferroelectrics, which are supposed to be insulators, the charge that leaks through them is a big problem. By bombarding ferroelectrics with beams of high energy ions, we knew we could make them better insulators, ”said Jieun Kim, PhD student in Martin’s group and lead author of the paper. “We then asked if we could use the same approach to make a ferroelectric relaxor withstand larger voltages and electric fields before it fails catastrophically.”

The answer was “yes”. Kim first made thin films from a prototypical ferroelectric relaxor called lead magnesium niobite-lead titanate. He then targeted the high-energy helium ion films in the ion beam analysis facility operated by the Accelerator Technology and Applied Physics (ATAP) division at the Berkeley Lab. The helium ions repelled target ions from their locations to create point defects. Measurements showed that the ion-bombarded film had more than twice the energy storage density of the values ​​given above and 50% higher efficiency.

“We originally expected the effects to be mainly due to the reduction in leakage for isolated point defects. However, we found that the shift in the relationship between polarization and electric field is equally important because of some of these defects, ”said Martin. “This shift means that ever larger applied voltages are required to achieve the maximum change in polarization.” The result suggests that ion bombardment can help overcome the compromise between high polarizability and easy breakage.

The same ion beam approach could improve other dielectric materials to improve energy storage and provides researchers with a tool to repair problems in already synthesized materials. “It would be great to see people using these ion beam approaches to post-heal materials in devices when their synthesis or production process isn’t perfect,” said Kim.

Reference: “Ultra-high capacitive energy density in ferroelectric relaxor films with ion bombardment” by Jieun Kim, Sahar Saremi, Megha Acharya, Gabriel Velarde, Eric Parsonnet, Patrick Donahue, Alexander Qualls, David Garcia and Lane W. Martin, July 3, 2020; science.
DOI: 10.1126 / science.abb0631

This research was supported by the DOE Office of Science and grants from the National Science Foundation.

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