New Berkeley Study Rewrites the Narrative on CapacitorsAugust 21, 2020 by Jake Hertz
Researchers from Berkeley have come up with a method to create ultra-high energy density capacitors, combining the virtues of both batteries and capacitors.
Capacitors are one of the most fundamental electronic components. Any engineer who has taken EE 101 has likely studied their characteristics. Capacitors have a wide variety of uses in electronic systems, but they are specifically ubiquitous in power system designs.
Decoupling capacitors being used in a system from chip to board level. Image used courtesy of Takashi Endo et al.
Be it in a regulator or just for decoupling, you’d be hard-pressed to find an electronic system without capacitors.
Capacitor vs. Battery
The main functional difference between a capacitor and a battery is two-fold:
- Capacitors store low amounts of energy for short periods of time while batteries store large amounts of energy for long periods of time.
- Capacitors are meant to charge and discharge rapidly while batteries will undergo damage if they charge/discharge too quickly.
Capacitors offer weight, cost, and charge speed benefits over batteries. Image used courtesy of Explain That Stuff
It’s with this difference in mind that researchers set out to create the perfect hybrid of the two: a device that can rapidly charge and discharge while also storing large amounts of energy.
Berkeley Researchers Leverage Relaxor Ferroelectrics
Just this week, researchers at Berkeley Lab have announced a new material they claim can be used to achieve ultra-high, energy-dense capacitors. The material leverages relaxor ferroelectrics, which, according to Berkeley, are ceramic materials that undergo a rapid mechanical or electronic response to an external electric field and are commonly used as a capacitor in applications such as ultrasonics, pressure sensors, and voltage generators.
Relaxor ferroelectrics undergo a change in electron orientation and energy storage when exposed to an electric field. In this work, the scientists at Berkeley explored ways to manipulate relaxor ferroelectrics so they can charge and discharge rapidly without sustaining damage.
Introducing Defects to Increase the Electric Field
In charging a ferroelectric material, an external electric field causes charge to build up. During discharge, the amount of energy available depends on how strongly the material’s electrons become polarized in response to the field.
The challenge is that most materials can’t withstand a large electric field before the material fails. Therefore, the researchers had to find a way to increase the maximum possible electric field without sacrificing the polarization.
Ion beam deposition onto the thin film material. Image used courtesy of Lane Martin and Berkeley Lab
The scientists were able to successfully accomplish this goal by introducing local defects into the material via an ion beam. The method consisted of exposing the thin-film material to a stream of high energy particles, successfully creating local defects on the material.
Adding these defects successfully decreased the material's conductivity, trapping the electrons and preventing their motion. In this way, the researchers could effectively maintain polarization and decrease leakage in the material while applying an even larger electric field (i.e storing more energy).
The Best of Capacitors and Batteries
The results from the paper are impressive, showing that materials exposed to the ion beam had more than twice the energy storage density and 50% higher efficiencies than those unexposed.
In some cases, they were able to achieve energy storage densities as high as ~133 joules per cubic centimeter with efficiencies exceeding 75%. Going forward, researchers hope the new material could ultimately combine the efficiency, reliability, and robustness of capacitors with the energy storage capabilities of larger-scale batteries.
They believe that applications include personal electronic devices, wearable technology, and car audio systems.