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New dielectric helps capacitor to record energy storage level

30 July 2015

Researchers have developed a new capacitor dielectric material that provides an electrical energy storage capacity rivalling certain batteries.

Samples of the new hybrid sol-gel material are shown placed on a clear plastic substrate for testing (photo: John Toon, Georgia Tech)

Using a hybrid silica sol-gel material and self-assembled monolayers of a common fatty acid, researchers have developed a new capacitor dielectric material that provides an electrical energy storage capacity rivalling certain batteries, with both a high energy density and high power density.

If the material can be scaled up from laboratory samples, devices made from it could surpass traditional electrolytic capacitors for applications in electromagnetic propulsion, electric vehicles and defibrillators, where capacitors often complement batteries because they can provide large amounts of current quickly.

The new material is composed of a silica sol-gel thin film containing polar groups linked to the silicon atoms and a nanoscale self-assembled monolayer of an octylphosphonic acid, which provides insulating properties. The bilayer structure blocks the injection of electrons into the sol-gel material, providing low leakage current, high breakdown strength and high energy extraction efficiency.

The need for efficient, high-performance materials for electrical energy storage has been growing along with the ever-increasing demand for electrical energy in mobile applications. Dielectric materials can provide fast charge and discharge response, high energy storage, and power conditioning for a range of applications. However, it has been challenging to find a single dielectric material able to maximise permittivity, breakdown strength, energy density and energy extraction efficiency.

Using an aluminised mylar film coated with the hybrid sol-gel capacitor material, the researchers showed that the capacitor could be rolled and re-rolled several times while maintaining high energy density, demonstrating its flexibility. But they were still seeing high current leakage. To address that, they deposited a nanoscale self-assembled monolayer of n-octylphosphonic acid on top of the hybrid sol-gel. Less than a nanometre thick, the monolayer serves as an insulating layer.

In their structures, the researchers demonstrated maximum extractable energy densities up to 40 joules per cubic centimetre, an energy extraction efficiency of 72 percent at a field strength of 830V per micron, and a power density of 520W/cc2. The performance exceeds that of conventional electrolytic capacitors and thin-film lithium ion batteries, though it currently doesn’t match the lithium ion battery formats commonly used in electronic devices and vehicles.

“This is the first time I’ve seen a capacitor beat a battery on energy density,” says Georgia Tech's Professor Joseph Perry. “The combination of high energy density and high power density is uncommon in the capacitor world.”

Researchers in Perry’s lab have been making arrays of small sol-gel capacitors in the lab to gather information about the material’s performance. The devices are made on small substrates about an inch square. The next step will be to scale up the materials to see if the attractive properties transfer to larger devices. If that is successful, Perry expects to commercialise the material.

The research is publishd in the journal, Advanced Energy Materials.

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