Researchers at the Lawrence Berkeley Lab, California, have developed a promising material for energy storage. Polymer film capacitors made with this ‘polysulphate compound’ can store large quantities of electricity.

Notably, this compound has been synthesised using ‘click chemistry’ — a means of getting two materials to bind together — for which three scientists received the 2022 Nobel Prize in Chemistry.

Capacitors, like batteries, are energy storage devices. Polymer film capacitors are known to be good storage devices, accounting for over half the high-voltage capacitor market, owing to their light weight, low cost and mechanical flexibility. However, their performance suffers with increase in voltage and temperature.

Scientists at Berkeley Lab explored the use of new materials with improved tolerance for heat and electric fields. They then decided to create special polymers.

They cast polysulphates with good thermal properties into flexible films. High-temperature, high-voltage capacitors made with this film show energy storage properties at 150 degrees C. The work has been reported in the latest issue of the  Joule magazine; the paper has been authored by 22 scientists, including Dr Barry Sharpless, the two-time Nobel laureate.

An article shared by Berkeley Lab says such power capacitors promise to improve the energy efficiency and reliability of integrated power systems in applications such as electrified transportation.

“Our work adds a new class of electrically robust polymers to the table. It opens many possibilities to the exploration of more robust, high-performing materials,” said Yi Liu, one of the researchers and the facility director of organic and macromolecular synthesis at the lab’s Molecular Foundry.

A capacitor should be a ‘dielectric’ material — it should not conduct electricity even when high voltages are applied. There are not many materials that can be thermally stable while also maintaining high dielectric strength.

“Improving the thermal stability of existing films while retaining their electrical insulating strength is an ongoing challenge,” says Liu. The big challenge in finding such materials has been the lack of efficient ways of synthesising them. The paper notes that an added reason is the lack of understanding of the relationship between the polymer’s structure and its properties.

Building anew

Polysulphates have outstanding dielectric properties, especially at high electric fields and temperatures. “Several commercial and lab-generated polymers are known for their dielectric properties, but polysulphates had never been considered. The marriage between polysulphates and dielectrics is one of the novelties here,” says He Li, a postdoctoral researcher at the Molecular Foundry and material sciences division, and lead author of the study.

The Berkeley Lab’s write-up describes how scientists zeroed in on the polysulphate compound. Inspired by the excellent baseline dielectric properties offered by polysulphates, the researchers deposited extremely thin layers of aluminium oxide onto thin films of the material to engineer capacitor devices with enhanced energy storage performance.

They discovered that the fabricated capacitors exhibited excellent mechanical flexibility, withstood electric fields of more than 750 million volts per metre, and performed efficiently at temperatures up to 150 degrees C.

In comparison, today’s benchmark commercial polymer capacitors only function reliably at temperatures below 120 degrees C. Beyond that temperature, they can withstand electric fields smaller than 500 million volts per metre, and the energy efficiency is halved.

“We have provided deep insight into the underlying mechanisms that contribute to the material’s excellent performance,” said Wu, one of the researchers.

It is in the synthesising of the polysulphate polymer that ‘click chemistry’ came in handy. The polymer strikes a balance between electrical, thermal, and mechanical properties, which is due to the sulphate linkages introduced by the ‘click chemistry’ reaction.

However, this is not the end of the research. The same method of synthesising polymers can be used to make new polymers with better performance.