Packed with power

‘Power Paper’ is based on cellulose fibres, broken down by hydroentanglement at the nanocellulose pilot facility of forestry products developer Innventia, to become as thin as 20 nm in diameter. With the cellulose fibres in a solution of water, an electrically charged polymer, also provided in a water solution, is added. The polymer then forms a thin coating around the fibres.

“The covered fibres are in tangles, in which the liquid in between the spaces functions as an electrolyte,” explains Hjalmar Granberg of Invenntia, who is leading the Power Papers project. “One of the keys to our breakthrough was working with the polymers in a wet state, which makes the cellulose flexible and adaptable, and enabled us to build thick layers. We have also created sufficient mechanical properties to be able to handle the material.”

In addition to Innventia, the Power Papers project involves researchers from KTH Royal Institute of Technology and Linköping University in Sweden, the Technical University of Denmark and the University of Kentucky.A sheet fifteen centimetres

in diameter and a few tenths of a millimetre thick has already been produced which can store as much energy as the supercapacitors currently on the market. The material can be recharged hundreds of times and charging only takes a couple of seconds. In fact, Power Paper has already broken four world records with:

• The highest measured current in an organic conductor.
• The highest capacity to simultaneously conduct ions and electrons.
• The highest transconductance in a transistor.
• The highest charge and capacitance in organic electronics.

The new cellulose-polymer material opens the door to continued development towards even higher capacity. Unlike the batteries and capacitors currently on the market, Power Paper is produced from simple materials – renewable cellulose and an easily available polymer. It is light in weight, requires no dangerous chemicals or heavy metals, and is waterproof.

Nanocellulose is derived from wood fibres and has exceptional strength characteristics on a par with Kevlar. However, in contrast to Kevlar and other materials based on fossil fuels, it is completely renewable.

Previously, the production process was too energy-intensive to make the commercialisation of nanocellulose a viable option. Recent process developments at Innventia, however, have enabled energy consumption to be reduced by a total of 98%, representing a saving of 29,000 kWh per tonne. By comparison, heating a normal sized house requires approximately 18,000 kWh per year.

Fully delaminated nanocellulose consists of long nanofibers which are 5-20 nm in diameter and has the appearance of a highly viscous, transparent gel.

Modulit

Following the success of the Power Papers development, the focus of a second research project called Modulit will be to demonstrate the manufacturing of complete energy storage modules in a roll-to-roll process.

Instead of separate manufacturing units, roll-to-roll manufacturing will involve producing both energy storage cells and balancing circuits on the same substrate, which it is believed will radically reduce the manufacturing costs for energy storage.

One long-term goal is to exploit the large-scale production capacity from paper manufacturing – from which, of course, wetlaid nonwovens production has also been adapted – along with large format printed electronics. The intention for the cellulose-based material is not to act as a passive substrate, but to be an integrated element of the process solution and to contribute to the electrical performance.

The new SEK 34 million Modulit project is a collaboration between Innventia, Acreo Swedish ICT, Linköping University, Mid Sweden University, KTH Royal Institute of Technology and SICS Swedish ICT. The four-year project is being funded by the Swedish Energy Agency.

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