The unpredictable nature of renewable energy limits the growth of technologies such as wind and solar power. This limitation means that electricity production units which utilize fossil-fuels are still necessary as they can generate energy in 10 seconds to a minute from start-up.

A solution is needed to bridge the gap between intermittent renewable energy and fossil fuel energy generation. This challenge requires an energy storage system which can deliver the large amount of energy necessary to fill the gap between a break in renewable energy production and the start-up of an emergency fossil fuel-based generator.

Increasing the lifetime of energy storage systems

When combined with a battery, a supercapacitor can absorb sharp and repetitive peaks in power generation or demand. This protects the battery which would otherwise overheat and wear-out prematurely.

Supercapacitors – alone or in combination with batteries – are an excellent solution as they increase the lifetime of energy storage systems. They can be used as buffer storage to recuperate braking energy in an electric or hybrid vehicle, or offer quick charge/discharge cycles to manage peaks in production and demand.

Existing supercapacitors have a very poor energy-to-weight ratio. Solving this issue was the main objective of the Energy Caps project. With our combined knowledge of lithium-ion (Li-ion) batteries, Solvay and our partners decided to explore whether it was possible to use this technology to create an alternative supercapacitor. The new supercapacitor should be able to store more energy in a lighter device, resulting in a more environmentally friendly solution.

Lithium Ion Capacitor

Recycling key consideration from the start

Together with partners from academia and industry, Solvay formed the Energy Caps project in December 2011. Researchers were seconded to the project from Poznan University of Technology (Poland), Kyiv National University of Technologies and Design (Ukraine), and Solvay.

Our industry partners were Yunasko (Ukraine) who created the Li-ion supercapacitor prototype, and Recupyl (France) who established a recycling process for the technology. In addition to contributions from Solvay and our partners, Energy Caps received funding of €2 million under the European Union’s Marie Curie Industry-Academia Partnerships and Pathways (IAPP) program.

In the very early stages of the project, the research team decided to focus on the development of new components for the supercapacitor. This included the electrodes, a high-performance polymer separator, and an optimized electrolyte mixture. We also decided to pay special attention to solutions which offered the lowest environmental impact and cost.

As we developed the individual components, they were tested for performance and combined with other components in a lab cell. Yunasko then developed the best solutions into larger prototypes. The recyclability of each component was assessed throughout by Recupyl. 

High temperature performance

Over the course of four years, we developed and tested many individual components. Solvay products were used to create new electrodes, separators, and electrolytes. LiTFSI, a Solvay product, was found to be the best lithium salt for the electrolyte. It can operate at temperatures above 45°C, a key limitation of existing high-power batteries. Combined with another solvent, we could demonstrate continuous high temperature operation at ±65°C.

During the last year of the project, Recupyl evaluated the environmental impact of manufacturing and recycling the final Li-ion supercapacitor. We were also able to demonstrate recycling efficiency for the capacitor of more than 50% using a clean, cost-effective, and safe recycling process.

The prototype created by Yunasko has enabled us to demonstrate that it is possible to develop a safe, sustainable, and recyclable Li-ion supercapacitor. It combines the electrostatic storage of supercapacitors and the electrochemical storage of lithium ion batteries into a single device. In tests, the device provided an energy density similar to that of a lead-acid battery, with a charging time as low as one minute. The number of cycles and power capacity has been improved by at least a factor of 100.

The Energy Caps project ended in December 2015 with the successful testing of the prototype. The partners are now looking for commercial support to scale-up the prototype into the world’s first fully operational Li-ion supercapacitor to achieve this level of performance. It will be a significant step forward for renewable energy and towards creating a much more sustainable future.