Sustainability as a key point
But not everything is about energy, power and cyclability. Environmental sustainability is an indispensable criterion for ensuring a future for coming generations.
For this reason, Li-ion capacitors developed at CIC energiGUNE are based on carbons obtained from recycling bio-waste, with the aim of creating a technology with the lowest possible environmental impact. Specifically, the coals that have been used for the development of our device have been obtained from recycling olive pits, a waste generated by the olive oil industry, thus promoting circular economy.
Through carbonization process of the olive pit at high temperature (700ºC) we obtain a hard carbon, which is used as an anode in the negative electrode. The hard carbon stores the charges in a faradic way, providing energy to the device.
The same hard carbon, through a subsequent chemical activation, is transformed into an activated carbon, with a specific surface area that can reach up to 3000 m2 g-1. The activated carbon is used as a cathode in the positive electrode and stores the electrical charges in a capacitive way, providing power to the device.
Thanks to the fact that the device consists only of carbon, it implies a total recyclability.
However, not all are advantages.
The toll to be paid for developing a carbon-only device is the absence of the ions in the system to store the electrical charges. In other words, our Li-ion capacitor lacks Lithium ions.
In order to solve this paradox, we have developed a lithium salt that we call "sacrificial salt". This salt decomposes at 3.8 V .vs Li/Li+ giving lithium ions and CO2 gas (in very reduced quantities), which is evacuated from the system after its generation.
Thus, in the first charge of the storage system, which goes up to 4 V, lithium ions released from the incorporation of the sacrificial salt in the positive electrode travel to the negative electrode where they are inserted in the hard carbon. This process is reversed when the device is discharged.
And so, again and again, for hundreds of thousands of cycles.
Today, lithium-ion capacitors offer 3-4 times more energy than supercapacitors at similar powers, however, they still have a long way to go to approach the energy levels of batteries, which are still about 5-10 times higher.
Therefore, although the world market for supercapacitors generated 3.27 billion dollars in 2019, and it is estimated that it will reach 16.95 billion dollars in 2027, in the case of lithium-ion capacitors the growth that is estimated in the same period is somewhat more contained, at 25% (Allied Market Research).
Application of lithium-ion capacitors in the market
From the market and technology transfer point of view, one of the objectives of CIC energiGUNE is to contribute to the competitiveness of the companies in the closest environment, applying the results in the market. In this context, there are many niche applications in which batteries are oversized or the power level required from a supercapacitor is not so high, making li-ion capacitors the ideal solution.
For example, as a support to the batteries of the electric vehicle, assisting in the recovery of kinetic energy during braking (KERS) or in the current peaks generated during starting, or in the use of auxiliary systems to achieve a more efficient, longer lasting and more autonomous storage.
Also, in renewable energies, such as wind power, where it would be possible to take advantage of the current peaks generated by strong winds that the battery will lose by not reaching those currents. Or solar energy, where it would take advantage of even the smallest current generated in low light conditions that the battery cannot absorb because it does not reach a voltage treshold.
Apart from the opportunities in large markets such as urban electric mobility and stationary storage in renewable energies, there are many applications such as vertical mobility, or the Internet of Things where a solution based on Li-ion capacitors would provide simplicity and economic viability.
Not in vain, Aurea mediocritas is the Latin term that alludes to the pretension of reaching a desired balance between the extremes and that Aristotle associated posteriori with virtue.
The development carried out at CIC energiGUNE in this area may represent an interesting advance for those companies working in the field of capacitors such as Skeleton and Murata, as well as for Basque companies such as Koneika, or Cegasa.
Surely, in the future, Li-ion capacitors will be the talk of the town, because virtue lies in balance.