Hybrid lithium-ion supercapacitors offer several distinct advantages compared to conventional supercapacitors. Like other supercapacitors, they can charge and discharge very quickly a very large number of times.
However, lithium-ion hybrid supercapacitors do not have the issue self-discharging (they retaining a charge over time), and unlike lithium-ion batteries, they do not suffer from the possibility of dangerous thermal runaway.
Also unlike batteries, the component materials for lithium-ion supercapacitors do not contain lithium ions (or electrons). A so-called pre-lithiation step is required to add lithium ions so that the device can operate.
Today, two major strategies are envisioned. In one strategy, either one of the constituent materials of the supercapacitor is pre-lithiated before being integrated. The other method is to redistribute an additive that is rich with lithium ions throughout the supercapacitor materials during the first charge.
Nonetheless, these processes are both expensive and complex, and they can decrease the capacity of the device.
Also, most of the available pre-lithiation additives tend to degrade in air and/or in contact with the solvents for making the lithium-ion supercapacitor. So, the fabrication of such hybrid lithium-ion supercapacitors has up until now at best has been very complex and extremely costly.
Using one additive means that the additive must meet the demands of price, chemical stability, and performance.
Researchers from the Jean Rouxel Materials Institute (CNRS / University of Nantes) in collaboration with the Münster Electrochemical Energy Technology (University of Münster), tried using not one but two additives coupled by a subsequent chemical reaction. They published their findings in Advanced Energy Materials.
Each of the additives has a role. One provides the lithium ions and the other provides the electrons.
The researchers assert that the use of two additives, each with a specific role, offers much more latitude because they can be selected independently for their price, their chemical properties, and performance.
During charging of the lithium-ion supercapacitor, the first additive, pyrene (naturally present in some types of coal) releases electrons and protons. The second additive, Li3PO4 (mass produced in the glass industry for example), captures these protons and exchanges them for lithium ions, which are then available for pre-lithiation.
Another advantage of this method, according to the researchers, results from the fact that after the pre-lithiation, the residue of one of the two additives used, pyrene, aids in the storage of the charges, thereby increasing the amount of electrical energy stored in the device.
The researchers claim that the efficiency and versatility offered with this new approach open the way to an inexpensive pre-lithiation solution that could lead to lithium-ion supercapacitors that can store more energy.
Overall, it is thought that the lifting of the technological bottleneck should allow faster marketing of the devices.
Anothumakkool, B., Simon Wiemers-Meyer, D., Guyomard, Winter, M., Brousse, T., and Gaubicher, J. Cascade-type pre-lithiation approach for Li-ion capacitors. Advanced Energy Materials, June 5, 2019. DOI: 10.1002 / aenm.201900078