IMMOSTORE was focused on development of an electric energy storage system and finally offered a novel type of hybrid electrochemical capacitor (HEC) that stores energy quickly (due to high power), efficiently (due to reversible (inter)reactions inside) and demonstrates satisfactory performance over thousands of charging/discharging cycles. Typical electrochemical capacitors (ECs) store energy in so-called electric double-layer, by attracting the ions to the electrode of opposite polarization. As one can assume, the energy stored in this way is rather moderate and cannot be compared to that of the batteries. Such a comparison, nevertheless, is pointless, as batteries have a completely different application niche and perform perfectly in applications that ECs will never reach. However, if electric double-layer storage is boosted by fast and reversible redox reactions (in certain cases called pseudocapacitive phenomena), the energy stored in the EC increases remarkably. Merging the capacitive and redox-based mechanisms allows for the design of a hybrid system that combines the advantages (unfortunately, disadvantages too) of both approaches. Li-ion capacitors (LIC) combine one capacitive electrode and one typical of Li-ion battery. The capacitive electrode is usually made of activated carbon, with a well-developed surface area for efficient electrostatic charge accumulation. The battery-like electrode could be graphite or so-called hard carbon, and it requires intercalation by Li ions to reach low potential values, which then allows the operating voltage to be extended. On the laboratory scale, this intercalation is performed in one cell, then the electrode is transferred to the another cell, and then is combined with activated carbon. From a practical point of view, this is almost impossible in industrial reality as it requires more than one cell for assembly. Furthermore, it requires manipulation with metallic lithium, raising serious safety issues. Several approaches have been proposed to date to address this problem – sourcing lithium from an electrolyte solution, using sacrificial Li foil as auxiliary electrode, or exploiting the Li-rich materials on the counter electrode. All these concepts demonstrate serious drawbacks: sourcing Li from an electrolyte aggravates the electrolyte conductivity, the presence of Li foil raises safety and economic issues, and Li-rich composites remain in the cell as a dead mass after intercalation and deteriorate the specific energy. Thus, the lack of convenient and industrially feasible method for intercalation of graphite as a HEC’s negative electrode is an obstacle in their further development and wide, industrial application.
Resigning from metallic foil in the cell, sophisticated composites in the counter electrode, or highly concentrated electrolyte that could induce viscosity-related problems seems to be a rational approach and was the major challenge of the project. IMMOSTORE furthemore changed the object of interest from the electrode to the electrolyte and played the game at the electrode/electrolyte interface. The idea assumed that if the electrolyte is responsible for providing an additional charge from the redox-active ion, there is no need to exploit complicated solutions. The idea we presented is to use a Li-based salt, with a redox-active anion, that could play a multifunctional role: to ensure the right concentration of Li cations in the electrolyte and prevent loss of its conductivity, as well as to counterbalance the charge of the positive electrode from the redox activity of the anion.
