Periodic Reporting for period 1 - IMMOSTORE ('It yet remains to see...' - Hybrid electrochemical energy storage system of high power and improved cycle life)
Reporting period: 2024-03-01 to 2025-08-31
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.
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.
As the first electrochemical tests were done in the primary project, here we focused on features important for further development. The first issue identified was the corrosion of the electrode (carbon), current collector, and device case. This initially aggravated the cycle life of the product and destroyed the concept of sustainability. Although quickly resolved at the lab scale, once translated to the device level, the parasitic reactions triggered by the cell elements became a serious issue. We optimised the cell with electrolyte modification by providing anti-corrosive additives. Another challenge concerned the electrode engineering and scale effect. The production process involves several steps, and certain additives (like binders) affect the porosity of the electrode and the surface activity. We observed noticeable capacitance fade for selected binders and tried to solve it by designing the binder-free self-standing electrodes; however, their stability still needs improvement. The next challenge concerned the safety and stability of the system at high voltages (at high state of charge). We solved that by shifting to another redox-active species (acetates). Furthermore, we performed an in-depth study on the impact of lithiation degree and current loads during cell conditioning on the final performance. As expected, the outcome triggered next concepts and ideas that are currently further investigated.
We presented the metal-ion capacitor that takes advantage of a redox-active electrolyte and does not exploit metallic electrodes or solid-state sacrificial materials for the initial insertion. This substantially simplifies the production process and decreases the final price of the device. Moreover, it opens the door to further developments in the battery field, as redox-active electrolytes could be an interesting alternative to the well-known cathodes in metal-ion batteries. This means that we could base our energy storage technologies on the resources we have available in Europe.
In our concept, we focused on the electrolyte that plays multiple roles: ensures high electrochemical stability (up to 4.5 V) and is a source of additional capacitance; thus, it boosts the energy density in two ways. Moreover, our device is the first one on the market that will exploit the negative electrode insertion without adding the less-conductive materials to the counter electrode or using the metal foil for primary insertion. By doing so, we would like to minimize the impact of critical and EU-unavailable raw materials (like metallic lithium or cobalt/nickel-based cathodes) on the product that will demonstrate competitive characteristics in terms of energy/power densities, cycle life, and unit cost.
In our concept, we focused on the electrolyte that plays multiple roles: ensures high electrochemical stability (up to 4.5 V) and is a source of additional capacitance; thus, it boosts the energy density in two ways. Moreover, our device is the first one on the market that will exploit the negative electrode insertion without adding the less-conductive materials to the counter electrode or using the metal foil for primary insertion. By doing so, we would like to minimize the impact of critical and EU-unavailable raw materials (like metallic lithium or cobalt/nickel-based cathodes) on the product that will demonstrate competitive characteristics in terms of energy/power densities, cycle life, and unit cost.