New electrolytes for capacitive electrochemical energy storage – NECES

Because of the limited number of partners, the Consortium decided to keep the structure of the project as simple as possible, were the contribution of each partner is clearly identified. As a result, the research program is divided in 2 scientific WPs (1 and 2) and one WP (3) for exploitation of results. WP1 will be dedicated to the investigation of the electrochemical properties of nanoporous carbon with tailored pore size in the new, high voltage electrolytes. FSU FSU-Jena group will design a number of non-aqueous electrolytes and test them in combination with AC-based electrodes, while the UPS Toulouse group will synthesize the porous carbon materials with controlled pore size below 2 nm where ion desolvation occurs; electrochemical behavior of these materials will be further investigated in the new electrolytes. Furthermore, UPS Toulouse will investigate the ion dynamics into porous carbon electrodes with tailored and controlled pore size, especially the ion desolvation process, via in-situ EQCM. These solvents will be used in combination with the conventional salt Et4NBF4 as well as with the alternative salts PYR14BF4, Et4N TFSI and PYR14TFSI. WP2 will be dedicated to the electrochemical characterizations of MXene electrodes in non-aqueous electrolytes to investigate charge storage mechanism. In addition to the new (solvent plus salt) electrolytes developed in WP1, FSU FSU-Jena group will design and characterize protic ionic liquids, while the UPS Toulouse group will synthesize MXenes and will investigate their electrochemical behavior in these various electrolytes. Within this WP, FSU FSU-Jena will also investigate the influence of the electrolyte composition on the electrochemical behavior of MXenes at various temperatures, by using TGA/DSC coupled with GS-MS and IR spectrometer. The experimental results will be critically discussed in WP3 dedicated to exploitation of the results, giving rise to common publications and presentations at international conferences.

During the first 18 months, conventional materials and electrolytes were tested to establish a reference database (UPS-UT3). The materials tested were mainly carbonaceous materials: YP50F, CMK3, CDCs. YP50F is a commercial nanoporous carbon, CDC a nanoporous carbon prepared in the lab with a pore size of 0.8 nm, and CMK3 a carbon of pore size 3 nm of synthetic origin. These materials have a variable porosity and surface condition. The reference electrolytes tested are tetraethylammonium tetrafluoroborate salt (Et4NBF4) in acetonitrile (ACN) and propylene carbonate (PC). At FSU-Jena, work during this period consisted in developing GC-MS (gas chromatography coupled with mass spectroscopy) equipment to be able to continuously record and analyze (operando) the decomposition products (gases) of electrolytes.
These studies were conducted with reference electrolytes and carbons, in addition to the work carried out by UPS-UT3. An aging protocol was then developed and used to gain additional information on storage-related degradation mechanisms (UPS-UT3). Each aging lasts about 6 weeks (profile made of galvanostatic cycling with periods of potentiostatic maintenance) and the electrodes undergo post-mortem analyzes (gas adsorption, ATG, Raman, etc ...). The reference materials (YP50F and CMK3) in 1.5M Et4NBF4 were tested. The results highlighted the difference in degradation processes between a material that has many surface functions such as YP50F (carboxyl acid functions, lactones, etc. and basic chromene, ketones, pyrone) and a material that contains only few or no surface groups CMK3 (synthetic origin, pore size 3 nm).
At FSU-Jena, the materials were tested in conventional electrolytes using the GC-MS technique. The results show a time- and voltage-dependent evolution of various decomposition products in the liquid electrolyte. We were able to distinguish degradation reactions occurring at the positive and/or negative electrode. In addition, we were able to observe that degradation is strongly affected by the surface chemistry of carbon electrodes. We have also started the synthesis and characterization of MXene materials in the various electrolytes, with delay.

During the second period (18-36 months), the aging of CMK3 and YP50F materials will be done in advanced electrolytes developed at FSU-Jena, such as the adiponitrile (DNA) 0.7M And4NBF4 solutions which has a higher electrochemical stability window. In parallel, the surface functions present on the surface of materials will be quantified using the Boehm method. Aging on oxidized YP50F (oYP50F) and reduced YP50F (YP50F) will also be performed to know the influence of the presence of oxygen functions on ageing mechanisms. Ageing studies will be initiated with CDC 0.8nm which has a smaller pore size and surface area free of oxygen function, in order to study the influence of pore size (CMk3/CDC comparison) and surface functions (CDC/YP50F) on aging.
This CDC 0.8nm material will also allow for discriminating the influence of the pore size distribution, the latter having a very tight distribution compared to YP50F. At FSU-Jena, advanced electrolyte formulations will be tested in GC-MS with YP50 and CMK3 carbons, and the results compared with aging in reference electrolytes. One PhD student from UPS-UT3 PhD will join FSU-Jena to do GC-MS and Raman spectroscopy in-situ. A similar exchange is planned with a PhD student from FSU-Jena. Thanks to the combination of operando analyses by GC-MS (FSU-Jena) and analyses made at UPS-UT3 (gas adsorption, ATG, Raman, etc.) in advanced electrolytes at FSU-Jena, we hope to be able to propose avenues for the development of stable electrolytes with high potentials. We will also focus on the characterization of the aging of MXenes

“Fast Charging Materials for High Power Applications”
B. Babu, P. Simon, A. Balducci, Advanced Energy Materials (2020) 10 2001128

In the last years, tremendous efforts have been dedicated toward the development of advanced materials suitable for electrochemical energy storage devices such as Electrochemical Capacitors (ECs).

Novel electrolytes as well as novel active materials have been proposed, and it has been shown that these materials display favorable properties, which could indeed allow the realization of new storage systems. Nevertheless, it is now necessary to deeply understand the storage process which is taking place when these novel electrolytes and materials are used together.

The aim of this collaborative project between the research groups of Prof. Balducci at the Friedrich Schiller University of Jena (Germany) and that of Prof. Simon at the Paul Sabatier University of Toulouse (France) is to synthesize new electrolytes for designing high energy density ECs; this will need to understand the charge storage mechanism of carbonaceous and pseudo-capacitive materials in these electrolytes.

In order to reach these challenging objectives, the two group are proposing a project divided in 3 work packages (WP).

WP1 will be dedicated to the investigation of the electrochemical properties of nanoporous carbon in new high voltage electrolytes. FSU Jena group will design a number of non-aqueous electrolytes, while the UPS Toulouse group will synthesize the carbonaceous materials and will investigate their electrochemical behavior in the new electrolytes. Furthermore, UPS Toulouse will investigate the ion dynamics into porous carbon electrodes, especially the ion desolvation process, via advanced electrochemical methods like in-situ Electrochemical Quartz-Crystal Microbalance.

WP2 will be dedicated to the investigation of the influence of non-aqueous electrolytes on the storage mechanism of MXenes. FSU Jena group will design and characterize protic ionic liquids, while the UPS Toulouse group will prepare MXenes and investigate their electrochemical behavior in the new électrolytes prepared by Jena (including those prepared in WP1). Within this WP, FSU Jena will also investigate the influence of the electrolyte composition on the electrochemical behavior of MXenes at various temperatures at both discharged and charged states. This will be done by using thermogravimetric analyses byTGA/DSC coupled with mass spectroscopy (GS-MS) and Infra-Red measurements.

The experimental results will be critically discussed in WP3, giving rise to common publications and presentations at international conferences.