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Versatile Ionomers for DIvalent CAlcium baTteries

Periodic Reporting for period 2 - VIDICAT (Versatile Ionomers for DIvalent CAlcium baTteries)

Reporting period: 2020-03-01 to 2021-08-31

VIDICAT aims at developing Calcium Rechargeable Batteries through the invention of Versatile Ionomers for Divalent Calcium batteries: why batteries? Transport, housing and industry increase Green House Gas emissions adversely impacts on global warming. While fully decarbonized energy as nuclear electricity and hydroelectricity produce unceasing electricity, the intermittency of renewable electricity requires an efficient storage to smooth production. As for the electrification of vehicle fleets, it will progressively suppress urban pollution and then, depending on the well to wheel efficiency, more or less decrease the CO2 emissions. The recent dramatic war against Ukraine banning, at short-term, Russian gas and petroleum should accelerate, together with new nuclear plants, energy transition towards renewable energies (solar PV and wind farms) both demanding electricity storage i.e. batteries to smooth their discontinuous electricity production.
Why post-lithium batteries? The huge energy storage-demand involves an explosion of the battery production, which will face the problem of the limited resources in lithium ores, non-located in Europe. Rechargeable calcium batteries, CaB, are very promising in terms of theoretical energy density, safety, and sustainability. Calcium is very abundant in European ores as both limestone and gypsum and is not ranked as a Critical Raw Material (CRM).
The very high melting temperature of calcium metal, Ca0, which exceeds 830ºC, as compared to 180 and 96ºC for Li0 and Na0 respectively, is an indisputable safety asset for CaB. However, the lack of reliable electrolytes so far impedes the practical research on CaB. VIDICAT aims at developing new material concepts based on multifunctional electrolyte and electrode binder. Such approach will provide at the interface with Ca0, chemically, electrochemically and thermally stable electrolytes.
VIDICAT is a FETOPEN project whose objective is the development, at the European level of disruptive, thus risky, research approaches. In agreement with FETOPEN philosophy, VIDICAT has proposed, in a bottom-up approach from monomer to electrolyte film and electrodes, the invention of a single-cation conducting polymer endowed with ionic functions as well as with solvating and polar ones. This conception-driven approach pretends avoiding (i) possible dendrite growth (ii) Ca0 interface degradation and (iii) accommodating electrode volumic changes while improving the CaB safety. To compensate the unavoidable conductivity drop due to the loss of anionic conductivity, a sharp slimming of the electrolyte film was scheduled. Very thin electrolytes are targeted, with cationic conductance close to standard lithium ones. From the beginning of the project, new ionic monomers and ionomers have been prepared and paved the way for future Innovative Polymer Electrolytes. In the frame of developing this groundbreaking electrolyte concept, VIDICAT also searches for positive electrodes towards reliable and safe CaB. Besides, preliminary results allow Ca/S to be considered and Calcium vanadates have also demonstrated interesting cyclic ability.
VIDICAT objective is achieving a CaB prototype with energy density similar to State of Art LiB. Multidisciplinary, the VIDICAT project aims at increasing EU capacity in building low-carbon energy. Based on sustainable materials and approaches, VIDICAT will prevent the loss of non-renewable chemicals and paves the way for Europe energetic independence. FETOPEN philosophy addresses disruptive research and technology but also pays attention to the Impacts of the projects on existing technologies e.g. Lithium-ion batteries or Lithium polymer batteries. In that sense, many of the new molecular and macromolecular products could advantageously be used in lithium, sodium or magnesium batteries and a few of them could be tested as electrolyte membrane of Proton Exchange Membrane Fuel Cell. The patentability of some of these innovations is currently considered.
From the beginning of VIDICAT, creativity has never been neglected and several ionic monomers, polar monomers and their related ionomers have been invented. We upscaled at the level of tens of grams, even hundreds of grams, the syntheses of new molecular and macromolecular materials. We evidenced that several calcium salts can be dissolved in commercial poly(oxyethylene), POE or PEO, up to fairly high salt concentrations. We scrutinized the possible microphase separation leading to highly concentrated salt solutions dispersed in salt-free polymer, a source of conductivity instability. The conclusion of our study is that the Ca salts/POE electrolytes do not undergo, up to more than 100°C, any microphase separation. Due to the crystallinity of the POE-based calcium polymer electrolytes, CAPE, the CAPEs exhibit low ionic conductivities below roughly 50°C. On contrary, above 50°C (melting of crystalline phase), ionic conductivities close or higher than the lithium ones have been obtained. Due to the linear nature of the POE host polymer and to its low glass transition temperature, the CAPEs start creeping around 90°C: this is a clear safety issue. To suppress the crystallinity at low temperatures while improving the thermomechanical stability we prepared CAPEs based on home-made POE networks, POE-3D. Their crystallinity vanishes from ambient temperatures while, due to their 3 dimensionality, creeping does not occur. As a result the ionic conductivity of POE-3D based CAPEs is significantly increased and the safety is preserved. Based on the same type of polymeric skeleton new Calcium-conducting Ionomers have been prepared and investigated as solvent-free electrolytes. In parallel, in a step-by-step approach the new POE-3D based CAPEs have been used to prepare solvent-free composite cathodes. Now mastered, we are currently moving from these CAPEs based composite cathodes to Calcium Ionomers based electrodes. In addition to the design, preparation of new calcium electrolytes and related composite electrodes, a great attention has been paid to the understanding of calcium electrolytes using, in particular, oligoethers mimicking the macromolecular backbone of calcium ionomers. The physicochemical studies, based on Electrochemical Impedance Spectroscopy, on RAMAN spectroscopy and on theoretical calculations, allow the solvation/de-solvation of Ca2+/glymes to be understood.
The lockdown has significantly impacted the production of Ionomers, since it demands laboratory access. Nonetheless several new monomers endowed with ionic functions or polar ones have been created. If the project management implies selecting rapidly some materials to be supplied to the whole of the partners, the other materials remain of great interest and their assessment will be continued even after the end of the project. As specified in the FETOPEN guidelines, it is essential to develop materials, systems or concepts usable in mature technologies or in technologies close to the market. In that sense, some cathode materials can be used both in calcium cells and in lithium cells. The new Ionomers are versatile ionomers that can be turned in single alkaline cation-conducting polymers (Li+, Na+, K+) but also in single alkaline-earth cation-conducting polymers (Mg2+, Ca2+). The progresses in upscaling of several molecular and macromolecular materials pave the way to future collaboration (after patents filing) with battery manufacturers.