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Solvated Ions in Solid Electrodes: Alternative routes toward rechargeable batteries based on abundant elements

Periodic Reporting for period 1 - SEED (Solvated Ions in Solid Electrodes: Alternative routes toward rechargeable batteries based on abundant elements)

Reporting period: 2020-06-01 to 2021-11-30

The transition from fossil to renewable energies is one of the largest and most urgent challenges humanity faces. The energy transition will be only successful if energy can be stored in a safe, efficient and sustainable way. This especially relates to electrifying the mobility sector (EVs) and storing renwable energy. Rechargeable batteries are the key towards this.
Since its commercialization in 1991, the lithium-ion battery technology is a real success story. Costs have been dramatically reduced and the energy density increased from less than 100 Wh/kg to almost 300 Wh/kg today. On the other hand, the technology is expected to reach its physical limits within the next years. At the same time, the demand for LIBs is projected to increase manifold times in the near future. Concerns have been raised whether this demand will create issues related to element resources, market access and supply chains on the long-term. The global distribution of lithium and cobalt resources are highly unequal with Europe being in an inferior situation. Numerous (sometimes controversial) reports on the availability of “battery materials” were published recently. Although long-term predictions are naturally difficult, there is broad consensus that the development of alternative batteries based on abundant elements is an important strategy to minimize the risk of resource depletion or of restricted access. This has recently resurged a lot of interest in alternatives to LIBs. A major approach is to adopt the LIB rocking chair concept to other working ions, i.e. replacing Li+ by more abundant ions such as Na+, K+, Mg2+, Ca2+ or Al3+, while at the same time also avoiding other critical elements. An additional argument for multivalent ions is that their use is one of the very few options to theoretically surpass the charge/energy density of LIBs.
On the other hand, the electrochemistry of alternative ions can be an extreme challenge, especially considering multivalent ions. Unconventional approaches are therefore needed to overcome unfavourable ion size and/or polarization effects.
The SEED project is such an unconventional approach, breaking with what has been common sense in the design of electrode materials so far: Instead of intercalating ions into solid electrodes, the SEED project aims at intercalating solvated ions into solid electrodes.

Overall objectives are:
- Understand the intercalation of solvated ions into solid electrodes
- Starting from graphite, explore the concept for other materials
- Starting with abundant Na+, explore the concept for multivalent ions
- Combine experiment with theory.
The SEED project started on time. Despite the challenges related to the Covid-19 pandemic, all staff was hired (2 postdocs, 2 PhD students) and all equipment has been purchased. Further supported is provided by two other group members that contribute part-time to the project (a postdoc and a technician).

Progress and major achievements:

WP1: Different solvents were systematically studied with graphite as the host material. With Na+, ethylenediamine was successfully reported as co-solvent that helped to reduce the expansion of the graphite (M1). This work was disseminated over one conference. Electrolytes using glymes with other co-solvents have also been investigated and we have seen positive preliminary results. In the case of multivalent ions (M2), Mg co-intercalation was studied with graphite electrodes in different electrolytes that resulted in a Master thesis. The study of the phase behaviour and kinetics has recently started by one of the PhD student.

WP2: This work package is continuously developing with carbon materials with different loadings and binders in addition to new electrode materials (e.g. layered sulfides, Prussian Blue Analogues, polyanionic compounds (M8)). In situ/operando experiments such as dilatometry, Raman and XRD has been used to monitor the structural changes (M7).

WP3: The study on the side reactions and interfaces was continuously monitor with electrochemical measurements (impedance, galvanostatic cycling), SEM and variation of pressure operando measurements. The sub package DEMS analysis has not been started yet but is planned to start in the near future.

WP4: A library with key properties of the different solvents has been built and is continuously extended. This library contains both our experimental measurements and static DFT calculations and we are looking for key features of the solvent molecules that enables us to predict if a solvent will co-intercalation.

WP5: Ab initio molecular dynamics simulations on structure and stability of the solvation shells in glyme based electrolytes have finished and the results are being analysed. Simulations on new electrolytes identified as promising in WP1 started have recently started. Computational resources for simulations of electrodes and electrode-solvent interactions and the charge transfer process are in the process of being acquired.

One master thesis (Junlin Chen, Studies on Mg storage in Graphite through solvent co-intercalation) was completed in 10/2021. A master thesis by Moritz Exner on the use of in situ Raman spectroscopy for characterizing co-intercalation reactions will be submitted 12/2021. A bachelor thesis will be completed in 2022 and another master student project is scheduled to start in 3/2022.

Overall, the work accomplished in this period resulted in the publication of two articles. Moreover, the results were disseminated at 5 conferences with 1 oral presentation and 4 poster presentations. In addition, the PI presented the concept and results of the project on various conferences.
The SEED project explores the concept of using solvated ions in solid electrodes for the reversible storage of a variety of ions. As the solvation shell acts as electrostatic shield and can be tuned in its composition, lattice polarization and charge transfer resistance can be minimized.
Using this effect, the SEED project aims at reversible charge storage of multivalent ions in solids with properties far beyond current state-of-the art.
Classical ion storage vs. SEED project