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

Periodic Reporting for period 2 - SEED (Solvated Ions in Solid Electrodes: Alternative routes toward rechargeable batteries based on abundant elements)

Reporting period: 2021-12-01 to 2023-05-31

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 is allowing a better understanding and expanding the concept of intercalation of solvated ions in electrodes. It was shown that other solvents besides glymes can also co-intercalate, which significantly expands the chemical scope of the concept. It is particularly noteworthy that the concept was successfully transferred to a cathode material (TiS2). Thus, the functional principle of a cointercalation battery (CoIB) could be demonstrated for the first time. In this type of battery, diglyme was used as a solvent, which intercalates with sodium ions in graphite (anode) and TiS2 (cathode) during charging and discharging of the battery. The course of the reaction was investigated "in operation" using various methods and supported by theoretical calculations.
In the project, the state-of-the-art is clearly being exceeded. For example, it was possible to significantly reduce the expansion of the electrode during the intercalation of solvated ions by around 30 %. This was achieved by combining different solvents. Furthermore, the concept of a cointercalation battery (CoIB) was demonstrated for the first time. Further information is given in this article. doi:10.1002/aenm.202202377. The results show that the concept of a CoIB can work in principle. In the second phase of the project, various measurements will be investigated for further improvement. In addition, a better understanding on the charge transfer at the interface between electrolyte and electrode will be achieved which will provide a clearer picture on how well the concept will work for high-power applications.
Classical ion storage vs. SEED project