Following the preliminary results that have been reported in the Description of the (DoA, Action (Annex 1 GA), i.e. the experimental and theoretical proof that Fe-doped CeO2 provides a novel Li+ charge storage mechanism involving the atom-level reduction of Fe to the metallic state while the surrounding CeO2 crystal structure is well maintained (see the schematic illustration in Figure 1) [1], we have subsequently confirmed that this new charge storage mechanism occurs also for Na+ and K+ as alternative charge carriers [2].
[Figure 1]
Figure 1. Illustration of the Li+ storage mechanism in Ce0.9Fe0.1O2-δ [1].
Interestingly, the comparison of the results obtained for Li+, Na+ and K+ reveals a clear size effect with regard to the diameter of the charge carrier cations, i.e. the amount of the charge carrier that can be reversibly inserted into the electrode material host structure decreases with an increasing size of the charge carrier cation in the order Li+ > Na+ > K+. Such a clear and straightforward impact of the ionic radius of the charge carrier is generally expected, but there are only very few examples for which this has been observed so far – and none for which this has been observed in such “simplicity” and “straightforwardness”. These findings marked a first large step towards our key objective to develop an in-depth understanding of the reaction mechanism depending on the charge carrier chemistry.
In a subsequent step, we replaced Fe by several other ARCs. This replacement did not have any significant effect on the mechanism, confirming that the RACER mechanism is not limited to Fe-doped CeO2.
In parallel to these studies towards an in-depth understanding of the new reaction mechanism and the impact of the ARC and charge carrier chemistry, we explored the impact of the eventual particle size, since the so far synthesized nanoparticles are highly suitable for fundamental studies on the reaction mechanism, but less appropriate for a potential commercialization owing to the low packing density. As some first preliminary experiments revealed a somewhat inferior effect when increasing the average diameter to the micrometer scale, we came up with the idea to synthesize advanced poly-crystalline materials instead – also with an average diameter that approaches the micrometer scale. Remarkably, these advanced polycrystals show a superior performance compared to the “simple” nanoparticles – at least in the case of neat CeO2. Further improvement can be achieved by introducing Fe and/or other ARCs, as perfectly in line with all previous results.
Last but not least, we explored also other metal oxides as potential basis for developing novel RACER materials showing the same reaction mechanism. One potential candidate is Nb2O5. As the reaction mechanism and the potentially successful doping depend intrinsically on the crystal structure, while the reduction of the ARC to the metallic state occurs commonly at lower potentials, we first synthesized a set of different crystal structures with a focus on monoclinic [3] and orthorhombic Nb2O5 [4] and conducted an in-depth investigation of the charge storage mechanism with a focus on the low potential region. At this rather early stage, still, we may briefly highlight already the great potential of such materials for (very) low temperature applications such as, for instance, in the case of batteries made for space applications. We showed this exemplarily for orthorhombic Nb2O5 (T-Nb2O5), comparing the performance of state-of-the-art graphite-based anodes with T-Nb2O5-based ones [4]. The results reveal a substantially better capacity retention at 0 °C and -20 °C (Figure 6), eventually resulting in a superior specific energy of full-cells with a “theoretical” LNMO cathode (Figure 7).
References
[1] Y.-J. Ma, Y. Ma, G. Giuli, H. Euchner, A. Groß, G.O. Lepore, F. d’Acapito, D. Geiger, J. Biskupek, U. Kaiser, A. Carlsson, T. Diemant, R.J. Behm, M. Kuenzel, S. Passerini, D. Bresser, Advanced Energy Materials 2020 (10) 25, 2000783.
[2] Y.-J. Ma, Y. Ma, H. Euchner, X. Liu, H. Zhang, B. Qin, D. Geiger, J. Biskupek, A. Carlsson, U. Kaiser, A. Groß, S. Indris, S. Passerini, D. Bresser, ACS Energy Letters 2021 (6) 915-924.
[3] X. Xue, J. Asenbauer, T. Eisenmann, G.O. Lepore, F. d’Acapito, S. Xing, J. Tübke, A. Mullaliu, Y. Li, D. Geiger, J. Biskupek, U. Kaiser, D. Steinle, A. Birrozzi, D. Bresser, Small Structures 2024 (5) 6, 2300545.
[4] X. Xue, J. Asenbauer, D. Bresser, IEEE Xplore, Proceedings of the 13th European Space Power Conference (ESPC), Elche, Spain, 02.-06.10.2023 DOI: 10.1109/ESPC59009.2023.10412706.
