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PRINCIPAL PUBLICATION AND AUTHORS

Isovalent vs. aliovalent transition metal doping of zinc oxide lithium-ion battery anodes in-depth investigation by ex situ and operando X-ray absorption spectroscopy, A. Trapananti (a), T. Eisenmann (b,c), G. Giuli (d), F. Mueller (b,c), A. Moretti (b,c), S. Passerini (b,c), D. Bresser (b,c), Mater. Today Chem. 20, 100478 (2021); https:/doi.org/10.1016/j.mtchem.2021.100478 (a) School of Science and Technology, Physics Division, University of Camerino (Italy) (b) Helmholtz Institute Ulm (HIU), Ulm (Germany) (c) Karlsruhe Institute of Technology (KIT), Karlsruhe (Germany) (d) School of Science and Technology, Geology Division, University of Camerino (Italy)

REFERENCES [1] M. Armand et al., J. Power Sources 479, 228708 (2020). [2] D. Bresser et al., Energy Environ. Sci. 9, 3348 (2016). [3] G. Giuli et al., Inorg. Chem. 54, 9393 (2015). [4] C.J. Pelliccione et al., J. Electrochem. Soc. 162, A1935 (2015).

which indicates the formation of the LixZn alloy when the discharge is completed. At the end of the charge process, the restored spectral features of the oxide phase indicate a high degree of reconversion to Zn2+. The quantitative analysis via linear combination fitting (LCF) allowed to quantify the metallic and oxide fractions along the lithiation/de-lithiation reaction (Figure 139c and 139e). At the end of the first cycle, the fraction of re-oxidised zinc is almost 80% for both dopants. This fraction is substantially higher than what has been reported for pure ZnO [4], revealing the beneficial impact of the doping on the reversibility of the conversion reaction. The re-oxidation occurs at almost the same level, independent of the chemical nature and oxidation state of the dopant.

The analysis of the EXAFS signals (Figure 140) provides further insights into the metallic phases formed along the lithiation reaction. The reduction yields highly damped single-shell EXAFS signals of metallic Fe and Co phases, and the quantitative analysis indicates the formation of very small (few nm) and/or highly defective metallic

nanoclusters. Eventually, the initial oxidation state of the metal dopant affects the initial reduction kinetics, with the Fe (and also Zn) reduction occurring at significantly higher potentials (Figures 139b, 139d and Figure 140), at least partially owing to the initial Li+ insertion favoured by vacancies associated with the aliovalent doping. The results of the measurements under operando conditions are fully consistent with the ex-situ data and, therefore, any potential impact of the ex-situ sample processing could be ruled out.

The overall results of this study provide fundamental insights into the mechanism of the conversion-alloying reaction taking place for these two active materials, which is crucial for their potential application and further development. The comparative investigation allowed to establish that isovalent (Co2+) and aliovalent (Fe3+) doping influence the initial lithiation kinetics, while both dopants generally enable a greatly increased re-oxidation of zinc compared to pure zinc oxide and, thus, a substantially higher reversible capacity.

Fig. 140: Magnitude of the Fourier transform of k2-weighted EXAFS signals collected ex-situ at the Co K-edge on Zn0.9Co0.1O anodes (a) and Fe K-edge on carbon-coated Zn0.9Fe0.1O anodes (b) recovered at the indicated potential values along the first discharge (adapted from principal publication with permission from Elsevier).