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

The precise determination of de-/lithiation mechanisms in alternative lithium-ion battery electrode materials is crucial for their potential future success, but quite challenging — e.g., due to the occurrence of multiple crystalline and (frequently) amorphous phases. Herein, we report an in-depth ex situ/operando characterization of (carbon-coated) Fe- and Co-doped zinc oxide anodes via X-ray absorption spectroscopy to probe the oxidation state and local structural environment of the different metals upon de-/lithiation. The results provide fundamental insights into the mechanism of the conversion and alloying reaction taking place for these two active materials. In addition, this comparative investigation allows for an evaluation of the impact of isovalent (Co2+) and aliovalent (Fe3+) doping on the lithiation mechanism, having an impact on the initial lithiation kinetics, while both dopants generally enable a greatly increased re-oxidation of zinc compared to pure zinc oxide and, thus, a more reversible conversion reaction.