Structural and Electrochemical Characterization of ZnFeO-Effect of Aliovalent Doping on the Li⁺ Storage Mechanism.

In order to further improve the energy and power density of state-of-the-art lithium-ion batteries (LIBs), new cell chemistries and, therefore, new active materials with alternative storage mechanisms are needed. Herein, we report on the structural and electrochemical characterization of Fe-doped ZnO samples with varying dopant concentrations, potentially serving as anode for LIBs (Rechargeable lithium-ion batteries). The wurtzite structure of the ZnFeO samples (with x ranging from 0 to 0.12) has been refined via the Rietveld method. Cell parameters change only slightly with the Fe content, whereas the crystallinity is strongly affected, presumably due to the presence of defects induced by the Fe substitution for Zn. XANES (X-ray absorption near edge structure) data recorded ex situ for ZnFeO electrodes at different states of charge indicated that Fe, dominantly trivalent in the pristine anode, partially reduces to Fe upon discharge. This finding was supported by a detailed galvanostatic and potentiodynamic investigation of ZnFeO-based electrodes, confirming such an initial reduction of Fe to Fe at potentials higher than 1.2 V (vs. Li⁺/Li) upon the initial lithiation, i.e., discharge. Both structural and electrochemical data strongly suggest the presence of cationic vacancies at the tetrahedral sites, induced by the presence of Fe (i.e., one cationic vacancy for every two Fe present in the sample), allowing for the initial Li⁺ insertion into the ZnO lattice prior to the subsequent conversion and alloying reaction.