Electrochemical Reactivity with Lithium of Spinel-type ZnFe_2–yCr_yO₄ (0≤ y≤ 2)

Members of the spinel solid solution series ZnFe_2–yCr_yO₄ (y = 0, 0.5, 1.0, 1.5, and 2) were synthesized using high-energy ball milling followed by annealing at 1000 °C. The structural study of the samples was performed by Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), and powder X-ray diffraction (XRD). While XRD verified the formation of single spinel phases with lattice parameters reduced by increasing Cr substitution, FTIR and XAS provided insight into the subsequently increased covalence of the chemical bonding of the spinels. The mixed transition-metal spinel oxides were employed as working electrodes in Li metal batteries. In agreement with the literature, the spinel oxides experience amorphization during the first discharge, as shown by ex situ XRD and selected area electron diffraction (SAED). The electrochemical activity of the spinel oxides was found to diminish with Cr content so that ZnCr₂O₄ is completely inactive even when the material is nanosized and in the presence of a large amount of conductive additive. Comparison with mixtures of ZnO and Cr₂O₃ led to the conclusion that the conducting band of the ternary oxide, which would be injected with electrons during reduction, is raised with respect to the individual binary oxides to the point that the overpotential required to drive a conversion reaction displaces the experimental electrochemical potential to be extremely close to, or even lower than, that of Li metal.